Tumor-infiltrating human CD4+ regulatory T cells display a distinct TCR repertoire and exhibit tumor and neoantigen reactivity

CD4(+) regulatory T (T-reg) cells have an essential function in maintaining self-tolerance; however, they may also play a detrimental role in antitumor immune responses. The presence of elevated frequencies of T-reg cells in tumors correlates with disease progression and poor survival in patients with cancer. The antigen specificity of T-reg cells that have expanded in the tumor microenvironment is poorly understood; answering this question may provide important insights for immunotherapeutic approaches. To address this, we used a novel combinatorial approach to characterizing the T cell receptor (TCR) profiles of intratumoral T-reg cells from patients with metastatic melanoma, gastrointestinal, and ovarian cancers and elucidated their antigen specificities. The TCR repertoires of tumor-resident T-reg cells were diverse yet displayed significant overlap with circulating T-reg cells but not with conventional T cells in tumor or blood. TCRs isolated from T-reg cells displayed specific reactivity against autologous tumors and mutated neoantigens, suggesting that intratumoral T-reg cells act in a tumor antigen-selective manner leading to their activation and clonal expansion in the tumor microenvironment. Tumor antigen-specific T-reg-derived TCRs resided in the tumor and in the circulation, suggesting that both T-reg cell compartments may serve as a source for tumor-specific TCRs. These findings provide insights into the TCR specificity of tumor-infiltrating human T-reg cells that may have potential implications for cancer immunotherapy.