Background Patients with sepsis-induced acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) commonly suffer from severe pulmonary thrombosis, but clinical trials of anti-coagulant therapies in sepsis and ARDS patients have failed. ARDS patients with thrombocytopenia also exhibit increased mortality, and widespread pulmonary thrombosis is often seen in coronavirus disease 2019 (COVID-19) ARDS patients.

Methods Employing different amounts of microbeads to induce various levels of pulmonary thrombosis. Acute lung injury was induced by either lipopolysaccharide i.p. or cecal ligation and puncture. Endothelial cell (EC)-targeted nanoparticle coupled with CDH5 promoter was employed to delivery plasmid DNA expressing the CRISPR/Cas9 system for EC-specific gene knockout or expressing Alox15 for EC-specific overexpression. Additionally, thrombocytopenia was induced by genetic depletion of platelets using DTRPf4Cre mice by breeding Pf4Cre mice into the genetic background of DTR mice.

Results We show that while severe pulmonary thrombosis or thrombocytopenia augments sepsis-induced ALI, the induction of mild pulmonary thrombosis conversely reduces endothelial cell (EC) apoptosis, ALI, and mortality via sustained expression of endothelial arachidonate 15-lipoxygenase (Alox15). Endothelial Alox15 knockout via EC-targeted nanoparticle delivery of CRISPR/Cas9 plasmid DNA in adult mice abolished the protective impact of mild lung thrombosis. Conversely, overexpression of endothelial Alox15 inhibited the increases in ALI caused by severe pulmonary thrombosis. The clinical relevance of the findings was validated by the observation of reduced ALOX15-expressing ECs in lung autopsy samples of ARDS patients. Additionally, restoration of pulmonary thrombosis in thrombocytopenic mice also normalized endotoxemia-induced ALI.

Conclusion We have demonstrated that moderate levels of thrombosis protect against sepsis-induced inflammatory lung injury via endothelial Alox15. Overexpression of Alox5 inhibits severe pulmonary thrombosis-induced increase of ALI. Thus, activation of ALOX15 signaling represents a promising therapeutic strategy for treatment of ARDS, especially in sub-populations of patients with thrombocytopenia and/or severe pulmonary thrombosis.

Y.Y. Zhao reported pending patent applications entitled "PLGA-PEG nanoparticles and methods of uses", "Cationic polymer-formulated nanoparticles and methods of use". Y.Y. Zhao is the founder and chief scientific officer of Mountview Therapeutics LLC. The authors have no additional competing interests.

This work was supported in part by NIH grants R01HL123957, R01HL133951, R01HL148810, R01HL162299, and R01HL164014 to Y.Y.Z and by an AHA Career Development Award 19CDA34500000 to C.E.E

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