T lymphocyte subsets in cancer immunity: Friends or foes

Although immune-based therapy is proving to be a success in several cancer types, only a set of patients appear to respond to immune checkpoint blockade including PD-1 and CTLA-4. A better understanding of the crucial components of cancer immunity is therefore necessary. T lymphocytes, a key element, are found within the tumor microenvironment and seem to be critical in determining the efficacy of immune surveillance. In this review, we will depict the pro- and antitumor roles of major T cell subsets in distinct cancer tissues. The central role of the mainly antitumor subsets, cytotoxic T cells and Th1 cells, will be delineated. Subsequently, we will indicate how other subsets including Th2, Th17, and T regulatory cells exhibit ambivalent roles. We will also describe the emerging and favorable role of Th9 cells in cancer immunity. In parallel, we will go through main mechanisms by which these cells operate, and will pinpoint pathways, which could be used as potential therapeutic targets in order to positively impact the immune response and ameliorate patients' clinical outcome.