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5068.Billiau A. Vandenbroeck K. - IFNg .pdf

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IFN
Alfons Billiau* and Koen Vandenbroeck
Rega Institute, University of Leuven, Minderbroedersstraat 10, B-3000 Leuven, Belgium
* corresponding author tel: +32-16-337349, fax: +32-16-337340,
e-mail: Alfons.Billiau@RegaKULeuven.ac.be
DOI: 10.1006/rwcy.2000.07002.
SUMMARY
Interferon (IFN) is produced mainly by activated
lymphocytes and has receptors on virtually all cell
types of the body. It thus exerts a multitude of cellular
biological effects. Among cytokines, IFN is the main
activator of macrophages. In addition it also activates
endothelial cells. Together, these two activities are the
basis for assigning a proinflammatory role to IFN.
The regulatory role of IFN on the antigen-specific
phases of the immune response derive from its effects
on antigen-presenting cells and on B and T lymphocytes. IFN augments expression of MHC molecules
in professional as well as nonprofessional antigenpresenting cells; its effects on B and T lymphocyte
proliferation and differentiation are complex. Production of IFN by T helper cells is a hallmark of
the TH1-type phenotype. Thus, high-level production
of IFN is typically associated with effective host
defense against intracellular pathogens, and with
immune and autoimmune pathology that depends
upon delayed-type hypersensitivity.
Genetic defects in the IFN system are rare; genetic
polymorphism exists and its impact on disease susceptibility is under study. Despite the powerful immunoregulatory potential of IFN and the availability
of preparations for administration to patients, clinical
applications are limited.
The reader is referred to previously published
reviews on IFN by Young and Hardy (1995), Young
(1996), Billiau (1996a, 1996b), Boehm et al. (1997),
and Stark et al. (1998).
BACKGROUND
Discovery
The protein which we now call interferon (IFN)
was discovered independently by two groups of
investigators and was originally given two different
names after the biological activities studied: (1) type II
or immune interferon, or IFN, and (2) macrophageactivating factor (MAF). Homonymy of IFN with
IFN and IFN does not imply molecular relationship but merely reflects sharing of the biological
property to be able to protect cells against virus
infection. The corresponding bioassay is the so-called
`antiviral assay', which consists in demonstrating that
cultured cells exposed to IFN resist destruction by
a standard challenge virus. Historically, the name
`interferon' refers to the phenomenon of interference,
i.e. the fact that cells infected with any one virus
species are relatively resistant to infection with viruses
of other species. However, whereas IFN and IFN
(collectively called type I interferons) do play a role in
such interference, IFN is not involved.
In 1965 Wheelock and coworkers (Wheelock, 1965),
demonstrated that an interferon-like antiviral activity, different from classical interferon by its lability in
acid, appears in supernatants of mononuclear cells
exposed to a mitogen. In fact, a 1964 report by
Gresser and Nacify mentions the occurrence of acidlabile interferon in cerebrospinal fluids of patients
with infectious and noninfectious neurological diseases (Gresser and Nacify, 1964). In the early 1970s
the terms `type II interferon' (Youngner and Salvin,
1973) and `immune interferon' (Falcoff, 1972) were
coined. The term immune interferon remained in use
for some time in recognition of the awareness that the
activity is associated with a protein physicochemically
distinct from that responsible for the then classical
interferon, that production of the factor is the prerogative of immune competent cell types, and that
the factor possesses immune regulatory properties
distinct from those of classical interferon. In 1980 an
International Interferon Nomenclature Committee
(Stewart et al., 1980) agreed on a new name for all
interferons. Type I interferons, then known as leukocyte and fibroblast interferons, were renamed to
642 Alfons Billiau and Koen Vandenbroeck
IFN and IFN, respectively; immune interferon was
renamed to IFN.
Availability of pure preparations of IFN and of
monospecific antibodies made it possible to prove
that MAF activity in biological fluids is largely if not
entirely accounted for by IFN (Nathan et al., 1983).
The term MAF appears in the literature around 1980,
referring to the ability of supernatants of mitogenor antigen-challenged mononuclear cell cultures to
augment various biological activities of macrophages.
That stimulated lymphocytes' supernatants have these
macrophage-stimulating abilities was first reported
in 1966 by Bloom and Bennett. MAF bioassays have
variably relied on intracellular killing of parasites or
increased oxidative metabolism (Nathan et al., 1983),
enhanced expression of class II antigens (Steeg et al.,
1982) or enhanced tumor cell killing (Weinberg et al.,
1978). Characterization of MAF with monoclonal
antibodies soon revealed its identity with IFN
(Nathan et al., 1983).
Alternative names
For names used previously see section on Discovery.
Structure
The size, amino acid sequence, and glycosylation of
IFN are well conserved among animal species, and
most studies on the structure of the glycoprotein have
been done on either human or mouse IFN. In its
biologically active form IFN is a 34 kDa homodimer stabilized by noncovalent forces. The peptide is
N-glycosylated on two sites. Natural IFN is heterogeneous in size and charge, due to enzymatic trimming of the C-terminus and to variation in degree of
glycosylation. This heterogeneity seems to be unimportant for biological activity on cells but might well
influence the dynamics of tissue distribution.
X-ray crystallographic analysis (Ealick et al., 1991)
has revealed the subunits to consist of six helices,
accounting for 62% of the molecule, with no sheet
domains. The two subunits, each of which is elongated in shape, are held together in an antiparallel
configuration by intertwining of the helical domains.
The N-terminus of each chain is juxtaposed to the
C-terminus of the opposing chain. The symmetry in
this structure allows IFN to bind a pair of identical
receptor peptides, as has been demonstrated by
analysis of the crystal structure of an IFN/IFNR
complex. Peptide stretches at both the N- and the
C-termini of IFN seem to have binding properties
for the receptor, as demonstrated by competition-type
experiments using the corresponding peptides (Griggs
et al., 1992). The N-terminal binding region has been
identified (Van Volkenburg et al., 1993).
Main activities and
pathophysiological roles
IFN is a typical lymphokine, i.e. a product mainly
but not exclusively of lymphocytes: T cells and NK
cells. Under normal circumstances only minute but
perhaps physiologically important quantities are produced. Larger quantities are produced only under
pathologic circumstances such as trauma, infection,
cancer, and autoimmunity. IFN acts on cells by
inducing increased expression of several genes, the
spectrum of which varies depending on the cell type
concerned and on the presence of other cytokines,
some of which (e.g. TNF) synergize with IFN, while
others (e.g. IL-4) antagonize its actions.
Receptors for IFN occur on virtually all cells of
the body, so that many organs and systems undergo
the action of IFN. IFN regulates cellular activities
responsible for inflammation: the activation state of
macrophages and endothelial cells. It also regulates
the antigen-specific immune response by affecting
both antigen-presenting cells and antigen-recognizing
lymphocytes. Thus IFN is important in the initial
phase of immune reactions (aspecific inflammatory
and antigen-presenting), in the middle phase (expansion and differentiation of antigen-reactive lymphocyte clones), and in the end phase (sustained or final
inflammation).
Defects in the IFN mechanism are associated with
severe impairment of resistance to infections caused
by viruses and certain bacteria, in particular those
which are normally killed by activated macrophages.
Although IFN is often quoted to be a `proinflammatory' cytokine, the production of IFN during
infections or as a result of immune reactions can under
some circumstances enhance and in others inhibit
inflammation and accompanying tissue damage.
However, massive production of IFN, occurring as
part of so-called acute cytokine release syndromes, is
often associated with severe systemic manifestations,
such as generalized bleedings and lethal shock.
GENE AND GENE REGULATION
Accession numbers
Accession numbers for some mammalian and bird
IFN cDNA (and genomic DNA) sequences are
given in Table 1.
IFN
Table 1 Accession numbers for IFN DNA of some mammalian and bird species (EMBL/
GenBank/DDBJ)
Species
Genome
cDNA or CDS
V00536
X01992
Primates
Human
J00219
X13274
a
M37265
X87308
M29383
V00543
Common marmoset (Callithrix jacchus)
X64659
±
Olive baboon (Papio hamadryas anubis)
±
To be added
Crab-eating macaque (Macaca fascicularis)
±
D89985
Rhesus monkey (Macaca mulatta)
±
L26024
Red-crowned mangabey (Cercocebus torquatus)
±
L26025
Pig-tailed macaque (Macaca nemestrina)
±
L26026
±
X86972
Carnivores
Cat (Felis catus)
D30619
±
Y11647b
Cow (Bos taurus)
Z54144
M29867
Goat (Capra hircus)
±
U34232
Eurasian badger (Meles meles)
Artiodactyla
c
Z92887
X52640
Z73273a
A19173
Red deer (Cervus elaphus)
±
X63079
Pig (Sus scrofa)
X53085
S63967
A11777
U04050
Sheep (Ovis aries)
L07502
Perissodactyla
Horse (Equus caballus)
D28520
Rodentia
House mouse (Mus musculus)
±
K00083
M28621
Norway rat (Rattus norvegicus)
X02325
d
X02326d
X02327
M29317
AF010466
d
Mongolian gerbil (Meriones unguiculatus)
±
L37782
Woodchuck (Marmota monax)
±
Y14138
±
AB010386
Common turkey (Meleagris gallopavo)
±
AJ000725
Helmeted guinea fowl (Numida meleagris)
±
AJ001263
Lagomorpha
Rabbit (Oryctolagus cuniculus)
Aves
643
644 Alfons Billiau and Koen Vandenbroeck
Table I (Continued )
Species
Genome
cDNA or CDS
Chicken (Gallus gallus)
Y07922
U27465
X99774
Japanese quail (Coturnix japonica)
±
AJ001678
Ring-necked pheasant (Phasianus colchicus)
±
AJ001289
a
VNTR polymorphisms in intron 1 of human and ovine IFN genes.
b
Incomplete sequences.
c
Promoter fragment of the ovine IFN gene.
d
Unassembled exon sequences of the rat IFN gene.
In all species characterized to date, IFN is found
to be coded for by a single-copy gene consisting of
four exons. From 20 mammalian and five avian species, IFN-coding DNA sequences (CDS) are now
available (Table 1). In addition, genomic regions
spanning the structural gene of human, marmoset,
cow, horse, pig, and chicken IFN have been sequenced. Comparison of these genes reveals a highly
conserved overall structure with closely matching
exon (and intron) sizes (Kaiser et al., 1998).
the IFN gene was physically localized to chromosome band 5p1.2-q1.1, close to the centromere
(Johansson et al., 1993).
Chromosome location
Regulatory sites and corresponding
transcription factors
Polymorphic sequences have been identified in the
first intron of the human, bovine, and ovine, and in
the third intron of the porcine IFN gene (Dijkmans
et al., 1990; Ellegren et al., 1993). The polymorphism
type and number of alleles, however, are different
(Table 2). In the early 1980s, the human IFN gene
was assigned to chromosome band 12q24.1 by in situ
hybridization (Trent et al., 1982). This localization
was only recently corrected by Bureau et al. (1995),
who mapped the gene close to the D12S335 and
D12S313 microsatellites on both the genetic and
physical maps, and showed physical linkage with the
MDM2 oncogene on chromosome band 12q15. This
coincides with the assignment of the gene to 12q14 by
fluorescence in situ hybridization (Zimonjic et al.,
1995). In the mouse, the Mdm1, Ifg, and Myf6 loci
were shown to be linked to each other in the telomeric
part of chromosome 10. Other linkages in the same
region include those between Ifg, Mdm1, Mdm2, and
Mdm3. Thus, the similarity between the telomeric
mouse chromosome 10 region and human chromosome band 12q15 is suggestive for a syntenic
conservation during evolution of a large chromosome
region that encompasses the IFN gene. In the pig,
Relevant linkages
Relevant linkages are discussed under the section on
Chromosome location. See also Table 2.
An overview of the most important regulatory sites
is given in Table 3. The current state of evidence
suggests a very complex mechanism of IFN gene
transcription that takes place on three different levels
of regulation: methylation, binding of activating
transcription factors, and binding of repressors (for
extensive review see Ye and Young, 1997, and Young,
1996). First, hypomethylation of CG islands in the
proximal promoter region and the first intron correlates with expression of the gene. A significant difference in methylation status was observed in human
CD4+ memory T cells (IFN producers) versus
nonproducing thymocytes, neonatal cells, and adult
naõÈ ve T cells (Melvin et al., 1995). The same holds
true for mice where the promoter is not methylated in
TH1 cells (IFN producers), but methylated in nonproducing TH2 cells (Young et al., 1994). Second,
two distinct activation-specific regulatory elements
are present in the human promoter region (pos. ÿ108
to ÿ40 bp), which are essential for induction by PMA
and ionomycin (Phenix et al., 1993) (see Table 3 for a
description). The first intron contains several important sites for control of transcriptional regulation.
IFN
645
Table 2 Chromosomal location, relevant linkages, and polymorphisms of the IFN gene
Chromosomal location
Polymorphisms
Species
Chromosome
Relevant linkages
Types
Alleles
Location
Human
12q15 (centromeric)
MDM2, D12S335,
D12S313
CA repeat
11
intron 1
Mouse
10 (telomeric)
Mdm1, Mdm2, Mdm3,
Myf6
RFLP; SphI
2
±
Pig
5p1.2-q1.1
(centromeric)
Mucin, diacylglycerol
kinase
[(GA)10(GAAG)8]n
n ˆ 1 or 2
intron 3
Sheep
±
±
(GTTT)n
n ˆ 5 or 6
intron 1
Table 3 Identification of important functional regulatory sites in the promoter and 1st intron of the IFN genea
Core motif
Position
Regulatory site
Transcription or
regulatory factors
Remarks
GGGGAGTTCC
(r.o.)b
ÿ786 to ÿ776
IFN B site
NFB
Complex formed after
T cell activation
AAAATATTCC
(r.o.)
ÿ772 to ÿ762
C3-1P
NFB
Complex formed after
T cell activation
AAAAATTTCC
ÿ278 to ÿ268
C3-3P
NF-ATc and NFB
Cyclosporin A-sensitive
site; enhanced transcription
through calcineurin-inducible
and NFB transcription
factors
ÿ251 to ÿ215
`Silencer' element
YY-1 and AP-2-like
Repression of basal and
induced transcription
repressor factors
CTATCT (r.o.)
ÿ95 to ÿ90
GATA site
GATA-3
Function not clear,
but present in a complex
from Jurkat nuclear extracts
TGTCACCA
ÿ90 to ÿ83
AP-1/CREB site
Dexamethasone inhibition
of promoter activity
TACGTAA
ÿ57 to ÿ51
AP-1/CREB site
Dexamethasone inhibition of
promoter activity
GTCACCAT
ÿ89 to ÿ82
GM-CSF/MIP-1
motif
Possibly related to expression
in memory T cells
ACGTAATCC
ÿ56 to ÿ48
NFIL2A-site
Binding of transcription
factors in Jurkat T cells;
no binding of Oct-1
TAC*GTA (SnaB1)c
ÿ54
Methylation
Methyltransferase
TATAA
ÿ28 to ÿ23
TATA box
TFIIB/TBP
GAATTTTCC
460 to 468
C3 1st intron
c-Rel and NFB
Hypomethylation correlates
to IFN expression
Located in the first intron;
complexes induced by
PMA/ionomycin; works in
concert with the promoter
IFN B and C3-3P sites to
enhance transcription
646 Alfons Billiau and Koen Vandenbroeck
Table 3 (Continued )
Core motif
Position
Regulatory site
Transcription or
regulatory factors
Remarks
CC*GG (HpaII)c
767
Methylation
Methyltransferase
Located in 1st intron
801±950
STAT sites
Located in 1st intron;
adjacent and overlapping sites
for STAT1, STAT4, STAT5,
and STAT6
a
Regulatory sites represented in this table were collected from the papers referred to in the text.
b
r.o., reverse orientation.
c
Methylation-sensitive restriction site.
Not only does it contain a c-Rel and NFB p65/p50binding element (Brown et al., 1992; Sica et al., 1992)
that enhances promoter activity, but also a pattern of
adjacent sites for STAT1, STAT4, STAT5, and
STAT6 has been identified in it (Xu et al., 1996).
NFB and NF-ATc-binding sites are found in the
promoter. NFB and calcineurin-inducible transcription factors form complexes with these regions upon
T cell activation (Sica et al., 1997). Third, several lines
of research have pointed to a crucial role for repressor
nuclear factors in silencing IFN gene transcription.
A silencer element (pos. ÿ251 to ÿ215 bp) has been
identified to which both the nuclear repressor YY-1
(yin-yang-1) and an AP-2-like repressor can bind.
This cooperative binding results in inhibition of IFN
expression (Chrivia et al., 1990; Ye et al., 1994).
Remarkably, a second YY-1-binding site has been
identified that overlaps with an AP-1-binding
enhancer site (Ye et al., 1996). While YY-1 activity
is not modulated upon activation of T cells, that of
AP-1 is upregulated. Thus, another mechanism for
repression may consist of YY-1 blocking constitutive
IFN gene transcription in resting T cells by competition for the overlapping binding site with the
enhancer AP-1.
Suppression of IFN transcription through glucocorticoids follows yet a different pathway. Dexamethasone-specific inhibition was localized to two
enhancer CREB/AP-1 sites in the proximal promoter
region (Cippitelli et al., 1995). As follows, the factors
needed for glucocorticoid-mediated inhibition of
IFN transcription utilize the same regulatory sites
required for induction of the promoter.
In conclusion, both enhancing and repressing
transcription factors appear to regulate expression
of the IFN gene through interaction with promoter
and intronic cis-elements in a concerted manner.
PROTEIN
Accession numbers
Accession numbers to IFN protein sequences
(SwissProt) and crystal structures (Brookhaven
Protein Database) are given in Table 4.
Sequence
See Figure 1.
