Antigen - specific B Cell Memory : Expression and Replenishment of a Novel B 220 2 Memory B Cell Compartment

The mechanisms that regulate B cell memory and the rapid recall response to antigen remain poorly defined. This study focuses on the rapid expression of B cell memory upon antigen recall in vivo, and the replenishment of quiescent B cell memory that follows. Based on expression of CD138 and B220, we reveal a unique and major subtype of antigen-specific memory B cells (B220 2 CD138 2 ) that are distinct from antibody-secreting B cells (B220 1 / 2 CD138 1 ) and B220 1 CD138 2 memory B cells. These nonsecreting somatically mutated B220 2 memory responders rapidly dominate the splenic response and comprise . 95% of antigen-specific memory B cells that migrate to the bone marrow. By day 42 after recall, the predominant quiescent memory B cell population in the spleen (75–85%) and the bone marrow ( . 95%) expresses the B220 2 phenotype. Upon adoptive transfer, B220 2 memory B cells proliferate to a lesser degree but produce greater amounts of antibody than their B220 1 counterparts. The pattern of cellular differentiation after transfer indicates that B220 2 memory B cells act as stable self-replenishing intermediates that arise from B220 1 memory B cells and produce antibody-secreting cells on rechallenge with antigen. Cell surface phenotype and Ig isotype expression divide the B220 2 compartment into two main subsets with distinct patterns of integrin and coreceptor expression. Thus, we identify new cellular components of B cell memory and propose a model for long-term protective immunity that is regulated by a complex balance of committed memory B cells with subspecialized immune function.