Identification of a carbohydrate-based endothelial ligand for a lymphocyte homing receptor

The human peripheral lymph node vascular addressin. An inducible endothelial antigen involved in lymphocyte homing

The extravasation of blood-borne lymphocytes into organized lymphoid tissues and sites of chronic inflammation is directed in part by interactions of lymphocyte surface adhesion molecules, known as homing receptors, with tissue-selective endothelial ligands called vascular addressins. In mice and humans, lymphocyte L-selectin and the peripheral lymph node addressin (PNAd) form a homing receptor-endothelial ligand pair involved in lymphocyte traffic to peripheral lymph node (PLN). We have examined the tissue distribution and function of human PNAd, using monoclonal antibody MECA-79 and in vitro assays of L-selectin-dependent lymphocyte binding. We demonstrate that PNAd is expressed by human high endothelial venules (HEV) in lymphoid tissues which support lymphocyte adhesion via a PLN-associated recognition system. MECA-79 inhibits adhesion to these HEV of a cell line that binds predominantly via the PLN-homing receptor, L-selectin, but has no effect on adhesion by a mucosal HEV-binding cell line. Furthermore, MECA-79 blocks binding of human peripheral blood mononuclear cells to both PLN and tonsil HEV, but not significantly to HEV in the appendix. In addition, we demonstrate PNAd induction on venules at chronic inflammatory sites in humans, particularly sites with severe or long-standing chronic inflammatory involvement. These results confirm that PNAd functions as a PLN vascular addressin in humans, and that in addition to directing normal lymphocyte recirculation to lymph nodes and tonsils, this addressin likely participates in lymphocyte recruitment to sites of chronic inflammation.

Expression of an adhesion molecule and homing in B-cell chronic lymphocytic leukaemia: II. L-selectin expression mediated cell adhesion revealed by immobilized analogue carbohydrates in B-cell chronic lymphocytic leukaemia and monoclonal lymphocytosis of undetermined significance

The L-selectin mediated adhesion of freshly isolated peripheral blood mononuclear cells (PBMCs) to phosphonomonoester core polysaccharide (PPME) and fucoidin derivatized gels was investigated in seven cases of monoclonal lymphocytosis of undetermined significance (B-MLUS) and 12 cases of chronic lymphocytic leukaemia: B-CLL, patients with peripheral lymphocytosis (LY-patients), lymph node enlargement (LN-patients) and splenomegaly (SM-patients). PBMCs isolated from the peripheral blood of 10 healthy donors served as controls. The binding to PPME and fucoidin correlated well (n = 19, P = 0.01). Adhesion of PBMCs from B-MLUS and B-CLL showed a greater variability than controls. A higher number of cells, on average, bound to PPME and fucoidin derivatized polyacrylamide gels in B-MLUS than in B-CLL. However, the differences observed were not statistically significant. In four cases with B-CLL, the stimulatory effect of interferon-alpha on the function of L-selectin and some other accessory molecules was also studied. The increased binding of PBMCs to immobilized analogue molecules (PPME, fucoidin) and to high endothelial venules (HEVs) in the in vitro HEV-binding assay supports the notion that interferon-alpha not only increases the expression of the adhesion molecules, but also results in an enhanced adhesive function.

Binding of L-selectin to the vascular sialomucin CD34

The adhesive interactions between leukocyte L-selectin and the endothelium are involved in the migration of lymphocytes through peripheral lymph nodes and of neutrophils to sites of inflammation. A recombinant L-selectin stains high endothelial venules (HEVs) in lymph nodes and recognizes sulfated carbohydrates found on two endothelial glycoproteins, Sgp50 and Sgp90. Amino acid sequencing of purified Sgp90 revealed a protein core identical to that CD34, a sialomucin expressed on hematopoietic stem cells and endothelium. A polyclonal antiserum to recombinant murine CD34 stains peripheral lymph node endothelium and recognizes Sgp90 that is functionally bound by L-selectin. Thus, an HEV glycoform of CD34 can function as a ligand for L-selectin.

Expression of an adhesion molecule and homing in B-cell chronic lymphocytic leukaemia: I. Application of the HEV-binding assay to a clinical series

High endothelial venule (HEV)-binding of peripheral blood mononuclear cells (PBMCs) from 43 patients with B-cell chronic lymphocytic leukaemia (B-CLL) was investigated with a HEV-binding in vitro assay. Immunophenotyping of HEV-adherent PBMCs proved that most of them belonged to the B-cell proliferation. B-CLL cells stringently expressed CD44 molecules (Hermes-1, -3 and H90). The patients were subgrouped according to Binet's classification, as well as according to the organ manifestations, i.e. patients with B-cell monoclonal lymphocytosis of undetermined significance (B-MLUS) and patients with lymphocytosis (LY), lymph node enlargement (LN) and splenomegaly (SM). The HEV-binding activity of the cells was the highest in Binet stage A patients and in patients with B-MLUS (p < 0.05 in B-MLUS versus B-CLL LY, LN, SM). Based on these findings it is suggested that B-CLL patients show not only a clinical and immunophenotypical heterogeneity, but a diverse function of adhesion molecules.

