|Detection of Siglec‑1/CD169 in Human PBMCs by Flow Cytometry. Human peripheral blood mononuclear cells (PBMCs), (A) resting, or (B) treated with 20 ng/mL Recombinant Human IFN‑ gamma (Catalog # 285-IF) for 24 hours, were stained with Mouse Anti-Human Siglec‑1/CD169 Alexa Fluor® 488‑conjugated Monoclonal Antibody (Catalog # FAB5197G) and Mouse Anti-Human CD14 APC‑conjugated Monoclonal Antibody (Catalog # FAB3832A). Quadrant markers were set based on control antibody staining (Catalog # IC002G). View our protocol for Staining Membrane-associated Proteins.|
Siglecs are sialic acid specific I-type lectins that belong to the immunoglobulin superfamily. Structurally, they are transmembrane proteins with an N-terminal Ig-like V‑set domain followed by varying numbers of Ig-like C2-set domains (1, 2). Human Siglec-1, also known as sialoadhesin and CD169, is a 175‑185 kDa glycoprotein. It contains a 1622 amino acid (aa) extracellular domain (ECD) with one Ig-like V‑set domain and 16 Ig-like C2-set domains, a 21 aa transmembrane segment, and a 47 aa cytoplasmic domain (3). Within the ECD, human Siglec-1 shares approximately 70% aa sequence identity with mouse and rat Siglec-1. Alternate splicing generates a potentially soluble form of the ECD, while a second isoform shows a substituted cytoplasmic domain. Siglec-1 expression occurs on monocytes, dendritic cells, and macrophages of various organs and tissues, including liver, spleen, lung, and gut. The adhesive function of Siglec-1 is supported by the N-terminal Ig-like domain which shows a selectivity for alpha 2,3‑linked sialic acid residues (3‑5). Siglec-1 binds a number of sialylated molecules including the mannose receptor, MGL1, MUC1, PSGL-1, and different glycoforms of CD43 (6‑9). Its binding capacity can be masked by endogenous sialylated molecules (10, 11). The sialylated and sulfated N-linked carbohydrates that modify Siglec-1 itself are required for ligand binding (6, 7). Siglec-1 is expressed on dendritic cells following rhinovirus exposure, and these DC promote T cell anergy (12). It is also induced on circulating monocytes during systemic sclerosis and HIV-1 infection (13‑15). Siglec-1 can trap HIV-1 particles for trans infection of permissive cells (14).
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