IDO1 and cancer: Aberrant activation of IDO1 by cells located in tumours or found in the tumour microenvironment (TME) (e.g. dendritic cells, cancer-associated fibroblasts, and macrophages) is known to contribute to cancer immune escape through the combined effect of tryptophan depletion and accumulation of kynurenine and its metabolites in the tumour region (Cheong and Sun 2018).