Robert Feulgen Lecture 1993. L-selectin and its biological ligands

This review considers the leukocyte adhesive receptor known as L-selectin. This protein, belonging to the selectin family of cell-cell adhesion receptors, mediates adhesion by virtue of a C-type lectin domain at its amino terminus. The protein was discovered as a lymphocyte homing receptor involved in the attachment of lymphocytes to high endothelial venules (HEV) of lymph nodes. Its widespread distribution on all leukocyte populations underlies a more general role in a variety of leukocyte-endothelial interactions. In the HEV interaction, cognate HEV ligands for L-selectin have been identified as two sulfated, sialylated, and fucosylated glycoproteins, known as GlyCAM-1 and Sgp90. These ligands have mucin-like domains which confer important properties for their proposed adhesive function. The carbohydrate features of these ligands, essential for their interaction with L-selectin, are reviewed. The existence of extralymphoid ligands for L-selectin is also discussed.

Bovine L-selectin: a peripheral lymphocyte homing receptor

The cDNA clone encoding for the bovine peripheral lymph node homing receptor (L-selectin) was isolated and sequenced. The predicted amino acid sequence of bovine L-selectin showed an overall high identity with that of human and murine L-selectin. However, the cytoplasmic tail of bovine L-selectin showed little similarity to that of human and murine L-selectin. The monoclonal antibody DREG-56, which recognizes human L-selectin, blocked the binding of bovine peripheral blood lymphocytes to high-walled endothelial venules in murine peripheral lymph nodes. Surface expression of bovine L-selectin was high in lymphocytes isolated from peripheral lymph nodes and low in lymphocytes isolated from Peyer's patches. This evidence strongly suggests that bovine L-selectin is a peripheral lymphocyte homing receptor.

Glycosylation-dependent cell adhesion molecule 1 (GlyCAM 1) mucin is expressed by lactating mammary gland epithelial cells and is present in milk

Glycosylation-dependent cell adhesion molecule 1 (GlyCAM 1) is a mucinlike endothelial glycoprotein that acts as an adhesive ligand for L selectin by presenting one or more O-linked carbohydrates to the lectin domain of this leukocyte cell surface selectin. The GlyCAM 1 glycoprotein has been previously shown to be expressed specifically by the endothelial cells of peripheral and mesenteric lymph nodes and in an unknown site in lung. Here we report that this protein is also expressed during lactation by mammary epithelial cells. Northern blot analysis has shown that the mRNA for GlyCAM 1 appears to be induced during pregnancy in a manner similar to that previously described for hormonally induced milk proteins. In situ hybridization analysis reveals that the site of GlyCAM 1 synthesis in the mammary gland is in the epithelial cells that produce these same milk proteins. Immunohistochemistry of mammary glands using antisera directed against GlyCAM 1 peptides demonstrates that these epithelial cells contain GlyCAM 1 protein, and that this protein is also found lumenally in the milk of the secreting mammary gland. Analysis of murine milk shows that immunoreactive GlyCAM 1 is found in the soluble whey fraction. Finally, labeling analysis of milk GlyCAM 1 has demonstrated that this form of the glycoprotein lacks the sulfate-modified carbohydrate that has recently been shown to be required for the ligand binding activity to L selectin. The nonsulfated mammary GlyCAM 1 is unable to interact with L selectin, consistent with the hypothesis that milk GlyCAM 1 has a different function than endothelial GlyCAM 1. These data thus suggest that milk GlyCAM 1 is a hormonally regulated milk protein that is part of the milk mucin complex. In addition, the finding that the mammary form of GlyCAM 1 contains different carbohydrate modifications than the endothelial form suggests that this glycoprotein may be a scaffold for carbohydrates that mediate functions in addition to cell adhesion.

