IDO1 and cystic fibrosis: As 5-HT repletion by exogenous administration of 5-HTP improved resistance to infection and inflammation in CftrF508del mice (Fig. 10e–g) and promoted IDO1/AhR activity balance (Fig. 10h), these results suggest that the benefit of 5-HT restoration in CF may go beyond mood control to include an action on Trp partitioning and immune homeostasis in the lung.