Neutrophil cell death, activation and bacterial infection in cystic fibrosis

Background: Cystic fibrosis (CF) is characterised by chronic endobronchial bacterial infection and neutrophil mediated inflammation. Neutrophil apoptosis is essential for the resolution of inflammation. This study assessed the relationship between levels of neutrophil apoptosis and sputum microbiology in matched clinically stable patients with CF. Methods: Sputum was induced from 34 patients ( nine with no Gram negative infection, 10 colonised with Pseudomonas aeruginosa, 10 with Burkholderia cenocepacia, and five with other infections). Apoptotic neutrophils measured by flow cytometric Annexin V/propidium iodide staining and morphology were similar in all groups. Results: Patients infected with P aeruginosa or B cenocepacia had a significantly lower percentage of viable neutrophils in the sputum than those with no Gram negative infection (Kruskal-Wallis p = 0.01, median (interquartile range (IQR)) 14.2% (9.4 - 21.6), 15.8% (12.3 - 19.5), and 48.4% (23.0 - 66.4); p = 0.003 and p = 0.002, respectively). They also had significantly higher levels of secondary necrotic granulocytes in sputum than patients with no Gram negative infection (Kruskal-Wallis p< 0.0001, median (IQR) 55.5% (48.4 - 64.5), 50.4% (44.6 - 61.9), and 24.8% (14.4 - 30.5); p< 0.0001 and p< 0.0001, respectively). Neutrophils (x 10(6)/g sputum) in P aeruginosa infected patients (Kruskal-Wallis p = 0.05, median (IQR) 6.3 (3.5 - 12.7)) and B cenocepacia infected patients (5.7 (1.5 - 14.5)) were significantly higher than in the group with no Gram negative infection (0.5 ( 0.5 - 4.3), p = 0.03 and 0.04, respectively). Conclusion: These results suggest that cell death and clearance may be altered in patients with CF colonised with P aeruginosa and B cenocepacia compared with those with no Gram negative infection.