CD80 and Arthritis: Seminal examples include the observation that B cell-deficient mice do not develop collagen-induced arthritis (7), that CD80/CD86 expression on B cells is required for autoreactive T cell activation and the development of arthritis (8), and that chimeric mice lacking IL-10 expressing B cells develop an exacerbated arthritis that is driven by an expanded Th17 compartment (9).