IDO1 and neoplasm: Major components of the TME of solid tumours include anti-tumour immune cells together with stromal cells, such as cancer-associated fibroblasts (CAFs), TREG cells, myeloid-derived suppressor cells (MDSCs) and tumour-associated macrophages (TAMs), that function in suppressing anti-tumour immunity.55–57 Immunosuppressive cytokines and soluble factors including adenosine, IL-10, transforming growth factor β (TGF-β), and indoleamine 2,3-dioxygenase (IDO) also reside in the hostile TME.