Interferon regulatory factor 1 mediates ROS-dependent release of NETs in LPS-induced mouse pneumonia by interacting with matrix metalloproteinase 9

While interferon regulatory factor 1 (IRF1) has been implicated in reactive oxygen species (ROS)-dependent neutrophil extracellular trap (NET) release during acute lung injury, its regulatory role in mitochondrial dysfunction-driven NETosis specific to pediatric pneumonia remains unexplored. Neutrophils were isolated from the bronchoalveolar lavage fluid (BALF) of pediatric pneumonia patients and the bone marrow of mice. IRF1 expression was quantified via qRT-PCR and Western blotting. Mitochondrial ROS (mtROS) and total ROS were measured by flow cytometry. Mitochondrial dysfunction was assessed by ATP quantification. NETosis was evaluated through immunofluorescence staining and ELISA quantification of neutrophil elastase (NE), myeloperoxidase-DNA (MPO-DNA), and citrullinated histone H3 (Cit-H3) levels. For in vivo studies, pneumonia-related lung injury was induced by intratracheal LPS instillation in mice, with pathological severity graded by H&E staining, pulmonary edema quantified via the wet/dry weight ratio, and inflammation assessed by the BALF protein concentration. Mechanistically, the IRF1/MMP9 interaction was predicted by bioinformatics (STRING database) and validated by co-immunoprecipitation (Co-IP) and immunofluorescence staining, while MMP9 overexpression was achieved via lentivirus transduction to delineate pathway regulation. IRF1 was significantly upregulated in BALF neutrophils from patients and correlated with elevated ROS production and mitochondrial dysfunction, as well as NETs release. IRF1 knockdown attenuated ROS-driven NETosis in vitro. Matrix metalloproteinase 9 (MMP9) was predicted to interact with IRF1, and MMP9 overexpression effectively reversed the beneficial effects of IRF1 deficiency on ROS release, mitochondrial dysfunction, neutrophil apoptosis, and NETosis. Consistently, in mouse models, MMP9 overexpression abolished the protective effects of IRF1 deficiency, exacerbating acute lung injury and restoring NETs levels in BALF.