The role of the distal lung in asthma

Pro-Resolving Effects of Resolvin D2 in LTD4 and TNF-α Pre-Treated Human Bronchi

Inflammation is a major burden in respiratory diseases, resulting in airway hyperresponsiveness. Our hypothesis is that resolution of inflammation may represent a long-term solution in preventing human bronchial dysfunctions. The aim of the present study was to assess the anti-inflammatory effects of RvD₂, a member of the D-series resolving family, with concomitant effects on ASM mechanical reactivity. The role and mode of action of RvD₂ were assessed in an in vitro model of human bronchi under pro-inflammatory conditions, induced either by 1 μM LTD₄ or 10 ng/ml TNF-α pre-treatment for 48h. TNF-α and LTD₄ both induced hyperreactivity in response to pharmacological stimuli. Enhanced 5-Lipoxygenase (5-LOX) and cysteinyl leukotriene receptor 1 (CysLTR1) detection was documented in LTD₄ or TNF-α pre-treated human bronchi when compared to control (untreated) human bronchi. In contrast, RvD₂ treatments reversed 5-LOX/β-actin and CysLTR1/β-actin ratios and decreased the phosphorylation levels of AP-1 subunits (c-Fos, c-Jun) and p38-MAP kinase, while increasing the detection of the ALX/FPR2 receptor. Moreover, various pharmacological agents revealed the blunting effects of RvD₂ on LTD₄ or TNF-α induced hyper-responsiveness. Combined treatment with 300 nM RvD₂ and 1 μM WRW4 (an ALX/FPR2 receptor inhibitor) blunted the pro-resolving and broncho-modulatory effects of RvD₂. The present data provide new evidence regarding the role of RvD₂ in a human model of airway inflammation and hyperrresponsiveness.

[Inflammation and remodeling of the distal airways: studies in humans and experimental models]

Asthma is characterized by inflammation and remodeling of the airways, giving rise to airway obstruction and symptoms of wheezing, chest tightness, cough and dyspnea. Most of these observations arise from the study of samples obtained from the central airways by distinct methods. However, it is currently accepted that this inflammatory process occurs not only in the central airway but also in the small airway and even in the pulmonary parenchyma of all asthmatic patients, even those with mild asthma. CD4+ lymphocytes, activated eosinophils and IL-5 mRNA expression are present in a greater quantity in the small airways. Also present is remodeling, with an increase in submucosal thickness, the muscular layer and adventitia. This inflammatory process causes a disconnection between the pulmonary parenchyma and the airway, giving rise to obstruction of the small airway, which is currently considered to be predominant in asthmatic patients. Likewise, studies of experimental asthma in animals support the substantial role of the distal airway. Recognition that asthma affects the entire airway could be clinically important and lead to the distal lung being considered as a target in any effective therapeutic strategy. However, longitudinal studies are required to evaluate the impact of distal airway inflammation and its treatment in asthma.

Disparity between proximal and distal airway reactivity during methacholine challenge

There is an increasing awareness of the role of distal airways in the pathophysiology of obstructive lung diseases including asthma and chronic obstructive pulmonary disease. We hypothesize that during induced bronchoconstriction: 1) disparity between distal and proximal airway reactivity may occur; and 2) changes in distal airway function may explain symptom onset in subjects with minimal FEV(1) change. 185 subjects underwent methacholine challenge testing (MCT). In addition to spirometry, oscillometry was performed at baseline and after maximum dose of methacholine; 33/185 also underwent oscillometry after each dose. Oscillometric parameters included resistance at 5 and 20 Hz (R(5), R(20)) and heterogeneity of distal airway mechanics assessed by frequency dependence of resistance 5-20 Hz (R(5-20)) and reactance area (AX). R(5) varied widely during MCT (range -0.8 - 11.3 cmH(2)O/L/s) and correlated poorly with change in FEV(1) (r = 0.17). Changes in R(5) reflected changes in both R(20) and R(5-20) (r = 0.59, p<0.05; r = 0.87, p<0.0001). However, R(20) increased only 0.3 cmH(2)O/L/s, while R(5-20) increased 0.7 cmH(2)O/L/s for every 1cmH(2)O/L/s change in R(5,) indicating predominant effect of distal airway mechanics. 9/33 subjects developed symptoms despite minimal FEV(1) change (<5%), while R(5) increased 42% due to increased distal airway heterogeneity. These data indicate disparate behavior of proximal airway resistance (FEV(1) and R(20)) and distal airway heterogeneity (R(5-20) and AX). Distal airway reactivity may be associated with methacholine-induced symptoms despite absence of change in FEV(1). This study highlights the importance of disparity between proximal and distal airway behavior, which has implications in understanding pathophysiology of obstructive pulmonary diseases and their response to treatment.

