OBJECTIVE: To investigate the characteristics of airway inflammation in severe asthma and the association with interleukin-17 (IL-17). METHODS: Sixteen patients with mild persistent asthma, 14 patients with moderate persistent asthma and 18 patients with severe persistent asthma, as well as 15 normal control subjects, were included in this study. At baseline, asthma symptom score was recorded, and lung function was measured. Induced sputum was obtained and processed for cell differential and the supernatant was assayed for the concentrations of IL-17 and IL-8 by sandwich enzyme-linked immunosorbent assay (ELISA), eosinophil cationic protein (ECP) by the immuno-CAP system, and neutrophil myeloperoxidase (MPO) by a colorimetric method, and the values were normalized to the protein content. Then 15 mild to moderate and 15 severe asthmatic patients received inhaled corticosteroid (Pulmicort, Astra-Zeneca) therapy for 4 weeks, and the above measurements were repeated. RESULTS: The percentage of sputum eosinophils and the level of ECP were significantly increased in mild [0.0670 +/- 0.0740, (155 +/- 91) x 10(-6) g/g protein], moderate [0.0830 +/- 0.0440, (180 +/- 83) x 10(-6) g/g protein] and severe [0.1240 +/- 0.1430, (191 +/- 87) x 10(-6) g/g protein] asthmatic patients, as compared to the normal controls [0.0000 +/- 0.0010, (44 +/- 25) x 10(-6) g/g protein, P < 0.01)]; but there was no significant difference among mild, moderate and severe asthma (P > 0.05). On the contrary, the percentage of sputum neutrophils was significantly increased in patients with severe asthma (0.589 +/- 0.203) as compared to patients with mild (0.455 +/- 0.154) and moderate (0.449 +/- 0.194) asthma and normal controls (0.313 +/- 0.134, P < 0.01). The level of MPO was also increased in severe asthma [(31 +/- 10 ) U/g protein] as compared to mild [(12 +/- 4) U/g protein] and moderate [(22 +/- 7) U/g protein] asthma and normal controls [(10 +/- 4) U/g protein, P < 0.01]. The sputum level of IL-17 in severe, moderate asthma [(264 +/- 137) x 10(-9) g/g protein, (172 +/- 65) x 10(-9) g/g protein] was significantly higher than those in mild asthma [(126 +/- 52) x 10(-9) g/g protein] and normal controls [(56 +/- 20) x 10(-9) g/g protein, P < 0.01]. As compared to the normal controls [(83 +/- 36) x 10(-9) g/g protein], the level of IL-8 in severe asthma [(531 +/- 321) x 10(-9) g/g protein] was significantly increased (P < 0.01), and those in mild [(410 +/- 181) x 10(-9) g/g protein] and moderate [(438 +/- 148) x 10(-9) g/g protein] asthma were also increased, although the difference was not statistically significant (P > 0.05). The level of IL-17 was correlated positively with IL-8, neutrophils and MPO (r = 0.525, 0.349, 0.602, all P < 0.01). After steroid therapy, the percentage of eosinophils, the levels of ECP, MPO, IL-17 and IL-8 decreased significantly in all patients combined, but the percentage of neutrophils remained unchanged (P > 0.05) and still significantly higher in severe asthmatic patients (0.642 +/- 0.157) as compared to mild and moderate asthmatic patients (0.394 +/- 0.147, P < 0.01). CONCLUSIONS: Airway neutrophilia is a feature of severe asthma. The IL-17/IL-8 pathway may be involved in the initial neutrophil influx into the airways.