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*
Department of Immunology and Bacteriology and
Centre for Rheumatic Diseases, University of Glasgow, Glasgow, G11 6NT, United Kingdom
| Abstract |
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, TNF-
, IL-6, and IL-12) production by spleen and
lymph node cells in response to bovine type II collagen (CII) in vitro
compared with wild-type mice, paralleled in vivo by a significant
reduction in serum anti-CII IgG2a Ab level. Treatment with rIL-18
completely reversed the disease of the IL-18-/- mice to
that of the wild-type mice. These data directly demonstrate a pivotal
role of IL-18 in the development of inflammatory arthritis and suggest
that antagonists to IL-18 may have therapeutic potential in rheumatic
diseases. | Introduction |
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IL-18 acts in both acquired and innate immunity (6, 7, 8). It
costimulates Th1 cells, activates NK cells to express the Fas ligand,
and acts directly as a proinflammatory cytokine by inducing CC and CXC
chemokines and IL-18 itself. IL-18 synergizes with IL-12 in the
induction of Th1 cells by reciprocal regulation of receptors (9, 10). It also induces production of GM-CSF, IL-2, IL-12R
,
TNF-
, PGE2, and inducible NO synthase by
mononuclear and mesenchymal cells (11, 12).
To provide direct evidence for a role of IL-18 in inflammatory arthritis, we investigated the induction of collagen-induced arthritis (CIA) in IL-18-/- mice of the DBA/1 background. We report here that IL-18-/- mice developed markedly reduced incidence and severity of CIA compared with wild-type mice. This was accompanied in vitro by significantly reduced Ag-specific T cell proliferation and proinflammatory cytokine production by lymphoid cells from the IL-18-/- mice compared with control wild-type mice. These results provide definitive evidence that IL-18 plays a crucial role in the development and sustenance of inflammatory arthritis.
| Materials and Methods |
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IL-18-/- mice were constructed as described previously (13). To generate mice with MHC haplotype susceptible to CIA, IL-18-/- mice were backcrossed into the DBA/1 strain. Fifth generation mice were used for experiments. Mice were typed by PCR (13) during backcrossing. Homozygous (IL-18-/-) mice were obtained by intercrossing IL-18+/- DBA/1 mice and progeny littermates were used for experiments. Both wild-type and heterozygous mice were used as controls. All mice were 810 wk old at the time of immunization and were maintained at the Central Animal Facilities (University of Glasgow). All animals were cared for in accordance with the guidelines of the Home Office, U.K.
Induction of CIA
Male mice received 200 µg of bovine type II collagen (CII; Sigma, Poole, U.K.) in Freunds complete adjuvant (Difco, Detroit, MI) by intradermal injection (day 0). Collagen (200 µg in PBS) was given again on day 21 by i.p. injection. Mice were monitored daily for signs of arthritis, for which severity scores were derived as follows: 0, normal; 1, erythema; 2, erythema plus swelling; 3, extension/loss function, and total score, sum of four limbs. Paw thickness was measured with a dial caliper (Kroeplin, Munich, Germany). For histological assessment, mice were sacrificed and the hind limbs from arthritic IL-18-/- and wild-type mice were removed and fixed in 10% neutral-buffered Formalin, and 5-µm sections were stained with hematoxylin and eosin (Sigma). The quantification of arthritis was performed by two treatment-blinded observers as previously described (14). For reconstitution experiments, IL-18-/- or wild-type mice were immunized (day 0) and boosted (day 21) as above. The mutant mice were injected i.p. with 100 ng/mouse/day of rIL-18 (R & D Systems, Oxon, U.K.) for 8 days, starting from day 29. Disease score was recorded as above. T cell proliferation, cytokine synthesis, and Ab production were also analyzed at the end of experiment (days 37 and 55).
Cell culture
Spleen and draining lymph node cells were cultured at 2 x 106 cells/ml for up to 96 h in medium (RPMI 1640; Life Technologies/BRL, Paisley, Glasgow, U.K.) supplemented with 2 mM L-glutamine, 100 IU/ml penicillin, 100 µg/ml streptomycin, 25 mM HEPES buffer, and 10% FCS (all Life Technologies/BRL)) at 37°C in 5% CO2. Cells were stimulated with graded concentrations of CII. Proliferation assays were performed in triplicate in U-bottom 96-well plates (Nunc, Roskilde, Denmark) as previously described (5). Supernatants from parallel triplicate cultures were stored at -70°C until estimation of cytokine content by ELISA.
Cytokine assays
Murine TNF-
, IFN-
(R&D Systems), IL-4, IL-6, IL-10, and
IL-12(p40 + p70) (PharMingen, San Diego, CA) were assayed by ELISA
using paired Abs according to the manufacturers instructions. Lower
limits of detection were as follows: IL-4, IL-6, IL-12, and TNF-
all
at 10 pg/ml and IL-10 and IFN-
at 80 pg/ml.
