Germinal Center Selection and Plasma Cell Differentiation.

Germinal centers (GCs) are specialized microenvironments in which B cells undergo affinity maturation through iterative cycles of somatic hypermutation, proliferation and selection, generating antibodies with improved antigen-binding properties. Although GC B cells do not themselves secrete antibody, they differentiate into memory B cells and plasma cells that provide long-term humoral immunity. GC selection has traditionally been viewed as a process that preferentially expands B cells expressing the highest-affinity B cell receptors. However, recent studies have revealed that physiological GC responses preserve substantial clonal and affinity diversity, and generate plasma cells spanning a broad range of antibody affinities. Here, we review current understanding of the mechanisms governing GC B cell selection and discuss an alternative framework in which T cell help quantitatively refuels GC B cells rather than acting as a binary gate for cyclic re-entry. We further explore how antigen organization, antibody feedback and the biophysical context of antigen recognition may reconcile the apparent discrepancy between affinity-based selection and the maintenance of clonal diversity. Finally, we consider how these concepts may alter our expectations for the plasma cell populations emerging from GCs and discuss their implications for rational vaccine design.