Reduced airway surface pH impairs bacterial killing in the porcine cystic fibrosis lung

Abstract

Cystic fibrosis (CF) is a life-shortening disease caused by mutations in the cystic fibrosis transmembrane conductance regulator ( CFTR ) gene 1 . Although bacterial lung infection and the resulting inflammation cause most of the morbidity and mortality, how loss of CFTR first disrupts airway host defense has remained uncertain 2 – 6 . We asked what abnormalities impair eradication when a bacterium lands on the pristine surface of a newborn CF airway? To investigate these defects, we interrogated the viability of individual bacteria immobilized on solid grids and placed on the airway surface. As a model we studied CF pigs, which spontaneously develop hallmark features of CF lung disease 7 , 8 . At birth, their lungs lack infection and inflammation, but have a reduced ability to eradicate bacteria 8 . Here we show that in newborn wild-type pigs, the thin layer of airway surface liquid (ASL) rapidly killed bacteria in vivo , when removed from the lung, and in primary epithelial cultures. Lack of CFTR reduced bacterial killing. We found that ASL pH was more acidic in CF, and reducing pH inhibited the antimicrobial activity of ASL. Reducing ASL pH diminished bacterial killing in wild-type pigs, and increasing ASL pH rescued killing in CF pigs. These results directly link the initial host defense defect to loss of CFTR, an anion channel that facilitates HCO 3 − transport 9 – 13 . Without CFTR, airway epithelial HCO 3 − secretion is defective, ASL pH falls and inhibits antimicrobial function, and thereby impairs killing of bacteria that enter the newborn lung. These findings suggest that increasing ASL pH might prevent the initial infection in patients with CF and that assaying bacterial killing could report on the benefit of therapeutic interventions.

Low pH Enhances the Action of Maximin H5 against Staphylococcus aureus and Helps Mediate Lysylated Phosphatidylglycerol-Induced Resistance

“…Low pH can exert a protective effect by promoting the membrane anchoring of cell wall enzymes and thereby shield these organisms from AMPs 108,109 . Acid conditions have also been shown to promote resistance to AMPs in soil bacteria 92 and to inhibit the antibacterial activity of endogenous human peptides, such as defensins and LL-37, to organisms found in lungs 110,111 . On the other hand, PMF dependent efflux pumps also appear able to import porcine, bovine and other AMPs, thereby promoting their activity [112][113][114] .…”

Inhibitors of pendrin anion exchange identified in a small molecule screen increase airway surface liquid volume in cystic fibrosis

“…The activity of several antimicrobial constituents in the ASL, including lysozyme and lactoferrin, is reduced by acidic pH (43). Antimicrobial activity was reduced in tracheas from non-CF pigs after ASL acidification (by increasing CO 2 ), whereas ASL alkalinization in tracheas from CF pigs (by addition of NaHCO 3 ) elevated antimicrobial activity (43).…”

The spatial profiles and metabolic capabilities of microbial populations impact the growth of antibiotic-resistant mutants

“…This would prevent inhibition. Inhaled bicarbonate therapy is a possible remediation for this and is already being studied as a therapy for CF because it may help restore the innate antimicrobial activity of airway surface fluids as well as facilitate the thinning and clearing of airway mucus [53,54]. Our results suggest a potential additional benefit for bicarbonate therapy in CF, where it could augment the activity of aminoglycosides and help curtail antibiotic resistance.…”

“…However, CF results from a genetic defect in the CF transmembrane receptor, which causes decreased bicarbonate ion transport and acidification of CF airway fluids [52,53]. This would prevent inhibition.…”