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The controversial role of tumor necrosis factor ╨Ю┬▒ in fibrotic diseases.

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Vol. 58, No. 8, August 2008, pp 2228–2235
DOI 10.1002/art.23645
© 2008, American College of Rheumatology
The Controversial Role of Tumor Necrosis Factor ␣ in
Fibrotic Diseases
Jörg H. W. Distler,1 Georg Schett,1 Steffen Gay,2 and Oliver Distler2
physiologic tissue structure, often causing severe functional impairment of the involved organs.
Tumor necrosis factor ␣ (TNF␣) is produced by a
wide variety of hematopoietic and nonhematopoietic
cells, such as macrophages, CD4⫹ and CD8⫹ T cells,
B cells, natural killer cells, neutrophils, smooth muscle
cells, endothelial cells, and fibroblasts (4). TNF␣ exists
in 2 forms, a membrane-bound form and a soluble form,
both of which are functional and contribute to the
effects of TNF␣ (5,6). Human TNF␣ is expressed as a
precursor protein with a molecular mass of 26 kd, which
is then enzymatically cleaved to yield a 17-kd active
Under physiologic conditions, TNF␣ forms homotrimers, which interact and crosslink with the respective receptors. TNF␣ binds with high affinity to 2
transmembrane receptors, TNF receptor I (TNFRI; also
referred to as TNFR␤, p55, or CD120a) and TNFRII
(also referred to as TNFR␣ or p75) (7). The homology
between the 2 receptors is limited, especially within their
intracellular regions (7,8). The expression of TNFRI and
TNFRII is differentially regulated. The expression of
TNFRI seems to be controlled by a housekeeping promoter with high basal activity and only a weak response
to stimulation. In contrast, the expression of TNFRII is
strongly influenced by external factors (9,10). TNFRI
can mediate almost all activities of TNF␣, whereas
TNFRII transduces signals at physiologic levels only in a
few cell types such as T cells (11). However, TNFRII
plays an important role in fine-tuning TNFRI signaling.
Due to its higher affinity, TNFRII binds TNF␣ preferentially at low concentrations. TNF␣ is then passed to
neighboring TNFRI molecules, which allows TNF␣ to
exert its effects at lower concentrations (12).
The efficacy of TNF␣ antagonists in inflammatory diseases such as rheumatoid arthritis (RA), spondylarthropathies, and Crohn’s disease has been demonstrated in numerous clinical trials and plays an important
role in the treatment of patients with these diseases
Fibrosis is characterized by an excessive deposition of extracellular matrix (ECM) components, predominantly of collagens. Fibrotic changes have been
observed in various diseases affecting different organs,
such as the skin, lung, liver, and kidney. Current concepts of fibrogenesis suggest a cascade of events similar
to those in wound healing (1,2). An initial trigger or
injury activates resident cells, which produce proinflammatory mediators. The resulting gradient of chemotactic
cytokines guides the infiltration of inflammatory cells,
which in turn, produce profibrotic mediators such as
transforming growth factor ␤ (TGF␤), platelet-derived
growth factor, and interleukin-4 (IL-4) (3). Depending
on the organ involved, these cytokines activate fibroblasts, hepatic stellate cells, mesangial cells, or tubular
epithelial cells, leading to an increased production of
ECM proteins. In later stages of the fibrotic diseases,
profibrotic cytokines are often produced by resident
cells. In addition to an increased production of ECM
proteins, an imbalance between matrix-degrading enzymes and their inhibitors might contribute to the
development of fibrosis. The matrix components accumulate in the extracellular compartment and disrupt the
Dr. Jörg H. W. Distler’s work was supported by the Interdisciplinary Center of Clinical Research (IZKF) in Erlangen (grant A20)
and by the Career Support Award of Medicine of the Ernst Jung
Jörg H. W. Distler, MD, Georg Schett, MD: University of
Erlangen–Nuremberg, Erlangen, Germany; 2Steffen Gay, MD, Oliver
Distler, MD: University Hospital Zurich, Zurich, Switzerland.