Description of protein
The precursors of human and mouse IFN consist of
166 and 155 amino acids, respectively (Figure 1).
Following removal of the signal peptide, mature
forms with theoretical molecular weights of 17,100
(human IFN: 143 aa) and 15,900 (murine IFN:
133 aa) are generated. While mature MuIFN
contains a single cysteine residue at its C-terminal
position, HuIFN is devoid of cysteine. This excludes
a role of disulfide bonds in tertiary structure formation. Most mammalian IFN species possess two
N-glycosylation sequons, albeit at unconserved positions. Comparison of IFN sequences of various
species shows that the most conserved sequences are
found in helices C and F, which are the two most
buried ones in the dimer. The most striking amino
acid sequence conservation, however, is seen at the
C-terminus. Here, a basic stretch of alternating lysine
and arginine residues is found (K-128-RKRS) that is
conserved in birds and mammals. During receptor
binding, residues 130±132 of this cluster are probably
targeted to an acidic stretch on the R C-terminal
IFN
647
Table 4 Accession numbers for IFN protein sequences and crystal structures for some mammalian
and bird species
Species
Protein sequence
(SwissProt)
Crystal structure
(PDB)
P05179
1HIG
Primates
Human
Common marmoset (Callithrix jacchus)
P28341
±
Crab-eating macaque (Macaca fascicularis)
P42163
±
Rhesus monkey (Macaca mulatta)
P42163
±
Red-crowned mangabey (Cercocebus torquatus)
P42162
±
Pig-tailed macaque (Macaca nemestrina)
P42163
±
P46402
±
Cow (Bos taurus)
P07353
1RFB
Goat (Capra hircus)
P79154
±
Sheep (Ovis aries)
P17773
±
Carnivores
Cat (Felis catus)
Artiodactyla
Red deer (Cervus elaphus)
P28333
±
Pig (Sus scrofa)
P17803
±
P42160
±
House mouse (Mus musculus)
P01580
±
Norway rat (Rattus norvegicus)
P01581
±
Mongolian gerbil (Meriones unguiculatus)
Q62574
±
P30123a
2RIG
P49708
±
Perissodactyla
Horse (Equus caballus)
Rodentia
Lagomorpha
Rabbit (Oryctolagus cuniculus)
Aves
Chicken (Gallus gallus)
a
Incomplete sequence.
domain, resulting in high-affinity binding of IFN on
its receptor (Griggs et al., 1992).
The secondary structure of the human IFN
subunit contains six helices, A±F, comprising
about 60% of the overall structure, but no sheets
(Figure 1). Apart from the AB loop, which is 13
residues long, all helices are connected by rather short
loops of 3±5 amino acids (Ealick et al., 1991).
Discussion of crystal structure
Information on crystal structures is available for
human, bovine, and rabbit IFN (see Table 4 for
accession numbers to the Brookhaven Protein
Database). Human IFN occurs in its natural conformation as a V-shaped globular dimer formed by
association of two identical subunits that are both
characterized by an extended flattened elliptical shape
(Ealick et al., 1991); spatial dimensions of 60 AÊ 40 AÊ 30 AÊ). All 12 helices in the dimer are located
parallel to the dimer 2-fold axis. Helices A, B, C, and
D of each subunit form a cleft in which the C-terminal
helix F of the other chain is packed. The latter helix
is structurally different from the others because it
shows a bend of about 125 at Glu112. This bend
is necessary to make helix F fit into the cleft. Both
N-glycosylation sites are on the surface of the dimer
648 Alfons Billiau and Koen Vandenbroeck
Figure 1 Sequence and structure of human IFN. Boxed residues are
conserved between human, baboon, bovine, ovine, porcine, murine, and rat
species. Numbering starts with the first amino acid of the mature protein. The
position of the six helices in the crystal structure (Ealick et al., 1991) is
indicated below the sequence. The C1 domain and a putative Grp78-binding
site (Vandenbroeck and Billiau, 1998a) are indicated also. Asterisks indicate
Asn25 and Asn97 N-glycosylation sites; filled circles indicate residues
conserved in the dimer folds of all IFN and IL-10 amino acid sequences
(Walter and Nagabhushan, 1995).
and thus exposed to the solvent. Receptor binding
involves formation of a 2 : 1 R : IFN dimer complex.
The receptor-binding interface of IFN consists of
two discontinuous polypeptide chain segments, the
first comprising helix A, the AB loop, and helix B, the
second comprising helix F and part of the C-terminus.
The long flexible loop that connects helices A and B is
highly variable, and may, because of its essential role
in receptor binding, explain some of the speciesspecificity of the molecule. While this loop exhibits no
clear secondary structure in unbound IFN, receptor
binding induces a conformational change including
formation of a 310 minihelix (Walter et al., 1995).
Important homologies
The IFN/IL-10 Fold and Topological Similarity
The IFN dimer interface, centered at a pair of C±C0
helices, shows extensive interdigitation of both
subunits. This type of intertwined interface is unique
to a relatively rare subclass (7%) of symmetrical
homodimers with high helical content, in which
both chains together fold into a single globular
domain, i.e. the cytokines IL-5, IL-10, the mnt, arc,
trp, and met repressors, and the fis, HU, and histone
B DNA-binding proteins (Larsen et al., 1998). In this
subclass the contact surfaces at the interface are
highly hydrophobic, thus providing stability reminiscent of monomeric proteins with an internally packed
hydrophobic core. In contrast, the interfaces of the
most common class of homodimers (65%) have a
highly polar and hydrated nature, and both subunits
form a separate compact domain. Tsai and Nussinov
(1997) have postulated that the latter category folds
by a three-state process, during which each subunit
folds into a stable monomer that then combines into a
dimer. IFN/IL-5/IL-10-type dimers are thought to
form through a two-state process. Indeed, the highly
intertwined structure of each subunit can be formed
only upon dimerization, thus excluding the possibility
of free monomers in solution. Similarly, any separation of the dimer must result in a pronounced
disruption of the tertiary structure of the subunits.
The dimer structure of IL-10, in particular, bears a
striking resemblance to that of IFN, and five
conserved residues are present in the dimer folds of all
IFN and IL-10 sequences (Figure 1) (Zdanov et al.,
1995; Walter and Nagabhushan, 1995).
Although it is not yet known whether, and how,
folding of this particular class of dimers is regulated
in vivo, we have recently shown that both Hsp60type (GroEL) and Hsp70-type (DnaK) chaperon
IFN
complexes can facilitate in vitro folding and dimerization of IFN at physiological temperature, a condition which is not permissive for spontaneous folding
of the protein (Vandenbroeck and Billiau, 1998;
Vandenbroeck et al., 1998a). A putative binding motif
for the ER chaperon Grp78/BiP is located in helix C
(Vandenbroeck and Billiau, 1998). Remarkably,
IFN/ subtypes, though biologically related to
IFN, are structurally different, in that they are not
clearly dimeric in solution, but associate into 1 : 1 : 1
heterotrimeric complexes with R and R subunits.
A Retroviral Homolog of IFN?
The p17 HIV matrix protein bears some structural
similarity to IFN (Matthews et al., 1994): it has a
similar helical/loop topology and also contains a
flexible C-terminus. It is not known whether this
resemblance has any functional significance.
Posttranslational modifications
N-Glycosylation
The primary sequences of murine, human, and porcine IFN contain two potential N-linked glycosylation sequons. Natural lymphocytic human IFN is
heterogeneously glycosylated, and forms that are not
glycosylated, or glycosylated either at a single site
(20 kDa) or at both sites (25 kDa) exist (Yip et al.,
1982; Kelker et al., 1984; Rinderknecht et al., 1984;
Sareneva et al., 1996). Posttranslational modifications ± with an emphasis on glycosylation ± of
recombinant human, mouse, and porcine IFN have
been studied by using mammalian (CHO cells, transgenic mice) and baculovirus/insect cell expression
systems (Scahill et al., 1983; Dijkmans et al., 1987;
Sareneva et al., 1994; Vandenbroeck et al., 1994; James
et al., 1995). Analysis of CHO cell-expressed recombinant MuIFN revealed that the highly heterogeneous nature of the secreted protein originates from
three cumulative posttranslational modifications: Cterminal processing, N-glycosylation, and dimerization (Dijkmans et al., 1987). Similar observations
were made later for human (Curling et al., 1990) and
porcine IFN (Vandenbroeck et al., 1994). Since
unglycosylated and N-glycosylated human IFN
bind with practically identical affinities to the same
receptors (Littman et al., 1985), glycosylation is
unlikely to exert dramatic effects on the biological
response. Neither does glycosylation alter or stabilize
the conformation of the molecule, as shown by
analysis of circular dichroic spectra (Arakawa et al.,
1986). Thus, the relevance of IFN glycosylation
should be sought 'upstream' from receptor binding.
In this connection, it was demonstrated recently that
649
glycans at Asn25, but less at Asn97, are essential for
protease resistance (Sareneva et al., 1995) and dimerization of IFN (Sareneva et al., 1994). Glycosylation
at Asn25 occurs cotranslationally, while glycosylation
at Asn97 occurs after dimerization. These observations are in line with the finding that in diverse
expression systems, glycosylation occurs more efficiently at Asn25 than at Asn97 (James et al., 1995).
In addition, the secretion level of a mutant IFN in
which both N-glycosylation sites are abolished is
much (10- to 100-fold) lower than that of the wildtype form (Sareneva et al., 1994). Taken together, the
data indicate that glycosylation at Asn27 is critical for
the formation of a secretion-competent form in the
endoplasmic reticulum of producer cells that is resistant to proteolytic degradation. Detailed sugar chain
analysis has been reported for CHO cell-derived
(Mutsaers et al., 1986) and natural (Sareneva et al.,
1996) human IFN, and revealed the presence of
complex/sialylated and hybrid or high-mannose-type
structures.
Modulation of C-terminal Processing
C-terminal truncation of IFN occurs at dibasic
amino acid sites (James et al., 1996), and is responsible for the series of low molecular weight variants
generally seen in the secreted pool. Proteolytic processing at the C-terminus results in a severe loss of
receptor binding and biological activity (Leinikki
et al., 1987), and may thus comprise a mechanism to
modulate IFN activity. This can be understood in
view of the observation that part of the C-terminal
polybasic cluster K-125TGKRKR (C1 domain,
Figure 1) is essential for binding to the receptor
chain (Griggs et al., 1992). The current state of evidence is suggestive of a more subtle regulation of IFN
activity through its C-terminus. The C1 domain was
recently found to bind with high affinity to heparin
and heparan sulfate. Heparin-binding shifts the extent
of proteolytic degradation from 18 to 10 C-terminal amino acids (Lortat-Jacob and Baltzer, 1996).
While the molecule is inactivated in the former case,
C-terminal removal of 10 amino acids increases IFN
activity by 600%. In receptor-binding experiments,
the C1 domain functions by enhancing the on rate
of the IFN±IFNR binding reaction (Sadir et al.,
1998). IFN bound to heparin displays, however, a
strongly reduced affinity for its receptor. It follows
that the interaction of heparin with the C1 domain is
regulatory on two levels: first, heparin controls
C-terminal truncation and hence intrinsic activity of
IFN, and second, it modulates receptor interaction
by competitive binding to the C1 domain (Sadir et al.,
1998).
650 Alfons Billiau and Koen Vandenbroeck
CELLULAR SOURCES AND
TISSUE EXPRESSION
Cellular sources that produce
IFN is a typical lymphokine, being produced almost
exclusively by NK cells and certain subpopulations of
T lymphocytes. Production takes place if these cells
are properly activated, either in vivo or after in vitro
cultivation. In the human system, T cells which
express the activation-dependent CD30 membrane
antigen have been identified as the principal IFNproducing subset (Alzona et al., 1994). Both CD4+
and CD8+ lymphocytes can produce IFN. Cloned
lines of T cells, especially those of murine origin, fall
into categories that do (T helper 1 or TH1 lines) and
others that do not (TH2 lines) produce IFN following appropriate stimulation. Sporadic literature
reports describe production of IFN by cultured
mononuclear phagocytes (Fultz et al., 1995; Gessani
and Belardelli, 1998), neutrophil granulocytes
( Yeaman et al., 1998), and neurons (Neumann et al.,
1997) or cell lines of neuronal origin (Watanabe et al.,
1989).
Tissue expression can be studied by various
approaches, e.g. (1) immunofluorescent staining of
producer cells tissue sections, (2) determination of the
protein in tissue extracts, and (3) quantitation of
IFN mRNA in tissue extracts. In vivo, IFN is
produced (1) in tissues which are infiltrated by activated lymphocytes, in particular in foci of acute
inflammation due to aspecific or antigen-specific
immune responses (e.g. the central nervous system in
experimental allergic encephalomyelitis (EAE), the
joints in autoimmune arthritis), and (2) in peripheral
lymphoid organs during local or generalized inflammation (e.g. as part of the acute phase response which
follow exposure to endotoxin). Whenever IFN is
produced somewhere in the body, it also enters the
bloodstream but is quickly cleared, so that it becomes
detectable in serum in usually small concentrations
and for relatively short durations.
Eliciting and inhibitory stimuli,
including exogenous and
endogenous modulators
NK and T cells do not produce IFN as long as they
are in a resting state. Many agents can activate them
to produce the cytokine. Although it has become
customary to distinguish between those that can
trigger production all by themselves (often referred to
as `inducers'), and those that can only enhance
production that is being or has been triggered, this
distinction is usually difficult to make. The main
reason is that proper activation of NK or T cells by
an exogenous agent requires cooperation of accessory
cells, e.g. mononuclear phagocytes, which also need
to be in some state of activation. These cells produce
soluble factors, in fact cytokines (e.g. TNF, IL-12,
IFN, and others), which act as endogenous inducers
or enhancers of IFN production. In addition, these
cells make physical contact with the lymphocytes
through membrane-attached molecules, e.g. the
CD40/CD40L system, and thereby provide so-called
costimulatory signals which are needed for optimal
activation.
Exogenous Stimuli
Exogenous stimuli which can elicit IFN production
are very diverse: in principle they are ligands of
receptor or receptor-like molecules on producer cells.
Some arise in vivo in the cells' environment as a result
of naturally occurring situations such as trauma,
infection, cancer, allergy, or autoimmunity. Others
are known in the literature because they have been
used as artificial inducers in experiments.
Any substance recognized by T lymphocytes as
`non-self ' can activate these cells and, as a result,
induce production of IFN, together with a number
of other cytokines. A classical system consists of
immunizing mice with a potent T cell antigen, e.g.
killed mycobacteria, then culturing spleen or lymph
node cells and stimulating these cultures with the
same antigen. The in vivo preimmunization is necessary for clonal expansion of the reactive T cells to
take place so that the cultures will contain sufficient
numbers of memory T cells to generate measurable
IFN yields.
Some microorganisms that infect humans or
animals produce superantigens, e.g. staphylococcal
enterotoxins. Like antigens, these substances bind to
the T cell receptor, but do so through a less variable
part of the molecule, so that more clones are initially
engaged in the reaction and so that clonal expansion
is not required for sizeable IFN production. Hence
superantigens can induce sizeable IFN production in
fresh cultures of mononuclear leukocytes taken from
nonimmunized animals or humans. As an exception
to the general rule that IFN induction in NK cells
requires help from accessory cells, it has been
reported that the superantigen SEB can independently induce IFN production in human NK cells
(D'Orazio et al., 1995).
The lipopolysaccharide (LPS) of the outer membrane of gram-negative bacteria, also designated as
IFN
endotoxin, is a potent inducer of IFN. Receptor
systems for LPS are present on many cell types,
ranging from leukocytes to fibroblasts. Injection of
LPS in animals results in an IFN response which
peaks at about 6 hours and wanes within 24 hours.
IFN produced in response to injection of LPS
apparently originates from NK cells as well as CD4+
and CD8+ T cells, since the mRNA is detectable in
all three populations (Heinzel et al., 1994). A comparison of IFN production by cultured murine
splenocytes of responsive and nonresponsive strains
led to the conclusion that for gram-negative bacteria
to induce IFN, the presence of mononuclear
phagocytes and production of IFN is required
(Yaegashi et al., 1995). Also, in endotoxin-injected
mice, release into the circulation of IL-12, a prominent NK cell-stimulatory monokine, precedes that
of IFN, and pretreatment with anti-IL-12 antibody
inhibits such production of IFN (Heinzel et al., 1994).
However, not only monocyte-derived cytokines, but
also IL-2, produced by TH1 cells can stimulate IFN
production by NK cells (Scharton and Scott, 1993).
It is not excluded that bacterial components other
than those mentioned can act as triggers of IFN
production. In gram-positive-bacterial infections, e.g.
in listeriosis, NK cells take a large part in IFN
production, which accounts for resistance of the host
to the infection (Dunn and North, 1991). Similarly, in
parasitic infections, e.g. in leishmaniasis models, early
production of IFN, which is of crucial importance
for the further course of the infection, is accounted
for by NK cells (Scharton and Scott, 1993).
Vegetal lectins, in particular phytohemagglutinin A
(PHA) and concanavalin A (Con A), by virtue of
their affinity for lymphocyte membrane glycoproteins, are potent mitogens and cytokine inducers.
Accordingly, they have been used as artificial
inducers of IFN in leukocyte cultures.
Similarly, antibodies directed at lymphocyte membrane components can be used experimentally as
inducers of lymphokines, including IFN. Polyclonal
antilymphocyte serum (ALS) as well as monoclonal
antibodies, in particular those directed at the universal T lymphocyte antigen CD3, can be used, either
in vitro or in vivo. These artificial inducers of IFN
have been especially helpful in the elaboration of
semi-large-scale production systems of the IFN protein and in analysing the role of different cell popuations, cytokines and other mediators as regulatory
factors in IFN production.
