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.
CD50 is a 120-140 kD type I membrane protein also known as intercellular adhesion molecule 3 (ICAM-3). It is broadly expressed on leukocytes, endothelial cells, and Langerhans cells. CD50 is a member of the Ig superfamily. It provides costimulatory signal for T cells and regulates cell adhesion through interaction with LFA-1 (CD11a/CD18, αL/β2) integrin.
ICAM proteins are ligands for the leukocyte adhesion protein LFA-1 (integrin alpha-L/beta-2). ICAM3 is also a ligand for integrin alpha-D/beta-2. In association with integrin alpha-L/beta-2, contributes to apoptotic neutrophil phagocytosis by macrophages.
CD53 is a member of the transmembrane 4 superfamily, also called the tetraspanin family. Most of these members are cell-surface proteins that are characterized by the presence of four hydrophobic domains. These proteins mediate signal transduction events that play a role in the regulation of cell development, activation, growth and motility. CD53 is a cell surface glycoprotein that is known to complex with integrins. Familial deficiency of CD53 gene has been linked to an immunodeficiency associated with recurrent infectious diseases caused by bacteria, fungi and viruses. CD53 contributes to the transduction of CD2-generated signals in T cells and natural killer cells and has been suggested to play a role in growth regulation.
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.
CD55 is a 60-70 kD glycosylphosphatidylinositol (GPI)-anchored single chain glycoprotein also known as decay-accelerating factor (DAF). It is expressed on hematopoietic cells including erythrocytes and many non-hematopoietic cells. CD55 accelerates the dissociation of the components of the C3-convertases (namely C2a from C4b in the C4bC2a complex, a C3-convertase of the classical pathway, and factor Bb from the C3bBb complex, a C3-convertase of the alternative pathway) to protect cells from inappropriate damage caused by autologous complement. CD55 has been reported to reduce the efficiency of NK cell lysis and induce signal transduction in T cells. CD55 has also been shown to interact with CD97 and bind to Coxsackie and Echovirus.
Ligand of the T-lymphocyte CD2 glycoprotein. This interaction is important in mediating thymocyte interactions with thymic epithelial cells, antigen-independent and -dependent interactions of T-lymphocytes with target cells and antigen-presenting cells and the T-lymphocyte rosetting with erythrocytes. In addition, the LFA-3/CD2 interaction may prime response by both the CD2+ and LFA-3+ cells.
CD59 is a widely expressed membrane regulatory protein, and its C-terminus is fixed on the cell surface via a GPI anchor. CD59 has different names due to its functions, such as homologous restriction factor-20, protectin and membrane inhibitor of reactive lysis (MIRL). The main physiological function of CD59 is to prevent the complement membrane attack complex (MAC) from lysing and destroying cells of the same species or its own, that is, homologous species restriction (HSR).
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