IR MacGregor, AM Perrie, SC Donnelly and C Haslett
National Science Laboratory, Scottish National Blood Transfusion Service, and the Respiratory Medicine Unit, University of Edinburgh, United Kingdom.

Pulmonary microvascular injury, one of the earliest events in adult respiratory distress syndrome (ARDS), is caused by the release of injurious products from stimulated neutrophils and other inflammatory cells in the lung microvessels. An increased level of the endothelial cell-surface anticoagulant protein thrombomodulin (TM) in plasma from patients with ARDS as shown in this study may be one consequence of this, and our objective in this investigation was to define the mechanisms by which TM is modulated by neutrophils, using an endothelial tissue culture system. Human neutrophils in contact with endothelium caused a fourfold reduction in cell-surface TM activity, but only after prior neutrophil priming and activation. Neutrophil release products elastase and cathepsin G caused rapid dose-related reductions in endothelial cell-surface TM activity, with complete abolition at 5 microg/ml in the absence of endothelial detachment. H2O2 also reduced TM activity. TM antigen accumulated in culture supernatant after treatment of endothelium with cathepsin G, indicating proteolytic release of cell-surface TM. We conclude that primed activated neutrophils are potent modulators of endothelial TM in vitro.

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