Neutrophil elastase-induced elastin degradation mediates macrophage

23 Neutrophil (PMNL) influx precedes lung macrophage (LM) influx into the lung following 24 exposure of newborn pups to 60% O2. We hypothesized that PMNL were responsible for the 25 signals leading to LM influx. This was confirmed when inhibition of PMNL influx with a CXC 26 chemokine receptor-2 antagonist, SB-265610, also prevented the 60% O2-dependent LM influx, 27 LM-derived nitrotyrosine formation and pruning of small arterioles. Exposure to 60% O2 was 28 associated with increased lung contents of neutrophil elastase and α-elastin, a marker of 29 denatured elastin, and a decrease in elastin fibre density. This led us to speculate that neutrophil 30 elastase induced elastin fragments were the chemokines that led to a LM influx into the 60% 31 O2-exposed lung. Inhibition of neutrophil elastase with sivelestat or elafin attenuated the LM 32 influx. Sivelestat also attenuated the 60% O2-induced decrease in elastin fibre density. Daily 33 injections of pups with an antibody to α-elastin prevented the 60% O2-dependent LM influx, 34 impaired alveologenesis and impaired small vessel formation. This suggests that neutrophil 35 elastase inhibitors may protect against neonatal lung injury not only by preventing structural 36 elastin degradation, but also by blocking elastin fragment-induced LM influx, thus preventing 37 tissue injury from LM-derived peroxynitrite formation. 38 39