Monitoring airway remodeling in asthma

Authors


  • Conflicts of interest
    This article forms part of a supplement sponsored by GlaxoSmithKline. Kjell Larsson has, during the last 5 years, on one or more occasion served on an advisory board and/or served as speaker and/or participated in education arranged by AstraZeneca, Boehringer Ingelheim, GlaxoSmithKline, MSD, Nycomed and Pfizer. He has also received unrestricted research grants from Boehringer Ingelheim, GlaxoSmithKline and AstraZeneca during the last 5 years

Kjell Larsson, MD, Unit of Lung and Allergy Research, National Institute of Environmental Medicine, Karolinska Institutet
SE 171 77 Stockholm, Sweden.
Tel: +46 8 52 48 74 33
Fax: +46 8 300 619
email: kjell.larsson@ki.se

Abstract

Introduction:  Airway remodeling occurs in both mild and severe forms of asthma but, from a clinical point of view, airway remodeling in asthma is difficult to monitor.

Objectives:  The objective of this overview is to make an inventory of which methods could be possible to monitor airway remodeling in asthma.

Methods:  Access to airway tissue through biopsies or material from surgery enables direct assessment of airway remodeling but there are no specific inflammatory markers obtained from, for example, sputum, lavage fluid, blood, exhaled air, exhaled breath condensate, urine or saliva that reflect certain aspects of airway remodeling. Physiological measures such as changes in lung function and bronchial responsiveness over time co-varies with changes in airway structure but these interactions are complex and non-specific. Novel imaging techniques have shown promising results and recent studies have demonstrated how structural airway and lung changes can be detected on computerized tomography.

Results and Conclusion:  Today, there are no available techniques for monitoring airway remodeling in daily clinical practice, but further development within this area and studies on co-variation between physiologic, inflammatory and visual abnormalities will likely enable us to better monitor airway remodeling in the future.

Please cite this paper as: Larsson K. Monitoring airway remodeling in asthma. Clin Respir J 2010; 4 (Suppl. 1): 35–40.

Introduction

‘Remodeling is an alteration in the size, mass, or number of tissue structural components that occurs during growth or in response to injury and/or inflammation’(1). Remodeling, thus, includes a number of morphologic alterations in the airways and is a general consequence of repeated tissue injury (Table 1). Apart from the airways (e.g. asthma, chronic obstructive pulmonary disease (COPD), cystic fibrosis) remodeling is observed in other tissues such as the skin (e.g. scleroderma), gut (e.g. inflammatory bowel disease), in wound repair, etc. In asthma, remodeling mostly occurs in central airways whereas in COPD the lung parenchyma and small airways are affected to a larger extent. There are a number of potential approaches when assessing structural airway changes and remodeling. Tissue can be obtained at autopsy and surgery and by endo- and transbronchial biopsies. Material can also be obtained from bronchoalveolar lavage fluid, sputum, blood, saliva, exhaled air, exhaled breath condensates and urine, and analyzed for cell and mediator content. In addition, information about rebuilding of the airways can be obtained by imaging techniques such as computerized tomography and endobronchial ultrasound and by physiologic methods, like spirometry, response to bronchodilators and bronchial challenge (Table 2).

Table 1.  Morphologic airway alterations in asthma
Loss of epithelial integrity
 Epithelial fragility
 Goblet cell hyperplasia
Subepithelial fibrosis
 Thickening of the lamina reticularis
 Deposition of extracellular matrix
Increased smooth muscle layer
 Smooth muscle hyperplasia
 Smooth muscle hypertrophy
 Migration of smooth muscle cells to the subepithelial area
Increased mucus secretion due to
 Goblet cell enlargement
 Submucous glands enlargement
Decreased cartilage integrity
Increased airway vascularity
 Increased vascular size
 Angiogenesis
Table 2.  Potential approaches for assessment of airway remodeling
Analyses of airway/lung tissues and cells
 Tissue from autopsy
 Tissue from surgery
 Endobronchial biopsies
 Transbronchial biopsies
 Bronchoalveolar lavage
 Sputum
Physiology
 Spirometry
 Bronchodilator response
 Bronchial challenges
Imaging
 Computerized tomography
 Endobronchial ultrasound
Other tissue/cell related analyses
 Exhaled air
 Exhaled breath condensates
 Blood
 Urine
 Saliva

Tissue remodeling

Airway smooth muscles are organized as spirals around the airways and muscle contraction leads to airway constriction as well as airway shortening. A direct consequence of airway remodeling is that the airways become rigid, which is caused by edema, increased thickening of the reticular basement membrane (Rbm), increased collagen deposition and/or vascular congestion and increased thickness of the smooth muscle layer. One of the consequences of these changes is that airway smooth muscle contraction results in airway constriction rather than airway shortening (2). The tissue alterations in asthma are also characterized by airway wall thickening, airway fibrosis, smooth muscle mass increase, increased vascularity, increased mucus glands and altered extracellular matrix composition (3).

