IDO1 and neoplasm: And even though treatment of tumor-bearing mice with STING agonists leads to reduced levels of tumor-associated myeloid-derived suppressor cells (MDSC) and Treg cells (107, 108), these regimens promote compensatory activation of immune regulatory pathways by augmenting expression of arginase-2 (ARG2), cyclooxygenase-2 (COX2/PTGS2), indoleamine 2,3 dioxygenase (IDO), programmed death-1 (PD-1), programmed death ligand-1 (PD-L1) and prostaglandin E synthase (PTGES) within the TME (67, 109–111).