PDCD1 and head and neck squamous cell carcinoma: High expression of indoleamine 2,3-dioxygenase in HNSCC catalyzes the conversion of tryptophan to l-kynurenine, thereby suppressing T cell proliferation and function.207, 208, 209, 210, 211 Further studies have shown that kynurenine up-regulates PD-L1 expression on granulocytic-myeloid-derived suppressor cells, macrophages, and dendritic cells and enhances PD-1 expression on CD8+ T cells in various malignancies.