Differences in airway remodeling between subjects with severe and moderate asthma

Airway remodeling in asthma comprises a range of structural changes. Several studies have suggested an association between these changes and disease severity. The relationship between the extent of remodeling and lung function is not well defined.

We sought to contrast the structural changes in the airways of well-defined groups of subjects with severe and moderate asthma and to correlate the extent of remodeling with disease severity.

Endobronchial biopsy specimens were obtained from 15 subjects with severe and 13 subjects with moderate asthma. Epithelial integrity, cell-layer areas, subepithelial fibrosis, and the distance between epithelial and airway smooth muscle (ASM) layers were measured by means of image analysis. Collagen was identified by using Van Giesen stain, and ASM was defined by using smooth muscle alpha-actin immunostaining. Specific immunostains were performed for the evaluation of RANTES, IL-8, and eotaxin expression as markers of ASM phenotype.

ASM area was greater in subjects with severe (0.24+/- 0.03 mm(2)) than in subjects with moderate (0.05+/- 0.01 mm(2)) asthma (P<.001). The distance between the epithelial and ASM layers was less in the severe group (0.12+/- 0.01 mm) than in the moderate group (0.24+/- 0.02, P<.001). A trend toward greater subepithelial fibrosis in subjects with severe asthma did not reach statistical significance. IL-8 and eotaxin expression, but not RANTES expression, were increased in the ASM of subjects with severe asthma compared with in subjects with moderate asthma.

Smooth muscle alteration is the key structural change that distinguishes severe from moderate asthma, and phenotypic change in ASM might contribute to the difficulty in obtaining adequate control in some subjects with severe asthma.