Adhesion molecules and their role in cancer metastasis

This article describes various adhesion molecules and reviews evidence to support a mechanistic role for adhesion molecules in the process of cancer metastasis. A variety of evidence supports the involvement of specific adhesion molecules in metastasis. 1. For example, some cancer cells metastasize to specific organs, irrespective of the first organ encountered by the circulating cancer cells. This ability to colonize a specific organ has been correlated with the preferential adhesion of the cancer cells to endothelial cells derived from the target organ. This suggests that cancer cell/endothelial cell adhesion is involved in cancer cell metastasis and that adhesion molecules are expressed on the endothelium in an organ-specific manner. 2. Further, inclusion of peptides that inhibit cell adhesion, such as the YIGSR- or RGD-containing peptides, is capable of inhibiting experimental metastasis. 3. Metastasis can be enhanced by acute or chronic inflammation of target vessels, or by treatment of animals with inflammatory cytokines, such as interleukin-1. In vitro, cancer cell/endothelial cell adhesion can be enhanced by pretreating the endothelial cell monolayer with cytokines, such as interleukin-1 or tumor necrosis factor-alpha. This suggests that, in addition to organ-specific adhesion molecules, a population of inducible endothelial adhesion molecules is involved and is relevant to metastasis. 4. Further support for this model is found in the comparison to leukocyte/endothelial adhesion during leukocyte trafficking. Convincing evidence exists, both in vivo and in vitro, to demonstrate an absolute requirement for leukocyte/endothelial adhesion before leukocyte extravasation can occur. The relevance of this comparison to metastasis is reinforced by the observation that some of the adhesion molecules involved in leukocyte/endothelial adhesion are also implicated in cancer cell/endothelial adhesion. The involvement of adhesion molecules suggests a potential therapy for metastasis based on interrupting adhesive interactions that would augment other treatments for primary tumors.

Calcium-dependent heparin-like ligands for L-selectin in nonlymphoid endothelial cells

L-Selectin is a calcium-dependent mammalian lectin that mediates lymphocyte trafficking by recognizing sialylated ligands on high endothelial venules in lymph nodes. Although L-selectin probably mediates neutrophil extravasation into nonlymphoid tissues, no corresponding ligand has been characterized. Staining of cultured endothelial cells with an L-selectin chimera (LS-Rg) showed an internal pool of ligands. Metabolic labeling with sulfur-35-labeled sulfate revealed heparin lyase-sensitive ligands that bound LS-Rg in a calcium-dependent, sialic acid-independent manner. A fraction of commercial heparin bound to LS-Rg and LS-Rg bound to heparin-agarose, both in a calcium-dependent manner. Thus, L-selectin recognizes endothelial heparin-like chains, which could be physiological ligands mediating leucocyte trafficking.

Alpha 4 beta 7 integrin mediates lymphocyte binding to the mucosal vascular addressin MAdCAM-1

The mucosal vascular addressin, MAdCAM-1, is an immunoglobulin superfamily adhesion molecule for lymphocytes that is expressed by mucosal venules and helps direct lymphocyte traffic into Peyer's patches (PP) and the intestinal lamina propria. We demonstrate that the lymphocyte integrin alpha 4 beta 7, also implicated in homing to PP, is a receptor for MAdCAM-1. Certain antibodies to alpha 4 and beta 7 integrin chains but not to the beta 2 integrin LFA-1 inhibit lymphocyte binding to purified MAdCAM-1 and to MAdCAM-1 transfectants. Lymph node lymphocytes, alpha 4 beta 7+ TK1 lymphoma cells, and a beta 7-transfected variant of an alpha 4+ B cell line, 38C13, bind constitutively to MAdCAM-1. Binding is enhanced by Mn(++)-induced integrin activation. The related integrin alpha 4 beta 1 supports efficient binding to VCAM-1 but not to MAdCAM-1, even after integrin activation, indicating that MAdCAM-1 is a preferential ligand for alpha 4 beta 7. Alpha 4 beta 7 can also bind VCAM-1, but this requires greater integrin activation than binding to MAdCAM-1. The findings imply a selective role for the interaction of alpha 4 beta 7 and MAdCAM-1 lymphocyte in homing to mucosal sites.

Attenuation of colitis in the cotton-top tamarin by anti-alpha 4 integrin monoclonal antibody

Recent studies have demonstrated the induced expression of endothelial adhesion molecules including E-selectin (also called endothelial leukocyte adhesion molecule-1), vascular cell adhesion molecule and intercellular adhesion molecule in actively involved mucosa of patients with ulcerative colitis and Crohn's disease. Similar induction has been demonstrated in the colon of the Cotton-top tamarin (CTT), a New World primate that experiences a spontaneous acute and chronic colitis resembling ulcerative colitis. To assess the potential importance of leukocyte adhesion as a necessary step in acute colitis, the effect of parenteral mAb directed against adhesion molecules on CTT colitis was evaluated in placebo-controlled blinded trials. Serial administration of either of two anti-E-selectin mAb designated BB11 and EH8 effectively coated endothelial surfaces expressing this vascular adhesion molecule. Although colitis activity was slightly diminished after the 10-d treatment period in CTT receiving either BB11 or EH8, this reduction was not significantly different than that seen in animals given a placebo control when assessed by a previously validated standardized scale of inflammatory activity: mean histologic activity grade 2.2 +/- 0.2 pretreatment vs 1.5 +/- 0.5 posttreatment in group receiving mAb and 2.1 +/- 0.1 pretreatment vs 1.3 +/- 0.5 posttreatment in the placebo group (P > 0.2). In contrast, administration of an anti-alpha 4 integrin mAb designated HP1/2 that binds VLA4 (alpha 4 beta 1) and presumably alpha 4 beta 7 integrins resulted in significant attenuation of acute colitis when compared to both pretreatment activity index (P = 0.005) and the placebo control group (P < 0.01): mean histologic activity grade 1.6 +/- 0.3 pretreatment vs 0.2 +/- 0.1 posttreatment in the group receiving HP1/2 and 1.8 +/- 0.5 pretreatment and 1.2 +/- 0.2 posttreatment in the placebo control group. These studies using a model of spontaneous colitis in the CTT demonstrate the feasibility of modulation of leukocyte-vascular adhesion and/or other integrin-mediated events possibly including T cell aggregation and T cell-stromal interactions, as well as lymphocyte homing. These results suggest both that these processes are important and possibly essential elements in sustaining acute colitis and that their disruption may result in therapeutic benefit.