Dr. Jörg H. W. Distler has received consulting fees, speaking
fees, and/or honoraria from Abbott, Actelion, and Encysive (less than
$10,000 each) and has received support from Novartis for a clinical
trial studying systemic sclerosis. Dr. Gay has received consulting fees,
speaking fees, and/or honoraria from Wyeth (less than $10,000).
Address correspondence and reprint requests to Oliver Distler, MD, Center of Experimental Rheumatology and Center of
Integrative Human Physiology, Department of Rheumatology, University Hospital Zurich, CH-8091 Zurich, Switzerland. E-mail:
Submitted for publication October 15, 2007; accepted in
revised form April 18, 2008.
Table 1. Overview of in vitro studies on the profibrotic and antifibrotic effects of tumor necrosis factor ␣*
Cell type
In vitro studies suggesting
antifibrotic effects
Dermal fibroblasts
Dermal, gingival, and synovial fibroblasts
Uterine fibroblasts
Neonatal foreskin fibroblasts
Dermal fibroblasts
In vitro studies suggesting
profibrotic effects
Intestinal myofibroblasts
Swiss 3T3 cells and primary
mouse fibroblasts
Mechanism of action (ref.)
Inhibition of transcription of
fibronectin, type I
collagen, and type III
collagen (25,26)
Induction of MMP-1 (31,32)
Reduction of expression of
TIMP-1 (33)
Inhibition of TGF␤-mediated
activation of Smad3 (30)
Mediator of inhibitory effects
of CD4 T cells on collagen
production (27)
Stimulation of collagen
synthesis, increased
expression of TIMP-1, and
decreased activity of
MMP-2 (35)
Induction of TGF␤ via ERKdependent pathways (36)
Inhibition of phagocytosis of
collagen (38)
* MMP-1 ⫽ matrix metalloproteinase 1; TIMP-1 ⫽ tissue inhibitor of
metalloproteinases 1; TGF␤ ⫽ transforming growth factor ␤.
lagen as a biomarker for fibrosis decreased under treatment in 1 study.
While definite conclusions cannot be drawn from
open-label, uncontrolled case series, these data suggest
that TNF␣ antagonists might play a role in the treatment
of fibrotic diseases. However, TNF␣ has long been
considered an antifibrotic cytokine, and treatment with
TNF␣ antagonists might not be safe in SSc patients.
Accordingly, another case report showed that treatment
with adalimumab was associated with fatal exacerbation
of fibrosing alveolitis in a patient with SSc (23). Similarly, TNF␣ antagonists have been associated with fibrosing alveolitis in patients with RA in a number of case
reports (24). Thus, before TNF␣ antagonists can be
considered for the treatment of patients with fibrotic
diseases, the following open questions have to be addressed: What is the experimental evidence for TNF␣
antagonists for the treatment of fibrotic disorders? Do
molecular studies demonstrate a profibrotic role of
TNF␣, or do they rather show antifibrotic effects of
TNF␣? In this review, we discuss in detail the results of
in vitro and in vivo studies analyzing the role of TNF␣
signaling in organ fibrosis, with a focus on skin and
pulmonary fibrosis, to provide a basis for further clinical
studies in rheumatic diseases.
Effects of TNF␣ on ECM remodeling in vitro
(13–15). Recent reports suggest that TNF␣ antagonists
might also be beneficial for the treatment of fibrotic
disorders. In a retrospective case series, treatment with
etanercept appeared to be efficacious in improving
active inflammatory joint disease in patients with systemic sclerosis (SSc). Etanercept was generally well
tolerated, and skin fibrosis was reduced under treatment
in this uncontrolled study (16). Similarly, investigators in
another case series reported on outcomes in 4 patients
with SSc and erosive arthritis treated with TNF␣ antagonists (17). TNF␣ antagonists not only improved arthritis, but the modified Rodnan skin thickness score
(MRSS) (18), used as a clinical score for the quantification of skin fibrosis in SSc patients, also decreased in all
patients, with reductions of ⬎50% in 3 patients. Another
case report showed stabilization of lung fibrosis in a
patient with SSc under treatment with infliximab (19).
Three additional open-label studies with etanercept or
infliximab have been reported in abstract form (20–22).
