3.2 T cell reactivity to SARS-CoV-2 in 2-year convalescents after inactivated vaccines
We also compared the SARS-CoV-2-specific T cell responses among the 2-year convalescents and healthy controls after inactivated vaccines, assessed by ELISpot and ICS assays (Figure 3A). In the ex vivoELISpot assay based on freshly-isolated PBMCs, both two groups have low-frequency antigen-specific T cell responses, with no obvious difference between 2-year convalescents and healthy controls (Figure 3B). After incubating with viral S1, S2, M, and N peptides in vitro , the positive proportion with IFN-γ responses in the population showed a noteworthy increase (Figure 3C). 92.2% (47/51) of the 2-year convalescents had positive T-cell response memory to at least one of these four antigens, which was markedly higher than the corresponding proportion (54.3%) of vaccinated healthy controls (Figure 3F and Supplementary Table 2). Meanwhile, the altitudes of T cell responses to M and N peptide pools, as well as S1 and S2, were greater in the 2-year convalescents (median 500 SFU/106 PBMCs [IQR 190-1582.5] for M; median 485 SFU/106 PBMCs [IQR 200-1487.5] for N) than in the healthy controls following vaccination (median 20 SFU/106 PBMCs [IQR 0-105] for M; median 20 SFU/106 PBMCs [IQR 0-222.5] for N) (Figure 3C). We also compared the T cell immune responses between the four peptide pools and found that proteins M and N overlapping peptide pools-specific T cell responses were much higher than S protein among the 2-year convalescents after vaccination (Figure 3C). 2-year convalescents after 2 doses immunization can produce even significantly higher IFN-γ responses than healthy controls inoculated with the 3 doses vaccination, which further underlined the significance of memory T-cell responses induced by the previous infection, and boosted by the following vaccination (Supplementary Figure 2A, B). But there were no significant differences in T cell immune responses between the 2-year convalescents with different severity during the acute infection (Figure 3D, E).
To further investigate the SARS-CoV-2-specific CD8+and CD4+ T cells in participants, we stimulated PBMCs with peptide pools of four antigens (S1, S2, M, and N) and measured cytokine production (IFN-γ, IL-2, and TNF-α) using flow cytometry (Figure 4A). Though without statistical significance, the median ratios of cytokine-secreting T cells against four peptide pools were higher in the 2-year convalescents than in healthy controls who had been vaccinated (Figure 4B, C). Furthermore, specific T cells among both groups developed stronger cellular immunity than individuals with neither infection nor vaccination (Figure 4C).