First printed in R&D Systems' 1996 Catalog.
Cell adhesion molecules (CAMs) are cell surface proteins involved in the binding of cells, usually leukocytes, to each other, to endothelial cells, or to extracellular matrix. Specific signals produced in response to wounding and infection control the expression and activation of certain of these adhesion molecules. The interactions and responses then initiated by binding of these CAMs to their receptors/ligands play important roles in the mediation of the inflammatory and immune reactions that constitute one line of the body's defense against these insults. Most of the CAMs characterized so far fall into three general families of proteins: the immunoglobulin (Ig) superfamily, the integrin family, or the selectin family.
The Ig superfamily of adhesion molecules, including ICAM-1, ICAM-2, ICAM-3, VCAM-1, and MadCAM-1, bind to integrins on leukocytes and mediate their flattening onto the blood vessel wall with their subsequent extravasation into the surrounding tissue. Chemokines such as MCP-1 and IL-8 cause a conformational change in integrins so that they can bind to their ligands.
The integrin family of CAMS serve as receptors for the ICAMs and VCAMs. The integrins are heterodimeric proteins consisting of an alpha and a beta chain that mediate leukocyte adherence to the vascular endothelium or other cell-cell interactions. Different sets of integrins are expressed by different populations of leukocytes to provide specificity for binding to different types of CAMs expressed along the vascular endothelium.
The selectin family members, L-Selectin, P-Selectin, and E-Selectin, are involved in the adhesion of leukocytes to activated endothelium. This adhesion is initiated by weak interactions that produce a characteristic "rolling" motion of the leukocytes on the endothelial surface. P-Selectin and L-Selectin, acting in concert, have been implicated in the mediation of these initial interactions. Stronger interactions, probably involving E-Selectin, follow the initial interactions, leading eventually to extravasation throught the blood vessel walls into lymphoid tissues and sites of inflammation.
Some other proteins that are functionally classified as CAMs are involved in strengthening the association of T cells with antigen-presenting cells or target cells and/or in T cell activation. Another type of adhesion molecule, CD44, has been implicated in lymphocyte homing, whereby lymphocytes recirculate via the lymphatic system back to the circulation so they are always available to be presented with antigen. At least twenty different forms of CD44, arising from differential splicing of up to ten alternative exons (v1-v10) have been identified to date. Variant forms of CD44 may play an important role in tumor metastasis.
L-Selectin [leukocyte selectin, LAM-1, LECAM-1, LECCAM-1, TQ1, mLHR (mouse), gp90MEL (mouse), gp100MEL (mouse), gp110MEL (mouse), CD62L, Leu-8, DREG, lymph node homing receptor, MEL-14 antigen] is a cell surface glycoprotein expressed constitutively on a wide variety of leukocytes.1-3
A member of the Selectin family of cell surface molecules, L-Selectin consists of an NH2-terminal lectin type C domain, an EGF-like domain, two complement control domains, a 15 amino acid residue spacer, a transmembrane sequence and a short cytoplasmic domain.1,4,5 Human and murine L-Selectins show the same domain structure and are 77% identical at the amino acid level and 79% identical at the nucleotide level.4,6 Two forms of L-Selectin have been reported apparently arising as a result of post-translational modifications. The lymphocyte form shows an apparent molecular weight of 74,000 while the neutrophil form is 95-100,000 in molecular weight.7
Three ligands for L-Selectin on endothelial cells have been identified, all containing O-glycosylated mucin or mucin-like domains.9 The first ligand, GlyCAM-1, is expressed almost exclusively in peripheral and mesenteric lymph node high endothelial venules.9,10 The second L-Selectin ligand, originally called sgp90, has now been shown to be CD34.11 This sialomucin-like glycoprotein, often used as a surface marker for the purification of pluripotent stem cells, shows vascular expression in a wide variety of nonlymphoid tissues, as well as on the capillaries of peripheral lymph nodes.12 The third ligand for L-Selectin is MadCAM 1, a mucin-like glycoprotein found on mucosal lymph node high endothelial venules.13
L-Selectin plays a role in the migration of lymphocytes into peripheral lymph nodes and sites of chronic inflammation and of neutrophils into acute inflammatory sites. Acting in cooperation with P-Selectin, L-Selectin mediates the initial interaction of circulating leukocytes with endothelial cells that produces a characteristic "rolling" of the leukocytes on the endothelium. This initial interaction is followed by a stronger interaction, probably involving E-Selectin, that leads eventually to extravasation through the blood vessel wall into lymphoid tissues and to sites of inflammation.8
L-Selectin is shed by proteolytic cleavage from the surfaces of lymphocytes and neutrophils in vitro following activation by a variety of agents (phorbol esters, LPS, f-met-leu-phe, etc.) and in vivo from neutrophils during inflammation2,14-16 and it has been suggested that loss of surface L-Selectin might be necessary to allow leukocytes to migrate through the endothelium.14,15 Soluble L-Selectin (sL-Selectin) derived from lymphocytes is about 62 kDa in size while the fragment derived from neutrophils is 75-100 kDa in size.17 Soluble L-Selectin retains bioactivity and at high concentrations can inhibit binding of lymphocytes to endothelium17, suggesting a possible role for soluble L-Selectin in modulating this binding in vivo.
