IL-35 is a novel cytokine with therapeutic effects against collagen-induced arthritis through the expansion of regulatory T cells and suppression of Th17 cells

Epstein-Barr virus-induced gene 3 (EBI3) and the p35 subunit of IL-12 have been reported to form a heterodimeric hematopoietin in human and mouse. We have constructed a heterodimeric protein covalently linking EBI3 and p35, to form a novel cytokine which we now call IL-35. The Fc fusion protein of IL-35 induced proliferation of murine CD4⁺CD25⁺ and CD4⁺CD25^– T cells when stimulated with immobilized anti-CD3 and anti-CD28 antibodies <i>in vitro</i>. The IL-35-expanded CD4⁺CD25⁺ T cell population expressed Foxp3 and produced elevated levels of IL-10, whereas the IL-35-induced CD4⁺CD25– T cells produced IFN-γ but not IL-4. The in vitro expanded CD4⁺CD25⁺ T cells retained their suppressive functions against CD4⁺CD25^– effector cells. Furthermore, when cultured with soluble anti-CD3 antibody and antigen-presenting cells, IL-35 suppressed the proliferation of CD4⁺CD25– effector cells. Moreover, IL-35 inhibited the differentiation of Th17 cells<i> in vitro</i>. <i>In vivo</i>, IL-35 effectively attenuated established collagen-induced arthritis in mice, with concomitant suppression of IL-17 production but enhanced IFN-γ synthesis. Thus, IL-35 is a novel anti-inflammatory cytokine suppressing the immune response through the expansion of regulatory T cells and suppression of Th17 cell development