Elevated expression of interleukin‐9 mRNA in the bronchial mucosa of atopic asthmatics and allergen‐induced cutaneous late‐phase reaction: relationships to eosinophils, mast cells and T lymphocytes

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Abstract
Background Interleukin‐9 is a T cell‐derived Th2‐type cytokine that has been linked to airway hyper‐responsiveness, mucus hypersecretion and mast cell infiltration in animal models. We recently demonstrated the potential for IL‐9 to act in human eosinophil development and survival. Objectives The aims of this study were: (i) to compare IL‐9 mRNA expression in bronchial biopsies between atopic asthmatics and normal controls, (ii) to investigate kinetic expression of IL‐9 mRNA in skin biopsies after allergen challenge; and (iii) to relate IL‐9 expression to infiltration of eosinophils, mast cell and T lymphocytes in local tissue. Methods Bronchial biopsies were obtained from atopic asthmatics (n = 12) and normal non‐asthmatics (n = 12) at baseline. Skin biopsies were obtained from atopic subjects (n = 11) at 1, 3, 6, 24, 48 and 72 h after allergen challenge. Diluent challenge sites at 24 h were used as controls. IL‐9 mRNA was identified using the technique of in situ hybridization. The numbers of eosinophils, mast cells and T cells were evaluated by immunohistochemistry. Results The numbers of IL‐9 mRNA+ cells present in the bronchial mucosa were significantly greater in atopic asthmatics than those in normal controls (P = 0.003). The numbers of eosinophils, but not mast cells, were also significantly higher in asthmatics (P < 0.005). The numbers of IL‐9 mRNA+ cells present in the airway of asthmatics significantly correlated with the numbers of eosinophils (r = 0.623, P = 0.03), but not mast cells or T cells. Compared with diluent challenge, the numbers of IL‐9 mRNA+ cells were significantly elevated at all allergen‐challenged sites in the skin, with maximal signals at 48 h (P < 0.005). At 72 h, the numbers of IL‐9 mRNA+ cells significantly correlated with the numbers of eosinophils (r = 0.707, P = 0.015). Conclusion Elevated expression of IL‐9 in allergic inflammation may contribute to local eosinophil infiltration and survival in asthma and other allergic atopic diseases.

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