Overall, treatment was well tolerated in these case
series, including long-term application for a mean of 24
months in 1 study. The MRSS was stable or improved,
and levels of N-terminal propeptide of type III procol-
Evidence for antifibrotic effects of TNF␣. The
effects of TNF␣ on the production of collagen and the
turnover of ECM have been analyzed in numerous
studies (Table 1). Mauviel and coworkers (25,26) demonstrated that TNF␣ inhibits the synthesis of type I
collagen in cultured dermal fibroblasts on the transcriptional level, resulting in a dose-dependent reduction of
the production of type I collagen as measured by
radioimmunoassay and gel electrophoresis. In addition,
TNF␣ inhibited the synthesis of type III collagen and
fibronectin (25). TNF␣ also plays an important role in
the inhibitory effects of CD4⫹ T cells on collagen
production in dermal fibroblasts. Th1 and Th2 cells
activated by CD3 crosslinking as well as preparations of
Th1 and Th2 plasma membranes significantly decreased
the production of collagen in dermal fibroblasts from
healthy controls (27). Antagonism of TNF␣ by the
addition of soluble TNFRI, but not neutralization of
IL-4 and interferon-␥, significantly reduced the inhibitory activity of T cells on matrix production. Addition of
Th2 membranes to fibroblasts stimulated with the profibrotic cytokines TGF␤ and IL-4 abrogated the increased production of collagen, demonstrating that the
inhibition by Th2 membranes was dominant over the
stimulatory effects of TGF␤ and IL-4. Interestingly,
these antifibrotic mechanisms of TNF␣ via T cell membranes might not be fully active in fibrotic diseases,
because SSc fibroblasts were resistant to inhibition by
Th2 membranes and less sensitive to inhibition by Th1
membranes compared with healthy fibroblasts.
The inhibitory effects of TNF␣ on the production
of collagen are partially mediated by NF-␬B. Using
electrophoretic mobility shift assays (EMSAs) and supershift assays, Kouba and coworkers (28) demonstrated
that RelA–NF-␬B (p65–p50) complexes bind to a cis
element adjacent to the promoter of COL1A2. Mutation
of the NF-␬B binding site almost completely prevented
the inhibitory effects of TNF␣. The crucial role of
NF-␬B in the down-regulation of COL1A2 synthesis was
confirmed using fibroblasts deficient in the NF-␬B–
essential modulator (NEMO) (29). NEMO is activated
in response to inflammatory stimuli and activates the
associated I␬B kinases (IKKs). In turn, IKKs phosphorylate I␬B. Phosphorylated I␬B dissociates from NF-␬B,
which can then enter the nucleus and activate transcription of NF-␬B–dependent genes. NEMO deficiency
prevented the inhibitory effects of TNF␣ on COL1A2
transcription. Similar results were also obtained in dermal fibroblasts transfected with dominant-negative
forms of IKK␣ (29).
In addition to direct effects on the transcription
of collagens, TNF␣ might exert antifibrotic properties by
interfering with TGF␤ signaling cascades (30). Incubation of neonatal human foreskin fibroblasts with TNF␣ 1
hour prior to stimulation with TGF␤ prevented Smadspecific gene transcription, as analyzed with reporter
constructs of the Smad3/4-binding element (SBE). The
inhibitory effect of TNF␣ on the activation of TGF␤dependent Smad signaling was not mediated by induction of the inhibitory Smad7, but rather, was inhibited by
activator protein 1 (AP-1)–dependent pathways. JunB
and c-Jun, 2 members of the AP-1 family, are strongly
induced in dermal fibroblasts upon stimulation with
TNF␣. Overexpression of JunB and c-Jun abrogated the
Smad3-dependent transactivation of the SBE reporter
construct in a dose-dependent manner. The important
role of AP-1 signaling for the inhibitory effect of TNF␣
was further supported by experiments demonstrating
that expression of antisense c-Jun messenger RNA
(mRNA) prevented the inhibition of TGF␤/Smad signaling by TNF␣.
