Germinal center entry not selection of B cells is controlled by peptide-MHCII complex density

B cells expressing high affinity antigen receptors are advantaged in germinal centers (GC), perhaps by increased acquisition of antigen for presentation to follicular helper T cells and improved T-cell help. In this model for affinity-dependent selection, the density of peptide/MHCII (pMHCII) complexes on GC B cells is the primary determinant of selection. Here we show in chimeric mice populated by B cells differing only in their capacity to express MHCII (MHCII +/+ and MHCII +/− ) that GC selection is insensitive to halving pMHCII density. Alone, both B cell types generate identical humoral responses; in competition, MHCII +/+ B cells are preferentially recruited to early GCs but this advantage does not persist once GCs are established. During GC responses, competing MHCII +/+ and MHCII +/− GC B cells comparably accumulate mutations and have indistinguishable rates of affinity maturation. We conclude that B-cell selection by pMHCII density is stringent in the establishment of GCs, but relaxed during GC responses. It has been thought that germinal center (GC) B cells with the highest density of peptide-MHCII molecules receive the most T cell help. Here the authors use heterozygous MHCII inactivation to instead show that after recruitment into the GCs, BCR affinity increases over time independent of the MHC density.