Endogenous Stimuli
Two purely endogenous factors, IL-12 and IL-18
have so far been described as `IFN inducers',
651
implying that they can trigger IFN production independently from the presence of exogenous stimuli,
though perhaps not of other endogenous factors.
IL-12, also known as a `natural killer cell stimulatory factor' or `cytotoxic lymphocyte maturation
factor', is a heterodimeric cytokine made up of a
40 kDa (p40) and a 35 kDa (p35) subunit (reviews in
references Bruna, 1994; Trinchieri, 1995). It is produced mainly by activated macrophages and B lymphocytes and exerts multiple effects on T cells and
NK cells. It is a potent inducer of IFN production
by these cells. In addition, IL-12 plays a major role
in promoting the development and differentiation of
IFN-producing TH1 lymphocytes (Manetti et al.,
1993; Trinchieri, 1993). IL-18 (reviewed by Dinarello
et al., 1998) is a monomeric protein (18±19 kDa),
originally described as `IFN-inducing factor' (IGIF).
Its folding pattern resembles that of IL-1. IL-18
is produced as a precursor by several cell types,
including mononuclear phagocytes and keratinocytes. Pro-IL-18 is cleaved into active IL-18 by
caspase 1; active IL-18 is deactivated by caspase 3.
IL-18 induces or augments IFN production in
the presence of other cytokines, namely IL-2 and/or
IL-12. The role of IL-12 seems to consist in inducing expression of the IL-18 receptor complex in
T cells.
Numerous studies have demonstrated the crucial
role of these two cytokines for (optimal) in vivo
induction of IFN by various exogenous agents.
Exogenous and Endogenous Modulators
of Production
Whereas certain cytokines enhance production of
IFN, and may in fact be crucial for any IFN
production to take place, one cytokine in particular,
IL-10, is a strong inhibitor of IFN production. IL-10
(for review, see Moore et al., 1993) was originally
described as `cytokine synthesis inhibitory factor'
produced by TH2 cells. However, monocytes and
B lymphocytes can also produce IL-10. Production of
IL-10 by mononuclear phagocytes seems to be
induced mainly in an autocrine fashion by TNF
and may represent an important negative feedback
pathway (Wanidworanun and Strober, 1993). The
powerful inhibitory effect of IL-10 on IFN production is illustrated by the observation that IL-10
administration to mice can inhibit shock induced by
SEB in mice (Florquin et al., 1994).
The antagonistic effect of IL-10 on IFN is reciprocal. IL-10 production by monocytes is inhibited
by IFN (Chomarat et al., 1993). It seems likely
therefore that the balance between IL-10 and IFN in
the initial stages of an immune response is of crucial
652 Alfons Billiau and Koen Vandenbroeck
importance to determine the further course of the
response. For instance, selective stimulation of IL-10
production has been proposed as a strategy used
by certain microorganisms, in particular helminths
and protozoans, to evade the IFN-mediated host
defense.
Production of IFN by T cells is also under
regulatory control of prostaglandins. PGE2 inhibits
production by TH1 cells of IFN and IL-2 but not
production of IL-4 by TH2 cells. cAMP is involved in
this control mechanism, which allows PGE2 to skew
immune responses in the TH2 direction (Snijdewint
et al., 1993).
RECEPTOR UTILIZATION
The IFN receptor complex on human mononuclear
leukocytes consists of at least three distinct proteins
(Finbloom et al., 1991). One of these, the IFN
receptor chain (IFNR), is the membrane protein
that primarily binds IFN with high affinity. It
encompasses three domains (Aguet et al., 1988; Hemmi
et al., 1989): the extracellular ligand-binding, the
transmembrane, and the intracellular domain. For
signals to be transmitted, the extracellular domain
must interact with one or more species-matching
proteins (Hibino et al., 1992). This protein, termed
IFNR, has a structure resembling that of IFNR.
Interaction of the chain extracellular domain with
this protein is sufficient for transmission of the signal
to express MHC class II molecules, but insufficient
for expression of the antiviral state. Therefore, interaction with a third protein is surmised. IFN binding
to the receptor complex causes its dimerization, which
in turn leads to intracellular signaling.
IN VITRO ACTIVITIES
In vitro findings
Biochemical Changes in Cells
One of the proteins induced by IFN is the enzyme
indoleamine-2,3-dioxygenase (IDO). IDO is the first
enzyme of the kynurenine pathway which links tryptophan to alanine and acetyl CoA. It catalyzes the
oxidative cleavage of the pyrrole ring in tryptophan
to yield N-formyl-kynurenine. Kynurenine is transformed by enzymes in liver and brain to alanine and
metabolites, some of which, such as quinolinic acid,
have neuroactive or toxic potential. Induction of IDO
by IFN explains why exposure to endogenous or
exogenous IFN is often associated with decreased
serum tryptophan levels and increased levels of
kynurenine in serum and urine. Induction of IDO
by IFN may indirectly function as a scavenger
mechanism of the superoxide anion, as this anion is
used in the IDO-catalyzed conversion of tryptophan to N-formyl-kynurenine. The cell- and tissuedamaging effect of IFN may thereby be dampened.
IFN-induced depletion of tryptophan has also been
speculated to contribute to the antiproliferative and
immunosuppressive effects of IFN.
IFN is well known to potentiate respiratory burst
responsiveness of macrophages to stimulants, resulting in increased production of highly reactive oxidants such as H2O2 and the superoxide anion (O2ÿ)
(Nathan et al., 1983). These effects of IFN are
believed to be due to regulation of the transcription of
genes coding for enzymes of the NADPH oxidase
system (Cassatella et al., 1990). This membrane-associated system is dormant in resting cells, but becomes
activated during phagocytosis or upon interaction
with certain soluble stimuli. Besides, IFN also stimulates production of NO, which in turn may react
with H2O2 to generate reactive oxygen.
IFN plays a role in the synthesis of tetrahydrobiopterin, a limiting factor in the synthesis of NO (see
below). Pteridins are synthesized from GTP, the first
step being conversion to 7,8-dihydroneopterin 30 -triphosphate by the enzyme GTP cyclohydrolase I
(Werner et al., 1993). The synthesis of this enzyme
is upregulated by IFN. Further steps leading to
production of tetrahydrobiopterin are catalyzed by
enzymes that appear not to be under control of cytokines. The tetrahydrobiopterin concentration in cells
is therefore mainly dependent on the level of GTP
cyclohydrolase, although part of the primary product
of this enzyme may also leave the cells after dephosphorylation. The latter escape pathway explains the
increased levels of neopterin in urine and tissues of
patients with inflammatory and infectious diseases.
IFN has an important role in the generation of
nitric oxide (NO). Various cells synthesize and release
endogenous NO as a short-distance and short-lived
messenger (Moncada and Higgs, 1993). NO is produced by NO synthases, which convert L-arginine to
L-citrulline and NO. NO synthase occurs in at least
three isoforms. Two forms are `constitutive', being
produced mainly by endothelial cells and platelets on
the one hand and by neurons on the other hand. The
inducible NO synthase (iNOS) occurs, in vitro, in
mononuclear phagocytes, in granulocytes, in fibroblasts, Kupffer cells, hepatocytes, endothelial cells,
vascular smooth muscle cells and, probably, in many
other cell types. The enzyme is not detectable in
uninduced cells and differs from the constitutive type
in that its induced release requires protein synthesis,
IFN
that it can act in the absence of Ca2+ and that its
activity is stimulated by flavin adenine dinucleotide
(FAD) and reduced glutathione.
In macrophages and fibroblasts, the known natural
inducers of the enzyme are cytokines and endotoxin.
Maximal activation of the cells to produce NO via
this pathway is obtained by their exposure to a combination of IFN and endotoxin, IFN and TNF, or
IFN and IL-1. As expected, macrophages of mice
with a targeted disruption of the IFN receptor gene
were found not to produce NO in response to IFN.
Other cytokines, TNF and IFN/, could only marginally substitute for IFN, indicating that IFN
is indeed the major cytokine controlling the NOgenerating pathways (Kamijo et al., 1993b).
The physiological receptor for NO is the intracellular soluble guanylyl cyclase, which generates cGMP.
The enzyme is activated by binding of NO to its heme
iron. Effects of NO mediated via this signaling pathway are: vasodilation, platelet inhibition, cell adhesion, and neurotransmission. By nitrosylating free
thiols, NO can regulate the activity of certain enzymes
and thereby exert physiological regulatory functions.
Thus, the reaction of NO with cell surface thiols has
been associated with antimicrobial effects, modulation of ligand-gated receptor (NMDA) activity, and
alterations of smooth muscle function. The antimicrobial effect of NO may also be due to loss of iron
from infected host cells (Weinberg, 1993).
In vitro, cells producing NO, following exposure to
IFN, can die a suicide-like death (Drapier and
Hibbs, 1988; Dijkmans and Billiau, 1991), in particular when they have no access to glucose or when
glycolysis is blocked, so that the respiratory chain is
the only pathway for ATP generation. However, aside
from being cytotoxic, NO can also exert cytostatic
activity by causing arrest of DNA synthesis. This
inhibition precedes inhibition of mitochondrial respiration and may be due to impairment of the enzyme
ribonucleotide reductase which contains at least three
targets for NO: a tyrosyl radical, cysteines, and an
iron center.
Macrophages in which NO production is induced
by IFN have cytocidal activity towards other cells,
e.g. tumor cells, in which no NO can be generated.
Cell death in this case can be due to interruption of
mitochondrial respiration (Henry et al., 1993), but
generation of peroxynitrite may also be involved.
Production of NO in phagocytic cells is associated
with reduced survival of ingested microorganisms.
The role of the IFN-induced NO synthase is
therefore assumed to consist in augmented defense
against infection with bacteria, molds, or protozoa
(for review see Green et al., 1991). However, the
cytotoxic effect of induced NO may also cause
653
undesirable cell and tissue damage and may thus
account for some of the deleterious in vivo effects of
endogenous IFN.
In view of the effects of NO on the vasculature, one
may propose that NO produced by IFN-activated
macrophages is in part responsible for local vasodilation in the inflammatory focus, and possibly for the
hyperdynamic circulation response associated with
inflammation. Also hypotension occurring as part of
the septic shock syndrome seems in part to be due to
excessive production, through the endotoxin/cytokine-induced pathway, of NO.
Finally, NO production has also been found to
contribute to the antiviral effects of IFN in macrophages infected with ectromelia virus, vaccinia virus,
or herpes simplex 1 virus (Karupiah et al., 1993b). In
the macrophage-like cell line, RAW 264.7, IFNinduced NO was found to inhibit vaccinia virus DNA
replication, late viral protein synthesis and particle
assembly, but not to affect early protein synthesis.
Virus replication was inhibited not only in the iNOSproducing macrophages themselves but also in
bystander cells cocultured with the IFN-pretreated
macrophages (Harris et al., 1995). Remarkably, this
apparently paracrine effect was not seen when the
cells were separated from each other by a semipermeable membrane, suggesting that, in addition to NO,
cell±cell contact is also necessary for transfer of the
antiviral state.
Aside from the IFN-induced proteins and enzymes
already mentioned, several others have been reported.
Some of these proteins have been found serendipitously or by molecular screening strategies such as
subtraction cloning. Often, the function of these
proteins has remained unknown for some time. One
example is the murine protein Mg21, identified by
subtraction cloning of cDNA from peritoneal macrophages treated with IFN (Lafuse et al., 1995). This
protein appears to belong to a family of GTP-binding
proteins which also encompasses IRG-47 (Gilly and
Wall, 1992), which is induced by IFN in B lymphocytes, and Mx which is an IFN/-induced protein
responsible for the antiviral effect against influenza
virus. GBP-1 is a GTP-binding protein induced by
IFN in human fibroblasts (Cheng et al., 1991) and in
mouse macrophages (Wynn et al., 1991); however its
sequence is unrelated to those of IRG-47 and Mg21.
Expression of Membrane Proteins
One of the best documented actions of IFN is the
induction of MHC class II antigens on many but not
all cell types in culture. IFN thus has the ability to
enhance or to induce these cells' ability to present
foreign antigens. MHC class I antigen expression can
654 Alfons Billiau and Koen Vandenbroeck
also be enhanced under the influence of IFN. Cells
in which this effect occurs may thus become better
targets for cytotoxic T cells recognizing viral, tumor
or autoantigens present in such cells.
IFN regulates the expression on phagocytes of the
high-affinity Fc receptor I (FcRI). Resting monocytes express both FcRI and the low-affinity receptors FcRII and FcRIII; resting polymorphonuclear
cells express only the low-affinity receptors. IFN
induces expression of FcRI on neutrophils (Petroni
et al., 1988) and augments its expression on mononuclear phagocytes (Guyre et al., 1983). Ligand binding to FcRs is widely recognized to stimulate
effector functions of phagocytes, such as phagocytosis, tumor cell killing, and inflammatory mediator
release. Therefore, augmented FcR expression is one
of the pathways by which IFN can act as a proinflammatory cytokine. Isolated rat brain microglia
also display enhanced expression of Fc receptors after
treatment with IFN, TNF, or IL-1. Remarkably,
the combination of IFN and TNF has been shown
to inhibit Fc receptor expression (Loughlin et al., 1992).
IFN augments expression of Fc"R on the human
mononuclear cell line U937 (Naray Fejes-Toth and
Guyre, 1984) and on platelets (Pancre et al., 1988). In
view of the important role of IgE in resistance to
parasitic diseases and in type I allergic reactions, these
effects of IFN need to be taken into account when
considering the role of IFN in these diseases.
IFN is among the cytokines that augment
expression of the adhesion molecule ICAM-1 on
various cell types, including cultured endothelial cells
and epidermal keratinocytes (Dustin et al., 1988),
resulting in increased adhesiveness for leukocytes
expressing the integrin LFA-1. The significance of this
effect may be that IFN produced early in an aspecific inflammatory focus (e.g. by NK cells) is
co-responsible for firm adhesion of granulocytes to
endothelial cells in postcapillary veins as a prelude to
their spreading and diapedesis. Similarly, this mechanism is believed to promote proximity of dendritic
cells, epidermal keratinocytes, and lymphocytes during
antigen presentation in the skin (Dustin et al., 1988).
Another important membrane protein induced by
IFN is the B7 antigen, whose ligand on T cells is the
CD28 molecule (Freedman et al., 1991). The presence
of B7 on antigen-presenting cells is indispensable
for them to avoid delivering an anergizing signal
(Harding et al., 1992).
IFN has also been reported to augment expression
and/or shedding of tumor-associated antigens by
tumor cells, thereby modulating their targetability for
the corresponding antibodies or sensitized T cells
(Marth et al., 1989; Leon et al., 1989; Greiner et al.,
1990).
Bone marrow-derived macrophages express a
protein with an epitope recognized by a monoclonal
antibody specific for a peptide of the mycobacterial
heat shock protein, hsp60. Exposure of the cells to
IFN was found to result in increased expression of
this crossreactive epitope. Antibodies recognizing heat
shock proteins are believed to play a role in autoimmune diseases. Therefore, the augmenting effect of
IFN on expression of hsp60-crossreactive proteins
may be of relevance to the pathogenesis of such
diseases (Wand-WuÈrttenberger et al., 1991).
Whereas, as a general rule, IFN augments expression of membrane molecules involved in immune
responses, there are some exceptions. Expression of
gp39 (CD40 ligand) by activated murine TH1 cell and
TH2 cell clones and by activated splenic CD4+ cells
is inhibited by IFN (Roy et al., 1993). IFN was
also reported to inhibit the ability of human dendritic
cells to express CD1A, CD80, and CD4, whereas the
expression of other membrane antigens is potentiated;
concomitantly the dendritic cells were functionally
depressed (Rungcun et al., 1998). Activation-induced
expression of E- and P-selectin on human endothelial
cells in culture was also found to be depressed
(Melrose et al., 1998). These effects may account in
part for the in vivo immunosuppressive role of IFN
in some systems (see section on immunosuppression
mediated by IFN).
Effects on Mononuclear Phagocytes
IFN has long since been recognized as the foremost
important cytokine converting macrophages from a
`resting' to an `activated' state. `Activation' is a rather
indiscriminately used term, which has meaning only
if placed in a context of a well-defined function or
functional ability that is considered. The mononuclear phagocyte (MPC) population, to which
macrophages belong, comprises cells in different
stages of differentiation and maturation, i.e. bone
marrow precursors, blood monocytes, and different
types of tissue macrophages (e.g. connective tissue
histiocytes, alveolar macrophages, Kupffer cells, exudate macrophages, microglial cells, osteoclasts, etc.).
These stages are usually considered as steps in a
process which, although regulated by environmental
signals, is in essence irreversible. Activation states of
tissue macrophages, as a contrast, are mostly seen as
reversible changes determined by the temporally
changing tissue microenvironment. IFN and other
cytokines, being constituents of the cellular microenvironment, play an important role both in the differentiation and maturation of MPCs and in
activation of tissue macrophages.
IFN
Circulating monocytes, when placed in culture,
undergo apoptosis unless provided with certain
stimuli, e.g. LPS and/or certain cytokines. Typically,
human blood monocytes will survive for more than
7 days in the presence of pure M-CSF, whereas in its
absence, they will die within 24 hours by apoptosis
and secondary necrosis. In the presence of M-CSF the
100% surviving monocytes become progressively
resistant against withdrawal of the growth factor
and can subsequently be activated by exposure to a
stimulus (e.g. serum-activated zymosan) to become
biologically active (as evident from adherence, phagocytic and respiratory activity). In the presence of not
only M-CSF but also IFN, resistance to cytokine
withdrawal develops more rapidly. Thus, although
IFN cannot by itself replace M-CSF, it can be seen
as a synergist for M-CSF to avoid apoptosis and
promote maturation into a macrophage. However,
remarkably, such monocytes which have avoided
apoptotic death in the presence of both M-CSF and
IFN, as opposed to those which have matured in the
presence of only M-CSF, do undergo apoptosis when
subsequently exposed to an activating stimulus (Munn
et al., 1995). The authors speculate that this yin-yang
type effect of IFN fulfils a useful function in host
defense against infection in that it allows for rapid
development of a microbicidal macrophage, but also
for subsequent elimination of the macrophage which,
by producing toxic metabolites, might otherwise
inflict undue damage on the host's own tissues.
