IDO1 and juvenile idiopathic arthritis: JIA patients with high disease activity are characterized by higher serum levels of both neopterin and kynurenine, and a higher ratio of both KYN/TRP and PHE/TYR and lower tryptophan serum levels than in clinically inactive JIA patients, reflecting increased activity of both IDO and GTP–CH1 pathways but decreased BH4 efficacy during chronic inflammation.