The Limit of MHC Class II-driven Selection in Germinal Centers

Abstract

Abstract B cells undergo iterative rounds of proliferation, hypermutation and selection in the germinal centers (GCs) to generate high affinity, protective antibody. It is now thought that GC B cells with higher affinity BCRs acquire more antigen for processing and presenting in association with MHC class II (MHCII) to T follicular helper (TFH) cells than lower affinity competitors. The resulting increase in T-cell help is hypothesized to be the major factor that drives affinity maturation. In this model for GC B cell selection, the density of peptide/MHCII (pMHCII) complexes available on the B-cell membrane is the major determinant of affinity-dependent selection. To quantify the limits of this model for GC selection, we generated short- and long-term chimeric mice with equal numbers of isogenic, mature B cells that differed by their capacity to express MHCII (MHCII+/+ and MHCII+/−). Separately, both B cell types generated virtually identical humoral responses, including affinity maturation, in response to NP-OVA. In competition, MHCII+/+ B cells were preferentially recruited to early GCs consistent with a critical role for pMHCII density in T:B collaboration. However, once established in GCs, no further competitive advantage was observed during the full course (24 days) of the GC response. During this period, both MHCII+/+ and MHCII+/− GC B cells comparably accumulated VH gene segment mutations and exhibited comparable rates of affinity maturation to the NP hapten. These data indicate that B-cell selection determined by MHCII density is more stringent prior to the establishment of GCs than in the GC itself. This observation may be relevant to our recent observation of the persistence of low affinity GC B cells.