Pathological features and inhaled corticosteroid response of eosinophilic and non-eosinophilic asthma

Background

Non‐eosinophilic asthma is a potentially important clinicopathological phenotype since there is evidence that it responds poorly to inhaled corticosteroid therapy. However, little is known about the underlying airway immunopathology and there are no data from placebo‐controlled studies examining the effect of inhaled corticosteroids.

Methods

Airway immunopathology was investigated using induced sputum, bronchial biopsies, bronchial wash and bronchoalveolar lavage in 12 patients with symptomatic eosinophilic asthma, 11 patients with non‐eosinophilic asthma and 10 healthy controls. The patients with non‐eosinophilic asthma and 6 different patients with eosinophilic asthma entered a randomised, double‐blind, placebo‐controlled crossover study in which the effects of inhaled mometasone 400 μg once daily for 8 weeks on airway responsiveness and asthma quality of life were investigated.

Results

Patients with non‐eosinophilic asthma had absence of eosinophils in the mucosa (median 4.4 cells/mm² vs 23 cells/mm² in eosinophilic asthma and 0 cells/mm² in normal controls; p = 0.03) and normal subepithelial layer thickness (5.8 μm vs 10.3 μm in eosinophilic asthma and 5.1 μm in controls, p = 0.002). Non‐eosinophilic and eosinophilic asthma groups had increased mast cell numbers in the airway smooth muscle compared with normal controls (9 vs 8 vs 0 cells/mm², p = 0.016). Compared with placebo, 8 weeks of treatment with inhaled mometasone led to less improvement in methacholine PC₂₀ (0.5 vs 5.5 doubling concentrations, 95% CI of difference 1.1 to 9.1; p = 0.018) and asthma quality of life (0.2 vs 1.0 points, 95% CI of difference 0.27 to 1.43; p = 0.008).

Conclusions

Non‐eosinophilic asthma represents a pathologically distinct disease phenotype which is characterised by the absence of airway eosinophilia, normal subepithelial layer thickness and a poor short‐term response to treatment with inhaled corticosteroids.Clinicians have long regarded asthma as a heterogeneous disease,^1,2 although detailed clinicopathological studies have tended to emphasise the similarities in the underlying airway pathology and disordered function between patients.^3,4 The development of safe non‐invasive induced sputum techniques has provided the opportunity to study airway inflammation in a diverse range of patients. Using this technique, we and a number of other groups have identified a subset of adults who have clear physiological evidence of asthma but no induced sputum evidence of eosinophilic airway inflammation.^5,6,7 This asthma phenotype is potentially clinically important since several uncontrolled studies have suggested that it is associated with a poor short‐term and longer‐term response to inhaled corticosteroid.^5,8,9Non‐eosinophilic asthma is present in 53% of patients presenting to an adult respiratory clinic with symptomatic asthma.⁹ Other investigators have reported the absence of a sputum eosinophilia in up to 50% of patients with refractory asthma,⁹ patients studied during an asthma exacerbation¹⁰ and patients taking high doses of inhaled corticosteroids.⁶ In a recent longitudinal study of patients with severe asthma, the absence of sputum eosinophils has been reported to be a stable feature in a number of patients observed over 12 months;¹¹ another study showed that it was present in untreated symptomatic patients as well as those receiving inhaled corticosteroid therapy.⁹ These observations suggest that, in some patients at least, non‐eosinophilic asthma is a stable phenotype that is not solely explained by the effects of corticosteroid therapy.Several studies have noted that an airway neutrophilia is often present in patients with non‐eosinophilic asthma, and Wenzel et al⁷ reported a predominantly neutrophilic airway inflammatory response with an absence of eosinophils and normal basement membrane thickness in a subgroup of patients with refractory asthma from whom bronchial biopsy specimens were taken. These findings support the concept that non‐eosinophilic asthma is a pathologically distinct entity, although the extent to which these findings reflect the effects of treatment remains unclear.The aim of this study was to compare the immunopathology of eosinophilic and non‐eosinophilic asthma with normal controls in patients with symptomatic asthma who were not treated with inhaled corticosteroids. We also set out to compare the response to 8 weeks of treatment with the inhaled corticosteroid mometasone in a prospective randomised, double‐blind, placebo‐controlled crossover trial in patients with non‐eosinophilic asthma and in a subgroup with eosinophilic asthma.