Circulating Mitochondrial <i>N</i> -Formyl Peptides Are Associated with Acute Respiratory Distress Syndrome after Cardiopulmonary Bypass and Regulate Endothelial Barrier through FPR2

Cardiopulmonary bypass (CPB) increases the risk of acute respiratory distress syndrome (ARDS) because of endothelial cell (EC) barrier dysfunction. However, the specific role of mitochondrial <i>N</i>-formyl peptides (mtNFPs) in ARDS after CPB remains unexplored. Here, we investigated the differential expression of circulating mtNFPs in patients after CPB, focusing on the novel role of FPR2 (formyl-peptide receptor 2) in ECs. Concentrations of circulating mtNFPs were assessed using ELISA. Several mtNFPs (ND4 [nicotinamide adenine dinucleotide dehydrogenase subunit 4], ND5, ND6, and Cox1) were significantly upregulated in patients with ARDS at Day 1 after CPB compared with patients without ARDS. Higher concentrations of ND6 were correlated with worse ratios of arterial oxygen pressure to fraction of inspired oxygen (<i>r</i> = -0.2219; <i>P</i> < 0.0001) and cardiac troponin T (<i>r</i> = 2.107; <i>P</i> < 0.0001). Using patient-derived serum and a rat lung ischemia-reperfusion injury model, we observed a positive correlation between serum ND6 concentration and ARDS, which is also associated with EC barrier dysfunction. <i>In vitro</i> experiments, using transendothelial electric resistance measurements and fluorescence microscopy with FITC-labeled vascular endothelial cadherin, demonstrated that ND6 disrupts the EC barrier through FPR2. Furthermore, FPR2 controls the release of ND6 out of mitochondria and cytoplasm under hypoxia-reoxygenation. Activated FPR2 leads to the upregulation of NF-κB by inducing IκBα phosphorylation, promoting ICAM1 (intercellular cell adhesion molecule-1) and VCAM1 expression, thereby compromising EC barrier integrity. Circulating proinflammatory and barrier-disruptive mtNFPs, particularly ND6, are associated with ARDS in patients undergoing CPB. The novel ND6-FPR2 axis regulates inflammation and EC permeability through the NF-κB pathway.