IDO1 and neoplasm: For another, CAFs secret various cytokines [e.g., IL-6, macrophage colony-stimulating factor (M-CSF)], growth factors (TGF-β, EGFR, HGF, IGF, VEGF), chemokines (e.g., CCL2, CCL5, CXCL12), exosomes and other effector molecules [e.g., indoleamine 2,3-dioxygenase (IDO), prostaglandin E2 (PGE2)], consequently shaping an immunosuppressive TME that enables tumor cells to evade surveillance of the immune system (19, 72).