B7x, an inhibitory member of the B7/CD28 superfamily, is highly expressed in a broad range of nonhematopoietic organs, suggesting a role in maintaining peripheral tolerance. As endogenous B7x protein is expressed in pancreatic islets, we investigated whether the molecule inhibits diabetogenic responses. Transfer of disease-inducing BDC2.5 T cells into B7x-deficient mice resulted in a more aggressive form of diabetes than in wild-type animals. This exacerbation of disease correlated with higher frequencies of islet-infiltrating Th1 and Th17 cells. Conversely, local B7x overexpression inhibited the development of autoimmunity, as crossing diabetes-susceptible BDC2.5/B6(g7) mice to animals overexpressing B7x in pancreatic islets abrogated disease induction. This protection was caused by the inhibition of IFN-γ production by CD4 T cells and not to a skewing or expansion of Th2 or regulatory T cells. The suppressive function of B7x was also supported by observations from another autoimmune model, experimental autoimmune encephalomyelitis, in which B7x-deficient mice developed exacerbated disease in comparison with wild-type animals. Analysis of central nervous system-infiltrating immune cells revealed that the loss of endogenous B7x resulted in expanded Th1 and Th17 responses. Data from these two autoimmune models provide evidence that B7x expression in the periphery acts as an immune checkpoint to prevent tissue-specific autoimmunity.