Clinical islet transplantation is a successful procedure that can improve the quality of life in recipients with diabetes. A drawback of the procedure is the need for chronic administration of immunosuppressive drugs that, among other side effects, are potentially diabetogenic. Definition of immunosuppressive protocols that utilize nondiabetogenic compounds could further improve islet transplantation outcome. We used the NOD mouse to assess the effect of targeting the T-lymphocyte surface receptors CD45RB and CD154 in preventing loss of allogeneic islet grafts as a result of recurrence of autoimmunity and allorejection. Administration of the two antibodies led to significantly prolonged allograft survival, with a percentage of grafts surviving long-term. The therapeutic efficacy of the treatment was paralleled by a shift in CD45RB isoform expression on T-lymphocytes, increased in vitro responsiveness to interleukin-7, and increased in vitro gamma-interferon production after anti-CD3 antibody stimulation. Furthermore, graft infiltration by CD8+ T-cells was remarkably reduced. Recipient mice bearing functioning allografts were otherwise immunocompetent, as assessed in vivo and in vitro by numerous tests, including intragraft cytokine production, responsiveness to polyclonal stimulation and alloantigens, and analysis of cell subset phenotype. These data show that nondiabetogenic regimens of immunomodulation can lead to prolonged islet allograft survival in the challenging NOD mouse model.