The role of SLAM family receptors in germinal center-mediated autoimmunity

Systemic lupus erythematosus (SLE) is a complex, polygenic autoimmune disease that exhibits a high female gender bias and possesses a highly heterogeneous clinical presentation. B6.Sle1b mice, a congenic strain that carries the Sle1b sub-locus containing the SLAM genes derived from lupus-associated New Zealand Mix mice (NZM2410) on the C57BL/6 (B6) background, spontaneously develop high titers of autoantibodies (ANAs). However, the role and mechanism(s) involved in the alteration of the germinal center (GC) tolerance checkpoint in the development of ANAs in these mice is not defined. These studies demonstrate that the elevated ANA-specific antibody-forming cells (AFCs) and ANA titers exhibited by aged female B6. Sle1b mice are directly correlated with increased spontaneous GC (spt-GC) formation. Through the use of a DNA and p-azophenylarsonate dual-reactive B cells from Ig V(H) knock-in mice (HKIR), we determined that the attenuated GC response was relieved in adoptively transferred B cell recipients harboring Sle1b. Mixed bone marrow chimeric experiments showed that Sle1b perpetuates the breach in peripheral tolerance through altering the germinal center (GC) checkpoint in a B cell autonomous manner. The Sle1b sublocus harbors the signaling lymphocyte activation molecule (SLAM) family genes and numerous studies have implicated different ones in driving SLE pathogenesis including Ly108, CD150, Ly9, and CD48. However, it is unclear which specific genes on the Sle1b locus are the strongest mediators in the breach in B cell tolerance. Furthermore, the mechanism by which polymorphisms in the SLAM and/or non-SLAM genes in Sle1b alter GC B cell tolerance is not clear. In order to definitely identify which genes located on the Sle1b sublocus is responsible for mediating the loss of B cell tolerance, we utilized a bacteria artificial chromosome (BAC)-transgenic rescue approach where we generated several BAC transgenic mice that overexpress B6 alleles of SLAM and non-SLAM genes in autoimmune-prone B6.Sle1b mice. B6.Sle1b mice overexpressing B6 alleles of the SLAM genes Ly108 and CD84 ( Sle1b-BAC90) showed a significant reduction in the spt-GC response and consequently ANA-specific Ab-forming cells and ANA titers compared to B6.Sle1b mice. Overexpression of B6-derived Ly108 alone in B6.Sle1b mice partially restored tolerance to ANA. Furthermore, Cre-mediated deletion of the BAC transgene containing B6-derived CD84 and Ly108 revealed a GC B cell-specific effect of these genes on reestablishing tolerance in B6.Sle1b mice. Mechanistically, Sle1b B cells escaped tolerance due to lower BCR signaling, reduced proliferative capacity, decreased apoptosis, and impaired B cell-T cell conjugate formation, all of which were restored in with the overexpression of B6-derived CD84 and Ly108. Collectively, these results implicate the critical role of both Ly108 and CD84 in the maintenance of GC tolerance and the regulation of autoimmunity. These results also potentially implicate therapeutic targets centered on GC B cell-specific CD84 and Ly108 for treatment of SLE