Hypereosinophilic syndrome is a group of diseases defined by marked eosinophilia in blood or tissue and eosinophil-related clinical manifestations. Human eosinophils express interleukin-5 receptor alpha (IL5RA). Benralizumab (MEDI-563; Fasenra, MedImmune/AstraZeneca) is a humanized, afucosylated monoclonal antibody against IL5RA that targets IL5RA-bearing cells for enhanced antibody-dependent cellular cytotoxicity. Benralizumab was utilized in a randomized, double-blinded, placebo-controlled, phase 2 clinical trial. A series of three monthly subcutaneous injections of either benralizumab or placebo in 20 symptomatic patients who had PDGFRA-negative hypereosinophilic syndrome and an absolute eosinophil count of at least 1000 cells/mL3 was performed; all the patients were receiving stable therapy (drugs or dietary changes) for this disease. This regimen was followed by an open-label phase, during which the patient's background therapy could be tapered as tolerated, and an extension phase. The primary end point of the randomized phase was a reduction of at least 50% in the absolute eosinophil count at week 12. The exploratory end points included an assessment of clinical and laboratory predictors of response. During the randomized phase, the primary end point occurred in more patients in the benralizumab group than in the placebo group (9 of 10 patients 90% vs. 3 of 10 patients 30%, P=0.02). Bone marrow aspirates and biopsy samples were obtained at baseline and at week 12. The numbers of bone marrow eosinophils, eosinophil precursors, and blood and bone marrow basophils were significantly decreased at week 12 in all the patients in the benralizumab group. Interestingly, the number of mast cells and serum tryptase levels were unchanged. During the open-label phase, clinical and hematologic responses were observed in 17 of 19 patients (89%) and were sustained for 48 weeks in 14 of 19 patients (74%); in the latter group, in 9 of 14 patients (64%), background therapies could be tapered. Bone marrow and tissue eosinophilia were also suppressed in these patients. The most common drug-related adverse events, headache and an elevated lactate dehydrogenase level, occurred in 32% of the patients after the first dose of benralizumab and resolved within 48 hours in all patients. Other adverse events occurred with similar frequency in the two groups. Of the many potential predictors of response that were examined, only clinical disease subtype appeared to be associated with the initial response or relapse. In a separate study, we investigated Sialic acid-binding immunoglobulin-like lectin (Siglec) 8 expression in patients with eosinophilic disorders. Sialic acid-binding immunoglobulin-like lectin (Siglec) 8 is selectively expressed on eosinophils, mast cells, and basophils and, when engaged on eosinophils, can cause cell death. We sought to characterize surface and soluble Siglec-8 (sSiglec-8) levels in normal donors (NDs) and eosinophilic donors (EOs) and assess the efficacy of anti-Siglec-8 antibodies in inducing eosinophil cell death in vitro. Eosinophil expression of Siglec-8 was assessed by using flow cytometry and quantitative PCR. Serum sSiglec-8 levels were measured by means of ELISA. Induction of eosinophil death by IgG4 (chimeric 2E2 IgG4) and afucosylated IgG1 (chimeric 2E2 IgG1 c2E2 IgG1) anti-Siglec-8 antibodies was evaluated in vitro by using flow cytometry and in vivo in humanized mice. Siglec-8 was consistently expressed on eosinophils from NDs and EOs and did not correlate with absolute eosinophil count or disease activity. sSiglec-8 levels were measurable in sera from most donors unrelated to absolute eosinophil counts or Siglec-8 surface expression. c2E2 IgG1 and chimeric 2E2 IgG4 were equally effective at inducing cell death (Annexin-V positivity) of purified eosinophils from NDs and EOs after overnight IL-5 priming. In contrast, killing of purified eosinophils without IL-5 was only seen in EOs, and natural killer cell-mediated eosinophil killing was seen only with c2E2 IgG1. Finally, treatment of humanized mice with anti-Siglec antibody led to robust depletion of IL-5-induced eosinophilia in vivo. Siglec-8 is highly expressed on blood eosinophils from EOs and NDs and represents a potential therapeutic target for eosinophilic disorders. Enhanced killing of eosinophils in the presence of IL-5 might lead to increased efficacy in patients with IL-5-driven eosinophilia.