In studies using helical computed tomography (CT), airway wall thickening has been clearly demonstrated in asthma and the increased thickness may be related to disease severity (4–6). The most important cause of the airway thickening is the increased smooth muscle mass because of both hypertrophy and hyperplasia. There is also an increase of the collagen layer, which is approximately 5 µm in healthy subjects and may be over and above 20 µm in asthmatic subjects (3).

Thickening of the Rbm is a characteristic feature of asthma. Subepithelial fibrosis contributes to the alteration of airway distensibility in asthma (7) and seems to be related to airway hyperresponsiveness (8). There does not seem to be a close relationship between the degree of subepithelial fibrosis and the duration of the disease (9–11) or the severity of airway inflammation (9, 10). It has, however, been claimed that there is a relationship between the thickness of the Rbm and the duration and severity of asthma (12). In the study by Shiba et al., a negative relationship between FEV1 and bronchial responsiveness on one hand and Rbm thickness on the other hand was shown (12). The thickening of Rbm occurs at an early stage of the asthma disease and may be present even before asthma is diagnosed. The thickening of the Rbm is thus a feature not restricted to asthma. The tenascin-specific immuno-reactivity band in the Rbm was thicker in cross country skiers than in controls irrespective of the presence or absence of bronchial hyperresponsiveness and asthma (13). It is not known whether or not these athletes later developed clinical asthma.

Also, the increased vascularity contributes to the thickening of the airway wall in asthma (14) and seems to be related to disease severity (15). There is a clear relationship between increased vascularity in the airway mucosa and Rbm thickening (16). Angiogenesis is a characteristic feature of severe asthma but is also observed in mild disease (17).

Goblet cell hyperplasia is not only a characteristic feature of chronic bronchitis but is also present in asthma (18). In asthma, unlike COPD, goblet cells are not increased in small airways but in the central airways. In asthma, there is also a hyperplasia of submucosal glands, similar to what is found in chronic bronchitis.

Inflammatory mediators

There are a number of mediators contributing to remodeling of the airways in asthma. The mediators may be multifunctional and exhibit a number of different effects and one mediator may emanate from a number of different cells. Thus, transforming factor-β1 (TGF-β1), of which one function is to stimulate fibroblast and smooth muscle proliferation, is produced by most cells within the lungs, i.e. macrophages, eosinophils, lymphocytes, epithelial cells and fibroblasts (Fig. 1). The expression of TGF-β1 is related to subepithelial fibrosis (19, 20) and it has been demonstrated that alveolar macrophages from asthmatic subjects produce more TGF-β1 than do macrophages from healthy controls (21, 22).

Figure 1.

Number of cells contributing to subepithelial fibrosis in asthma (here exemplified by the central role of a growth factor, TGF-β).

In animal experiments, it has been shown that IL-13 causes an inflammatory response with eosinophilic infiltration, tissue inflammation, subepithelial fibrosis, goblet cell hyperplasia, mucus secretion, airway obstruction and bronchial hyperresponsiveness (3, 23). In studies on transgenic mice, it has been demonstrated that IL-13 is involved in cellular airway infiltration and development of airway hyperresponsiveness(23). It has been established that IL-13 mediates increased bronchial responsiveness through a signaling molecule, signal transducer and activator of transcription 6 (STAT6), which has shown an increased expression in airway epithelial cells from asthmatic subjects (24). In a study by Mullings et al. it was demonstrated that the expression of STAT6 is increased in patients with severe asthma compared with those with mild asthma and healthy controls (25). The cytokine IL-13 thus seems to be of great importance for airway responsiveness whereas its ‘close relative’ IL-4, which is important for eosinophil influx into the airways in asthma, does seem to be of less importance for the development of airway hyperresponsiveness (26).

Physiology

An increased rate of lung function decline with age has been demonstrated in adult asthmatics, compared with healthy individuals, indicating progressive airway changes over time (27). This observation is supported by the finding of a relationship between the response to inhaled steroid treatment and the duration symptoms prior to commencement of steroid treatment, likely indicating that the absence of steroid therapy allows airway remodeling, such as fibrosis, and the development of irreversible airway obstruction (28). In line with these findings, there are data indicating that delayed inhaled steroid treatment may contribute to the development of airway hyperresponsiveness and that bronchial hyperresponsiveness improves less if the commencement of steroid treatment is delayed (29).

Bronchial hyperresponsiveness to direct stimuli is a function of airway geometry, airway inflammation with mucosal thickening and airway remodeling. Airway remodeling is not a prerequisite for increased bronchial responsiveness to direct stimuli as acute exposure to organic material can induce substantial increase in bronchial responsiveness in healthy, non-asthmatic subjects (30, 31). It is thus obvious that both short-term changes following an acute exposure as well as long-term restructuring of the airways lead to increased airway responsiveness to direct stimuli. In asthmatic subjects, there is a positive correlation between airway responsiveness, on one hand, and Rbm thickening and the number of fibroblasts in bronchial biopsies, on the other hand (21).

