Purcarea and Jarosch et al. showed that partial protection from a neoantigen+ tumor was provided by transfer of as few as 128 neoantigen-specific T cells into irradiated host mice. PD-1, TIM-3, and LAG-3 had concerted expression on transferred T cells, which was highest on exhausted TILs. TCR avidity of transferred cells correlated with tumor protective peripheral blood T cells expressing PD-1 levels lower than in TILs, reflecting recent activation, but not with abundance or phenotype of neoantigen-specific TILs. In two patients with melanoma, neoantigen-specific TCRs were enriched among T cells expressing RNA and protein signatures of recent T cell activation.
Contributed by Paula Hochman
ABSTRACT: 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.