Frequency and phenotype of SARS-CoV-2-specific CD8⁺ T cells in healthy and convalescent COVID-19 individuals

COVID-19, an infectious disease caused by SARS-CoV-2, has caused the current global pandemic. Despite the number of infections nearing 100 million people worldwide, the CD8⁺ T cells against SARS-CoV-2 in healthy adults as well as in convalescent COVID-19 patients are not fully characterized. Here, we report an analysis of the frequency and phenotype of circulating CD8⁺ T cells recognizing seventeen epitopes from three SARS-CoV-2 proteins (S, N, and M) in HLA-A2+ healthy adults and COVID-19 convalescent patients by multi-color flow cytometry with antigen-specific tetramers. Through the analysis of 65 healthy adults ranging between 17 to 98 years of age, we found that the frequencies of CD8⁺ T cells recognizing SARS-CoV-2 ranged from 10⁻² to undetectable. We did not find a generalized age-related change in the frequency of SARS-CoV-2-recognizing CD8⁺ T cells. Interestingly, we also did not find increased frequencies of SARS-CoV-2-recognizing CD8⁺ T cells from convalescent patients compared to the healthy controls. Phenotypical analysis showed that SARS-CoV-2-recognizing CD8⁺ T cells had a mixed naive and memory phenotype in healthy adults whereas convalescent patients mainly had a memory phenotype. Currently, we are identifying the TCRs of these SARS-CoV-2-recognizing CD8⁺ T cells. Taken together, our findings have identified the frequency and phenotype of CD8⁺ T cells which recognize SARS-CoV-2-specific epitopes in humans, and future TCR research will further reveal the binding specificity and functionality of these SARS-CoV-2-specific CD8⁺ T cells.