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Interleukin-6 modulates production of T lymphocytederived cytokines in antigen-induced arthritis and drives inflammation-induced osteoclastogenesis.

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ARTHRITIS & RHEUMATISM
Vol. 54, No. 1, January 2006, pp 158–168
DOI 10.1002/art.21537
© 2006, American College of Rheumatology
Interleukin-6 Modulates Production of T Lymphocyte–Derived
Cytokines in Antigen-Induced Arthritis and
Drives Inflammation-Induced Osteoclastogenesis
Peter K. K. Wong,1 Julian M. W. Quinn,2 Natalie A. Sims,3 Annemarie van Nieuwenhuijze,1
Ian K. Campbell,1 and Ian P. Wicks1
Objective. To determine the cellular mediators of
antigen-induced arthritis (AIA) and the relative contribution of members of the interleukin-6 (IL-6) family
and tumor necrosis factor (TNF) in AIA.
Methods. AIA was induced in mice deficient in T
and B lymphocytes, IL-6 (IL-6ⴚ/ⴚ), TNF (TNFⴚ/ⴚ),
IL-11 receptor, and oncostatin M receptor, by immunization with methylated bovine serum albumin (mBSA)
followed 7 days later by intraarticular injection of
mBSA. Arthritis severity was assessed histologically,
and T lymphocyte responses were assessed in vitro.
Anti-TNF neutralizing antibody was administered to
wild-type mice during AIA. Bone marrow osteoclasts
were generated in vitro via culture with RANKL and
macrophage colony-stimulating factor.
Results. AIA was dependent on CD4ⴙ T lymphocytes, but not CD8ⴙ T lymphocytes or B cells. IL-6ⴚ/ⴚ
mice had reduced AIA severity and fewer osteoclasts at
sites of bone erosion. This protective effect was not seen
with a deficiency of other IL-6 family members and was
similar to that in TNFⴚ/ⴚ mice or wild-type mice
receiving TNF blockade treatment. IL-6ⴚ/ⴚ CD4ⴙ T
lymphocytes from draining lymph nodes had reduced
antigen-induced proliferation and produced less IL-17
and less RANKL, relative to osteoprotegerin, than cells
from wild-type mice. Bone marrow from IL-6ⴚ/ⴚ mice
generated fewer osteoclasts in vitro than bone marrow
from either wild-type or TNFⴚ/ⴚ mice.
Conclusion. AIA is driven by CD4ⴙ T lymphocytes. IL-6 is an important mediator of bone destruction
in AIA because it regulates T lymphocyte production of
key osteoclastogenic cytokines and inflammationinduced bone marrow osteoclast differentiation. These
findings have implications for reducing bone and joint
damage in rheumatoid arthritis.
Rheumatoid arthritis (RA) is an autoimmune
inflammatory disease characterized by sustained local
overproduction of cytokines such as tumor necrosis
factor (TNF) (1) and interleukin-1 (IL-1) (2). While
targeted blockade of these cytokines has been a major
therapeutic advance, many patients fail to respond to
either TNF (3) or IL-1 blockade treatment (4). It is
possible that other inflammatory mediators, such as IL-6
(5), may also be important in driving joint inflammation.
Results from early clinical trials of targeted IL-6 blockade treatment in RA appear promising (6,7).
RA patients have elevated levels of IL-6 in
synovial fluid and serum compared with patients with
osteoarthritis or healthy controls (8,9). Production of
IL-6 by RA synoviocytes is up-regulated by TNF and
IL-1 (10). Serum levels of IL-6 and IL-6 receptor
(IL-6R) correlate with radiographic joint destruction in
RA (8). Matrix metalloproteinases (MMPs) are important mediators of joint destruction in RA (11). IL-6 has
been found to enhance IL-1–induced production of
proMMP-1 and proMMP-3 and production of tissue
inhibitor of metalloproteinases by RA synovial fibroblasts (12).
Bone loss is a prominent feature of RA and may
Supported by the Reid Charitable Trusts, the Arthritis Foundation of Australia, and the National Health and Medical Research
Council of Australia.
1
Peter K. K. Wong, MBBS, FRACP, PhD, Annemarie van
Nieuwenhuijze, MSc, Ian K. Campbell, PhD, Ian P. Wicks, MBBS,
FRACP, PhD: The Walter and Eliza Hall Institute of Medical
Research, Parkville, Victoria, Australia; 2Julian M. W. Quinn, DPhil:
St. Vincent’s Institute of Medical Research, Fitzroy, Victoria, Australia; 3Natalie A. Sims, PhD: St. Vincent’s Hospital, University of
Melbourne, Fitzroy, Victoria, Australia.
Address correspondence and reprint requests to Ian P. Wicks,
MBBS, FRACP, PhD, Reid Rheumatology Laboratory, Division of
Autoimmunity and Transplantation, The Walter and Eliza Hall Institute of Medical Research, 1G Royal Parade, Parkville, Victoria 3050,
Australia. E-mail: wicks@wehi.edu.au.
Submitted for publication April 7, 2005; accepted in revised
form September 29, 2005.
158
ROLE OF IL-6 IN AIA
occur focally as periarticular erosions or more generally
throughout the skeleton, resulting in osteoporosis. Bone
loss is due to enhanced osteoclast activity modulated by
RANKL. RANKL is expressed by synovial fibroblasts
(13), osteoblasts (14), and T lymphocytes (15). The
action of RANKL is inhibited by the decoy receptor
osteoprotegerin (OPG), which is produced by RANKLexpressing cells (16). Osteoclasts are found at sites of
bony erosion in RA (17). Proinflammatory cytokines,
such as TNF (18), IL-1 (19), IL-6/soluble IL-6R (20),
and IL-17 (21), act on stromal/osteoblastic cells to
up-regulate RANKL expression and promote osteoclastogenesis. T lymphocyte production of RANKL (22) and
IL-17 (23) is therefore likely to be involved in the
pathogenesis of bone destruction in RA.
