The relationship between eosinophilia and airway remodelling in mild asthma

Eosinophilia is a marker of corticosteroid responsiveness and risk of exacerbation in asthma; although it has been linked to submucosal matrix deposition, its relationship with other features of airway remodelling is less clear.

The aim of this study was to investigate the relationship between airway eosinophilia and airway remodelling.

Bronchial biopsies from subjects (n = 20 in each group) with mild steroid-naïve asthma, with either low (0-0.45 mm(-2)) ) or high submucosal eosinophil (23.43-46.28 mm(-2) ) counts and healthy controls were assessed for in vivo epithelial damage (using epidermal growth factor receptor staining), mucin expression, airway smooth muscle (ASM) hypertrophy and inflammatory cells within ASM.

The proportion of in vivo damaged epithelium was significantly greater (P = 0.02) in the high-eosinophil (27.37%) than the low-eosinophil (4.14%) group. Mucin expression and goblet cell numbers were similar in the two eosinophil groups; however, MUC-2 expression was increased (P = 0.002) in the high-eosinophil group compared with controls. The proportion of submucosa occupied by ASM was higher in both asthma groups (P = 0.021 and P = 0.046) compared with controls. In the ASM, eosinophil and T-lymphocyte numbers were higher (P < 0.05) in the high-eosinophil group than both the low-eosinophil group and the controls, whereas the numbers of mast cells were increased in the high-eosinophil group (P = 0.01) compared with controls.

Submucosal eosinophilia is a marker (and possibly a cause) of epithelial damage and is related to infiltration of ASM with eosinophils and T lymphocytes, but is unrelated to mucus metaplasia or smooth muscle hypertrophy.