Preterm birth is an increasing statistic among American mothers. Young infants are at increased risk for developing severe manifestations of Paramyxoviral infection, including respiratory syncytial virus (RSV) and parainfluenza virus-3 (PIV3). Expression of innate pulmonary antimicrobial molecules such as beta-defensins have been shown to be developmentally regulated and immature expression may underlie susceptibility. It is my hypothesis that age-dependent susceptibility to severe Paramyxoviral disease is, in part, due to immature beta-defensin expression and that supplemental beta-defensin expression by gene therapy can reduce the severity of Paramyxoviral infection.;An animal model is lacking for studying the age-dependent susceptibility to Paramyxoviral infection seen in preterm infants. Lambs are naturally susceptible to Paramyxoviral infection (bovine) RSV and are proven in use as perinatal pulmonary models of disease. In this study, RSV infected preterm lambs had clinical features of RSV infection and at seven days of infection had lesions consistent with severe RSV seen in preterm infants. Immunohistochemical staining of cells and syncytial cell formation was greater in the preterm versus neonates with the same or higher dose inoculum. This work characterizes a viable preterm animal model for study of age-dependent severity of Paramyxoviral infection in preterm infants.;Next, the developmental expression and cellular localization of sheep beta-defensin-2 (SBD2) in fetal, neonatal and adult sheep was determined. SBD2 expression was prominent in the gastrointestinal tract with no consistent lung expression. The fetal and neonatal lambs had a wider tissue distribution SBD2 than adult sheep. The levels of SBD2 mRNA and peptide were increased with age suggesting developmental regulation. This is the first study to characterize the developmental expression and distribution of SBD2 in sheep.;Lastly, adenoviral-mediated gene therapy was used to express human beta-defensin-6 (HBD6) in neonatal lambs concurrently infected with PIV3. HBD6 expression enhanced neutrophil recruitment to the lung and increased the severity of clinical parameters. The adenoviral gene therapy, regardless of gene insert, enhanced cellular PIV3 antigen staining and syncytial cell formation. This work showed adenoviral-mediated HBD6 expression did not reduce clinicopathological features of PIV3 infection, but rather accentuated it.