Upregulated Fcrl5 disrupts B cell anergy and causes autoimmunity

B cell anergy plays a critical role in maintaining self-tolerance by inhibiting autoreactive B cell activation to prevent autoimmune diseases. Here, we demonstrated that Fc receptor-like 5 (Fcrl5) upregulation contributes to autoimmune disease pathogenesis by disrupting B cell anergy. Fcrl5--a gene whose homologs are associated with human autoimmune diseases--is highly expressed in age/autoimmunity-associated B cells (ABCs), an autoreactive B cell subset. By generating B cell-specific Fcrl5 transgenic mice, we demonstrated that Fcrl5 overexpression in B cells caused systemic autoimmunity with age. Furthermore, Fcrl5 upregulation in B cells exacerbated the systemic lupus erythematosus-like disease model and increased the levels of ABCs and activated T cells. Mechanistically, an increase in Fcrl5 expression broke B cell anergy, activating autoreactive B cells in the presence of a self-antigen. Fcrl5 facilitated toll-like receptor signaling, reactivating anergic B cells. Thus, Fcrl5 is a potential regulator of B cell-mediated autoimmunity by regulating B cell anergy. This study provides important insights into the role of Fcrl5 in breaking B cell anergy and its effect on the pathogenesis of autoimmune diseases.