Part III: Location of asthma inflammation and the distal airways: clinical implications

Accumulating evidence suggests that the airway inflammation and remodeling characteristic of asthma occur not only in the central airways, but also in the distal lung and the lung parenchyma. The distal airways are increasingly being recognized as important sites of airflow obstruction. Research indicates that distal inflammation may play a crucial role in airway hyperresponsiveness, nocturnal asthma, and spontaneous exacerbations of asthma symptoms. Although the effectiveness of inhaled corticosteroids in improving asthmatic symptoms and preventing exacerbations is well established, the ability of conventional formulations to reach the distal airways is limited. The impaired ability of these formulations to reach the distal airways may contribute to the observation that inhaled corticosteroids do not always provide adequate control of asthma symptoms. Newer formulations of inhaled corticosteroids that use hydrofluoroalkane (HFA) propellants in solution have greater access to the distal airways and produce beneficial changes in lung function. Due to their highly targeted delivery systems, newer HFA-based formulations have the potential to effectively treat asthma at reduced doses.

Part II: Inhaler technique and adherence to therapy

Although the importance of effective asthma management has been well established, in many patients asthma control remains suboptimal. While numerous factors likely contribute to inadequate control of asthma symptoms, studies have demonstrated that many patients fail to use their inhaler devices correctly, which may compromise asthma treatment. To ensure that patients use their inhaler devices appropriately, the choice of inhaler should be matched to each individual patient's capabilities and preferences. Thorough patient education regarding the effective use of the selected inhaler, repeated demonstrations on proper inhalation technique, and frequent patient follow-up is necessary to ensure that inhalers are operated correctly. Patient adherence to therapy, which traditionally has been poor, is another factor that may negatively affect asthma control. Patient attitudes and perceptions regarding the effectiveness of therapy, the risks of inadequate asthma control, and the need for daily controller medications all influence compliance. Consequently, effective education regarding the benefits of treatment and the risks of nonadherence is critical to enhancing patient adherence, self-care skills, and, potentially, asthma control.

Regional differences in the pattern of airway remodeling following chronic allergen exposure in mice

Background

Airway remodeling present in the large airways in asthma or asthma models has been associated with airway dysfunction in humans and mice. It is not clear if airways distal to the large conducting airways have similar degrees of airway remodeling following chronic allergen exposure in mice. Our objective was to test the hypothesis that airway remodeling is heterogeneous by optimizing a morphometric technique for distal airways and applying this to mice following chronic exposure to allergen or saline.

Methods

In this study, BALB/c mice were chronically exposed to intranasal allergen or saline. Lung sections were stained for smooth muscle, collagen, and fibronectin content. Airway morphometric analysis of small (0–50000 μm²), medium (50000 μm²–175000 μm²) and large (>175000 μm²) airways was based on quantifying the area of positive stain in several defined sub-epithelial regions of interest. Optimization of this technique was based on calculating sample sizes required to detect differences between allergen and saline exposed animals.

Results

Following chronic allergen exposure BALB/c mice demonstrate sustained airway hyperresponsiveness. BALB/c mice demonstrate an allergen-induced increase in smooth muscle content throughout all generations of airways, whereas changes in subepithelial collagen and fibronectin content are absent from distal airways.

Conclusion

We demonstrate for the first time, a systematic objective analysis of allergen induced airway remodeling throughout the tracheobronchial tree in mice. Following chronic allergen exposure, at the time of sustained airway dysfunction, BALB/c mice demonstrate regional differences in the pattern of remodeling. Therefore results obtained from limited regions of lung should not be considered representative of the entire airway tree.