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Atopic dermatitis (AD) as well as other type 2 immune response (T2) diseases are often linked to elevated eosinophil (Eos) levels in the blood. Although the role of Eos in AD pathophysiology is suspected, it remains unclear. The development of new treatments targeting the T2 response, particularly cytokines involved in Eos activation and chemotaxis, makes it necessary to identify potential Eos profiles in AD that may respond to these treatments. A prospective study was conducted comparing blood Eos phenotypes in moderate-to-severe AD patients (n=19) without recent systemic treatment to healthy individuals (HI, n=19). The primary outcome was the membranous phenotypic signature of Eos, assessed by flow cytometry. Most AD patients (84%) had early onset in childhood, a severe disease (mean SCORAD of 57.5), and elevated blood Eos counts (310 per µl in AD vs 120 in HI, p<0.0001). AD patients exhibited lower CRTH2 on Eos but higher levels of HLA-DR and Siglec-8 compared to HI. Other surface proteins showed no significant differences. Clustering analysis confirmed increased Siglec-8 in AD patients. Additionally, AD patients had higher serum levels of T2 immune response-markers as eotaxin-2, IL-5, IL-3, and TARC. Circulating Eos in AD patients show a distinct phenotypic profile, suggesting a role in AD pathophysiology and potential involvement in differential treatment responses., (© The Author(s) 2025. Published by Oxford University Press on behalf of Society for Leukocyte Biology. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

Background: Hypereosinophilic syndromes (HES) are a heterogenous group of eosinophilic disorders. To date, only retrospective studies of limited sample-size and/or follow-up duration are available., Methods: The COHESion study is a national prospective multicenter multidisciplinary cohort recruiting both adults or children with the spectrum of eosinophilic disorders (including reactive HE/HES [HE/HES-R], idiopathic HES [HES-I], lymphocytic HES [HES-L], neoplastic HE/HES [HE/HES-N], HE of unknown significance [HE-US], as well as IgG4-related disease [IgG4RD] or ANCA-negative eosinophilic granulomatosis with polyangiitis [EGPA] overlaps). Patients are followed-up yearly. All data about final diagnosis, organ involvement assessments, and outcome profiles in HES-I were captured and analyzed centrally by HES expert centers., Results: From May 2019 to November 2023, 779 patients were included. For this preliminary analysis, 550 cases were available for centralized review (mean ± SD age: 56 ± 18 years, 42% of female patients). The final diagnoses were HES-I (47%), HE/HES-R (16%), HE-US (15%), HE/HES-N (7%), HE/HES-L (6%), IgG4RD (2%), and ANCA-negative EGPA (7%). In the 258 HES-I patients, outcome profiles were classified as follows: 16.3% had a "single-flare" without further relapse, 28.3% had a "relapsing-remitting" disease when there was at least a 6-month period free of symptoms between two flares, 46.1% had a "persistent disease" requiring continuous treatment to avoid relapses (9.3% remained unclassified because of insufficient follow-up)., Conclusions: The COHESion cohort is the first nationwide prospective multicenter study collecting data on the full spectrum of HE/HES disorders. This preliminary analysis confirms that idiopathic HES patients have various outcome profiles, suggesting different underlying pathophysiological mechanisms and the need of patient-specific management., Trial Registration: ClinicalTrials.gov identifier: NCT04018118., (© 2025 European Academy of Allergy and Clinical Immunology and John Wiley & Sons Ltd.)