basal, ciliated, serous, goblet, mucous, non-ciliated-bronchiolar epithelial cells (Clara cells), and 
neuroepithelial bodies (NEBS). In the human lung, the trachea and proximal bronchi contain 
numerous tracheal-bronchial glands that are not features of the rodent airway. In the alveolar 
region, only two epithelial cell types are readily apparent, Type II epithelial cells and Type I 
epithelial cells. Type I cells are terminally differentiated cells derived from Type II cells that 
comprise approximately 90-95 % of the surface area of the alveolar gas exchange. Type II cells 
produce pulmonary surfactant and are considered the progenitor cells of the most distal 
respiratory epithelium. The functions of the distinct cells in the respiratory tract are highly 
specialized, species specific and subject to developmental regulation. Complex changes in the 
morphological characteristics of these cells are associated with marked changes in gene 
expression during development and in response to injury of the airway. Because of this 
heterogeneity of cellular types and functions it is unclear at present in which cells CFTR defect 
causes lung disease and whether all or some cells must be treated to effect an amelioration of 
disease. 
1.8 Gene Therapy to the Airway Epithelium: In patients with cystic fibrosis, lung 
development proceeds normally until after birth, when mucous plugging, air trapping and 
infection begin to cause chronic lung injury that usually accelerates in the second decade of life. 
Therapy for the pulmonary complications of cystic fibrosis has been focused on the treatment 
of the secondary effects of the disease, thought related to abnormal function of the CFTR. 
Primary therapy may now be considered by transferring the wild-type hCFTR cDNA to the 
epithelial cells of the respiratory tracts of patients with cystic fibrosis. Biochemical evidence 
to date confirms the clinical observation that cystic fibrosis is a recessive trait and that 
expression of the mutant CFTR does not confer a dominant abnormality of cellular function in 
cells transfected with CFTR. Thus, it is likely that restoration of cAMP-dependent chloride 
transport, or other more subtle abnormalities of cellular function such as abnormal acidification 
of intracellular organelles (Barasch et.al., 1991), may be accomplished by transfer of the CFTR 
gene to cells of the lung. Complicating the therapeutic strategy for gene transfer to the 
epithelium is the imprecision qn our present understanding of the identity of epithelial cells 
critical to the pathogenesis of the cystic fibrosis defect in the lung. The CFTR gene is expressed 
at very low levels in the respiratory tract in both normals and individuals with cystic fibrosis 
(Trapnell et.al., 1991a, Tresize et.al., 1991) and pulmonary epithelial cells express human 
CFTR at levels below that readily detected by in situ hybridization or immunocytochemistry. 
The precise cellular and intracellular sites of normal and mutant CFTR proteins in the lung have 
not been clarified at present. While alveolar function is, in general, well maintained in patients 
with cystic fibrosis, the small airways are involved early in the pathogenesis of the disease as 
evidenced by the hyperaeration on the chest radiograph commonly seen within the first year of 
life. The clinical findings in cystic fibrosis, therefore, focus attention to the need to complement 
the CFTR in the epithelial cell lining of the bronchial and bronchiolar regions of the lung at an 
early age, before mucous plugging and infection cause irreversible lung disease and make access 
via the respiratory tract impossible. If cell-specific, temporal and spatial regulation of the 
hCFTR is required for normal lung function, direct transfer of the gene by plasmid, adenovirus, 
retrovirus or other vectors, may be greatly complicated and strategies for gene therapy to the 
respiratory epithelium must consider the possibility that unregulated or imprecisely regulated 
expression of CFTR itself may be detrimental to lung function, growth or development. 
However, experiments from this laboratory and others, as outlined later in the present RAC 
application, support the concept that high level or promiscuous expression of CFTR in 
I 
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