Identification and regulation of circulating tumor TCR-matched cytotoxic CD4+ lymphocytes by KLRG1 in bladder cancer

While cytotoxic CD4+ tumor-infiltrating lymphocytes have anti-cancer activity in patients, whether these can be non-invasively monitored and how these are regulated remains obscure. By matching single cells with T cell receptors (TCR) in tumor and blood of bladder cancer patients, we identified distinct pools of tumor-matching cytotoxic CD4+ T cells in the periphery directly reflecting the predominant antigenic specificities of intratumoral CD4+ TILs. On one hand, the granzyme B (GZMB)-expressing cytotoxic CD4+ subset proliferated in blood in response to PD-1 blockade, but was separately regulated by the killer cell lectin-like receptor G1 (KLRG1) which inhibited their killing by interacting with E-cadherin. Conversely, a clonally related, granzyme K (GZMK)-expressing circulating CD4+ population demonstrated basal proliferation and a memory phenotype that may result from activation of GZMB+ cells, but was not directly mobilized by PD-1 blockade. As KLRG1 marked the majority of circulating tumor TCR-matched cytotoxic CD4+ T cells, this work nominates KLRG1 as a means to isolate them from blood and provide a window into intratumoral CD4+ recognition, as well as a putative regulatory receptor to mobilize the cytolytic GZMB+ subset for therapeutic benefit. Our findings also underscore the ontogenic relationship of GZMB- and GZMK-expressing populations and the distinct cues that regulate their activity.