T-cell Immunity in Infected Individuals
To explore the breadth and depth of memory T-cell immunity against SARS-CoV-2, we examined responses in 34 selected patients with documented SARS-CoV-2 infection (Figure 2A). T-cell immune responses to peptide pools of 5 major SARS-CoV-2 proteins (Spike, VME, NCAP, AP3A, and NS7A) were analyzed. In healthy control individuals, no significant CD4+ T-cell responses to the 5 SARS-CoV2 proteins were seen (Figure 2B, IL-2+TNF-α+(%) in CD4+ <0.05%, mean = 0.01%). However, 20% of healthy individuals showed heterogenous TNF-α+/IFN-γ+ CD8+ T-cell (>0.05%) responses to the 5 SARS-CoV-2 proteins, which could represent cross-reaction of CD8+ T-cells generated from previous endemic coronavirus infection (Figure 2B)9,10. Based on the background level of CD4+ T-cell response in healthy controls, we set 0.05% of dual-positive CD4+ and CD8+ T-cells as the cutoff level determining positive T-cell immunity against SARS-CoV-2. Overall, we observed 88% (30 of 34) infected patients had either positive CD4+ or CD8+ T-cell immunity to one or more of 5 CoV-2 peptides. Most patients showed positive CD4+ T-cell immunity (85%, 29 of 34), and CD4+ T-cells demonstrated immunodominant responses to Spike peptides as previously described11,12 (Figure 2A). CD8+ T-cells showed similar responses to the 5 proteins; 68% (23 of 34) had positive CoV-2 specific CD8+ T-cells to one or more of 5 CoV-2 proteins.