Detection of SARS-CoV-2-Specific Humoral and Cellular Immunity in COVID-19 Convalescent Individuals

Abstract

The World Health Organization has declared SARS-CoV-2 virus outbreak a worldwide pandemic. However, there is very limited understanding on the immune responses, especially adaptive immune responses to SARS-CoV-2 infection. Here, we collected blood from COVID-19 patients who have recently become virus-free, and therefore were discharged, and detected SARS-CoV-2-specific humoral and cellular immunity in eight newly discharged patients. Follow-up analysis on another cohort of six patients 2 weeks post discharge also revealed high titers of immunoglobulin G (IgG) antibodies. In all 14 patients tested, 13 displayed serum-neutralizing activities in a pseudotype entry assay. Notably, there was a strong correlation between neutralization antibody titers and the numbers of virus-specific T cells. Our work provides a basis for further analysis of protective immunity to SARS-CoV-2, and understanding the pathogenesis of COVID-19, especially in the severe cases. It also has implications in developing an effective vaccine to SARS-CoV-2 infection.

Keywords: COVID-19 patients; SARS-CoV-2; SARS-CoV-2-specific T cells; SARS-CoV-2-specific antibody; adaptive immunity.

Figure 1

SARS-CoV-2 NP- and S-RBD-Specific Antibodies in COVID-19 Convalescent Individuals (A) Titration of individual serum samples. (B) Serological responses of 14 COVID-19 patients to recombinant NP (top) and S-RBD (bottom). Dilution of 1:50 was used for IgM and 1:450 for IgG. (C) Data from the same experiments with (A) were presented as AUC. (D) IgG isotypes of 14 COVID-19 patients to recombinant NP and S-RBD. NP, nucleocapsid protein; S-RBD, receptor binding domain of spike protein; HD, healthy donor; Pt, patient; AUC, area under curve. The experiment was performed in duplicates. Date are presented as mean ± SEM. For HD 1–3, the sera were collected in 2018. For HD 4–6, the sera were from close contacts and collected in 2020. ^∗p < 0.05, 0.05 < ^∗∗p < 0.001, ^∗∗∗p < 0.001.

Figure 2

Measurement of Neutralizing Antibody Titers in COVID-19 Convalescent Individuals (A) Correlation analysis of neutralizing antibody titers in COV1D-19 patients measured by pseudovirus and live SARS-CoV-2 (n = 20). (B) Neutralizing curves of 14 COVID-19 patients measured by pseudovirus-based assay. The experiment with patients was performed in triplicates. The experiment with healthy donors was performed in duplicates. (C) Measurement of neutralizing antibody titers of 14 COVID-19 patients by pseudovirus-based assay. (D) Correlation between NAT50 and AUC of anti-S-RBD (left) and anti-NP (right) IgG (n = 14). HD, healthy donor; Pt, patient; AUC, area under curve; NAT50, neutralizing antibody titers. Date are presented as mean ± SEM. ^∗p < 0.05, 0.05 < ^∗∗p < 0.001, ^∗∗∗p < 0.001.

Figure 3

T cell Responses to Recombinant SARS-CoV-2 Proteins in COVID-19 Convalescent Individuals (A) Phenotypic analysis of PBMCs from representative COVID-19 patients. (B) Summarized data on the frequencies of different immune cell subsets in COVID-19 patients. HD, healthy donors (n = 2); D-Pt, discharged patients (n = 3); F-Pt, follow-up patients (n = 5). (C) IFN-γ ELISpot analysis of COVID-19 patients to recombinant proteins. The experiments were performed in duplicates. (D) Correlation analysis of the NAT50 and the numbers of NP-specific T cells (n = 14). M protease, main protease; NP, nucleocapsid protein; S-RBD, receptor binding domain of spike protein; NAT50, neutralizing antibody titers. Date are presented as mean ± SEM.