Recent Progress in the Methodologies to Identify Physiological Ligands of Siglecs

Quantitative Analysis of Siglec Ligands by Flow Cytometry

Siglecs (sialic acid-binding, immunoglobulin superfamily, lectins) are a family of transmembrane receptor-type glycan recognition proteins in vertebrates that are primarily expressed on leukocytes and regulate immune responses. Siglecs are involved in several diseases, such as cancer and neurodegenerative diseases. Most Siglecs suppress the activation of leukocytes by recognizing ligands containing sialic acid, a group of acidic sugars commonly found in vertebrate glycans, but rare among microbes. Siglec ligands are critical in the interaction between leukocytes and target cells. The abundance of the Siglec ligand is influenced by both the abundance of the glycoconjugate carrier (glycoprotein or glycolipid) and that of the terminal glycan epitope directly recognized by the Siglec. Therefore, a direct approach to evaluate the expression level of a Siglec ligand on cells of interest is to analyze the binding of recombinant Siglec protein to these cells. In this article, we describe a protocol for semi-quantitatively analyzing the expression level of Siglec ligands via flow cytometry using recombinant Siglec-Fc fusion protein. Support protocols describe how to remove sialic acids from the cell surface with sialidase under mild conditions to demonstrate the sialic acid dependence of Siglec binding, and the preparation of recombinant Siglec-Fc fusion proteins by transient transfection of mammalian cells. © 2023 Wiley Periodicals LLC. Basic Protocol: Quantitative analysis of Siglec ligands on mammalian cells via flow cytometry with recombinant Siglec-Fc fusion protein Support Protocol 1: Sialidase treatment of mammalian cells Support Protocol 2: Preparation of recombinant Siglec-Fc fusion protein via transient transfection of mammalian cells.

Roles of Siglecs in neurodegenerative diseases

Microglia are resident myeloid cells in the central nervous system (CNS) with a unique developmental origin, playing essential roles in developing and maintaining the CNS environment. Recent studies have revealed the involvement of microglia in neurodegenerative diseases, such as Alzheimer's disease, through the modulation of neuroinflammation. Several members of the Siglec family of sialic acid recognition proteins are expressed on microglia. Since the discovery of the genetic association between a polymorphism in the CD33 gene and late-onset Alzheimer's disease, significant efforts have been made to elucidate the molecular mechanism underlying the association between the polymorphism and Alzheimer's disease. Furthermore, recent studies have revealed additional potential associations between Siglecs and Alzheimer's disease, implying that the reduced signal from inhibitory Siglec may have an overall protective effect in lowering the disease risk. Evidences suggesting the involvement of Siglecs in other neurodegenerative diseases are also emerging. These findings could help us predict the roles of Siglecs in other neurodegenerative diseases. However, little is known about the functionally relevant Siglec ligands in the brain, which represents a new frontier. Understanding how microglial Siglecs and their ligands in CNS contribute to the regulation of CNS homeostasis and pathogenesis of neurodegenerative diseases may provide us with a new avenue for disease prevention and intervention.

Sialylated IgG in epithelial cancers inhibits antitumor function of T cells via Siglec-7

Although effective, immune checkpoint blockade induces response in only a subset of cancer patients. There is an urgent need to discover new immune checkpoint targets. Recently, it was found that a class of sialic acid-binding immunoglobulin-like lectins (Siglecs) expressed on the surface of T cells in cancer patients inhibit T cell activation through their intracellular immunosuppressive motifs by recognizing sialic acid-carrying glycans, sialoglycans. However, ligands of Siglecs remain elusive. Here, we report sialylated IgG (SIA-IgG), a ligand to Siglec-7, that is highly expressed in epithelial cancer cells. SIA-IgG binds Siglec-7 directly and inhibits TCR signals. Blocking of either SIA-IgG or Siglec-7 elicited potent antitumor immunity in T cells. Our study suggests that blocking of Siglec-7/SIA-IgG offers an opportunity to enhance immune function while simultaneously sensitizing cancer cells to immune attack.

Molecular Basis and Role of Siglec-7 Ligand Expression on Chronic Lymphocytic Leukemia B Cells

Siglec-7 (sialic acid-binding immunoglobulin-like lectin 7) is an immune checkpoint-like glycan recognition protein on natural killer (NK) cells. Cancer cells often upregulate Siglec ligands to subvert immunosurveillance, but the molecular basis of Siglec ligands has been elusive. In this study, we investigated Siglec-7 ligands on chronic lymphocytic leukemia (CLL) B cells. CLL B cells express higher levels of Siglec-7 ligands compared with healthy donor B cells, and enzymatic removal of sialic acids or sialomucins makes them more sensitive to NK cell cytotoxicity. Gene knockout experiments have revealed that the sialyltransferase ST6GalNAc-IV is responsible for the biosynthesis of disialyl-T (Neu5Acα2-3Galβ1-3[Neu5Acα2-6]GalNAcα1-), which is the glycotope recognized by Siglec-7, and that CD162 and CD45 are the major carriers of this glycotope on CLL B cells. Analysis of public transcriptomic datasets indicated that the low expression of GCNT1 (encoding core 2 GlcNAc transferase, an enzyme that competes against ST6GalNAc-IV) and high expression of ST6GALNAC4 (encoding ST6GalNAc-IV) in CLL B cells, together enhancing the expression of the disialyl-T glycotope, are associated with poor patient prognosis. Taken together, our results determined the molecular basis of Siglec-7 ligand overexpression that protects CLL B cells from NK cell cytotoxicity and identified disialyl-T as a potential prognostic marker of CLL.