A second issue relates to the fraction of cells within an airway region that 
must be corrected to restore normal electrolyte transport to airway epithelia. 
Recently, our laboratory reported findings that provide an estimate of this efficiency 
(49). In brief, using immortalized CF tracheal epithelial cells (CFT1 cells) that 
maintain the CF airway epithelial phenotype with a high degree of validity, aliquots 
of cells were either corrected with the introduction of the CFTR cDNA by retroviral 
techniques, or "marked" by the introduction of the truncated IL2R receptor, which 
permitted quantitation of cells that exhibited the CF phenotype by cell surface 
labelling (anti-IL2R antibodies) techniques. These cells were admixed together in 
defined percentages, allowed to re-form functioning monolayers on collagen 
matrices in vitro, and assayed for restoration of Cl' transport. As summarized in the 
manuscript in Appendix B, it appeared from these studies that a degree of 
correction could be effected when as little as 3% of the cells in the monolayer were 
corrected. Correction of 10% of the cells routinely produced full correction of 
electrolyte transport in CF epithelia. The mechanism by which the amplification of 
correction to adjacent cells was effected was investigated. In a series of studies 
designed to test the hypothesis that cell-cell communication via gap junctions may 
account for this phenomenon, the degree of gap junctional coupling between cells in 
the monolayer was assessed by direct injection of lucifer yellow. These studies 
revealed that each cell was coupled to approximately 10 to 15 adjacent cells. Thus, 
it appears that the mechanism for the so-called "bystander" effect with respect to 
correction is cell-cell coupling. These data likely can be extrapolated to the in vivo 
situation because a similar degree of cell-cell coupling has been reported for intact 
airway epithelial cells by freeze-fracture techniques. 
II.C. The nose as a model for CF epithelial dysfunction 
Cystic fibrosis is an epithelial disease. Ample evidence has accrued that 
many of the functions and activities of the upper (nasal) and lower respiratory 
epithelium are very similar. Morphologically, the upper and lower respiratory 
mucosal surfaces have a similar distribution of epithelial cell types in the superficial 
epithelium, i.e., ciliated, goblet, and basal cells, and similar complements of 
submucosal glands (50). A fundamental activity of the epithelia in both these 
regions, i.e., mucociliary transport, requires coordination of cilial beat frequency, 
secretion of a biorheologically favorable mucin, and regulation of a periciliary liquid 
layer to an appropriate depth and low viscosity (51,52). This latter activity appears 
to be regulated by ion transport systems located in the superficial epithelia, which, 
upon direct comparison, appear to be similar in nasal and lower airway regions (53- 
55). 
The ion transport activities of respiratory epithelia are "electrogenic", i.e., 
they are associated with the development of transepithelial electrical potential 
differences. A major advance for the clinical investigation of CF was the 
development of techniques to measure this bioelectric correlate of ion transport, the 
transepithelial electric potential difference (PD), in vivo (53,54). The capacity to 
make this measurement in vivo allowed investigators to bridge data developed from 
in vitro Ussing chamber techniques to those directly manifested in the airway 
epithelium in situ (17,56). The development of the in vivo PD technology permitted 
the first definitive identification of an epithelial defect in the lung in CF by members 
[446] 
Recombinant DNA Research, Volume 17 
