IDO1 and neoplasm: Moreover, changes in the signal transduction molecules, loss of tumor-specific antigens (TSAs), stimulation of the inhibiting receptor cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) on T-cells as well as some soluble molecules (interleukin (IL)-10, type I interferons (IFNs), Indoleamine 2,3-Dioxygenase (IDO), adenosine, vascular endothelial growth factor A (VEGF-A), transforming growth factor-beta (TGF-β), and IL-35) secreted by tumor cells or non-tumor cells in the TME mediate immune cell dysfunction [9].