Suppression mediated by regulatory T cells (T reg cells) represents a unique, cell-extrinsic mechanism of in-trans negative regulation that restrains multiple types of immune cells. The loss of T reg cells leads to fatal, highly aggressive, and widespread immune-mediated lesions. This severe autoimmunity may be driven by commensal microbiota, the largest source of non-self ligands activating the innate and adaptive immune systems. Alternatively, T reg cells may primarily restrain T cells with a diverse self–major histocompatibility complex (MHC)–restricted T cell receptor repertoire independently of commensal microbiota. In this study, we demonstrate that in germ-free (GF) mice, ablation of the otherwise fully functional T reg cells resulted in a systemic autoimmune lympho- and myeloproliferative syndrome and tissue inflammation comparable with those in T reg cell–ablated conventional mice. Importantly, there were two exceptions: in GF mice deprived of T reg cells, the inflammation in the small intestine was delayed, whereas exocrine pancreatitis was markedly accelerated compared with T reg cell–ablated conventional mice. These findings suggest that the main function of T reg cells is restraint of self-MHC–restricted T cell responsiveness, which, regardless of the presence of commensal microbiota, poses a threat of autoimmunity.