Foxp3+ regulatory T (Treg) cells are dominant suppressor cells which regulate conventional T (Tconv) cells. Inside tumor microenvironment, Treg cells have been known to become potent in suppressing Tconv cell responses, thereby enabling tumor cells to circumvent immune response. However, the underlying mechanism by which tumor-infiltrating Treg cells display enhanced suppressive function is still unresolved. To understand characteristics and function of tumor-infiltrating Treg cells as well as Tconv 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+ Treg as well as CD4+Foxp3- Tconv cells. Moreover, tumor infiltrating Treg 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 Treg cells is related with their enhanced suppressive function.
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