The inhibitory effects of JunB and c-Jun on Smad
signaling are mediated by 2 distinct mechanisms. Coprecipitation assays and EMSAs demonstrated that JunB
and c-Jun form heterocomplexes with Smad3 and that
JunB and c-Jun reduce the binding of Smad3 to its
cognate cis DNA element. These findings suggest that
Smad3–AP-1 interactions may sequester Smad3 and
compete against Smad3 for binding to DNA. Furthermore, JunB and c-Jun bind p300, a cofactor essential for
optimal transcription of Smad-regulated genes. Because
the amount of p300 in the nucleus is limited, competition of JunB and c-Jun with Smad3 for p300 can
efficiently prevent Smad signaling (30).
In addition to its direct effects on the synthesis of
ECM proteins, TNF␣ regulates the expression of matrixdegrading enzymes and their inhibitors. TNF␣ induced
matrix metalloproteinase 1 (MMP-1) mRNA and protein in a dose-dependent manner in dermal, gingival,
and synovial fibroblasts (31,32). Furthermore, higher
concentrations of TNF␣ reduced the expression of tissue
inhibitor of metalloproteinases 1 (TIMP-1), thereby
promoting the degradation of ECM proteins (33). The
effects of TNF␣ on collagen synthesis and on the
production of MMPs and TIMPs are not restricted to
fibroblasts and are also found in other cell types (34).
Evidence for profibrotic effects of TNF␣. In
contrast to the results discussed above, a recent study
suggested that TNF␣ might promote a profibrotic phenotype in murine intestinal myofibroblasts in vitro (35).
In vimentin- and ␣-smooth muscle actin–positive myofibroblasts isolated from wild-type (WT) mice, murine
TNF␣ stimulated collagen synthesis, increased expression of TIMP-1, and decreased activity of MMP-2.
Similar results were also obtained with myofibroblasts
from mice homozygous for the disruption of the gene for
TNFRI (TNFRI–/– mice). However, no induction of
collagen was observed in myofibroblasts from TNFRII–/–
mice and from TNFRI–/– and TNFRII–/– doubleknockout mice (TNFRI⫺/⫺/II⫺/⫺ mice), suggesting that
the profibrotic effect of TNF␣ on myofibroblasts is
mediated by signaling via TNFRII. It remains unclear
why the effects of TNF␣ on the production of ECM in
that study differed from those found in previous studies.
The concentrations of TNF␣ used were within the range
of those used in other studies, and species-specific
differences can be excluded because murine fibroblasts
respond to TNF␣ with an up-regulation of collagen and
induction of MMPs similar to that found in human
fibroblasts (29). However, the effects of TNF␣ might be
cell type and organ specific, and intestinal myofibroblasts might react differently from other cell types.
Investigators in another in vitro study also proposed a profibrotic effect of TNF␣, indirectly via induction of TGF␤ (36). Incubation of Swiss 3T3 cells (a
mouse fibroblast cell line) and primary mouse fibroblasts with TNF␣ stimulated the expression of TGF␤
mRNA and protein. The induction of TGF␤ by TNF␣
was blocked by inhibitors of the ERK-specific MAPK
pathway. However, it remains unclear whether the induction of TGF␤ is sufficient to overcome the inhibitory
effects of TNF␣ on collagen production, since the
investigators did not analyze the expression of collagen.
The induction of TGF␤ upon stimulation with TNF␣
might therefore represent a counterregulatory mechanism to compensate for the inhibitory effects of TNF␣
on Smad signaling.
Phagocytosis of collagen has been suggested to
play a role in the homeostasis of the ECM, and inhibition of this phagocytosis might therefore lead to fibrosis
(37). Although this mechanism is currently not considered to play a major role, it might further enhance
fibrotic processes. TNF␣ has been suggested to inhibit
the phagocytosis of collagen by fibroblasts, and it might
also therefore enhance fibrotic processes via this mechanism. Incubation with TNF␣ increased slightly the
expression of ␣1␤1 integrin and ␣2␤1 integrin, which
bind collagen in human gingival fibroblasts, but the
binding of collagen was significantly decreased, suggesting an inactivation of the binding sites for collagen by
TNF␣ (38). Consistent with decreased binding, the
percentage of phagocytosed collagen beads per cell and
the proportion of phagocytically active gingival fibroblasts were reduced in a dose-dependent manner by
The role of TNF␣ in different in vivo models of tissue
Effects of overexpression of TNF␣ on the turnover of ECM. Results from animal studies of the overexpression of TNF␣ are inconsistent regarding the effects on ECM synthesis (Table 2). A number of studies
indicate profibrotic effects of TNF␣.
