Background Eosinophil peroxidase (EPO) is an eosinophilic basic protein, which leads to increased permeability and damage of bronchial epithelial cells in asthma.

Objective As little is known about its local expression and release in humans the intracellular expression in lung and peripheral eosinophils and the concentrations of EPO in bronchoalveolar lavage (BAL) fluid and serum was investigated in patients with asthma.

Methods Twelve mild atopic asthmatic and nine control subjects underwent segmental sham and allergen challenge. EPO concentrations in BAL fluid and serum were determined by immunoassay and flow cytometry was used to determine the intracellular expression of EPO in BAL‐derived and peripheral eosinophils.

Results In asthmatic patients a large increase in BAL eosinophils – total cells: median 9.5 × 10⁶ (range: 0.5 to 455.0 × 10⁶); relative: 38% (1 to 91%) – was detectable 24 h following allergen challenge, but peripheral blood eosinophil counts did not change. Concentrations of EPO in BAL fluid increased from 1 µg/L (1.0 to 6.8 µg/L) to 42 µg/L (5.6 to 379.6 µg/L; P < 0.01) after allergen but not after saline challenge (1.5 µg/L; 1.0 to 21.9 µg/L), whereas in control subjects all measurements were below the detection limit. Serum concentrations of EPO increased slightly from 18.3 µg/L (3.0 to 56.8 µg/L) to 27 µg/L (3.8 to 133.9 µg/L; P < 0.05) 24 h after allergen challenge in asthmatic patients. Furthermore, the intracellular expression of EPO (measured as mean fluorescence intensity) was decreased in BAL eosinophils compared with blood eosinophils (mean fluorescence intensity 29 (7 to 71) vs. 48 (20 to 85); P < 0.01) after allergen challenge.

Conclusion The finding of increased EPO concentrations in the BAL fluid and decreased intracellular EPO expression in pulmonary eosinophils of asthmatic patients reflects the allergen‐triggered release of EPO into the bronchial space.