Vascular endothelial growth factor in developing ovine lung: endogenous expression and effects of exogenous administration on respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis in children with nearly all children infected by two years of age. Preterm infants and infants with cardiopulmonary compromise or immunodeficiency are at increased risk for severe RSV disease. However, a high percentage of hospitalizations due to severe RSV disease occur in otherwise healthy infants. Severe RSV disease is characterized by bronchiolitis and airway obstruction secondary to sloughed epithelial cells, cellular debris and inflammatory cells that can partially occlude the airway lumen. There is currently no licensed vaccine to prevent RSV infection. The purpose of this study was to develop a model of RSV infection in the newborn lamb using a human strain of RSV, strain A2. The pulmonary pathology and cellular localization of antigen was determined, in addition to the viral effect on the expression of epithelial innate immune genes, surfactant proteins A and D (SP-A and SP-D respectively) and sheep beta-defensin-1 (SBD-1). Pulmonary lesions were characterized by suppurative bronchiolitis with multifocal alveolar consolidation and peak pulmonary lesions at day 6 post-infection. RSV infection increased expression of SP-A and SBD-1, which demonstrates alteration of innate immune responses caused by RSV A2. Using this model, lambs were pretreated with exogenous human recombinant vascular endothelial growth factor (rhVEGF) prior to RSV infection. VEGF is a known endothelial mitogen, but also plays an important pulmonary role in surfactant protein production, epithelial cell proliferation, and epithelial cell survival. VEGF administration prior to RSV infection decreased both viral load and pulmonary pathology at peak infection - 6 days post-infection. In addition, VEGF increased the expression of SP-A and SBD-1 showing that VEGF can induce expression of important epithelial innate immune genes. VEGF pretreatment altered epithelial cell proliferation in RSV infected animals at day 4 post-infection. In the final part of this study, the effect of ontogeny on pulmonary endogenous VEGF expression was investigated. The results of this study demonstrate that the expression of major VEGF isoforms are differentially regulated with high pulmonary VEGF mRNA expression in prenatal lamb lung and low expression in adult lung. However, mRNA expression diverged from the protein profile with low VEGF protein expression in prenatal lung and high VEGF protein expression in adult lung, demonstrating tight translational regulation to maintin appropriate VEGF concentrations during development