A major problem in the treatment of hemophilia A patients with factor VIIII (FVIII) is that up to 30% of these patients produce antibodies to therapeutic FVIII. These antibodies block (inhibit) the pro- coagulant function of FVIII and thus are termed inhibitors. The focus of our lab has been to develop novel approaches for the induction of tolerance so that it can be applied to the prevention or reversal of undesirable immune responses, including the formation of hemophilia inhibitors. We have used immunoglobulin (Ig) fusion proteins expressed in B lymphocytes for the induction of tolerance in animal models of hemophilia. This technology is based on the well-known tolerogenicity of IgG carriers, onto which we engineer multiple epitope-containing polypeptides in frame at the N-terminus. We know that CD4+CD25+ regulatory T cells (Tregs) are required in our tolerogenic protocol and have shown that that the IgG scaffold increases the efficacy of this tolerance process. Independent data suggests that IgG domains contain epitopes, termed Tregitopes, which can activate Tregs. Indeed, recent clinical studies with FVIII-Fc fusion proteins designed for longer half-life appear to be less immunogenic, and may be tolerogenic. We hypothesize that the presence of Tregitopes and IgG processing mechanisms promote tolerogenic presentation. We focus in this proposal to take advantage of B-cell tolerogenic presentation of IgG fusion proteins (including a B-cell directed soluble fusion protein using a single chain anti-CD20) that can be utilized to induce tolerance to FVIII and prevent/reverse inhibitor formation. Moreover, T-cell receptors (TCR) from hemophilia patients will be used to create antigen-specific effector and regulatory CD4 T cells in analogy to the CAR19 T cells designed to treat B-cell leukemias. These will be used to test novel tolerogenic therapy and the efficacy of engineered human Tregs to prevent and reverse inhibitor formation. Our ultimate Aims, thus, are: (1) to use B-cell targeted fusion proteins to modulate anti-FVIII responsiveness, and finally (2) to express human TCR specific for FVIII in expanded human regulatory T cells to effectively suppress inhibitor responses to FVIII. Elucidating the mechanisms of these tolerogenic approaches will enhance our ability to optimize the efficacy and safety of this approach for translation in humans.