The relationship of eosinophil granule proteins to ions in the sputum of patients with cystic fibrosis

Background Increased sputum levels of eosinophil granule proteins have been reported despite normal eosinophil numbers in peripheral blood and in the lung in cystic fibrosis (CF). Mechanisms of eosinophil priming and activation are still unclear in CF. Objective In the present study we investigated whether ion concentrations in the sputa of CF patients are related to eosinophil activity. Methods We assessed concentrations of eosinophil cationic protein (ECP), eosinophil protein X (EPX), major basic protein (MBP) and ions (Na⁺, Cl^-, Ca²⁺Mg²⁺ in sputum samples of 29 children with CF as well as in 10 controls with bronchial asthma. Results Patients with CF demonstrated significantly higher levels of ECP, Na⁺, Cl^- and Ca²⁺ levels than asthmatics (P < 0.04, P < 0.0001, P < 0.0001, P < 0.02). No differences were seen between concentrations of EPX and Mg²⁺ in the two groups. In CF, eosinophil granule proteins correlated significantly with Ca²⁺ and Mg⁺ concentrations (ECP, P < 0.0001, r = 0.65, P < 0.0001, r = 0.66; MBP, P < 0.03, r = 0.41, P < 0.03, r = 0.42), furthermore inversely with Cl^- concentrations (ECP, P < 0.0003, r = - 0.63; EPX, P < 0.02, r = - 0.45; MBP, P < 0.03, r = - 0.41) but not with Na⁺ levels. ECP, Na⁺ and Cl^- were also correlated with lung function variables (FVC, P < 0.04, r = - 0.38, P < 0.02, r = 0.44, P < 0.03, r = 0.41; FEV1, P < 0.007, r = - 0.49, P < 0.006, r = 0.5, P < 0.008, r = 0.48; MEF50, P < 0.003, r = - 0.54, NS, P < 0.03, r = 0.42; MEF25, P < 0.039, r = - 0.4, P < 0.005, r = 0.51, P < 0.05, r = 0.37). Conclusion Our results demonstrated a significant relationship of eosinophil degranulation and ions in CF, indicating that ion composition in CF sputa may be at least partly be responsible for high levels of eosinophil products despite low eosinophil numbers.