|Detection of Siglec‑11 in Human Peripheral Blood Mononuclear Cell Monocytes and Granulocytes by Flow Cytometry. Human peripheral blood mononuclear cell monocytes and granulocytes were stained with Mouse Anti-Human CD14 APC‑conjugated Monoclonal Antibody (Catalog # FAB3832A) and either (A) Goat Anti-Human Siglec‑11 PE‑conjugated Antigen Affinity-purified Polyclonal Antibody (Catalog # FAB3258P) or (B) Normal Goat IgG Phycoerythrin Control (Catalog # IC108P). View our protocol for Staining Membrane-associated Proteins.|
Siglecs (Sialic acid binding Ig-like lectins) are I-type lectins that belong to the immunoglobulin superfamily. They are characterized by an N‑terminal Ig-like V-set domain which mediates sialic acid binding, followed by a varying numbers of Ig-like C2-set domains. Siglecs‑3 and -5 through -13 constitute the CD33/Siglec-3 related group, which are defined by their sequence homology and differential expression in the hematopoietic system. Mature human Siglec-11 is an 85-90 kDa, type I transmembrane glycoprotein that consists of a 534 amino acid (aa) extracellular domain (ECD), a 23 aa transmembrane segment, and a 114 aa cytoplasmic domain. The ECD contains one Ig-like V-set domain, and three Ig-like C2-set domains. The cytoplasmic domain contains two Immunoreceptor Tyrosine-based Inhibitory Motifs (ITIMs). A splice variant of Siglec-11 has a deletion of nearly 100 aa in the extracellular juxtamembrane region. Among siglecs, the ECD of Siglec-11 is most closely related to that of Siglec-10 (82% aa sequence identity). The cytoplasmic domains of these proteins are only 20% identical. Siglec-11 is closely related to the pseudogenes Siglec-14 and Siglec-16. Human Siglec-11 shares 90%‑96% aa sequence identity with Siglec-11 from great apes. Rodent orthologs of Siglec-11 have not been identified. In human, Siglec-11 is expressed in tissue macrophages, brain microglia, and inflammatory site monocytes. Strong microglial expression is specific to humans, as it is less prominent or absent in chimpanzees and orangutans. Siglec-11 forms 180 kDa disulfide-linked dimers. It shows a strong binding preference for sialic acid in alpha 2-8 linkage which is unusual for siglecs. Notably, this applies to polysialyated NCAM residues on neurons where Siglec-11 activation imparts neuroprotection towards neurons in response to neuroinflammation.
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