Background: Severe eosinophilic asthma (SEA) is a T2^high-endotype with recurrent exacerbations and poor disease control. Mepolizumab (anti-IL5 mAb) decreases the number of peripheral blood eosinophils, but the effect on their function is poorly understood. Our aim was to identify changes in the phenotype or activation of eosinophils in response to mepolizumab in patients with SEA.

Methods: Blood samples were collected from healthy controls (HC; n=10) and SEA patients (n=12) before (T0) and after mepolizumab treatment (T4, 16, 32 weeks). Expression of proteins related to eosinophil activation (CD11b, CD44, CD48), immunomodulatory function (galectins 1/10), and eosinophil subsets, including resident (rEOS; CCR3⁺CD62L^hiIL-3R?^lo) and inflammatory (iEOS; CCR3⁺CD62L^loIL-3R^hi) eosinophils, were analyzed by flow cytometry.

Results: Eosinophils from SEA display higher levels of Siglec-8, IL-5R?, CD11b and CD44 (p<0.05) compared to HC, but no changes in regulatory proteins. Treatment with mepolizumab led to a decrease in eosinophil number (from 545.0 cells/?L at T0 to 133.1 cells/?L at T32; p<0.001) and a clear reduction of CD44 (p<0.01), together with a transition from rEOS to iEOS. All these changes were observed at early time point (T4) and maintained at T32.

Conclusions: The number and activation status of eosinophils in peripheral blood is decreased by mepolizumab treatment and restored to HC levels. This treatment also induces a transition of iEOS to rEOS. Altogether, our results demonstrate a significant effect of mepolizumab on the function of eosinophils, which can be used as an indicator of response to this treatment. GSK provided funding for this study.