Challenges Facing Airway Epithelial Cell-Based Therapy for Cystic Fibrosis

Abstract

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause the life-limiting hereditary disease, cystic fibrosis (CF). Decreased or absent functional CFTR protein in airway epithelial cells leads to abnormally viscous mucus and impaired mucociliary transport, resulting in bacterial infections and inflammation causing progressive lung damage. There are more than 2000 known variants in the CFTR gene. A subset of CF individuals with specific CFTR mutations qualify for pharmacotherapies of variable efficacy. These drugs, termed CFTR modulators, address key defects in protein folding, trafficking, abundance, and function at the apical cell membrane resulting from specific CFTR mutations. However, some CFTR mutations result in little or no CFTR mRNA or protein expression for which a pharmaceutical strategy is more challenging and remote. One approach to rescue CFTR function in the airway epithelium is to replace cells that carry a mutant CFTR sequence with cells that express a normal copy of the gene. Cell-based therapy theoretically has the potential to serve as a one-time cure for CF lung disease regardless of the causative CFTR mutation. In this review, we explore major challenges and recent progress toward this ambitious goal. The ideal therapeutic cell would: (1) be autologous to avoid the complications of rejection and immune-suppression; (2) be safely modified to express functional CFTR; (3) be expandable ex vivo to generate sufficient cell quantities to restore CFTR function; and (4) have the capacity to engraft, proliferate and persist long-term in recipient airways without complications. Herein, we explore human bronchial epithelial cells (HBECs) and induced pluripotent stem cells (iPSCs) as candidate cell therapies for CF and explore the challenges facing their delivery to the human airway.

Keywords: cell-based therapy; cystic fibrosis; engraftment; human bronchial epithelial cells; induced pluripotent stem cells.

FIGURE 1

Normal vs. CF Airway. Schematic drawings of normal and CF airways, illustrating selected components of epithelial apical ion transport. (A) Secreted mucins are produced by submucosal glands in the large airways and surface goblet cells, forming the overlying, mucin-rich component of the airway surface layer (ASL). This layer is normally transported by effective beating of cilia in the periciliary layer (PCL). ASL hydration is largely controlled by the balance of chloride secretion through CFTR and sodium reabsorption by the epithelial sodium channel (ENaC). (B) Mucin overproduction from hypertrophic submucosal glands and hyperplastic surface goblet cells characterize the CF large airways. Non-functional or absent CFTR leads to diminished chloride transport and increased sodium transport through ENaC. Airway dehydration reduces the PCL, impairing mucociliary transport. Accumulation of thick, sticky mucus increases overall ASL thickness and promotes a low pH environment favoring chronic bacterial infection and inflammation. The CF ASL environment is a likely barrier for cell therapy.

FIGURE 2

A Cell-Based Therapy Model for CF. (A) The pre-treatment CF large airway. (B–D) A depiction of cell-based therapy in three steps: (B) Mucus removal and transient preconditioning of the airway epithelium, potentially requiring luminal epithelial surface cell loss and partial epithelial denudation; (C) Delivery of CFTR-corrected stem and progenitor cells to the surface epithelium and possibly the submucosal glands; (D) Differentiation of delivered CFTR-corrected cells into all relevant cell types, restoration of functional CFTR ion-transport and a return to normal airway physiology.