Heterogeneous Activated B Cell Compartments Arising Early and Transiently After SARS‐CoV‐2 Vaccination

In humans, the stages and dynamics of B cell development after antigen encounter remain unclear. Identifying early B cell differentiation stages could reveal biomarkers for humoral immunity and potential targets to prevent unwanted antibody responses. We characterized antigen-specific B cell responses longitudinally after SARS-CoV-2 mRNA vaccination using multiparameter spectral flow cytometry. Spike-specific IgG⁺ CD27⁺ CD71⁺ activated B cells (ActBCs), presumed to be germinal center-derived and IgG⁺ DN2 extrafollicular B cells, dominated the early antigen-specific B cell response, while memory B cells were the main population 6 months after vaccination. Within the IgG⁺ ActBC compartment, we delineated six novel clusters with specific contraction dynamics. Following the second vaccination, certain ActBC clusters displayed sustained expansion over time, being phenotypically similar to memory B cells, while others strongly expanded and subsequently contracted. Several of the rapidly contracting ActBC clusters expressed CD11c, a defining marker for atypical B cells, suggesting a possible extrafollicular origin of these clusters. The transient presence of heterogeneous ActBC clusters was also observed for total B cells when gated in an antigen-independent manner. Characterization of novel ActBC clusters early after antigen encounter helps delineate and dissect the complexity of B cell differentiation, which is vital for understanding unwanted B cell responses.