IDO1 and lung cancer: Originally identified as a major contributor to immune tolerance at the maternofoetal interface (108), IDO enzymatic activity is now recognized as one of the “metabolic checkpoints” in tumors such as melanoma and lung cancer: IDO catabolises tryptophan, which is also a “fuel” for proper T-cell activation and proliferation, into kynurenines that act as T-cell toxic metabolites.