Antigen-induced arthritis (AIA) is a widely used
murine model of inflammatory arthritis that is induced
by intradermal (ID) immunization with methylated bovine serum albumin (mBSA) in the presence of Freund’s
complete adjuvant (CFA), followed by intraarticular
(IA) challenge with mBSA 7 or more days later. AIA has
many histologic features of RA, such as a leukocyte
infiltration, synovitis, and cartilage and bone destruction. AIA is dependent on T lymphocyte activity (24,25),
specifically CD4⫹ T lymphocytes (26–28). The presence
of anti-mBSA antibodies in murine AIA has been reported, but the functional contribution to the disease
was not defined (29). Similarly, the role of CD8⫹ T
lymphocytes and B cells in AIA has not been directly
addressed.
Mice deficient in IL-6 were protected against
severe joint inflammation and destruction in AIA, even
though TNF and IL-1 were produced in the synovium
(29,30). Cells from draining lymph nodes (LNs) of
arthritic IL-6⫺/⫺ mice mounted a weaker antigenspecific proliferative response, but whether this defect
was due to defective CD4⫹ T lymphocytes or to antigenpresenting cells (APCs) was not examined (29,30). This
is important because APCs in IL-6⫺/⫺ mice have been
reported to be less efficient stimulators of T lymphocyte
activation (31,32). In another study of AIA in IL-6⫺/⫺
mice, transient joint inflammation developed, but failed
to progress to long-term infiltration (33), suggesting that
IL-6 was important in converting acute inflammation to
a chronic phase. Expression of IL-6 was found in the
bone marrow before and during the course of AIA, and
AIA could be adoptively transferred into wild-type mice,
but not into IL-6⫺/⫺ mice, by bone marrow cells from
arthritic wild-type mice (34).
We began this study by undertaking a detailed
characterization of the cellular mediators of AIA using a
159
variety of mice deficient in T and B lymphocytes,
because the contributions of these cell types have not
been fully defined. We confirmed that AIA is dependent
on CD4⫹ T lymphocytes, but found that it does not
require CD8⫹ T lymphocytes or B cells. We next
examined the role of IL-6 family cytokines in AIA,
including a comparison of the relative importance of
IL-6 and TNF in this disease model. Finally, we examined the effect of IL-6 on T lymphocyte production of
pro-osteoclastogenic cytokines and inflammationinduced osteoclastogenesis during AIA. We found that
IL-6 may be at least as important as TNF in AIA, and
exerts major effects on T lymphocyte production of
IL-17 and RANKL/OPG. Finally, IL-6 was implicated in
regulating bone marrow osteoclastogenesis during peripheral joint inflammation in AIA.
MATERIALS AND METHODS
Mice. IL-6⫺/⫺ mice (35) were obtained from W. Alexander (Walter and Eliza Hall Institute of Medical Research
[WEHI]). Mice deficient in TNF (36) were obtained from J.
Sedgwick (Centenary Institute, Sydney, New South Wales,
Australia). Recombination-activating gene 1 (RAG-1)–
deficient mice (37) were provided by L. Corcoran (WEHI).
GK mice, which are deficient in CD4⫹ T lymphocytes, were
provided by Y. Zhan (WEHI) (38). Mice deficient in ␤2microglubulin (␤2m) (39) were provided by L. Harrison
(WEHI). B lymphocyte–deficient ␮MT/␮MT mice (40) were
provided by D. Tarlinton (WEHI). IL-11 receptor–deficient
(IL-11R1⫺/⫺) mice (41) and mice deficient in both IL-6 and
IL-11R (IL-6⫺/⫺/IL-11R1⫺/⫺) were obtained from L. Robb
(WEHI). Mice deficient in the oncostatin M receptor (OSMR)
(42) were provided by A. Miyajima (Tokyo, Japan). All
gene-knockout mice were either derived from C57BL/6 (B6)
embryonic stem cells or were backcrossed onto B6 mice for
ⱖ10 generations. All mice were ⱖ8 weeks of age at the time of
experimentation. B6 mice, obtained from WEHI Animal Services (Kew, Victoria, Australia), were used as wild-type controls in all experiments. All animal procedures were approved
by the institutional ethics committee.
Antigen-induced arthritis. Methylated BSA (2 mg/ml;
Sigma, St. Louis, MO) was emulsified in an equal volume of
CFA containing 1 mg/ml heat-killed Mycobacterium tuberculosis (strain H37Ra; Difco, Detroit, MI). On day 0, mice were
primed by ID injection of 100 ␮l mBSA/CFA emulsion into the
base of the tail. On day 7, mice were anesthetized and injected
IA with 10 ␮l of a 20-mg/ml solution of mBSA (in sterile
saline) into the knee joints. Control joints received the same
volume of vehicle (normal saline). Mice were killed 7 days
after the IA injections (day 14), and the rear limbs were
removed and processed, as previously described (43), for
histologic assessment of hematoxylin and eosin (H&E)–
stained frontal knee joint sections. Sections were assessed by 2
independent observers with no knowledge of the experimental
groups (PKKW, IKC) and graded in severity from 0 (normal)
to 5 (severe) for the following components: joint exudate,
160
synovitis, pannus, cartilage destruction, and bone destruction.
The overall mean total histologic severity score (of a maximum
possible score of 25) of an experimental group was calculated
by averaging the sum of the 5 histologic features, with each
individual feature scored at 4 section depths per joint.
Osteoclast numbers were measured using standard histomorphometric procedures (44) on H&E-stained frontal knee
joint sections. Measurements were carried out from the chondro–
osseous junctions of both the proximal and distal articular cartilage with the lateral and medial periosteal surfaces. Each measurement followed the periosteal surface for a maximum of 300
␮m. A minimum of 3 sites was measured per animal.
TNF neutralization in AIA. Wild-type arthritic mice
were administered an anti-TNF neutralizing antibody (XT22;
0.5 mg/day) or an isotype control antibody on days 6, 9, and 11
of AIA. The dose of XT22 was previously shown to be effective
in collagen-induced arthritis (45). Mice were killed on day 14,
and joints were processed and scored as described above.
