Inflammation as a Regulator of the Airway Surface Liquid pH in Cystic Fibrosis

Abstract

The airway surface liquid (ASL) is a thin sheet of fluid that covers the luminal aspect of the airway epithelium. The ASL is a site of several first-line host defenses, and its composition is a key factor that determines respiratory fitness. Specifically, the acid-base balance of ASL has a major influence on the vital respiratory defense processes of mucociliary clearance and antimicrobial peptide activity against inhaled pathogens. In the inherited disorder cystic fibrosis (CF), loss of cystic fibrosis transmembrane conductance regulator (CFTR) anion channel function reduces HCO₃^- secretion, lowers the pH of ASL (pH_ASL), and impairs host defenses. These abnormalities initiate a pathologic process whose hallmarks are chronic infection, inflammation, mucus obstruction, and bronchiectasis. Inflammation is particularly relevant as it develops early in CF and persists despite highly effective CFTR modulator therapy. Recent studies show that inflammation may alter HCO₃^- and H⁺ secretion across the airway epithelia and thus regulate pH_ASL. Moreover, inflammation may enhance the restoration of CFTR channel function in CF epithelia exposed to clinically approved modulators. This review focuses on the complex relationships between acid-base secretion, airway inflammation, pH_ASL regulation, and therapeutic responses to CFTR modulators. These factors have important implications for defining optimal ways of tackling CF airway inflammation in the post-modulator era.

Keywords: airway epithelium; airway surface liquid; cystic fibrosis; host defense; inflammation; pH.

Figure 1

pH_ASL influences respiratory host defenses. The acid–base balance of ASL controls key first-line airway host defenses including secreted antimicrobial peptide activity and synergism against inhaled bacteria; mucus viscosity and elasticity; ciliary beat frequency (CBF)^#; innate immune cell activities such as phagocytosis and extracellular killing of microbes through release of chromatin; activities of apical channels (e.g., acidic pH_ASL inhibits short palate lung and nasal epithelial clone 1-mediated inhibition of ENaC, promoting increased Na⁺ absorption [28]; extracellular HCO₃⁻ concentration, sensed by soluble adenylyl cyclase, regulates CFTR expression [33,34]); and entry of respiratory viruses into airway epithelial cells (e.g., pH-dependent entry of SARS-CoV-2 in TMPRSS2-expressing cells [35])^##. ASL = airway surface liquid, ENaC = epithelial Na⁺ channel, CFTR = cystic fibrosis transmembrane conductance regulator, TMPRSS2 = transmembrane serine protease 2. ^#The mechanism by which pH_ASL alters CBF is not clear. In one study, CBF in bronchial cells increased as extracellular pH increased from 6 to 7.5 [26]. However, pH outside this range reduced CBF. Interestingly, the effect was less prominent in small airway ciliated cells. ^##Most studies of airway physiology use proximal (large) airway cells. As CF airway disease involves distal (small) airways, regional differences in pH_ASL regulation and host defense mechanisms require further attention.

Figure 2

Acid–base transporters that control pH_ASL differ between human and mouse airways. Models show key transport mechanisms that determine pH_ASL in human and mouse airway epithelia. Left panel (human): (A) a model of non-CF airway epithelium with ASL overlying the apical membrane; (B) loss of CFTR-mediated HCO₃⁻ secretion resulting in a lower pH_ASL; (C) inhibition of basolateral NBC diminishes HCO₃⁻ secretion and lowers pH_ASL despite intact apical CFTR channels. Right panel (mouse): (D) a model of non-CF (wild type) mouse airway epithelium. Note the absence of ATP12A and the expression of non-CFTR (CaCC) HCO₃⁻ channels; (E,F) in contrast to humans, loss of CFTR fails to lower pH_ASL in CF mice, providing one explanation for lack of spontaneous airway disease. However, exogenous ATP12A expression increases H⁺ secretion and lowers CF mouse pH_ASL; (G) SLC4A4^−/− mice phenocopy human CF. For simplicity, only the chief acid–base transport mechanisms controlling pH_ASL are shown. We do not show Na⁺/H⁺ exchangers, Cl⁻/HCO₃⁻ exchangers, Na⁺ and K⁺ channels, or Na⁺/K⁺-ATPase, which may also influence the movement of acid–base equivalents into or out of ASL. See legend and text for details. CA = carbonic anhydrase.

Figure 3

Inflammatory cytokines (IL-17/TNFα) regulate pH_ASL in human CF and non-CF airway epithelia. Control CF epithelia lack CFTR-mediated HCO₃⁻ secretion and have a lower pH_ASL than control non-CF epithelia. IL-17/TNFα upregulate pendrin, an apical Cl⁻/HCO₃⁻ exchanger, and thereby increase CF pH_ASL. Non-CF epithelia exposed to IL-17/TNFα have a higher pH_ASL compared to similarly treated CF epithelia. Interestingly, restoring CFTR channel function in IL-17/TNFα-treated CF epithelia further increases pH_ASL. Thus, maximal ASL alkalinization response involves two apical HCO₃⁻ transporters, CFTR and pendrin. Modified from Rehman et al. [39] and reproduced with permission.

Figure 4

WNK kinases regulate HCO₃⁻ versus Cl⁻ secretion across human airway epithelia. Model shows the with-no-lysine [K] (WNK) kinase signaling pathway in human CF airway epithelia lacking functional CFTR. Left panel: WNK1 and WNK2 signal via intermediate Ste20/SPS1-related proline-alanine-rich protein kinase/oxidative stress responsive 1 kinase (SPAK/OSR1) to regulate basolateral Na⁺-K⁺-2Cl⁻ cotransporter (NKCC1) activity. Right panel: reducing WNK activity, inhibiting NKCC1, or removing Cl⁻ from basolateral solution lower the intracellular [Cl⁻]. At the same time, these interventions also increase HCO₃⁻ secretion and alkalinize ASL. Higher pH_ASL improves epithelial host defenses which are otherwise impaired in CF. The mechanism by which WNK kinases and intracellular [Cl⁻] regulate apical and/or basolateral HCO₃⁻ transporters (CaCC, pendrin, NBC) remain unknown. Modified from Rehman et al. [101] and reproduced with permission. Copyright © 2022 American Thoracic Society. All rights reserved. The American Journal of Respiratory Cell and Molecular Biology is an official journal of the American Thoracic Society.