Summary: Role of IL-7R in CNS Autoimmunity IL-7 is an essential cytokine for the development of T and B cells and plays an important role in inflammation by stimulating Th1, Th17 cells and GM-CSF production. The IL-7 receptor (IL-7R) complex consists of IL-7R alpha-chain (IL-7Rα) and common γ-chain (γc) and polymorphisms in the IL-7Rα gene are associated with an increased risk for the development of multiple sclerosis (MS), an inflammatory demyelinating disease of the central nervous system (CNS). Although these observations suggest that signaling through IL-7R has a pathogenic role in CNS autoimmunity, its underlying mechanisms are unclear. In this regard, we have made the following preliminary observations: 1) Dendritic cells (DCs) express IL-7Rα in homeostatic conditions and during experimental autoimmune encephalomyelitis (EAE), an animal model of MS; 2) conditional knockout (cKO) mice lacking the IL-7Rα gene in DCs are protected from EAE; 3) IL-7Rα cKO EAE mice have a significant increase in IL-9+Foxp3+ regulatory T (Treg) cells; 4) IL-7Rα cKO DCs produce the vasoactive polypeptide Endothelin-1 (Edn-1); 5) DC-derived Edn-1 induces IL-9 production by T cells; and 6) Ab-mediated depletion of Treg cells or IL-9 restores EAE susceptibility in IL-7Rα cKO mice. Together, these and other observations form the bases of our hypothesis that IL-7Rα in DCs enhances EAE severity by suppressing the development of Edn-1- induced IL-9+ Treg cells. We will test this hypothesis in three Specific aims: 1) To dissect the mechanisms by which IL-7Rα downregulates Edn-1 production by DCs and promotes EAE. We will study the mechanisms by which IL-7Rα suppresses Edn-1 in mouse and human DCs, and the role of DC- derived Edn-1 on neuroinflammation by genetic ablation and inducible expression. 2) To investigate the signaling pathways by which Edn-1 induces IL-9+Foxp3+ Treg cells. We will investigate the mechanisms of IL-9 induction in Foxp3+ Treg cells by ETBR signaling by genetic approaches in vivo and in vitro, and study the effect of dimethyl fumarate, a common therapy for MS, on the expression profile of IL-7Rα+DCs, IL-9+Foxp3+ Treg cells and Edn-1 and its receptors in MS patients. 3) To test the therapeutic effect of Edn-1 and its analogues in EAE. This will be tested in myelin-reactive T cells, and multiple EAE models, including relapsing-remitting and chronic progressive EAE. Overall impact: Experiments in this study focusing on IL-7Rα, a genetic risk factor for MS, will enhance our understanding of the pathways and cellular interactions that promote CNS autoimmunity as well as investigate novel approaches to treat MS.