T cell receptor (TCR) avidity is assumed to be a major determinant of the spatiotemporal fate and protective capacity of tumor-specific T cells. However, monitoring polyclonal T cell responses with known TCR avidities in vivo over space and time remains challenging. Here, we investigated the fate and functionality of tumor neoantigen–specific T cells with TCRs of distinct avidities in a well-established, reductionist preclinical tumor model and human patients with melanoma. To this end, we used polyclonal T cell transfers with in-depth characterized TCRs together with flow cytometric phenotyping in mice inoculated with MC38 OVA tumors. Transfer of T cells from retrogenic mice harboring TCRs with high avidity resulted in best tumor protection. Unexpectedly, we found that both high- and low-avidity T cells are similarly abundant within the tumor and adopt concordant phenotypic signs of exhaustion. Outside the tumor, high-avidity TCR T cells were not generally overrepresented but, instead, selectively enriched in T cell populations with intermediate PD-1 protein expression. Single-cell sequencing of neoantigen-specific T cells from two patients with melanoma—combined with transgenic reexpression of identified TCRs by CRISPR-Cas9–mediated orthotopic TCR replacement—revealed high-functionality TCRs to be enriched in T cells with RNA signatures of recent activation. Furthermore, of 130 surface protein candidates, PD-1 surface expression was most consistently enriched in functional TCRs. Together, our findings show that tumor-reactive TCRs with high protective capacity circulating in peripheral blood are characterized by a signature of recent activation.

The avidity of TCRs for peptide-major histocompatibility complexes (pMHCs) is a governing factor in how T cells respond to antigen. TCR avidity is generally linked to T-cell functionality and there is growing evidence for distinct roles of low and high avidity T cells in different phases of immune responses. While physiological immune responses and many therapeutic T-cell products targeting infections or cancers consist of polyclonal T-cell populations with a wide range of individual avidities, the role of T-cell avidity is usually investigated only in monoclonal experimental settings. In this report, we induced polyclonal T-cell responses with a wide range of avidities toward a model epitope by altered peptide ligands, and benchmarked global avidity of physiological polyclonal populations by investigation of TCR-pMHC k off -rates. We then investigated how varying sizes and avidities of monoclonal subpopulations translate into global k off -rates. Global k off -rates integrate subclonal avidities in a predictably weighted manner and robustly correlate with the functionality of murine polyclonal T-cell populations in vitro and in vivo. Surveying the full avidity spectrum is essential to accurately assess polyclonal immune responses and inform the design of polyclonal T-cell therapeutics.

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