The relationship between eosinophilia and airway remodelling in mild asthma

Abstract

Background: 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.

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

Methods: 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.

Results: 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.

Conclusion: 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.

Keywords: asthma; eosinophil; epithelium; goblet cell; inflammation; remodelling; smooth muscle.

Figure 1. Eosinophil distribution in steroid naïve asthma

Subjects were stratified based on submucosal eosinophil numbers. The range for the whole cohort was zero to 73.21 eosinophills mm⁻². 20 subjects were randomly selected from the lowest quartile (0–2.21 eosinophils mm⁻²)(low eosinophils) and 20 from the uppermost quartile (18.41–73.21 eosinophils mm⁻²) (high eosinophils).

Figure 2. Assessment of epithelial integrity

Staining for EGFR was used to distinguish epithelium that had sustained damage in vivo (EGFR+)(a: middle plate) from that that had been damaged during bronchoscopy or processing (EGFR−)(a: right hand plate). Intact undamaged epithelium was EGFR negative (a: left hand plate). The total epithelial length and the lengths of intact, in vivo and ex vivo damaged epithelium were measured using computer assisted image analysis (b).

Figure 3. Integrity of the epithelium

The percentage of intact epithelium in the high eosinophil group is decreased compared to both healthy controls and asthmatics with low eosinophil counts (a). The in vivo damaged epithelium is increased in the eosinophil high group only in comparison to the eosinophil low group (b).

Figure 4. Epithelial mucin phenotype

Epithelial expression of MUC2 is higher in asthmatics with high eosinophil counts compared to healthy controls but not those with low eosinophil counts (a). Immunohistochemical staining (brown) for MUC2 can be seen within the goblet cells. Scale bar is 50μm.

Figure 5. Influx of inflammatory cells into the airway smooth muscle

Immunohistochemical staining showing the influx of eosinophils (a), T cells (b) and mast cells (c) into the smooth muscle. Inflammatory cells are stained red and those within the ASM bundle were counted (↑). Scale bar is 50μm.

Figure 6. Smooth muscle proportion and inflammatory cell influx

The proportion of airway smooth muscle (ASM)(a) is increased in asthmatics with low and high eosinophil counts compared to healthy controls. Positive immunostained nucleated cells were counted in the ASM. More eosinophils (b) and T cells (c) are observed in the ASM of the asthmatics with high eosinophils counts compared to those with low counts and to the healthy controls. Mast cell numbers (d) are increased in subjects with high eosinophil counts compared to healthy controls but not low eosinophil counts.

Figure 6. Smooth muscle proportion and inflammatory cell influx

The proportion of airway smooth muscle (ASM)(a) is increased in asthmatics with low and high eosinophil counts compared to healthy controls. Positive immunostained nucleated cells were counted in the ASM. More eosinophils (b) and T cells (c) are observed in the ASM of the asthmatics with high eosinophils counts compared to those with low counts and to the healthy controls. Mast cell numbers (d) are increased in subjects with high eosinophil counts compared to healthy controls but not low eosinophil counts.