Mucosal vascularity seems to be inversely related to lung function and airway responsiveness and treatment with beclomethasone for 6 months led to decreased vascularity and improved lung function and bronchial responsiveness (16). In another study, a negative correlation between airway wall thickness and bronchial responsiveness to methacholine was clearly demonstrated (32). Taking all this information together, it could be speculated that it is not primarily the forces leading to airway contraction that are of most importance for the development of bronchial hyperresponsiveness but rather the properties of the airway wall and its ability to stay patent despite contracting forces. Thus, if airway wall characteristics define airway responsiveness, it may, at least to some extent, explain the link between airway remodeling and bronchial hyperresponsiveness.

In summary, there are a number of alterations in airway physiology relating to airway remodeling. Impaired ventilation, increased mucus secretion and a positive correlation between subepithelial basement membrane thickening and airway hyperresponsiveness seem to be related whereas the relationship between airway wall thickness in general and bronchial hyperresponsiveness is less obvious. The changes seem to be, at least partly, reversible.

Imaging

Computerized tomography (CT) has been used for qualitative descriptions of airways and lung alterations in asthma. Thus, airway wall thickening, mucoid impaction, atelectasis, airspace opacities and bronchial dilatation have been described in asthma (6). During recent years, quantitative estimations of CT abnormalities have also become available and airway wall thickness has been evaluated in several studies (5, 33, 34). In order to reduce the time of investigation and the radiation dose, standards to focus on one airway rather than to investigate multiple airways have been developed. Thus, thin-section helical scans of the right upper lobe bronchus were suggested as the target for examination (35, 36). This technique is very work consuming and demands careful standardization procedures. As previously mentioned, the main factors contributing to increased airway wall thickness in asthma are vascular proliferation, smooth muscle hypertrophy and hyperplasia, increased thickness of the sub-basement membrane because of deposition of extra-cellular matrix in the airway wall, edema and infiltration of inflammatory cells. It has been demonstrated that airway wall thickness relates to the Rbm thickness (34). Airway wall thickening is not only related to duration and severity of asthma but is also observed in mild disease as assessed by CT (34, 36). There are no clear relationship between airway wall thickness and markers of airway inflammation, implicating that there is no easily accessible sputum or blood marker that can be used as a surrogate marker for airway wall thickening.

It could be anticipated that airway wall thickening would be strongly related to bronchial hyperresponsiveness but studies in this area are not fully conclusive (5, 37). Niimi et al. showed that bronchial responsiveness to a direct stimulus even is negatively related to airway wall thickness, likely because of the increase airway wall stiffness (32).

In a study by Gono et al., a clear relationship between airway wall thickness and bronchial obstruction was found, indicating that physiological outcome measures directly reflect remodeling of the airways (38). In that study, airway obstruction was assessed as air trapping identified by CT performed at full inspiration and expiration. The relationship between airway wall thickness and air trapping was more clear in patients who did not respond well to bronchodilators than in those who had reversible airway obstruction (38).

Other tissue/cell related analyses

The measurement of exhaled nitric oxide (NO) has been widely used as a marker of airway inflammation in asthma and seems to be more closely related to allergy than to asthma per se(39–41). High exhaled NO levels are almost instantaneously lowered and may even be normalized when treatment with inhaled steroid commences in asthmatic subjects. Although airway inflammation in asthma is closely related to airway remodeling, there is no clear relationship between exhaled NO levels and indices of airway remodeling.

Exhaled breath condensates have been only sparsely studied in this respect and there are no conclusive data. In a study by Lex et al., it was indicated that cysteinyl-leukotrienes in exhaled breath condensate are related to thickness of Rbm as assessed by bronchial biopsies in asthmatic children (42). Endothelin-1 in exhaled breath condensate was found to be increased in asthmatic subjects compared with healthy controls and higher in those with unstable disease than in stable asthma (43).

Apart from the assessment of specific IgE and cellular pattern, blood analyses are of minor importance in asthma and there are no blood markers to be use with the aim of monitoring airway remodeling in asthma. Although there are some studies in which analyses, mainly of arachidonic acid metabolites, in urine and saliva have been performed in asthma (44–46), there are no specific markers suitable for monitoring of airway remodeling at present.

Effects of steroids

It has been shown that treatment with inhaled glucocorticosteroids reduces airway wall thickening in asthmatic patients (47) and Rbm thickening was reduced following long-term treatment with inhaled steroids as shown in a 1-year placebo controlled study of fluticasone (48). As the effect of steroids in this respect seems to be inversely related to the duration of the disease, it could be hypothesized that airway wall thickening is one factor contributing to the development of irreversible airflow obstruction. Six-month treatment with inhaled steroids decreased vessel number and vascularity in the airway wall as assessed by biopsies (16). The changes induced by steroid treatment were accompanied by decreased thickness of the sub-epithelial basement membrane and diminished bronchial responsiveness (16). In a study by Chetta et al., the vascular area, number of vessels and thickness of sub-epithelial basement membrane, respectively, decreased following 6 weeks of treatment with an inhaled steroid (49).

Ancillary