Regulatory B (Breg) cells producing interleukin-10 (IL-10) suppress autoimmune disease, but the mechanisms mediating their induction remain unclear. Charles E. Egwuagu and colleagues report that IL-35 induces Breg cells and also promotes their production of IL-35. In vivo IL-35 suppresses autoimmune disease in an IL-35 receptor– and IL-10–dependent manner. Adoptive transfer of IL-35–induced Breg cells inhibits established autoimmune disease, suggesting that IL-35 and IL-35–producing Breg cells may be used to treat inflammatory disease. Interleukin-10 (IL-10)-producing regulatory B (Breg) cells suppress autoimmune disease, and increased numbers of Breg cells prevent host defense to infection and promote tumor growth and metastasis by converting resting CD4+ T cells to regulatory T (Treg) cells. The mechanisms mediating the induction and development of Breg cells remain unclear. Here we show that IL-35 induces Breg cells and promotes their conversion to a Breg subset that produces IL-35 as well as IL-10. Treatment of mice with IL-35 conferred protection from experimental autoimmune uveitis (EAU), and mice lacking IL-35 (p35 knockout (KO) mice) or defective in IL-35 signaling (IL-12Rβ2 KO mice) produced less Breg cells endogenously or after treatment with IL-35 and developed severe uveitis. Adoptive transfer of Breg cells induced by recombinant IL-35 suppressed EAU when transferred to mice with established disease, inhibiting pathogenic T helper type 17 (TH17) and TH1 cells while promoting Treg cell expansion. In B cells, IL-35 activates STAT1 and STAT3 through the IL-35 receptor comprising the IL-12Rβ2 and IL-27Rα subunits. As IL-35 also induced the conversion of human B cells into Breg cells, these findings suggest that IL-35 may be used to induce autologous Breg and IL-35+ Breg cells and treat autoimmune and inflammatory disease.

Th17 cells are implicated in CNS autoimmune diseases. We show that mice with targeted-deletion of Stat3 in CD4⁺ T cells (CD4^Stat3−/−) do not develop experimental autoimmune uveoretinitis (EAU) or experimental autoimmune encephalomyelitis. Defective Th17 differentiation noted in CD4^Stat3−/− mice is compensated by exaggerated increases in Foxp3-, IL-10-, IL-4-, and IFN-γ-expressing T cells, suggesting critical roles of STAT3 in shaping Ag-specific CD4⁺ T cell repertoire. In mice with EAU, a high percentage of IL-17-expressing T cells in their peripheral lymphoid organs also secrete IFN-γ while these double-expressors are absent in CD4^Stat3−/− and wild-type mice without EAU, raising the intriguing possibility that uveitis maybe mediated by Th17 and IL-17-expressing Th1 cells. Resistance of Stat3-deficient mice to EAU derives in part from an inability of uveitogenic Th17 and Th1 cells to enter eyes or brain of the CD4^Stat3−/− mouse because of the reduction in the expression of activated α4/β1 integrins on CD4^Stat3−/− T cells. Adoptive transfer of activated interphotoreceptor retinoid-binding protein-specific uveitogenic T cells induced in CD4^Stat3−/− mice a severe EAU characterized by development of retinal folds, infiltration of inflammatory cells into the retina, and destruction of retinal architecture, underscoring our contention that the loss of STAT3 in CD4⁺ T cells results in an intrinsic developmental defect that renders CD4^Stat3−/− resistant to CNS inflammatory diseases. STAT3 requirement for IL-17 production by Th17, generation of double positive T cells expressing IL-17 and IFN-γ, and for T cell trafficking into CNS tissues suggests that STAT3 may be a therapeutic target for modulating uveitis, sceritis, or multiple sclerosis.

Interleukin 35 (IL-35) is a heterodimeric cytokine composed of IL-12p35 and Ebi3 subunits. IL-35 suppresses autoimmune diseases while preventing host defense to infection and promoting tumor growth and metastasis by converting resting B and T cells into IL-10-producing and IL-35-producing regulatory B (Breg) and T (Treg) cells. Despite sharing the IL-12p35 subunit, IL-12 (IL-12p35/IL-12p40) promotes inflammatory responses whereas IL-35 (IL-12p35/Ebi3) induces regulatory responses, suggesting that IL-12p35 may have unknown intrinsic immune-regulatory functions regulated by its heterodimeric partner. Here we show that the IL-12p35 subunit has immunoregulatory functions hitherto attributed to IL-35. IL-12p35 suppresses lymphocyte proliferation, induces expansion of IL-10-expressing and IL-35-expressing B cells and ameliorates autoimmune uveitis in mice by antagonizing pathogenic Th17 responses. Recapitulation of essential immunosuppressive activities of IL-35 indicates that IL-12p35 may be utilized for in vivo expansion of Breg cells and autologous Breg cell immunotherapy. Furthermore, our uveitis data suggest that intrinsic immunoregulatory activities of other single chain IL-12 subunits might be exploited to treat other autoimmune diseases.

Interleukin‐12 family cytokines have emerged as critical regulators of immunity with some members (IL‐12, IL‐23) associated with disease pathogenesis while others (IL‐27, IL‐35) mitigate autoimmune diseases. Each IL‐12 family member is comprised of an α and a β chain, and chain‐sharing is a key feature. Although four bona fide members have thus far been described, promiscuous chain‐pairing between alpha (IL‐23p19, IL‐27p28, IL‐12/IL‐35p35) and beta (IL‐12/IL‐23p40, IL‐27/IL‐35Ebi3) subunits, predicts six possible heterodimeric IL‐12 family cytokines. Here, we describe a new IL‐12 member composed of IL‐23p19 and Ebi3 heterodimer (IL‐39) that is secreted by LPS‐stimulated B cells and GL7⁺ activated B cells of lupus‐like mice. We further show that IL‐39 mediates inflammatory responses through activation of STAT1/STAT3 in lupus‐like mice. Taken together, our results show that IL‐39 might contribute to immunopathogenic mechanisms of systemic lupus erythematosus, and could be used as a possible target for its treatment.