Secreted microvesicles (MVs) are potent inflammatory triggers that stimulate autoreactive B and T cells， causing Type 1 Diabetes in non-obese diabetic (NOD) mice. Proteomic analysis of purified MVs released from islet cells detected the presence of endogenous retrovirus (ERV) antigens， including Env and Gag sequences similar to the well-characterized murine leukemia retroviruses. This raises the possibility that ERV antigens may be expressed in the pancreatic islets via MV secretion. Using virus-like particles produced by co-expressing ERV Env and Gag antigens， and a recombinant gp70 Env protein， we demonstrated that NOD but not diabetes-resistant mice developed anti-Env autoantibodies that increase in titer as disease progresses. A lentiviral-based RNA interference knockdown of Gag revealed that Gag contributes to the MV-induced T-cell response， whose diabetogenic function can be demonstrated via cell-transfer into immune-deficient mice. Finally， we observed that Gag and Env are expressed in NOD islet-derived primary mesenchymal stem cells (MSCs). However， MSCs derived from the islets of diabetes-resistant mice do not express the antigens. Taken together， abnormal ERV activation and secretion of MVs may induce anti-retroviral responses to trigger autoimmunity.