Nonasthmatic eosinophilic bronchitis is a condition characterized by the presence of eosinophilic airway inflammation in the absence of airflow obstruction or airway hyperresponsiveness. In asthma, the T_H2-type cytokine IL-13 has been implicated in the development of airway inflammation and hyperresponsiveness. Whether the expression of IL-13 is different between these 2 conditions is unknown.

We sought to investigate whether IL-13 expression is increased in asthma compared with eosinophilic bronchitis.

Sputum samples from subjects with mild asthma (n=30) and eosinophilic bronchitis (n=15) and normal controls (n=16) were dialyzed, and IL-13 concentration was measured by ELISA. In a subgroup of these patients, IL-13 protein expression in bronchial biopsies was assessed by immunohistochemistry.

The concentration of sputum IL-13 was higher in patients with mild asthma than in normal controls (P=.03) and in patients with eosinophilic bronchitis (P=.03). The median (interquartile range) number of IL-13⁺ cells/mm² submucosa was significantly higher in asthma 4 (8) than eosinophilic bronchitis 1.7 (1.9) and normal controls 0.5 (1.1; P=.004). Eighty-three percent of the cells expressing IL-13 in the submucosa were eosinophils, and 8% were mast cells. The median (interquartile range) proportion of eosinophils that expressed IL-13 was higher in the subjects with asthma, 16 (10)%, than those with eosinophilic bronchitis, 7 (3)% (P=.02).

The increased expression of IL-13 in asthma compared with eosinophilic bronchitis supports the concept that IL-13 may play a critical role in the pathophysiology of asthma.