Exploring the role of the Th9/IL-9 axis in atopic dermatitis: analysis of Th9 cells, PU.1 expression and serum IL-9 levels.

Atopic dermatitis (AD) is a chronic inflammatory skin disease resulting from complex interactions between genetic, environmental, and immunological factors, particularly the dysregulation of T helper (Th) cell subsets. While Th2 responses have been central to AD research, evidence highlights the role of Th9 cells and their signature cytokine, interleukin-9 (IL-9), in allergic inflammation. However, the involvement of the Th9/IL-9 axis in AD remains insufficiently characterized. This study investigates the role of the Th9/IL-9 axis in AD through an integrated clinical, immunological, and molecular approach. The study comprised a total of 89 subjects, consisting of 54 AD cases and 35 healthy controls. Disease severity and allergen sensitivity were assessed using SCORAD scores and skin prick testing, respectively. PU-1 mRNA was quantified using real-time PCR. Th9 cell frequency was determined through flow cytometry, vitamin D levels were measured by chemiluminescence, and total IgE and IL-9 levels were estimated using ELISA. AD patients demonstrated a significant increase in circulating Th9 cell frequency (11.5 ± 0.3% vs. 5.0 ± 0.5%, p < 0.0001), PU.1 mRNA expression with an average 2.5-fold upregulation (p < 0.001), and elevated serum IL-9 concentrations (18.6 ± 6.3 pg/mL vs. 8.3 ± 1.9 pg/mL, p < 0.0001) compared to healthy controls. Stratified analysis revealed significant associations of PU.1 mRNA and IL-9 levels with rural residence, vitamin D deficiency, and peripheral eosinophilia (p < 0.05). Allergen sensitivity, as determined by skin prick testing, was positive in 50% of AD patients, and 66.7% exhibited elevated total serum IgE levels. Our findings underscore the involvement of the Th9/PU.1/IL-9 axis in the immunopathogenesis of AD. The observed upregulation of Th9 cell frequency, PU.1 expression, and IL-9 levels suggests a contributory role of Th9-mediated pathways in disease development and severity. These results provide a foundation for future studies investigating the dynamic crosstalk between Th9 cells, other immune cell subsets, and epidermal components, with the aim of identifying novel immunomodulatory targets for precision therapy in AD.