Overexpression of TNF␣ in the lungs has been
linked to fibrosis. Using a replication-deficient adenovirus, a prolonged overexpression of TNF␣ for 7–10 days
was achieved in the lungs of rats (39). This local
overexpression resulted in severe infiltration of the lungs
by neutrophils, macrophages, and lymphocytes. Later
on, increased expression of TGF␤ was detected and
myofibroblasts appeared. After the inflammation subsided, a patchy fibrosis developed that persisted beyond
day 64. These data are supported by the analysis of
peripheral CD4⫹ T cells from patients with idiopathic
pulmonary fibrosis. Compared with those from normal
Table 2. Overview of studies on the role of TNF␣ in animal models
of skin and pulmonary fibrosis*
In vivo studies suggesting
antifibrotic effects
In vivo studies suggesting
profibrotic effects
Model (ref.)
Reduction by TNF␣ antagonists
of accumulation of
extracellular matrix in mouse
models of silica- and
pulmonary fibrosis (48,49)
Protection from experimental
pulmonary fibrosis induced
by silica, asbestos, and
bleomycin in TNFRI⫺/⫺/II⫺/⫺
mice (52–54)
Reduced fibroproliferative
lesions after allogeneic
transplantation of rat
tracheas by treatment with
TNF␣ antagonists (50)
Decreased collagen synthesis in
the skin of nude mice
injected with Chinese
hamster ovary cells
overexpressing TNF␣ (47)
Induction of pulmonary fibrosis
in mice overexpressing TNF␣
Induction of interstitial
myocardial fibrosis by
overexpression of TNF␣ (42)
Increased deposition of
collagen after subcutaneous
infusion of TNF␣ (44)
* TNF␣ ⫽ tumor necrosis factor ␣.
subjects, these CD4⫹ T cells synthesized higher levels of
TNF␣, as analyzed by intracellular fluorescenceactivated cell sorting staining (40). The role of TNF␣ in
pulmonary fibrosis has also been analyzed with transgenic mice overexpressing murine TNF␣ in the lungs
under the control of the human surfactant protein C
(SP-C) promoter. These mice spontaneously developed
a chronic lymphocytic alveolitis, and its severity correlated with the expression of TNF␣ mRNA in the lungs
(41). The transgenic mice also had greater lung volumes,
pulmonary hypertension, and decreased elastic recoil
compared with control mice. Histologically, the investigators observed thickened alveolar walls due to accumulation of desmin-containing fibroblasts and collagen
fibers, a phenotype resembling idiopathic pulmonary
Similar profibrotic effects of TNF␣ were observed in animal models of congestive heart failure using
modified TNF␣-transgenic mice (42,43). Accordingly,
continuous subcutaneous infusion of TNF␣ led to abundant infiltration with polymorphonuclear leukocytes and
stimulated the growth of dermal fibroblasts, resulting in
an increased local deposition of collagen (44).
While these studies suggest that TNF␣ is a profibrotic cytokine, other studies in mice overexpressing
TNF␣ indicate antifibrotic effects and a protective role
of TNF␣ in models of experimental fibrosis. A more
recent study by a different group on the same transgenic
mice with lung-specific TNF␣ expression under the
control of the human SP-C promoter led to different
conclusions. Fujita and colleagues (45) confirmed the
presence of chronic pulmonary inflammation and increased lung volumes with an increase in the total
amount of hydroxyproline in the lungs. However, when
the hydroxyproline content was normalized for the lung
weight, no differences were observed between TNF␣transgenic mice and controls. Accordingly, Fujita et al
observed only very little fibrosis in the lungs of transgenic animals. Furthermore, TNF␣-transgenic mice
were resistant to experimental pulmonary fibrosis induced by bleomycin or by overexpression of TGF␤. In
contrast to WT mice, no increase in the hydroxyproline
content and no fibrotic changes in histologic sections
were observed in TNF␣-transgenic mice.
