IDO1 and cancer: In cancer, the corresponding signal transduction after PRR stimulation amplifies the effect of the immunosuppressive response of MDSCs by producing a variety of proteins, including arginase-1, iNOS, IDO-1, prostaglandin E2, PD-L1, CD40, TNF-α, IL-1β, IL-6, cyclooxygenase-2, and others.