PhD ThesisControlled opening of endothelial-cell (EC) junctions is vital in regulating vascular permeability and neutrophil transendothelial migration (TEM) during acute inflammation. Although both phenomena can occur independently, as supported by distinct molecular pathways, the potential inter-play of these two responses requires further exploration. In this thesis, we investigated the impact of microvascular leakage on neutrophil TEM and the potential downstream pathophysiological consequences.
To this aim, as part of this project, a confocal intravital microscopy platform was developed for simultaneous analysis of neutrophil TEM and vascular permeability within the murine cremaster muscle microcirculation. The inflammatory reactions employed were driven by locally administered LTB4, or IL-1β ± vasoactive agents (e.g. histamine/VEGF), or by a model of IR-injury. The findings provide direct evidence for the ability of inflammatory reactions characterised by enhanced microvascular leakage to promote an aberrant mode of neutrophil TEM, known as reverse (r)TEM. This response is characterised by neutrophils that have partially breached the endothelium and move in a retrograde mode, thus returning into the lumen. Interestingly, genetic functional deficiency or pharmacological blockade of VE-cadherin-dependent hyper-permeability reduced the frequency of neutrophil rTEM. Mechanistically, this migration behaviour was driven by excessive diffusion of tissue-derived CXCL1 through EC junctions into the plasma, resulting in a disrupted chemotactic gradient across the endothelium. Development of a novel tracking method allowed us to demonstrate that rTEM neutrophils exhibited a pro-inflammatory phenotype and disseminated into the blood and lung circulation. Presence of these cells in lungs was associated with vascular damage. Finally, we identified distinct roles for TNF-receptors in controlling vascular permeability and neutrophil migration during IR-injury. Collectively, the findings of this thesis provide a causal link between increased local microvascular leakage induction and disrupted localisation of chemotactic directional cues across the endothelial barrier, resulting in aberrant mode of neutrophil migration and subsequent distant organ damage
