Modulation of Peripheral B Cell Tolerance by CD72 in a Murine Model

B cell self tolerance is maintained by several mechanisms, including deletion, receptor editing, and anergy (1–4). Mechanisms of peripheral tolerance are thought to be important, because a considerable proportion of self-reactive B cells escape central tolerance mechanisms and emerge into the periphery (5). Breakdown of peripheral B cell tolerance results in autoantibody production, which contributes to autoimmune diseases such as systemic lupus erythematosus (SLE), multiple sclerosis, Sjogren's syndrome, and rheumatoid arthritis. SLE is an autoimmune disorder characterized by multiorgan inflammation and deregulated production of autoantibodies, including antinuclear antibodies (ANAs) and anti–double-stranded DNA (anti-dsDNA) antibodies (6). B cell–targeted therapies have shown great promise in the treatment of SLE (7). B cell coreceptors, such as CD19, CD22, CD72, Fcγ receptor type IIb (FcγRIIb), CD45, and CD5, tightly regulate B cell receptor (BCR) signaling to fine-tune B cell responses in encounters with antigens in the periphery (8,9). In fact, several of these coreceptors, including CD22, FcγRIIb, and CD45, have been directly linked to SLE (10–13). CD72, a 45-kd type II transmembrane glycoprotein, is constitutively expressed on developing and mature B cells, but not on terminally differentiated plasma cells. The cytoplasmic domain of CD72 contains an immunoreceptor tyrosine–based inhibition motif (ITIM) that binds SH2-containing protein tyrosine phosphatase 1 (SHP-1), and an ITIM-like motif that binds Grb2 (14). Our recent data indicate that CD72 plays a negative role in regulating BCR-induced signaling in primary mature B cells (9), but CD72 may also transmit a positive signal independent of the BCR (15). CD100, a ligand for CD72, can turn off the negative effect of CD72 by inhibiting phosphorylation of CD72 and, in turn, disrupting the SHP-1/CD72 complex (16,17). Although a link between CD72 and SLE has been suggested (18–20), the role of CD72 in autoimmunity is not well defined and the molecular mechanisms by which CD72 might influence B cell tolerance are unknown. Cbl-b, an E3 ubiquitin ligase, has been shown to regulate B cell tolerance. Cbl-b–deficient B cells are hyperreactive to BCR ligation, and mice lacking Cbl-b produce anti-dsDNA autoantibodies (21). Clinically, Cbl-b–deficient mice are susceptible to spontaneous and induced autoimmune diseases (22–24), and mice with B cell–specific ablation of both Cbl and Cbl-b manifest SLE-like autoimmune disease (25), suggesting that Cbl-b modulates peripheral T cell and B cell tolerance. However, the proteins that anchor Cbl-b to the membrane in close proximity to the BCR signalosome and that regulate Cbl-b function in B cells have not been identified. In the present study we examined the direct role of CD72 in peripheral B cell tolerance and in the development of autoimmunity. Our findings indicate that CD72 is an essential negative regulator of BCR signaling in self-reactive B cells, and that CD72 interacts with and regulates Cbl-b. As indicated by multiple criteria, CD72 functions as a physiologic regulator of B cell anergy, and inappropriate regulation of the CD72-signaling network may contribute to the development of autoimmune diseases such as SLE.