AHR and rheumatoid arthritis: Clinical studies have found lower levels of tryptophan metabolites in the synovial fluid of RA patients compared to that in osteoarthritis patients (160) and reduced levels of ligands that activate AHR, leading to insufficient Treg cell generation and disruption of the Treg/Th17 balance; these findings indicate the importance of tryptophan supplementation for improving the Treg/Th17 balance in RA.