Neutrophil Elastase-Mediated Lung Disease

Abstract

Elastases of both the neutrophil and macrophage have been implicated in lung disease initiation and progression. Although it is unlikely that these proteases evolved for the purpose of injuring lung tissue, the elastin-rich connective tissue framework of the lungs appears to be particularly susceptible to the action of elastolytic proteases. Assuming that neutrophil elastase most likely plays a role in the migration of neutrophils toward a site of inflammation and degradation of proteins from invading organisms or other products of the inflammatory response, it is the role of inhibitors of this protease to protect normal tissues from its effects. In alpha-1 antitrypsin deficiency we find an experiment of nature that disrupts this protease-anti-protease balance, resulting in an increased risk of destructive lung disease.

Keywords :

• alpha-1 antitrypsin deficiency
• neutrophil elastase
• emphysema

Introduction

Elastases of both the neutrophil and macrophage have been implicated in lung disease initiation and progression. Although it is unlikely that these proteases evolved for the purpose of injuring lung tissue, the elastin-rich connective tissue framework of the lungs appears to be particularly susceptible to the action of elastolytic proteases.

Assuming that neutrophil elastase most likely plays a role in the migration of neutrophils toward a site of inflammation and degradation of proteins from invading organisms or other products of the inflammatory response, it is the role of inhibitors of this protease to protect normal tissues from its effects. In alpha-1 antitrypsin deficiency we find an experiment of nature that disrupts this protease-anti-protease balance, resulting in an increased risk of destructive lung disease(1).

Historical perspectives

Our understanding of the role of elastase in the development of lung injury closely parallels our understanding of alpha-1 antitrypsin deficiency (AATD) both temporally and mechanistically. Although a variety of animal models of emphysema predated the first description of AATD, many were based on now-outdated theories of the mechanism of disease, such as the placement of one-way valves in the airways of rabbits to mimic the air trapping observed in patients with chronic obstructive pulmonary disease (COPD).

In the period of the late 1950s and early 1960s, in the United States, studies were exploring the role of proteolytic enzymes such as pancreatic elastase and clostridial collagenase as tools to determine their effects on specific connective tissue components such as elastin and collagen. Cowdrey conducted a detailed histological study of elastin in pulmonary emphysema which demonstrated specific abnormalities in elastic fiber structure (2) but the basis of the anatomic abnormalities could not be defined.

At approximately the same time that Eriksson and Laurell in Malmo, Sweden were evaluating the clinical effects of the deficiency of alpha-1 antitrypsin (AAT), Gross et al. in Pittsburg were evaluating a model of pulmonary fibrosis created by silica inhalation in rats (3). In an attempt to reverse the pathologic changes seen in these rats, Gross instilled the proteolytic enzyme papain intratracheally in affected animals. Rather than reversing the fibrotic changes, animals with silicosis developed additional anatomic changes consistent with pulmonary emphysema following papain instillation.

Similar emphysematous changes were seen following papain administration into the lungs of healthy rats. As this animal model of emphysema became widely used, investigators identified variability in the potency of papain preparations as an emphysema-promoting agent. When this variability was linked to the elastin degrading capacity of the various papain preparations (4), attention turned toward elastolytic proteases, in general, as potential emphysema producing agents.

Elastases from many sources including those of bacterial, plant, and animal origin were able to promote destructive lung disease in a variety of laboratory animal species including mice, hamsters, guinea pigs, and dogs (5–7). It became apparent that, if a protease could degrade elastin, it could cause the anatomic changes, and in some cases the physiologic changes, associated with pulmonary emphysema. A question that plagued all of these models of emphysema was that none of the elastases evaluated in this range of studies seemed to have access to human lungs. Investigators even sought elastolytic activity in cigarette smoke but none was found. Some questioned the applicability of these animal studies to human disease. One study did demonstrate that the plasma of patients with COPD and alpha-1 antitrypsin deficiency could not inhibit an elastase of pancreatic origin, which suggested a pathogenic role specifically for elastase in COPD(8).

In 1967, Janoff and Scherer (9) published the first description of a potent elastolytic protease found in the human polymorphonuclear neutrophilic leukocyte or neutrophil. This elastase is synthesized during the promyelocytic phase of white cell maturation and packaged within the azurophilic granules of the mature neutrophil. It was named human neutrophil elastase (HNE). At last a potential source of elastase with access to the human lung was identified. Studies in laboratory animals confirmed that HNE is a potent emphysema-promoting agent (10). In addition, both the activity of HNE (11) and porcine pancreatic elastase (12, 13) was shown to persist in association with lung elastin for as long as 96 hours after a single intratracheal administration.

