Single-Cell Sequencing Reveals Varying Cell-Mediated Immunity Profiles in Individuals With Distinct Antibody Responses Following Ad5-nCoV Booster

BACKGROUND: Vaccination is critical for controlling infectious diseases; however, assessing efficacy solely through neutralizing antibody titers oversimplifies immune protection. The cellular and molecular mechanisms driving variable vaccine-induced immune responses remain underexplored, limiting comprehensive vaccine evaluation. METHODS: Using single-cell sequencing, we profiled and compared cellular dynamic, transcriptomic profiles, the immune repertoire, and cellular communication in vaccine recipients with high and low antibody titers after Ad5-nCoV booster vaccination, using peripheral blood mononuclear cell samples collected from a cohort of 144 participants. RESULTS: Cellular profiling revealed no significant changes in most immune cell subsets. However, high responders exhibited increased cytotoxic CD8+ lymphocytes and natural killer T cells, with reduced CD8+ mucosal-associated invariant T cells. This group showed up-regulated immune and effector genes, down-regulated proinflammatory genes, and elevated inhibitory gene expression in clonal natural killer cells, accompanied by enhanced CD8+ T-cell clonal expansion. In addition, the variable and joining (VJ) gene usage of B and T cells in the high-response group showed biased patterns, with preferential expression of IGLJ3, IGKJ3, IGHV3-33, and IGHJ4 in B cells and TRAV27, TRAJ33, TRBV9, and TRBJ2-7 in T cells. Cell communication also indicated balanced proinflammatory and anti-inflammatory cytokine expression in high responders. CONCLUSIONS: These findings elucidate the cellular and molecular mechanisms underlying robust immune responses to the Ad5-nCoV booster, highlighting the critical role of cellular immunity. By integrating humoral and cellular insights, this study offers a new perspective of comprehensive framework for evaluating vaccine efficacy.