Background: Activated macrophages can produce macrophage extracellular traps (METs) - DNA strands coated with cytotoxic components that are released to immobilize pathogens and control infection. However, METs can also contribute to the inflammatory process and lead to severe tissue damage. Bronchopulmonary dysplasia (BPD) is a chronic lung disease in premature infants initiated by inflammation and oxidative stress. Gram-negative bacteria and candida increase the risk of developing BPD and are strong inducers of extracellular traps.  Macrophages are the most abundant immune cells in the neonatal lung tissue, responsible for controlling the inflammatory process. We aimed to determine whether inflammation triggers METosis in a neonatal model of lipopolysaccharide (LPS)-induced lung injury.

Methods: Bronchoalveolar Lavage Fluid (BALF) from neonatal Sprague Dawley rat pups was collected to obtain alveolar macrophages (AMs). In vitro postnatal day (PND) 6/8/22 AMs were incubated with METosis agonists (LPS, phorbol myristate acetate (PMA)) or control (media) for 4 hours. Cells were fixed and imaged for markers of METosis (myeloperoxidase (MPO) and citrullinated histone H3 (CitH3)) and DNA (4’,6-diamidino-2-phenylindole) using immunofluoresence. In vivo PND6-9 pups were treated with LPS or saline intrapharyngeally and BALF was collected on PND10. Immunofluoresence analysis was used to examine the presence of DNA, MPO and CitH3 and supernatant was analyzed using a picogreen assay.

Results: Immunofluorescence imaging of PND8 AMs showed increased fluorescence of METosis markers and MET-like structures with in vitro exposure of PMA (n = 3) and LPS (n = 3) in comparison to control. In vivo, exposure to LPS (n = 3), significantly increased extracellular trap formation in the lung of PND10 rats in comparison to saline-instilled rats (n = 3; p = 0.0019); increased fluorecesnce of METosis markers was also observed with imaging. During lung inflammation, macrophages predominate in the tissue during neonatal periods, whereas neutrophil influx predominates later in the life. Therefore, we sought to explore whether the capacity of AMs to undergo METosis is unique to the neonatal periods. Immunofluorescence images of isolated AMs showed less METosis in the adult (PND22) AMs in comparison to the neonatal (PND6) AMs. Thus, this capacity of AMs to undergo METosis is decreased in adulthood, indicating its relevance predominantly in the neonatal period.

Conclusions: To the best of our knowledge, this study is first to establish (i) rat alveolar macrophages undergo METosis (ii) a role for METosis in lung injury (iii) the relevance of METosis in lung injury in early vs late life. Reducing the effects of METosis could translate to therapies for neonates with BPD.