IDO1 and infection: As the disruption of the epithelial barrier is implicated in many human diseases, including respiratory allergies, encephalitis, and neurodegenerative diseases, the possibility to target both the host and the microbiota by modulating the cross-regulatory circuit between AhR and IDO1 in human infections and inflammatory diseases makes the microbial indoles, including 3-IAld, one of the greatest challenges in the field.