B cell counterpart of Treg cells: As a new target for autoimmune disease therapy

B cells have been considered a positive immune regulator by producing antibodies and presenting antigens to T cells. Recently, it has been shown that B cells comprise rare but potent subset capable of inhibiting immune responses via diverse regulatory mechanisms. This subset of B cells, known as regulatory B cells (Bregs), negatively regulates immune responses and contributes to immune tolerance. Among several regulatory B cell subsets, interleukin 10 (IL-10)-producing regulatory B cells are intensively investigated. Studies in experimental animal models, as well as in patients with autoimmune diseases, have identified Breg subsets exhibiting a powerful therapeutic tool for severe immune disorders. Now, human regulatory B cells are becoming an active area of research. Clear criteria to identify these cell subsets and the key molecular mechanisms underlying their physiological features are required for understanding the complete picture of regulatory B cells. This review highlights the current knowledge on regulatory B cells, mainly IL-10-producing regulatory B cells.