Figure 4: The mechanism of IDO1 regulation on viral infectious diseases. Innate defense occurs when pathogens contact or invade host cells and elicit the production of cytokines and chemokines, which in turn induce an influx of immune cells that affect pathogen clearance. Type I IFNs are critical mediators of innate immunity and limit disease caused by many viruses [73, 86]. The enhancement of TRP breakdown by IDO1 regulates the signal pathway for IFNs production, and TRP metabolites might contribute to the function of IFNs producing cells.