Pathological studies of bronchial biopsy specimens have confirmed the apparent thickening of lamina reticularis of the epithelial basement membrane. Corticosteroids have proven to be most effective in modifying airway inflammation. However, there is not much data on the effects of corticosteroid‐treatment on the basement membrane.

To investigate the effects of inhaled beclomethasone dipropionate (BDP) on the thickness of basement membrane and cellular infiltration into the bronchial mucosa, and the expression of growth factors in patients with asthma.

We studied bronchial biopsies from 24 asthmatic patients before and after treatment with inhaled BDP, 400 μg twice a day or placebo, for 6 months in a double‐blind manner. Each subject recorded daily asthma symptoms and peak expiratory flow (PEF). Lung function and bronchial responsiveness to methacholine were measured before and after treatment. The thickness of the basement membrane was determined by electron microscopy. Inflammatory cells and the expression of growth factors were examined by immunohistochemistry in endobronchial biopsy specimens.

After 6 months of treatment, we observed a significant improvement of asthma symptoms (P < 0.01), PEF (P < 0.01), diurnal variation of PEF (P < 0.05), and airway responsiveness (P < 0.05) in the BDP group compared with the placebo group. This was accompanied by a significant decrease in the thickness of the lamina reticularis (P < 0.001), and in the number of activated eosinophils (P < 0.01), T‐lymphocytes (P < 0.01), and fibroblasts (P < 0.05) in BDP‐treated patients. There was also a reduction in the expression of insulin‐like growth factor (IGF)‐I (P < 0.01). Significant correlation was found between the IGF‐I expression and collagen thickening (r_s = 0.34, P < 0.01), and the number of fibroblasts (r_s = 0.45, P < 0.01).

These results suggest that corticosteroid treatment in asthma can reduce the lamina reticular thickness by modulation of IGF‐I expression with consequent inhibition of the airway infiltration by inflammatory cells, and therefore may help to prevent remodelling of the airways.