Increased sputum and bronchial biopsy IL-13 expression in severe asthma

Background: The importance of IL-13 in the asthma paradigm is supported by increased expression in human subjects, particularly in patients with mild-to-moderate asthma. However, the role of IL-13 in severe asthma needs to be further defined. Objective: We sought to assess IL-13 expression in sputum and bronchial biopsy specimens from subjects with mild-to-severe asthma. Methods: Sputum IL-13 concentrations were measured in 32 control subjects, 34 subjects with mild asthma, 21 subjects with moderate asthma, and 26 subjects with severe asthma. Enumeration of mast cells, eosinophils, and IL-13(+) cells in the bronchial submucosa and airway smooth muscle (ASM) bundle was performed in 7 control subjects, 14 subjects with mild asthma, 7 subjects with moderate asthma, and 7 subjects with severe asthma. Results: The proportion of subjects with measurable IL-13 in the sputum was increased in the mild asthma group (15/34) and severe asthma group (10/26) compared with that seen in the control group (4/32; P =.004). IL-13(+) cells were increased within the submucosa in all asthma severity groups compared with control subjects (P =.006). The number of IL-13+ cells were increased within the ASM bundle in the severe asthma group compared with that seen in the other groups (P <.05). Asthma control questionnaire scores positively correlated with sputum IL-13 concentrations (R-s = 0.35, P =.04) and mast cells in the ASM bundle (R-s = 0.7, P =.007). IL-13(+) cells within the submucosa and ASM correlated with sputum eosinophilia (R-s = 0.4, P <=.05). Conclusions: IL-13 overexpression in sputum and bronchial biopsy specimens is a feature of severe asthma.