Mepolizumab does not alter blood basophil count in severe asthma

Introduction: Mepolizumab (anti-human IL5) reduces blood eosinophils, however, it is less well known that blood basophils, also IL-5 receptor positive, are also decreased in routine clinical tests. Aims and Objectives: Our objective was to determine whether mepolizumab treatment reduces blood basophil levels in severe asthma as an additional potential efficacy mechanism. Methods: Asthma patients receiving mepolizumab in a single-centre, difficult asthma clinic were recruited for blood collection prior to and ~16 weeks after treatment (n=24). Samples from healthy subjects, collected at parallel time-points but without an intervention, acted as controls. The concentration of basophils, eosinophils (positive control) and other Type-2 lymphocytes were measured (cells/100μl blood) by flow cytometry (Attune NxT) and compared to routine clinical measurements using an Advia 2120. Results: Following 16-weeks of treatment, eosinophils were reduced [Δ -6,442±6,852, p Conclusion: These data suggest that the basophil concentration reported clinically should be interpreted with caution and that basophil numbers are unlikely to be affected in response to mepolizumab in severe asthma.