Macrophages activated by IFN or other agents
differ from resident ones by enhanced endocytic
capacity, as manifested by increased pinocytosis and
phagocytosis via receptors for complement and IgG2a.
Such enhanced endocytic capacity does not, however,
apply to all ligands. Expression of Fc receptors for
other immunoglobulin classes, for instance, has been
shown to be reduced in the activated state. Significantly, macrophages activated by IFN have been
found to have reduced ability to ingest a variety of
obligately intracellular microorganisms, e.g. rickettsiae, Trypanosoma cruzi, and Leishmania amastigotes.
In the case of Leishmania in mouse macrophages,
reduced binding was demonstrated to be due to
reduced expression of lectin-like receptors for the
organisms (Mosser and Handman, 1992). Reduced
uptake of intracellular parasites by IFN-activated
macrophages may represent one of the mechanisms
by which IFN contributes to both specific and
aspecific host resistance to these pathogens.
IFN also affects accessory cell function of
mononuclear phagocytes. In vivo, endogenous IFN
seems to promote helper and to counteract suppressive circuitry, as evident from experiments in which
anti-IFN antibody was injected in carrier-primed
655
mice, whose spleen cells were subsequently immunized in vitro with carrier±haptene complex (Frasca
et al., 1988). However, IFN may also activate suppression circuits in mixed mononuclear cell populations (Noma and Dorf, 1985; Ishikura et al., 1989).
IFN regulates production of chemokines by
macrophages (and some other cells) (Table 5). It is a
potent inducer of IP-10 mRNA expression in
human and murine macrophages (Luster et al.,
1985; Hamilton et al., 1989). IP-10 belongs to the
chemokine family. Human IP-10 has been shown
to exhibit chemoattractant activity for monocytes and
activated T cells and to promote T cell adhesion to
endothelial cells (Taub et al., 1993). On the other
hand, IFN was found to suppress LPS-induced expression of certain other related chemokines, namely
MCP-1 and KC (GRP/melanoma growth-stimulating
activity) in mouse peritoneal macrophages (Ohmori
and Hamilton, 1994). Similarly, induction of IL-8 by
IL-2 or IL-1 in human monocytes was found to be
inhibited by IFN (Gusella et al., 1993). However,
IFN is not to be regarded as a general inhibitor of
IL-8 expression, since it synergizes with TNF to
induce IL-8 in other cells. Clearly, IFN has the
ability to either stimulate or inhibit production of
proinflammatory mediators by macrophages.
Effects on Endothelial Cells
Endothelial cells are of particular importance in the
local inflammatory response. IFN, as well as IL-1
and TNF, augment expression of the adhesion
molecule ICAM-1 on cultured endothelial cells of
extracerebral (Dustin et al., 1993) as well as cerebral
origin (McCarron et al., 1993), resulting in increased
adhesiveness for leukocytes expressing the integrin
LFA-1 (Yu et al., 1985).
IFN also enhances the capacity of endothelial
cells for producing IL-1 in response to LPS
(Arenzana-Seisdedos et al., 1985; Haq et al., 1985;
Miossec and Ziff, 1986). A similar effect has been
noted in cultures of synovial cells (Johnson et al.,
1990). IL-1, in turn, acts as an inducer of other
inflammatory mediators, such as procoagulants and
prostaglandins.
Chemokine secretion by endothelial cells is under
regulatory control by cytokines, in particular IFN
and TNF: barely any RANTES was found to be
produced by human vascular endothelial cell cultures
treated with either IFN, TNF, or IL-1. However,
the combination TNF+IFN was highly effective
in inducing RANTES production: pretreatment with
IFN sensitized the cells to induction with TNF.
This regulatory control was found to be exerted at the
transcriptional level (Marfaing-Koka et al., 1995).
Table 5 Influence of IFN on chemokine production
Cells tested
Inducer
Chemokine
Effect
Comment
References
Human fibroblastoid
synoviocytes
IFN (+TNF)
MCP-1
Induction
Synergy with TNF
Hachicha et al., 1993
Human leukocytes
LPS + IFN
MIP-1, MIP-1,
(IL-8)
Early inhibition;
later enhancement
IFN by itself inactive;
anti-TNF abrogates
enhancement
Kasama et al., 1995
Mouse peritoneal
macrophages
LMW hyaluronan
MIP-1, MIP-1
Inhibition
Production of IL-12 enhanced
Hodge-Dufour et al., 1998
Human umbilical vein
endothelial cells
TNF + IFN
RANTES
Enhancement
Single cytokines inactive;
IFN pretreatment sensitizes;
IL-4 and IL-13 inhibit;
IL-10 has no effect
Marfaing-Koka et al., 1995
Human fibroblasts;
Hep-2 and
MG-63 tumor cells
IFN
MCP-1, MCP-2
Induction
MCP-2 more responsive to
IFN; synergy with IL-1
Van Damme et al., 1994;
Van Coillie et al., 1997;
Struyf et al., 1998
Human monocytes
-amyloid peptide+
IFN
MCP-1
Induction and
enhancement
Human monocytes
IL-2+IFN
IL-8
Inhibition
IFN by itself inactive
Gusella et al., 1993
Murine peritoneal cells
LPS+IFN
KC/GRO/MGSA
Inhibition
Cell-specific effect
(not in endothelial or 3T3 cells)
Ohmori and Hamilton, 1994
LPS+IFN
JE
Inhibition
Cell-specific effect
(not in endothelial or 3T3 cells)
Ohmori and Hamilton, 1994
LPS+IFN
IP-10
No effect
Cell-specific effect
(not in endothelial or 3T3 cells)
Ohmori and Hamilton, 1994
CC chemokines
Meda et al., 1996
CXC chemokines
IFN
Effects on T Cell Proliferation and Apoptosis
Whereas IFN acts as a mild inhibitor of proliferation for most cell types, it stimulates proliferation of
mitogen-triggered primary T cells as well as a variety
of T cell lines (see, for example, Landolfo et al., 1988).
Remarkably, the antiviral effect of IFN is not
expressed in these cells. It has been considered that
this exceptional situation is due to modulation of
signal transduction in T cells and that enhancement of
IL-2 production and IL-2R expression are involved
(Landolfo et al., 1988). In studies using murine T cell
clones, it was found that IFN exerts a slight suppressive effect on IL-2- and IL-4-mediated proliferation of TH2 but not TH1 clones (Fernandez-Botran
et al., 1988; Gajewski and Fitch, 1988).
One aspect of the regulatory effect of IFN on
lymphocytes is its ability to promote apoptosis under
specific conditions. Thus, blockage of IFN inhibits
cell death induced in effector T cells by TCR linkage
in the absence of accessory cells (Liu and Janeway,
1990). Furthermore, both in normal and in cultured
malignant lymphocytes, IFN has been shown to
exert contrasting effects, i.e. apoptosis or proliferation, depending on the level of expression of IFN
receptors: high-level expression is associated with an
apoptotic response, low-level expression with a proliferative one (Novelli et al., 1994). In vivo apoptosis
of thymocytes after treatment with anti-CD3 antibody is more pronounced in mice which are deficient
in expression of the IFNR chain than in their wildtype counterparts, indicating that in this system IFN
triggers an antiapoptotic pathway (Matthys et al.,
1995a).
IFN may play a certain role in the differentiation,
either in vitro or in vivo, of T cells into populations
with either a TH1 or TH2 cytokine production profile. The cytokines with the highest impact on this
differentiation are IL-12 and IL-4. IL-12 is a strong
inducer of IFN and a promotor of the TH1 track.
IL-4, conversely, is an antagonist of IFN and a
strong promoter of TH2 responses. Many effects of
IL-12 have been shown to be mediated by IFN.
Hence it is possible that IFN may act in this process
as a mediator of the IL-12 effect. In fact, a basic difference between TH1 and TH2 lymphocytes appears
to be the absence of the chain of the IFN receptor
in the TH1 population (Pernis et al., 1995), implying
that IFN should be inactive on these cells.
IFN and the Generation of
Cytotoxic Lymphocytes (CTLs)
Exogenous IFN can augment the development of
cytotoxic T lymphocyte activity in mixed lymphocyte
cultures, and neutralizing antibodies to IFN can
657
inhibit the development of CTL activity in antigen- or
lectin-stimulated lymphocyte cultures (Siegel, 1988).
Anti-IFN was also found to inhibit both proliferation and activation of CTLs in the primary in vitro
mixed lymphocyte reaction (Landolfo et al., 1985;
Simon et al., 1986), an observation not subsequently
confirmed under apparently similar conditions (Bucy
et al., 1988). A contribution of IFN to the generation of CTLs may, in principle, result from stimulatory effects on mononuclear phagocytes, on helper
T cells, or on CTL precursors, or from inhibitory
effects on suppressor T cells. In human mixed lymphocyte cultures the augmentation by IFN of CTL
generation was found to result mainly from a direct
effect on CD8+ T cells (Siegel, 1988). In a system
suitable for the expansion of single murine CD4±
xCD8+ T cells into clones, both IL-2 and IFN were
found to be required (Maraskowsky et al., 1989).
Effects on B Lymphocytes
Reports on the effects of IFN on B cells are somewhat contradictory. In early studies recombinant
murine IFN was found to possess B cell maturation
factor activity for resting splenic B cells and the
comparable B cell tumor line WEHI-279.1 (Sidman
et al., 1984). However, on the other hand, IFN was
found to inhibit LPS-induced IgM production in
murine spleen-derived B cells, by reducing the number
of IgM-forming cells and without affecting overall
proliferation (Abed et al., 1994b). In fact, in B cells, as
opposed to T cells, the stage of differentiation seems
to codetermine the type of response to IFN. Resting
B cells seem not to be affected by IFN, while preactivated B cells are inhibited from further differentiation. These cells also show increased expression
of IFN receptors (Abed et al., 1994a, 1994b). Also,
cultured normal murine pre-B cells (Grawunder et al.,
1993; Garvy and Riley, 1994) or the human pre-B cell
line (Trubiani et al., 1994) which are exposed to IFN
undergo apoptosis, while human CD5+ chronic B
lymphocytic leukemia cells, as a contrast, are protected from apoptosis by IFN (Buschle et al., 1993).
IFN has been found to stimulate polyclonal
immunoglobulin production by resting or activated
human B cells (Sidman et al., 1984; Nakagawa et al.,
1985). In cultured human PBMCs addition of IFN
has been found to promote an anti-IFN antibody
to inhibit spontaneous late production of IgG2; in
PWM-stimulated cultures, exogenous IFN inhibited
and anti-IFN stimulated early production of IgG1
(Kawano et al., 1994). In this system IFN did not
seem to act as a IgG2 switch factor, since the IgG2promoting effect disappeared when the culture system
had been depleted of sIgG2+ cells. However, in a
658 Alfons Billiau and Koen Vandenbroeck
culture system in which T cells are eliminated, IFN
seemed to possess IgG2-switching activity (Kitani and
Strober, 1993).
IFN in Hematopoiesis
Effects of IFN on hematopoiesis have been demonstrated in many studies, using different experimental
settings, which invariably employ one or several
hemopoietic factors and/or cytokines. The question
as to whether the IFN effects depend on the presence of accessory cells (stroma cells, monocytes or
lymphocytes) has been a matter of controversy in
early studies (Broxmeyer et al., 1983; Mamus et al.,
1985) and can as yet not be considered as being
resolved. Also, IFN induces or enhances production
of various other cytokines, and its effects on hematopoiesis may therefore be indirect. For instance, the
IFN-inducible chemokine IP-10 has been shown to
inhibit colony formation from early hematopoietic
progenitors, apparently by counteracting r-steel factor
(rSLF) (Sarris et al., 1993).
In bone marrow cultures prepared from normal
mice IFN was found to inhibit granulocyte-macrophage colony growth. However, in cultures from mice
pretreated with post-5-fluorouracil it promoted
colony formation. It was suggested that primitive
progenitors require stem cell factor in the early stages
and IFN for subsequent growth (Shiohara et al.,
1993). In the human system IFN has similarly been
shown by many studies to counteract the proliferative
activity of colony-stimulating factors. However in
cultures of pure CD34+ progenitor cells, IFN was
found to synergize with IL-3 (Caux et al., 1992).
IFN by itself did not affect proliferation, and in the
presence of IL-3, while augmenting the number of
colonies, failed to affect their size, indicating that it
acted only on early progenitors. In fact, in long-term
cultures, once mature cells appeared, IFN inhibited
further proliferation. In a study on human bone
marrow cultures, both IFN and NO were found to
suppress colony formation, and pharmacological
blockage of NO could partially prevent the suppressive effect of IFN (Maciejewski et al., 1995).
IFN has been found to suppress erythropoiesis
both in vitro and in vivo, an effect that may play a role
in anemia which accompanies chronic infections or
autoimmune diseases. In affecting erythropoiesis
in vivo, IFN undoubtedly interacts with several
other hematopoietic factors and cytokines. Studies
have been aimed at revealing the most important of
these interactions and at defining the most sensitive
stages of erythropoiesis. In EPO-containing human
CFU-E or BFU-E cultures addition of IFN was
found to inhibit colony formation, an effect that
could be reversed by increasing EPO concentrations
(Means and Krantz, 1991). Similarly, in murine
macrophage-containing bone-marrow cultures supplemented with EPO, addition of IFN was found
to suppress formation of both BFU-E and CFU-E
colonies (Wang et al., 1995). However, the dose
required was lower for suppression of BFU-E, and
the overall effect was more pronounced as IFN was
added earlier after culture initiation. The effect of
IFN was not prevented by addition of single or
combined antibodies against TNF, IL-1, or GMCSF. Accordingly, increased production of these
cytokines in IFN-treated cultures, or synergy of these
cytokines with IFN seemed not to play a significant
role in these macrophage-containing cultures. Nevertheless, in macrophage-depleted cultures, IFN was
shown to synergize with the antierythropoietic effect
of TNF. Therefore, and also because cytokines
other than those which were examined may be involved, it should not be concluded that IFN exerts a
direct inhibitory effect on BFU-E colony formation
independently of other cells and cytokines.
Effects on CNS Cells
The two main types of CNS targets for IFN are
microglia and astroglia. Microglial cells fulfil such
functions as antigen presentation, phagocytosis, cytocidal activity, and production of tissue-degradative
activity. Astrocytes act as regulators of the ionic
balance in the CNS and neurotransmitter distribution. When cultured in vitro these cells spontaneously
express MHC antigens. Exposure to IFN greatly
enhances this expression. However, in astrocytes, as
opposed to microglia, the effect of IFN can be
counteracted by various factors including IFN,
IL-1, TGF, glutamate and cAMP agonists, as well
as contact with neurons. Normal human CNS astrocytes, when cultured, spontaneously express MHC
antigens as well as ICAM-1. However this expression
is greatly increased in the presence of IFN (Pulver
et al., 1987; Mauerhoff et al., 1988; Frohman et al.,
1989; Satoh et al., 1991; Yong et al., 1991; Aloisi et al.,
1992). This enhancing effect of IFN on expression of
class II MHC antigen, but not on expression of class I
MHC antigen or ICAM-1, was found to be counteracted by IFN (Barna et al., 1989; Satoh et al., 1995).
In a murine oligodendroglioma cell line (MOCH-1)
IFN was found to induce a morphological change
from a small round cell with thin branches to a large
fibroblast-like cell. IFN also stimulated markedly
enhanced expression of the astrocyte marker protein
GFAP (Li et al., 1995).
IFN
IFN and Connective Tissue
Maintenance of intact and remodeling of traumatized
or inflamed connective tissue is increasingly recognized to be controlled in part by the cytokine network. The contribution of IFN to this control
mechanism derives at least in part from its ability to
inhibit the synthesis of collagen and fibronectin by
fibroblasts or chondrocytes in vitro (Jiminez et al.,
1984; Amento et al., 1985). This inhibition is associated with reduction in mRNA levels (Rosenbloom
et al., 1984; Goldring et al., 1986). Systemic administration of IFN was found to inhibit collagen
synthesis in murine models: tissue reaction to a subcutaneous foreign body (Granstein et al., 1987) or to
skin wounds (Granstein et al., 1989), and to alleviate
pulmonary fibrosis induced by bleomycin (Giri et al.,
1986).
Effects on Endocrine Cells
In cultured human thyrocytes IFN has been found
to inhibit expression of thyrotropin receptors
(Nishikawa et al., 1993), production of triiodothyronin (Kraiem et al., 1990) and transcription of the
thyroglobulin gene (Kung and Lau, 1990). It also
reduces basal thyroid peroxidase content and inhibits
thyrotropin-induced increase of the enzyme (Asakawa
et al., 1992). Similar effects have been noted to occur
in a rat thyroid cell line (FRTL-5), although the
effects were minor unless IFN was given in combination with TNF (Tang et al., 1995). These effects
may in part explain the decrease in thyroid function in
autoimmune thyroiditis.
