B-lymphocyte-specific membrane protein that plays a role in the regulation of cellular calcium influx necessary for the development, differentiation, and activation of B-lymphocytes. Functions as a store-operated calcium (SOC) channel component promoting calcium influx after activation by the B-cell receptor/BCR.
This gene encodes a membrane protein, which functions as a receptor for Epstein-Barr virus (EBV) binding on B and T lymphocytes. Genetic variations in this gene are associated with susceptibility to systemic lupus erythematosus type 9 (SLEB9). Alternatively spliced transcript variants encoding different isoforms have been found for this gene.
CD22 is a member of the immunoglobulin superfamily, SIGLEC family of lectins. It is first expressed in the cytoplasm of pro-B and pre-B cells, and on the surface as B cells mature to become IgD+. CD22 serves as an adhesion receptor for sialic acid-bearing ligands expressed on erythrocytes and all leukocyte classes. In addition to its potential role as a mediator of intercellular interactions, signal transduction through CD22 can activate B cells and modulate antigen receptor signaling in vitro. The phenotype of CD22-deficient mice suggests that CD22 is primarily involved in the generation of mature B cells within the bone marrow, blood, and marginal zones of lymphoid tissues. CD22 recruits the tyrosine phosphatase Src homology 2 domain-containing phosphatase 1 (SHP-1) to immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and inhibits B-cell receptor (BCR)-induced Ca2+ signaling on normal B cells. CD22 interacts specifically with ligands carrying alpha2-6-linked sialic acids. As an inhibitory coreceptor of the B-cell receptor (BCR), CD22 plays a critical role in establishing signalling thresholds for B-cell activation. Like other coreceptors, the ability of CD22 to modulate B-cell signalling is critically dependent upon its proximity to the BCR, and this in turn is governed by the binding of its extracellular domain to alpha2,6-linked sialic acid ligands. However, genetic studies in mice reveal that some CD22 functions are regulated by ligand binding, whereas other functions are ligand-independent and may only require expression of an intact CD22 cytoplasmic domain at the B-cell surface. CD19 regulates CD22 phosphorylation by augmenting Lyn kinase activity, while CD22 inhibits CD19 phosphorylation via SHP-1.
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. Human Siglec-2, also known as B-cell antigen CD22 or Blymphocyte cell adhesion molecule (BL-CAM), is a B-cell restricted glycoprotein that is expressed in the cytoplasm of progenitor B and pre-B cells and on the surface of mature B cells. Two distinct human Siglec2/CD22 cDNAs that arise from differential RNA processing of the same gene have been isolated. Siglec2/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.
This gene encodes Lutheran blood group glycoprotein, a member of the immunoglobulin superfamily and a receptor for the extracellular matrix protein, laminin. This protein may play a role in epithelial cell cancer and in vaso-occlusion of red blood cells in sickle cell disease.
CD23 (also named B cell differentiation antigen) is a member of subgroup II of the C-type (Ca++-dependent) lectin superfamily. Human CD23 is a 47 kDa type II transmembrane glycoprotein that is expressed by a wide variety of cell types. The full-length receptor is 321 amino acids (aa) in length and contains a 274 aa extracellular region, a 26 aa transmembrane segment, and a 21 aa cytoplasmic domain. The extracellular region contains a C-type lectin domain and a connecting stalk with coiled-coil topography. The lectin domain binds both protein and carbohydrate in an apparently Ca++ independent manner. The coiled-coil region contributes to oligomerization. The lectin domain in human CD23 (aa 162‑284) is 64%, 62% and 68% aa identical to the lectin domains in mouse, rat and bovine CD23, respectively. In the cytoplasmic region, two FC isoforms exist which arise from alternate start sites. The “a” (or long) isoform begins with the sequence MEEGQYS and is constitutively expressed by B cells. It is believed to participate in IgE-mediated endocytosis. The “b” (or short) isoform begins with MNPPSQ and is induced on a wide variety of cell types by IL-4. Fcb reportedly contributes to IgE-mediated phagocytosis (13). Fcb expressing cells include eosinophils, monocytes, visceral smooth muscle and intestinal epithelium. At least four soluble forms of CD23 are known to exist. They range in molecular weight from 25 kDa to 37 kDa, with the 25 kDa form predominating in sera. Soluble CD23 (sFc) is generated by metalloprotease (ADAM8; ADAM15; ADAM28) and cysteine-protease activity. Cleavage usually occurs between aa 150‑160. It is unclear if sequential metalloprotease-cysteine protease activity is necessary for the generation of all soluble forms. Both soluble and membrane-bound CD23 show bioactivity. Ligands for CD23 include CD21, IgE, CD11b, and CD11c. CD23 binding to CD11b and Cd11c on monocytes results in oxidative product generation and proinflammatory cytokine release. On B cells, sCD23 induces IgE secretion by binding CD21. Conversely, secreted IgE will, in turn, bind B cell membrane CD23, rendering it unavailable for cleavage, and thus shutting down IgE production.
CD23, also known as FcεRII, is a trimeric type II transmembrane glycoprotein member of the calcium-dependent (C-type) lectin family. CD23 is expressed in B and T lymphocytes, monocytes, polymorphonuclear leukocytes, follicular dendritic cells, intestinal epithelial cells, and bone marrow stromal cells. CD23 was first identified as a low-affinity receptor for IgE and the engagement of membrane bound CD23 leads to suppressed IgE production by B lymphocytes. CD23 can be cleaved to form a soluble protein which can drive cytokine release in monocytic cells, nitric oxide (NO) production, and the synthesis of cyclic adenosine 3′5′-monophosphate (cAMP). Soluble CD23 signals through integrins, activating MAPK and NF-κB pathways. CD23 is a useful marker in the prognosis of neoplastic disease, is elevated in a variety of autoimmune and inflammatory conditions, and is being investigated as a therapeutic target for IgE-mediated allergy, arthritis, and B cell chronic lymphocytic leukemia (CLL).
Low-affinity receptor for immunoglobulin E (IgE) and CR2/CD21. Has essential roles in the regulation of IgE production and in the differentiation of B-cells (it is a B-cell-specific antigen).
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