Chronic obstructive pulmonary disease * 3: experimental animal models of pulmonary emphysema. (Review Series).

The use of genetically manipulated mice together with traditional animal studies are steadily increasing our knowledge of the factors important in determining alveolar formation and destruction in emphysema. A review of the animal models used to study emphysema is presented.

Chronic obstructive pulmonary disease (COPD) is one of the commonest reasons for ill health worldwide, with 16 million individuals affected in the USA alone. It is ranked as the fourth and fifth highest cause of death in the USA and UK, respectively, with a mortality rate at least 14 times that of asthma. (1 2) The single most important factor in the development of emphysema is cigarette smoke. Inhalation of cigarette smoke causes a chronic pulmonary inflammatory infiltrate of macrophages, neutrophils and CD8+ cells that persists long after smoking cessation. In susceptible individuals this ultimately leads to irreversible destruction and dilatation of the terminal airspaces of the lung, chronic disability due to respiratory failure, and premature death.

Animal studies have been critical in shaping contemporary views regarding the pathogenesis of emphysema. Almost 40 years ago Gross reported that intratracheal instillation of the plant proteinase papain into rats resulted in emphysema. This, combined with the clinical observation by Eriksson that deficiency of the antiproteinase [[alpha].sub.1] -antitrypsin was associated with early onset panlobular emphysema in humans, (3 4) fuelled the concept that an imbalance within the lung favouring proteinases over their inhibitors resulted in emphysema. As more sophisticated animal models have evolved, they have continued to develop fresh insights into lung biology and enabled the testing of novel treatments for emphysema. While the initial association of emphysema with [[alpha].sub.1]-antitrypsin deficiency suggested that neutrophil derived proteinases were critical to the development of emphysema, recent studies--largely in animal models--have broadened the scope of cells and proteinases that may cause emphysema. Pa rticular attention has been given to macrophage derived matrix metalloproteinases (MMPs). (5) Furthermore, stemming from the study of alveogenesis in experimental animals, retinoic acid treatment has been shown to regenerate alveoli in rats both during development and following experimental emphysema. (6) It is now clear that complex interacting pathways are involved in the initiation, progression, and the failure of correct repair in emphysema.

While hypotheses and mechanisms of emphysema can be addressed in the test tube and cell culture, ultimately these ideas need to be tested in mammals to elucidate the complex pathophysiology of COPD. Rodents, dogs, guinea pigs, monkeys, and sheep have all been used to study emphysema. With the advent of genetic engineering in mice, this species offers the greatest ability to dissect pathogenetic pathways in mammals. In addition, more is known about its genome than any other animal and cDNA and antibody probes are abundant. They also offer the practical benefits of short breeding times, large litters, and relatively cheap housing. The main issue confounding the use of mice and other animals as a template for human disease is that we do not know how accurately they reflect human biology and pathology. Mouse and man clearly share many basic physiological processes, but the details of how gas exchange is achieved will determine how closely findings in mice can be applied to man. Each animal model should be viewed as one component of the process for studying human disease and not viewed in isolation or extrapolated directly to humans.

Emphysema can be modelled in many ways. Exogenous administration of proteinases, chemicals, particulates, and exposure to cigarette smoke result in features characteristic of human COPD. Genetic manipulation itself can result in airspace enlargement during development and throughout life. These different approaches each have their own merits and limitations and require individualised interpretation. The methods also often complement each other so their usefulness can be enhanced by using a combination of approaches to study the disease. The field has also not infrequently been stimulated by the inadvertent generation of emphysema in animals where experiments were initially undertaken for another reason.

EMPHYSEMA PRODUCED BY CHALLENGE WITH EXOGENOUS AGENTS

Intrapulmonary challenge with injurious proteins, chemicals, particulates, and other compounds into lungs of animals has been used to cause emphysema directly. Compounds have also been administered that inhibit protein function (loss of function models), resulting in airspace enlargement.

A single intrapulmonary challenge with proteinases including porcine pancreatic elastase (PPE), papain, and human neutrophil elastase causes panacinar emphysema. (7) Their effectiveness was directly related to their elastolytic activity, while instillation of bacterial collagenases did not cause emphysema. PPE mediated emphysema was accompanied by secretory cell metaplasia and abnormalities of pulmonary function, hypoxaemia, and right ventricular hypertrophy that are characteristic of human COPD. Following an intratracheal bolus of PPE, there is an initial loss of elastin and collagen. Over time, elastin and glycosaminoglycans return to normal and collagen is enhanced, yet intraparenchymal extracellular matrix (ECM) remains diminished and distorted and the architecture of the lung is grossly and permanently abnormal. (8) Airspace enlargement develops immediately after elastolytic injury, followed by inflammation. The subsequent progression of emphysema over the next few months is probably caused by the destru ctive effect of host inflammatory proteinases. Although repair mechanisms fail to establish the normal lung structure, impairment of elastin and collagen cross linking with the addition of the lathyrogen [beta]-aminopropionile (BAPN) worsens the emphysema, (9) suggesting that some repair occurs. Instillation of elastases remains a useful tool, particularly to study effects downstream of proteolytic injury. Yet inflammation and host injury following instillation suggests that this model might also provide insights into upstream events such as inflammation and endogenous proteinases. However, extrapolating these findings to the slowly developing smoking induced disease in humans is not straightforward as additional mediators are likely to be involved.

Various other agents have also been used in an attempt to recreate inflammation and lung injury. For example, repeated endotoxin administration recruits neutrophils and activates macrophages with resultant airspace enlargement. (10) Administration of oxidants such as nitrogen dioxide and ozone, the two major airborne pollutants, causes lung injury. Repeated long term administration of nitrogen dioxide results in mild focal emphysema, while ozone results in fibrosis. (11) Administration of cadmium chloride, a constituent of cigarette smoke, also results in primarily interstitial fibrosis with tethering open of airspaces simulating emphysema. While this mechanism differs from airspace enlargement secondary to matrix destruction that characterises emphysema, we now appreciate that airway fibrosis is a significant factor in centrilobular emphysema seen in human smokers. (12) Interstingly, a combination of cadmium and BAPN enhanced the emphysematous changes. (13)

Coal dust and silica result in focal emphysema, and animal models have uncovered complex inflammation and oxidant injury with connective tissue breakdown that was attributed to neutrophil mediated injury. (14)

Intravascular administration of a vascular endothelial cell growth factor receptor-2 (VEGFR-2) blocker has recently been used to generate a…