Cellular basis of enhanced humoral immunity to SARS-CoV-2 upon homologous or heterologous booster vaccination analyzed by single-cell immune profiling

SARS-CoV-2 vaccine booster dose can induce a robust humoral immune response, however, its cellular mechanisms remain elusive. Here, we investigated the durability of antibody responses and single-cell immune profiles following booster dose immunization, longitudinally over 6 months, in recipients of a homologous BBIBP-CorV/BBIBP-CorV or a heterologous BBIBP-CorV/ZF2001 regimen. The production of neutralizing antibodies was dramatically enhanced by both booster regimens, and the antibodies could last at least six months. The heterologous booster induced a faster and more robust plasmablast response, characterized by activation of plasma cells than the homologous booster. The response was attributed to recall of memory B cells and the de novo activation of B cells. Expanded B cell clones upon booster dose vaccination could persist for months, and their B cell receptors displayed accumulated mutations. The production of antibody was positively correlated with antigen presentation by conventional dendritic cells (cDCs), which provides support for B cell maturation through activation and development of follicular helper T (Tfh) cells. The proper activation of cDC/Tfh/B cells was likely fueled by active energy metabolism, and glutaminolysis might also play a general role in promoting humoral immunity. Our study unveils the cellular mechanisms of booster-induced memory/adaptive humoral immunity and suggests potential strategies to optimize vaccine efficacy and durability in future iterations.