L-Selectin (also known as Leukocyte Selectin, LAM-1, LECAM-1, LECCAM-1, TQ1, Leu-8, MEL-14 antigen, DREG, lymph node homing receptor, CD62L) is a member of the Selectin family of cell surface molecules which include E-Selectin and P-Selectin. All Selectins have an extracellular domain composed of an amino-terminal calcium-dependent lectin domain, an epidermal growth factor (EGF)-like domain, two to nine short consensus repeat (SCR) units, a transmembrane domain, and a cytoplasmic tail. L-Selectin expression is limited to hematopoietic cells, with most leukocytes expressing L-Selectin at some stage of differentiation. The majority of myeloid cells, B cells, and virgin T cells express L-Selectin, while only a sub-population of memory T cells and NK cells express L-Selectin. Lymphocytes and neutrophils exhibit a reversible loss of L-Selectin after cellular activation that results from endoproteolytic release of the extracellular portion of receptor from the cell surface. Cleavage of L-Selectin from the cell surface results in a high circulating level of functionally active soluble L-Selectin. All selectins bind sialytated and fucosylated oligosaccharides that are linked to glycoproteins and glycolipids. L-Selectin specifically binds to at least three different heavily glycosolylated mucin-like proteins: GlyCAM-1, CD34, and MAdCAM-1. Multiple studies indicated that L-Selectin, P-Selectin E-Selectin collaborate to mediate the initial binding of leukocytes to endothelium at sites of tissue injury and inflammation, producing the characteristic “rolling” of leukocytes along the endothelium. L-Selectin knockout mice have a 70% decrease in rolling leukocytes in exposed mesentery and have impaired neutrophil and monocyte migration into areas of inflammation. Additionally, L-Selectin knockout mice have relatively few lymphocytes present in peripheral lymph nodes and Peyer’s patches. Short-term in vivo homing experiments in L-Selectin deficient mice demonstrate that L-Selectin is involved in lymphocyte homing to Peyer’s patches and mesenteric lymph nodes in the gut. Rat and human L-Selectin share 77% amino acid sequence homology. Rat and mouse L-Selection share 83% amino acid sequence homology.
The cluster of differentiation (CD) system is commonly used as cell markers in immunophynotyping. Different kinds of cells in the immune system can be identified through the surface CD molecules which associating with the immune function of the cell. There are more than 320 CD unique clusters and subclusters have been identified. Some of the CD molecules serve as receptors or ligands important to the cell through initiating a signal cascade which then alter the behavior of the cell. Some CD proteins do not take part in cell signal process but have other functions such as cell adhesion. Cluster of differentiation 63 (CD63) is a member of the CD family and the transmembrane 4 superfamily,also known as the tetraspanin family. CD63 is a cellular surface glycoprotein characterized by the presence of four bydrophobic domains. CD63 had functions in mediating signal transduction processes and then regulate variety of cellular processes such as cell proliferation, activation and motility. It has reported that CD63 protein associated with tumor progression and served as a blood platlet activation marker and the deficiency of this protein may be associated with Hermansky-Pudlak syndrome.
Macrosialin, also known as CD68 and Gp110, is a single-pass type I membrane protein which belongs to theLAMP family. CD68 is highly expressed by blood monocytes and tissue macrophages. It is also expressed in lymphocytes, fibroblasts and endothelial cells. CD68 is expressed in many tumor cell lines which could allow them to attach to selectins on vascular endothelium, facilitating their dissemination to secondary sites. CD68 plays a role in phagocytic activities of tissue macrophages, both in intracellular lysosomal metabolism and extracellular cell-cell and cell-pathogen interactions. It is a commonly used marker for macrophages. However, a number of studies have shown that CD68 antibodies react with other hematopoietic and non-hematopoietic cell types, suggesting that CD68 may not be a macrophage-specific antigen. CD68 binds to tissue- and organ-specific lectins or selectins, allowing homing of macrophage subsets to particular sites. Rapid recirculation of CD68 from endosomes and lysosomes to the plasma membrane may allow macrophages to crawl over selectin-bearing substrates or other cells.
T-cell differentiation antigen CD6, also known as TP120 and CD6, is a single-pass type I membrane protein which contains threeSRCR domains. CD6 / TP120 is a cell surface glycoprotein expressed primarily on T cells, it may function as a costimulatory molecule and may play a role in autoreactive immune responses. CD6 / TP120 is expressed by thymocytes, mature T-cells, a subset of B-cells known as B-1 cells, and by some cells in the brain. CD6 ligand termed CD166 (previously known as activated leukocyte cell adhesion molecule, ALCAM ) has been identified and shown to be expressed on activated T cells, B cells, thymic epithelium, keratinocytes, and in rheumatoid arthritis synovial tissue. CD6 / TP120 binds to activated leukocyte cell adhesion molecule ( CD166 ), and is considered as a costimulatory molecule involved in lymphocyte activation and thymocyte development. CD6 / TP120 partially associates with the TCR / CD3 complex and colocalizes with it at the center of the mature immunological synapse (IS) on T lymphocytes. During thymic development CD6-dependent signals may contribute both to thymocyte survival, and to the overall functional avidity of selection in both man and mouse.
CD73, also known as Ecto-5'-nucleotidase, is an ectoenzyme that is attached to the cell membrane by a glycosyl phophatidylinositol anchor. The enzyme is expressed by most cell types. The 5 ’ -Nucleotidase activity of CD73 converts extracellular nucleoside-5 ’ -monophosphates to nucleosides. CD73 is one of several enzymes responsible for the production of extracellular adenosine, a signaling molecule that is involved in responses to inflammation and tissue injury. CD73 hydrolyzes extracellular nucleotides into membrane permeable nucleosides.
5'-nucleotidase, also known as NT5E, NTE, and CD73, is a cell membrane protein which belongs to the5'-nucleotidase family. CD73 is a glycosyl phosphatidylinositol (GPI) anchored purine salvage enzyme expressed on the surface of human T and B lymphocytes. CD73 catalyzes the conversion of purine and pyrimidine ribo- and deoxyribonucleoside monophosphates to the corresponding nucleosides. CD73 serves as a costimulatory molecule in activating T cells. CD73 generated adenosine functions in cell signalling in many physiologic systems, including intestinal epithelium, ischemic myocardium, and cholinergic synapses. CD73 might mediate lymphocyte-stromal cell interactions or condition the local microenvironment to facilitate lymphocyte development and/or function. In CD73-depleted cells, surface levels of the leukocyte adhesion molecules ICAM-1, VCAM-1 and E-selectin increase. CD73 produces extracellular adenosine, which then acts on G protein-coupled purigenic receptors to induce cellular responses. CD73 has also been reported to regulate expression of pro-inflammatory molecules in mouse endothelium.
Transfers sialic acid from the donor of substrate CMP-sialic acid to galactose containing acceptor substrates.
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