TGF-β1, expressed in the pancreatic islets, protects the nonobese diabetic (NOD) mouse from insulin-dependent diabetes mellitus (IDDM). The islet antigen-specific T cell response of ins-TGF-β1 mice relied on different antigen-presenting cells (APC) from those used by NOD T cells. T cells from NOD mice utilized B cells to present islet antigen, whereas T cells from ins-TGF-β1 mice utilized macrophages. In addition, the islet antigen-specific T cell repertoire of ins-TGF-β1 mice was distinct and deviated toward an IL-4-producing Th2 phenotype. When ins-TGF-β1 mice were treated with anti-IL-4 antibody, islet antigen-specific IFNγ-producing Th1 cells were unleashed, and the incidence of diabetes increased to the level of NOD mice. This suggests active suppression of a diabetogenic T cell response. This study describes a novel mechanism in which expression of TGF-β1 in the context of self-antigen shifts APC preference, deviating T cell responses to a Th2 phenotype, preventing IDDM.