Interleukin-21 is secreted by activated T cells, including the Th1, Th2 and Th17 cell subsets.24 However, relative to the Th1 and Th2 subset, Th17 cells secrete significantly higher amounts of IL-21.24 IL-21 ACP-196 plays an important role as an autocrine signal for the differentiation of Th17 cells and the absence of the IL-21 receptor leads to a reduction in activated Th17 cells.25 IL-21 plays pleiotropic effects within the immune system where it mediates autoantibody production on B cells, generates mature cytotoxic natural killer cells, and enhances

CD8+ T cell activity.43 Interleukin-22 is secreted by Th17 cells in response to IL-23.27 The receptor for IL-22 is expressed on epithelial and endothelial cells but not on immune cells.44 It is believed that Th17 cells use IL-22 to mediate local INCB018424 in vitro tissue inflammation as seen in mouse models of psoriasis45 or possibly facilitate the influx of Th17 cells as IL-22 helps disrupt the blood-brain barrier and promotes Th17 cell infiltration into the central nervous system.46 IL-22 also plays protective roles in acute liver inflammation47 and can induce lipopolysaccharide-binding protein from hepatocytes.48 Secretion of IL-9 has been reported by Th2,49 Treg cells50 and more recently by Th17 cells.28 IL-9 plays protective effects against nematode infections when secreted by Th2 cells,49 suppresses EAE when secreted by Treg cells,50 and mediates EAE when

secreted by Th17 cells.28 Differentiated Th17 cells express the IL-9 receptor

and IL-9 may act as an autocrine signal amplifying Th17-mediated disease, as the transfer of IL-9R-deficient T cells into wild-type mice delayed the onset of EAE.28 IL-9, however, also enhances the suppressive effects of Treg cells and IL-9R deficient mice develop more severe EAE.51 The polarization of naive CD4+ Th cells into Treg cells and Th17 requires TGF-β. Unopposed TGF-β stimulation in the context of antigen presentation induces Foxp3 expression and Treg commitment and immunoregulation. However, in the context of inflammation signalled by the presence of IL-6, TGF-β drives Th17 differentiation and inflammation.52 Furthermore, IL-6 inhibits the generation of Foxp3 and the differentiation of Tregs. It also facilitates Th17 effector cells by reducing the functional capacity of Tregs.53 These observations suggest a reciprocal relationship Dehydratase between Tregs and Th17 differentiation depending on the presence of inflammatory danger signal IL-6.54 This reciprocity of activation/deactivation of inflammation may explain the prominence of Th17 pathway in the development of autoimmunity. The reciprocity between Th17 and Treg cell development is seen at multiple levels of CD4+ activation. At the level of T subset pathway regulators, RORγt (the critical Th17 pathway inducing transcription factor) and Foxp3 (critical transcription factor for Treg cells) have been shown to interact physically and inhibit one another.