Effects of ion composition and tonicity on human neutrophil antibacterial activity

Infants with cystic fibrosis (CF) often are infected with Staphylococcus aureus (S. aur.), which is followed by colonization with Pseudomonas aeruginosa (P. aerug.). In spite of an excessive, neutrophil-dominated inflammatory response in the respiratory tract, patients with CF often succumb to pulmonary infections with P. aerug. Because peripheral blood neutrophils of these patients have normal functions, we examined whether hypothesized alterations of the airway surface liquids (ASL) in these patients significantly impair neutrophil bactericidal activity in the microenvironment of the CF lung. The ionic composition of CF ASL is still not entirely defined and has been speculated to be abnormally high or abnormally low in Na+ and Cl- concentrations; estimates of osmolarities have ranged from 200 (hypo-osmolar) to 285 (iso-osmolar) to > 300 meq/L (hyper-osmolar). Our data indicate that bacterial killing activity of human peripheral blood neutrophils against P. aerug. or S. aur. is not decreased in buffers in which NaCl was replaced with equimolar concentrations of choline Cl, KCI, or N-methyl-D-glucamine chloride to maintain isotonicity. Amiloride or benzamil, known modulators of Na+ transport in neutrophils, did not interfere with this neutrophil function. Deviations from isotonicity of +/- 50% also failed to diminish bactericidal activity of neutrophils significantly. In contrast, superoxide production and enzyme secretion in response to the chemotactic peptide N-formylmethionylleucylphenylalanine appeared to be sensitive to the ionic milieu of the assay buffers. Our results suggest that the postulated alterations in the ionic composition of ASL in CF lungs are insufficient to explain why neutrophils fail to clear infections with P. aerug. in these patients.