Ultrastructural characterization of cystic fibrosis sputum using atomic force and scanning electron microscopy

Cystic fibrosis (CF) lung disease is characterized by perpetuated neutrophilic inflammation with progressive tissue destruction. Neutrophils represent the major cellular fraction in CF airway fluids and are known to form neutrophil extracellular traps (NETs) upon stimulation. Large amounts of extracellular DNA-NETs are present in CF airway fluids. However, the structural contribution of NETs to the matrix composition of CF airway fluid remains poorly understood. We hypothesized that CF airway fluids consist of distinct DNA-NETs that are associated to subcellular structures.

We employed atomic force microcopy (AFM) and scanning electron microcopy to ultrastructurally characterize the nature of CF sputum and the role of NETs within the extracellular CF sputum matrix. These studies demonstrate that CF sputum is predominantly composed of a high-density meshwork of NETs and NETosis-derived material. Treatment of CF sputum with different DNases degraded CF NETs and efficiently liquefied the mucous-like structure of CF sputum. Quantitative analysis of AFM results showed the presence of three globular fractions within CF sputum and the larger two ones featured characteristics of neutrophil ectosomes.

These studies suggest that excessive NET formation represents the major factor underlying the gel-like structure of CF sputum and provide evidence that CF-NETs contain ectosome-like structures that could represent targets for future therapeutic approaches.