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

Objective. B cells play a dominant role in the pathogenesis of several autoimmune diseases, including systemic lupus erythematosus. It is not well understood how B cell signaling contributes to autoantibody production. The goal of this study was to elucidate the role of CD72 in modulating B cell receptor (BCR)-mediated tolerogenic signaling and peripheral B cell tolerance. Methods. A mouse model utilizing hen egg lysozyme (HEL) anergic B cells was studied. CD72-deficient mice carrying the BCR-specific Ig(HEL) and/or soluble HEL (sHEL) transgenes were generated by breeding Ig(HEL)-transgenic MD4 mice and/or sHEL-transgenic ML5 mice with congenic, CD72-deficient C57BL/6J mice. Normal and anergic B cells were isolated for analyses of B cell signaling. Aged wild-type and CD72-deficient mice were also examined for autoimmune phenomena. Results. In the absence of CD72, anergic B cells inappropriately proliferated and survived in response to stimulation with self antigen. Biochemical analyses indicated that in anergic B cells, CD72 dominantly down-regulated BCR signaling to limit the antigen-induced elevation in [Ca2+](i) and the activation of NFATc1, NF-kappa B, MAPK, and Akt. Mechanistically, CD72 was associated with, and regulated, the molecular adaptor Cbl-b in anergic B cells, suggesting that Cbl-b may play a role in mediating the negative effects of CD72 on BCR signaling. Moreover, in aged CD72-deficient mice, spontaneous production of antinuclear and anti-double-stranded DNA autoantibodies and features of lupus-like autoimmune disease were observed. Conclusion. CD72 is required to maintain B cell anergy and functions as a regulator of peripheral B cell tolerance. Thus, altered CD72 expression may play a role during the development of systemic lupus erythematosus.