Background
The upper airways (UAW) are a niche and a reservoir of Pseudomonas aeruginosa strains that cause chronic infection of the lower airways (LAW) in cystic fibrosis (CF). Here, we assessed the role of anti‐P. aeruginosa immunoglobulin A (IgA) and IgG antibodies in upper and lower airway infections in cystic fibrosis patients.

Methods
Nasal lavage fluid and induced sputum samples of 40 CF patients were microbiologically cultured. We searched for correlations between anti‐P. aeruginosa IgA and IgG levels, measured by enzyme‐linked immunosorbent assay (optical density), and unspecific immune mediators in both specimens.

Results
Anti‐P. aeruginosa IgA (median optical density: 0.953 vs 0.298) and IgG (0.120 vs 0.059) were significantly higher in nasal lavage than in sputum, but not significantly different between patients with and without chronic P. aeruginosa infection in UAW. Matrix metallopeptidase‐9 (MMP‐9) in nasal lavage and neutrophil elastase (NE) in sputum were predictors of IgA in nasal lavage and IgA in sputum, respectively. IgA was a predictor of myeloperoxidase (MPO) in nasal lavage. Tissue inhibitor of metalloproteinases‐1 (TIMP‐1) was a predictor of IgG in sputum. IgG, TIMP‐1, and NE in sputum were predictors of IgG in nasal lavage.

Conclusion
The anti‐P. aeruginosa IgA response was more prominent in CF patients' UAW, indicating a lower degree of inflammatory responses. Proteases may play a role in the anti‐P. aeruginosa humoral response in the upper and LAW, and anti‐P. aeruginosa IgG may be involved in the crosstalk between upper and lower airways in cystic fibrosis patients.