Draining LN T lymphocyte proliferation assay. Popliteal and inguinal LN cells (4 ⫻ 105 cells/well) from arthritic
mice were cultured for 72 hours with mBSA (0–50 ␮g/ml) or
concanavalin A (Con A; 2 ␮g/ml), as previously described (43).
Incorporation of tritiated thymidine was determined on a
microplate scintillation counter (Canberra Packard, Melbourne, Victoria, Australia) as a measure of T lymphocyte
proliferation.
Antigen-specific proliferation of CD4ⴙ T lymphocytes.
CD4⫹ T lymphocytes, isolated from draining LNs by staining
with fluorescein isothiocyanate (FITC)–conjugated CD4 and
anti-FITC microbeads (Miltenyi Biotec, Auburn, CA), were
positively selected using an AutoMACS according to the
instructions of the manufacturer (Miltenyi Biotec) and resuspended in 100 ␮l T lymphocyte medium (RPMI 1640 supplemented with 10% fetal calf serum [FCS] and 50 ␮M
2-mercaptoethanol with 1 mM sodium pyruvate) at 1 ⫻ 105
cells/100 ␮l in 96-well plates. To determine purity, cells were
analyzed by flow cytometry on a FACScan using CellQuest
software (Becton Dickinson, San Jose, CA). Wild-type B6
splenocytes were irradiated (2,000 rads) after overnight incubation at 37°C with 1 mg/ml mBSA in RPMI 1640 supplemented with 10% FCS in the presence of 5% CO2. Splenocytes
were then resuspended in 100 ␮l T lymphocyte medium to give
a final ratio of 1–10 CD4⫹ T lymphocytes per APC. CD4⫹ T
lymphocytes and APCs were cocultured for 72 hours at 37°C in
the presence of 5% CO2. Tritiated thymidine incorporation
was used to measure cellular proliferation.
Antigen presentation by dendritic cells (DCs). Sevenweek-old naive wild-type and IL-6⫺/⫺ mice were injected
intravenously with whole ovalbumin (OVA; 3 mg per mouse)
(Sigma) or phosphate buffered saline vehicle. Splenic DCs
were isolated 16 hours later (purity 50–70% CD11c⫹) and DC
phenotypic markers were assessed by flow cytometry, as previously described (45,46). Irradiated (2,000 rads) DCs were
cultured (0–4 ⫻ 104 DCs/well) with OVA-specific OVAtransgenic I (OT-I) CD8⫹ T and OT-II CD4⫹ cells (1 ⫻ 104
cells/well). OT-I and OT-II T lymphocyte proliferation was
measured after 3–4 days by tritiated thymidine incorporation
for 18 hours.
Cytokine enzyme-linked immunosorbent assays
(ELISAs). IL-17 and interferon-␥ (IFN␥) ELISAs were performed using the appropriate matched antibodies (BD Phar-
WONG ET AL
Mingen, San Jose, CA). The optical density at 450 nm was read
using a Multiskan Ascent photometer (Thermo Electron,
Beverly, MA).
Semiquantitative polymerase chain reaction (PCR).
Total RNA was extracted from draining LN T lymphocytes
cultured with mBSA using RNAzol B reagent (Tel-Test,
Friendswood, TX). Complementary DNA was generated with
Moloney murine leukemia virus reverse transcriptase, using 0.5
␮g total RNA and oligo primers diluted to a final volume of 20
␮l. To determine the expression of RANKL and OPG relative
to ␤-actin, semiquantitative PCR was performed using a
LightCycler FastStart DNA Master SYBR Green I kit (Roche
Diagnostics, Mannheim, Germany) according to the manufacturer’s instructions, and a LightCycler (Roche Diagnostics),
with primers specific for RANKL (forward 5⬘-GGTCGGGCAATTCTGAATT-3⬘, reverse 5⬘-GGGAATTACAAAGTGCACCAG-3⬘); OPG (forward 5⬘-ACCTCACCACAGAGCAGCTT-3⬘, reverse 5⬘-GTGCAGGAACCTCATGGTCT-3⬘);
and ␤-actin (forward 5⬘-ATGGATGACGATATCGCTG-3⬘, reverse 5⬘-ATGAGGTAGTCTGTCAGGT-3⬘). Cycling conditions
(40 cycles) for RANKL relative to ␤-actin and for OPG relative to
␤-actin were 95°C for an initial 12 minutes, then 95°C for 5
seconds, 55°C for 15 seconds, and 72°C for 60 seconds (RANKL)
or 45 seconds (OPG).
Osteoclast cultures. The osteoclastogenic potential of
bone marrow and bone marrow macrophage precursors from
wild-type, TNF⫺/⫺, and IL-6⫺/⫺ mice with and without AIA
was determined as previously described (47). Briefly, bone
marrow preparations flushed from femora and tibiae of mice
were plated at a density of 105 cells/well and cultured in the
presence of soluble recombinant RANKL and macrophage
colony-stimulating factor (M-CSF; R&D Systems, Minneapolis, MN), or in coculture with a stromal cell line (Kusa O) in the
presence of 1,25-dihydroxyvitamin D3 (1,25[OH]2D3; 10 nM)
with prostaglandin E2 (100 nM) (47,48). Tartrate-resistant acid
phosphatase (TRAP)–positive multinucleated cells were
counted on days 7 and 11 (49).
Statistical analysis. Results are expressed as the
mean ⫾ SEM. The Mann-Whitney U test was used to compare
mean histologic severity scores of test and control groups.
Student’s 2-tailed t-test was used to analyze differences in
tritiated thymidine incorporation and TRAP⫹ cell numbers.
P values less than 0.05 were considered significant.