High levels of soluble L-Selectin have been found in plasma drawn from apparently normal individuals.17 A number of studies have reported that levels of L-Selectin in biological fluids may be elevated or lowered in subjects with a variety of pathological conditions.18-23
P-Selectin (GMP-140, LECAM-3, PADGEM, CD62, CD62P) is a cell surface glycoprotein that plays a critical role in the migration of lymphocytes into tissues.1-5 P-Selectin is found constitutively in a pre-formed state in the Weibel-Palade bodies of endothelial cells and in the alpha granules of platelets.4 This stored P-Selectin is mobilized to the cell surface within minutes in response to a variety of inflammatory or thrombogenic agents.4 The mobilized P-Selectin is apparently present on the cell surface for only a few minutes after which it is recycled to intracellular compartments.4 Additional evidence indicates that transcription of P-Selectin mRNA can be activated in the endothelium by treatment with inflammatory mediators.6
P-Selectin, a member of the Selectin family of cell surface molecules, consists of an NH2-terminal lectin type C domain, an EGF-like domain, nine complement control domains, a transmembrane domain, and a short cytomplasmic domain.1,2 The murine equivalent of P-Selectin shows a similar organization of functional domains and an overall sequence identity of approximately 73%.5,6 However, murine P-Selectin contains only eight complement control domains, suggesting that the absolute number of these domains is not crucial for function.6 The molecular weight predicted from the cDNA for P-Selectin is approximately 86 kDa.1,2 The observed molecular weight on reducing SDS-PAGE, however, is approximately 140 kDa.2
The tetrasaccharide sialyl Lewisx (sLex) has been identified as a ligand for both P- and E-Selectin, but P-, E-, and L-Selectin can all bind sLex and sLea under appropriate conditions.4,5 P-Selectin also reportedly binds selectively to a 160 kDa glycoprotein present on murine myeloid cells and to a glycoprotein on myeloid cells, blood neutrophils, monocytes, and lymphocytes termed P-Selectin glycoprotein ligand-1 (PSGL-1), a ligand that also can bind E-Selectin.4,8,9 P-Selectin-mediated rolling of leukocytes can be completely inhibited by a monoclonal antibody specific for PSLG-1, suggesting that even though P-Selectin can bind to a variety of glycoproteins under in vitro conditions, it is likely that physiologically important binding is more limited.9
A variety of evidence indicates that P-Selectin is involved in the adhesion of myeloid cells, as well as B and a subset of T cells, to activated endothelium.4 P-Selectin is also involved in the adhesion of platelets to monocytes and neutrophils, playing a central role in neutrophil accumulation within thrombi.4 The adhesion of leukocytes and neutrophils to the endothelium is initiated by weak interactions that produce a characteristic "rolling" motion of the leukocytes and neutrophils on the endothelial surface.3-5,7 P-Selectin, acting in cooperation with L-Selectin, is implicated in the mediation of these initial interactions.3-5,7 Stronger interactions, probably involving E-Selectin, follow the initial interactions, leading eventually to extravasation through the blood vessel walls into lymphoid tissues and to sites of inflammation.4,8
P-Selectin is found in the plasma of normal individuals at ng/ml concentrations.10 Circulating P-Selectin appears to be slightly smaller than native P-Selectin. An alternatively spliced mRNA encoding a form of human P-Selectin lacking the transmembrane anchoring domain has been reported for both megakaryocytes and endothelial cells.5,10 and recent evidence suggests that the majority of circulating soluble P-Selectin arises in this manner.10,11 A number of studies have reported that levels of soluble P-Selectin in biological fluids may be elevated in subjects with a variety of pathological conditions.10,12-16