CD48

CD48 antigen (Cluster of Differentiation 48) also known as B-lymphocyte activation marker (BLAST-1) or signaling lymphocytic activation molecule 2 (SLAMF2) is a is_associated_with::protein that in humans is encoded by the CD48 is_associated_with::gene.

CD48 is a member of the is_associated_with::CD2 subfamily of the is_associated_with::immunoglobulin superfamily (IgSF) which includes SLAM (signaling lymphocyte activation molecules) proteins, such as is_associated_with::CD84, is_associated_with::CD150, is_associated_with::CD229 and is_associated_with::CD244. CD48 is found on the surface of is_associated_with::lymphocytes and other immune cells, is_associated_with::dendritic cells and is_associated_with::endothelial cells, and participates in activation and differentiation pathways in these cells.

CD48 was the first B-cell-specific is_associated_with::cellular differentiation is_associated_with::antigen identified in transformed B is_associated_with::lymphoblasts.

Structure
The gene for CD48 is located in is_associated_with::chromosome 1q23 and contains 4 exons, each exon encoding one of the 4 domains of CD48: is_associated_with::signal peptide, variable (V) domain, constant 2 (C2) domain and the is_associated_with::glycophosphatidylinositol anchor (is_associated_with::GPI anchor). The cDNA sequence of 1137 nucleotides encodes a 243 amino acid polypeptide of about 45 kDa. It consists of a 26 amino acid is_associated_with::signal peptide, 194 amino acids of mature CD48 (V and C2 domains) and the C-terminal 23 amino acid segment comprising the is_associated_with::GPI anchor. The GPI linkage of CD48 to the cell surface is through serine residue 220. CD48 does not have a is_associated_with::transmembrane domain, however, but is held at the cell surface by a GPI anchor via a is_associated_with::C-terminal domain which can be cleaved to yield a soluble form of the receptor. The CD48 protein is heavily is_associated_with::glycosylated, with five possible asparagine-linked is_associated_with::glycosylation sites at positions 40, 44, 104, 162 and 189, respectively. Approximately 35-40% of the total molecular weight is attributed to the carbohydrate side chains.

Interactions
CD48 was found to have a very low affinity for is_associated_with::CD2 with is_associated_with::dissociation constant ($$K_{D}$$) < 0.5 mM. It was found that the preferred ligand of CD48 is is_associated_with::2B4 (is_associated_with::CD244), which is also a member of the CD2 subfamily SLAM of is_associated_with::IgSF expressed on is_associated_with::natural killer cells (NK cells) and other leukocytes. The affinity of is_associated_with::CD244 for CD48 is at $$K_{D}$$ = 8 μM which is about 5 - 10 times stronger than for CD2.

Cell distribution
CD48 is expressed on all is_associated_with::peripheral blood lymphocytes (PBL) including is_associated_with::T cells, is_associated_with::B cells, is_associated_with::Null cell and is_associated_with::thymocytes. It is also found on the surface of activated T cells, is_associated_with::monocytes and is_associated_with::granulocytes. Like all other GPI anchor protein (GPI-AP), CD48 is deficient in is_associated_with::erythrocytes (red blood cells).

T cell activation
CD48 and CD2 molecular coupling together with other interaction pairs of CD28 and CD80, TCR and peptide-MHC and LFA-1 and ICAM-1 contribute to the formation of an immunological synapse between a T cell and an is_associated_with::antigen presenting cell. CD48 interaction with CD2 has been shown to promote lipid raft formation, T cell activation and the formation of caveolae for is_associated_with::macrophages through cell signal transductionthe via GPI moieties.