Identification of a glycoprotein ligand for E-selectin on mouse myeloid cells

E-selectin is an inducible endothelial cell adhesion molecule for neutrophils which functions as a Ca(2+)-dependent lectin. Using a recombinant, antibody-like form of mouse E-selectin, we have searched for glycoprotein ligands on mouse neutrophils and the neutrophil progenitor cell line 32D cl 3. We have identified a 150-kD glycoprotein as the only protein which could be affinity-isolated with soluble E-selectin from [35S]methionine/[35S]cysteine-labeled 32D cl 3 cells. Binding of this protein was strictly Ca(2+)-dependent, was blocked by a cell adhesion-blocking mAb against mouse E-selectin, and required the presence of sialic acid on the 150-kD ligand. This glycoprotein was also affinity-isolated from mature neutrophils, in addition to a minor component at 250 kD, but could not be isolated from several other non-myeloid cell lines. The 150-kD glycoprotein was the only protein from 32D cl 3 cells, which was detectable by silver-staining after a one-step affinity-isolation.

Lectin binding to chronic inflammatory gingival tissue: possible adhesion mechanisms based on lectin-carbohydrate interactions

In this study, we analyzed the expression of different leukocyte surface antigens, of the adhesion molecules ELAM-1 and GMP-140 and binding of various lectins and neoglycoproteins in inflamed gingival tissue. Cell suspensions from collagenase-digested gingiva were analyzed by flow cytometry in a FACScan. The expression of ELAM-1, GMP-140, carbohydrate structures and lectins in gingival specimens was also studied by immunohistochemistry. Gingival tissue of patients with active periodontal disease contained between 5% and 50% CD45+ mononuclear cells, consisting mainly of CD19+ cells (B lymphocytes). CD62, resembling GMP-140, and ELAM-1 were strongly expressed on endothelial cells of these patients. Control subjects usually contained almost no CD45+ cells in their gingiva and no CD62+ or ELAM-1-positive endothelial cells could be found in 5 of 6 control persons. Analysis of the glycosylation pattern revealed staining of infiltrating cells by peanut agglutinin (PNA; specificity for galactose), whereas soy bean agglutinin (SBA; specificity for N-acetyl-galactosamine) bound to epithelial cells. An endogenous lactosyl-specific lectin could be detected on endothelial cells by binding of lactosyl-BSA. Ulex europeus I agglutinin (UEA-1, specific for fucose) showed selective staining of endothelial and epithelial cells. Expression of a fucose-binding lectin, demonstrated by binding of fucosylated BSA, could be found on infiltrating cells. The adhesion molecules ELAM-1 and GMP-140 seem to be involved in cell adhesion during chronic inflammation of the gingiva. Interaction of other carbohydrate residues with endogenous lectins might resemble additional adhesion mechanisms in inflamed gingiva.

P- and E-selectin use common sites for carbohydrate ligand recognition and cell adhesion

The selectins are a family of three calcium-dependent lectins that mediate adhesive interactions between leukocytes and the endothelium during normal and abnormal inflammatory episodes. Previous work has implicated the carbohydrate sialyl Lewis(x) (sLe(x); sialic acid alpha 2-3 galactose beta 1-4 [Fucose alpha 1-3] N-acetyl glucosamine) as a component of the ligand recognized by E- and P-selectin. In the case of P-selectin, other components of the cell surface, including 2'6-linked sialic acid and sulfatide (galactose-4-sulfate ceramide), have also been proposed for adhesion mediated by this selectin. We have recently defined a region of the E-selectin lectin domain that appears to be directly involved with carbohydrate recognition and cell adhesion (Erbe, D. V., B. A. Wolitzky, L. G. Presta, C. R. Norton, R. J. Ramos, D. K. Burns, R. M. Rumberger, B. N. N. Rao, C. Foxall, B. K. Brandley, and L. A. Lasky. 1992. J. Cell Biol. 119:215-227). Here we describe a similar analysis of the P-selectin lectin domain which demonstrates that a homologous region of this glycoprotein's lectin motif is involved with carbohydrate recognition and cell binding. In addition, we present evidence that is inconsistent with a biological role for either 2'6-linked sialic acid or sulfatide in P-selectin-mediated adhesion. These results suggest that a common region of the E- and P-selectin lectin domains appears to mediate carbohydrate recognition and cell adhesion.

