Sialic acid-binding immunoglobulin-like lectin (Siglec) 8 in patients with eosinophilic disorders: Receptor expression and targeting using chimeric antibodies

Background: 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. Objective: 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. Methods: 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 IgG(4) (chimeric 2E2 IgG(4)) and afucosylated IgG(1) (chimeric 2E2 IgG(1) [c2E2 IgG(1)]) anti-Siglec-8 antibodies was evaluated in vitro by using flow cytometry and in vivo in humanized mice. Results: 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 IgG(1) and chimeric 2E2 IgG(4) 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 IgG(1). Finally, treatment of humanized mice with anti-Siglec antibody led to robust depletion of IL-5-induced eosinophilia in vivo. Conclusions: 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.