New perspectives on basic mechanisms in lung disease. 4. Why are the airways so vascular?

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

MECHANICAL VENTILATION FOR THE LUNG TRANSPLANT RECIPIENT

Mechanical ventilation (MV) is an important aspect in the intraoperative and early postoperative management of lung transplant (LTx)-recipients. There are no randomized-controlled trials of LTx-recipient MV strategies; however there are LTx center experiences and international survey studies reported. The main early complication of LTx is primary graft dysfunction (PGD), which is similar to the adult respiratory distress syndrome (ARDS). We aim to summarize information pertinent to LTx-MV, as well as PGD, ARDS, and intraoperative MV and to synthesize these available data into recommendations. Based on the available evidence, we recommend lung-protective MV with low-tidal-volumes (≤6 mL/kg predicted body weight [PBW]) and positive end-expiratory pressure for the LTx-recipient. In our opinion, the MV strategy should be based on donor characteristics (donor PBW as a parameter of actual allograft size), rather than based on recipient characteristics; however this donor-characteristics-based protective MV is based on indirect evidence and requires validation in prospective clinical studies.

Asthma is not only an airway disease, but also a vascular disease

Multiple studies have identified an expansion and morphological dysregulation of the bronchial vascular network in the airways of asthmatics. Increased number, size and density of blood vessels, as well as vascular leakage and plasma engorgement, have been reported in the airways of patients with all grades of asthma from mild to fatal. This neovascularisation is an increasingly commonly reported feature of airway remodelling; however, the pathophysiological impact of the increased vasculature in the bronchial wall and its significance to pulmonary function in asthma are unrecognised at this time. Multiple factors capable of influencing the development and persistence of the vascular network exist within asthmatic airway tissue. These include structural components of the altered extracellular matrix (ECM), imbalance of proteases and their endogenous inhibitors, release of active matrikines and the dysregulated levels of both soluble and matrix sequestered growth factors. This review will explore the features of the asthmatic airway which influence the development and persistence of the increased vascular network, as well as the effect of enhanced tissue perfusion on chronic inflammation and airway dynamics. The response of cells of the airways to the altered vascular profile and the subsequent influence on the features of airway remodelling will also be highlighted. We will explore the failure of current asthma therapeutics in "normalising" this vascular remodelling. Finally, we will summarize the outcomes of recent clinical trials which provide hope that anti-angiogenic therapies may be a potent asthma-resolving class of drugs and provide a new approach to asthma management in the future.

Neuronal modulation of airway and vascular tone and their influence on nonspecific airways responsiveness in asthma

The autonomic nervous system provides both cholinergic and noncholinergic neural inputs to end organs within the airways, which includes the airway and vascular smooth muscle. Heightened responsiveness of the airways to bronchoconstrictive agents is a hallmark feature of reactive airways diseases. The mechanisms underpinning airways hyperreactivity still largely remain unresolved. In this paper we summarize the substantial body of evidence that implicates dysfunction of the autonomic nerves that innervate smooth muscle in the airways and associated vasculature as a prominent cause of airways hyperresponsiveness in asthma.

The role of the bronchial microvasculature in the airway remodelling in asthma and COPD

In recent years, there has been increased interest in the vascular component of airway remodelling in chronic bronchial inflammation, such as asthma and COPD, and in its role in the progression of disease. In particular, the bronchial mucosa in asthmatics is more vascularised, showing a higher number and dimension of vessels and vascular area. Recently, insight has been obtained regarding the pivotal role of vascular endothelial growth factor (VEGF) in promoting vascular remodelling and angiogenesis. Many studies, conducted on biopsies, induced sputum or BAL, have shown the involvement of VEGF and its receptors in the vascular remodelling processes. Presumably, the vascular component of airway remodelling is a complex multi-step phenomenon involving several mediators. Among the common asthma and COPD medications, only inhaled corticosteroids have demonstrated a real ability to reverse all aspects of vascular remodelling. The aim of this review was to analyze the morphological aspects of the vascular component of airway remodelling and the possible mechanisms involved in asthma and COPD. We also focused on the functional and therapeutic implications of the bronchial microvascular changes in asthma and COPD.

