Salivary kynurenine pathway metabolites as potential non-invasive markers of glandular dysfunction in Sjögren’s disease

Sjögren’s disease (SjD) is an autoimmune disorder characterized by salivary gland dysfunction and systemic manifestations. This study aimed to evaluate kynurenine (Kyn) pathway metabolites in saliva and investigate their clinical relevance in SjD. Saliva samples were collected from 39 SjD patients and 32 healthy controls (HCs). Concentrations of tryptophan, Kyn, quinolinic acid (QA), and kynurenic acid (KA), as well as inferred enzyme activities, were measured using ELISA. Receiver operating characteristic curve analyses were conducted, and correlations with disease activity indices and unstimulated whole salivary flow rates (UWSFR), were assessed. SjD patients exhibited significantly higher salivary KA levels (p < 0.001) and lower QA levels (p < 0.0001) compared with HCs. Ratios of QA to Kyn and KA to Kyn were also significantly altered in the SjD group. Salivary QA demonstrated excellent discriminatory ability (area under the curve > 0.85, sensitivity 0.87, specificity 0.76, p < 0.001) for distinguishing SjD from HCs. Among various clinical parameters, salivary QA levels showed a strong inverse correlation with UWSFR (r = − 0.596, p < 0.001). Salivary Kyn pathway metabolites, particularly QA, may serve as non-invasive biomarkers reflecting salivary gland dysfunction in SjD.