Background
Neutrophils that respond to airway inflammation cause tissue damage via the production and release of oxygen radicals, proteases, and soluble mediators of inflammation. Inhaled irritants such as cigarette smoke, ozone, and bacterial endotoxin can produce inflammation through nonallergic mechanisms. Endotoxin [or lipopolysaccharide (LPS)], a component of the cell wall of gram-negative bacteria, is ever-present in the environment, with high concentrations in organic dusts, such as dust from grain, and air-pollution particles.
Several studies have demonstrated that inhalation of air that is contaminated with endotoxin is associated with the classic features of asthma, including reversible airflow obstruction and inflammation and persistent airway hyperreactivity and remodeling. Epidemiological studies have shown that the concentration of inhaled endotoxin in the air is strongly and consistently associated with reversible airflow obstruction among cotton workers, agricultural workers, and fiberglass workers.
Previous studies have shown that the concentration of endotoxin in the air is the most important occupational exposure associated with airway disease found in agricultural workers. Experimentally, inhalation of endotoxin can cause reversible airflow obstruction and airway inflammation in previously unexposed healthy study subjects. In fact, healthy study subjects challenged with dust from animal-confinement buildings develop airflow obstruction and an increase in the serum concentration of neutrophils and interleukin-6 (IL-6), all of which are most strongly associated with the concentration of endotoxin (not dust). In endotoxin-sensitive (C3H/HeBFeJ) but not endotoxin-resistant (C3H/HeJ) mice, subchronic inhalation of grain dust causes persistent airway hyperreactivity and remodeling, which suggests that endotoxin is one of the principal components of grain dust that causes the development of chronic airway disease.
Although thickening of the subepithelial region of the airway is a consistent feature in the cellular structure of asthma, cystic fibrosis, and chronic obstructive lung disease and is directly related to the clinical severity of these diseases, the biological factors that lead to a localized fibrotic response following chronic airway inflammation have not been well defined.
The Study
To elucidate whether neutrophils are essential to the development of chronic LPS-induced airway disease, researchers used PMN antiserum to produce neutrophil-depleted mice and examined the LPS-induced changes in those animals compared to similarly exposed mice that were not neutropenic.
The researchers hypothesized that antiserum to PMNs would substantially minimize the acute inflammatory response to inhaled LPS and in doing so would substantially inhibit subepithelial thickening and alter the development of chronic LPS-induced airway disease.
The authors of "Neutrophils Play a Critical Role in Development of LPS-Induced Airway Disease," are Jordan D. Savov, David M. Brass and David A. Schwartz, all from the Pulmonary and Critical Care Division, Department of Medicine, Duke University Medical Center and Veterans Affairs Medical Center, Durham, NC. Additional authors are Stephen H. Gavett and Daniel L. Costa, both from the Pulmonary Toxicology Branch, Experimental Toxicology Division, National Health and Environmental Effects Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC.
The findings of their study appear in the November 2002 edition of the American Journal of Physiology--Lung Cellular and Molecular Physiology. The journal is one of 14 scientific publications published each month by the American Physiological Society (APS).
Methodology
The study examined the impact of systemic neutrophil depletion on chronic LPS-induced airway hyperresponsiveness, inflammation, and remodeling in 44 laboratory mice to determine whether neutrophils are essential to the development of LPS-induced airway disease. Physiological, biological, and morphological measures were performed at three time points: before the inhalation challenge, immediately after completion of the four week inhalation challenge, and another four weeks after. Airway responsiveness (AR) to methacholine (MCh) aerosol was determined, lung inflammation was assessed in whole lung lavage fluid, and airway remodeling was estimated by light microscopic morphometry, Masson-Trichrome staining, and transforming growth factor(TGF-â1) immunohistochemistry.
The 44 mice were randomly assigned to two experimental groups: one group received rabbit anti-mouse PMN antiserum, and the second group received normal rabbit serum. In each treatment group, 16 animals were exposed to LPS and six were used as controls, which were exposed to filtered air. Mice in each group were evaluated before the exposure, immediately after the four-week exposure, and four weeks after the end of exposure.
Results
The results of the present study indicate that the development of chronic LPS-induced airway disease is dependent on the presence of neutrophils. Mice that were depleted of circulating neutrophils had markedly less lung inflammation and demonstrated little evidence of LPS-induced airway hyperreactivity or airway remodeling (expansion of the subepithelial matrix). In contrast, neutrophil-replete mice developed airway inflammation, hyperreactivity, and remodeling after a prolonged exposure to LPS.
These results provide support for the hypothesis that chronic airway disease in individuals exposed to dust contaminated with LPS may be largely mediated by PMNs and suggest a direct association between airway inflammation and remodeling.
The study also demonstrates that a four week exposure to LPS by inhalation causes development of chronic airway disease in LPS-responsive mice. In the evolution of this condition, two phases could be recognized:
- · the early or direct phase (during and immediately after the exposure), induced by the consistent and repetitive presence of the stimulus and during which infiltration and activation of inflammatory cells (mostly neutrophils) took place without accompanying enhancement in airway reactivity; and
· the late or indirect phase (four weeks after the exposure), which was characterized by fibrotic airway remodeling and hyperresponsiveness.
Finally, the results showed that inhibition of neutrophil recruitment in the early phase mitigates the chronic inflammatory response to inhaled endotoxin (LPS) and subsequently attenuates the late-phase airway hyperreactivity and remodeling in LPS-responsive mice.
Conclusions
According to the authors, these data are the first to show a temporal and causative relationship between neutrophil recruitment in the lung provoked by repetitive stimuli and the later development of airway hyperreactivity and remodeling. However, because the PMN antiserum used was effective but not completely selective for neutrophils, the results do not rule out the possibility that in addition to neutrophils, lymphocytes might also be involved in the development of the chronic LPS induced airway disease.
The findings indicate that after chronic endotoxin exposure, neutrophils can contribute to airway remodeling and hyperreactivity. This suggests that control of neutrophil recruitment in the airway or the neutralization of TGF-â1 may limit the extent of chronic LPS-induced airway disease.
Source: November 2002 edition of the American Journal of Physiology--Lung Cellular and Molecular Physiology.
The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.