Glycan Array Screening Reveals a Candidate Ligand for SigIec-8.

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[Fucα13]GlcNAc, also referred to in the literature as 6'-sulfosLex. Notably, neither unsulfated sLex (NeuAcα23Galβ1-4[Fucα1-3]GlcNAc) nor an isomer with the sulfate on the 6-position of the GlcNAc residue (6-sulfosLex, 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 sLex and 6-sulfo-sLex failed to support detectable Siglec-8 binding, 6'-sulfosLex supported significant binding with a Kd 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-sLex of < 5 pmol/spot. In contrast, sLex and 6-sulfo-sLex did not support any Siglec-8 binding at the highest concentration tested (300 pmol/spot). We conclude that Siglec-8 binds preferentially to the sLex structure bearing an additional sulfate ester on the galactose 6-hydroxyl. [ABSTRACT FROM AUTHOR]