The human airway is a pretty inhospitable place for microbes. There are numerous immune defense mechanisms poised to kill or remove inhaled bacteria before they can cause problems. But cystic fibrosis disrupts these defenses, leaving patients particularly susceptible to airway infection, which is the major cause of disease and death in cystic fibrosis.
Using a unique animal model of cystic fibrosis, a team of scientists from the University of Iowa has discovered a difference between healthy airways and airways affected by cystic fibrosis that leads to reduced bacterial killing in cystic fibrosis airways. The finding directly links the genetic cause of cystic fibrosis—mutations in a channel protein called cystic fibrosis transmembrane conductance regulator (CFTR)—to the disruption of a biological mechanism that protects lungs from bacterial infection.
The study, published in the July 5 issue of Nature, shows that the thin layer of liquid coating the airways is more acidic in newborn pigs with cystic fibrosis than in healthy newborn pigs, and that the increased acidity (lower pH) reduces the ability of the liquid to kill bacteria. Moreover, making the airway liquid less acidic with a simple solution of baking soda restores bacterial killing in cystic fibrosis airways to almost normal levels.
Although the findings suggest that therapies that raise the pH of the airway surface liquid may help prevent infection in cystic fibrosis, the researchers strongly caution that this discovery is at an early stage.
"Some have asked us if people with cystic fibrosis should inhale an aerosol that would raise the pH of the airway surface liquid," says Joseph Zabner, M.D., UI professor of internal medicine and senior study author. "At this point, we have no idea if that would help. And more importantly, it could be harmful."
"This was a very surprising finding," adds Alejandro Pezzulo, M.D., UI postdoctoral fellow and co-lead author of the study. "There have been many ideas as to what goes wrong in cystic fibrosis, but lack of a good experimental model has made it difficult to gain insight into how the disease gets started."
Unlike mouse models of the disease, the cystic fibrosis pigs develop lung disease that closely mimics what is seen in humans. Previous studies from the UI lab showed that although the airways of cystic fibrosis pigs are infection-free at birth, they are less able to get rid of bacteria than healthy airways and quickly become infected.
Testing bacterial killing in airways
The UI team, including Pezzulo and co-lead author Xiao Xiao Tang, Ph.D., a Howard Hughes Medical Institute postdoctoral research associate at the UI, developed a simple experiment to study bacterial killing by the airway surface liquid. They immobilized bacteria on a tiny gold grid and exposed these bacteria to airway surface liquid from cystic fibrosis-affected and healthy pigs.
The airway surface liquid from normal airways killed most of the bacteria very rapidly, whereas the airway surface liquid from cystic fibrosis-affected airways only killed about half of the bacteria, suggesting that in cystic fibrosis airways some bacteria would survive and go on to cause infection.
Further investigation showed that although many characteristics of the airway surface liquid in cystic fibrosis and noncystic fibrosis pigs are similar, the airway surface liquid from cystic fibrosis airways is more acidic than the liquid from healthy airways.
When the scientists raised the pH of the airway surface liquid in cystic fibrosis pigs through inhalation of a solution of sodium bicarbonate (baking soda), the treated airway surface liquid was capable of killing most of the bacteria on the grid (just like airway surface liquid from normal airways). Conversely, lowering the pH of airway surface liquid from normal airways reduced bacterial killing. The finding confirms that pH is a critical factor for bacterial killing.
"This study explains why a defect in the CFTR channel protein leads to reduced bacterial killing and an airway host defense defect," Tang says. "Impaired bicarbonate transport because of the defective CFTR could cause increased acidity in the airway surface liquid, which the study shows reduces the airway surface liquid bacterial killing capability."
Potential clinical use
Although the approach is not ready for clinical application, the study indicates that pH is a contributing factor in airway infection, suggesting that therapies that modify airway pH may potentially be helpful in preventing infection in cystic fibrosis patients.
In addition, the researchers believe that using the bacteria-coated grids to measure bacterial killing in airways might provide a simple way to test the effectiveness of other new cystic fibrosis therapies that are being developed.
The UI team also included Mark Hoegger; Mahmoud Abou Alaiwa; Shyam Ramachandran; Thomas Moninger; Phillip Karp, Christine Wohlford-Lenane; Henk Haagsman; Martin van Eijk; Botond Banfi; Alexander Horsewill; David Stoltz; Paul McCray; and Michael Welsh.
The work was supported in part by grants from the National Heart, Lung, and Blood Institute, and the Cystic Fibrosis Foundation. Welsh is a Howard Hughes Medical Institute (HHMI) investigator.