IDO1 and cancer: As the major killers fighting against cancer cells, the presence of cytotoxic CD8+ T lymphocytes is correlated with better NSCLC patient outcomes.101 The loss or downregulation of antigen-presenting machinery, such as MHC-I, along with the secretion and expression of immunosuppressive factors, like TGF-β, IL-6, IL-10, IDO and PD-L1, are often described as mechanisms through which metastatic cancer cells avoid T cell recognition and killing or compromise T cell activation and proliferation.102