RESULTS
Necessity of CD4ⴙ T lymphocytes for AIA. The
relative importance of T lymphocyte subsets and B cells
in AIA has not been fully evaluated, but is important for
understanding the effect of specific cytokines. Consistent with findings of previous studies (37), RAG-1⫺/⫺
mice were completely resistant to AIA (Table 1, experiment 1). The importance of CD4⫹ T lymphocytes in
AIA was investigated next. GK mice, which were rendered CD4⫹ T lymphocyte–deficient by transgenic expression of an anti-CD4⫹ antibody (38), failed to develop significant AIA (Table 1, experiment 2). The role
ROLE OF IL-6 IN AIA
161
Table 1. Dependence of AIA on CD4⫹ T lymphocytes*
Cellular defect of mutant
Mean ⫾ SEM
total histologic
score
WT
RAG-1⫺/⫺
No mature T or B lymphocytes
17.2 ⫾ 0.8
1.3 ⫾ 0.4†
WT
GK mice
No CD4⫹ T lymphocytes
17.1 ⫾ 0.8
4.0 ⫾ 0.9†
WT
␤2m⫺/⫺
No CD8⫹ T lymphocytes
18.1 ⫾ 0.6
16.2 ⫾ 1.1
WT
␮MT/␮MT
No B lymphocytes or IgM or
IgG antibodies
17.7 ⫾ 0.5
15.7 ⫾ 1.0
Experiment,
genotype
1
2
3
4
* Antigen-induced arthritis (AIA) was induced as described in Materials and Methods. Frontal sections from arthritic knee joints (at least
10 joints and 6 mice per group) were assessed by an investigator with
no knowledge of the experimental groups, and graded in severity from
0 (normal) to 5 (severe) for the following components: joint exudate,
synovitis, pannus, cartilage destruction, and bone destruction. The
mean total histologic severity score in each experimental group
(lymphocyte-deficient and wild-type [WT]) was calculated by averaging the sum of the score for these 5 histologic features (maximum
possible score ⫽ 25). All mutant mice were either derived on or
backcrossed onto a C57BL/6 background for at least 10 generations
and were age- and sex-matched with WT controls. RAG-1⫺/⫺ ⫽
recombination-activating gene 1 deficient; ␤2m⫺/⫺ ⫽ ␤2 microglobulin
deficient.
† P ⬍ 0.001 versus WT mice.
of CD8⫹ lymphocytes in AIA was assessed by using
␤2m⫺/⫺ mice, which lack CD8⫹ T lymphocytes (39).
There was no reduction in AIA in ␤2m⫺/⫺ mice compared with wild-type mice (Table 1, experiment 3). To
evaluate the role of B lymphocytes in AIA, ␮MT/␮MT
mice, which lack mature B lymphocytes and do not
produce IgM or IgG antibodies (40), were examined.
AIA in these mice was similar to that in control animals
(Table 1, experiment 4). These data confirm that CD4⫹
T lymphocytes are essential for disease in the acute
phase of AIA and that neither CD8⫹ T lymphocytes nor
B cells are involved.
Greater protective effect of IL-6 deficiency than
TNF deficiency. AIA was induced in wild-type, IL-6⫺/⫺,
and TNF⫺/⫺ mice. The overall severity of AIA was
reduced by ⬃30% in IL-6⫺/⫺ mice compared with
wild-type controls (Figures 1A and B). In particular,
there was a marked reduction in osteoclast recruitment
to inflamed joints, as measured by the number of
osteoclasts per millimeter of bone perimeter (mean ⫾
SEM 2.7 ⫾ 0.5 in wild-type mice versus 1.1 ⫾ 0.5 in
IL-6⫺/⫺ mice; P ⬍ 0.01) (Figure 1C). Since TNF is
important in the pathogenesis of RA (1), we compared
the protective benefits of IL-6 and TNF deficiency in
Figure 1. Reduction in antigen-induced arthritis and osteoclast recruitment to the arthritic joint in interleukin-6–deficient (IL-6⫺/⫺)
mice. Mice were killed 7 days after intraarticular injection of methylated bovine serum albumin (day 14 from initial treatment) and the
knee joints were removed and processed as described in Materials and
Methods. A, Frontal sections of knee joints from a naive wild-type
(WT) C57BL/6 mouse (i), an arthritic WT mouse (ii), and an IL-6⫺/⫺
mouse (iii) (hematoxylin and eosin [H&E] stained; original magnification ⫻ 40). Magnified views of pannus (PN) adjacent to the patella
(P) in an arthritic WT mouse (iv) and an IL-6⫺/⫺ mouse (v),
respectively, are shown (H&E stained; original magnification ⫻
100). Representative section of a knee joint of an arthritic WT
mouse shows multinucleated osteoclasts (arrows) resorbing bone
(Bn) (H&E stained; original magnification ⫻ 200). Severity of the
arthritic changes was graded from 0 (normal) to 5 (severe) for
exudate, synovitis (S), pannus, cartilage (C) destruction, and bone
destruction. E ⫽ exudate; F ⫽ femur. B, Total histologic score
(mean and SEM; n ⫽ 15 joints per group, n ⫽ 10 mice per group
pooled from 2 independent experiments). C, Number of osteoclasts
relative to bone surface, measured using standard histomorphometric procedures as described in Materials and Methods (mean and
SEM; n ⫽ 15 joints per group, n ⫽ 10 mice per group pooled from
2 independent experiments). BPm ⫽ bone perimeter. ⴱ ⫽ P ⬍ 0.01
versus WT mice.
162
inflammatory arthritis by inducing AIA in IL-6⫺/⫺ and
TNF⫺/⫺ mice. Both types of mutant mice had less
disease compared with wild-type mice (mean ⫾ SEM
total histologic score 21.5 ⫾ 0.5 in wild-type mice [n ⫽
14 joints]; P ⬍ 0.001 versus TNF⫺/⫺ and IL-6⫺/⫺ mice)
(Figure 2A). IL-6⫺/⫺ mice had reduced disease compared with TNF⫺/⫺ mice (mean ⫾ SEM total histologic
scores 17.4 ⫾ 0.5 in TNF⫺/⫺ mice and 13.8 ⫾ 0.8 in
IL-6⫺/⫺ mice [n ⫽ 16 joints per group]; P ⬍ 0.01)
(Figures 2A and B).
