P1.154: Defining patterns of lung injury and repair following early life influenza virus infection

Background: Viral respiratory infections early in life are associated with significant pediatric morbidity and mortality and have long-term consequences for lung health into adulthood. Despite pediatric respiratory infections being the leading cause of hospitalization in children, little is known about the injury and repair mechanisms that operate in the lung following illness. The primary objective of this study was to define the patterns of lung injury and repair using a neonatal mouse model of influenza virus lower respiratory tract infection.

Methods: Wildtype outbred p2 neonatal CD-1 mice were intranasally inoculated with influenza A virus (H1N1) strain A/PR/8/34 under isoflurane anesthesia at various TCID50. Littermate controls were inoculated with phosphate-buffered saline. Following inoculation, pups were weighed daily and assessed clinically for signs of respiratory distress. Pups were sacrificed early if moribund, displaying severe respiratory distress, or experiencing significant weight loss. Kaplan-Meier curves were plotted to assess survival following infection at different TCID50. To characterize clinical consequence of infection, vital signs were monitored using the MouseOX pulse oximeter 7 days post infection to assess heart rate, oxygen saturation, and respiratory rate. To assess histological patterns of injury, pups were sacrificed at 14 or 28 days post infection (or earlier if meeting the predefined humane endpoints for sacrifice). Lungs were harvested and processed for H&E staining or immunohistochemistry using antibodies specific for influenza or lung cell subtypes. All statistical analysis were performed using GraphPad Prism software with statistic tests indicated in figure legend.

Results: Influenza virus infection of neonatal mice was associated with significant mortality, even at low TCID50. Newborn mice inoculated with sublethal doses of influenza virus displayed clinical signs of respiratory distress with tachypnea, increased work of breathing, and stagnation in weight gain. Pulse oximetry at 7 days post infection showed significant hypoxemia in influenza-infected pups. Histology during the acute phase of infection (4-6 days post inoculation) showed areas of predominant cellular infiltration and interstitial inflammation as well as other areas displaying patterns of alveolar simplification. Histology done at 14- and 28-days post infection showed a resolution of areas of significant cellular infiltration and persistence of emphysematous changes seen in the early stages of infection.

Conclusion: Neonatal mice develop severe lower respiratory tract disease following influenza virus infection that is associated with significant architectural changes in the lung. There are pathologic histological changes following neonatal influenza infection that persist into adulthood, indicating that early life respiratory tract infections cause long-term damage.