Characterization of a heterogenous activated B cell compartment arising early after antigen exposure preceding long-lived memory B cell formation

Once formed, plasma cells and memory B cells (MBCs) are difficult to eradicate, posing a problem in the context of unwanted antibody responses. Characterizing early B cell differentiation stages after antigen encounter is thus crucial to target and prevent unwanted antibody formation. Here, we unravelled in-depth antigen-specific B cell responses longitudinally after SARS-CoV-2 mRNA vaccination in healthy individuals using multiparameter spectral flow cytometry. The early antigen-specific B cell response was dominated by spike-specific IgG + CD27 + CD71 + activated B cells (ActBCs), previously assigned as germinal center-derived and DN2 extrafollicular B cells. Within the early IgG + ActBC compartment, six distinct clusters were identified with specific contraction dynamics, whereby some of these clusters were more closely related to pre-ASCs and others more to long-lived MBCs. Some of the highly contracting ActBC clusters expressed CD11c, a marker previously used to define atypical B cells. The transient presence of different ActBC clusters could also be observed in total B cells when gated in an antigen- independent manner. Our results thus delineate the early stages of the antigen-specific B cell response, with a further dissection of the CD71 + ActBC compartment. Detection of ActBC clusters early after antigen encounter in total B cells opens avenues for future evaluation of their potential to serve as a proxy for antigen-reactive B cells in autoimmunity or other unwanted B cell responses.