Antigen-Specific B Cell Memory: Expression and Replenishment of a Novel B220⁻ 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⁻CD138⁻) that are distinct from antibody-secreting B cells (B220^+/−CD138⁺) and B220⁺CD138⁻ memory B cells. These nonsecreting somatically mutated B220⁻ 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⁻ phenotype. Upon adoptive transfer, B220⁻ memory B cells proliferate to a lesser degree but produce greater amounts of antibody than their B220⁺ counterparts. The pattern of cellular differentiation after transfer indicates that B220⁻ memory B cells act as stable self-replenishing intermediates that arise from B220⁺ memory B cells and produce antibody-secreting cells on rechallenge with antigen. Cell surface phenotype and Ig isotype expression divide the B220⁻ 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.