AHR and rheumatoid arthritis: in rheumatoid arthritis (Moulin et al., 2024), while the use of exogenous catalysts conversion of Kyn and 3-HK to produce XA and KA via the recombinant enzyme aminoadipate aminotransferase (AADAT) directly regulates endogenous tryptophan metabolism through AhR activation and cellular metabolic reprogramming, thereby reducing the severity of inflammatory bowel disease and rheumatoid arthritis (Michaudel et al., 2023; Moulin et al., 2024).