Anti-Inflammatory Influences of Cystic Fibrosis Transmembrane Conductance Regulator Drugs on Lung Inflammation in Cystic Fibrosis

Abstract

Cystic fibrosis (CF) is caused by a defect in the cystic fibrosis transmembrane conductance regulator protein (CFTR) which instigates a myriad of respiratory complications including increased vulnerability to lung infections and lung inflammation. The extensive influx of pro-inflammatory cells and production of mediators into the CF lung leading to lung tissue damage and increased susceptibility to microbial infections, creates a highly inflammatory environment. The CF inflammation is particularly driven by neutrophil infiltration, through the IL-23/17 pathway, and function, through NE, NETosis, and NLRP3-inflammasome formation. Better understanding of these pathways may uncover untapped therapeutic targets, potentially reducing disease burden experienced by CF patients. This review outlines the dysregulated lung inflammatory response in CF, explores the current understanding of CFTR modulators on lung inflammation, and provides context for their potential use as therapeutics for CF. Finally, we discuss the determinants that need to be taken into consideration to understand the exaggerated inflammatory response in the CF lung.

Keywords: CFTR modulator; anti-inflammatory treatment; cystic fibrosis; inflammation; ivacaftor; lumacaftor; lung inflammation.

Figure 1

Cystic fibrosis transmembrane conductance regulator (CFTR) protein structure. Protein consists of two membrane-spanning domains (MSD), typically comprising of six transmembrane regions each which form the channel pore allowing chloride and bicarbonate transport; two nucleotide binding domains (NBD) which bind and hydrolyse ATP allowing for the channel to open; a single regulatory (R) domain, containing numerous charged amino acid and phosphorylation sites (star representing a phosphorylation site). The channel opens when R domain is phosphorylated, and NBDs are ATP-bound (adapted from Sheppard et al Physiol Rev. 1999 January 79 (1 Suppl): S23–S45).

Figure 2

Simplified schematic of a healthy and CF airway. (A) Normal functioning CFTR channel in healthy airway enables efficient Chloride (Cl⁻) and bicarbonate (HCO₃⁻) transport, as well as regulated sodium (Na⁺) reabsorption through epithelial sodium channel (ENaC), resulting in homeostatic airway microenvironment, with appropriate pH, airway surface liquid (ASL) hydration and mucus viscosity, promoting efficient mucociliary transport. (B) Dysfunctional CFTR protein in cystic fibrosis airway results in inhibited chloride and bicarbonate ion transport and increased sodium reabsorption which leads to reduced volume of periciliary layer, increased viscosity of mucus layer and acidic airway microenvironment contributing to mucostasis and inefficient mucociliary transport. Increased mucus viscosity promotes mucus plugging in submucosal gland, further preventing mucus clearance (not shown). Reduced mucus clearance and acidic airway pH contribute to a pro-inflammatory airway microenvironment consisting of increased influx of inflammatory cells and accumulation of bacteria within mucus, accelerating pathogenesis.

Figure 3

Simplified schematic outlining the complex immune cell interaction involved in producing the hyperinflammatory response and chronic infection in CF. In the CF airway, airway epithelial cells (AECs) secrete increased levels of IL-8, resulting in enhanced neutrophil migration. Perturbed secretion of prostaglandins and glutathione from AECs promotes inflammation. Neutrophils are the predominant driver of airway inflammation through multiple mechanisms including increased inflammasome activation, increased secretion of neutrophil elastase (NE), and a bias towards neutrophil extracellular trap (NET)-mediated cell death (NETosis). Neutrophils also possess impaired bacterial phagocytosis and killing leading to insufficient bacterial clearance and chronic infection. CF-macrophages also secrete increased levels of IL-8 and other proinflammatory mediators (not shown), promoting neutrophil infiltration and further inflammation directly. CF-macrophages also undergo impaired bacterial killing encouraging persistent infections. T helper cells in CF are skewed towards Th2 and Th17 differentiation rather than a Th1 phenotype. Th17 cells secrete IL-17, promoting neutrophil infiltration, as well as other proinflammatory cytokines which contribute to the hyperinflammatory response (not shown). Th2 cells stimulate a pro-allergic response involving increased secretion of IL-4 and IL-13 leading to IgE production. Th2 cells also secrete IL-10, dampening the expression of co-stimulatory molecules on macrophages causing decreased antigen presentation and impaired bacterial clearance.

Figure 4

Simplified schematic outlining the ability of CFTR to negatively regulate the pro-inflammatory response of airway epithelial cells. CFTR prevents the overproduction of hypophosphorylated IκΒ-β, preventing the overactivation of the NFκΒ pathway. CFTR also negatively regulates the PGE₂-mediated cyclooxygenase-2 (COX-2) positive feedback loop, preventing the excessive production of prostaglandin (PG) E2.

Figure 5

Simplified apoptotic and NETosis pathways in neutrophils. Normal apoptotic pathway involves neutrophil degranulation and phagocytosis of pathogens, initiating neutrophil apoptosis. The apoptotic neutrophil is then phagocytosed by macrophages promoting inflammatory resolution. CF neutrophils are biased towards undergoing NETosis in response to bacteria and other inflammatory stimuli, rather than the apoptotic pathway. In response to infectious stimuli, CF neutrophils expel NETs containing DNA, histone fragments and various pro-inflammatory neutrophil contents, such as neutrophil elastases which can directly or indirectly, through cell interactions, induce inflammation. Macrophages interact with the NETs, resulting in stimulation of Th17 cells, leading to neutrophil influx and further inflammation. Progressive inflammation triggered by sustained NET formation and release of toxic contents can damage the lung architecture.

Figure 6

Simplified schematic of LPS-induced inflammatory pathway. Toll-like receptor 4 (TLR4) recognises and binds lipopolysaccharide (LPS). Binding activates two signal transduction pathways through the stimulation of myeloid differentiation primary response gene 88 (MyD88) and TIR domain-containing adaptor inducing IFN-β (TRIF) adaptor molecules. MyD88 and TRIF activates two transcription factors, nuclear factor-κB (NFκB) and interferon response factors (IRFs). IRFs enhance the expression of Type I interferon genes, resulting in the secretion of type I interferons. NFκB promotes transcription of pro-inflammatory genes, such as cytokines and chemokines, resulting in acute inflammation and initiation of the adaptive immunity. NFκB signalling also induces NOD-, LRR-, and pyrin domain–containing protein 3 (NLRP3) gene expression, essential for inflammasome formation. The NLRP3-inflammasome cleaves and activates caspase-1, which cleaves the inactive precursor pro-IL-1β into its biologically active and secreted form, IL-1β. IL-1β secretion results in enhanced inflammation and immune cell infiltration. Caspase-1 also cleaves and activates Gasdermin D (GSDMD), leading to pore formation in the plasma membrane, triggering pyroptosis and further inflammation. This inflammatory pathway is a key regulator of the inflammation associated with CF.

Figure 7

Effect of cystic fibrosis transmembrane conductance regulator (CFTR) potentiator ivacaftor on dysfunctional CFTR. Ivacaftor binds directly to CFTR protein, increasing the time which activated CFTR channel remains open at the apical membranes. Results in increased chloride and bicarbonate ion transport at these sites, possibly improving the clinical outcome in CF patients.