IDO1 and viral infectious disease: In this setting, L-kynurenine will increase the activation of the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor widely expressed by immune cells and with a key role in the host response to viral infections and gut immune homeostasis (62).This activation in T-effector cells promotes the transformation into Treg cells and IDO induction, which maintains immunosuppression (63).