Competition for refuelling rather than cyclic re-entry initiation evident in germinal centers

Antibody affinity maturation occurs in germinal centers (GCs) through iterative rounds of somatic hypermutation and proliferation in dark zones (DZs) and selection in light zones (LZs). GC B cells exit cell cycle a number of hours before entering LZs, therefore continued participation in responses requires they subsequently re-enter cell cycle and move back to DZs, a process known as cyclic re-entry. Affinity enhancements are thought to arise by B cells having to compete to initiate cyclic re-entry each time they enter LZs, with T cell help being determining, however direct proof is lacking. Using Fucci2 mice, we confirmed an association between B cell receptor affinity and the first step of cyclic re-entry, S phase initiation from a resting LZ state. However, neither T cell ablation nor MHCII deletion prevented resting LZ cells from re-entering cell cycle, and this late G1-S transition was also not detectably restricted by competition. In contrast, using BATF induction as exemplar, we found that T cells “refuelled” LZ cells in an affinity-dependent manner that was limited both by competition and cells’ intrinsic antigen-acquiring abilities. Therefore, cyclic re-entry initiation and B cell refuelling are independently regulated in GCs, which may contribute to permitting cells of different competencies to be sustained alongside each other and allow T cell support to be provided across a dynamic range commensurate with affinity. We speculate that this less binary selection mechanism could help GCs nurture complex antibody maturation pathways and support the clonal diversity required for countering fast evolving pathogens.