IL-25 promotes Th2-type reactions and correlates with disease severity in the pathogenesis of oral lichen planus

•IL-25 promotes Th2-type reaction in oral lichen planus.•IL-25 correlates with disease severity in oral lichen planus.•IL-25 highly involved in reticular subtype of oral lichen planus than erosive ones. The aim of the present study was to investigate the correlation between IL-25 expression and disease severity, and the potential immunoregulatory role of IL-25 expression in oral lichen planus (OLP). The oral mucosal tissue samples obtained from OLP patients and healthy controls (HCs) were analyzed for IL-25 expression by real-time quantitative PCR (qPCR) and immunohistochemistry. Recombinant IL-25 was used to stimulate OLP patient-derived CD4 + T cells, and then IL-4 secretion and mRNA expression were evaluated by ELISA and qPCR, respectively. The efficiency of the siRNA-mediated knockdown of IL-25R expression in oral keratinocytes was determined by qPCR and Western blotting. Human oral keratinocyte cells were cultured with the recombinant human cytokines IL-25, IL-17 A and IL−17 F. The production of associated cytokines by keratinocytes was determined by qPCR. Statistical analyses of quantitative data were performed using SPSS software. The IL-25 and IL-4 mRNA levels were elevated and correlated significantly with each other in specific OLP subtype lesions compared to HCs, while the numbers of IL-25 positive cells were also increased in local OLP lesions as compared to HCs. In vitro culture with recombinant IL-25 could significantly promote CD4 + T cells from both subtypes of OLP to produce IL-4 mRNA and remarkably elevate supernatant IL-4 levels in reticular OLP CD4 + T cell cultures, which may be attributed to the elevated expression of IL-25R in local OLP lesions. Statistical analyses demonstrated that the simultaneously increased levels of IL-4, CXCL8 and CCL20 in keratinocytes were induced by IL-25 but not IL-17 A or IL−17 F. Decreasing IL-25R subunit expression by siRNA-mediated knockdown significantly blocked the expression of all cytokine-produced inflammatory mediators in oral keratinocytes. In OLP lesions, IL-25 can function to mediate the Th2 response in specific disease subtypes, which may be an important cause of OLP disease chronicity and persistent inflammation.