Two distinct durable human switched memory B cell populations are induced by vaccination and infection

The traditional definition of memory lymphocytes encompasses antigen-experienced, durable subsets that persist in the absence of antigen and are capable of responding to the antigen on subsequent encounters. We examined the relative durability of eight different non-overlapping class-switched human B cell sub-populations (excluding plasmablasts) that together encompass 100% of all human class switched B cells and identified only two long-lived B cell populations that persist after vaccination in the relative absence of their cognate antigen. In addition to canonical switched memory B cells (SWM) with an IgD- CD27+ CXCR5+ phenotype, a second, non-canonical but distinct memory population of IgD- CD27- CXCR5+ (double negative 1, DN1) B cells is also durable. Beyond surface phenotyping, DN1 memory B cells intriguingly exhibit lower levels of replacement mutations in complementarity determining as well as framework regions of their B cell receptors and have longer CDRH3 sequences relative to canonical SWM B cells. This non-canonical, human memory B cell subset is distinguished by a distinct TP63 driven transcriptional signature that has been linked to extended cell survival and these B cells persist in the circulation at least as long as canonical SWM B cells. Non-canonical human DN1 memory B cells therefore represent a second, distinct, durable, less somatically hypermutated, class-switched human memory B cell subset defined by a unique transcriptomic signature, that likely arises outside germinal centers and may have evolved to preserve immunological breadth.