While the CF Cl- channel defect has been shown to be reversible in cell culture many additional extracellular barriers exist in vivo as the airway epithelia have evolved to prevent penetration by foreign materials (McCluskie & Davis 2000).
Initially the GTA carrying the transgene needs to reach the pulmonary airways where it will encounter the mucus membrane complex, which may bind the gene transfer agent (GTA) and clear it from the lung by mucociliary clearance (Boucher 1999).
If not immediately cleared from the lungs, the GTA must penetrate the mucus to gain access to the surface of the cell and then penetrate the plasma membrane (Yonemitsu et al. 2000).
In the airway, generally only the apical surface of cells is available to a GTA as tight junctions between cells restricts access to the basolateral surface.
Once the GTA has entered the cell, it must cross the cytoplasm and the DNA must enter the nucleus. Therefore a successful strategy for long term gene transfer to the apical surface of the airway epithelia must be able to overcome these barriers.