CD8+ T cells activated during viral infection migrate to infected skin in an antigen-independent manner. Local recognition of antigens drives the differentiation into Trm CD8+ T cells.

Spontaneous germinal center (Spt-GC) B cells and follicular helper T cells (Tfh) generate high affinity autoantibodies involved in the development of systemic lupus erythematosus (SLE). Toll like receptors (TLRs) play a pivotal role in SLE pathogenesis. While previous studies have focused on the B cell intrinsic role of TLR-MyD88 signaling on immune activation, autoantibody repertoire and systemic inflammation, a thorough investigation of the mechanisms by which TLRs control the formation of Spt-GCs remains unclear. Using non-autoimmune C57BL/6 (B6) mice deficient in MyD88, TLR2, 3, 4, 7 or 9, we identified B cell-intrinsic TLR7 signaling as a prerequisite to Spt-GC formation without the confounding effects of autoimmune susceptibility genes and the overexpression of TLRs. TLR7 deficiency also rendered autoimmune B6.Sle1b mice unable to form Spt-GCs, leading to markedly decreased autoantibodies. Conversely, B6.yaa and B6.Sle1b.yaa mice expressing an extra copy of TLR7 and B6.Sle1b mice treated with a TLR7 agonist had increased Spt-GCs and Tfh. Further, TLR7/ MyD88 deficiency led to compromised B cell proliferation and survival after B cell stimulation both in vitro and in vivo. In contrast, TLR9 inhibited Spt-GC development. Our findings demonstrate an absolute requirement of TLR7 and a negative regulatory function for TLR9 in Spt-GC formation under non-autoimmune and autoimmune conditions. Our data suggest that, under non-autoimmune conditions, Spt-GCs initiated by TLR7 produce protective antibodies. However, in the presence of autoimmune susceptibility genes, TLR7 dependent Spt-GCs produce pathogenic autoantibodies. Thus, a single copy of TLR7 in B cells is the minimal requirement for breaking the GC-tolerance checkpoint.

Germinal centers (GC) are specialized microenvironments that generate high-affinity antibody-forming (AFCs) and memory B cells. Many B cells undergo apoptosis during B-cell clonal selection in GC. Although the factors that regulate the AFC and GC responses are not precisely understood, it is widely believed that dysregulated AFCs and GCs contribute to autoimmunity. The Mer receptor tyrosine kinase (MerTK or Mer) facilitates macrophage clearance of apoptotic cells. The TAM (Tyro-3, Axl, and Mer) receptors, including Mer, suppress Toll-like receptors (TLRs) and cytokine-mediated inflammatory responses. We report here that tingible body macrophages (TBMϕs) in GC express Mer. Compared to C57BL/6 (B6) controls, Mer deficient (Mer−/−) mice had significantly higher AFC, GC and Th1-skewed antibody (IgG2c) responses against the T-dependent Ag (4-hydroxy-3-nitrophenyl) acetyl (NP)-chicken gamma globulin (CGG). Mer−/− mice had significantly higher percentage of GC B cells on days 9, 14 and 21 post-immunization compared to B6 controls. Significantly increased numbers of apoptotic cells accumulated in Mer−/− GCs than in B6 GCs, while the number of TBMϕs remained similar in both strains. Our data are the first to demonstrate a critical role for Mer in GC apoptotic cell clearance by TBMϕs and have interesting implications for Mer in the regulation of B cell tolerance operative in the AFC and GC pathways.

C57BL/6 (B6) mice carrying the Sle1b sub-locus (named B6.Sle1b), which harbors the lupus-associated NZM2410/NZW SLAM family genes, produce anti-nuclear antibodies (ANA). 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. Here we show significantly higher spontaneously formed GCs (Spt-GCs) in B6.Sle1b female mice compared to B6 controls. We also found a significant increase in CD4+CXCR5hiPD-1hi spontaneously activated follicular helper T (Spt-TFH) cells in B6.Sle1b female mice. Compared to B6 controls, B6.Sle1b female mice had increased numbers of proliferating B cells predominantly located in Spt-GCs. The elevated Spt-GCs in B6.Sle1b female mice were strongly associated with increased ANA-specific antibody forming cells (AFCs) and ANA titers. The increased numbers of Spt-GCs and Spt-TFH cells in B6.Sle1b mice were not the result of a generalized defect in B cells expressing Sle1b. Consistent with the elevated spontaneous response in B6.Sle1b mice, the attenuated GC response characteristic of DNA and p-azophenylarsonate (Ars) reactive B cells from Ig VH knock-in mice (termed HKIR) were relieved in adoptively transferred recipients in the presence of Sle1b. Finally, by generating mixed bone marrow chimeras, we showed that the effect of Sle1b on Spt-GC, TFH cell and autoantibody responses in B6.Sle1b mice was B cell autonomous. These data indicate that the NZM2410/NZW-derived Sle1b sub-locus in conjunction with the female sex primarily affects B cells leading to the alteration of the GC tolerance checkpoint and the generation of ANA-specific AFCs.