Anti-collagen Ab ELISA
Serum anti-collagen II Ab titers were measured by ELISA. Briefly, 96-well plates (Maxisorb; Nunc) were coated with CII (2 µg/ml in 0.1 M NaH2CO3) overnight at 4°C, blocked, and serial dilutions of sera were added. Bound IgG1/2a was detected with biotin-conjugated anti-mouse IgG1 or IgG2a (PharMingen), respectively, and developed as previously described (5). Plates were read at 630 nm.
Statistical analysis
This was performed using Minitab software for Macintosh
(Cupertino, CA). Clinical and histological scores were analyzed with
the nonparametric Mann-Whitney U test. Differences between
cumulative incidences at a given time point were analyzed by the
2 contingency analysis. Cytokine and
collagen-specific IgG levels were compared using Students
t test. Bonferronis correction for multiple comparison was
applied to Fig. 1
.
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| Results and Discussion |
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To determine whether the lack of IL-18 altered erosive articular
disease, histological examination of the hind limb joints was performed
on day 37. As expected, the arthritic wild-type mice revealed extensive
mononuclear and polymorphonuclear infiltration of the synovial membrane
with synovial hyperplasia and adjacent cartilage and bone erosion. In
contrast, the mutant mice show only a mild disease which was markedly
less severe compared with that of the wild-type mice (Fig. 2
).
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, TNF-
, and IL-6 were
detected in wild-type mice. These were present as significantly reduced
concentrations in the IL-18-/- mice (Table I
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, TNF-
, IL-6, and IL-12 compared with similarly treated cells
from the arthritic wild-type mice in both the acute and the chronic
phases of the disease (Fig. 4
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CIA represents an ideal system to explore the diverse inflammatory
effects of IL-18 in an inducible autoimmune model. CIA has proven a
useful surrogate in which to investigate the role of cytokines in RA.
Th1 responses are generally implicated in the pathogenesis of RA
(20). Furthermore, proinflammatory cytokines including
IL-12, IL-6, TNF-
, and IFN-
play a pathogenic role in CIA
(21, 22, 23, 24). However, the role of IL-12 and IFN-
is
complex. The levels of IL-12 and IFN-
induced during CIA are
variable and their effects on CIA development are dependent on the time
of administration (21, 23). Our data clearly demonstrate
that IL-18 is required for the optimal induction of Th1 cells and
proinflammatory cytokines. Although we have not measured the level of
IL-18 in mice with CIA, we have previously shown that IL-18R is present
on macrophages of RA patients and that IL-18 can induce production of
proinflammatory cytokines, particularly TNF-
in vitro
(4). Hence, IL-18 likely mediates inflammatory arthritis
not only by enhancing Th1 activity but also by directly inducing
proinflammatory cytokine production from diverse cell types of the
innate immune system. We have earlier demonstrated an important role
for IL-15 in RA (25, 26, 27). IL-12 has also been implicated
to contribute to the induction and manifestation of CIA
(21). The fact that IL-18-/- mice
still developed CIA, albeit at a significantly reduced level compared
with intact mice, suggests that RA pathogenesis likely involves a
number of proinflammatory cytokines, including IL-12, IL-15, and IL-18,
which synergistically or independently activate transcription factors
such and NF-
B (28).
A major feature of our finding is the sustained reduction of chronic inflammatory response in arthritic IL-18-/- mice compared with intact mice. IL-18 is known not to activate Th1 cell differentiation alone, but synergizes with IL-12 in the expansion of Th1 cells (29, 30). However, recent reports also indicate that IL-18 can also activate naive CD4+ cells along the Th2 pathway (Ref. 31 and our unpublished data). Nevertheless, differentiated Th2 cells lost their IL-18R (9). Thus, IL-18 not only plays a crucial role in inducing proinflammatory cytokines which are likely responsible for the pathological destruction of the joint in RA, it is also required for the sustained Th1 response and hence chronic inflammation in this and other related arthritic diseases. Our results provide direct evidence for a proinflammatory role of IL-18 in arthritis. Antagonists of IL-18 are therefore likely of considerable therapeutic potential against this pervasive clinical disability.
| Acknowledgments |
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| Footnotes |
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2 X.q.W. and B.P.L. contributed equally to this work. ![]()
3 Address correspondence and reprint requests to Dr. Foo Y. Liew, Department of Immunology and Bacteriology, University of Glasgow, Glasgow G11 6NT, U.K. ![]()
4 Abbreviations used in this paper: RA, rheumatoid arthritis; CIA, collagen-induced arthritis; CII, type II collagen. ![]()
Received for publication July 12, 2000. Accepted for publication September 27, 2000.
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