Proinflammatory role of the Th17 cytokine interleukin‐22 in collagen‐induced arthritis in C57BL/6 mice

OBJECTIVE: To investigate the role of interleukin-22 (IL-22) in collagen-induced arthritis (CIA), an animal model of rheumatoid arthritis. METHODS: C57BL/6 mice were immunized with type II collagen (CII) in Freund's incomplete adjuvant with added Mycobacterium tuberculosis, and levels of IL-22 and its specific receptor, IL-22 receptor type I (IL-22RI), were measured in sera and tissue by enzyme-linked immunosorbent assay and real-time quantitative polymerase chain reaction analysis. Clinical and histologic signs of arthritis were recorded and compared with those in C57BL/6 mice deficient in the IL-22 gene (IL-22(-/-)). Humoral and cellular immune responses against CII were analyzed. In vitro osteoclastogenesis assays were performed on splenocytes. RESULTS: Upon immunization with CII in Freund's incomplete adjuvant plus heat-killed Mycobacterium tuberculosis, sera from C57BL/6 mice were found to contain high levels of IL-22, and the specific IL-22RI was expressed in lymphoid tissue, including splenocytes. IL-22(-/-) mice were less susceptible to CIA than were wild-type mice, as evidenced by their decreased incidence of arthritis and decreased pannus formation. Remarkably, the less severe form of arthritis in IL-22(-/-) mice was associated with increased production of CII-specific and total IgG antibodies, whereas cellular CII responses were unchanged. In vitro, IL-22 was found to promote osteoclastogenesis, a process that might contribute to its proinflammatory activity in CIA. CONCLUSION: Endogenous IL-22 plays a proinflammatory role in CIA in C57BL/6 mice. Our data also indicate that IL-22 promotes osteoclastogenesis and regulates antibody production.