The effects of IFN on the function of the anterior
pituitary have been studied in an in vitro system consisting of organotypic anterior pituitary cell aggregates (Vankelecom et al., 1990, 1992). In such
aggregates secretion of prolactin and ACTH in
response to hypothalamic stimulatory factor was
found to be inhibited by IFN. This inhibitory effect
was, however, codetermined by the composition of
the aggregates. In particular it required the presence
of the folliculo-stellate (FS) cell component. In in vitro
systems, the FS cells have been shown to mitigate
responses of secretory cells to stimulatory as well as
inhibitory hypothalamic signals. One of their in vivo
functions may therefore consist of avoiding overly
brisk fluctuations in hormone levels. The FS cells also
have properties and activities which resemble those
of mononuclear phagocytes or glial cells: expression
of typical macrophage and dendritic cell markers,
phagocytosis and secretion of IL-6 (Vankelecom et al.,
1989). The apparent kinship of FS cells with other
typical IFN target cells, and the FS cell-dependent
659
inhibitory effect of IFN on the anterior pituitary
secretory activity invites speculation that IFN,
produced during inflammatory or immune responses,
uses the FS cell to act on the neuroendocrine axis
(Vankelecom and Billiau, 1992).
Effects on Adipocytes
Dissipation of fat stores during cachexia may in part
be due to direct effects of IFN (and other cytokines)
on adipocytes. For instance, treatment of cultured
adipocytes to IFN results in reduction of the amount
of lipoprotein lipase activity releasable from the cells
by incubation with heparin (Patton et al., 1986;
Doerrler et al., 1994). This enzyme is responsible for
hydrolyzing the triglycerides in circulating lipoproteins and is a major determinant of fat accumulation
in adipocytes. IFN has also been shown to reduce
the rate of fatty acid synthesis (Patton et al., 1986;
Doerrler et al., 1994) and to cause increased
hydrolysis of endogenous intracellular triglyceride in
adipocytes (Feingold et al., 1992; Doerrler et al.,
1994). In accord with these effects, IFN was found
to reduce the level of lipoprotein lipase and fatty acid
synthetase mRNAs. However, the levels of acetyl
CoA carboxylase mRNA was unaffected and the level
of hormone-dependent lipase mRNA was decreased
rather than increased. Therefore posttranscriptional
as well as transcriptional regulatory pathways seem to
be involved (Doerrler et al., 1994).
IFN and Epithelial Barriers
Epithelia represent an example of tight but flexible
permeability barriers within the body. In an in vitro
model IFN was shown to modulate permeability
of an epithelial layer. IFN was found to be rather
unique in this respect, as the effect was not seen with
IFN. Also, only the basolateral side and not the
apical side of the epithelial layer was found responsive
(Adams et al., 1993).
Most epithelial cells contain cytokeratins, a family
of proteins forming cytoskeletal filaments. The expression of at least one of these molecules, the acid
cytokeratin K17, was shown to be enhanced in HeLa
cells treated with IFN (Flohr et al., 1992), and the
promoter of this protein was found to contain three
putative GAS elements (Vogel et al., 1995). However,
the exact mode of action of the promoter remains to
be elucidated.
Effects on Virus Replication
Cells respond to IFN by entering a state of relative
resistance to virus infection. The molecular and
660 Alfons Billiau and Koen Vandenbroeck
cellular mechanisms underlying this resistance are
similar to those initiated by IFN or IFN. Detailed
information on these mechanisms comes almost
exclusively from studies with the latter interferons
and can be found in the relevant chapters.
Only few comparative studies on antiviral action
mechanisms of IFN and IFN/ are available. Both
in the human and in the murine system, the specific
antiviral activity (number of antiviral units per
milligram of protein) is 10- to 100-fold lower for
IFN than for IFN/. Like IFN and IFN, IFN
induces derepression of the genes for 20 ,50 -oligoadenylsynthase and dsRNA-dependent protein kinase,
which account to a large degree for the broadspectrum antiviral effect in cells. However, IFN fails
to induce the Mx protein, which accounts for an
added antiviral effect against some virus families, e.g.
influenza virus and Togotovirus. Protein synthesis
inhibitors block induction of some genes by IFN,
leaving the antiviral response to IFN unaffected
(Kelly et al., 1985; Faltynek et al., 1985; Kusari and
Sen, 1986). It is therefore possible that, in a first
round of protein synthesis, IFN induces production
of other proteins which in turn act as interferons. In
particular, it has been shown that IFN induces
production of IFN in the murine L929 (Hughes and
Baron, 1987) and Friend erythroleukemia cell lines
(Marziali et al., 1991) and in mouse macrophages
(Hughes and Baron, 1989). In Friend cells the genes
for IFN and IFN were found to be constitutively
transcribed, and the mRNAs appeared to accumulate
in response to IFN. Mutant Friend cell lines that are
resistant to type I interferons retained responsiveness
to IFN, indicating that induced production of IFN
is only partially responsible for the antiviral effect
of IFN.
There are examples of IFN being more antivirally
active in cells of heterologous animal species. For
instance, this is the case for porcine IFN which is
more active in bovine than in porcine cells, suggesting
that in this species or in general, the direct antiviral
effect of IFN is not a property among those that are
most essential for its role in host defense against virus
infections.
A number of studies have been devoted to the
possible effects of IFN on the replication of HIV in
monocytic cells. The results have been confusing, as
both inhibitory and stimulatory effects have been
seen, each by several independent workers. IFN was
found to inhibit HIV replication in U937 histiocytic
lymphoma cells if added before and kept present
continuously after virus inoculation; withdrawal of
IFN was followed by resumption of virus replication
and addition of IFN to U937 cells that were
persistently infected with HIV had no effect (Hammer
et al., 1986). Similarly, in primary human monocytes
IFN was reported to inhibit virus production in a
dose-dependent fashion (Kornbluth et al., 1989; Fan
et al., 1994; Dhawan et al., 1994). Time kinetic study
indicates that virus production in these primaryinfection systems appears to result from multiple
cycles. Virus replication per infected cell appears not
to be inhibited by IFN. The antiviral action seems
rather to be directed against virus in the fluid phase.
Stimulatory effects of IFN on HIV replication in
mononuclear phagocytic cells have also been reported
(Koyanagi et al., 1988; Biswas et al., 1992). Although
the mechanism of this effect remains obscure, it is
speculated that activation of monocytes by IFN
may go hand in hand with activation of the proviral
genome. Support for this concept has come from a
study on monuclear phagocytes of a transgenic mouse
strain the genome of which contains the long terminal
repeat sequence (LTR) of HIV linked to the bacterial
gene encoding chloramphenicol acetyltransferase
(CAT). Because the LTR contains the transcription
regulatory elements of HIV, alterations in HIV-LTRdirected expression of CAT can be surmised to be
analogous to changes in HIV expression in cells
harboring the provirus. In this study it was found that
IFN by itself had little effect on CAT expression.
However, combinations of IFN with LPS, IL-6, or
TNF led to significant synergistically augmented
levels of CAT expression. The synergic effect was also
evident when the macrophages were first treated with
IFN and then exposed to LPS, IL-6, or TNF
(Warfel et al., 1995).
Aside from exerting direct effects on HIV replication IFN may, in conjunction with other cytokines,
play a role in AIDS by influencing immunopathogenesis. In particular, aberrant cytokine production due
to retrovirus infection may promote the predominance
of TH2- over TH1-type cytokines which is characteristic for AIDS. In accordance with this concept is the
observation that, in a murine model for AIDS,
administration of IFN together with anti-IL-4 antibody retards development of the disease manifestations (Wang et al., 1994).
Poxviruses of different animal species, having coevolved with their host under intermittent pressure of
the antiviral effect of IFN, have acquired the genetic
information to instruct infected cells to produce soluble IFN receptor homologs. An example is the
rabbit myxoma virus. Cells infected with this virus
release a protein, M-T7, which can bind rabbit IFN
and inhibit its biological activity. The reactivity was
found to be animal species-specific (Upton et al.,
1995). Poxviruses also express proteins which interfere with the biological activity of cytokines other
than IFN, in particular IL-1 and TNF. Mutant
IFN
viruses which lack the ability to produce these
proteins replicate normally in cells in vitro, but tend
to be attenuated in vivo (for review, see Gooding,
1992).
Regulatory molecules: Inhibitors
and enhancers
Synergy between IFN and TNF
Many instances have been reported in which IFN
synergizes with TNF either in vitro or in vivo: in vitro
cytotoxicity for certain tumor cells, induction of
microbicidal activity in macrophages, induction of
NO release by various cell types, expression of cell
surface adhesion molecules, in vivo antitumor effects,
in vivo induction of other cytokines, systemic toxicity
and lethality.
The subcellular mechanism(s) underlying this synergy are poorly understood. IFN has been found to
augment expression TNF receptors by certain cell
lines (Aggarwal et al., 1985; Tsujimoto and Vilcek,
1986; Ruggiero et al., 1986). However, this is not the
general rule and cannot explain all synergistic effects
described. In explanted murine peritoneal macrophages IFN, when added at relatively high concentrations, was found to inhibit rather than enhance
expression of TNF receptors: in freshly explanted
macrophages it prevented the appearance of receptors, and in mature macrophages it downregulated
receptors already expressed (Drapier and Wietzerbin,
1991). This inhibitory effect was not considered to be
contradictory to the synergy between TNF and IFN,
since such synergy was only seen at doses of IFN
lower than those inhibiting TNF receptor expression.
This dose-dependent difference in interaction between
the two cytokines may be of particular importance for
the interpretation of often contradictory effects of
both cytokines seen in inflammatory models in vivo.
It should also be mentioned that in these studies no
distinction was made between effects of IFN on
the low versus the high molecular weight receptor
of TNF.
Natural IFN Antagonists
In several systems IL-4 and IFN have been found to
have opposite effects and to antagonize each other
when they are both present. Typical effects of IFN
on monocytes and macrophages (induction of IFNresponsive genes, H2O2 production, intracellular
antimicrobial activity) are counteracted by IL-4
(Gaspari et al., 1988; Gautam et al., 1992), although
661
synergy has also been reported (Belosevic et al., 1988).
Also, the antiviral effect IFN on L929 cells has
been found to be antagonized by IL-4 (Lohoff et al.,
1990). IL-4 inhibits induction of chemokines by
IFN (Larner et al., 1993; Marfaing-Koka et al.,
1995).
In reverse, typical IL-4 actions on B lymphocytes
(isotype switch in favor of IgE) and T lymphocytes
(inhibition of thymocyte proliferation) are counteracted by IFN (PeÁne et al., 1988a, 1988b; Plum et al.,
1991). IL-4, finally, antagonizes IFN by inhibiting
its production by peripheral blood mononuclear cells
(Peleman et al., 1989; Wagner et al., 1989; Vercelli
et al., 1990).
The subcellular mechanism of antagonism between
IFN and IL-4 in monocytic cells involves inhibition
by IL-4 of transcriptional activation of the IFNinducible proteins, as was shown to be the case for the
chemokine IP-10. The target for inhibition seems to
be the ISRE (interferon-sensitive response element) in
the promoter of the IP-10 gene (Larner et al., 1993).
However, rather than inhibiting activation of the
ISRE-binding transcription factor, ISFGF-3, IL-4
seems to induce a distinct ISRE-binding factor that
functions to inhibit IFN-driven, ISRE-dependent
transcription (Deng et al., 1994).
The production of IFN by antigen-stimulated
CD4+ T cells depends on the presence of IL-2 both
during the priming and the expression phases. As a
contrast, production of IL-4 requires IL-2 only during
the priming phase. This difference in requirement of
IL-4 and IFN production may be part of the
mechanism directing the immune response towards
TH1 or TH2 predominance. Type I interferons (
or ) counteract induction by IFN of MHC class II
antigens in murine macrophages (Ling et al., 1985;
Fertsch et al., 1987). The antagonistic effect has also
been observed in cultured human astrocytes (Barna
et al., 1989) and astrocytoma cells, but not in human
monocytes (Ransohoff et al., 1991). As documented
by nuclear run-on experiments, the antagonistic effect
is exerted at the transcriptional level (Ransohoff
et al., 1991).
Although TGF1 can by itself induce transcription
of cytokine genes, it mostly inhibits production of cytokines, such as IL-1, TNF, and IFN, induced by
other agents, such as LPS or PHA. The mechanism
of this inhibitory effect remains unclear; synthesis of
PGE2 or cAMP, which have both been implicated in
posttranscriptional control of cytokine expression,
seem not to be involved. TGF1 can also counteract
IFN action as it was found to downregulate constitutive as well as IFN-induced expression of MHC
class II antigens on a human melanoma cell line
(Czarniecki et al., 1988). Another IFN-controlled
662 Alfons Billiau and Koen Vandenbroeck
activity which is counteracted by TGF is induction
of nitrogen oxide synthesis in macrophages (Ding
et al., 1990).
In reverse, IFN can antagonize actions of TGF.
Thus, it has been reported that IFN reverses the
stimulation by TGF of the collagen gene but not
that of the fibronectin gene expression in normal
human fibroblasts (Varga et al., 1990).
Although IL-10, like all cytokines, possesses
multiple biological activities, its main effect remains
its capacity to inhibit the synthesis of IFN. In addition, however, IL-10 also counteracts the action of
IFN, e.g. as it inhibits IFN-induced nitrogen oxide
production (Gazinelli et al., 1992). The powerful
inhibitory effect of IL-10 on IFN production is
illustrated by the observation that IL-10 administration to mice can inhibit shock induced by staphylococcal enterotoxin B (SEB) (Florquin et al., 1994).
The antagonistic effect of IL-10 on IFN is reciprocal. IL-10 production by monocytes is inhibited by
IFN (Chomarat et al., 1993). It seems likely, therefore, that the balance between IL-10 and IFN in the
initial stages of an immune response is of crucial
importance to determine the further course of the
response. For instance, selective stimulation of IL-10
production has been proposed as a strategy used by
certain microorganisms, in particular helminths
and protozoans, to evade the IFN-mediated host
defense.
TNF synergizes with IFN in many in vitro
test systems. However, this is by no means a general
rule. With regard to two important activities, enhancement of expression of MHC class II molecules
and of Fc receptors, the combination of IFN and
TNF has been reported to be less active than
either cytokine alone, both in rat peritoneal macrophages and in microglia (Zimmer and Jones, 1990;
Loughlin et al., 1992). In fact, whether IFN and
TNF synergize or antagonize with each other may
critically depend on the state of differentiation of
the cells. Thus, TNF was found to enhance
IFN-induced MHC class II molecule expression in
undifferentiated macrophages and to inhibit such
enhancement in mature macrophages (Watanabe and
Jacob, 1991).
IL-6 has been found to act as an antagonist for the
toxoplasmacidal activity of IFN in murine peritoneal macrophages in vitro (Beaman et al., 1994).
Combining TNF with IL-6 pretreatment resulted in
restoration of the toxoplasmacidal activity, whereas
addition of anti-TNF antibody to this combination
resulted in enhancement of the IL-6-mediated impairment of IFN function. It would appear, therefore,
that in this system IFN and IL-6 interact at the level
of TNF triggering.
IN VIVO BIOLOGICAL
ACTIVITIES OF LIGANDS IN
ANIMAL MODELS
Normal physiological roles
Nonviral Infections
IFN is generally assumed to play a primordial role
in defense against pathogens that reside in and use the
intracellular environment as a shield against microbicidal antibodies. Specific host defense against these
organisms is therefore mainly dependent on cellular
immunity mechanisms. Many of these pathogens
have their habitat in mononuclear phagocytes. The
traditional view holds that CD4+ T helper cells which
recognize microbial antigen on the class II antigenpositive infected phagocytes are activated to produce
cytokines including IFN. This IFN then triggers
several microbicidal mechanisms in the phagocyte,
e.g. tryptophan oxidase and reactive oxygen and
nitrogen. It should be noted, however, that this
CD4+ T cell- and cytokine-dependent pathway is
complemented by the cytotoxic T cell pathway, which
kills phagocytes or other cells that harbor microbial
pathogens (Ottenhof and Mutis, 1995).
In a great variety of in vivo models for infectious
diseases IFN has been demonstrated to influence
the outcome of experimental infection. The two
principal approaches used in these studies have been
to demonstrate alterations in the course of infection
under the influence of exogenous IFN, or conversely, an effect of blockage of endogenous IFN by
administration of a neutralizing anti-IFN antibody.
These data have been confirmed and extended by the
use of mice which are engineered to be deficient in
producing IFN or in expressing the IFN receptor.
For instance, such mice were found to have lost
the ability to mount a protective host response to
Mycobacterium tuberculosis. Although granulomas
and delayed-type hypersensitivity did develop, the
mice succumbed to the infection (Cooper et al., 1993;
Flynn et al., 1993).
The cellular or physiological mechanisms by which
IFN exerts its advantageous actions are diverse.
Reduced binding of the parasite to the host cell has
been described to be a possible mechanism in
Leishmania infection (Mosser and Handman, 1992).
Reduced availability of tryptophan, due to IFNinduced enhanced production of indoleamine oxidase,
has been pinpointed as a mechanism for reduced
intracellular growth of Toxoplasma (Pfefferkorn,
1984; Byrne et al., 1986; Murray et al., 1989). Intracellular killing of organisms has been invoked as a
IFN
mechanism of action against several agents. Oxygendependent killing by peroxide generation has been
reported as a mechanism involved in killing of
Listeria (Peck, 1989) and Salmonella (Kagaya et al.,
1989) by IFN-activated macrophages. Production of
NO reportedly contributes to IFN-mediated killing
of Trypanosoma cruzi (Mayer et al., 1993), Mycobacterium tuberculosis (Flynn et al., 1993), and
Histoplasma capsulatum (Nakamura et al., 1994). It
should be noted, however, that at least some IFNactivated macrophage lines can kill Listeria by a
reactive nitrogen-independent mechanism (Leenen
et al., 1994).
