Experimental Theories

Information compiled from the article "Cystic Fibrosis Airway Epithelia Fail to Kill Bacteria Because of Abnormal Airway Surface Fluid" by Jeffery J. Smith, Sue M.Travis, E. Peter Greenberg, and Michael Welsh in the April 19, 1996 issue of Cell magazine.

Introduction: Lung disease is characterized by bacterial colonization and chronic airway infection. Though many organisms may be involved, Pseudomonas aeruginosa and Staphylococcus aureus are particularly prominent for a number of reasons. Antibiotics used to treat lung infections may select for these organisms. Systems that serve as a second line of defense may have difficulty eliminating these particular bacteria. These organisms may also possess certain properties such as increased adherence to CF epithelia that enhance colonization.

I. Normal but Not CF Epithelia Kill Bacteria Applied to the Apical Surface

Question Posed:

What happens when P. aeruginosa and S. aureus are each placed on normal epithelia?


It was found that both, P. aeruginosa and S. aureus were killed when placed on the apical surface of normal airway epithelia. However, none of the bacteria was killed when placed on the apical surface of CF epithelia. The defect in killing P. aeruginosa was corrected when CFTR was expressed in CF epithelia using a recombinant adenovirus. No bacteria, however, was killed when the epithelia was treated with an adenovirus expressing B-galactosidase.Graphical Summary

When more than 10^3 P. aeruginosa was added to the apical surface of normal airway epithelia the channel could no longer kill the bacteria. When less than 10^3 P. aeruginosa was added antibacterial activity proceeded as normal. Graphical Summary

When P. aeruginosa was added to the basolateral surface of normal airway epithelial tissue, none of the bacteria was killed, suggesting that the antibacterial activity was localized to the apical surface of the epithelia. Graphical Summary

II. Normal and CF Airway Surface Fluids Contain Bactericidal Activity

Question Posed:

Is there bactericidal activity in the airway surface fluid of normal epithelia?


The airway surface fluid from normal airway epithelia killed both P. aeruginosa and S. aureus. Graphical Summary

Once the normal airway epithelia had been washed with water, it lost its ability to kill the bacteria. Graphical Summary

The airway surface fluid collected from CF airway epithelia killed P. aeruginosa. Graphical Summary

III. Airway Surface Fluid from CF Epithelia Has an Abnormally Increased Cl- Concentration

Question Posed:

How do the Cl- concentrations of normal airway surface fluid and CF epithelia compare?


The CF epithelia had much greater Cl- concentrations than the normal airway epithelia. Graphical Summary

IV. An Increased NaCl Concentration Inhibits Bactericidal Activity in Airway Surface Fluid

Question Posed:

Does the amount of NaCl in the airway surface fluid affect its ability to kill P. aeruginosa and S. aureus?


As the NaCl concentration increased in the airway surface fluid, the amount of bacteria killed decreased in the surface fluid of the epithelia. Graphical Summary

V. Reduction of the NaCl Concentration Allows CF Epithelia to Kill P. aeruginosa.

Question Posed:

If the electrolyte concentration is altered, will bactericidal activity be altered as well?


As the Cl- concentration in airway surface fluid collected from normal epithelia increased, the bactericidal activity decreased. Graphical Summary

As the Cl- concentration in airway surface fluid collected from CF epithelia increased the bactericidal activity decreased. Graphical Summary

Discussion: The test results suggest that airway epithelia secrete a bactericidal substance into the thin layer of fluid covering the apical surface and its activity is dependent on a low salt concentration. In CF epithelia, however, loss of CFTR Cl channels produces an abnormally high salt concentration, reducing bactericidal activity. Loss of bactericidal activity could explain why CF patients have lung disease. According to the data collected through the experiments described in the paper, it seems possible that a second line of defense, neutrophils and macrophages, may kill the bacteria and cause an inflammatory environment. The abundant inflammatory mediators and chronic infection may stimulate hypertrophy of submucisal glands and cause mucus hypersecretion. Inflammation and infection then lead to progressive lung destruction.

Other Theories: One alternative theory suggests that impaired phagocytosis of P. aeruginosa might be the basis for all infections. This conclusion is based on the decreased phagocytosis of P. aeruginosa by a transformed CF epithelial cell line grown on tissue culture plates. It has also been reported that adherence of P. aeruginosa to CF airway epithelia is slightly increased on the surface of a CF cell because submucosal glands express high levels of CFTR. Their secretions may be abnormal in CF, leading to predisposition to infection. Although the data in the paper does not exclude any role of the theories in infection, it shows that electrolyte composition affects bacterial survival which provides an explanation for why CF airways are not maintained as a sterile environment. The data speculates that the bactericidal factor produced by airway epithelia may be a defensin-like molecule because it has characteristics of such factors. The results presented in the paper also link the molecular defect in CFTR Cl channels to the pathogenesis of CF lung disease. Most importantly the data suggests new approaches for therapy such as measurements of salt concentration and bactericidal activity may be clinically relevant assays for determining the effectiveness of potential therapeutic interventions. The data also raises the possibility that new interventions designed to correct the abnormally high salt concentration in CF fluid could be of benefit in treating or preventing airway infections in people with CF.