1. ABSTRACT (PROJECT 1) Gene transfer is a potential strategy to treat CF airways disease, yet we still lack answers to important questions to any gene transfer approach. What percentage of CF airway epithelial cells must be targeted and at what expression level is needed to correct relevant ion transport defects? What are the key CF host defense defects? Can we rescue those defects? Will targeting airway surface epithelia in vivo rescue host defense? Clinical CF lung disease? Our program made advances in three areas that allow us to answer these questions. First, we developed a model of CF in pigs. Within months of birth, they develop features of CF lung disease. Second, we discovered that loss of CFTR impairs two key respiratory host defense mechanisms, mucociliary transport (MCT) and antibacterial activity in airway surface liquid (ASL). Impaired CFTR-mediated HCO3- secretion that results in abnormally acidic ASL causes both of these defects. Third, we developed viral vectors: AAV evolved to infect airway epithelia, and GP64-pseudotyped FIV that can deliver CFTR cDNA to airway epithelia. Our preliminary data indicate that these vectors generate airway epithelial HCO3- secretion and correct the acidic ASL pH. Now is the time to answer questions that have nagged the field since we first began thinking about gene therapy. Here we propose three specific aims that will inform strategies for CF gene transfer. Aim 1. What factors determine whether gene transfer to pig and human CF airway epithelia in vitro rescues anion secretion and host defense properties? We will investigate the relationships between transduction and expression data (the % of cells expressing CFTR, the level of CFTR expression) and the physiological consequences. Aim 2. Will CFTR gene transfer to CF airway surface epithelia correct ASL pH, bacterial killing, and MCT defects in vivo? Aim 3. Will gene transfer to airway surface epithelia prevent the airway inflammation, infection, and remodeling associated with early CF lung disease? These data may help us understand the relative importance of the CF host defense defects, will suggest whether targeting surface epithelia has clinical benefit, and will guide future gene transfer approaches.