Tumor-infiltrating regulatory T cells delineated by upregulation of PD-1 and inhibitory receptors

Foxp3⁺ regulatory T (T_reg) cells are dominant suppressor cells which regulate conventional T (T_conv) cells. Inside tumor microenvironment, T_reg cells have been known to become potent in suppressing T_conv cell responses, thereby enabling tumor cells to circumvent immune response. However, the underlying mechanism by which tumor-infiltrating T_reg cells display enhanced suppressive function is still unresolved. To understand characteristics and function of tumor-infiltrating T_reg cells as well as T_conv cells in the tumor site, we analyzed their phenotypes either within tumor burden or at distant site of tumor using both heterotopic and orthotopic mouse cancer models. Compared to CD8⁺ T cells at distant site of tumor, tumor-infiltrating CD8⁺ T cells dramatically upregulated programmed death 1 (PD-1) and other inhibitory receptors, thereby being more exhausted functionally. Tumor-infiltrating CD4⁺ T cells also expressed higher level of PD-1 than CD4⁺ T cells at distant site of tumor but very surprisingly, upregulation of PD-1 occurred in CD4⁺Foxp3⁺ T_reg as well as CD4⁺Foxp3^- T_conv cells. Moreover, tumor infiltrating T_reg cells upregulated other inhibitory receptors such as T cell immunoglobulin mucin 3 (TIM-3), cytotoxic T lymphocyte antigen-4 (CTLA-4), glucocorticoid-induced tumor necrosis factor receptor (GITR), and lymphocyte activation gene-3 (LAG-3). These results suggest that upregulation of PD-1 and other inhibitory receptors on tumor-infiltrating T_reg cells is related with their enhanced suppressive function.