Production of IFN by NK-like cell populations in
the early stages of bacterial or protozoal infections is
now generally accepted to constitute a crucial element
in defense, because it generates a transient aspecific
resistance. For instance, the IFN-dependent temporary resistance of mice to Toxoplasma infection was
found to result from stimulation of CD4ÿCD8ÿ cells
(Johnson et al., 1993). Not unexpectedly, IFN
knockout mice succumb rapidly to infection with
otherwise avirulent strains of Toxoplasma. In such
mice neutralization of IL-12 activity by antibody
blocked NK cell activity but did not further reduce
resistance to infection, suggesting that all early
resistance is mediated through IFN (Scharton et al.,
1996). The IFN-producing cell population involved
in this early resistance may not only be of the NK
type but may also include CD4+ T cells, as evident
from studies with cytokine receptor c chain-deficient
mice (Scharton-Kersten et al., 1998).
Another type of mechanism enabling early IFN
production to determine the outcome of infections
is the regulation of the balance between TH1 and
TH2 lymphocyte development. A single administration of anti-IFN antibody before an experimental
Leishmania infection in mice promotes susceptibility
(Belosevic et al., 1989) and inhibits development of
TH1 cells (Scott, 1991). Conversely, injection of IFN
early in infection switches the early cytokine pattern
in susceptible mice from a TH2 to a TH1 profile
(Scott, 1991). The producers of early IFN in this
model were found to be NK cells (Scharton and Scott,
1993). Mice from a genetically resistant background,
lacking the IFN receptor gene, were found to be
susceptible to infection but nevertheless to mount a
TH1 type response (Swihart et al., 1995).
Listeria infection is another example: treatment of
inherently resistant mice with neutralizing anti-IFN
antibodies resulted in delayed elimination of bacteria
from spleens and livers, and in increased production
of circulating IL-4 and IL-10 (TH2 cytokines) late in
infection. However, additional treatment with antiIL-4 antibody restored resistance, suggesting that
663
endogenous IFN is needed to avoid a TH2 response
(Nakane et al., 1996).
Remarkably, there are a few distinct exceptions
to the general rule that endogenous IFN exerts a
beneficial effect in infections. These exceptions can
be divided in two categories: (1) adaptation of the
microbial agent and (2) overproduction of the
cytokine.
Certain agents seem to have adapted so well to the
immune system of the host that they have succeeded
in perverting the cytokine network so that IFN acts
to their advantage. Trypanosoma infections in rodents
exemplify perversion of IFN by the parasite. These
infections in general are associated with nonspecific
immunosuppression, and various mechanisms have
been shown to be involved. However, in the case
of T. brucei and T. congolense infections in rats or
mice, IFN production is a crucial element in bringing about immunosuppression. Lymphocytes from
T. brucei-infected mice, while unable to express IL-2
receptor and to proliferate after in vitro stimulation
with antigen, do produce normal quantities of IFN
(Sileghem et al., 1987). The presence of macrophages
was required for this state of immunosuppression to
occur. In a culture system T. brucei was found to
install a similar state of macrophage-dependent
inhibited mitogen responsiveness with intact production of IFN (Darji et al., 1993). Remarkably, interference with endogenous IFN was found to relieve
suppression. Similarly, in rat mononuclear cell cultures, T. brucei was found to induce a rapid,
nonantigen-specific release of IFN which was dependent on the presence of CD8+ cells. A proliferative
response was seen only if endogenous IFN was
blocked by antibody. The parasites, conversely, proliferated more profusely in the presence than in the
absence of IFN. Furthermore, by immunochemical
staining, rat IFN was found to bind and possibly to
be ingested by T. brucei (Olsson et al., 1991). Thus,
IFN may act as a direct growth factor for the
parasite. Remarkably, IFN of human origin did not
bind to or affect growth of the organism. The
immunosuppressive effect of IFN in T. congolense
infection is illustrated by the observation that
treatment with anti-IFN antibody protects highly
susceptible Balb/c mice from infection (Uzonna et al.,
1998).
Two other agents that, at least in certain in vitro
systems, thrive better in the presence than in the
absence of IFN are Mycobacterium lepraemurium
(Mor et al., 1989) and Candida albicans (Garner et al.,
1989).
Overproduction of IFN, in association with that
of other cytokines, can act to the disadvantage of the
host in two possible ways: (1) IFN may enhance the
664 Alfons Billiau and Koen Vandenbroeck
production of other cytokines, e.g. TNF, which may
cause tissue damage and death, or (2) IFN may act
synergistically with such cytokines. Shock caused in
mice by endotoxin is aggravated by endogenous
as well as exogenous IFN. Thus, treatment with
neutralizing antibody against IFN protects mice
against induction of the lethal generalized Shwartzman
reaction, and this protective effect is associated with
lower TNF and IFN/ levels in serum (Billiau
et al., 1987; Heremans et al., 1990; Billiau and
Vandekerckhove, 1991). It can be inferred that in
some instances of gram-negative sepsis endogenous
IFN may contribute to, rather than antagonize fatal
outcome (Silva and Cohen, 1994). Chronically
infected mice develop a state of hypersensitivity to
the toxic effects of endotoxin or TNF. Hypersensitivity to endotoxin does not, however, develop
in BCG-infected mice which lack the receptor for
IFN (Kamijo et al., 1993a), and endotoxin hypersensitivity in Propionibacterium acnes-infected mice
can be blocked with neutralizing antibody against
IFN (Katschinsky et al., 1992). Similarly, hypersensitivity of BCG-infected mice to the lethal effect of
human TNF does not occur in mice which are deficient in expressing the IFN receptor; as a contrast,
sensitization for TNF by tumors does occur in the
IFN receptor-deficient mice (Cauwels et al., 1995). It
should be noted that toxicity, as well as some other
systemic effects of TNF are governed by multiple
factors. In mice, whether toxicity occurs critically
depends on which of the two TNF receptors becomes
activated. Human TNF happens to trigger only the
55 kDa receptor (TNFRI, p55), whereas mouse TNF
hits both this receptor and TNFRI (p75). This
difference is correlated with a dramatically higher
toxicity of homologous TNF than that of human
TNF (Brouckaert et al., 1992).
Chronic infections are often accompanied by
cachexia. Overproduction of cytokines, in particular
TNF (cachectin), has been incriminated as an
important element in the pathogenesis of cachexia
that develops in association with cancer (Tracey et al.,
1988; Sherry et al., 1989). However, at least in tumor
models in rats and mice, cachexia has been
demonstrated to be due as much to IFN as to
TNF (Langstein et al., 1991; Matthys et al., 1991a,
1991b; Billiau and Matthys, 1992).
Viral infections
IFN has the potential to protect the host against
virus infection by virtue of its direct antiviral effect on
most types of cells and by its regulatory activity on
immunocytes. Not unexpectedly, exogenously administered IFN has been found to act prophylactically
against a variety of experimental virus infections, e.g.
murine cytomegalovirus (CMV) infection in mice
(Fennie et al., 1988) or rat CMV infection in rats
(Haagmans et al., 1994). Of more fundamental
importance are the studies analyzing production of
IFN and the effects of its neutralization in model
virus infections in mice and rats. Generally speaking
these studies have indicated that endogenous IFN
is essential for adequate host defense against virus
infection, i.e. for elimination of the virus following
primary infection and, in some instances, also for
establishment of adequate immunity against reinfection. However, a question which remains largely
unsolved is whether the in vivo antiviral effects of
endogenous IFN are due to its direct antiviral effects
on cells or to its immunomodulatory activities, such
as activation of NK cells or maturation of cytotoxic
T lymphocytes.
Much relevant information has come from studies
on lymphocytic choriomeningitis (LCM) virus infection in mice. LCM virus is indigenous to mice.
Typically, the virus causes lethal choriomeningitis
when inoculated intracerebrally in adult mice, but is
poorly pathogenic when inoculated by other routes
and/or at young age. Cell-mediated immunity is
required for elimination of the virus in mice (LehmanGrube et al., 1985): in young or immunosuppressed
mice, the infection tends to be inapparent and to
become persistent. Cell-mediated antiviral immunity,
when it arises, is also instrumental in bringing about
tissue damage in mice infected under conditions that
favor LCM pathology, e.g. in adult immunocompetent mice infected intracerebrally.
In adult mice infected with LCM virus by inoculation in the footpad, the number of IFN-producing
cells in the spleen increases significantly above low
constititive levels without IFN becoming detectable
in the circulation (Gessner et al., 1989, 1990).
Pretreatment of the mice with anti-IFN antibodies
delays virus elimination (Leist et al., 1989; Wille et al.,
1989). Also, virus replicates to higher titers in organs
of IFN receptor knockout mice than in those of
wild-type counterparts (MuÈller et al., 1994). However,
anti-IFN also converts an aggressive into an inapparent infection (Leist et al., 1989). The conclusion of
these studies is that IFN contributes to cell-mediated
immunity against the virus, the result of which may be
favorable or unfavorable to the host depending on the
circumstances.
In mice infected with murine cytomegalovirus,
clearance of the virus from the salivary glands was
shown to depend on endogenous IFN (Lucin et al.,
1992). SCID mice infected with murine CMV have
activated macrophages with increased MHC gene
product and ICAM-1 expression. Treatment with
IFN
anti-IFN antibody prevented activation from occurring, caused virus replication in the spleen and liver
to be increased, and pathology in spleen to be more
pronounced (Heise and Virgin, 1995). Since SCID
mice do not possess specific antigen-reactive T cells, it
was concluded that IFN-dependent macrophage
activation in this model of CMV infection occurred
independently of T cells, implying that other IFNproducing cells, conceivably NK cells, control macrophage activation in the infection.
In rats and rat cells infected with a rat cytomegalovirus IFN affects virus replication in opposite
ways depending on the dose and the circumstances
(Haagmans et al., 1994). Rat CMV replicates in
immunocompetent as well as immunosuppressed mice
but is pathogenic only for the latter ones. High doses
of IFN were found to act prophylactically both
in vitro and in vivo in immunosuppressed rats. However, low doses of IFN caused enhanced replication
in both fibroblast and macrophage cultures. Also, in
immunocompetent rats, neutralization of endogenous
IFN with antibody was associated with reduced
replication of the virus in the spleen and helped to
protect immunosuppressed rats partially reconstituted with sensitized T cells.
The enhancing effect of IFN on rat CMV replication, especially in mononuclear phagocytes, is
reminiscent of similar effects of IFN on replication
of HIV in mononuclear leukocytes (Koyanagi et al.,
1988; Biswas et al., 1992). The effect may be direct
and virus-specific, e.g. if IFN action would activate
promoter regions in the viral genome by the same
pathways as it derepresses cellular IFN-inducible
genes. A precedent for cytokines to act in this way is
available: TNF and TGF have been shown to
activate the immediate early genes of human CMV
(Haagmans et al., 1992; Alcami et al., 1993). Alternatively, the effect may be indirect as is suggested by
the fact that it is more pronounced in macrophages.
IFN may alter production of other cytokines or may
affect cellular receptiveness to such cytokines which
have an influence on virus replication.
Another mouse-indigenous virus is MHV (mouse
hepatitis virus), a coronavirus of which there exist
many strains. The virus is widespread among mouse
colonies. Again the severity of experimental infection
differs, depending on a number of defined variables.
For instance, one colony of mice (A/J strain) was
found to develop mild rather than severe disease after
infection with MHV3, due to immunity resulting
from inadvertent preceding infection with related
coronaviruses. Interestingly, however, this partial
immunity could be abrogated by pretreatment with
neutralizing anti-IFN antibodies (Lucchiari et al.,
1991, 1992). The findings imply that acquired
665
resistance to virus infections can in some instances
be infection-permissive and depend on early elimination of the challenge virus by cellular immunity.
In mice infected in the footpad with the murine
poxvirus (ectromeliavirus) treatment with anti-IFN
antibody was found to result in enhanced spread of
the virus to and efficient replication in the spleen,
lungs, ovaries and, especially, liver (Karupiah et al.,
1993a). Similarly, neutralization of endogenous IFN
resulted in higher mortality in mice infected with
human herpes virus (Stanton et al., 1995). Also, in
IFN receptor gene knockout mice, a normally nonlethal vaccinia virus infection causes death (Huang
et al., 1993; MuÈller et al., 1994). Vesicular stomatitis
virus (VSV) infection (Huang et al., 1993) and Semliki
Forest virus infection (MuÈller et al., 1994), however,
were found to take a normal course.
Mice infected with influenza virus via the respiratory route develop pneumonia. Recovery from this
experimental infection depends on the development of
virus-specific cytolytic T cell clones. Although IFN
can in general terms affect development of cytolytic
T cells, a study employing IFN gene knockout mice
has indicated that, in mice infected with influenza
virus, endogenous IFN plays little if any role in
cellular immunity to the virus (Graham et al., 1993).
As a contrast, the humoral immune response to the
viral antigens was different in the knockout mice in
that the proportion of IgG1 antibodies was increased.
In mice lacking CD8+ T cells as a result of 2microglobulin gene disruption, neutralization of IFN
was found to be associated with delayed virus clearance and increased proportions of lymph node B cells
producing virus-specific antibodies of the IgG2a
subclass (Sarawar et al., 1994).
Cancer
Rejection of some experimental tumors is associated
with the presence of IFN in the tumor tissue. That
this IFN contributes to this process is evident from
studies with immunogenic autologous or isologous
tumors, the rejection of which is abrogated by
administration of neutralizing anti-IFN antibodies
(Prat et al., 1987; Jarpe et al., 1989). Investigators
have also inserted the IFN gene into nonimmunogenic murine tumor cells with high metastasizing potential and found that the IFN-secreting
cells, when injected in syngeneic mice, had lesser
ability than the parental cells to develop into tumors
(Watanabe et al., 1989; Gansbacher et al., 1990); this
suppression of tumorigenicity was reversed by the
administration of anti-IFN or anti-CD8 antibodies.
In a model of metastasis following operative removal
of the primary tumor (Lewis lung), immunization
666 Alfons Billiau and Koen Vandenbroeck
with an irradiated, highly productive IFN-transfected line of the tumor was found to be able to
cure micrometastases (Porgador et al., 1993). Lowproducer lines and lines transfected with other IL-2 or
IL-6 were ineffective.
Dependence of tumor rejection on the presence
of endogenous IFN and the generation of cytotoxic
T cells was also demonstrated by the use of a
transplantable mouse fibrosarcoma cell line whose
IFN receptor was engineered so as to generate
sublines that were either sensitive or insensitive to
IFN (Dighe et al., 1994). The IFN-insensitive line
was more tumorigenic and tumors induced by this
line were more resistant to induction of rejection by
LPS. Moreover, the IFN-insensitive line retained
tumorigenic potential in vaccinated mice, and was
itself unable to induce immunity to challenge with the
standard line. More relevant for the natural occurrence of tumors was the approach in which primary
tumors were induced by application of a carcinogen
or by making use of an incapacitated anti-oncogene
(Kaplan et al., 1998). IFN receptor-deficient as well
as STAT1-deficient mice were found to develop
methylcholanthrene-induced tumors more rapidly and
with greater frequency than normal mice. Similarly,
inactivation of the p53 anti-oncogene resulted in a
larger number and a wider variety of tumors in these
mice. From transplantation experiments with cells
from carcinogen-induced tumors generated in IFNinsensitive mice, it appeared that IFN acts at least in
part by directly affecting the tumor cells, rendering
them more immunogenic.
Discordant with these observations are reports
describing enhancement by IFN of tumor growth or
metastatic potential of experimental tumors. A metastasizing murine mammary carcinoma (TS/A-pc), productively transfected with the IFN gene was found
to metastasize more extensively than the untransfected tumor line (Ferrantini et al., 1994). Also,
treatment of mice bearing Lewis lung tumors with
anti-IFN antibodies was found to reduce tumor
outgrowth (Matthys et al., 1991a). In vitro treatment
of carcinoma cells with IFN prior to their inoculation in mice has been reported to enhance metastatic potential (Ramani and Balkwill, 1987); the
mechanism involved in this model appeared to be
augmentation by IFN of the tumor cells' resistance
to the cytolytic effect of NK cells.
immunization systems testing for primary IgM antibody responses to sheep erythrocytes, blockage of
endogenous IFN with antibodies that neutralize
macrophage activation by IFN have indeed been
found to reduce the amount of antibody produced
(White-Helman and Wallace, 1989). IFN was also
found to be a necessary component of T cell-derived
helper factors for antibody induction in in vitro
immunization systems (Brunswick and Lake, 1985).
In contrast to this immunization-promoting effect
of endogenous IFN, of which only minute amounts
are actually detectable during the process, exogenous
IFN suppresses early antibody formation (WhiteHelman and Wallace, 1989).
IFN and IL-4 antagonize each other in a variety
of systems, and it has become general knowledge that
antibody responses depend in fact on the balance
between two categories of cytokines, IFN and IL-2
belonging to the first one (the TH1 cassette) and IL-4,
IL-5, IL-6, and IL-10 belonging to the second one
(the TH2 cassette). In this setting the role of IFN
consists in suppressing IgG1 and IgE antibody
formation and stimulating IgG2a antibody formation (Finkelman et al., 1988; Snapper et al., 1993).
Following infection with influenza virus (Graham
et al., 1993) the virus-specific IgG1 response was
found to be significantly higher in IFN-deficient than
in normal mice, probably reflecting increased production of IL-4. Treatment with neutralizing antiIFN antibody in mice vaccinated with influenza
virus antigens resulted in increased levels of antigenspecific IgG1 and IgE but reduced levels of IgG2 and
IgG3 (Dobber et al., 1995). In IFN receptor knockout mice the IgG1 antibody response to ovalbumin
was not different from that in normal mice, but the
IgG2a isotype was reduced (Huang et al., 1993).
When antigen in adjuvant (oil or alum) is injected,
a very early change (day 3) is the appearance at the
injection site of IFN-producing NK cells. Only later
(day 7) do cytokine-producing T cells appear in
draining lymph nodes. At this site IL-2 and IL-4 predominate over IFN (Bogen et al., 1993). The
possibility is considered that early IFN production
by NK cells is, in fact, secondary to IL-12 production
by macrophages that respond to the antigen and/or
the adjuvant. Early IL-12 and IFN may therefore
play the crucial role in directing the immune response
towards TH1 or TH2 predominance.
