Peptide-driven identification of TCRs reveals dynamics and phenotypes of CD4 T cells in tuberculosis.

Assigning antigen specificity to T cell receptor (TCR) sequences is challenging due to the TCR repertoire's diversity and the complexity of TCR-antigen recognition. We developed the peptide-driven identification of TCRs (PDI-TCR) assay that combines in vitro expansion of cells with peptide pools, bulk TCR sequencing, and statistical analysis to identify antigen-specific TCRs from human blood. A key feature of PDI-TCR is the ability to distinguish true antigen-specific TCR clonotypes from TCRs associated with unspecific bystander activation by comparing responses to nonoverlapping peptide pools. We applied PDI-TCR to tuberculosis (TB) patients, sampling blood at diagnosis and throughout treatment, and Mycobacterium tuberculosis (Mtb)-sensitized healthy individuals (IGRA+). We identified hundreds of Mtb-specific TCRs, as well as unspecific TCRs, and characterized their phenotype in each cohort by single-cell RNA sequencing ex vivo. Mtb-specific T cells were highly diverse, with short-lived effector phenotypes only present in TB at diagnosis, while memory phenotypes were maintained through treatment. In contrast, unspecific expanded T cells were more clonally restricted, had a cytotoxic phenotype, and were maintained throughout treatment. While the PDI-TCR parameters used in this study are specific to Mtb, the underlying approach is broadly applicable to the study of antigen-specific T cells and can be adapted as needed for other antigen systems. Thus, PDI-TCR is a powerful tool for identifying antigen-specific TCRs and enables direct ex vivo identification and monitoring of antigen-specific T cells.