Accumulating evidence indicates that the type IIB Fc receptor for IgG (FcgammaRIIB) plays a pivotal role in maintaining peripheral tolerance by suppressing excessive humoral and cellular immune responses. However, little is known about the mechanism by which the autoreactive B cells develop in the periphery in FcgammaRIIB-deficient mice. To clarify the role of FcgammaRIIB in the emergence of autoreactive B cells, we analyzed B-cell compartments in the autoimmune arthritis-susceptible DBA/1 mice devoid of FcgammaRIIB (DBA.IIB-/-) during the induction of collagen-induced arthritis (CIA). We found that DBA.IIB-/- showed an increase in the number of peripheral immature type 2 transitional (T2) B cells after immunization with type II collagen (C-II), followed by the enhanced severity of CIA with higher autoantibody titers to mouse C-II than those of wild-type DBA/1. In addition, elevated secretion of IL-1alpha by peritoneal macrophages from DBA.IIB-/- on stimulation with IgG immune complexes in vitro suggested the augmented effector cell responses in the CIA course of DBA.IIB-/-. These findings suggest that the FcgammaRIIB-dependent triple regulation in the peripheral T2 B cells, in the antibody production, and in the effector cell responses is crucial for suppressing CIA.