Project summary description In cystic fibrosis (CF), lung damage is responsible for the majority of disease morbidity and mortality. While CF lungs host polymicrobial infections, lung disease has only been linked to a few bacterial pathogens including Staphylococcus aureus. S. aureus is the most prevalent infectious agent in the respiratory tract of CF patients. Neutrophil granulocytes represent the most powerful component of the healthy immune system to kill S. aureus. In CF, though, neutrophils fail to eliminate this bacterium despite their robust recruitment to the airways. It remains largely unknown why neutrophils in CF cannot eliminate S. aureus. Our published results show that the CF airway environment reduces the ability of human neutrophils to kill CF isolates of S. aureus. In contrast, the CF sputum does not affect S. aureus phagocytosis by neutrophils, extrusion of neutrophil extracellular traps or neutrophil viability. Our additional data demonstrate that CF blood neutrophils are not impaired in their abilities to kill S. aureus in vitro but the CF sputum also reduces their bactericidal activities against this bacterium. Our long-term goal is to identify why CF patients are unable to clear S. aureus from their airways. Our goal in this proposal is to determine which components of the CF sputum are responsible for its inhibitory action on neutrophils’ S. aureus killing. The rationale of the proposed work is that a deeper understanding of S. aureus pathogenesis in CF will aid to develop new, anti-staphylococcal intervention strategies for CF in the near future. Our central hypothesis is that a concerted action of IgG immune complexes and exosomes present in the CF sputum mediate the inhibitory action of CF sputum on S. aureus killing by neutrophils. Our proposed work will be expected to determine the exact component(s) of the CF sputum that compromise(s) the ability of neutrophils to kill S. aureus. This knowledge is essential for our better understanding of airway inflammation, host-pathogen interactions and S. aureus pathogenesis in CF. In addition, this exploratory work will open future studies focusing on the exact mechanism of how the CF sputum components identified here impair neutrophil functions and on exploring new, neutrophil-targeting therapies for CF.