Background Preterm birth followed by prolonged mechanical ventilation often results in chronic lung disease (CLD) of prematurity. CLD is characterized by alveolar simplification, which is manifest, in part, by thick, cellular mesenchyme in the distal airspace walls. The thick, cellular mesenchyme is postulated to persist during the evolution of CLD because apoptosis (programmed cell death) is reduced. Markers of apoptosis are nicked DNA (assessed by the TUNEL method), and expression of p53 and its downstream targets, such as caspase-3. Whether apoptosis in the walls of the distal airspaces is affected by preterm birth followed by prolonged ventilation has not been determined.
Hypothesis Apoptosis of mesenchymal cells in the distal airspace walls is reduced during the evolution of CLD.
Methods Preterm lambs (,130d gestation) were managed by conventional mechanical ventilation (CV) or nasal continuous positive airway pressure (nCPAP) for 24h or 72h (n=4-6/group). We used nCPAP as the positive control (gold-standard) for outcome in chronically ventilated preterm lambs (Albertine, Pediatr Res 55:438A, 2004). Lung tissue was analyzed for caspase-3 protein abundance (immunoblot analysis) and topographic distribution (immunohistochemistry).
Results Immunoblot analysis showed that caspase-3 protein abundance in lung tissue homogenates from preterm lambs was significantly lower in the CV group compared to the nCPAP group, at both 24h and 72h. Immunohistochemistry showed that caspase-3 protein was localized in fewer mesenchymal cells.
Conclusion Our results show that markers of apoptosis (caspase-3 protein abundance and immunolocalization) are less in the lungs of preterm lambs that are managed by CV compared to nCPAP. We conclude that CLD is associated with reduced apoptosis of mesenchymal cells in the distal airspace walls and consequently, thick, cellular mesenchyme. NCPAP permits alveolar formation, apparently through apoptotic loss of mesenchymal cells during the first few days of life, leading to thinner, less cellular mesenchyme in the future alveolar walls. [Children's Health Research Center, and NIH grants R01 HL62875 and T35 HL07744]