IL-38: A new factor in rheumatoid arthritis

• The soluble form of IL-38 is detected in the sera obtained from of RA patients.
• IL-38 protein was highly expressed in the synovial lining of RA synovium.
• IL-38 expression was up-regulated during arthritis in mice at the mRNA level.
• IL-38 may attenuate joint inflammation by inhibiting the IL-1 induced inflammation.

The newly characterized cytokine IL-38 (IL-1F10) belongs to the IL-1 family of cytokines. Previous work has demonstrated that IL-38 inhibited Candida albicans-induced IL-17 production from peripheral blood mononuclear cells. However, it is still unclear whether IL-38 is an inflammatory or an anti-inflammatory cytokine. We generated anti-human IL-38 monoclonal antibodies in order to perform immunohistochemical staining and an enzyme-linked immunosorbent assay. While human recombinant IL-38 protein was not cleaved by recombinant caspase-1, chymase, or PR3 in vitro, overexpression of IL-38 cDNA produced a soluble form of IL-38 protein. Furthermore, immunohistochemical analysis showed that synovial tissues obtained from RA patients strongly expressed IL-38 protein. To investigate the biological role of IL-38, C57BL/6 IL-38 gene-deficient (−/−) mice were used in an autoantibody-induced rheumatoid arthritis (RA) mouse model. As compared with control mice, IL-38 (−/−) mice showed greater disease severity, accompanied by higher IL-1β and IL-6 gene expression in the joints. Therefore, IL-38 acts as an inhibitor of the pathogenesis of autoantibody-induced arthritis in mice and may have a role in the development or progression of RA in humans.