Large pH oscillations promote host defense against human airways infection

Abstract

The airway mucosal microenvironment is crucial for host defense against inhaled pathogens but remains poorly understood. We report here that the airway surface normally undergoes surprisingly large excursions in pH during breathing that can reach pH 9.0 during inhalation, making it the most alkaline fluid in the body. Transient alkalinization requires luminal bicarbonate and membrane-bound carbonic anhydrase 12 (CA12) and is antimicrobial. Luminal bicarbonate concentration and CA12 expression are both reduced in cystic fibrosis (CF), and mucus accumulation both buffers the pH and obstructs airflow, further suppressing the oscillations and bacterial-killing efficacy. Defective pH oscillations may compromise airway host defense in other respiratory diseases and explain CF-like airway infections in people with CA12 mutations.

Breathe—Your immune system is counting on it

“…For this mechanism to be functional, the tissue must be able to accommodate rapid CO 2 hydration and dehydration reactions at rates that are relevant to the respiratory cycle. These reactions are catalyzed by carbonic anhydrase, which the authors showed to be tethered to the epithelial surface (Kim et al, 2021). Inhibitors of carbonic anhydrase greatly dampened the rate of alkalinization of artificial ASL upon reduction of CO 2 abundance.…”

“…In a highly mechanistic study reported here (Kim et al, 2021), an investigative team led by Dr. John Hanrahan (McGill University) has explored an important physiological function not previously considered but likely to be relevant to CF and a variety of other pulmonary diseases: respiratory cycling of airway surface pH, which contributes to host defense mechanisms.…”

“…Prior work from other laboratories relied primarily upon measurements of apical fluid pH in airway cells ex vivo under static conditions, sometimes via indirect assays, which led to the conclusion that ASL in CF cells was at best slightly acidic compared with ASL in non-CF cells. In contrast, Hanrahan and colleagues used flexible, doublebarreled, ultrafine capillary microelectrodes to directly measure pH in the ASL of human nasal mucosa in vivo (Kim et al, 2021). It is well accepted that nasal epithelium is a good surrogate for distal airway epithelium, at least with respect to CFTR function (Brewington et al, 2018).…”

“…With this apparatus, the team showed that the pH of nasal mucosa of healthy subjects oscillated widely between ∼7.5 and 9 during exhalation and between ∼8.5 and 9 during inhalation. Following up on this surprising observation, Hanrahan and colleagues used fluorescent pH indicators to measure ASL pH in cultures of primary human bronchial epithelial cells (a mix of ciliated and goblet cells) exposed to gases in a waveform designed to mimic tidal breathing (Kim et al, 2021). They found that oscillations in CO 2 in this system, as seen in tidal breathing, resulted in reciprocal pH changes.…”

“…Loss of CFTR results in greatly reduced capacity for bicarbonate secretion into the ASL. Indeed, Hanrahan and colleagues compared breathing-induced pH changes in the nasal mucosa of CF and non-CF subjects using their in vivo microelectrode system (Kim et al, 2021). They found that peak and trough pH values measured during inhalation and expiration were 1.3 and 0.5 pH units lower, respectively, in CF subjects compared with healthy controls.…”