The Lupus-Associated Fcγ Receptor IIb-I232T Polymorphism Results in Impairment in the Negative Selection of Low-Affinity Germinal Center B Cells Via c-Abl in Mice.

Abstract

Fcγ receptor IIb (FcγRIIb) is an essential negative regulator of B cells that blocks B cell receptor (BCR) signaling and triggers c-Abl-dependent apoptosis of B cells. FcγRIIb-deficient mice display splenomegaly with expansion of B cells, leading to lupus. FcγRIIb-I232T is a hypofunctional polymorphism associated with lupus susceptibility in humans, an autoimmune disease linked to diminished deletion of autoreactive B cells. In the context of the FcγRIIb-I232T polymorphism, we investigated the role of FcγRIIb in the deletion of low-affinity germinal center (GC) B cells, an important mechanism for preventing autoimmunity. We generated FcγRIIb^232T/T mice to mimic human FcγRIIb-I232T carriers and immunized mice with chicken gamma globulin (CGG)-conjugated NP, a T cell-dependent antigen, to examine the response of GC B cells. Compared to wild-type (WT) mice, FcγRIIb^232T/T mice showed increased numbers of low-affinity NP-specific IgG and NP-specific B cells and plasma cells; additionally, the expression of a somatic mutation (W33L) in their V_H 186.2 genes encoding high-affinity BCR was reduced. Notably, FcγRIIb^232T/T mice had a higher number of GC light zone B cells and showed less apoptosis than WT mice, despite having equivalent follicular helper T cell numbers and function. Moreover, phosphorylation of c-Abl was reduced in FcγRIIb^232T/T mice, and treatment of WT mice with the c-Abl inhibitor nilotinib during the peak of GC response resulted in reduced affinity maturation reminiscent of FcγRIIb^232T/T mice. Our findings provide evidence of a critical role of FcγRIIb/c-Abl in the negative selection of GC B cells in FcγRIIb^232T/T mice. Importantly, our findings indicate potential benefits of up-regulating FcγRIIb expression in B cells for treatment of systemic lupus erythematosus.