Loss of endothelial cell (EC) barrier function is important in the development of indirect acute lung injury (ALI). We have shown, in a novel murine model of hemorrhage (shock) with a subsequent septic challenge caused by cecal ligation and performation (CLP), that neutrophil (PMN) interactions with resident pulmonary cells are central to this pathology. In ALI, unresolved inflammation elicits a pathological process with loss of EC barrier integrity and impaired lung function. EC growth factors, Angiopoietin (Ang)-i and 2, under physiological conditions, maintain vascular homeostasis through competitive interactions wdth the t3T:osine kinase receptor, Tie2, expressed on ECs. Ang-1/Tie2 binding has been shown to stabilize vessels and stimulate down stream pro-survival/anti-inflammatory signaling, in contrast, Ang-2, released from storage granules of activated ECs, destabilizes vessels. Recent findings report that plasma Ang-2 levels are significantly elevated in patients that develop ALI. We find similar elevation in the lungs and plasma in our shock/CLP model, and we have found that depletion of PMNs prior to shock abrogates Ang-2 elevation. We propose the following central hypothesis: Ang-2 causes loss of pulmonary EC barrier function in ALI due to shock/CLP, initiated by EC interaction with shock-primed PMNs. We propose the following specific aims: Aim 1 will determine the kinetics of change in Ang-1:Ang-2 and Ang-2 expression and re-synthesis as well as its relationship to changes in indices of inflammation. We will use Ang-2 (si)RNA to suppress lung tissue expression, Ang-2 protein specific inhibition, and Ang-1 competitive inhibition of Ang-2/Tie2 binding to assess the contribution of Ang-2 release in shock priming for the development of ALI. Aim 2 will determine mechanisms by which Ang-2 changes pulmonary EC phenotype/activation in response to plasma from mice with ALI/ARDS. Aim 3 will determine mechanisms by which Ang-2 mediates changes in EC phenotype /activation in cultured mouse ECs following co-culture with shock-primed PMNs. The studies in this proposal will provide novel insights into the mechanisms of PMN associated, Ang-2 mediated ALI and will elucidate pathways that hold potential for therapeutic intervention.