Antibody Formation
Immunosuppression Mediated by IFN
IFN augments expression of MHC class II antigen
expression and may therefore be expected to facilitate
antibody induction in systems in which antigen
presentation is the limiting factor. In murine in vitro
The predominant responses of leukocytes and
endothelial cells to IFN (enhanced expression of
MHC molecules and of cell adhesion molecules,
priming for production of other cytokines such as
IFN
IL-1, TNF, and IL-12) are such that IFN is generally considered as an immunostimulant. Therefore,
reference to immunosuppressive effects of IFN is
counterintuitive. Yet, there are multiple reports of
in vivo or ex vivo experimental systems in which
higher production of IFN is associated with lesser
responsiveness to antigens or mitogens.
Graft-versus-host disease (GVHD), a major complication of bone marrow transplantation, is associated with suppression of cellular immune responses,
as evident from reduced proliferative responses of
lymphocytes to mitogens. Addition of monoclonal
antibodies against IFN has been shown to relieve
suppression, implying that endogenous IFN is
involved (Wall et al., 1988). The target cell for
IFN in this system is believed to be a so-called
natural suppressor cell (Huchet et al., 1993).
In fresh adherent mouse spleen macrophages as
well as in macrophage hybridomas, IFN induces
suppressive activity for proliferative responses of
T cells. This induction is MHC class II antigenrestricted and specifically depends on expression of
I-J region-coded antigens, which is stimulated by
IFN (Noma and Dorf, 1985; Ishikura et al., 1989).
Macrophage-like suppressor cells which inhibit mitogen-induced lymphocyte proliferation also occur in
mice infected with Mycobacterium lepraemurium
(Gosselin et al., 1995a) or Mycobacterium avium
complex (Tomioka et al., 1995). Induction of these
cells in vivo is independent of IFN but their suppressive action in vitro does require IFN. Acquisition
of the suppressor activity requires cell-to-cell contact
between the adherent-type and nonadherent-type leukocytes, the latter being different from classical T, B,
and NK cells (Gosselin et al., 1995b).
Another example in which IFN mediates generalized immunosuppression is in infection with
Trypanosoma brucei in mice. A macrophage hybridoma, upon interacting with T. brucei was found to
acquire the ability to suppress mitogenic responses
of T lymphocytes (Darji et al., 1993). This suppressive
activity was accompanied by decreased expression
of IL-2 receptors but increased production of IFN.
Addition of anti-IFN antibody to the system prevented suppression of the mitogenic responses and
of IL-2 receptor expression, implying that IFN was
instrumental in bringing about the suppressive
activity. In chronic infection in mice, both spleen
and lymph node cells acquire suppressive activity for
mitogen-induced T cell proliferation (Sileghem et al.,
1987). By the use of IFN knockout mice and
inhibitors of iNOS, it could be demonstrated that, in
this system, splenocytes acquire suppressive activity
independently of IFN and of NO production
(Beschin et al., 1998). In other studies using
667
Trypanosoma brucei, NO production was found to
indeed contribute to immunosuppressive effects of
IFN-activated macrophages (Mabbott et al., 1995).
In Trypanosoma congolense infections in highly susceptible Balb/c mice, immunosuppression by endogenous IFN may be mediated by IL-10: in vivo
treatment with anti-IFN antibody was found to
reduce plasma levels of IL-10 as well as secretion of
IL-10 by splenocytes (Uzonna et al., 1998).
Finally, IFN seems to be able to induce suppressor cells for delayed-type hypersensitivity (DTH)
reactions. Splenic adherent cells incubated with haptene and then injected in mice were found to suppress
DTH and induce appearance of haptene-specific
suppressor cells, demonstrable by transfer into mice
that were subsequently tested for DTH responsiveness. On prolonged culture the splenic cells lost their
ability to induce suppression, but addition of IFN
could restore this ability (Noma and Dorf, 1985).
A possible mechanism of suppression by mononuclear phagocytes is generation of H2O2 and prostaglandins, since both catalase and indomethacin can
alleviate suppression (Metzger et al., 1980). Boraschi
et al. (1984), on the other hand, found that IFN
reduces rather than stimulates murine macrophage
suppressive activity by inhibiting PGE2 release and
inducing IL-1 induction. Another pathway used by
suppressor macrophages that can be activated by
IFN, is the generation of nitric oxide (Mills, 1991;
Albina et al., 1991). IFN also enhances release by
mononuclear phagocytes of TGF (Twardzik et al.,
1990), which is generally known as an anti-inflammatory cytokine. Another mechanism of immunosuppression by IFN may be inhibition of expression
of CD40 ligand (Roy et al., 1993). Other mechanisms
of immunosuppression by IFN include inhibitory
effects on the expression of membrane-associated
molecules and on production of chemokines.
Acute Inflammation
Application of various methods has documented
production of IFN in lymphoid organs and in local
sites of inflammation, e.g. in the CNS during
experimental autoimmune encephalomyelitis (reviewed
in Heremans and Billiau, 1997). However, most of
our current understanding of the role of IFN in
inflammation stems from experiments employing
monoclonal antibodies to neutralize IFN in vivo,
and from experiments involving the use of IFN or
IFN receptor knockout mice, the former being
unable to produce IFN, the latter being unable to to
respond to it.
A model of local inflammation is the local
Shwartzman reaction elicited by a single injection of
668 Alfons Billiau and Koen Vandenbroeck
endotoxin in the mouse footpad (Heremans et al.,
1987). Pretreatment of the mice with monoclonal
antibody to IFN resulted in a modification of the
footpad swelling reaction: the early edema was
reduced, whereas the later phase, consisting of cellular
infiltration and intravascular thrombosis remained
largely unchanged. This result indicates that IFN
that is produced subsequent to the inflammatory
stimulus exerts a local proinflammatory effect.
However, it is unclear whether this IFN came from
a local source (sporadic NK or T cells in the footpad)
or had its origin in the spleen or lymph nodes. The
local endotoxin injection most probably sufficed to
induce a generalized cytokine response, as it was
found to prime for a generalized Shwartzman reaction induced by a second systemic endotoxin injection
(Heremans et al., 1990). Remarkably, pretreatment of
the mice with exogenous IFN, instead of augmenting the local reaction, also inhibited it. A possible
interpretation is that the effect of IFN differs
depending on whether it hits the local site prior to or
subsequent to application of endotoxin. Opposing
effects of IFN depending on time of entry into the
system is a recurrent theme in studies with animal
models.
This example illustrates the difficulty of distinguishing local from generalized pathogenetic inflammatory events. Model systems for generalized acute
inflammation have been extremely popular in IFN
studies. One of the first to be studied was the generalized lethal effect of endotoxin. Pretreatment of mice
with anti-IFN antibody was found to make mice
completely resistant to the generalized Shwartzman
reaction. This resistance seemed to be related to a
reduced systemic production of TNF (Heremans et al.,
1990). As already mentioned, IFN and TNF synergize at the levels of both production and action. These
early results from treatment with anti-IFN antibody
have subsequently been reinforced by studies on
knockout mice (Kamijo et al., 1993a; Car et al., 1994).
Other models of acute inflammation in which the
antibody approach led to evidence for a proinflammatory effect of IFN are the superantigen-induced
shock syndrome (Matthys et al., 1995b), the anti-CD3
antibody-induced syndrome (Matthys et al., 1993),
tumor-associated cachexia (Matthys et al., 1991a,
1991b) and the Con A-induced lethal hepatitis syndrome (Tagawa et al., 1997). In each of these
instances, pretreatment with anti-IFN antibody protected the animals against the toxic manifestations of
the inflammatory reaction.
Of special note is the anti-CD3 syndrome, because
the results and conclusions from antibody-mediated
ablation of IFN were contradicted by the results
from experiments with knockout mice. The CD3
membrane molecule is present on all T cells. Injection
of the antibody initiates a response of all T cells,
resulting in massive production of several cytokines
including not only IFN, but also IL-2, IL-4, TNF,
and several others. The result is a shock-like
syndrome characterized by hypomotility and piloerection, hypothermia, and hypoglycemia. Depending
on inherent sensitivity of the mice and on the dose of
antibody, the syndrome may be self-contained or
lethal. Mouse strains which are good IFN producers, e.g. Balb/c mice, were found to be more sensitive
than others. Pretreatment of the mice with anti-IFN
antibody prior to injection of the anti-CD3 antibody
was found to provide near complete protection
against the disease manifestations (Matthys et al.,
1993), indicating that IFN produced as a result of
the anti-CD3 challenge contributes to the severity of
the syndrome. Studies with IFN receptor knockout
mice, however, yielded data that led to a different
conclusion (Matthys et al., 1995a). These studies were
done with mice of the 129 background. Wild-type 129
mice are poorly sensitive to the anti-CD3 syndrome;
the IFNR knockout mice, however, turned out to be
more sensitive, indicating that, in this situation, an
IFN-dependent protective pathway is operational.
Obviously, IFN generates more than one pathway
in the pathogenesis of the syndrome; some pathways
make the syndrome worse, others rather provide
protection. The question then is to know why the
protective pathway is to prevail in one case and the
disease-promoting pathway in the other. An obvious
difference between blockage with anti-IFN antibody
and blockage by knocking out the IFN receptor, is
that the first one affects only IFN formed after antiCD3 challenge, while the second one affects all IFN,
including any of it that is formed prior to the antiCD3 challenge. IFN produced in the animal's life
time before exposure to the challenge with anti-CD3
antibody may be important for the protective pathway to be available; IFN produced as a result of the
anti-CD3 challenge may be critical for full-blown
disease to develop. Again, as was the case in the
localized Shwartzman reaction, the effect of IFN
seems to differ depending on the time of its entry into
the system.
In Vivo Modulation of Immune Pathology by IFN
In various animal model systems IFN has been
found to protect against immune pathology (Table 6).
For instance, IFN seems to be able to induce
suppressor cells for DTH reactions. Splenic adherent
cells incubated with haptene and then injected in mice
were found to suppress DTH and induce appearance
of haptene-specific suppressor cells, demonstrable by
IFN
669
Table 6 Animal model systems of immunopathology in which IFN acts as an immunosuppressant
Model
Observation
References
DTH reaction
Haptene-specific suppressor cells
lose in vivo suppressor activity
upon prolonged culture
Addition of IFN restores
suppressive activity
Noma and Dorf, 1985
Skin or heart allograft
Rejection prolonged by blockage
of CD40±CD40L or B7±CD28
pathways
Rejection not prolonged in
IFN knockout mice
Saleem et al., 1996;
Konieczny et al., 1998
Experimental autoimmune
encephalomyelitis (EAE)
Immunization with CNS antigens
causes CNS inflammation
and demyelination
Anti-IFN antibody
enhances disease
Billiau et al., 1988;
Duong et al., 1992, 1994;
Lublin et al., 1993;
Heremans et al., 1996b
Exogenous IFN alleviates
disease
Billiau et al., 1998
IFN ligand and/or receptor
knockout mice are more
sensitive
Ferber et al., 1996
Anti-IFN antibody enhances
disease
Caspi et al., 1994b
Exogenous IFN inhibits
disease
Caspi et al., 1994a
Experimental autoimmune
uveitis (EAU)
Immunization with bovine
retinal antigen
transfer into mice that were subsequently tested for
DTH responsiveness. On prolonged culture, the
splenic cells lost their ability to induce suppression,
but addition of IFN could restore this ability (Noma
and Dorf, 1985).
Treatment of skin allograft recipient mice with
anti-IFN antibody has been found to prolong rejection if the graft is MHC class II antigen-incompatible,
but not if it is only MHC class I-incompatible
(Rosenberg et al., 1990), suggesting that, if endogenous IFN contributes to the rejection of a skin allograft, it does so because it induces class II expression
on keratinocytes. More recent evidence from allograft
rejection studies is rather indicative of a suppressive
effect of endogenous IFN. The rejection rate of
cardiac transplants was found to be similar in IFN
gene knockout mice and in wild-types (Konieczny
et al., 1998). However, in the knockout mice, and also
in wild-type ones treated with neutralizing anti-IFN
antibody, it proved impossible to prolong graft
survival by blockage of the B7±CD28 or the CD40±
CD40L pathways, as it could be done in the plain
control wild-types (Saleem et al., 1996). Thus, it
appears that endogenous IFN mediates a suppressive circuit when costimulation is inadequate.
Experimental autoimmune encephalomyelitis (EAE)
and experimental autoimmune uveitis (EAU) in mice
are two examples in which the net effect of endogenous IFN, produced in the course of the immune
process, is immunosuppression. In several variant
models of the disease, treatment of the mice with
neutralizing antibody against IFN was found to
result in augmented symptoms and mortality
(reviewed in Billiau, 1996b). Furthermore, both IFN
and IFN receptor knockout mice were found to be
more sensitive to induction of EAE than their wildtype counterparts (Ferber et al., 1996; Heremans et al.,
1996a).
Examples of models in which IFN accelerates
or intensifies immune pathology are autoimmune
diabetes and lupus-like syndrome in NZB/W mice, and
inflammatory bowel disease (Table 7). Reports on
the role of IFN in autoimmune diabetes models
are controversial. Streptozotocin-induced diabetes, as
assessed by hyperglycemia and body weight loss, was
found to be more severe in mice that also received
IFN injections (Campbell et al., 1988). Anti-IFN
antibody pretreatment was found to reduce the incidence and severity of diabetes in nonobese diabetic
(NOD/Wehi) mice in which occurrence of diabetes is
boosted by cyclophosphamide (Debraye-Sachs et al.,
1991; Campbell et al., 1991a). Counter to expectation,
administration of IFN in this mouse model of
diabetes did not affect blood glucose profiles. In fact,
in combination with TNF, IFN treatment was
associated with a reduction in severity of islet inflammation, although this treatment caused moderate to
severe pancreatitis and several other pathologic
670 Alfons Billiau and Koen Vandenbroeck
Table 7 Animal model systems of immunopathology in which IFN acts as an immunopotentiator
Model
Observation
References
Streptozotocin-induced
Exogenous IFN aggravates
disease
Campbell et al., 1988
Spontaneous or cyclophosphamideboosted diabetes in NOD mice
Anti-IFN antibody alleviates
disease
Campbell et al., 1991a;
Debraye-Sachs et al.,
1991
IFN receptor knockout mice
are protected
Wang et al., 1997
Anti-IFN antibody or soluble
IFN receptor alleviate disease
Jacob et al., 1987;
Ozmen et al., 1995
IFN receptor knockout mice
are protected
Haas et al., 1998
MRL-lpr mice
IFN receptor knockout mice
are protected
Balomenos et al., 1998
IL-10 knockout mice
Anti-IFN antibody prevents
disease
Berg et al., 1996;
Rennick et al., 1997
SCID mice reconstituted with
CD45Rbhi CD4+ T cells
Anti-IFN antibody prevents
disease
Powrie et al., 1994
IL-12-induced enteropathy
IFN knockout mice are
insensitive; exogenous IFN
induces enteropathy
Guy-Grand et al., 1998
Experimental
autoimmune neuritis
Immunization of rats with
peripheral nerve antigen
in CFA
Anti-IFN antibody alleviates
disease
Hartung et al., 1990;
Strigard et al., 1989;
Tsai et al., 1991
Experimental
autoimmune orchitis
Immunization with syngeneic
testicular germ cells
(no adjuvant)
Late (day 20) treatment with
anti-IFN antibody
alleviates disease
Itoh et al., 1998
Autoimmune diabetes
Lupus-like disease
Inflammatory bowel
disease
Female (NZB NZW)F1 mice
changes (Campbell et al., 1991b). Breeding of a null
mutation of the IFN receptor into the NOD mice
resulted in a drastic reduction of insulitis and diabetes
(Wang et al., 1997). Thus, the weight of the evidence
favors the view that endogenous IFN stimulates
autoimmune diabetes. The underlying mechanisms
are considered to be (a) upregulation of MHC class I
molecules, which could augment targeting of cytotoxic T cells to the islet cells, and (b) increased production of aspecific inflammatory mediators, e.g. NO.
Female (NZB NZW)F1 mice spontaneously
develop a lupus-like syndrome, which has long been
considered as a prototype autoantibody-mediated
autoimmune disease. This view is reinforced by the
cytokine production profile in these mice, which is of
the TH2 type. Moreover, interventions which interfere with TH2-type cytokines, e.g. administration of
anti-IL-10 antibodies, were found to alleviate disease.
Endogenous or exogenous IFN would therefore
be expected to also play a disease-alleviating role.
However, the experiments turned out to yield the opposite result. Treatment with neutralizing anti-IFN
antibody (Jacob et al., 1987) or soluble IFN receptor
(Ozmen et al., 1995) was found to prevent disease.
These observations were recently reinforced by the
report that IFN receptor-deficient (NZB NZW)F1 mice are insensitive to the disease (Haas
et al., 1998). The difference in sensitivity between the
wild-type and mutant mice could entirely be explained
by the different levels of autoantibody, indicating that
IFN uses this pathway to accelerate disease. An
obvious implication is that the TH1/TH2 concept
fails to provide a suitable framework to explain the
role of IFN in the NZB/NZW lupus model.
Inflammatory bowel diseases (IBD) are now
considered as disturbances in the delicate balance
between immune reactivity and anergy towards
microbial antigens and toxins present in the gut
lumen. Few studies have been done on the role of
IFN in models of IBD (see Table 7). The weight of
the evidence is in favor of a disease-promoting role
of endogenous IFN. Intestinal mucosal involvement
in mice with graft-versus-host disease in SCID mice
reconstituted with CD45Rbhi CD4+ T cells, and in
IFN
IL-10 knockout mice was found to be alleviated by
treatment with anti-IFN antibody (Powrie et al.,
1994; Berg et al., 1996; Rennick et al., 1997). Also,
IFN knockout mice were found to be resistant to
IL-12-induced small bowel enteropathy, and exogenous IFN was found to be able by itself to cause
mucosal epithelial damage (Guy-Grand et al., 1998),
indicating that IFN produced by intraepithelial lymphocytes has direct cytotoxic effects on epithelial cells.
