Mechanisms of neutrophil transmigration across the vascular endothelium in COPD

Chronic obstructive pulmonary disease (COPD) is a common and important disease. Neutrophils have been shown to play a fundamental role in its development and progression. Understanding the mechanisms underlying the trafficking of neutrophils across the vascular endothelium into the lung could potentially allow the development of targeted biological treatments. The early stages of neutrophil tethering, adherence to and rolling on the endothelium have been determined. The later stages of diapedesis through the glycocalyx, endothelial cell (EC) layer and basement membrane, which are less well characterised, have been reviewed here. Evidence obtained from in vitro and in vivo work, concerning the implicated adhesion molecules on the neutrophil and endothelium, the mechanisms for neutrophil navigation through the EC junction (paracellular route) and evidence for transmigration through the body of an EC itself (transcellular route), is considered. The mechanisms are complex and are often disease and stimulus specific. There is evidence that a significant degree of redundancy occurs. Transmigration in the lung differs from that in other organs in that the neutrophil can exit the circulation either through the postcapillary venule in the systemic circulation or through the capillary in the pulmonary circulation. A number of factors make the mechanisms of transmigration within the lung and COPD model unique. These include physical differences between the flow through the capillary and the postcapillary venule, the modulating effect of the alveolar epithelium and other cells such as the macrophage, the presence of a 'diseased' neutrophil and indeed the presence or absence of acute, acute on chronic or chronic pulmonary disease.