Investigation of pulmonary microcirculatory dysfunction in sepsis-induced ALI/ARDS

Abstract

The lung is the most vulnerable organ during sepsis, but disturbances in pulmonary microcirculation at the cellular level are poorly understood due to the limitation of adequate imaging tool. Here, we implemented a intravital lung imaging system integrated to a custom-design video-rate laser-scanning confocal microscopy platform to perform direct real-time visualization of pulmonary microcirculation. We successfully observed rapidly flowing erythrocytes in pulmonary capillaries by fluorescence angiography using a dextran conjugated fluorophore. By delineating the red blood cell (RBC) exclusion zone inside ca-pillary, we defined the endothelial glycocalyx layer (EGL). Then we identified degradation of EGL in sepsis model, which represents the destructed barrier of vascular endothelium. We observed that prolonged neutrophil entrapment within capillaries induced the development of dead space during sepsis-induced acute lung injury (ALI). We derived the functional capillary ratio (FCR) as a novel parameter for assessing the fraction of functional microvasculature in the pulmonary microcirculation and demonstrated that FCR rapidly decreased in the early stage of systemic inflammation. This decrease was the result of extended sequestration time and the arrest-like dynamic behavior of neutrophils, which triggered neutrophil thrombosis inside capillaries and arterioles. Finally, we found that Mac-1 (CD11b/CD18) was upregulated in neutrophils sequestered in the lung and that a Mac-1 inhibitor restored the FCR, suggesting that this inhibitor has potential as a therapeutic drug for sepsis-induced acute respiratory distress syndrome (ARDS).