The production of immunosuppressive metabolite kynurenine due to the upregulation of indoleamine-2,3-dioxygenase (IDO1) in tumor, tumor associated macrophages (TAMs), and monocytes prevents the activation of effector T cells, inhibits NK cell function, supports Treg activation, and promotes the expansion and activation of dendritic cells (DCs) and MDSCs [125]. Here, IDO1 is linked to neoplasm.