Non-classical CD45RBlo memory B-cells are the majority of circulating antigen-specific B-cells following mRNA vaccination and COVID-19 infection.

Resting memory B-cells can be divided into classical and non-classical groups based on differential expression of markers such as CD27 and CD11c, while activated memory B-cells express a combination of markers, making their ontogeny hard to determine. Here by longitudinal analysis of COVID-19, bacterial sepsis, and BNT162b2 mRNA vaccine recipients by mass cytometry and CITE-seq we describe a three-branch structure of resting B-cell memory consisting of “classical” CD45RB⁺ memory and two branches of CD45RB^lo memory further defined by expression of CD23 and CD11c respectively. Stable differences in CD45RB upon activation allowed tracking of activated B-cells and plasmablasts derived from CD45RB⁺ classical and CD45RB^lo non-classical memory B-cells. In both COVID-19 patients and mRNA vaccination, CD45RB^lo B-cells formed the majority of SARS-CoV2 specific memory B-cells while CD45RB⁺ memory was most strongly activated by bacterial Sepsis. These results suggest that diverse non-classical CD45RB^lo memory B-cells consisting of branches of CD11c⁺Tbet⁺ and CD23⁺ fractions form a critical part of responses to viral infection and vaccination.