It needs to be stressed that the determination of
the hydroxyproline content is often misleading, especially under inflammatory conditions, in which inflammatory cells produce C1q (which also contains hydroxyproline). The fact that type III collagen contains
30% more hydroxyproline than does type I collagen also
contributes to this problem. The antifibrotic role of
TNF␣ in experimental pulmonary fibrosis in these animals was further supported by experiments demonstrating that application of recombinant human TNF␣ attenuated bleomycin-induced pulmonary fibrosis in WT
An antifibrotic effect of TNF␣ was also reported
by Buck and coworkers (46). They demonstrated that
inoculation of nude mice with Chinese hamster ovary
cells secreting TNF␣ resulted in a decreased collagen
synthesis in the skin, which also led to impaired wound
healing and was paralleled by decreased expression of
TGF␤. Similar results in the liver were obtained by the
same group (47).
Studies with inhibitors of TNF␣. Studies by
Piguet and coworkers (48,49) demonstrated that TNF␣
antagonists can significantly reduce the accumulation of
ECM in mouse models of silica- and bleomycin-induced
pulmonary fibrosis. Silica and bleomycin stimulate the
expression of TNF␣ and lead to a patchy fibrosis after 15
days. Infusion of anti-TNF␣ antibodies or recombinant
soluble TNFRI prevented the development of fibrosis in
both models. Treatment of established fibrosis with
soluble TNFRI was also effective, as demonstrated by
experiments in which soluble TNFRI was administered
25 days after bleomycin and silica. Accordingly, continuous infusion of recombinant TNF␣ strongly augmented
the fibrotic process (48).
Beneficial effects of TNF␣ inhibitors were also
found in a transplantation model (50). When tracheas
from Brown-Norway rats were transplanted into Lewis
rats, obliterative airway disease with inflammatory infiltrates and fibroproliferative lesions developed. Luminal
occlusion occurred in 58% of untreated animals after 14
days. When animals were treated with antibodies against
TNF␣ or with RDP-58, a translational inhibitor of
TNF␣, fibroproliferative lesions were strongly reduced
and the percentage of luminal occlusion decreased from
58% to 32%. Similar antifibrotic effects with anti-TNF␣
antibodies were observed in experimental crescentic
glomerulonephritis as a model for kidney fibrosis (51).
Lessons from studies of mice deficient in TNF
receptors. Numerous studies have analyzed outcomes in
mice deficient in TNFRI (TNFRI–/– mice), TNFRII
(TNFRII–/– mice), or both receptors (TNFRI⫺/⫺/II–/–
mice) in different models of organ fibrosis. In contrast to
experiments with TNF␣-transgenic mice, these studies
demonstrated uniformly that inhibition of signaling via
TNFRI and TNFRII prevents the development of fibrosis. For instance, TNFRI⫺/⫺/II–/– mice were protected
from experimental pulmonary fibrosis induced by silica,
asbestos, and bleomycin (52–54). In contrast to WT
mice, TNFRI⫺/⫺/II–/– mice did not demonstrate significant inflammation or increased proliferation in the lungs
after exposure to profibrotic agents, the induction of
TNF␣ was diminished, and there was no increased
deposition of ECM. Similar results were obtained with
TNFRI–/– and TNFRI⫺/⫺/II–/– mice in different models
of liver and kidney fibrosis, while TNFRII–/– mice were
not protected from kidney and liver fibrosis (55–59).
Summary and conclusion
Results of in vitro and in vivo experiments concerning the role of TNF␣ in fibrosis are in part contradictory and do not allow definite conclusions about the
role of TNF␣ as a profibrotic mediator or an antifibrotic
cytokine. The majority of in vitro studies show antifibrotic effects of TNF␣, in that it suppresses the production of collagen, reduces the expression of TIMPs, and
stimulates the release of MMPs, thereby preventing the
Figure 1. Tumor necrosis factor ␣ (TNF␣) exerts prominent proinflammatory properties and activates inflammatory cells in the initial
phases of experimental models of fibrosis. Inflammatory cells secrete
profibrotic mediators, which activate fibroblasts. Activated fibroblasts
produce increased amounts of extracellular matrix, resulting in tissue
fibrosis in later stages. However, TNF␣ directly inhibits the synthesis
of extracellular matrix proteins in fibroblasts in vitro in most studies.
