IDO1 and neoplasm: MDSCs are considered the main immune components involved in the metabolic crosstalk between tumor cells and the immune system; immunosuppression takes place by the metabolic reprogramming of MSDCs via Arg1, iNOS, and indoleamine 2,3 dioxygenase 1 (IDO1) inhibition, which determine amino-acid depletion and CD8+ and natural killer cells’ suppression [113].