['The endopeptidase dipeptidyl peptidase-IV (DPP-IV) has been shown to NH2-terminally truncate incretin hormones, glucose-dependent insulinotropic polypeptide, and glucagon-like peptide-1, thus ablating their ability to potentiate glucose-stimulated insulin secretion. Increasing the circulating levels of incretins through administration of DPP-IV inhibitors has therefore been introduced as a therapeutic approach for the treatment of type 2 diabetes. DPP-IV inhibitor treatment has also been shown to preserve islet mass in rodent models of type 1 diabetes. The current study was initiated to define the effects of the DPP-IV inhibitor sitagliptin (MK0431) on transplanted islet survival in nonobese diabetic (NOD) mice, an autoimmune type 1 diabetes model.', 'Effects of MK0431 on islet graft survival in diabetic NOD mice were determined with metabolic studies and micropositron emission tomography imaging, and its underlying molecular mechanisms were assessed.', 'Treatment of NOD mice with MK0431 before and after islet transplantation resulted in prolongation of islet graft survival, whereas treatment after transplantation alone resulted in small beneficial effects compared with nontreated controls. Subsequent studies demonstrated that MK0431 pretreatment resulted in decreased insulitis in diabetic NOD mice and reduced in vitro migration of isolated splenic CD4+ T-cells. Furthermore, in vitro treatment of splenic CD4+ T-cells with DPP-IV resulted in increased migration and activation of protein kinase A (PKA) and Rac1.'] Does dipeptidyl peptidase IV inhibition with MK0431 improve islet graft survival in diabetic NOD mice partially via T-cell modulation?