ITK: an emerging target for treatment of T cell-mediated autoimmune diseases Lymphocyte-mediated autoimmune diseases arise from a breakdown of self-tolerance. One of the key proteins regulating self-tolerance is the inhibitory T cell protein, CTLA-4. When CTLA-4 is absent, mice succumb to a rapid and fatal multi-organ autoimmune disease, and die by three weeks of age. Recent studies from our labs have demonstrated that CTLA-4 has two distinct functions in preventing autoimmunity. First, CTLA-4 is required for the function of FOXP3+ regulatory T cells in maintaining T cell tolerance. Second, CTLA-4 is required in conventional T cells, to block aberrantly activated self-reactive T cell accumulation in tissues under non-inflammatory conditions. It is unknown how costimulatory molecules control aberrantly activated self-reactive T cells from infiltrating and damaging tissues. Based on the observation that the null mutation of the Tec kinase Itk prevents activated T cell migration and autoimmune pathology of Ctla4-/- mice, we propose that ITK activation is critical for T cell-mediated autoimmune diseases. We hypothesize that ITK is required for the synergistic activation of VAV1 and the pathways downstream of VAV1 leading to actin polymerization and cytoskeletal reorganization. Further, we propose that ITK activates VAV1 by phosphorylating and activating the Src family kinase, FYN, which in turn phosphorylates and activates VAV1. We will test this hypothesis by examining the migratory behavior of self-reactive T cells from Itk-/- mice in vitro and in vivo to determine the molecular mechanism by which ITK controls T cell movement into non-lymphoid tissues. We will also determine whether ITK regulates cytoskeletal reorganization in primary T cells by activating FYN, whether FYN is required for actin polymerization and cytoskeletal reorganization in primary T cells and contribute to autoimmune disease progression. Finally, we will investigate the efficacy and mechanism of suppression of Type I diabetes in animals by small molecule inhibitors of ITK, to begin to explore the utility of targeted ITK blockade in clinics to treat various organ-specific autoimmune diseases.