Effector CD4 and CD8 T Cells and Their Role in the Tumor Microenvironment

T cells in tumors—the so-called tumor infiltrating lymphocytes (TIL) have been studied intensively over the past years. Compelling evidence point to a clinical relevance for high numbers of T cells at the tumor site with CD8 memory T cells as a key denominator for overall survival (OS) in patients with colo-rectal cancer (CRC), and also for others solid cancers. These data goes hand in hand with studies of clonality of TIL showing the T cells among TIL are expanded clonally, and also that tumor specific T cells of CD4 as well as CD8 type are enriched at the tumor site. The tumor microenvironment is hostile to T cell function e.g., due to expression of enzymes that depletes the amino acids tryptophan and arginine, high concentration of tumor secreted lactate, and presence innate cells or regulatory T cells both with suppressive activity. Analyses of the specificity of TILs in melanoma demonstrate that quite few known antigens are in fact recognized by these cultures underscoring patient unique and/or mutated antigens may represent important target for recognition.