|Detection of Siglec‑2/CD22 in Mouse Splenocytes by Flow Cytometry. Mouse splenocytes were stained with Rat Anti-Mouse Siglec‑2/CD22 Fluorescein‑conjugated Monoclonal Antibody (Catalog # FAB2296F, filled histogram) or isotype control antibody (Catalog # IC006F, open histogram). View our protocol for Staining Membrane-associated Proteins.|
Siglecs (sialic acid binding Ig-like lectins) are I-type (Ig-type) lectins belonging to the Ig superfamily. They are characterized by an N‑terminal Ig-like V-type domain which mediates sialic acid binding, followed by varying numbers of Ig-like C2-type domains (1, 2). Eleven human Siglecs have been cloned and characterized. Among these are sialoadhesin/CD169/Siglec-1, CD22/Siglec-2 and CD33/Siglec-3. To date, no Siglec has been shown to recognize any cell surface ligand other than sialic acid, suggesting that interactions with glycans containing this carbohydrate are important in mediating the biological functions of Siglecs. The cDNA of mouse Siglec-2 (also known as B-cell antigen CD22), encodes an 862 amino acid (aa) protein that contains a 21 aa signal peptide, a 681 aa extracellular region, a 19 aa transmembrane region and a 141 aa cytoplasmic tail (3, 4). The extracellular region contains one N-terminal V-type Ig-like domain followed by six Ig-like C2-type domains. The cytoplasmic domain has 3 immunoreceptor tyrosine-based inhibition motifs (ITIMs). Two splice forms exist, both showing deletions in the V-type Ig domain of 30 aa and 60 aa each. There are also two alleles in mouse that account for a difference of 10 aa in the extracellular region. The extracellular region of mouse Siglec-2 is 60% aa identity to human extracellular Siglec-2. Expression of mouse Siglec-2/CD22 generates a 140 kDa integral membrane glycoprotein that is limited to the B cell compartment of lymphoid tissues. Its expression is upregulated by LPS activtion (5, 6). Siglec-2/CD22 is an adhesion molecule that preferentially binds alpha 2,6- linked sialic acid on the same (cis) or adjacent (trans) cells. Interaction of CD22 with trans ligands on opposing cells was found to be favored over the binding of ligands in cis (7).