IDO1 and cancer: IL-6, IL-8, SDF-1α and TNF-α, or tumour-derived agents, i.e., IL-10, IDO, and TGF-β, and in response to signals generated by direct contact of cancer cells and MSCs, mesenchymal stromal cells may adopt an immunosuppressive phenotype that affects both innate cells and development of adaptive mechanisms and facilitates the tumour’s escape from immune surveillance.