ABSTRACT: SARS-CoV-2 mRNA vaccines, including Pfizer/Biontech BNT162b2, were shown to be effective for COVID-19 prevention, eliciting both robust antibody responses in naive individuals and boosting pre-existing antibody levels in SARS-CoV-2-recovered individuals. However, the magnitude, repertoire, and phenotype of epitope-specific T cell responses to this vaccine, and the effect of vaccination on pre-existing T cell memory in SARS-CoV-2 convalescent patients, are still poorly understood. Thus, in this study we compared epitope-specific T cells elicited after natural SARS-CoV-2 infection, and vaccination of both naive and recovered individuals. We collected peripheral blood mononuclear cells before and after BNT162b2 vaccination and used pools of 18 DNA-barcoded MHC-class I multimers, combined with scRNAseq and scTCRseq, to characterize T cell responses to several immunodominant epitopes, including a spike-derived epitope cross-reactive to common cold coronaviruses. Comparing responses after infection or vaccination, we found that T cells responding to spike-derived epitopes show similar magnitudes of response, memory phenotypes, TCR repertoire diversity, and αβTCR sequence motifs, demonstrating the potency of this vaccination platform. Importantly, in COVID-19-recovered individuals receiving the vaccine, pre-existing spike-specific memory cells showed both clonal expansion and a phenotypic shift towards more differentiated CCR7-CD45RA+ effector cells. In-depth analysis of T cell receptor repertoires demonstrates that both vaccination and infection elicit largely identical repertoires as measured by dominant TCR motifs and receptor breadth, indicating that BNT162b2 vaccination largely recapitulates T cell generation by infection for all critical parameters. Thus, BNT162b2 vaccination elicits potent spike-specific T cell responses in naive individuals and also triggers the recall T cell response in previously infected individuals, further boosting spike-specific responses but altering their differentiation state. Overall, our study demonstrates the potential of mRNA vaccines to induce, maintain, and shape T cell memory through vaccination and revaccination.
