­­Rapid TCR:Epitope Ranker (RAPTER): A primary human T cell reactivity screening assay pairing epitope and TCR at single cell resolution
Identifying epitopes that T cells respond to is critical to understanding T cell-mediated immunity. Traditional multimer and other single cell assays often require large blood volumes and/or expensive HLA-specific reagents and provide limited phenotypic and functional information. Here, we present the Rapid TCR:Epitope Ranker (RAPTER) assay, a single cell RNA sequencing (scRNA-SEQ) method that uses primary human T cells and antigen presenting cells (APCs) to assess functional T cell reactivity at single cell resolution. Using hash-tag oligonucleotide (HTO) coding and T cell activation-induced markers (AIM), RAPTER defines paired epitope specificity and TCR sequence and can include RNA- and protein-level T cell phenotype information.  We demonstrate that RAPTER identified specific reactivities to viral and tumor antigens at sensitivities as low as 0.15% of total CD8+ T cells, and deconvoluted low-frequency circulating HPV16-specific T cell clones from a cervical cancer patient. The specificities of TCRs identified by RAPTER for MART1, EBV, and influenza epitopes were functionally confirmed in vitro. In summary, RAPTER identifies low-frequency T cell reactivities using primary cells from low blood volumes, and the resulting paired TCR: ligand information can directly enable immunogenic antigen selection for vaccine epitope inclusion, antigen-specific TCR tracking, and TCR cloning for further therapeutic development. Overall design: Human PBMCs were put through the RAPTER assay described in the manuscript then measured via scRNAseq