Increased expression of interleukin-9 in patients with allergic contact dermatitis caused by p-phenylenediamine

Background Objective Allergic contact dermatitis has been described as a type IV reaction caused by antigen-specific T cells. Central roles for CD8(+) cytotoxic T cells as effector cells and CD4(+) T cells as regulatory cells have been suggested. T helper (Th) 2 and Th1 cytokines have been implicated; however, the nature of the allergen influences the Th response. To determine the types of T cells and cytokines expressed in patients allergic to p-phenylenediamine (PPD). Methods Results Serial skin biopsies of areas with positive patch test reactions in 29 PPD-sensitized patients were collected. T cell markers and cytokine expression were analysed by flow cytometry and quantitative reverse transcription polymerase chain reaction in both skin and peripheral blood mononuclear cells (PBMCs) of sensitized patients. We observed increased expression of T cell markers and Th2/Th9-associated cytokines in both skin and stimulated PBMCs of PPD-allergic patients. Moreover, interleukin (IL)-9 was mainly produced by Th9 cells, in both skin and PBMCs. Further investigations showed that Il9r-deficient mice were more affected in a PPD contact hypersensitivity model than wild-type mice. Conclusion We did not confirm the preclinical presence of CD8(+) T cells. However, the expression of different T cell markers positively correlated with patch test reactions. IL-9 expression was strongly upregulated and directly related to patch test severity. In addition, we showed that IL-9 has an anti-inflammatory role in a mouse model of PPD contact hypersensitivity.