The encephalitogenicity of TH17 cells is dependent on IL-1- and IL-23-induced production of the cytokine GM-CSF

The development of experimental autoimmune encephalomyelitis has been attributed to cells of the TH1 or TH17 subset of helper T cells. Becher and Rostami and their colleagues show that IL-23-induced production of the cytokine GM-CSF underlies disease development and severity. Interleukin 17 (IL-17)-producing helper T cells (TH17 cells) require exposure to IL-23 to become encephalitogenic, but the mechanism by which IL-23 promotes their pathogenicity is not known. Here we found that IL-23 induced production of the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) in TH17 cells and that GM-CSF had an essential role in their encephalitogenicity. Our findings identify a chief mechanism that underlies the important role of IL-23 in autoimmune diseases. IL-23 induced a positive feedback loop whereby GM-CSF secreted by TH17 cells stimulated the production of IL-23 by antigen-presenting cells. Such cross-regulation of IL-23 and GM-CSF explains the similar pattern of resistance to autoimmunity when either of the two cytokines is absent and identifies TH17 cells as a crucial source of GM-CSF in autoimmune inflammation.