Autoimmune-mediated cytotoxicity may cause pancreatic islet transplant failure, leading to recurrent diabetes. Protection of islet grafts depends on immunosuppressive control, which may also prevent autoimmune recurrence of diabetes. In this study, we compared the survival of syngeneic islet transplants using different strains of donor mice. We observed extended functional survival in the islet grafts from donors lacking the genetic background and potential of autoimmunity. Without immunosuppression, the islet grafts of NOR and immune-deficient NOD. Scid donors functioned up to 3 weeks in syngeneic islet transplants compared to 3-day survivals with the grafts from NOD donors. T-cell proliferation and activation markers, CD44 and CD69, were upregulated in NOD donors, suggesting that T-cell activation had occurred prior to pancreas procurement. Systemic delivery of a recombinant adenoassociated viral vector (AAV) encoding the viral (vIL-10) IL-10 gene (AAV vIL-10) in NOD recipients protected syngeneic islets from autoimmune destruction. Alternatively, pretreatment of NOD donor mice with AAV vIL-10 prolonged islet graft survival in untreated NOD recipients. Both studies indicate the effectiveness of vIL-10 gene therapy in autoimmune regulation. These results suggest that a donor factor may exist in autoimmune-prone donors. Therefore, autoimmune recurrence of diabetes may result from donor immune cells transferred during islet transplantation. The AAV vIL-10 gene therapy suppressed previously activated donor T cells and protected the grafted islets from autoimmune-mediated destruction.