Review
Effects of airway surface liquid pH on host defense in cystic fibrosis☆

https://doi.org/10.1016/j.biocel.2014.02.009Get rights and content

Abstract

Cystic fibrosis is a lethal genetic disorder characterized by viscous mucus and bacterial colonization of the airways. Airway surface liquid represents a first line of pulmonary defense. Studies in humans and animal models of cystic fibrosis indicate that the pH of airway surface liquid is reduced in the absence of cystic fibrosis transmembrane conductance regulator function. Many aspects of the innate host defense system of the airways are pH sensitive, including antimicrobial peptide/protein activity, the rheological properties of secreted mucins, mucociliary clearance, and the activity of proteases. This review will focus on how changes in airway surface liquid pH may contribute to the host defense defect in cystic fibrosis soon after birth. Understanding how changes in pH impact mucosal immunity may lead to new therapies that can modify the airway surface liquid environment, improve airway defenses, and alter the disease course.

This article is part of a Directed Issue entitled: Cystic Fibrosis: From o-mics to cell biology, physiology, and therapeutic advances.

Introduction

Cystic fibrosis (CF) is an autosomal recessive genetic disorder that affects multiple organs including the sweat duct, lungs, intestines, pancreas, and liver. The primary cause of morbidity and death in CF is progressive lung disease caused by chronic bacterial infection and inflammation. CF is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene which encodes an anion channel regulated by nucleotide binding and cAMP-mediated phosphorylation. CFTR is localized to the apical membrane of cells of the surface airway epithelium and submucosal glands. It is especially abundant in ciliated cells (Kreda et al., 2005). In the lung, CFTR conducts HCO3− and Cl− thereby helping regulate airway surface liquid (ASL) volume and composition. Loss of CFTR function has been implicated in an airways host defense defect, leading to impaired innate immunity and chronic bacterial colonization of the airways.

In the respiratory tract, ASL is a first line of defense against inhaled or aspirated pathogens including bacteria, fungi, and respiratory viruses. ASL comprises two layers: an aqueous (sol) layer and a mucus (gel) layer (Fig. 1). The aqueous periciliary layer covers the cilia, hydrating mucins and allowing for ciliary beating by distancing the mucus from the cell surface. The mucus layer is comprised of secreted and tethered mucins produced by surface goblet cells and submucosal gland epithelia. This material traps inhaled and aspirated microbes so they can be removed from the lung via mucociliary clearance. In addition to trapping pathogens, ASL also contains numerous antimicrobial peptides, proteins, and lipids, the secreted products of surface and submucosal gland epithelia and resident phagocytic cells (Bartlett and McCray, 2013).

While it has traditionally been thought that babies with CF are born with normal lungs, growing evidence indicates airway defenses are compromised early, perhaps as early as the first month (Khan et al., 1995, Armstrong et al., 1998). This defect contributes to lung disease progression during the first years of life and is characterized by colonization with bacteria (e.g. Haemophilus influenzae, Staphylococcus aureus, and Pseudomonas aeruginosa), and the onset of inflammation. In addition to difficulties with bacterial infections, infants with CF are more likely to suffer greater morbidity from common respiratory virus infections, though the total number of viral infections is not different from non-CF (Abman et al., 1988, Hiatt et al., 1999).

The availability of new CF animal models, including the pig (Rogers et al., 2008) and ferret (Sun et al., 2008), has facilitated study of the early events of CF lung disease at the molecular and cellular levels. While newborn CF pigs do not have pulmonary inflammation (Rogers et al., 2008), they exhibit an impaired ability to eradicate bacteria compared to their non-CF littermates (Stoltz et al., 2010). BAL and lung tissues removed from newborn CF pigs were less likely to be sterile than non-CF samples from non-CF littermates. CF pigs also exhibited a reduced ability to clear S. aureus when challenged via aerosol. A recent study by Pezzulo et al. indicates the CF host defense defect in newborns is caused, in part, by abnormal ASL pH (Pezzulo et al., 2012). These studies showed that a reduction in pH leads to decreased ASL antimicrobial activity in CF pigs (Pezzulo et al., 2012), though the mechanism(s) by which pH impair ASL antimicrobials is currently unknown. The CF ferret also demonstrates early abnormalities in host defense, with tracheal xenographs from newborn CF ferrets exhibiting defective cAMP-induced chloride permeability and decreased submucosal gland fluid secretion (Sun et al., 2010). Juvenile and adult CF ferrets have decreased mucociliary clearance, increased mucus obstruction, and increased bacterial infections compared to non-CF littermates (Sun et al., 2013).

The pathogenesis of CF lung disease is complex and changes as patients age. As the disease progresses, the secondary complications of chronic inflammation and the protease rich environment of the airways further compromise host defenses. In this review we focus primarily on early events linked to the CF host defense defect and the onset of lung disease.

Section snippets

The role of ASL in the defenses of the airways

ASL plays a key role in the initial defense of the airways from pathogens. In addition to acting as a physical barrier to infection, ASL contains a number of peptide and protein antimicrobials. Some of the antimicrobials found in ASL include LL-37, lactoferrin, lysozyme, β-defensins, secretory leukocyte peptidase inhibitor (SLPI), and surfactant proteins A and D (SP-A and SP-D). Many of these proteins possess both antibacterial and antiviral activity. Some phagocytic cells of the innate immune

Therapy

The work of Pezzulo et al. in the CF pig model suggests that increasing ASL pH may prevent or reduce airway infections (Pezzulo et al., 2012). A phase 2 clinical trial from 2006 reported that patients administered inhaled HCO3− expectorated three times more mucus than those given inhaled saline alone (ClinicalTrials.gov NCT00177645). However, to the best of our knowledge no published clinical studies demonstrate benefits of inhaled HCO3− or other pH-altering interventions in the treatment of

Conclusion

Studies in human cells, BAL, and lung tissues and CF animal models provide convergent evidence that airway defenses are not normal at birth in CF. ASL from newborn CF pigs has impaired antibacterial activity compared to the ASL of non-CF littermates (Pezzulo et al., 2012). CF ASL is also more acidic than non-CF ASL (Tate et al., 2002, Coakley et al., 2003, Song et al., 2006, Pezzulo et al., 2012, Abou Alaiwa et al., 2014) and the antibacterial activity of ASL is pH-sensitive (Pezzulo et al.,

Acknowledgements

We acknowledge the support of NIH P01 HL-51670, P01 HL-091842, and National Science Foundation Graduate Research Fellowship (Grant No. 1048957). This work was also supported by the Roy J. Carver Charitable Trust. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We thank Jennifer Bartlett and Shyam Ramachandran for critically reviewing the manuscript.

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    ☆

    This article is part of a Directed Issue entitled: Cystic Fibrosis: From o-mics to cell biology, physiology, and therapeutic advances.

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