Neutrophil–Endothelial Interactions Mediate Angiopoietin-2–Associated Pulmonary Endothelial Cell Dysfunction in Indirect Acute Lung Injury in Mice

Abstract

Unresolved inflammation in the lung is thought to elicit loss of endothelial cell (EC) barrier integrity and impaired lung function. We have shown, in a mouse model of shock/sepsis, that neutrophil interactions with resident pulmonary cells appear central to the pathogenesis of indirect acute lung injury (iALI). Normally, EC growth factors angiopoietin (Ang)-1 and Ang-2 maintain vascular homeostasis through tightly regulated interaction with the kinase receptor Tie2 expressed on ECs. Although Ang-1/Tie2 has been shown to promote vessel integrity, stimulating downstream prosurvival/antiinflammatory signaling, Ang-2, released from activated ECs, is reported to promote vessel destabilization. This mechanism of regulation, together with recent clinical findings that plasma Ang-2 levels are significantly elevated in patients who develop acute respiratory distress syndrome, has focused our investigation on the contribution of Ang-2 to the development of iALI. A murine model of hemorrhagic shock-induced priming for the development of iALI after subsequent septic challenge was used in this study. Our findings show that 1) Ang-2 is elevated in our experimental model for iALI, 2) direct EC/neutrophil interactions contribute significantly to EC Ang-2 release, and 3) suppression of Ang-2 significantly decreases inflammatory lung injury, neutrophil influx, and lung and plasma IL-6 and TNF-α. These findings support our hypothesis and suggest that Ang-2 plays a role in the loss of pulmonary EC barrier function in the development of iALI in mice resultant from the sequential insults of hemorrhagic shock and sepsis and that this is mediated by EC interaction with activated neutrophils.

Clinical and Biological Heterogeneity in Acute Respiratory Distress Syndrome

“…Similar to human ARDS 54 , higher levels of RAGE in the airspaces was associated with more severe lung injury in direct experimental lung injury 53 . In mice, indirect lung injury caused by hemorrhage followed by CLP showed that Ang-2 levels were increased systemically in affected mice, consistent with endothelial injury 55 . Furthermore, IP injection of Ang-2 alone was sufficient to cause increased pulmonary capillary leak 31 , supporting the concept that endothelial dysfunction contributes to lung injury.…”

Vascular endothelial cell Toll-like receptor pathways in sepsis

“…8,16,30,55,76,77 Studies suggest that activated neutrophils also independently promote endothelial dysfunction and contribute to barrier dysfunction. [78][79][80] The direct activation of endothelial TLR2 or TLR4 by bacterial lipopeptides or LPS, respectively, increases endothelial permeability. 8,16,30,55 In contrast, TLR9 agonists induce endothelial permeability in a neutrophil-dependent fashion.…”

Blockade of Endothelial Growth Factor, Angiopoietin-2, Reduces Indices of Ards and Mortality in Mice Resulting from the Dual-Insults of Hemorrhagic Shock and Sepsis

“…Ang-2/Tie2 binding is believed to produce vessel destabilization, pulmonary leakage and inflammation (16,17). Recent clinical findings from our trauma/surgical intensive care units and others have reported elevated EC growth factor, Ang-2, in the plasma from patients that develop ARDS (18–20). We have previously described similarly elevated Ang-2 in the plasma and lung tissue in our shock/sepsis model for the development of iARDS and demonstrated effective reduction in indices of inflammation and lung tissue injury following siRNA inhibition of Ang-2 protein synthesis (20).…”

“…Recent clinical findings from our trauma/surgical intensive care units and others have reported elevated EC growth factor, Ang-2, in the plasma from patients that develop ARDS (18–20). We have previously described similarly elevated Ang-2 in the plasma and lung tissue in our shock/sepsis model for the development of iARDS and demonstrated effective reduction in indices of inflammation and lung tissue injury following siRNA inhibition of Ang-2 protein synthesis (20). While this method is useful in identifying potential target proteins for therapeutic intervention, similar usage is not currently feasible for treating ARDS in the human patient population.…”

“…To test our hypothesis we used a mouse model of hemorrhagic shock induced “priming” followed by the induction of sepsis for the development of iARDS, a model with which we have considerable experience (9,10,20). This mouse model induces a decrease in the ratio of partial pressure arterial oxygen to fraction of inspired oxygen (arterial PO 2 /FIO 2 ) that corresponds with the Berlin criteria for mild to moderate ARDS (Fig.…”