Abstract T helper (h) 17 cells are a newly described subset of CD4+ T cells which has been implicated, along with Th1 cells in the development of autoimmune diseases such as multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE). Regulatory T cells, on the other hand, protect from the development of autoimmunity. The mechanisms by which Th17 cells and factors associated with their differentiation promote autoimmunity have not been fully elucidated. We have previously determined that the differentiation of Th17 cells required the combined action of TGF-2 plus IL-6 and later IL-21. On the other hand IL-23 p19 is critical for sustaining Th17 responses and for the development of EAE since mice deficient for IL- 23 p19 are resistant to EAE. We found that IL-23 receptor (IL-23R), besides being expressed on Th17 cells, is also expressed on cells of the innate immune system such as macrophages and microglia. However, the role of IL-23 on non-T cells is to date unknown. We have generated a novel IL-23R green fluorescent protein (GFP) knockin (KI) reporter mouse in which all cells expressing the IL-23R will concomitantly express the GFP and become non responsive to IL-23. The IL-23R GFP KI mice will help us determine the relative contribution of IL- 23 on Th17 cells and macrophages/microglia in the development and progression of EAE. We have further determined that Th17 cells migrate quickly to the central nervous system (CNS) at the onset of EAE and their numbers correlate with the disease severity. However, the number of Th17 cells rapidly decline while the number of Th1 cells is maintained suggesting that Th17 cells may have a dominant role early on at the initiation phase and that Th1 cells are more involved during the chronic phase of EAE. We have generated an IL-17F RFP/DTR mouse in which Th17 cells can be deleted at any given time through the injection of diphtheria toxin and we will be able to determine the role of Th17 at different stages of the disease. Finally, in our efforts to further determine the mechanisms by which Th17 cells have enhanced pathogenic activity, we have isolated a surface molecule selectively expressed on Th17 cells named podoplanin. The expression of podoplanin has never been reported on T cells before but podoplanin has previously been implicated in the migration of tumor cells. Our preliminary data, injecting anti-podoplanin antibody in vivo in mice immunized with myelin antigen show an increase number of Th17 cells present in the CNS of the treated animals compared to control animals suggesting that podoplanin may play a role in the migration of Th17 cells.