Higher-affinity oligosaccharide ligands for E-selectin

A series of synthetic oligosaccharides based on sialyl Lewis x (sLex; Neu5Ac alpha 2-3Gal beta 1-4[Fuc alpha 1-3]GlcNAc) and sialyl Lewis a (sLea; Neu5Ac alpha 2-3Gal beta 1-3[Fuc alpha 1-4]GlcNAc) was used to study the binding interactions of selectins. E-selectin-immunoglobulin fusion protein (E-selectin-Ig) bound to immobilized bovine serum albumin (BSA)-neoglycoproteins containing sLex or sLea in a Ca(2+)-dependent manner. Solution-phase sLex tetrasaccharide blocked this interaction by 50% at a concentration of 750 +/- 20 microM (IC50). sLea was more effective (IC50 = 220 +/- 20 microM), while nonsialylated, nonfucosylated derivatives showed little or no activity at concentrations up to 1 mM. Attachment of an 8-methoxycarbonyloctyl aglycone in a beta linkage to the anomeric carbon of the GlcNAc of sLex or sLea increased their blocking activity nearly twofold. Finally, replacement of the 2-N-acetyl substituent of the GlcNAc by an azido or amino group resulted in substantial increases in activity, with the most potent inhibitor being amino substituted sLea, which was 36-fold more active (IC50 = 21 +/- 3 microM) than the reducing tetrasaccharide sLex. In contrast to results obtained with E-selectin-Ig, P-selectin-Ig binding to immobilized BSA-sLea was blocked modestly by most oligosaccharides at 1 mM, with no substantial differences among them. IC50 values of soluble oligosaccharides determined in competitive binding studies accurately predicted blocking of leukocyte adhesion to recombinant E-selectin-Ig and to cytokine-activated endothelium.

The HNK-1 reactive sulfoglucuronyl glycolipids are ligands for L-selectin and P-selectin but not E-selectin

E-selectin, L-selectin, and P-selectin are related cell adhesion molecules that bind via their lectin domains to sialyl Lewis x and related carbohydrate determinants. Reports have indicated that sulfated glycolipids and polysaccharides also bind selectins. To extend these findings, we compared binding of selectin-IgG chimeras to immobilized sulfated and sialylated glycosphingolipids. E-, L-, and P-selectin chimeras all bound to surfaces absorbed with 2,3-sialyl Lewis x glycolipid or sulfatide (galactosylceramide I3-sulfate) but not to surfaces adsorbed with control sulfated lipids (octadecyl sulfate, sphingosine sulfate). Notably, the L- and P-selectin chimeras but not E-selectin chimera bound to surfaces adsorbed with sulfoglucuronyl glycosphingolipids (SGNL lipids; e.g., IV3 glucuronylneolactotetraosylceramide V3-sulfate). These unusual lipids have been reported as antigenic determinants for monoclonal IgM antibodies produced in patients with neuropathy associated with paraproteinemia and react with the mouse monoclonal antibody HNK-1. Binding of L- and P-selectin chimeras to SGNL lipids was specifically inhibited by appropriate anti-selectin antibodies. While binding of all three selectin chimeras to sialyl Lewis x was blocked by removal of calcium, binding to SGNL lipid was only modestly reduced by EDTA. Chemically desulfated SGNL lipid retained binding activity for L- and P-selectin chimeras, while methyl esterification of the glucuronic acid eliminated binding. We conclude that SGNL lipids, unlike sialyl Lewis x and sulfatides, selectively support L- and P-selectin but not E-selectin chimera binding. The presence of SGNL lipids on brain microvascular endothelium (and other endothelia) may implicate these molecules in leukocyte trafficking to the nervous system and elsewhere.

Sulphation requirement for GlyCAM-1, an endothelial ligand for L-selectin

L-selectin participates in the initial attachment of leukocytes to the vascular endothelium. On lymphocytes, it mediates binding to high endothelial venules of lymph nodes. As a selectin it functions as a calcium-dependent lectin recognizing carbohydrate-bearing ligands on endothelial cells. Two lymph node ligands for L-selectin have been identified as sulphated glycoproteins of M(r) approximately 50K and approximately 90K, called Sgp50 and Sgp90 (ref. 10). The recently cloned Sgp50 (ref. 12), now designated GlyCAM-1, is a high endothelial venule-associated, mucin-like glycoprotein containing predominantly O-linked carbohydrate chains. Sialylation of GlyCAM-1 is necessary for its ligand activity and a role for fucosylation is suspected. We have used chlorate as a metabolic inhibitor of sulphation, and report here that GlyCAM-1 has an additional requirement for sulphate.

