II. B. Preclinical Studies 
II.B.1. Distribution of CFTR in Human Airway 
Design of rational approaches of gene therapy for CF lung disease requires an understanding of 
the distribution of CFTR expression in the non-CF lung. Extensive experience with the clinical 
and pathological manifestations of CF suggest that the primary abnormalities in the lung relate 
to the defects in mucociliary clearance (see section II. A). The prediction, therefore, is that the 
CF gene is normally expressed in the conducting airway rather than the airspace. 
The human airway is a network of branching structures that conduct air from the alveolar sacs 
where gas exchange occurs to the external environment. The general structure of the airway 
varies as it branches from the most proximal site below the larynx (i.e., the trachea) to the 
most distal terminal bronchioles. Proximal airways are lined with a surface epithelium that is 
pseudostratified, containing a layer of squamous cells resting on the basal lamina (called basal 
cells) with a variety of columnar cells including ciliated cells, goblet cells, undifferentiated 
cells and secretory cells with electron dense granules. Basal and goblet cells are believed to be 
progenitors that are capable of self-renewal and differentiation into ciliated cells. 
Areas of the proximal airway surrounded by cartilage also contain submucosal glands which 
consist of a network of secretory tubules and associated ducts [Meyrick et al., 1969]. These 
glands deliver into the airway lumen a complex seromucous secretion essential for mucociliary 
clearance. The most distal component of the submucosal glands consists primarily of serous 
tubules and serous acini. The serous cell has exocytitic vesicles that contain a mixture of 
bactericidal proteins. The mucous tubules, which lie proximal to the serous cells, secrete 
highly viscous mucous glycoproteins. All serous secretions must pass through the mucous 
tubules on the way to the airway. Seromucous secretions move from the tubules into collecting 
ducts which are lined by simple columnar epithelial cells rich in mitochondria. The high 
mitochondrial content of these cells suggests they may play a metabolically active role in 
regulating water and ion concentrations in ducts. Seromucous secretions reach the respiratory 
surface by passing from collecting ducts which, as extensions of the surface mucosa, are lined 
by a pseudostratified, ciliated epithelium. The structure of a submucosal gland is presented in 
Figure 1. 
The distal airway is normally void of submucosal glands and has a much simpler surface 
epithelium which is cuboidal in structure. There are essentially two types of cells in the distal 
airway: ciliated cells and clara cells which contain electron dense secretory granules and are 
believed to be progenitors. 
Critical to our understanding of the pathophysiology of CF lung disease is a precise definition of 
the cellular and subcellular localization of CFTR in the human airway. Previous studies have 
reported variable success in accomplishing this by direct localization. Immunocytochemical 
analyses of adult human tissues have successfully localized CFTR in epithelial cells of sweat 
ducts, small pancreatic ducts, kidney tubules and intestinal crypts, but failed to detect CFTR in 
the lung [Cohn et al., 1991 and Crawford et al., 1991]. Analysis of RNA extracted from human 
bronchial brushings of normal patients indicated that the CF gene is expressed at low levels in 
the cells released by this procedure, which were primarily ciliated and nonciliated columnar 
cells of the surface epithelium [Trapnell et al., 1991]. In situ hybridization studies of rat 
trachea show low levels of diffuse staining over the surface epithelium but these studies could 
make no conclusions about the presence of CFTR mRNA in submucosal glands which are 
extremely nonabundant in the rodent airway. More recently, Zeitlin et al. detected CFTR protein 
in primary nasal epithelial cells by Western blot analysis [Zeitlin et al., 1992]. Function of 
CFTR has been clearly demonstrated in a variety of cells isolated from the surface epithelium 
using electrophysiological techniques [Reviewed in Welsh, 1990]. 
Recombinant DNA Research, Volume 16 
