Signal Regulatory Protein α (SIRPα) is a monomeric approximately 90 kD type I transmembrane glycoprotein. The 504 amino acid human SIRPα contains two Ig-like C1-type domains and one Ig-like V-type domain. SIRPα can express in various tissues, mainly on brain and myeloid cells, including macrophages, neutrophils, dendritic and Langerhans cells. It also can detect in neurons, smooth muscle and endothelial cells. SIRPA is an immunoglobulin-like cell surface receptor for CD47. SIRPα acts as docking protein and induces translocation of PTPN6, PTPN11 and other binding partners from the cytosol to the plasma membrane. SIRPα shows adhesion of cerebellar neurons, neurite outgrowth and glial cell attachment. SIRPα engagement generally produces a negative regulatory signal; it may mediate negative regulation of phagocytosis, mast cell activation and dendritic cell activation.
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. CD180, also known as RP105, is a B-cell surface molecule belonging to the family of pathogen receptors, Toll-like receptors (TLR). CD180 has an extracellular leucine-rich repeats and a short cytoplasmic tail. CD180 / RP105 interact with an extracellular molecule named MD1 and then together form the cell surface receptor complex RP105 / MD1 which induces B-cell activation in humans and mice, leading to proliferation and up-regulation of a costimulatory molecule, B7.2 / CD86. CD180 / RP105 also has a role in LPS response because B cells lacking RP105 show hyporesponsiveness to LPS.
CD19 is a single-pass type I membrane protein containing 2 Ig-like C2-type (immunoglobulin-like) domains. CD19 is expressed on follicular dendritic cells and B cells. In fact; it is present on B cells from earliest recognizable B-lineage cells during development to B-cell blasts but is lost on maturation to plasma cells. CD19 primarily acts as a B cell co-receptor in conjunction with CD21 and CD81. Upon activation; the cytoplasmic tail of CD19 becomes phosphorylated; which leads to binding by Src-family kinases and recruitment of PI-3 kinase. CD19 Assembles with the antigen receptor of B lymphocytes in order to decrease the threshold for antigen receptor-dependent stimulation. Defects in CD19 are the cause of immunodeficiency common variable type 3 (CVID3) which is a primary immunodeficiency characterized by antibody deficiency; hypogammaglobulinemia; recurrent bacterial infections and an inability to mount an antibody response to antigen.
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 19 (CD19) is a member of CD system. CD19 is a cell surface molecule that assembles with the antigen receptor of B-cells. This results in a descent in threshold for antigen receptor-dependent stimulation. A simplified view holds that the ability of B-cells to respond to the various antigens in a specific and sensitive manner is achieved in the presence of low-affinity antigen receptors. CD19 primarily acts as a B-cell coreceptor in conjunction with CD21 and CD81. The formation of the receptor complex is induced by antigen and CD19; induced by exogenous antigen; has been found cytoplasmic tail phosphorylated and bind to sIg.
CD1B contains 1 Ig-like (immunoglobulin-like) domain and belongs to the CD1 family. CD1 family members are transmembrane glycoproteins, which are structurally related to the major histocompatibility complex (MHC) proteins and form heterodimers with beta-2-microglobulin. During protein synthesis and maturation, they bind endogenous lipids that are replaced by lipid or glycolipid antigens when the proteins are internalized and pass through endosomes, before trafficking back to the cell surface. CD1B localizes to late endosomes and lysosomes via a tyrosine-based motif in the cytoplasmic tail, and requires vesicular acidification to bind lipid antigens.. It is expressed on cortical thymocytes, epidermal Langerhans cells, dendritic cells, on certain T-cell leukemias, and in various other tissues. CD1B is an antigen-presenting protein that binds self and non-self lipid and glycolipid antigens and presents them to T-cell receptors on natural killer T-cells.
CD200 (OX-2) is a cell surface glycoprotein that imparts immune privileges by suppressing alloimmune and autoimmune responses through its receptor, CD200R, expressed primarily on myeloid cells. Signals delivered through the CD200:CD200R axis have been shown to play an important role in the regulation of anti-tumor immunity, and overexpression of CD200 has been reported in a number of malignancies, including CLL, as well as on cancer stem cells. The role of CD200-CD200R signaling in immune regulation of the central nervous system has become a popular field of research in recent years. Many studies have shown that there is a close correlation between CD200-CD200R, microglia activation, and Parkinson's disease (PD). The ability of CD200 to suppress myeloid cell activation is critical for maintaining normal tissue homeostasis but may also enhance the survival of migratory neoplastic cells. CD200 and CD200R associate via their respective N-terminal Ig-like domains. CD200 has been characterized as an important immunoregulatory molecule, increased expression of which can lead to decreased transplant rejection, autoimmunity, and allergic disease. Elevated CD200 expression has been reported to be associated with poor prognosis in some human malignancies. Besides, CD200 also plays an important role in prevention of graft rejection, autoimmune diseases and spontaneous abortion.
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