Since arginase and indoleamine-pyrrole 2,3-dioxygenase (IDO) were highly expressed in DCs, myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs), the absence of amino acids such as arginine and tryptophan could further impair metabolic fitness and alter the activation and differentiation of TILs [80]. Here, IDO1 is linked to neoplasm.