Interleukin 9 in Oral Lichen Planus: an immunohistochemical study before and after treatment by intralesional steroid injection

Oral Lichen Planus (OLP) is a chronic inflammatory mucocutaneous disorder of the oral mucosa. Th9 cells secrete IL9, which induces elevated levels of MMP9, exacerbating OLP disease severity. IL9 also increases Th17 levels in OLP lesions. This study aimed to detect IL9 tissue expression in patients with OLP compared to normal controls and to correlate its expression with disease severity and response to intralesional steroid therapy. This study involved 18 patients with OLP and 18 healthy, age- and sex-matched volunteers. The REU scoring system was used to monitor OLP lesions before and after treatment with 20 mg/mL intralesional triamcinolone acetonide every 2 weeks for four sessions. Biopsies for H&E and IL-9 expression were taken from patients and controls, with repeat biopsies after the fourth session for patients. A highly statistically significant increase in IL9 expression was observed in the patient group compared with the control group. A highly statistically significant decrease in both the REU score and post-treatment IL-9 expression was detected in the patient group. We can conclude that IL9 is a tissue marker of OLP activity. Future studies on therapies targeting IL-9 in OLP are needed.