Abstract
Background: Elderly individuals represent a population at disproportionate risk for severe outcomes following SARS-CoV-2 infection. The humoral immune response critically controls viral replication and disease progression. However, the antibody responses and B-cell subsets to mutant strains of SARS-CoV-2 in elderly patients have not yet been fully elucidated. This study aims to elucidate the humoral immune response and B-cell subsets distribution in elderly patients infected with SARS-CoV-2 Omicron variant, thereby providing insights for identifying prognostic biomarkers and developing therapeutic strategies. Methods: Using enzyme-linked immunosorbent assays and pseudotyped virus neutralization assays, we determined plasma levels of RBD-specific IgA, IgM, IgG, and neutralizing antibodies from 46 elderly patients with SARS-CoV-2 Omicron variant infection within the first two weeks post-symptom onset (PSO). Using a multicolor flow cytometry approach, we analyze the frequencies of different B-cell subsets and assess the functional characterization of B-cells. Result: In elderly non-severe patients, SARS-CoV-2 RBD-specific antibody levels (IgA, IgM, and IgG) increased progressively within the first two weeks post-symptom onset (PSO). In contrast, severe patients exhibited lower initial antibody levels during 0-3 days PSO but experienced a transient surge (11-fold for IgA, 12-fold for IgM, and 48-fold for IgG) during 4-7 days PSO, which was followed by a decline between 8-14 days. Despite this early elevated response in severe patients, both groups ultimately demonstrated generally weak neutralizing activity against the SARS-CoV-2 Omicron variant. Both elderly patient groups exhibited an higher proportion of plasmablasts (PB). Among these, class-switched IgM-IgG+ PB were significantly more abundant than IgM+IgG-PB. Notably, elderly severe patients showed a further lower in class-unswitched IgM+IgG-PB. Concurrently, IgA expression on PB was upregulated during early disease in all elderly patients. Beyond PB changes, IgG+double negative B (DNB) cells were higher than IgM+DNB cells in both groups. however, severe patients demonstrated a significant reduction in IgG+DNB cells frequencies. Furthermore, these elderly severe patients also exhibited a decline in co-stimulatory molecule expression (HLA-DR+CD80+) within both naive B (NB) and DNB cells, indicating a dysregulated humoral immune response. Conclusion: Elderly individuals generate RBD-specific and neutralizing antibody responses after SARS-CoV-2 Omicron variant infection, which correlate significantly with disease severity and infection duration. Clinically, these findings highlight the potential utility of antibody kinetics as prognostic biomarkers for stratifying elderly patients at high risk of severe outcomes, thereby informing tailored intervention strategies and vaccine booster optimization. Furthermore, SARS-CoV-2 Omicron variant infection affects the distribution of B-cell subsets, which may predict the long-term immune reconstitution capacity post-recovery. Notably, elderly patients exhibit pronounced PB expansion, a response pattern that facilitates antibody class-switching to enhance antiviral immunity. Such expansion represents a key immunologic feature in elderly patients with SARS-CoV-2 Omicron variant infection.