To confirm our finding of reduced disease in
TNF⫺/⫺ mice and IL-6⫺/⫺ mice, we induced AIA in
wild-type mice and administered a neutralizing antibody
to TNF (XT22) or isotype control antibody (GL113).
IL-6⫺/⫺ mice had a greater reduction in disease compared with wild-type mice that received TNF blockade
treatment (total histologic score 19.4 ⫾ 0.4 in wild-type
mice, 17.4 ⫾ 1.3 in GL113-treated wild-type mice,
14.5 ⫾ 0.5 in XT22-treated wild-type mice, and 12.9 ⫾
0.5 in IL-6⫺/⫺ mice [n ⫽ 10 joints per group]; P ⬍ 0.05
for IL-6⫺/⫺ versus XT22-treated wild-type mice and P ⬍
0.01 for IL-6⫺/⫺ and XT22-treated wild-type mice versus
wild-type or GL113-treated wild-type mice) (Figure 2C).
These findings show that IL-6 deficiency confers equal
or greater protection against inflammation in AIA compared with TNF deficiency or blockade treatment.
Independence of AIA from IL-11 and OSM signaling. To determine whether the protective effect of
IL-6 deficiency was specific, we induced AIA in the
absence of other IL-6 family members. There was no
difference in arthritis severity between wild-type and
IL-11R1⫺/⫺ mice (total histologic score 18.1 ⫾ 0.4 in
wild-type mice versus 18.6 ⫾ 0.6 in IL-11R1⫺/⫺ mice
[n ⫽ 10 joints per group]) (Figure 3A). The absence of
both IL-6 and IL-11 signaling in IL-6⫺/⫺/IL-11R1⫺/⫺
mice did not confer additional protection relative to that
seen in IL-6⫺/⫺ mice (total histologic score 8.9 ⫾ 0.4 in
IL-6⫺/⫺/IL-11R1⫺/⫺ mice versus 9.8 ⫾ 0.4 in IL-6⫺/⫺
mice [n ⫽ 10 joints per group]) (Figure 3A). There was
only a minor difference in disease severity between
wild-type mice and OSMR⫺/⫺ mice (total histologic
score 21 ⫾ 0.4 in wild-type mice versus 18 ⫾ 0.5 in
OSMR⫺/⫺ mice [n ⫽ 14 joints per group]) (Figure 3B).
These findings indicate that the protective effect of IL-6
deficiency in AIA is specific for IL-6, and that AIA is
probably independent of IL-11 and OSM signaling.
IL-6ⴚ/ⴚ T lymphocytes show reduced antigenspecific proliferation. Having established that AIA is
dependent on CD4⫹ T lymphocytes but not CD8⫹ T
lymphocytes or B cells, and that IL-6 deficiency confers
significant protection against AIA, we next investigated
WONG ET AL
Figure 2. IL-6 deficiency is more protective than tumor necrosis factor
deficiency (TNF⫺/⫺) in antigen-induced arthritis (AIA). Mice were killed
on day 14 of AIA, and the knee joints were removed and processed as
described in Materials and Methods. Severity of the arthritic changes in
frontal sections of knee joints was graded from 0 (normal) to 5 (severe)
for joint exudate, synovitis, pannus, cartilage destruction, and bone
destruction. The mean total histologic severity score was calculated by
averaging the sum of these 5 features. A, IL-6⫺/⫺ mice developed less
severe AIA than TNF⫺/⫺ mice (values are the mean and SEM total
histologic score; n ⫽ 14–16 joints per group, n ⫽ 9–10 mice per group
pooled from 2 independent experiments). ⴱ ⫽ P ⬍ 0.001 versus WT mice;
# ⫽ P ⬍ 0.01 versus TNF⫺/⫺ mice. B, Representative frontal sections of
knee joints from an arthritic TNF⫺/⫺ mouse (i) and an IL-6⫺/⫺ mouse (ii)
(H&E stained; original magnification ⫻ 100). C, IL-6⫺/⫺ mice had a
greater reduction in disease compared with WT mice who were administered a TNF-neutralizing antibody (0.5 mg/day XT22 on days 6, 9, and
11 of AIA) (values are the mean and SEM total histologic score; n ⫽ 10
joints per group, n ⫽ 7 mice per group). GL113 ⫽ isotype control
antibody (n ⫽ 10 joints per group, n ⴝ 7 mice per group). ⴱ ⫽ P ⬍ 0.01
versus WT and GL113-treated mice; # ⫽ P ⬍ 0.05 versus XT22-treated
mice. See Figure 1 for other definitions.
antigen-specific T lymphocyte proliferation in IL-6⫺/⫺
mice. T lymphocytes from draining LNs of wild-type or
IL-6⫺/⫺ mice with AIA were cultured in the presence of
ROLE OF IL-6 IN AIA
Figure 3. Independence of antigen-induced arthritis (AIA) from
IL-11 and oncostatin M (OSM) signaling. Mice were killed on day 14
of AIA, and the knee joints were removed and processed as described
in Materials and Methods. A, WT and IL-11 receptor antagonist
1–deficient (IL-11Ra1⫺/⫺) mice developed similar levels of disease
(values are the mean and SEM; n ⫽ 10 joints per group, n ⫽ 7 mice per
group). The absence of both IL-6 and IL-11 signaling in the IL-6⫺/⫺/
IL-11Ra1⫺/⫺ mice did not confer additional protection relative to that
seen in IL-6⫺/⫺ mice. ⴱ ⫽ P ⬍ 0.01 versus WT and IL-11Ra1⫺/⫺ mice.
