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.