Tumor microenvironment dictates regulatory T cell phenotype: Upregulated immune checkpoints reinforce suppressive function

BACKGROUND: Regulatory T (T(reg)) cells have an immunosuppressive function in cancer, but the underlying mechanism of immunosuppression in the tumor microenvironment (TME) is unclear. METHODS: We compared the phenotypes of T cell subsets, including T(reg) cells, obtained from peripheral blood, malignant effusion, and tumors of 103 cancer patients. Our primary focus was on the expression of immune checkpoint (IC)-molecules, such as programmed death (PD)-1, T-cell immunoglobulin and mucin-domain containing (TIM)-3, T cell Ig and ITIM domain (TIGIT), and cytotoxic T lymphocyte antigen (CTLA)-4, on T(reg) cells in paired lymphocytes from blood, peritumoral tissue, and tumors of 12 patients with lung cancer. To identify the immunosuppressive mechanisms acting on tumor-infiltrating T(reg) cells, we conducted immunosuppressive functional assays in a mouse model. RESULTS: CD8(+), CD4(+) T cells, and T(reg) cells exhibited a gradual upregulation of IC-molecules the closer they were to the tumor. Interestingly, PD-1 expression was more prominent in T(reg) cells than in conventional T (T(conv)) cells. In lung cancer patients(,) higher levels of IC-molecules were expressed on T(reg) cells than on T(conv) cells, and T(reg) cells were also more enriched in the tumor than in the peri-tumor and blood. In a mouse lung cancer model, IC-molecules were also preferentially upregulated on T(reg) cells, compared to T(conv) cells. PD-1 showed the greatest increase on most cell types, especially T(reg) cells, and this increase occurred gradually over time after the cells entered the TME. PD-1 high-expressing tumor-infiltrating T(reg) cells displayed potent suppressive activity, which could be partially inhibited with a blocking anti-PD-1 antibody. CONCLUSIONS: We demonstrate that the TME confers a suppressive function on T(reg) cells by upregulating IC-molecule expression. Targeting IC-molecules, including PD-1, on T(reg) cells may be effective for cancer treatment.