Viral infection activates AhR through an IDO1-AhR-IDO1-positive feedback loop, which eventually causes upregulation of downstream effectors, such as TCDD-inducible PARP (TiPARP), and enhances the expression of cytokines (e.g., interleukin IL-1β, IL-10, and TNF-α) [39]. This evidence concerns the gene IDO1 and viral infectious disease.