Memory B cells dominate the early antibody-secreting cell response to  SARS-CoV-2 mRNA vaccination in naïve individuals independently of their antibody affinity
Memory B cells (MBCs) formed over the individual's lifetime constitute nearly half of the adult peripheral blood B cell repertoire in humans. To assess their response to novel antigens, we tracked the origin and followed the differentiation paths of MBCs in the early anti-S response to mRNA vaccination in SARS-CoV-2-naïve individuals on single-cell and monoclonal antibody level. Newly generated and pre-existing MBCs differed in their differentiation paths despite similar levels of SARS-CoV-2 and common corona virus S-reactivity. Pre-existing highly mutated MBCs showed no signs of germinal center re-entry and rapidly developed into mature antibody secreting cells (ASCs). In contrast, newly generated MBCs derived from naïve precursors showed strong signs of antibody affinity maturation before differentiating into ASCs. Thus, although pre-existing human MBCs have an intrinsic propensity to differentiate into ASCs, the quality of the anti-S antibody and MBC response improved through the clonal selection and affinity maturation of naïve precursors. Overall design: Blood from five healthy volunteers (V1-5) was collected directly before and one, two, and three weeks (1w, 2w, 3w) after the first (I) and second (II) vaccination with BNT162b2. Donor V5 was excluded from analysis due to rapid anti-S IgG response indicating previous SARS-CoV-2 exposure. S-reacitve B cells (S+), activated B cells (CD71+) and plasmablasts (CD27+CD38+) were isolated from PBMC samples by fluorescence-activated cell sorting (FACS) and analyzed using scRNAseq