B lymphocytes are central mediators of humoral immunity. Aberrant B cell function also contributes to multiple autoimmune diseases, including rheumatoid arthritis (RA). In addition, we and others have recently found that B cells also serve critical negative regulatory functions during adaptive CD4+ T cell responses that can dampen both cellular and humoral immune responses, and the development of autoimmunity. This unexpected observation is explained in part by the identification of a potent regulatory B cell subset that dramatically attenuates Th1 immune responses and autoimmunity in mice. This regulatory B cell subset is uniquely CD1d+CD5+, produces IL-10. and represents 1-2% of total spleen B cells in wild type mice and <1% of circulating human B cells. We call this subset B10 cells to emphasize that they are the predominant, if not exclusive, B cell population that produces IL-10 and to distinguish them from other regulatory subsets that may also exist. B10 cell numbers within tissues increase significantly in mice with autoimmunity and age. In this proposal, we hypothesize that antigen-specific regulatory B10 cells influence autoimmune disease in both mice and humans. We will test this hypothesis and examine B10 cell generation, function, and mechanisms of action using the mouse collagen-induced arthritis (CIA) model of RA and B cells from patients with RA. In four specific aims, the proposed studies will identify the extent that the B10 subset modulates immune responses during autoimmunity, determine whether B10 cells can be manipulated for therapeutic benefit, and identify and characterize this unique B cell subset in normal humans and patients with autoimmunity. Specific Aim 1 will identify and characterize the B10 cell subset before, during and after CIA induction; Specific Aim 2 will characterize B10 cell function during CIA; Specific Aim 3 will develop an in vivo preclinical mouse model for B10 cell adoptive therapy; and Specific Aim 4 will identify and characterize the B10 subset during human autoimmune disease. These overlapping studies will significantly expand our knowledge of how B10 cells regulate both normal and abnormal immune responses in both species.