IL-9: basic biology, signaling pathways in CD4+ T cells and implications for autoimmunity.

CD4(+) T cell subsets play an important role in the adaptive immune response in human autoimmune diseases and in animal models of autoimmunity. In recent years, our knowledge of CD4(+) T cell differentiation has increased significantly, and new subsets continue to be recognized. Of significant importance is the recent discovery of Th9 cells, the CD4 + T cell subset that produces Interleukin-9. IL-9 has largely been regarded as a Th2 cytokine; however, it is now known that under specific conditions, Tregs, Th1, Th17 and the Th9 subset of T cells also produce IL-9. The STAT family of proteins plays a major role in the signaling pathways of these CD4(+)T subsets. Biological actions of IL-9 and the STATs signaling pathways in autoimmune diseases are continuing to be clarified. Investigation of IL-9-producing CD4(+)T cells, and elucidation of the mechanisms of IL-9-induced STATs signaling, in concert with other transcription factors, will provide a better understanding of the pathogenesis of various autoimmune diseases.