Siglec 5 to 11 share a high degree of sequence similarity with CD33/Siglec-3 both in their extracellular and intracellular regions. They are collectively referred to as CD33-related Siglecs. One remarkable feature of the CD33-related Siglecs is their differential expression pattern within the hematopoietic system. This fact, together with the presence of two conserved immunoreceptor tyrosine-based inhibition motifs (ITIMs) in their cytoplasmic tails, suggests that CD33-related Siglecs are involved in the regulation of cellular activation within the immune system. Mouse Siglec-F cDNA encodes a 569 amino acid polypeptide with a hydrophobic signal peptide, an N-terminal Ig-like V-type domain, three Ig-like C2-type domains, a transmembrane region and a cytoplasmic tail. The expression of Siglec-F is restricted to the cells of myelomonocytic lineage. Mouse Siglec-F is likely an ortholog of human Siglec-5. Unlike many human CD33-related Siglecs, which show similar binding to both alpha 2,3- and alpha 2,6-linked sialic acids, mouse Siglec-F preferentially recognize alpha 2,3-linked sialic acid.
Signal Transducer CD24 is a heavily and variably glycosylated GPI-linked sialoprotein. Human CD24 is expressed on B lineage cells and granulocytes, on epithelial, neuronal, and muscle cells, and on a range of tumor cells. CD24 expression is regulated during lineage development and with the activation of various cell types. Antibody crosslinking of CD24 enhances the induction of apoptosis in B and T lymphocytes which contributes to negative selection and the induction of immune tolerance. CD24 on antigen presenting cells cooperates with B7 molecules in the costimulation of T cells. CD24 associates in cis with Siglec10 and with the danger-associated molecules HMGB1, HSP70, or HSP90 which are released from necrotic or damaged cells. Formation of these ternary complexes fills a protective role: the resulting Siglec10 signaling inhibits inflammatory responses that are otherwise induced by extracellular DAMPs.
Mouse Signal Regulatory Protein α (SIRPα) is a type I transmembrane glycoprotein.It contains two Ig-like C1-type domains and one Ig-like V-type domain. Mouse SIRP alpha ECD shares 61%, 75%, 62%, 61%, and 59% aa sequence identity with human, rat, equine, bovine, and porcine SIRP alpha, respectively.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
SIRPα is a type I transmembrane glycoprotein.It contains two Ig-like C1-type domains and one Ig-like V-type domain. Mouse SIRP alpha ECD shares 61%, 75%, 62%, 61%, and 59% aa sequence identity with human, rat, equine, bovine, and porcine SIRP alpha, respectively.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.
SIRPB1A (Signal-regulatory protein beta 1A), also known as SIRP beta 1, belongs to signal-regulatory-protein (SIRP) family, and immunoglobulin superfamily. Signal-regulatory proteins (SIRPs) are cell-surface glycoproteins expressed on myeloid and neural cells that have been shown to recruit SH2 domain-containing protein phosphatase 1 (SHP-1) and SHP-2 and to regulate receptor tyrosine kinase-coupled signaling. SIRP are classified as SIRP alpha molecules, containing a 110- to 113-amino acid long, or SIRP beta molecules, with a 5-amino acid long intracytoplasmic domain. SIRP beta 1 is a new DAP12-associated receptor involved in the activation of myeloid cells, which contains a short cytoplasmic domain that lacks sequence motifs capable of recruiting SHP-1 and SHP-2. SIRP beta 1. SIRP beta 1 acts as an activating isoform of SIRP alpha molecules, confirming the co-existence of inhibitory ITIM-bearing molecules, recruiting SHP-1 and SHP-2 protein tyrosine phosphatases, and activating counterparts, whose engagement couples to protein tyrosine kinases via ITAM-bearing molecules.
CD229(SLAMF3) is a type I transmembrane glycoprotein in the SLAM subgroup of the CD2 family. Mature mouse CD229 consists of a 406 aa extracellular domain (ECD) with two Ig-like V-set and two Ig-like truncated C2-set domains, a 21 aa transmembrane segment, and a 180 aa cytoplasmic domain with two immunoreceptor tyrosinebased switch motifs ITSMs. Within the first two Ig-like domains that are common to all SLAM proteins, mouse CD229 shares 22%-36% aa sequence identity with mouse 2B4, BLAME, CD2F10,CD84, CRACC, NTBA, and SLAM. CD229 is expressed on T, B, and NK cells, thymocytes and monocytes. Homophilic binding between CD229 molecules is mediated by the N-terminal Ig-like domain. Human and mouse CD229 exhibit crossspecies binding. Antigen stimulation of lymphocytes induces CD229 clustering to sites of T cell-B cell contact. Antibody ligation of CD229 can inhibit T cell activation, but CD229 knockout mice show impaired T cell immune responses, suggesting a potential role for CD229 in T cell activation or costimulation.
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