Complex-type N-linked oligosaccharides of gp120 from human immunodeficiency virus type 1 contain sulfated N-acetylglucosamine

The major envelope glycoproteins gp120 and gp41 of human immunodeficiency virus type 1, the causative agent for human AIDS, contain numerous N-linked oligosaccharides. We report here our discovery that N-acetylglucosamine residues within the complex-type N-linked oligosaccharides of both gp120 and its precursor, gp160, are sulfated. When human Molt-3 cells persistently infected with human T-cell leukemia virus IIIB were metabolically radiolabeled with 35SO4, gp160, gp120, and to some extent gp41 were radiolabeled. The 35SO4-labeled oligosaccharides were quantitatively released by N-glycanase treatment and were bound by immobilized Ricinus communis agglutinin I, a lectin that binds to terminal beta-galactosyl residues. The kinetics of release of sulfate upon acid hydrolysis from 35SO4-labeled gp120 indicate that sulfation occurs in a primary sulfate ester linkage. Methylation analysis of total glycopeptides from Molt-3 cells metabolically radiolabeled with [3H]glucosamine demonstrates that sulfation occurs at the C-6 position of N-acetylglucosamine. Fragmentation of the gp120-derived 35SO4-labeled glycopeptides by treatment with hydrazine and nitrous acid and subsequent reduction generated galactosyl-anhydromannitol-6-35SO4, which is the expected reaction product from GlcNAc-6-sulfate within a sulfated lactosamine moiety. Charge analysis of the [3H]galactose- and [3H]glucosamine-labeled glycopeptides from gp120 and gp160 indicates that approximately 14% of the complex-type N-linked oligosaccharides are sulfated.

Selectins

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Direct demonstration of heterogeneous, sulfated O-linked carbohydrate chains on an endothelial ligand for L-selectin

We have previously identified endothelial ligands for L-selectin as sialylated, fucosylated and sulfated glycoproteins of approximately 50 kDa and 90 kDa (Sgp50 and Sgp90). In this report, we use the beta elimination reaction to demonstrate directly the presence of sulfated O-linked sugar chains on one of these ligands, after metabolic labeling with radiolabeled sulfate or fucose. All of the sulfated and the majority of the fucosylated O-linked sugar chains were shown to be sialylated by affinity chromatography on a Limax agglutinin column. Analyses by anion exchange and gel permeation chromatography revealed a complexity of sugar chains, which were heterogeneous both in charge and size. Charged groups other than sialic acid appeared to exert a predominant influence on the total charge of the sugar chains. The probable existence of a varying number of sulfate modifications per sugar chain is discussed.

Enhanced interaction of L-selectin with the high endothelial venule ligand via selectively oxidized sialic acids

The selectins are adhesion receptors that play key roles in leukocyte trafficking. Each has an N-terminal C-type lectin domain that binds to specific carbohydrates in a calcium-dependent manner. L-selectin recognizes sulfated, sialylated ligands on lymph node high endothelial venules. This recognition is abolished by strong periodate oxidation under conditions that destroy oligosaccharides. In contrast, mild periodate oxidation, which selectively oxidizes the side chain of sialic acid residues without affecting the underlying oligosaccharide, markedly enhances this interaction. The enhancement is calcium dependent, indicating that lectin recognition is maintained. Reduction of the sialic acid aldehydes generated by mild periodate to alcohol groups abolishes this effect. Covalent cross-linking of the oxidized ligand to L-selectin can be demonstrated, suggesting Schiff base formation between lysine residues of the selectin and the newly formed aldehydes. Such selectively oxidized sialylated ligands could be used to probe the lectin domains of the selectins and to identify lysine residues near the binding site. Also, this approach could be used to design drugs for disrupting leukocyte-endothelial interactions leading to pathological inflammation.

Human epidermal keratinocyte expression of sialyl-Lewis X

Cell-surface oligosaccharides can function as ligands for intercellular adhesion receptors, matrix proteins, and growth factors. We report that human neonatal and adult epidermal keratinocytes (KC) express sialyl Lewis X [s-Le(x); SA alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1-3R], a ligand for endothelial and platelet selectins. Freshly isolated or cultured KC bind FH6 monoclonal antibody (MoAb), which is specific for s-Le(x)-containing oligosaccharides. The relevant epitope is bona fide s-Le(x), because sialidase treatment of KC suspensions abrogates FH6 binding while generating de novo KC reactivity with anti-Le(x). KC stained in ice-cold suspension display a knobby membrane distribution of s-Le(x) detectable by immunofluorescence microscopy. As others have reported, FH6 appeared not to bind KC in perpendicular skin sections. However, basal KC in intact epidermal sheets exhibited obvious "honeycomb" reactivity with FH6 when stained and viewed en face, suggesting that s-Le(x) in intact epidermis may occur in bands that parallel the major tissue axis. FH6 specifically immunoprecipitated proteins of Mr 34 kd, 44 kd, and 56 kd from [35S]-labeled KC, and anti-Le(x) precipitated similar proteins from sialidase-treated KC. The enzymatic basis for KC s-Le(x) expression was studied by analyzing acceptor specificities and other properties of KC fucosyltransferases. Results indicate that KC express both Lewis- and myeloid-type alpha 1-3fucosyltransferases. KC s-Le(x) could be an important element of the epithelial milieu, because both epithelial cells and immune cells that home to epithelia express s-Le(x) and related structures, and because KC s-Le(x) is well positioned for selectin-mediated platelet binding after trans-cutaneous wounding. The apparent distributions of s-Le(x) in epidermis and on isolated KC are compatible with a functional role for s-Le(x) in these intercellular interactions.

