IDO1 and neoplasm: The tumor microenvironment is modulated by numerous factors including the expression of anergic and immunosuppressive proteins such as PD-L1, indoleamine-2,3-dioxygenase (IDO), immunosuppressive cytokines, and the infiltration of myeloid-derived suppressive cells (MDSCs), including macrophages and dendritic cells, as well as tumor-infiltrating T lymphocytes (TILs), neoantigen load, and mutation burden which positively regulate the immune response [28,29,30,31].