B, WT mice and mice deficient in OSM receptor (OSMR) developed
similar levels of disease (values are the mean and SEM; n ⫽ 14 joints
per group, n ⫽ 9 mice per group). See Figure 1 for other definitions.
mBSA for 72 hours. T lymphocytes from IL-6⫺/⫺ mice
exhibited ⬃30% reduction in antigen-specific proliferation compared with cells from wild-type mice (Figure
4A), with no difference in mitogenic response following
Con A stimulation (data not shown). IL-6 has been
reported to regulate DC differentiation (32), and DCs of
IL-6⫺/⫺ mice may induce less antigen-specific T lymphocyte activation (31). However, we found that reduced
proliferation in purified CD4⫹ T lymphocytes of IL6⫺/⫺ mice persisted following culture with mBSA-coated
APCs from wild-type mice (Figure 4B). There was no
difference in proliferation following culture with APCs
not coated with mBSA (Figure 4B) or following Con A
stimulation (data not shown).
DC antigen presentation function was directly
assessed by comparing the ability of DCs from IL-6⫺/⫺
and wild-type mice to present OVA to OVA-specific T
lymphocytes. OVA-pulsed DCs from IL-6⫺/⫺ mice produced equivalent proliferation of OT-I CD8⫹ T lymphocytes (Figure 4C) and OT-II CD4⫹ T lymphocytes
(Figure 4D) compared with OVA-pulsed DCs from
wild-type mice. These findings suggest that the reduced
antigen-specific proliferation of T lymphocytes in IL-6⫺/⫺
mice is not due to defective antigen presentation function by DCs.
T lymphocytes in IL-6ⴚ/ⴚ mice produce fewer
pro-osteoclastogenic cytokines. IL-17 is produced by T
lymphocytes and has been shown to promote osteoclastogenesis (15). Because IL-6⫺/⫺ mice showed reduced
163
osteoclast recruitment to erosive sites in arthritic joints
(Figure 1C), we investigated T lymphocyte production of
IL-17. T lymphocytes in IL-6⫺/⫺ mice produced 6-fold
less IL-17 compared with wild-type controls following
antigen-specific stimulation in vitro (with 50 ␮g/ml
mBSA, mean ⫾ SEM 1.2 ⫾ 0.5 ng/ml in wild-type mice
versus 0.2 ⫾ 0.1 in IL-6⫺/⫺ mice; P ⬍ 0.01) (Figure 5A).
There was no difference in IL-17 production following
Con A stimulation (data not shown), which indicates
that T lymphocytes in IL-6⫺/⫺ mice can respond to
mitogenic stimuli and that the reduced IL-17 production
was from antigen-specific T lymphocytes.
Figure 4. IL-6⫺/⫺ lymphocytes have reduced antigen-specific proliferation, but not due to defective antigen presentation by dendritic cells
(DCs). A, Lymphocytes from draining lymph nodes (LNs) of arthritic
WT mice and IL-6⫺/⫺ mice were cultured in the presence of methylated bovine serum albumin (mBSA) for 72 hours and pulsed with
tritiated thymidine for the last 8 hours. There was no difference in
mitogenic response following concanavalin A stimulation (data not
shown). LNs were pooled from 10 mice per group. ⴱ ⫽ P ⬍ 0.05 versus
WT mice. B, Purified CD4⫹ lymphocytes from draining LNs of
arthritic WT mice and IL-6⫺/⫺ mice were cultured for 72 hours in the
presence of irradiated WT antigen-presenting cells (APCs) coated with
mBSA. Tritiated thymidine incorporation was used to measure cellular
proliferation. Wells containing APCs not coated with mBSA or having
no APCs were used as negative controls. LNs were pooled from 8 mice
per group. ⴱ ⫽ P ⬍ 0.05 versus WT mice. C and D, Proliferation of C,
ovalbumin (OVA)–specific OVA-transgenic I (OT-I) CD8⫹ and D,
OT-II CD4⫹ T lymphocytes in response to OVA-pulsed DCs from
WT and IL-6⫺/⫺ mice. After 3–4 days, cellular proliferation was
measured by tritiated thymidine incorporation for the last 18 hours.
Values are the mean and SEM (n ⫽ 3 mice per group). PBS ⫽
phosphate buffered saline (see Figure 1 for other definitions).
164
Figure 5. Reduced production of pro-osteoclastogenic cytokines by
IL-6⫺/⫺ T lymphocytes. A, IL-17 in supernatants from the same
cultures used in the studies shown in Figure 4A, harvested at 72 hours,
was measured by enzyme-linked immunosorbent assay (ELISA).
There was no difference in IL-17 levels following concanavalin A
stimulation (data not shown). ⴱ ⫽ P ⬍ 0.01 versus WT mice. B, IL-17
in supernatants from the same cultures used in the studies shown in
Figure 4B, harvested at 72 hours, was measured by ELISA. CD4⫹ T
lymphocytes from IL-6⫺/⫺ mice produced no detectable IL-17 (ⴱ ⫽
P ⬍ 0.05 versus WT mice). Values are the mean and SEM. C, Total
RNA was extracted from T lymphocyte cultures used in the studies
shown in Figure 4A, and cDNA was generated via reverse
transcriptase–polymerase chain reaction. To determine expression of
RANKL and osteoprotegerin (OPG) relative to ␤-actin, semiquantitative polymerase chain reaction was performed with primers specific
for RANKL, OPG, and ␤-actin as described in Materials and Methods.
Following antigen-specific stimulation in vitro, the ratio of RANKL
mRNA to OPG mRNA was up to 10-fold higher in WT compared with
IL-6⫺/⫺ mouse lymphocytes from draining lymph nodes. Values are
the mean. mBSA ⫽ methylated bovine serum albumin; APC ⫽
antigen-presenting cells (see Figure 1 for other definitions).
