Introduction: During lung inflammation, large numbers of neutrophils infiltrate the airways. Conventional idea is that these neutrophils die by apoptosis in the airways and promote resolution. However, the neutrophil extracellular trap (NET) formation (NETosis), and factors that regulate this form of neutrophil death in the airways have not been clearly established.

Method: LPS instilled mice model used for the lung infection. Both SP-D^-/- and wild type mice were instilled with LPS and vehicle for 6, 12, 16, 24, 48, 72 hours and lavaged for the assessment of neutrophils, apoptosis and NETosis assays.

Results and conclusion: Data show that lipopolysaccharide instillation increases cytokine and chemokines (6, 12, 16, 24, 48 and 72 h post instillation; IL-1 beta, VEGF, GMCSF, IL-17, IL-23, MIP, KC) in wild type mouse airways, with higher concentrations measured in the Surfactant Protein-D deficient mice (SP-D^-/-). By contrast, lower numbers of neutrophils and increased amounts of NETs (DNA, citrullinated histone) were detected in the bronchoalveolar lavage and in lung sections of SP-D^-/- than wild-type mice at early time points (16-24 h). Higher amounts of reactive oxygen species (ROS) are detected in the SP-D^-/- mouse neutrophils, and release more NETs compared to SP-D^+/+ mouse neutrophils. Apoptotic cells are mostly detected during the late resolution phase (Annexin-V, TUNEL; 48 h).

Conclusions: Therefore, neutrophils infiltrating the airways primarily undergo NET formation during early stages of inflammation, likely with cytokines, SP-D and ROS as key regulators of the process, whereas apoptosis predominates at the late phase of airway inflammation.