Potential roles of IL-9 in the pathogenesis of systemic lupus erythematosus.

T helper (Th) cells and their cytokines play a pleiotropic role in the pathogenesis of systemic lupus erythematosus (SLE). Recently, a new effector T cell subset, Th9 cells, which preferentially secrete IL-9, has been identified. IL-9 is mainly produced by several T cell subsets including Th9 and Th17, and effective on the functions of Th cells and mast cell. However, there are no unambiguous conclusions that IL-9 contributes to the pathogenesis of SLE. Recently, IL-9 was reported to mediate profound anti-inflammatory effects in several cells or experimental autoimmune models. In particular, IL-9 production seemed to be important in mast cell recruitment. Defect in IL-9/IL-9R axis exhibited a more severe course of Experimental Autoimmune Encephalomyelitis (EAE) and enhanced activity of Tregs, phenotypes reminiscent of SLE. Consistently, IL-9 was implicated in the proliferation of several types of CD4+ T cells, indicating that IL-9 may be therapeutically relevant in SLE. In this article, we briefly discuss the biological features of IL-9 and summarize recent advances on the role of IL-9 in the pathogenesis and treatment of SLE.