The impact of mepolizumab on the sputum level of eosinophil-derived protein in three cases of severe asthma

Dear Editor, Mepolizumab, an interleukin 5 (IL-5) antagonist monoclonal antibody, has been shown to decrease asthma exacerbations and blood eosinophilia in patients with severe eosinophilic asthma. Mepolizumab consistently reduces blood eosinophilia, but its effect on the reduction of sputum eosinophils is controversial. The reduction of sputum eosinophils may be related to the dose of mepolizumab, however, at the licensed dose, eosinophil concentration may often remain unchanged. In this report, we monitored three patients who had initiated mepolizumab therapy for the treatment of severe asthma. These patients showed clinical improvement in symptoms and an obvious reduction in exacerbations, despite the insignificant reduction of sputum eosinophils. Analysis of sputum from these cases revealed that the levels of eosinophilic cationic protein (ECP) and Charcot-Leyden crystal (CLC) protein in sputum supernates were clearly decreased. Informed consent was obtained for all the patients involved in this study.