Selective expression of sialyl-Lewis x and Lewis a epitopes, putative ligands for L-selectin, on peripheral lymph-node high endothelial venules

High endothelial venules (HEV) lined by the high endothelium are the sites where leukocytes enter into the lymph nodes from the blood. Lymphocyte homing into lymph nodes is organ-selective, i.e., different molecules are involved in the lymphocyte homing to peripheral nodes compared with mucosa associated lymphoid tissue. The traffic into peripheral nodes is regulated by the expression of L-selectin on leukocytes and its ligand on HEVs. The ligand for L-selectin is suggested to be a 50, 90, or 105 kd glycoprotein, which is sulfated, fucosylated, and sialylated. The two other members of the selectin family (E- and P-selectin) recognize sialyl-Lewis x and -Lewis a (sLex and sLea, respectively) carbohydrate motifs, and there is preliminary data suggesting that this would also be the case for L-selectin. We have initiated a study to identify the expression of these sialylated structures on endothelial surfaces. We present data that show that HEVs in peripheral nodes, but not in the mucosa-associated lymphoid tissue, express large quantities of sLex and sLea identified by MAbs in immunohistology. Endothelium in capillaries or larger vessels in non-lymphoid tissues do not react with anti-sLex or -Lea mAbs. Only 1-2% of the lymphocytes in the peripheral blood express sLex and so far only the skin-homing lymphocytes are known to be sLex positive in larger quantities. We show that in many occasions the B cells in the peripheral lymph-node germinal centers are also sLex-, but not sLea-positive, and provide evidence of the restricted pattern of sLex and sLea expression on peripheral lymph-node HEVs. We propose that they are at least parts of the ligand for L-selectin.

The selectin family of carbohydrate-binding proteins: structure and importance of carbohydrate ligands for cell adhesion

Protein-carbohydrate interactions have been found to be important in many steps in lymphocyte recirculation and inflammatory responses. A family of carbohydrate-binding proteins or lectins, termed selectins, has been discovered and shown to be involved directly in these processes. The three known selectins, termed L-, E- and P-selectins, have domains homologous to other Ca(2+)-dependent (C-type) lectins. L-selectin is expressed constitutively on lymphocytes, E-selectin is expressed by activated endothelial cells, and P-selectin is expressed by activated platelets and endothelial cells. Here, we review the nature of the carbohydrate determinants in tissues recognized by these selectins. The expression of specific sialylated, fucosylated and sulfated carbohydrates in activated endothelium and high endothelial venules promotes interactions with L-selectin on leukocyte surfaces. In contrast, E- and P-selectins recognize specific carbohydrate determinants related to sialyl Le(x) antigen on neutrophil and monocyte surfaces. The discovery of the selectins has generated excitement among glycoconjugate researchers that other carbohydrate-binding proteins and their cognate ligands will be found to function in regulating many types of cellular interactions.

Selectins: interpreters of cell-specific carbohydrate information during inflammation

Although a bewildering array of cell surface carbohydrate structures have been described, the physiological relevance of any of these complex molecules has often eluded biologists. A family of cell surface glycoproteins, the "selectins," has a characteristic ability to use some of these carbohydrate structures in adhesive mechanisms that help localize leukocytes to regions of inflammation. This article will review the biology of these carbohydrate-binding adhesive proteins and discuss the potential for developing anti-inflammatory antagonists that could inhibit binding events that are selectin-mediated.

Identification of a soluble form of a ligand for the lymphocyte homing receptor

Lymphocytes are engaged in constant trafficking from the blood into secondary lymphoid tissues, such as peripheral lymph nodes (PLN), mesenteric lymph nodes (MLN), and Peyer's patches (PP). The initial step in this process is the binding of lymphocytes to high endothelial venules (HEV), and in the case of trafficking of cells to the PLN, it is required that they bear the L-selectin surface receptor. Using a chimeric protein, combining the extracellular domains of L-selectin with a human immunoglobulin (Ig) G1 Fc region (L-selectin-IgG), we have probed the expression of ligands for this receptor on HEV and in cell lysates. Two sulfated glycoproteins of 50 and 90 kD have been identified in lysates from PLN and MLN, but not PP. Here we show that the 50-kD molecule is secreted in organ cultures in vitro and is present in the blood of normal animals. Indeed, normal serum inhibits lymphocyte binding to HEV by approximately 50% in an in vitro assay. This inhibitory activity can be removed by passage of the serum over an L-selectin-IgG column and has a molecular mass of approximately 50 kD. We speculate on the possible reasons for secretion of a homing receptor ligand.

