Indoleamine 2,3-Dioxygenase 1 as an Adjunctive Histopathological Marker of Cholangitis Activity in Primary Biliary Cholangitis.

AIM: To investigate the expression and clinicopathological significance of indoleamine 2,3-dioxygenase 1 (IDO-1) in primary biliary cholangitis (PBC), particularly its potential role as an adjunctive histopathological marker for diagnosis in early or atypical cases and as an indicator of disease activity. METHODS: We retrospectively analyzed 83 liver biopsy specimens from patients clinically diagnosed with or suspected of having PBC between 2016 and 2025. IDO-1 expression in interlobular bile ducts was assessed by immunohistochemistry and correlated with clinical, biochemical, and histological features, including a new histological staging and grading system for PBC. Statistical analyses included univariate and multivariate logistic regression, and a subgroup analysis was performed for patients receiving ursodeoxycholic acid (UDCA). RESULTS: IDO-1 was expressed in 64 of 83 cases (77.1%). Our multivariate analysis identified UDCA treatment (OR = 0.105, 95% CI: 0.0138-0.794, p = 0.029) and the cholangitis activity (CA) score (OR = 2.91, 95% CI: 1.18-7.18, p = 0.021) as independent predictors of IDO-1 expression. IDO-1 positivity was significantly reduced in UDCA-treated patients (50.0%) compared with untreated patients (95.9%, p < 0.0001). In the UDCA subgroup, unlike other histological features, IDO-1 expression remained significantly associated with higher CA scores. CONCLUSIONS: IDO-1 reflects active cholangitis rather than chronic damage such as fibrosis or bile duct loss. It is a sensitive marker of dynamic immune activity that may serve as an adjunctive histopathological marker.