Thus, in IBD, IFN seems to act more by its inflammatory than by its immunoregulatory potential.
Table 7 reviews evidence for mixed immunosuppressive and immunostimulatory effects of IFN in
a number of animal models. Graft-versus-host disease
develops in irradiated mice reconstituted with allogeneic bone marrow. In several independent studies
(Mowat, 1989; Brok et al., 1993) the contribution
of this IFN to the disease manifestations has
been assessed by the use of neutralizing anti-IFN
antibodies. These studies are unanimous in observing
that blockage of IFN inhibits disease development,
in particular the lesions in the gut mucosa. On the
other hand, as an apparent paradox, it has been
reported that systemic administration of IFN inhibits disease development in much the same way as antiIFN antibody does. This inhibition was associated
with reduced numbers of IFN-producing cells (Brok
et al., 1993).
Another model of graft-versus-host disease consists of inoculating semiallogeneic lymphocytes into
neonatal mice, which allows for the persistence of the
donor cells in the host. These cells differentiate into
TH2-like cells as evident from predominance of IL-4
production over that of IL-2 and IFN. As an
apparent consequence, donor B cells differentiate to
produce large quantities of IgE and IgG1 autoantibodies, resulting in immune deposits and SLE-like
pathology. In this model, exogenous IFN was found
to prevent the disease, apparently by restoring the
ability of the lymphocytes to produce IL-2 and IFN
(Donckier et al., 1994).
In collagen-induced arthritis treatment with antiIFN antibody has been reported to either inhibit or
enhance disease, depending on the time of administration (Boissier et al., 1995). However, more recent
studies have provided evidence for a uniform diseaseaggravating effect, as evident from accelerated occurrence of symptoms and a higher cumulative incidence
(Williams et al., 1993; Vermeire et al., 1997; ManourySchwarz et al., 1997). Moreover, IFN receptor
knockout mice were found to be more sensitive than
corresponding wild types (Vermeire et al., 1997;
Manoury-Schwarz et al., 1997).
Remarkably, both experimental autoimmune encephalomyelitis (EAE) and collagen-induced arthritis
671
(CIA) were found to be completely inhibited by
treatment with anti-IL-12 antibody, indicating that,
in these models, IL-12 acts independently from, and
in opposite direction to IFN (Matthys et al., 1998;
Heremans et al., 1999). On the basis of cytokine production profiles, EAE and CIA are both considered
to result from TH1-type activity of autoreactive T cells.
These observations are therefore seen to be at variance with the tenet, based on in vitro observations,
that endogenous IFN participates in upregulating
TH1-type reactivity. Recently, a possible explanation
for the discrepancy has been proposed, based on the
observation that the protective effect of endogenous
IFN in CIA depends on the use of complete, as
opposed to incomplete Freund's adjuvant. CIA
induced by immunization with the aid of incomplete
adjuvant, i.e. not containing heat-killed mycobacteria, was less pronounced in IFN receptor knockout mice or in anti-IFN antibody-treated mice than
in corresponding controls. It was also found that the
severity of the disease critically depends on the induction by the mycobacteria of an expansion of the
Mac-1+ leukocyte population by intra- and extramedullary hematopoiesis, and that this myelopoiesis
is much less pronounced in IFN-competent than in
IFN-ablated mice (Matthys et al., 1999).
In experimental autoimmune thyroiditis (EAT), a
model which is operationally similar to EAE, EAU,
or CIA, the role of endogenous IFN varies with
genetic background and experimental conditions
(Table 8). Deletion of the IFN receptor gene in
H2k-haplotype mice has been reported to have little
effect (Alimi et al., 1998), but deletion of the IFN
ligand gene in H2q-haplotype mice resulted in more
severe thyroiditis with granulomatous lesions and
eosinophil infiltrations (Stull et al., 1992). On the
other hand, treatment with anti-IFN antibody was
found to inhibit actively induced disease (Tang et al.,
1993), but to enhance disease if used to treat donor
splenocytes in a transfer model (Stull et al., 1992).
PATHOPHYSIOLOGICAL ROLES
IN NORMAL HUMANS AND
DISEASE STATES AND
DIAGNOSTIC UTILITY
Normal levels and effects
Clinical evidence documenting the importance of
IFN in human diseases is scarce. One reason for this
is that determination of systemic or local IFN
production in patients is impractical.
672 Alfons Billiau and Koen Vandenbroeck
Table 8 Animal models of immunopathology in which IFN exerts mixed immunosuppressive and immunostimulatory
effects
Model
Graft-versus-host disease
Collagen-induced
arthritis (CIA)
Experimental autoimmune
thyroiditis (EAT)
Observation
References
Reconstitution of irradiated mice
with allogeneic bone marrow
Both IFN and anti-IFN
antibody alleviate disease
Mowat, 1989;
Brok et al., 1993
Allogeneic lymphocytes in
neonatal mice cause
TH2-associated
SLE-like pathology
Exogenous IFN prevents
disease, restores TH1
responsiveness
Donckier et al., 1994
Immunization with chicken
collagen causes joint
inflammation and deformity
Early anti-IFN antibody
reduces severity; late antibody
aggravates
Boissier et al., 1995
Anti-IFN antibody aggravates
disease
Williams et al., 1993;
Vermeire et al., 1997
IFN receptor knockout mice
are more sensitive
Manoury-Schwarz
et al., 1997;
Vermeire et al., 1997
In vivo anti-IFN inhibits
actively induced disease
Tang et al., 1993
Actively induced disease
attenuated in IFN receptor
knockout mice
Alimi et al., 1998
Anti-IFN-treated splenocytes
are more pathogenic in the
transfer model
Stull et al., 1992
IFN-deficient mice develop
severe adoptive disease with
eosinophilic infiltration
Tang et al., 1998
Immunization with thyroglobulin
in CFA induces mononuclear
infiltrate thyroiditis;
transferable by spleen cells
Role in experiments of nature
and disease states
Genetic Defects
Inherited deficiencies in the IFN receptor system
have been described in children suffering from
exceptional susceptibility to mycobacterial infections
(for review, see Ottenhof et al., 1998). Complete
IFNR1 deficiency, resulting from null mutations, is
characterized by infections with low-virulent mycobacteria in early childhood. The granulomata are
poorly differentiated and resemble those seen in
lepromatous leprosy. Patients with partial IFNR1
deficiency, resulting from point mutations affecting
the affinity of the receptor, have preserved the ability
to form well-differentiated granulomata, but still are
exceptionally susceptible to mycobacterial infections.
A single case of complete IFNR2 deficiency, again
suffering from early childhood mycobacterial infections, has also been described.
IFN Gene Polymorphisms and Disease
A variety of animal experimental models have
revealed a complex role, either disease-promoting or
disease-limiting, for IFN in inflammatory diseases.
Thus, the recent identification of a multiallelic
microsatellite polymorphism in the human IFN
gene has driven several research groups to investigate
a potential genetic association with diverse autoimmune-type diseases, i.e. insulin-dependent diabetes
mellitus, in Japanese (Awata et al., 1994) and Finnish
and Danish individuals (Pociot et al., 1997); multiple
sclerosis in German (Epplen et al., 1997), Finnish
(Wansen et al., 1997), Sardinian (Vandenbroeck et al.,
1998b) and Swedish (He et al., 1998) patients; and
rheumatoid arthritis in British patients (John et al.,
1998). The results of these analyses are summarized in Table 9. Though all these diseases show a
certain degree of linkage to specific HLA class II
alleles on chromosome 6, they are genetically highly
heterogeneous and complex. In general, no global
disease association with the IFN gene was seen in
IFN
673
Table 9 Linkage and association analysis between a multiallelic CA repeat polymorphism in the first intron of the IFN
gene and insulin-dependent diabetes mellitus (IDDM), multiple sclerosis (MS), and rheumatoid arthritis (RA)
Disease type
Ethnicity
(number of
alleles detected)
Datasets
(statistical tests used)
Statistically significant
associations
References
Insulin-dependent
diabetes mellitus
(IDDM)
Japanese (8)
Case-control (chi square)
Global disease association;
strong disease association
in patients with onset
10 years of age
Awata et al., 1994
Danish/Finnish (5)
Case-control (chi square)
Global disease association
in Finnish, but not Danish,
case-control dataset
Pociot et al., 1997
Multiplex families (TDT)
No associations found
Pociot et al., 1997
German (11)
Case-control (chi square)
No associations found
Epplen et al., 1997
Finnish (5)
Case-control (chi square)
Multiplex families
(two point linkage analysis)
No associations found
Wansen et al., 1997
Sardinian (4)
Case-control (chi square)
Simplex families (TDT)
Disease association in
patients with low HLA-DR
associated risk
Vandenbroeck
et al., 1998b
Swedish (4)
Case-control (chi square)
No associations found
He et al., 1998
Multiplex families
(two point linkage analysis)
Positive LOD score under
assumption of recessive
inheritance
Multiplex families
(MLS IBD)
Increase in allele sharing in
sibling pairs with an
affected male
Multiple sclerosis
(MS)
Rheumatoid
arthritis (RA)
British
John et al., 1998
TDT, transmission disequilibrium test; MLS IBD, maximum likelihood score method for sharing of alleles by identity by
descent.
any of the case-control data sets ± with the Japanese
and Finnish insulin-dependent diabetes mellitus
patients representing, however, a notable exception.
By restricting the analysis to clinically (e.g. early
disease onset) or genetically (e.g. family-based study
designs) more homogeneous subsets of patients,
significant associations were disclosed between
specific IFNG CA-repeat alleles and insulin-dependent diabetes mellitus, multiple sclerosis, and rheumatoid arthritis.
IgE Pathology
Association of increased IgE levels with imbalance
between IL-4 and IFN has been suggested to be
involved in the pathogenesis of elevated IgE levels
observed in patients with hyper-IgE recurrent infection
syndrome (HIE) (King et al., 1989), atopic dermatitis,
or helminth infections. In HIE, increased IL-4
production has not been observed. Conversely, some
but not all studies reported decreased production of
IFN. Mitogen-driven IL-4 production by PBMCs
of atopic subjects was found to be higher and
production of IFN lower than that of PBMCs of
normal subjects (Rousset et al., 1991). In patients with
helminth infections, IgE levels were found to correlate
with increased IL-4 and decreased IFN production
by parasite antigen-stimulated lymphocytes (King
et al., 1993). However, in a placebo-controlled trial,
exogenous IFN failed to affect clinical parameters or
IgE levels in patients with hayfever-type rhinitis due
to ragweed allergy (Li et al., 1990). As a contrast, in
a murine model for allergen sensitization, nebulized
but not parenteral IFN was found to decrease IgE
production and to normalize airway function (Lack
et al., 1994).
674 Alfons Billiau and Koen Vandenbroeck
Elevated levels of IgE are among the immunological characteristics of chronic atopic eczema. IFN
responses have been reported to be defective in these
patients (Reinhold et al., 1990). However, other
changes are diminished delayed hypersensitivity reactions, diminished in vitro responsiveness to mitogens
and recall antigens and a decreased proportion of
CD8+ T-cells.
IN THERAPY
Preclinical ± How does it affect
disease models in animals?
See section on In vivo biological activities of ligand in
animal models ± Normal physiological roles.
Clinical results
Anti-infectious Potential
Chronic granulomatous disease (CGD) is the name of
a group of inherited deficiencies in the multicomponent enzyme NADPH oxidase, which is essential
for the production by phagocytes of superoxide and
related oxygen intermediates. Phagocytes of CGD
patients ingest bacteria at a normal rate, but fail to
effectively kill them. The patients suffer from
recurrent and severe pyogenic infections that begin
in early life and may lead to death. Pretreatment of
normal phagocytes with IFN augments the respiratory burst triggered by other stimuli, such as LPS:
IFN is believed to regulate the transcription of genes
coding for the enzymes of the NADPH oxidase
system.
In a controlled clinical trial on CGD patients IFN
was found to reduce the frequency of serious
infections (International Chronic Granulomatous
Disease Study Group, 1991). Although this study
was initiated following reports (Ezekowitz et al.,
1988; Sechler et al., 1988) of increased oxidative
metabolism and bactericidal potential of CGD
phagocytes, provoked by in vitro or in vivo exposure
to IFN, there is controversy as to whether the
clinical benefit is entirely or even partially accounted
for by increased responsiveness of the NADPH oxidase system (International Chronic Granulomatous
Disease Study Group, 1991; Muhlebach et al., 1992;
Weening et al., 1996). The possibility is considered
that oxygen-independent bactericidal mechanisms are
involved. However, the clinical data have been confirmed (Weening et al., 1995) and extended to provide
evidence for a therapeutic effect in established hepatic
abcess (Hague et al., 1993), which is a frequent type
of infection in these patients. Obviously, in CGD
patients spontaneous endogenous IFN production
is insufficient to fully exploit what remains available
of bactericidal potential. Optimal dosage regimens
and differential responses of different categories of
CGD patients are still under study (Ahlin et al.,
1997).
Visceral leishmaniasis (kala-azar), caused mainly by
Leishmania donovani, is a severe disease characterized
by fever, hepatosplenomegaly, anemia, and leukopenia. If left untreated, kala-azar mostly ends in progressive emaciation and death. The infection is
endemic in most tropical and subtropical countries,
with a focal distribution corresponding to the prevalence of specific sandfly vectors. The classical treatment consists of pentavalent antimony compounds or
amphotericin B. Both types of treatment are frequently ineffective and mostly associated with toxic
side-effects. Treatment with IFN in combination
with antimony has been shown to provide a cure in
some of the antimony-resistant (8 out of 10) or
previously untreated (8 out of 9) cases (Badaro
et al., 1990). In regions where the infection is highly
resistant against antimony, the beneficial adjunctive
effect of IFN seems to be limited (Sundar et al.,
1997).
Multidrug-resistant tuberculosis is likely to become
an increasing concern in years to come. Preliminary
evidence is available that treatment with IFN via
aerosol may be part of alternative strategies (Condos
et al., 1997).
Prevention of infection in patients on cytostatic
regimens represents another potential field of application. Aggressive therapy for certain forms of
leukemia, while successful in children, fails in adults
due to neutropenia resulting in uncontrollable infections (mostly pneumonia) with Pseudomonas, Staphylococci, and fungi. If IFN is to protect neutropenic
patients it should do so mainly by acting on the
mononuclear phagocyte system.
A further proposed field of application of IFN is
the control of infections occurring in patients with
severe trauma, either accidental or resulting from
aggressive surgery. Support for this possibility
comes from results obtained in experimental wound
infections with Klebsiella or Pseudomonas in mice
(Hershman et al., 1988a, 1988b, 1988c). IFN might
synergize with antibiotics and might thereby not only
increase survival rates but also recovery rates. So
far, placebo-controlled, double-blinded studies have
failed to establish a clear-cut effect (Mock et al.,
1996); one major difficulty being the large number
and predominant impact of confounding variables
IFN
affecting outcome. A European multicenter trial on
the use of IFN to prevent burn-related infections
also failed to reveal any beneficial effect (Wasserman
et al., 1998).
Anticancer Potential
Ever since clinical-grade recombinant human IFN
has become available, various regimens of treatment
for malignant disease have been undertaken. It seems
fair to state that, despite a tremendous input of effort,
so far there have been no major breakthroughs in
the field. Here, no attempt will be made to review
these studies. Suffice it to state that the efforts
are being continued along four main lines of
investigation:
1. Systemic treatment of patients with recurrent or
metastatic malignancy ± IFN alone or as adjunct
to other drugs (IL-2, cytostatics) is being given
to patients with melanoma (Kim et al., 1996;
Kirkwood et al., 1997), colorectal cancer (Pavlidis
et al., 1996), renal cell carcinoma (Lummen et al.,
1996), small cell lung cancer (van Zandwijk et al.,
1997), and non-Hodgkin lymphoma post stem cell
transplantation (Nakao et al., 1997). These trials
so far have borne out that IFN can be given
without unsurmountable side-effects, for long periods of time, and at doses which affect immunological parameters.
2. IFN as an adjunct to TNF in isolated limb
perfusion in primary or recurrent melanoma ±
Isolated limb perfusion with TNF in conjunction
with IFN has been shown to be effective in
bringing about regression of melanoma skin
lesions (Lienard et al., 1998).
3. Immunization with IFN gene-modified autologous melanoma cells ± Several strategies are currently being developed to combat cancer by
immunization against cancer cell antigens. One
approach consists in administering irradiated
autologous tumor cells which have been engineered to express one or several immune-stimulating cytokines. In view of its immune-potentiating
effects IFN is a prime candidate (Abdel et al.,
1997).
4. Immunotherapy with autologous IFN-activated
macrophages ± Activated macrophages, by virtue of
their cytocidal capacities, are considered to be
essential players in host defense against cancer.
One strategy to put this anticancer potential to use
consists in harvesting monocytes from the patient's
blood and reinjecting them after they have been
cultured in the presence of IFN (for review, see
Andreesen et al., 1998).
675
Antiatopic Potential
In a double-blind, placebo-controlled trial in patients
with chronic atopic dermatitis (Hanifin et al., 1993)
therapy with recombinant IFN has been found to
result in improvement of the clinical parameters.
However, therapeutic failures in children with severe
refractory disease have also been reported (Horneff
et al., 1994). Long-term IFN therapy for severe
atopic dermatitis is considered a safe practise
(Schneider et al., 1998).
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