Integral membrane glycoprotein that plays an essential role in the immune response and serves multiple functions in responses against both external and internal offenses. In T-cells, functions primarily as a coreceptor for MHC class II molecule:peptide complex. The antigens presented by class II peptides are derived from extracellular proteins while class I peptides are derived from cytosolic proteins. Interacts simultaneously with the T-cell receptor (TCR) and the MHC class II presented by antigen presenting cells (APCs). In turn, recruits the Src kinase LCK to the vicinity of the TCR-CD3 complex. LCK then initiates different intracellular signaling pathways by phosphorylating various substrates ultimately leading to lymphokine production, motility, adhesion and activation of T-helper cells. In other cells such as macrophages or NK cells, plays a role in differentiation/activation, cytokine expression and cell migration in a TCR/LCK-independent pathway. Participates in the development of T-helper cells in the thymus and triggers the differentiation of monocytes into functional mature macrophages.
CD55, also well known as decay-accelerating factor (DAF), is a member of the RCA (regulators of complement activation) family characterized by four to 30 SCRs (short consensus repeats) in their plasma-exposed regions. It is a major regulator of the alternative and classical pathways of complement activation and is expressed on all serum-exposed cells. CD55 is physiologically acting as an inhibitor of the complement system, but is also broadly expressed in malignant tumours. DAF seems to exert different functions beyond its immunological role such as promotion of tumorigenesis, decrease of complement mediated tumor cell lysis, autocrine loops for cell rescue and evasion of apoptosis, neoangiogenesis, invasiveness, cell motility. It is commonly hijacked by invading pathogens, including many enteroviruses and uropathogenic Escherichia coli, to promote cellular attachment prior to infection. This 70-75 kDa glycoprotein CD55 containing four SCR modules is involved in the regulation of the complement cascade. It inhibits complement activation by suppressing the function of C3/C5 convertases, thereby limiting local generation or deposition of C3a/C5a and membrane attack complex (MAC or C5b-9) production. DAF has been identified as a ligand for an activation-associated, seven-transmembrane lymphocyte receptor, CD97, which is a receptor mediating attachment and infection of several viruses and bacteria. In addition, it has been shown that DAF regulates the interplay between complement and T cell immunity in vivo, and thus may be implicated in immune and tumor biology.
CD59 glycoprotein, also known as 20 kDa homologous restriction factor, HRF20, MAC-inhibitory protein, Membrane attack complex inhibition factor, Membrane inhibitor of reactive lysis, MIC11, MIRL and CD59, is a cell membrane protein which contains one UPAR/Ly6 domain. CD59 is a small, highly glycosylated, GPI-linked protein, with a wide expression profile. The soluble form of CD59 from urine retains its specific complement binding activity, but exhibits greatly reduced ability to inhibit MAC assembly on cell membranes. CD59 is a potent inhibitor of the complement membrane attack complex (MAC) action. CD59 was first identified as a regulator of the terminal pathway of complement. It acts by binding to the C8 and/or C9 complements of the assembling MAC, thereby preventing incorporation of the multiple copies of C9 required for complete formation of the osmolytic pore. This inhibitor appears to be species-specific. CD59 is involved in signal transduction for T-cell activation complexed to a protein tyrosine kinase. Defects in CD59 are the cause of CD59 deficiency (CD59D).
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. CD5 is a member of the CD system. CD5 was found to be widely distributed in T-cells and B1 cells which is a subset of IgM-secreting B cells. CD5 also was found expressed in small lymphocytic lymphoma, hairy cell leukaemia and mantle cell lymphoma cells. CD5 serves to weaken the activating stimulus from the BCR so that the B1 cells can only reflect to the very strong stimuli but not the normal tissue proteins.
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.
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