Surface expression of CD63 and HLA-DR in circulating eosinophils correlates with improved clinical control after treatment optimization in asthma

Eosinophils are key effectors in asthma, especially within the T2-high phenotype. Conventional biomarkers such as blood eosinophils, FeNO, and IgE incompletely reflect disease activity. Beyond eosinophil enumeration, qualitative assessment of eosinophil activation and immune engagement may provide complementary information. Surface markers like CD63 and HLA-DR might provide additional insight into eosinophil activation and treatment response. To evaluate whether CD63 and HLA-DR expression on circulating eosinophils correlates with clinical improvement after therapy optimization in severe asthma. In this pre-randomization, single-center analysis of 105 adults enrolled in an anti-IL5 trial (NCT05001529), patients underwent a 3-month run-in with optimized therapy per GINA guidelines. Clinical data, lung function, FeNO, IgE, and eosinophil counts were collected. Flow cytometry assessed CD63 and HLA-DR expression. Associations with clinical improvement, defined by ACT score and exacerbation frequency, were analysed. ACT scores improved from 21 to 24 and uncontrolled asthma prevalence dropped from 50% to 16%. HLA-DR expression declined significantly and correlated with improved asthma control (β = 0.03, p = 0.05). CD63 expression did not change overall but remained elevated in patients with persistent symptoms. Neither marker correlated with eosinophil count, FeNO, or IgE. HLA-DR and CD63 reflect functional eosinophil states that are modulated during standard therapy optimization. Eosinophil immunophenotyping may complement traditional biomarkers as an associative, non-predictive tool to support personalized asthma management.