Neutrophil Elastase Triggers the Release of Macrophage Extracellular Traps: Relevance to Cystic Fibrosis

Neutrophil extracellular traps increase cystic fibrosis (CF) airway inflammation. We hypothesized that macrophage exposure to neutrophil elastase (NE) would trigger the release of macrophage extracellular traps (METs), a novel mechanism to augment NE-induced airway inflammation in CF. Experiments were performed using human blood monocyte derived macrophages (hBMDM) from patients with and without CF to test specific mechanisms associated with MET release, and MET release by NE was confirmed in alveolar macrophages from Cftr-null and wild-type littermate mice exposed to intratracheal NE in vivo. Human BMDM were exposed to FITC-NE, and intracellular FITC-NE was localized to cytoplasmic and nuclear domains. Intracellular NE was proteolytically active as indicated by DQ-Elastin substrate cleavage. NE (100 to 500 nM) significantly increased extracellular PicoGreen fluorescence consistent with DNA release/ MET release from hBMDM in the absence of cell death. MET release was further confirmed by confocal microscopy in hBMDM treated with NE, and in alveolar macrophages from Cftr-null and wild-type littermate mice that had been exposed to intratracheal NE. NE-triggered MET release was associated with H3 citrullination detected by immunofluorescence assays and with partial cleavage of histone H3 but not H4. Exposure to NE caused release of METs from both CF and non-CF hBMDM in vitro and murine alveolar macrophages in vivo. MET release was associated with NE-activated H3 clipping, a mechanism associated with chromatin decondensation, a prerequisite for METs.