Dynamic regulation of somatic hypermutation in germinal centers enables rapid B cell clonal expansion without loss of affinity

In the course of antibody affinity maturation, germinal center (GC) B cells mutate their immunoglobulin (Ig) heavy and light chain genes in a process known as somatic hypermutation (SHM). Panels of mutant B cells with different binding affinities for antigen are then selected in Darwinian fashion, leading to a progressive increase in affinity among the population. As with any Darwinian process, rare gain-of-fitness mutations must be identified while common loss-of-fitness mutations are avoided. Progressive acquisition of mutations therefore poses a risk during large proliferative bursts, when GC B cells go through several cell cycles in the absence of affinity-based selection. Using a combination of in vivo mouse experiments and mathematical modeling, we show that GCs achieve this balance by strongly suppressing SHM during clonal burst-type expansion, so that a large fraction of the progeny generated during such bursts do not deviate from their ancestral genotype. Intravital imaging and image-based cell sorting of a new CyclinE(A)/cyclin-dependent kinase 2 (CDK2) reporter mouse strain showed that B cells actively undergoing proliferative bursts lack the transient CDK2low “G0-like” phase of the cell cycle in which SHM takes place. We propose a model in which inertially cycling B cells largely delay SHM until the G0-like phase that follows their final round of division in the DZ, thus maintaining affinity as they clonally expand in the absence of selection.