Relationships among CFTR expression, HCO ₃ ⁻ secretion, and host defense may inform gene- and cell-based cystic fibrosis therapies

Abstract

Cystic fibrosis (CF) is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. Airway disease is the major source of morbidity and mortality. Successful implementation of gene- and cell-based therapies for CF airway disease requires knowledge of relationships among percentages of targeted cells, levels of CFTR expression, correction of electrolyte transport, and rescue of host defense defects. Previous studies suggested that, when ∼10–50% of airway epithelial cells expressed CFTR, they generated nearly wild-type levels of Cl− secretion; overexpressing CFTR offered no advantage compared with endogenous expression levels. However, recent discoveries focused attention on CFTR-mediated HCO3− secretion and airway surface liquid (ASL) pH as critical for host defense and CF pathogenesis. Therefore, we generated porcine airway epithelia with varying ratios of CF and wild-type cells. Epithelia with a 50:50 mix secreted HCO3− at half the rate of wild-type epithelia. Likewise, heterozygous epithelia (CFTR+/− or CFTR+/∆F508) expressed CFTR and secreted HCO3− at ∼50% of wild-type values. ASL pH, antimicrobial activity, and viscosity showed similar relationships to the amount of CFTR. Overexpressing CFTR increased HCO3− secretion to rates greater than wild type, but ASL pH did not exceed wild-type values. Thus, in contrast to Cl− secretion, the amount of CFTR is rate-limiting for HCO3− secretion and for correcting host defense abnormalities. In addition, overexpressing CFTR might produce a greater benefit than expressing CFTR at wild-type levels when targeting small fractions of cells. These findings may also explain the risk of airway disease in CF carriers.

CORK Study in Cystic Fibrosis

“…That said a significant 76% reduction in intravenous antibiotic requirements was observed in the year after ivacaftor which also would contributed to this finding. Changes within the lung environment may alter the lung environment sufficiently to driving such change (24).…”

Paranasal sinus size is decreased in CFTR heterozygotes with chronic rhinosinusitis

“…suggest that alterations in bicarbonate transport and reduced ASL pH may play a role in defective innate immunity. In addition, mixing studies of null and wild‐type porcine airway epithelia suggest that 50% of CFTR function leads to an intermediate pH and bacteria‐killing phenotype …”

Highly Efficient Repair of the ΔF508 Mutation in Airway Stem Cells of Cystic Fibrosis Patients with Functional Rescue of the Differentiated Epithelia

“…Previous studies have attempted to ascertain the minimal level of correction necessary to restore normal Cltransport by co-culturing non-CF or corrected CF cells and CF cells in ALI. These reports indicate that 10-50% normal cells are sufficient to restore non-CF level Cltransport in F508 homozygotes (Johnson et al, 1992;Shah et al, 2016). Another study used a lentivirus based strategy to overexpress CFTR under the control of an RSV promoter in pig epithelia (Cooney et al, 2016).…”