Low CD21 identifies germinal center B cells failing antigen-driven positive selection destined for removal by FAS 2307103

Introduction Germinal centers (GC) drive affinity maturation of antibody responses. GC B cells lacking FAS death receptor escape affinity-based selection and persist despite carrying non-competitive low-affinity BCRs. It remains unclear how these FAS-deficient GC B cells can capture sufficient antigen to present to CD4+ follicular T cells and persist in competition against other high-affinity GC B cells. Defining how these low-affinity GC B cells persist within the competitive GC microenvironment may reveal additional mechanisms that enforce typical affinity-matured GC B cell responses. Methods Using adoptive transfer of FAS-deficient B cells possessing a starting specificity for hen egg lysozyme (Fas-/-.SWHEL B cells), we perform a series of single cell-omics and in vivo experiments to characterise the GC B cells that accumulate in FAS-deficient responses following immunisation. Results We find FAS-deficient GC B cells that escape affinity maturation display low surface CD21 and high B220 (CD21lo). Using this definition, we also identify rare, non-affinity matured CD21lo GC B cells in wild-type responses. Unlike their affinity-matured CD21+ GC B cell counterparts, CD21lo GC B cells are dark zone enriched, express low levels of surface BCR and show diverse variable immunoglobulin gene somatic hypermutations. Despite low affinities for foreign antigen, FAS-deficient CD21lo GC B cells are highly proliferative and depend on T cell signals. Transcriptomic profiling finds evidence of an analogous CD21lo population in human GCs. Conclusion Our findings suggest GC B cells enriched for reduced antigen reactivity that still receive follicular T cell help transition into CD21loB220hi GC B cells. These non-affinity matured CD21lo GC B cells normally fail to accumulate in typical GC responses due to FAS-mediated removal. As such, FAS serves as an additional checkpoint for GC B cells failing antigen-driven positive selection in the GC.