Sialic acid binding immunoglobulin-like lectin 8 (Siglec-8), which exists in 2 isoforms including one possessing cytoplasmic tyrosine motifs, is expressed only on human eosinophils, basophils, and mast cells. Until now, its function was unknown. Here we define a novel function of Siglec-8 on eosinophils. Siglec-8 cross-linking with antibodies rapidly generated caspase-3–like activity and reduced eosinophil viability through induction of apoptosis. The pancaspase inhibitor benzyloxycarbonyl (Cbz)–Val-Ala-Asp-(Ome)-fluoromethyl ketone (zVAD-FMK) completely blocked this response, implicating caspases in Siglec-8 cross-linking–induced apoptosis. Eosinophil survival-promoting cytokines such as interleukin 5 (IL-5) and granulocyte-macrophage colony-stimulating factor (GM-CSF) failed to block apoptosis and instead enhanced the sensitivity of eosinophils to undergo apoptosis in response to Siglec-8 antibody. Siglec-8 activation may provide a useful therapeutic approach to reduce numbers of eosinophils (and perhaps basophils and mast cells) in disease states where these cells are important.

The human eosinophil has long been thought to favorably influence innate mucosal immunity but at times has also been incriminated in disease pathophysiology. Research into eosinophil biology has uncovered a number of interesting contributions by eosinophils to health and disease. However, it appears that not all eosinophils from all species are created equal. It remains unclear, for example, exactly how having eosinophils benefits the human host when helminth infections in the developed world have become scarce. This review focuses on our current state of knowledge as it relates to human eosinophils. When information is lacking, we discuss lessons learned from mouse studies that may or may not directly apply to human biology and disease. It is an exciting time to be an “eosinophilosopher” because the use of biologic agents that selectively target eosinophils provides an unprecedented opportunity to define the contribution of this cell to eosinophil-associated human diseases.

Sialic acid-binding immunoglobulin-like lectin 8 (Siglec-8) is selectively expressed on human eosinophils, basophils, and mast cells, where it regulates their function and survival. Previous studies demonstrated sialic acid-dependent binding of Siglec-8 but failed to reveal significant substructure specificity or high affinity of that binding. To test a broader range of potential ligands, a Siglec-8-Ig chimeric protein was tested for binding to 172 different glycan structures immobilized as biotinylated glycosides on a 384-well streptavidin-coated plate. Of these, ∼40 structures were sialylated. Among these, avid binding was detected to a single defined glycan, NeuAcα2–3(6-O-sulfo)Galβ1–4[Fucα1–3]GlcNAc, also referred to in the literature as 6′-sulfo-sLe^x. Notably, neither unsulfated sLe^x (NeuAcα2–3Galβ1–4[Fucα1–3]GlcNAc) nor an isomer with the sulfate on the 6-position of the GlcNAc residue (6-sulfo-sLe^x, NeuAcα2–3Galβ1–4[Fucα1–3](6-O-sulfo)GlcNAc) supported detectable binding. Subsequent secondary screening was performed using surface plasmon resonance. Biotin glycosides immobilized on streptavidin biosensor chips were exposed to Siglec-8-Ig in solution. Whereas surfaces derivatized with sLe^x and 6-sulfo-sLe^x failed to support detectable Siglec-8 binding, 6′-sulfo-sLe^x supported significant binding with a K_d of 2.3 μm.In a separate test of binding specificity, aminopropyl glycosides were covalently immobilized at different concentrations on activated (N-hydroxysuccinimidyl) glass surfaces (Schott-Nexterion Slide H). Subsequent exposure to Siglec-8-Ig precomplexed with fluorescein isothiocyanate anti-human Fc resulted in fluorescent signals at immobilized concentrations of 6′-sulfo-sLe^x of <5 pmol/spot. In contrast, sLe^x and 6-sulfo-sLe^x did not support any Siglec-8 binding at the highest concentration tested (300 pmol/spot). We conclude that Siglec-8 binds preferentially to the sLe^x structure bearing an additional sulfate ester on the galactose 6-hydroxyl.

Eosinophilia and eosinophil activation are recurrent features in various reactive states and certain hematologic malignancies. In patients with hypereosinophilia (HE), HE‐induced organ damage is often encountered and may lead to the diagnosis of a hypereosinophilic syndrome (HES). A number of known mechanisms and etiologies contribute to the development of HE and HES. Based on these etiologies and the origin of eosinophils, HE and HES are divided into primary forms where eosinophils are clonal cells, reactive forms where an underlying reactive or neoplastic condition is detected and eosinophils are considered to be “non‐clonal” cells, and idiopathic HE and HES in which neither a clonal nor a reactive underlying pathology is detected. Since 2012, this classification and the related criteria have been widely accepted and regarded as standard. However, during the past few years, new developments in the field and an increasing number of markers and targets have created a need to update these criteria and the classification of HE and HES. To address this challenge, a Working Conference on eosinophil disorders was organized in 2021. In this conference, a panel of experts representing the relevant fields, including allergy, dermatology, hematology, immunology, laboratory medicine, and pathology, met and discussed new markers and concepts as well as refinements in definitions, criteria and classifications of HE and HES. The outcomes of this conference are presented in this article and should assist in the diagnosis and management of patients with HE and HES in daily practice and in the preparation and conduct of clinical trials.

Park YM, et al. Allergy Asthma Immunol Res. 2010 Apr;2(2):87-101. https://doi.org/10.4168/aair.2010.2.2.87