To further clarify the cellular origin of the defective IL-17 production, ELISA was used to measure
supernatants from purified CD4⫹ T lymphocytes from
draining LNs in coculture with mBSA-coated APCs in
wild-type mice. CD4⫹ T lymphocytes in wild-type mice
had a dose-dependent increase in IL-17 production
following stimulation with mBSA-coated APCs (Figure
5B). In comparison, IL-17 was undetectable in supernatants from antigen-stimulated CD4⫹ T lymphocytes in
IL-6⫺/⫺ mice (Figure 5B). One of the most important
determinants of osteoclastogenesis is the ratio of
RANKL to OPG (50). Following antigen-specific stimulation in vitro, the ratio of RANKL to OPG messenger
RNA was up to 10-fold higher in lymphocytes of wild-
WONG ET AL
type mice compared with T lymphocytes of IL-6⫺/⫺ mice
(when normalized to ␤-actin) (Figure 5C).
IFN␥ has been reported to have an inhibitory
effect on osteoclastogenesis (51). Hence, a possible
explanation for the observed reduction in osteoclast
recruitment to arthritic joints in IL-6⫺/⫺ mice may have
been elevated local production of IFN␥. However, there
was no difference in T lymphocyte production of IFN␥
from antigen-stimulated cultures (data not shown). This
finding demonstrates that reduced IL-17 production by
antigen-specific CD4⫹ T lymphocytes in IL-6⫺/⫺ mice is
not due to a generalized defect in cytokine production.
Taken together, these data suggest that IL-6 may modulate production of the pro-osteoclastogenic cytokines
IL-17 and RANKL (relative to OPG) by antigen-specific
T lymphocytes, which might help explain reduced bone
destruction in IL-6⫺/⫺ mice with AIA.
Osteoclast generation in bone marrow of arthritic
IL-6ⴚ/ⴚ mice. Naive IL-6⫺/⫺ mice have been shown to
have normal osteoclast numbers in vivo (52) and in vitro
(49). We hypothesized that the reduction in osteoclast
recruitment to inflamed joints in AIA may also relate to
reduced expansion of osteoclast precursors in bone
marrow of arthritic IL-6⫺/⫺ mice. Therefore, we examined osteoclast generation in bone marrow harvested
from arthritic mice. Following RANKL and M-CSF
stimulation for 11 days, bone marrow from IL-6⫺/⫺ mice
generated ⬃50% fewer osteoclasts than bone marrow
from wild-type mice or TNF⫺/⫺ arthritic mice (mean ⫾
SEM TRAP⫹ cells per well 135 ⫾ 24 in bone marrow of
wild-type mice, 175 ⫾ 24 in bone marrow of TNF⫺/⫺
mice, and 78 ⫾ 13 in bone marrow of IL-6⫺/⫺ mice; P ⬍
0.01, IL-6⫺/⫺ mice versus either wild-type or TNF⫺/⫺
mice) (Figure 6A).
To determine if this difference was present in
naive mice, we cultured bone marrow from nonarthritic
wild-type, TNF⫺/⫺, and IL-6⫺/⫺ mice in the presence of
RANKL and M-CSF. Bone marrow from naive IL-6⫺/⫺
mice generated ⬃50% fewer osteoclasts than that from
wild-type or TNF⫺/⫺ mice (data not shown). Although
TNF⫺/⫺ mice developed less severe AIA than wild-type
mice (Figure 2A), bone marrow from arthritic TNF⫺/⫺
and wild-type mice generated the same number of
osteoclasts in vitro (Figure 6A). These findings were
confirmed when bone marrow from arthritic wild-type,
TNF⫺/⫺, and IL-6⫺/⫺ mice was cultured for 7 days in the
presence of Kusa O stromal cells and 1,25(OH)2D3
(TRAP⫹ cells per well 504 ⫾ 84 in bone marrow of
wild-type mice, 498 ⫾ 71 in bone marrow of TNF⫺/⫺
mice, and 312 ⫾ 34 in bone marrow of IL-6⫺/⫺ mice; P ⬍
0.01, IL-6⫺/⫺ mice versus either wild-type mice or
ROLE OF IL-6 IN AIA
Figure 6. Reduced generation of osteoclasts in vitro by bone marrow
from arthritic IL-6⫺/⫺ mice. Bone marrow preparations flushed from
the femora and tibiae of arthritic WT mice, tumor necrosis factor
(TNF)–deficient mice, and IL-6⫺/⫺ mice (day 14) were plated at a
density of 105 cells/well A, in the presence of soluble recombinant
glutathione S-transferase–RANKL and macrophage colonystimulating factor or B, in coculture with a stromal cell line in the
presence of 1,25-hydroxyvitamin D 3 . Tartrate-resistant acid
phosphatase–positive multinucleated cells were counted on days 7 and
11. Values are the mean and SEM from 2 independent experiments;
bone marrow was pooled from a total of 10 mice per group. ⴱ ⫽ P ⬍
0.01 versus WT mice and TNF⫺/⫺ mice. See Figure 1 for other
definitions.
TNF⫺/⫺ mice) (Figure 6B). Our findings suggest that
IL-6 (but not TNF) may be implicated in the regulation
of osteoclast precursors in the bone marrow, before and
during inflammatory arthritis.
DISCUSSION
In these studies, we used a murine model of
acute, destructive inflammatory arthritis to further investigate the effects of endogenous IL-6. While AIA is a
widely used model of inflammatory arthritis, the cellular
mediators have not been completely defined. We used
gene-knockout mice to demonstrate that AIA, at least in
165
the acute phase, is completely dependent on CD4⫹ T
lymphocytes, but not CD8⫹ or B lymphocytes. These
findings do not, however, exclude a role for B cells or
CD8⫹ T lymphocytes in the chronic phase of AIA.
Having characterized acute AIA as a CD4⫹ T
lymphocyte–dependent model of disease, we investigated the role of IL-6 in detail. IL-6⫺/⫺ mice have
previously been shown to have reduced AIA, with
increased production of Th2 cytokines from draining
LNs (30,33). We confirmed that IL-6⫺/⫺ mice had
significantly reduced levels of AIA. Furthermore, we
found that IL-6 deficiency conferred at least as much
protection against AIA as the absence of TNF. These
findings strengthen the rationale for IL-6 blockade in
the treatment of inflammatory joint disease and encouraged us to undertake mechanistic studies.
