IDO1 and head and neck squamous cell carcinoma: Indeed, HNSCC tumors express various immunosuppressive cytokines such as transforming growth factor beta (TGF-β), IL-6 or IL-10 [44, 45] and enzymes which deprive the tumor microenvironment of essential nutrients for T cell function, such as indoleamine 2,3- dioxygenase (IDO)-mediated degradation of the amino acid tryptophan [46].