AbstractNeonatal islet-specific expression of IL-10 in nonobese diabetic (NOD) mice accelerates the onset of diabetes, whereas systemic treatment of young NOD mice with IL-10 prevents diabetes. The mechanism for acceleration of diabetes in IL-10-NOD mice is not known. Here we show, by adoptive transfers, that prediabetic or diabetic NOD splenocytes upon encountering IL-10 in the pancreatic islets readily promoted diabetes. This outcome suggests that the compartment of exposure, not the timing, confers proinflammatory effects on this molecule. Moreover, injection of IL-10-deficient NOD splenocytes into transgenic IL-10-NOD.scid/scid mice elicited accelerated disease, demonstrating that pancreatic IL-10 but not endogenous IL-10 is sufficient for the acceleration of diabetes. Immunohistochemical analysis revealed hyperexpression of ICAM-1 on the vascular endothelium of IL-10-NOD mice. The finding suggests that IL-10 may promote diabetes via an ICAM-1-dependent pathway. We found that introduction of ICAM-1 deficiency into IL-10-NOD mice as well as into NOD mice prevented accelerated insulitis and diabetes. Failure to develop insulitis and diabetes was preceded by the absence of GAD65-specific T cell responses. The data suggest that ICAM-1 plays a role in the formation of the “immunological synapse”, thereby affecting the generation and/or expansion of islet-specific T cells. In addition, ICAM-1 also played a role in the effector phase of autoimmune diabetes because adoptive transfer of diabetogenic BDC2.5 T cells failed to elicit clinical disease in ICAM-1-deficient IL-10-NOD and NOD mice. These findings provide evidence that pancreatic IL-10 is sufficient to drive pathogenic autoimmune responses and accelerates diabetes via an ICAM-1-dependent pathway.