Deep profiling of antigen-specific B cells from different pathogens identifies novel compartments in the IgG memory B cell and antibody-secreting cell lineages

A better understanding of the bifurcation of human B cell differentiation into memory B cells (MBC) and antibody-secreting cells (ASC) and identification of MBC and ASC precursors is crucial to optimize vaccination strategies or block undesired antibody responses. To unravel the dynamics of antigen-induced B cell responses, we compared circulating B cells reactive to SARS-CoV-2 (Spike, RBD and Nucleocapsid) in COVID-19 convalescent individuals to B cells specific to Influenza-HA, RSV-F and TT, induced much longer ago. High-dimensional spectral flow cytometry indicated that the decision point between ASC- and MBC-formation lies in the CD43+CD71+IgG+ Activated B cell compartment, showing properties indicative of recent germinal center activity and recent antigen encounter. Within this Activated B cells compartment, CD86+ B cells exhibited close phenotypical similarity with ASC, while CD86− B cells were closely related to IgG+ MBCs. Additionally, different activation stages of the IgG+ MBC compartment could be further elucidated. The expression of CD73 and CD24, regulators of survival and cellular metabolic quiescence, discerned activated MBCs from resting MBCs. Activated MBCs (CD73- CD24lo) exhibited phenotypical similarities with CD86− IgG+ Activated B cells and were restricted to SARS-CoV-2 specificities, contrasting with the resting MBC compartment (CD73-/CD24hi) that exclusively encompassed antigen-specific B cells established long ago. Overall, these findings identify novel stages for IgG+ MBC and ASC formation and bring us closer in defining the decision point for MBC or ASC differentiation. ### Competing Interest Statement The authors have declared no competing interest.