IDO1 and neoplasm: Tregs attenuate anti-tumour immune activity through a variety of mechanisms, including depleting IL-2 from the TME (70), pushing antigen-presenting cells (APCs) towards tolerogenic phenotypes characterized by the downregulation of CD80/86 and the upregulation of IDO1 (71), inducing apoptosis in CD8+ cells via expression of the Fas ligand (72) or granzyme and perforin (73), and secreting immunosuppressive cytokines, including IL-10 and TGF-β (74).