Analysis of B cell selection in the germinal center reaction during a T-dependent antibody response at a single cell level

The quasimonoclonal mouse is useful to examine B cell selection during T-dependent antibody (Ab) responses because of its limited B cell populations mainly expressing the knockin 17.2.25 VH-encoded H chain (VHT) paired with the λ1 or λ2 L chain. It has been reported that both two VHT/λ1 and VHT/λ2 B cell populations responded to a T-dependent antigen conjugated with a hapten p-nitrophenylacetyl (pNP), but only VHT/λ2 B cells differentiated to secrete high affinity anti-pNP IgG Abs by acquiring a critical mutation (T313A) in the VHT. The VHT/λ2 B cells may be more potent in migrating to the germinal centers (GCs) due to about 50-fold higher affinity for pNP than VHT/λ1 B cells. Here, to uncover how VHT/λ2 B cells were preferentially recruited for affinity maturation during the anti-pNP Ab response, we examined the L chain usage and mutation frequency of VHT+ GC B cells at a single cell level. VHT/λ2 B cells bearing the unmutated VHT gene were found in the GCs more frequently than VHT/λ1 and mutated VHT/λ2 counterparts in an early phase of the Ab response. In the course of the GC reaction, the number of VHT/λ2 B cells that mutated their VHT genes preferentially expanded, and finally VHT/λ2 B cells bearing the T313A mutation occupied VHT+ GC B cell population. Thus, it is suggested that B cells with a higher affinity were selected not only for entry to the GCs but also in the affinity maturation process during a T-dependent Ab response.