Systemic Activation of the Kynurenine Pathway in Graves Disease With and Without Ophthalmopathy

Abstract

Context: Graves disease (GD) is one of the most common autoimmune disorders. Recent literature has shown an immune response involving several different inflammatory related proteins in these patients.

Objective: This work aimed to characterize the kynurenine pathway, activated during interferon-γ (IFN-γ)-mediated inflammation and cellular (T-helper type 1 [Th1] type) immunity, in GD patients with and without thyroid eye disease (TED).

Methods: We analyzed 34 biomarkers by mass spectrometry in serum samples from 100 patients with GD (36 with TED) and 100 matched healthy controls. The analytes included 10 metabolites and 3 indices from the kynurenine pathway, 6 microbiota-derived metabolites, 10 B-vitamers, and 5 serum proteins reflecting inflammation and kidney function.

Results: GD patients showed significantly elevated levels of 7 biomarkers compared with healthy controls (omega squared [ω2] > 0.06; P < .01). Of these 7, the 6 biomarkers with the strongest effect size were all components of the kynurenine pathway. Factor analysis showed that biomarkers related to cellular immunity and the Th1 responses (3-hydroxykynurenine, kynurenine, and quinolinic acid with the highest loading) were most strongly associated with GD. Further, a factor mainly reflecting acute phase response (C-reactive protein and serum amyloid A) showed weaker association with GD by factor analysis. There were no differences in biomarker levels between GD patients with and without TED.

Conclusion: This study supports activation of IFN-γ inflammation and Th1 cellular immunity in GD, but also a contribution of acute-phase reactants. Our finding of no difference in systemic activation of the kynurenine pathway in GD patients with and without TED implies that the local Th1 immune response in the orbit is not reflected systemically.

Keywords: Graves disease; Th1 response; autoimmune; cellular immunity; kynurenine pathway; thyroid eye disease.

Figure 1.

Combination plot of effect size against significance level in biomarkers between GD and healthy controls. X-axis: effect size estimated by omega squared (ω²). Y-axis: significance level as −log 10 (adjusted P value). Blue line: represents significance level at P = .01. Blue dots: biomarker suppressed in GD. Red dots: biomarker elevated in GD. ω² and P values for difference in biomarkers were obtained from a linear regression model, adjusted for age, BMI, and smoking. AA, anthranilic acid; BMI, body mass index; CRP, C-reactive protein; FMN, flavin mononucleotide; GD, Graves disease; HK, 3-hydroxykynurenine; IAA, indole-3-acetate; IAld, indole-3-aldehyde; ILA, indole-3-lactate; IPA, indole-3-propionate; KA, kynurenic acid; KTR, kynurenine-tryptophan-ratio; Kyn, kynurenine; mNAM, N1-methylnicotinamide; NAM, nicotinamide; PA, 4-pyridoxic acid; Pic, picolinic acid; PL, pyridoxal; PLP, pyridoxal 5′-phosphate; QA, quinolinic acid; Qld, quinaldic acid; Ribo, riboflavin; SAA, serum amyloid A; S100A, calprotectin; Trp, tryptophan; XA, xanthurenic acid; 3IS, 3-indoxyl sulfate.

Figure 2.

Score plot from factor analysis showing scores from factor 1 (acute-phase inflammation) and 2 (Th1 cellular immunity) labeled by outcome status (GD or healthy control). The filled symbols indicate the central estimates, and the dark-shaded zones and the light-shaded zones the 50 percentile and the 95 percentile, respectively. The scale on each axis is SD of the factor scores. The difference between Graves disease (GD) and healthy control was 0.69 SD on factor 1 and 0.83 SD on factor 2 as indicated by dashed lines.