Furthermore, several mechanisms have been reported to be involved in immunosuppression, including expansion of myeloid-derived suppressor cells (MDSCs) [49], tumor-associated macrophages, and other myeloid cells [50,51], perturbation of cytokine networks [52], changes in host metabolism [53], and the production of amino acid-degrading enzymes and indoleamine 2,3-dioxygenase 1 (IDO1) [54]. Here, IDO1 is linked to neoplasm.