Expression of Programmed Death Ligand 1 and Indoleamine 2,3‐Dioxygenase in Oral Lichen Planus and Oral Lichenoid Lesions

ABSTRACT Background Oral lichen planus (OLP) and oral lichenoid lesions (OLL) are inflammatory T‐cell mediated disorders of the oral mucosa (OM). Both are associated with an increased risk of oral squamous cell carcinoma, with OLL possibly having a higher rate of malignant transformation than OLP. Programmed death ligand 1 (PD‐L1) and indoleamine 2,3‐dioxygenase (IDO) are immunosuppressive molecules possessing inhibitory effect on T‐cells and have been implicated in carcinogenesis. The aim of this study was to examine the expression of PD‐L1 and IDO in OLP and OLL. Methods Sixty‐eight formalin‐fixed, paraffin‐embedded tissue samples diagnosed as OLP, compatible with OLP, or OLL were divided into OLP (n = 39) or OLL (n = 29) groups based on both clinical and histopathological diagnostic criteria. Samples of healthy OM (n = 9) served as controls. Samples were immunohistochemically stained for PD‐L1 and IDO, and staining distribution and intensity were evaluated. Results Immunohistochemical expression of PD‐L1 was increased in the basal and intermediate layers of epithelium in OLP and in lamina propria in both OLP and OLL compared to controls. OLP and OLL showed increased expression of IDO in epithelium and lamina propria compared to controls. PD‐L1 staining intensity in the basal epithelial layer, and IDO staining intensity in lamina propria were increased in OLP compared to OLL. Conclusion The results indicate that the expression of PD‐L1 and IDO increases in OLP and OLL, suggesting that these molecules may play a role in the pathogenesis of both disorders.