Pseudomonas aeruginosa triggers CFTR-mediated airway surface liquid secretion in swine trachea

Cystic fibrosis (CF) is an autosomal recessive genetic disorder caused by mutations in the gene encoding for the anion channel cystic fibrosis transmembrane conductance regulator (CFTR). Several organs are affected in CF, but most of the morbidity and mortality comes from lung disease. Recent data show that the initial consequence of CFTR mutation is the failure to eradicate bacteria before the development of inflammation and airway remodeling. Bacterial clearance depends on a layer of airway surface liquid (ASL) consisting of both a mucus layer that traps, kills, and inactivates bacteria and a periciliary liquid layer that keeps the mucus at an optimum distance from the underlying epithelia, to maximize ciliary motility and clearance of bacteria. The airways in CF patients and animal models of CF demonstrate abnormal ASL secretion and reduced antimicrobial properties. Thus, it has been proposed that abnormal ASL secretion in response to bacteria may facilitate the development of the infection and inflammation that characterize CF airway disease. Whether the inhalation of bacteria triggers ASL secretion, and the role of CFTR, have never been tested, however. We developed a synchrotron-based imaging technique to visualize the ASL layer and measure the effect of bacteria on ASL secretion. We show that the introduction of Pseudomonas aeruginosa and other bacteria into the lumen of intact isolated swine tracheas triggers CFTR-dependent ASL secretion by the submucosal glands. This response requires expression of the bacterial protein flagellin. In patients with CF, the inhalation of bacteria would fail to trigger ASL secretion, leading to infection and inflammation.The human airway is normally protected from injury caused by microbial colonization and viral infection by a complex immune defense system. The cornerstone of airway defense is mucociliary clearance. Particles, including bacteria, are captured in mucus and removed by an efficient mucociliary clearance mechanism. Airway host defense is compromised in individuals with cystic fibrosis (CF), whose lungs are thus prone to chronic bacterial infections, frequently with Pseudomonas aeruginosa, and inflammation that may eventually cause lung tissue damage and respiratory failure (1, 2). The events leading from cystic fibrosis transmembrane conductance regulator (CFTR) gene mutation to airway disease are incompletely understood, but accumulating evidence suggests that CF airway disease results from abnormal microbial clearance (3, 4).Although chronic inflammation is a major aspect of CF lung disease, recent data show that the initial consequence of CFTR mutation is impaired ability to eradicate bacteria. In previous studies, lungs from animal models of CF (F508del and CFTR^−/− pigs) (5, 6) did not eradicate bacteria as effectively as lungs from WT littermates before the development of inflammation (3, 4). These results suggest that impaired bacterial elimination is the pathogenic event that initiates a cascade of inflammation and pathology in CF lungs (4).The failure to clear bacteria likely results from abnormal airway surface liquid (ASL) secretion and properties (6–10). The ASL consists of a layer of mucus that traps inhaled particles and a periciliary liquid layer that keeps the mucus an optimum distance from the underlying epithelia to maximize ciliary mobility (10, 11). The mucus layer is a complex mixture of water, salts, gel-forming mucins, and antimicrobial compounds that helps inactivate, kill, and trap pathogens and facilitates mucociliary clearance (10, 11). In CF airways, both the bacteria-killing properties and ASL secretion are abnormal (3, 9). The airway liquid produced by CFTR^−/− swine has weaker bactericidal properties compared with that produced by WT littermates, owing to abnormal pH (3, 4). In addition, human CF airways, 1-d-old CF piglets, newborn CFTR^−/− ferrets, and CFTR^−/− mice fail to respond to stimulatory signals that normally elicit strong ASL secretion (6–9). Consequently, it has been proposed that abnormal secretion of fluid and mucin in response to bacterial infection may contribute to the pathogenesis of CF lung disease (7–10, 12–15); however, the central questions of whether bacteria trigger ASL secretion in the airways, and the role of CFTR in such a process, have not been explored previously, owing to the lack of a suitable experimental technique.We have developed a novel synchrotron-based method to measure the height of the ASL layer covering the epithelium of intact, isolated swine trachea. We show that the introduction of P. aeruginosa into the lumen of intact isolated swine tracheas triggers CFTR-dependent ASL secretion by the submucosal glands. This is a local response that affects only the glands in close proximity to the bacteria and requires expression of the bacterial protein flagellin. We also show that Staphylococcus aureus and Haemophilus influenzae trigger CFTR-dependent ASL secretion, indicating that this response is not unique to P. aeruginosa. In patients with CF, the inhalation of bacteria would fail to trigger ASL secretion by submucosal glands, facilitating infection and inflammation.