Project 2: Affinity Maturation of the B-cell Repertoire

Abstract Naive B cells expressing low affinity antigen-receptors (BCRs) are activated by antigens (Ags), and enter germinal centers (GCs) where hypermutation of BCR gene rearrangements (SHM) drives selection for higher affinity mutants (affinity maturation). Specialized T cells, TFH, also enter GCs and are the agents of high-affinity selection. Inferences regarding the fates of GC B and T cells after influenza immunization or infection have been largely indirect. We will follow the fates of GC T and B cells directly by immunizing or infecting S1pr2CreERT2-RFPLSL mice. In this strain, GC B and T cells (and their progeny) can be labeled by giving mice Tamoxifen. This conditional marking will allow us to follow the fate decisions of GC B cells: whether they become antibody (Ab)-secreting plasmacytes (PCs) or quiescent memory B cells (Bmem). On re-exposure – boost immunizations or secondary infections – we can then follow the roles of primary post- GC B and T cells (RFP+) or newly recruited cells (RFP-) in the secondary responses. This analysis will be important for understanding “imprinting” by influenza HA and NA Ags, the persistence of Bmem cells at local and distal sites, and the persistence of long-lived PC in the bone marrow. The results of this research will also be important for understanding the evolutionary “arms race” between host immunity and mutating virus -- a major issue for vaccine development and for understanding the fundamental biology of humoral responses to variant pathogens.