Antibody (Ab)-based mmunotherapy has been used in the clinic to treat T cell-mediated pathologies including autoimmunity such as type 1 diabetes (T1D), allograft transplantation, and graft versus host disease (GVHD). Typically these Ab indiscriminately deplete T cells, which can lead to only short-term efficacy and complications related to immunosuppression. Application of nondepleting (ND) Ab offers an approach to modify pathogenic and regulatory T cells without systemic T cell loss. Indeed we recently reported that ND Ab specific for the CD4 and CD8? co-receptors reverses diabetes long-term in NOD mice. Despite the fact that all T cells are bound by the ND ?CD4 and ?CD8 Ab, induction and maintenance of tolerance are tissue-specific, and protective immunity is unaffected in treated NOD mice. To date, only ND Ab specific for human (hu) CD4 have been studied in a limited number of clinical trials for the treatment of autoimmunity. ND Ab specific for huCD8, however, have yet to be reported. It is very likely that combined ND ?huCD4 and ?huCD8 Ab therapy will be needed to effectively treat pathologies driven by both CD4+ and CD8+ T cells, such as T1D. With this in mind we recently established ND Ab specific for huCD8?. Immunoglobulin VL and VH gene segments derived from cDNA encoding huCD8?-specific murine IgG? were used to engineer ?huCD8? human IgG4 recombinants. Notably, IgG4 exhibits limited proinflammatory effector and depleting function. The objective of this R21 is to test the tolerogenic function of ND ?huCD8? IgG4. Humanized mouse models are being employed to study the immunoregulatory effects of ?huCD8? IgG4 in vivo in xenogeneic GVHD-mediated pancreatitis and human islet allograft rejection. In Aim 1, experiments will focus on defining mechanisms of tolerance induced by ND ?huCD8? IgG4 in the suppression of pancreatitis that develops in humanized mice. Aim 2 will investigate the tissue-specificity and therapeutic efficacy of ND ?huCD8? IgG4 in blocking human islet allograft rejection in humanized mice. Together these experiments will establish the tolerogenic properties and translational potential of ND ?huCD8? IgG4, as well as provide novel reagents to treat T cell-mediated autoimmunity and other pathologies in the clinic.