Complement C3 deficiency prevent against the onset of streptozotocin-induced autoimmune diabetes involving expansion of regulatory T cells

Recent studies have demonstrated that complement contributes to the development of autoimmune diabetes. However, the mechanisms remain unknown. Herein, using a model of streptozotocin (STZ)-induced diabetes, we found the presence of immune tolerance to self islet in complement C3-deficient mice after STZ. Higher number of CD4 + CD25+ regulatory T cells (Tregs) with characteristics of expressing Foxp3 was observed in C3−/− mice. These C3−/− Tregs exhibited enhanced suppressive capacity to effector cell proliferation. The central role of Tregs was further evidenced by that depleting these cells using anti-CD25 antibody dramatically abrogated the preventive effects of C3 deficiency on STZ-induced diabetes. Importantly, transforming growth factor-β (TGF-β) was a key factor for Treg-mediated immune suppression as blocking TGF-β activity reversed suppressive capacity of Tregs in vitro and diabetes-resistant effects of C3 deficiency in vivo. These findings suggest that resistance to overt diabetes in STZ-treated C3−/− mice involves a population of Tregs in TGF-β-dependent manner.