Marginal zone B cells mediate humoral immunity in vaccine-induced versus naturally acquired immunity against SARS-CoV-2

The role of marginal zone B (MZB) cells in SARS-CoV-2 immunity remains unclear, particularly in comparing vaccine-induced responses to natural infection. MZB cells are crucial for rapid antibody responses, but their contributions to vaccine-induced immunity, especially in the elderly, are not fully understood. MZB-deficient (FcµR-/-) mice were immunized with SARS-CoV-2 spike protein-conjugated virus-like particles (VLPs). B cell receptor repertoire (BCR) profiles and antibody levels were assessed using ELISA, flow cytometry and single-cell sequencing. Human peripheral blood samples were collected from vaccinated individuals and unvaccinated elderly patients with Omicron infection. Vaccinated blood samples were collected at four time points: Pre-vaccine, Post-2nd dose (7 days), 6 months post-2nd, Post-booster (7 days). Infected patient samples were collected during acute and recovery phases. All samples were analyzed using hypersensitive chemiluminescence immunoassays, protein microarrays and flow cytometry. MZB cell deficiency in mice reduced splenic BCR repertoire diversity. In vaccinated individuals, total SARS-CoV-2 antibodies, including IgG, IgM and RBD-ACE2 competitive neutralization surrogate response peaked at Post-2nd dose (7 days), declined at Post-2nd dose (7 days), and increased significantly at Post-booster (7 days). Protein microarray analysis confirmed vaccine-induced antibodies targeting RBD and spike proteins. Significant expansion was observed in the percentage of MZB cells (CD21+CD23-) and follicular helper T cells (Tfh, ICOS+CXCR5+) at Post-booster (7 days), accompanied by a decrease in the percentage of follicular B cells (FoB, CD21-CD23+) in PBMCs. In contrast, unvaccinated elderly Omicron-infected individuals showed increased MZB cells during recovery compared to acute infection. MZB cells contribute to humoral immunity after both SARS-CoV-2 vaccination and natural infection. The delayed immune response in elderly individuals with natural infection underscores the importance of prioritizing vaccination for timely protection in this vulnerable group.