Sympathetic nerve-dependent regulation of mucosal vascular tone modifies airway smooth muscle reactivity

The airways contain a dense subepithelial microvascular plexus that is involved in the supply and clearance of substances to and from the airway wall. We set out to test the hypothesis that airway smooth muscle reactivity to bronchoconstricting agents may be dependent on airway mucosal blood flow. Immunohistochemical staining identified vasoconstrictor and vasodilator nerve fibers associated with subepithelial blood vessels in the guinea pig airways. Intravital microscopy of the tracheal mucosal microvasculature in anesthetized guinea pigs revealed that blockade of α-adrenergic receptors increased baseline arteriole diameter by ~40%, whereas the α-adrenergic receptor agonist phenylephrine produced a modest (5%) vasoconstriction in excess of the baseline tone. In subsequent in vivo experiments, tracheal contractions evoked by topically applied histamine were significantly reduced (P < 0.05) and enhanced by α-adrenergic receptor blockade and activation, respectively. α-Adrenergic ligands produced similar significant (P < 0.05) effects on airway smooth muscle contractions evoked by topically administered capsaicin, intravenously administered neurokinin A, inhaled histamine, and topically administered antigen in sensitized animals. These responses were independent of any direct effect of α-adrenergic ligands on the airway smooth muscle tone. The data suggest that changes in blood flow in the vessels supplying the airways regulate the reactivity of the underlying airway smooth muscle to locally released and exogenously administered agents by regulating their clearance. We speculate that changes in mucosal vascular function or changes in neuronal regulation of the airway vasculature may contribute to airways responsiveness in disease.

Therapeutic perspectives in bronchial vascular remodeling in COPD

COPD may be characterized by significant changes in airway mucosal blood vessels, which may contribute to bronchial airway remodeling. The airway wall is more vascularized in COPD patients than in healthy subjects, though this phenomenon is less evident than in asthmatic patients. The vascular mucosal changes in the airways of patients with COPD are strictly linked to the inflammatory processes. The cellular mechanisms responsible for the microvascular changes are still unclear, however, pro-angiogenic factors, such as VEGF, TGF-beta, FGF, and proteolytic enzymes such as MMPs, may play a role. Up to now, the clinical and functional consequences of this phenomenon and the therapeutic approach have been scarcely investigated. Inhaled corticosteroids seem to have positive effects, by reducing the vascular area and growth factor expression. Specific antagonists to VEGF, TGF- beta, FGF, and MMPs could beneficially control chronic airway inflammation and vascular remodeling in COPD as well as slow down the progression of the disease. Several of these growth factor antagonists are being evaluated and some seem to be effective in reducing vascularity, however further studies are required to ascertain whether or not these antagonists may play a role in COPD therapy.

Airway vascular remodeling in asthma

Several characteristic changes occur in the bronchial wall in asthma, including specific changes to the vasculature. These result in an increase in vessel numbers per unit area, as well as increased vessel activity suggested by vasodilatation, vessel leakage, and cellular margination with transmigration to target tissues. This combined action in asthma leads to airway-wall thickening and reduced airflow. Each component of the vascular response has been shown to be controlled by a range of inflammatory mediators and growth factors. These factors are themselves regulated by a complex process initially involving gene expression, transcription, and translation at the molecular level, then subsequent protein release, binding to matrix elements, endothelial cell activation, and a proliferative endothelial response. Many commonly used airway medications are capable of modulating the vascular response to inflammatory stimuli. New therapies might improve airflow through better regulation of vessel growth, dilatation, and leakage in the airway wall.

Angiogenesis in paediatric airway disease

A number of characteristic changes occur in the bronchial wall in paediatric airway diseases. The process of remodelling is usually associated with specific changes to the vasculature, resulting in an increase in vessel numbers, vasodilatation, vessel leakage and cellular margination with transmigration to target tissues. This combined action in conditions such as asthma, cystic fibrosis and bronchiolitis lead to airway wall thickening and reduced airflow. Each component of the vascular response has been shown to be controlled by a range of inflammatory mediators and growth factors. These factors are regulated by a complex process involving gene expression, transcription and translation at the molecular level, protein release, binding to matrix elements and receptors on endothelial cells, then the endothelial response itself. A number of commonly used airway medications are potentially capable of modulating the vascular response to inflammatory stimuli. New therapies may be able to improve airflow through better regulation of vessel growth, dilatation and leakage in the airway wall.

