Blood leukocyte characterisation in Eosinophilic Granulomatosis with Polyangiitis (EGPA)

<bold>Background:</bold> Immunological mechanisms are poorly understood in EGPA, a rare disease whereby Severe Eosinophilic Asthmatics (SEA) develop blood & tissue eosinophilia, with associated comorbidities. Our objective was to measure blood leukocyte counts and activity from EGPA patients, including type 2-polarised Innate Lymphoid cells (ILC2s) & Type 2-T helper (Th) /cytotoxic (Tc) cell subsets, using chemokine receptor expression. <bold>Methods:</bold> Cytometry was performed on whole blood from stable EGPA (n=9), SEA (n=8) & healthy volunteers (n=10) to measure leukocyte populations, reported as /100µL blood, median [IQR]. EGPA/SEA patients were on OCS only, or with anti-IL5/IL5R. From PBMCs, we measured ILC subset counts (ILC1/ILC2/ILC3; n=5/group), reported /2x10⁶ PBMCs, and maturity status using CD45RO⁺/CD45RA⁺, expressed as % of each ILC subset, mean [SD]. <bold>Results:</bold> Type 2-polarised lymphocyte (Th/Tc2, CD161⁺ Th/Tc2) counts were similar between EGPA and SEA. Eosinophils, basophils, neutrophils, B cells, Th/c 1, 17 and 22 cell counts were also similar between EGPA and SEA. ILC2 counts were lower in EGPA (13.8[8.8-39.4]) & SEA (16.9[10.6-34.4]) compared to health (45[21.3-120.9]) in whole blood (p=0.0792; p=0.1363 respectively). In PBMCs, a greater proportion of ILC2s expressed CD45RO in EGPA (45.8[16.9]) versus SEA (23.3[14.4]) and health (22.7[4.4]) (p=0.0307; p=0.0259, respectively). <bold>Discussion:</bold> T2-polarised lymphocytes circulate at similar levels between health, SEA and EGPA. ILC2s in EGPA are decreased in blood, but show evidence of increased maturation compared to SEA, suggesting disease specific differences that require confirmation at the functional level.