Integrated Assessment of Humoral and Cellular SARS-CoV-2 Immunity Using ELISA and a Gold Nanoparticle-Based IGRA

Although SARS-CoV-2 immunity has been widely investigated, the relationship between antibody levels and cellular immune responses following different routes of exposure to SARS-CoV-2 remains not fully revealed. In this study, cellular immunity was assessed using a newly developed whole-blood interferon-gamma release assay (nano-IGRA) based on gold nanoparticle (AuNP)-conjugated SARS-CoV-2 antigens. AuNP-conjugated and soluble antigens were compared regarding interferon gamma (IFN-γ) responses and intra- and interassay reproducibility in 112 individuals with immunity acquired through infection, vaccination, or both. Humoral immunity was evaluated by enzyme-linked immunosorbent assay (ELISA). Overall, 88% of participants were seropositive. AuNP-conjugated Spike (S) and Nucleocapsid (N) SARS-CoV-2 antigens induced significantly higher IFN-γ responses than soluble antigens in all exposure groups (p < 0.001). AuNP conjugation reduced intraassay variability by 62.4% and 61.4% for S and N, respectively, and interassay variability by 71.7% and 68.3%, respectively. Antibody levels correlated significantly with IFN-γ responses to S (ρ = 0.358, p < 0.001) and N (ρ = 0.309, p < 0.001). An interesting finding is that cellular responses were also detected in some seronegative individuals. Nano-IGRA, based on a novel AuNP-based approach, provides antigen-specific cellular immune assessment, combining enhanced IFN-γ responses with improved reproducibility. Further studies should establish diagnostic cut-offs, sensitivity, specificity, and clinical performance.