All cytokines in the IL-6 family use the common
transmembrane receptor, gp130, as part of a multimeric
complex (for review, see ref. 53). Both IL-11 (54) and
OSM (55) have been implicated in the pathogenesis of
inflammatory arthritis. However, we found that AIA was
independent of both IL-11 and OSM signaling. Furthermore, the combined absence of both IL-6 and IL-11
conferred no additional protection relative to that seen
in the absence of IL-6 signaling alone. These findings
suggest that the protective effect of IL-6 deficiency in
AIA is quite specific.
Our finding that IL-6⫺/⫺ mice had markedly
reduced osteoclast recruitment to sites of joint disease in
AIA led us to investigate the effect of IL-6 on the
RANKL:OPG ratio and T lymphocyte production of
IL-17. IL-17 has been reported to induce IL-6 and IL-8
production by fibroblasts (56), enhance IL-1 and TNF
production by monocytes (57), and up-regulate nitric
oxide and prostaglandin production by chondrocytes
(58). Elevated levels of IL-17 have been found in RA
synovial fluid (21). RA synovial fluid induced osteoclastogenesis in cocultures of bone marrow cells with osteoblasts (21). IL-17 also promoted bone erosion in murine
CIA through modulation of the RANKL:OPG ratio
(23). RANKL can directly induce peripheral blood
monocyte differentiation into osteoclasts (15). Synovial
macrophages can undergo osteoclast differentiation in
the presence of both RANKL and M-CSF (59). Activated T lymphocytes from RA patients have been shown
to express RANKL (13).
We found that CD4⫹ T lymphocytes from draining LNs of arthritic IL-6⫺/⫺ mice had reduced antigenspecific proliferative responses, produced less IL-17, and
had a lower ratio of RANKL to OPG. Early studies
showed that IL-6⫺/⫺ mice had defective T helper–
166
dependent responses to viral infection (35). Subsequent
work demonstrated that T lymphocytes in IL-6⫺/⫺ mice
had impaired antigen-specific proliferation, probably
due to reduced IL-2 receptor expression (60) rather than
decreased IL-2 production (61). Given that IL-6 regulates DC differentiation (32) and function (31), we were
careful to exclude defective DC function as a possible
explanation for the reduced antigen-specific proliferation of T lymphocytes in IL-6⫺/⫺ mice. IFN␥ has been
reported to inhibit osteoclastogenesis by enhancing degradation of TNF receptor–associated factor 6 (51).
However, we found that the absence of IL-6 had no
effect on T lymphocyte production of IFN␥. Our findings suggest that IL-6 selectively modulates activated T
lymphocyte production of IL-17 and RANKL/OPG.
Targeted treatment with IL-6 blockade in RA may thus
have a particularly favorable effect on T lymphocyte–
mediated osteoclast recruitment and activation, thereby
reducing joint destruction.
Naive IL-6⫺/⫺ mice have been reported to have
normal osteoclast numbers in vivo (52) and in vitro (49).
We found that bone marrow from naive IL-6⫺/⫺ mice
generated fewer osteoclasts than bone marrow from
wild-type mice, possibly because the bone marrow was
cultured in the presence of RANKL and M-CSF, rather
than 1,25(OH)2D3, as in a previous study (49). Although
our data show a defect in basal osteoclastogenesis in
naive IL-6⫺/⫺ mice, this does not appear to be functionally important, since basal histomorphometric parameters were normal in naive IL-6⫺/⫺ mice (49).
Perhaps of greater interest is that IL-6⫺/⫺ mice
were protected against bone loss following estrogen
depletion (52) and ethanol ingestion (49). We found that
bone marrow from arthritic IL-6⫺/⫺ mice generated
fewer osteoclasts than that from wild-type controls after
culture in the presence of pro-osteoclastogenic stimuli.
IL-6 may therefore be more important in regulating
bone marrow osteoclastogenesis and systemic osteoporosis in disease states, such as inflammatory arthritis.
In contrast, bone marrow from arthritic TNF⫺/⫺ mice
generated the same number of osteoclasts as bone
marrow from arthritic wild-type mice. Therefore, although both TNF and IL-6 are important in the pathogenesis of inflammatory arthritis, IL-6 may selectively
mediate inflammation-induced expansion of osteoclast
precursors in the bone marrow.
These observations suggest that IL-6 may mediate bone destruction during AIA by 3 possible mechanisms. The reduced joint destruction observed in
IL-6⫺/⫺ mice during AIA may have been, at least
partially, a consequence of generally reduced joint in-
WONG ET AL
flammation. IL-6 may specifically modulate local T
lymphocyte production of IL-17, RANKL, and OPG
during inflammatory arthritis. Unopposed RANKL and
synergy between IL-17 and other proinflammatory cytokines, such as IL-1 and TNF, may then induce local
synovial monocyte/macrophage differentiation into osteoclasts. Our findings also suggest that IL-6 deficiency
leads to reduced osteoclastogenesis from bone marrow
precursors during inflammatory disease, such as AIA.
Systemic osteoporosis is a poorly understood feature of
RA. These findings raise the possibility that IL-6 plays a
major role in systemic bone loss, as well as local osteoporosis, during inflammatory joint disease. Our findings
strengthen the rationale for targeting IL-6 in inflammatory arthritis. Therapeutic inhibition of IL-6 may reduce
generalized osteoporosis as well as joint inflammation
and local joint destruction in RA.
ACKNOWLEDGMENTS
We thank S. Mihajlovic and E. Tsui for histologic
processing; W. Alexander, L. Corcoran, Y. Zhan, L. Harrison,
D. Tarlinton, L. Robb, and A. Miyajima for provision of mice;
and J. Morrow and J. Merryfull for animal husbandry.
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production, drive, lymphocytederived, inflammation, induced, modulate, antigen, arthritis, interleukin, cytokines, osteoclastogenesis
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