IL-35 ameliorates collagen-induced arthritis by promoting TNF--induced apoptosis of synovial fibroblasts and stimulating M2 macrophages polarization

Synovitis, the chronic inflammation of the synovial membranes, is a hallmark of rheumatoid arthritis, a chronic disease with profound impact on human health. Recently, interleukin-35 (IL-35), a new member of the IL-12 family, was identified as an anti-inflammatory and immunosuppressive cytokine and was shown to ameliorate collagen-induced arthritis (CIA) in mice. However, the mechanism by which IL-35 alleviates CIA remains unknown. In this study, we investigated the effect of IL-35 on the CIA microenvironment and, specifically, the tumor necrosis factor alpha (TNF-)-induced macrophage inflammatory response and apoptosis of fibroblast-like synoviocytes (FLSs). Firstly, using RT-PCR, western blot, and flow cytometry, we found that IL-35 suppressed TNF--induced inflammatory responses by down-regulating iNOS and COX-2 in peripheral blood monocyte-derived macrophages. IL-35 also activated alternative M2 macrophage polarization, as determined by evaluation of CCR7 and CD206 expression. Moreover, we showed that IL-35 enhanced TNF--induced FLS apoptosis. Using a panel of immunohistochemical and immunofluorescence analyses in a CIA model established in 18 DBA/1J mice, we demonstrated that IL-35 promotes synoviocyte apoptosis and alternative activation of macrophages to alleviate arthritis in vivo. Taken together, our results show that IL-35 promotes TNF--induced FLS apoptosis and modulates M2 macrophage polarization to ameliorate CIA inflammation both in vitro and in vivo.