De novo identification of the specificities of recurrent human T cell receptors.

T cell repertoires of different individuals occasionally converge on the same T cell receptor (TCR) sequence as a solution to target immunodominant epitopes. A complete mapping of these "public" TCR specificities may enable a global understanding of population-level immune histories. Here, we sought to determine the antigen specificities of public TCRs with unknown target identity. We developed a functional screening workflow in which we screen panels of TCRs for reactivity to individual viral genomes or to approximately 1,000 viral reference strains, and then sort out the immunogenic peptides by labeling antigen-presenting cells that are in proximity to activated T cells. Using this workflow, we identified the target specificities of T cells that are circulating in up to 14% of individuals, including a pre-COVID-19 seasonal coronavirus-reactive TCR that cross-reacts with peptides within pandemic coronaviruses; an influenza B-reactive TCR that targets a highly conserved epitope; and TCRs targeting Herpesviridae family viruses that cause long-term latent infections. Our results demonstrate an efficient strategy to reveal public T cell memories de novo, offering a window into shared immune exposures.