Probing the binding specificities of human Siglecs by cell-based glycan arrays

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Abstract
Siglecs are a family of sialic acid–binding receptors expressed by cells of the immune system and a few other cell types capable of modulating immune cell functions upon recognition of sialoglycan ligands. While human Siglecs primarily bind to sialic acid residues on diverse types of glycoproteins and glycolipids that constitute the sialome, their fine binding specificities for elaborated complex glycan structures and the contribution of the glycoconjugate and protein context for recognition of sialoglycans at the cell surface are not fully elucidated. Here, we generated a library of isogenic human HEK293 cells with combinatorial loss/gain of individual sialyltransferase genes and the introduction of sulfotransferases for display of the human sialome and to dissect Siglec interactions in the natural context of glycoconjugates at the cell surface. We found that Siglec-4/7/15 all have distinct binding preferences for sialylated GalNAc-type O-glycans but exhibit selectivity for patterns of O-glycans as presented on distinct protein sequences. We discovered that the sulfotransferase CHST1 drives sialoglycan binding of Siglec-3/8/7/15 and that sulfation can impact the preferences for binding to O-glycan patterns. In particular, the branched Neu5Acα2–3(6-O-sulfo)Galβ1–4GlcNAc (6′-Su-SLacNAc) epitope was discovered as the binding epitope for Siglec-3 (CD33) implicated in late-onset Alzheimer’s disease. The cell-based display of the human sialome provides a versatile discovery platform that enables dissection of the genetic and biosynthetic basis for the Siglec glycan interactome and other sialic acid–binding proteins. Significance Siglecs are immunomodulatory receptors that recognize sialic acid–carrying glycans with important functions in immunity. However, many of their natural ligands are poorly defined. We generated a cell-based glycan array comprised of a library of isogenic human cells displaying the greater complexity of sialic acids on diverse glycan structures and glycoconjugates in the natural context of the cell surface. We applied this array to reveal fine binding specificities of Siglecs for sialoglycans, informed of the underlying required glycosyltransferase genes, and provided evidence for selective binding context provided by glycan presentation on distinct protein sequences. Insight into the fine binding specificities of Siglecs will advance understanding their diverse biological functions and benefit therapeutic targeting in autoimmunity, inflammation, cancer, and Alzheimer’s disease.
Funding Information
  • European Commission (MSCA-IF 787684)
  • Lundbeck Foundation (na)
  • European Commission (H2020-MSCA-ITN-721297)
  • European Commission (H2020-MSCA-ITN-722171)
  • Danish National Research Foundation (DNRF107)
  • HHS | National Institutes of Health (U19AI136443)

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