Segmental allergen challenge in patients with atopic asthma leads to increased IL-9 expression in bronchoalveolar lavage fluid lymphocytes

Background: IL-9 is a T(H)2 cell-derived cytokine that might be involved in the pathophysiology of allergic diseases. Little is known about its expression and release during the allergic response in the human lung. Objective: The expression of IL-9 was measured in 10 atopic subjects with mild asthma and 5 nonatopic healthy control subjects at baseline and 24 hours after segmental sham and allergen challenge. Methods: IL-9 protein was measured in bronchoalveolar lavage (BAL) fluid by means of ELISA and detected within the BAL cells by means of immunocytochemistry. Furthermore, IL9 mRNA expression of BAL cells was detected by means of real-time PCR. Results: Although only low or undetectable amounts of IL9 mRNA and IL-9 protein were present in nonatopic control subjects and atopic asthmatic patients at baseline, there was an increase after segmental allergen challenge in the atopic subjects. Lymphocytes were identified as major cellular sources of IL-9 production by means of immunocytochemistry. Furthermore. IL-9 protein and IL9 mRNA expression correlated with eosinophil numbers in BAL fluid. Conclusions: These findings demonstrate that IL-9 is specifically upregulated after local allergen challenge in the lungs of atopic asthmatic patients. Lymphocytes are the major cellular source of IL-9. The increased expression and its correlation with eosinophil numbers suggest a potential role for IL-9 in the late phase of the allergic response. (J Allergy Clin Immunol 2003:111:1319-27.).