Remodeling in asthma and chronic obstructive lung disease

Asthma and chronic obstructive lung disease (COPD) are both inflammatory conditions of the lung associated with structural "remodeling" inappropriate to the maintenance of normal lung function. The clinically observed distinctions between asthma and COPD are reflected by differences in the remodeling process, the patterns of inflammatory cells and cytokines, and also the predominant anatomic site at which these alterations occur. In asthma the epithelium appears to be more fragile than that of COPD, the epithelial reticular basement membrane (RBM) is significantly thicker, there is marked enlargement of the mass of bronchial smooth muscle, and emphysema does not occur in the asthmatic nonsmoker. In COPD, there is epithelial mucous metaplasia, airway wall fibrosis, and inflammation associated with loss of surrounding alveolar attachments to the outer wall of small airways: bronchiolar smooth muscle is increased also. Emphysema is a feature of severe COPD: in spite of the destructive process, alveolar wall thickening and focal fibrosis may be detected. The hypertrophy of submucosal mucus-secreting glands is similar in extent in asthma and COPD. The number of bronchial vessels and the area of the wall occupied by them increase in severe corticosteroid-dependent asthma: it is likely that these increases also occur in severe COPD as they do in bronchiectasis. Pulmonary vasculature is remodeled in COPD. In asthma several of these structural alterations begin early in the disease process, even in the child. In COPD the changes begin later in life and the associated inflammatory response differs from that in asthma. The following synopsis defines and compares the key remodeling processes and proposes several hypotheses.

Microvascular anatomy of the nose

The microvasculature of the nose consists of: 1) A dense subepithelial network of capillaries, with fenestrations between the endothelial cells. This network provides nutrients to the epithelium and glands, and allows passage of water into the lumen for evaporation and air-conditioning. 2) A system of capacitance vessels or sinuses, which when they distend, block the nasal lumen, and when they empty, open the nasal passages. Changes in their volume will affect the filtering and air-conditioning functions of the nose. 3) Arteriovenous anastomoses which allow rapid passage of blood through the mucosa. They are probably important in air-conditioning, and in the countercurrent mechanisms that tend to keep the brain cool in a hot dry climate. The anatomical interrelationships between these different systems is not well understood, nor is their differential control in terms of actions of mediators and nerves. In neurogenic inflammation sensory nerves are excited and release local mediators such as substance P via axon reflexes. These sensory neuropeptides will cause vasodilatation, vascular congestion and extravasation of liquid from the postcapillary venules, with resultant oedema and exudate. They may also cause secretion from the submucosal glands.

Comparative morphology of the airways in asthma and chronic obstructive pulmonary disease

Asthma and chronic obstructive pulmonary disease (COPD) are complex conditions with imprecise definitions that make definitive morphological comparisons difficult. Asthma may be divided into extrinsic (allergic), intrinsic (late onset), and occupational forms. The airways in fatal asthma are occluded by markedly tenacious plugs of exudate and mucus; there is fragility of airway surface epithelium, thickening of the reticular basement membrane, and bronchial vessel dilatation, congestion, and edema. There is enlargement of bronchial smooth muscle and also of submucosal gland mass, particularly in medium-sized bronchi. There is increased inflammatory infiltrate comprising activated lymphocytes and eosinophils with release of granular content in the latter; the allergic inflammation is associated with gene expression for interleukins IL-4 and IL-5 (ie, TH2 phenotype) and also GMCSF and TNF alpha. Many of these changes are already present in mild forms of asthma. In comparison, three conditions contribute to COPD: (1) In chronic bronchitis there is inflammation associated with mucous hypersecretion, enlargement of tracheo-bronchial submucosal glands, and a disproportionate increase of acidic mucus. The reticular basement membrane is not consistently thickened. (2) In small (peripheral) airways disease there is a macrophage bronchiolitis, mucous metaplasia and hyperplasia, increased intraluminal mucus, increased wall muscle, fibrosis, and airway stenoses. Respiratory bronchiolitis and loss of alveolar wall attachments are important lesions. (3) Emphysema involves elastolytic destruction of the alveolar wall; its overall severity, rather than type, appears to be an important determinant of chronic irreversible deterioration of airflow in COPD.