Cross-reactive and mono-reactive SARS-CoV-2 CD4+ T cells in prepandemic and COVID-19 convalescent individuals.

Class II tetramer reagents for eleven common DR alleles and a DP allele prevalent in the world population were used to identify SARS-CoV-2 CD4+ T cell epitopes. A total of 112, 28 and 42 epitopes specific for Spike, Membrane and Nucleocapsid, respectively, with defined HLA-restriction were identified. Direct ex vivo staining of PBMC with tetramer reagents was used to define immunodominant and subdominant T cell epitopes and estimate the frequencies of these T cells in SARS-CoV-2 exposed and naïve individuals. Majority of SARS-CoV-2 epitopes identified have <67% amino acid sequence identity with endemic coronaviruses and are unlikely to elicit high avidity cross-reactive T cell responses. Four SARS-CoV-2 Spike reactive epitopes, including a DPB1*04:01 restricted epitope, with ≥67% amino acid sequence identity to endemic coronavirus were identified. SARS-CoV-2 T cell lines for three of these epitopes elicited cross-reactive T cell responses to endemic cold viruses. An endemic coronavirus Spike T cell line showed cross-reactivity to the fourth SARS-CoV-2 epitope. Three of the Spike cross-reactive epitopes were subdominant epitopes, while the DPB1*04:01 restricted epitope was a dominant epitope. Frequency analyses showed Spike cross-reactive T cells as detected by tetramers were present at relatively low frequency in unexposed people and only contributed a small proportion of the overall Spike-specific CD4+ T cells in COVID-19 convalescent individuals. In total, these results suggested a very limited number of SARS-CoV-2 T cells as detected by tetramers are capable of recognizing ccCoV with relative high avidity and vice versa. The potentially supportive role of these high avidity cross-reactive T cells in protective immunity against SARS-CoV-2 needs further studies. Author summary: Previous studies with activation induced marker assays in monitoring antigen-specific CD4+ T cells have shown that common cold coronavirus T cells can cross-react with SARS-CoV-2 antigens and these cross-reactive T cells are present in up to 60% of the unexposed population. In this current study, sets of overlapping peptides for Spike, Membrane, and Nucleocapsid proteins were used to identify epitopes across 11 HLA-DR and 1 HLA-DP alleles in SARS-CoV-2 convalescent samples using peptide-loaded MHC-II tetramers. Using these tetramers ex vivo, frequencies of these epitope-specific T cells were estimated in convalescent and pre-pandemic samples. Based on these frequencies, epitopes were stratified into immunodominant and subdominant epitopes. Amino acid sequences of epitopes identified were compared with 4 common cold coronaviruses. Potential cross-reactive epitopes were defined as having ≥67% sequence identity between common cold viruses and SARS-CoV-2. Four potential Spike specific cross-reactive epitopes were identified and functional cross-reactivity was demonstrated. Of the four cross-reactive epitopes identified, three were subdominant epitopes eliciting relatively low frequencies in both unexposed and convalescent subjects. In contrast to the results from the activation induced marker assays, the current data suggests that only a limited number of high avidity SARS-CoV-2 T cells as detected by tetramers are cross-reactive. [ABSTRACT FROM AUTHOR]