Soon after the description of HNE, several investigators connected the AATD story with the HNE story (8, 14). It was appreciated that the circulating human protein AAT was an effective inhibitor of HNE and could block its ability to produce emphysema in laboratory animals (15). Subsequently, it was shown that oxidants in cigarette smoke are capable of inactivating the HNE-inhibiting properties of AAT (16–19).

This knowledge led to the protease pathogenesis model of pulmonary emphysema (20), which proposed that all pulmonary emphysema is due to the action of primarily elastolytic proteases on lung connective tissue, and suggested that most emphysema in man is due to alpha-1 antitrypsin deficiency. In the majority of individuals with emphysema and normal circulating levels of AAT, this deficiency is functional and due to the action of cigarette smoke on the AAT molecule. In the minority of individuals whose emphysema is primarily related to AATD, the deficiency is genetic. Clinical experience has taught us that the combination of cigarette smoking and AATD can cause devastating precocious pulmonary emphysema.

Over the decades since this protease pathogenesis model was first proposed, the story of cigarette smoke-induced lung disease has become more complex with a variety of additional proteases, inhibitors, inflammatory mediators, and host responses appearing in pathway diagrams. Still, the story of COPD in AATD maintains a simpler, more direct route with neutrophil elastase maintaining a prominent role in the pathogenesis of parenchymal disease of the lung.

Inflammation plays a role, especially in view of the prominence of neutrophils in the inflammation seen the lungs of those with AATD (21). There is a growing appreciation of the association of AATD with airway disease, especially bronchiectasis, and this is likely the result of proteolytic injury to airway connective tissue, at least in part.

Augmentation therapy

Based on our understanding of the role of AAT in the protection of normal lung tissue and the precocious lung damage seen in many with a deficiency of this protein, the intravenous administration of purified, plasma-derived AAT protein has become standard therapy for individuals with AATD and pulmonary emphysema in several countries (32). A number of studies have documented beneficial effects of this therapy including slowing lung function decline and, in the largest of these studies, improved survival (22–25). The rationale for this therapy is not only that it is good to augment the quantity of a deficient circulating protein, but also that protection of lung connective tissue from proteolytic attack is key to reducing lung destruction in emphysema.

Current status

To evaluate the effects of cigarette smoke on the lungs, several centers have developed chronic cigarette smoke exposure systems for laboratory animals. These investigators were able to demonstrate destructive changes in the lungs of smoking animals. Investigators have found that knocking out the gene for macrophage elastase prevents this smoking-related injury in mice (26), while knocking out the neutrophil elastase gene was only partially protective. Of interest, mice have several redundant AAT genes and it has not been possible to knock out mouse AAT production using these techniques.

Unanswered questions remain in AATD-related lung disease. Why do some with severe AATD never develop clinically significant lung disease? While it is appreciated that environmental risk factors, such as smoking and occupational exposures, greatly increase the risk of disease, the identified risk factors don't explain individual variability in susceptibility to lung disease in AATD. The exact mechanisms accounting for the airway neutrophilia and increases in inflammatory mediators in bronchoalveolar lavage fluid in AATD remain elusive.

There is an appreciation that certain unusual infections, such as invasive pulmonary non-tuberculous mycobacteria, have a higher prevalence in those with AATD (27). It is unclear whether AATD makes the lung a more hospitable environment for these organisms or whether the high prevalence of bronchiectasis in those with AATD simply allows these organisms a foothold.

Future directions

The neutrophil and its array of proteolytic defenses have been partially ignored in recent decades as more complex and seemingly more elegant pathways relating to innate immunity have gained the spotlight. Recently, there has been renewed interest in the role of neutrophilic proteases as mediators and modulators of some of these processes. Through all this, AATD, an experiment of nature that leaves a substantial number of humans with reduced protection against many of these neutrophilic proteases and HNE in particular, remains a condition that has once again been given renewed attention. Novel elastase inhibitors are being evaluated, after a substantial hiatus, for possible therapeutic use. Gene therapy is moving forward with some human trials already completed (28, 29).

Recognizing that in vivo elastases degrade elastic tissue into fragments of various molecular weight and amino acid composition, recent publications have indicated that the biological activity of elastin fragments can produce chemotactic and antigenic effects which may have pathogenic significance in COPD (30, 31).

Clinician investigators are looking beyond lung injury in AATD and evaluating whether other conditions involving neutrophilic inflammation, such as arthritis and autoimmune disease, may be more severe in those with a deficiency of this HNE inhibitor.

Declaration of Interest Statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.