The pulmonary endothelial glycocalyx regulates neutrophil adhesion and lung injury during experimental sepsis

The glycocalyx is a layer of proteoglycans and complex carbohydrates that lines the endothelial cell surface in blood vessels. Schmidt et al. show that in mouse models of sepsis, lung inflammation and injury depend on glycocalyx degradation, which increases neutrophil access to endothelial adhesion molecules. The authors also provide data indicating the potential relevance of this mechanism of lung injury to humans with sepsis. Sepsis, a systemic inflammatory response to infection, commonly progresses to acute lung injury (ALI), an inflammatory lung disease with high morbidity. We postulated that sepsis-associated ALI is initiated by degradation of the pulmonary endothelial glycocalyx, leading to neutrophil adherence and inflammation. Using intravital microscopy, we found that endotoxemia in mice rapidly induced pulmonary microvascular glycocalyx degradation via tumor necrosis factor-α (TNF-α)-dependent mechanisms. Glycocalyx degradation involved the specific loss of heparan sulfate and coincided with activation of endothelial heparanase, a TNF-α–responsive, heparan sulfate–specific glucuronidase. Glycocalyx degradation increased the availability of endothelial surface adhesion molecules to circulating microspheres and contributed to neutrophil adhesion. Heparanase inhibition prevented endotoxemia-associated glycocalyx loss and neutrophil adhesion and, accordingly, attenuated sepsis-induced ALI and mortality in mice. These findings are potentially relevant to human disease, as sepsis-associated respiratory failure in humans was associated with higher plasma heparan sulfate degradation activity; moreover, heparanase content was higher in human lung biopsies showing diffuse alveolar damage than in normal human lung tissue.