Quantification of SARS-CoV-2 RBD IgG level and identification of SARS-CoV-2 reactive T cells in recovered children and adolescents
We detected the presence of SARS-CoV-2 RBD IgG antibodies in 30/31 recovered COVID-19 patients compared with the 20 healthy unexposed cases (p<0.001), with 1 patient showed negative in the RBD IgG antibodies test (Fig. 1A). Stimulation of CD4+ and CD8+ T cells with the mixed SARS-CoV-2 peptide pool (S + M + N peptides, representing the reactive epitopes of the SARS-CoV-2 virus) showed significantly higher numbers of CD69+, IFN-γ+, and double-positive CD69+/IFN-γ+ T cells in recovered patients compared with controls. In agreement with this observation, we also found significantly higher numbers of CD4+ and CD8+ T cells responding to stimulations by M, N and S peptide pools, with exception of CD8+CD69+IFN-γ+ subsets that showed statistically marginal differences. These results reveal a strong type 1 T cell memory response against the SARS-CoV-2 virus (Fig. 1C).
Next, the reactivity of the CD4+ and CD8+ T cells towards individual M, N, and S peptide pools were analyzed in convalescent patients. (Fig.2) Both CD4+ (Fig.2 left panel) and CD8+ (Fig.2 right panel) reactivity was readily detectable in all patients towards each structural protein; however, the CD4+ T cells responded more strongly to stimulation by S peptide than to N (Cohen’s d=0.53) or M peptides (Cohen’s d=0.34). On the other hand, CD8+ T cells responded less strongly to stimulation by M peptides compared with N peptides (Cohen’s d=-0.36) or S peptide (Cohen’s d=-0.23), where the difference in CD8+ T cell responses between S and N peptides was small (Cohen’s d=0.10).