Development of a rabbit model to investigate the effects of acute nitrogen dioxide intoxication

Do corticosteroids have a role in preventing or reducing acute toxic lung injury caused by inhalation of chemical agents?

OBJECTIVE

To assess the evidence that treatment with corticosteroids improves the outcome in those exposed to lung-damaging agents.

METHODS

We searched Pubmed, Toxnet, Cochrane database, Google Scholar, and Embase from 1966 to January 2010 using the search terms "steroid", "corticosteroid", "lung injury", "lung damage", and "inhalation". These searches identified 287 papers of which 118 contained information on animal studies. However, most were reviews or case reports and only a few were controlled animal experiments of which 13 were considered relevant. ROLE OF CORTICOSTEROIDS:

ANIMAL STUDIES

Corticosteroids have no beneficial effect at the alveolar level on acute lung injury, which is caused by inhalation of poorly water-soluble compounds (e.g. nitrogen dioxide, ozone, phosgene) or following severe exposure to water-soluble compounds (e.g. chlorine, ammonia). In the recovery phase, corticosteroids may even be harmful, because corticosteroids hamper the division of type II alveolar cells and hamper the differentiation from type II into type I alveolar cells. The latter is important for the re-epithelialization of the alveolus and removal of excess of water in the alveolus. Furthermore, the quality of animal studies does not always allow extrapolation to human exposures. Differences between humans and animals in anatomy, pulmonary defense systems, breathing physiology, as well as the way the animals have been exposed, and the timing and route of corticosteroids in animal studies make predictions difficult. ROLE OF CORTICOSTEROIDS:

HUMAN STUDIES

An abundance of uncontrolled case reports and a few human crossover studies have evaluated the outcome of human volunteers exposed to various lung-damaging agents. Only a few reports contained systematic information on corticosteroid treatment. Data on the efficacy of corticosteroids after human exposure to lung-damaging agents are inconclusive. Often the number of patients involved is small or the severity of exposure is unclear or not well determined. These reports are therefore limited in their ability to establish a cause-effect relationship for the treatments involved. In some studies involving mild to moderate exposure to water-soluble agents (e.g. chlorine, ammonia), corticosteroid treatment was beneficial for some physiological parameters, such as airway resistance or arterial oxygen tension. However, severe lung injury and inflammation appear not to be improved by corticosteroid treatment. The optimal duration of treatment to obtain these beneficial effects has not been assessed adequately, but it only seems to be useful in the first hours after exposure. Generally, studies evaluating exposure to water-soluble compounds have too short a follow-up, which hampers the evaluation of the efficacy of corticosteroid treatment. The results of studies with longer follow-up suggest that the initial slight improvement in some variables is lost several hours after exposure.

CONCLUSIONS

Clinical data on the efficacy of corticosteroids after human exposure to lung-damaging agents are inconclusive as the number of well-structured controlled studies is small and the indications for administration of corticosteroids are unclear. There have been no human controlled studies of high-dose exposure to lung-damaging agents. Furthermore, treatment with corticosteroids is limited by the potential side effects, such as prolonged neuromuscular weakness, deregulation of glucose metabolism, superinfection, and sepsis, which could diminish the chances for recovery.

Stem cells in the lung

The lung is composed of two major anatomically distinct regions-the conducting airways and gas-exchanging airspaces. From a cell biology standpoint, the conducting airways can be further divided into two major compartments, the tracheobronchial and bronchiolar airways, while the alveolar regions of the lung make up the gas-exchanging airspaces. Each of these regions consists of distinct epithelial cell types with unique cellular physiologies and stem cell compartments. This chapter focuses on model systems with which to study stem cells in the adult tracheobronchial airways, also referred to as the proximal airway of the lung. Important in such models is an appreciation for the diversity of stem cell niches in the conducting airways that provide localized environmental signals to both maintain and mobilize stem cells in the setting of airway injury and normal cellular turnover. Because cellular turnover in airways is relatively slow, methods for analysis of stem cells in vivo have required prior injury to the lung. In contrast, ex vivo and in vitro models for analysis of airway stem cells have used genetic markers to track lineage relationships together with reconstitution systems that mimic airway biology. Over the past decades, several widely acceptable methods have been developed and used in the characterization of adult airway stem/progenitor cells. These include localization of label-retaining cells (LRCs), retroviral tagging of epithelial cells seeded into xenografts, air-liquid interface cultures to track clonal proliferative potential, and multiple transgenic mouse models. This chapter reviews the biologic context and use of these models while providing detailed methods for several of the more broadly useful models for studying adult airway stem/progenitor cell types.