We do not yet have a clear picture of the net result of these contrary
effects in human fibrotic diseases, in which inflammation is often less
pronounced than in experimental models.
accumulation of ECM. The intracellular signaling pathways that mediate these effects have been identified and
include activation of JunB and c-Jun. However, under
certain circumstances, TNF␣ might have profibrotic
effects in vitro. Myofibroblasts in particular, which are
thought to play a central role in fibrotic disorders such as
SSc by releasing large amounts of ECM proteins, might
react to TNF␣ differently from resting fibroblasts.
While the in vitro studies overall favor TNF␣ as
an antifibrotic cytokine, different in vivo animal studies
with antagonists of TNF␣ and mice lacking TNFRI or
both TNF␣ receptors demonstrated that inhibition of
TNF␣ signaling can prevent fibrosis. However, studies
on TNF␣-transgenic mice yielded different results, with
most suggesting a profibrotic role of TNF␣.
In our opinion, these differences in the results
between in vitro and in vivo studies might be explained
by the inflammatory component in animal models of
experimental fibrosis (Figure 1). Most of these models
depend critically on a strong inflammatory component
at the initial stages as a trigger for the later development
of fibrosis. As a major proinflammatory stimulus, TNF␣
has been shown to play an important role in the activation of inflammatory cells. Inflammatory conditions and
cellular inflammatory infiltrates are absent in vitro, and
TNF␣ can exert its direct effects on the matrixproducing cells. In our view, a likely scenario is that the
direct antifibrotic effects of TNF␣ on fibroblasts might
be outweighed in experimental models of fibrosis by its
important role in driving inflammation.
Thus, to predict the effects of anti-TNF␣ treatments in human fibrotic diseases, the key question is
whether inflammation triggers and perpetuates the development of fibrosis in humans. The role of inflammation in human fibrotic diseases is less clear than was
initially thought; it might differ between the specific
organs and might also depend on other parameters such
as disease duration. For example, in SSc skin fibrosis,
inflammatory infiltrates are thought to play an important role in very early disease stages, but are rarely seen
in later stages of the disease. This might predict that
anti-TNF␣ treatments could be a promising antifibrotic
strategy for very early inflammatory skin fibrosis in SSc,
while it might even be deleterious for later noninflammatory stages of skin fibrosis.
Accordingly, the effects on lung fibrosis might be
different from those on skin fibrosis. Inflammation is
thought to play a more important role in interstitial
pulmonary fibrosis associated with SSc, with recurrent
alveolitis triggering progression of fibrosis during the
course of the disease. If this concept holds true, patients
with pulmonary fibrosis and alveolitis might benefit
from anti-TNF␣ treatments, while the effects on lung
fibrosis might again be deleterious in patients with no
evidence of inflammation/alveolitis. However, it must be
emphasized that this concept is theoretical, because so
far, no controlled study has looked at the effects of
TNF␣ in inflammatory phases of SSc.
Taken together, the results of available molecular
and cellular studies do not allow definite conclusions
about the role of TNF␣ in fibrotic diseases. More
experimental studies are necessary, with a particular
focus on fibrotic animal models that are independent of
inflammation as a trigger for the fibrotic process. Cellular studies have lacked a complete description of the
transcriptom after TNF or anti-TNF treatment, which
would provide a more comprehensive analysis and,
together with a proteomics study, could elucidate the
contradictory findings reported so far instead of analyzing only selective and expected pathways.
Open-label uncontrolled studies with TNF␣ inhibitors in SSc appear to be promising. However, from
the existing literature, there are concerns that treatment
with TNF␣ antagonists could lead to progression of
fibrosis. Therefore, TNF␣ antagonists should not be
used in daily clinical practice for the treatment of
patients with fibrotic diseases until these concerns are
cleared. To elucidate the definitive clinical effects of
TNF ␣ antagonists in fibrotic diseases, placebocontrolled trials need to be performed. Patients with
inflammatory stages of the fibrotic disease are most
likely to respond, while patients with noninflammatory
stages of fibrosis might even show deleterious effects.
Regarding safety issues, these trials need to focus on
fibrosis outcomes in addition to the conventional safety
issues such as infection and malignancy, and they need
to be designed with sufficient statistical power to exclude
potentially harmful effects.
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