Complex Antigens Drive Permissive Clonal Selection in Germinal Centers

Highlights

• Developed a single B cell culture to characterize the mouse B cell repertoire

• Affinity maturation of GC B cells is accompanied by increased V_H representation

• GCs permit a wide range of intra- and interclonal BCR affinities

• Substantial fractions of GC B cells do not detectably bind native antigen

Summary

Germinal center (GC) B cells evolve toward increased affinity by a Darwinian process that has been studied primarily in genetically restricted, hapten-specific responses. We explored the population dynamics of genetically diverse GC responses to two complex antigens—Bacillus anthracis protective antigen and influenza hemagglutinin—in which B cells competed both intra- and interclonally for distinct epitopes. Preferred V_H rearrangements among antigen-binding, naive B cells were similarly abundant in early GCs but, unlike responses to haptens, clonal diversity increased in GC B cells as early “winners” were replaced by rarer, high-affinity clones. Despite affinity maturation, inter- and intraclonal avidities varied greatly, and half of GC B cells did not bind the immunogen but nonetheless exhibited biased V_H use, V(D)J mutation, and clonal expansion comparable to antigen-binding cells. GC reactions to complex antigens permit a range of specificities and affinities, with potential advantages for broad protection.