Progression of Cystic Fibrosis Lung Disease from Childhood to Adulthood: Neutrophils, Neutrophil Extracellular Trap (NET) Formation, and NET Degradation

Abstract

Genetic defects in cystic fibrosis (CF) transmembrane conductance regulator (CFTR) gene cause CF. Infants with CFTR mutations show a peribronchial neutrophil infiltration prior to the establishment of infection in their lung. The inflammatory response progressively increases in children that include both upper and lower airways. Infectious and inflammatory response leads to an increase in mucus viscosity and mucus plugging of small and medium-size bronchioles. Eventually, neutrophils chronically infiltrate the airways with biofilm or chronic bacterial infection. Perpetual infection and airway inflammation destroy the lungs, which leads to increased morbidity and eventual mortality in most of the patients with CF. Studies have now established that neutrophil cytotoxins, extracellular DNA, and neutrophil extracellular traps (NETs) are associated with increased mucus clogging and lung injury in CF. In addition to opportunistic pathogens, various aspects of the CF airway milieux (e.g., airway pH, salt concentration, and neutrophil phenotypes) influence the NETotic capacity of neutrophils. CF airway milieu may promote the survival of neutrophils and eventual pro-inflammatory aberrant NETosis, rather than the anti-inflammatory apoptotic death in these cells. Degrading NETs helps to manage CF airway disease; since DNAse treatment release cytotoxins from the NETs, further improvements are needed to degrade NETs with maximal positive effects. Neutrophil-T cell interactions may be important in regulating viral infection-mediated pulmonary exacerbations in patients with bacterial infections. Therefore, clarifying the role of neutrophils and NETs in CF lung disease and identifying therapies that preserve the positive effects of neutrophils, while reducing the detrimental effects of NETs and cytotoxic components, are essential in achieving innovative therapeutic advances.

Keywords: DNase; NETosis; cystic fibrosis; cystic fibrosis transmembrane conductance regulator; lung disease; neutrophil extracellular traps (NETs); neutrophils.

Figure 1

Airway alterations and neutrophil extracellular traps (NET) formation in Cystic Fibrosis airways. In fetal stages and early childhood, neutrophilic inflammation at the peribronchial regions is identifiable in patients with CF. In early childhood, CF airways have excess mucus and obstructive secretions, but not persistent bacterial infections. CF-associated airway abnormalities continue to increase throughout the childhood and include lower airway inflammation. During the disease progression, various microbial components and inflammatory cytokines and lipid mediators induce NETosis. Excess NETs and their cytotoxic components combined with thick and dry mucus exacerbate CF airway disease. Even with therapeutic interventions, the airway destruction/complications progress with age, resulting in irreversible lung damage, which often requires lung transplantation for survival. Minimizing excess NET formation, while preserving antimicrobial functions of neutrophil is a therapeutic option to minimize NET-mediated destruction of CF lungs.

Figure 2

Pseudomonas aeruginosa, induces NETosis in CF neutrophils. Neutrophil purified from patients with CF has typical polymorphonuclear morphology (blue stain). Upon incubation of these neutrophils with P. aeruginosa (e.g., multiplicity of infection of 20), CF neutrophils readily release NETs-DNA (blue stain) coated with granular proteins (e.g., myeloperoxidase; immunostained with green fluorescence antibodies).