Enhanced Tryptophan-Kynurenine Metabolism Via Indoleamine 2,3-Dioxygenase 1 Induction in Dermatomyositis

Extrahepatic tryptophan (Trp)-kynurenine (Kyn) metabolism via indoleamine 2,3-dioxygenase 1 (IDO1) induction was found to be associated with intrinsic immune regulation. However, the Trp-Kyn metabolism–associated immune regulation in dermatomyositis (DM) remains unknown. Therefore, we aimed to investigate the clinical relevance of the Trp-Kyn metabolism via IDO1 induction in DM. Liquid chromatography-mass spectrometry (HPLC–MS) was used to examine the serum Kyn and Trp concentrations in DM. In addition, we used X-tile software to determine the optimal cutoff value of the Kyn/Trp ratio, a surrogate marker for Trp-Kyn metabolism. Spearman analysis was performed to evaluate the association of Trp-Kyn metabolism with muscle enzymes and inflammatory markers. DM patients had significantly higher serum Kyn/Trp ratio (× 10−3) when compared with the healthy controls. The serum Kyn/Trp ratio was positively correlated with the levels of muscle enzymes and inflammatory markers. In addition, the serum Kyn/Trp ratio significantly decreased (36.89 (26.00–54.00) vs. 25.00 (18.00–37.00), P = 0.0006) after treatment. DM patients with high serum Kyn/Trp ratio had a significantly higher percentage of muscle weakness symptoms (62.5% vs. 20.0%, P = 0.019) and higher levels of LDH (316.0 (236.0–467.0) vs. 198.0 (144.0–256.0), P = 0.004) and AST (56.5 (35.0–92.2) vs. 23.0 (20.0–36.0), P = 0.002)) than those with low serum Kyn/Trp ratio. Multiple Cox regression analyses identified ln(Kyn/Trp) (HR 4.874, 95% CI 1.105–21.499, P = 0.036) as an independent prognostic predictor of mortality in DM. DM patients with enhanced Trp-Kyn metabolism at disease onset are characterized by more severe disease status and poor prognosis. Intrinsic immune regulation function via enhanced Trp-Kyn metabolism by IDO1 induction may be a potential therapeutic target in DM.