High affinity binding of the leucocyte adhesion molecule L-selectin to 3'-sulphated-Le(a) and -Le(x) oligosaccharides and the predominance of sulphate in this interaction demonstrated by binding studies with a series of lipid-linked oligosaccharides

The binding of the leucocyte adhesion molecule L-selectin has been investigated toward several structurally defined lipid-linked oligosaccharides immobilized on silica gel chromatograms or plastic wells. In both assay systems the 3'-sulphated Le(a)/Le(x) type tetrasaccharides [formula: see text] were more strongly bound than 3'-sialyl analogues. A considerable binding was observed to the 3'-sulphated oligosaccharide backbone in the absence of fucose but not to a 3'-sialyl analogue or fuco-oligosaccharide analogues lacking sulphate or sialic acid. Affinity for other sulphated saccharides: 3'-sulphoglucuronyl neolactotetraosyl ceramide and glycolipids with sulphate 3'-linked to terminal or sub-terminal galactose or N-acetylgalactosamine was detected in the chromatogram assay only. These studies, together with earlier reports that L-selectin binding to endothelium is inhibited by sulphatide, highlight the relative importance of sulphate in the adhesive specificity of this protein.

Identification of an E-selectin region critical for carbohydrate recognition and cell adhesion

E-selectin elicits cell adhesion by binding to the cell surface carbohydrate, sialyl Lewis X (sLe(x)). We evaluated the effects of mutations in the E-selectin lectin domain on the binding of a panel of anti-E-selectin mAbs and on the recognition of immobilized sLe(x) glycolipid. Functional residues were then superimposed onto a three-dimensional model of the E-selectin lectin domain. This analysis demonstrated that the epitopes recognized by blocking mAbs map to a patch near the antiparallel beta sheet derived from the NH2 and COOH termini of the lectin domain and two adjacent loops. Mutations that affect sLe(x) binding map to this same region. These results thus define a small region of the E-selectin lectin domain that is critical for carbohydrate recognition.

Leukocyte-endothelial cell interactions

The movement of leukocytes from the blood circulation into organized lymphoid tissues or sites of inflammation requires cooperative interactions between signaling and adhesion molecules. Selectins mediate the initial rolling contacts of leukocytes with the endothelium. Following leukocyte activation, integrins strengthen adhesion and then direct migration beneath the endothelium. Unique combinations of signaling and adhesion molecules may regulate the subsets of leukocytes that are recruited into specific tissues.

Soluble L-selectin is present in human plasma at high levels and retains functional activity

L-selectin expressed by granulocytes, lymphocytes, and monocytes is responsible for initial leukocyte attachment to inflamed endothelium and high endothelial venules of peripheral lymph nodes. After leukocyte activation in vitro, L-selectin is rapidly shed from the cell surface. In this study, shed L-selectin (sL-selectin) from both lymphocytes and neutrophils was demonstrated to be present in high levels in human plasma by Western blot analysis and using a quantitative ELISA. In serum from normal human blood donors, a mean sL-selectin level of 1.6 +/- 0.8 micrograms/ml (n = 63) was found by ELISA. In addition, semipurified sL-selectin from plasma inhibited L-selectin-specific attachment of lymphocytes to cytokine-activated endothelium in a dose-dependent manner. L-selectin-dependent leukocyte attachment was completely inhibited at sL-selectin concentrations of 8-15 micrograms/ml, while physiological concentrations of sL-selectin caused a small but consistent inhibition of lymphocyte attachment. sL-selectin in plasma also inhibited anti-L-selectin mAb (2-5 micrograms/ml) binding to the surface of leukocytes. Interestingly, one epitope present within the EGF-like domain of L-selectin was lost in sL-selectin, suggesting a conformational change in the structure of the receptor after shedding. The presence of serum sL-selectin with functional activity indicates a potential role for sL-selectin in the regulation of leukocyte attachment to endothelium.

Expression of vascular adhesion molecules in inflammatory bowel disease

The expression of the vascular adhesion molecules ELAM-1 (endothelial leukocyte adhesion molecule 1) and VCAM-1 (vascular cell adhesion molecule 1) was evaluated in colonic mucosa of patients with inflammatory bowel disease and normal controls by immunocytochemistry. VCAM-1 was found to be constitutively expressed in lymphoid aggregates in normal colonic mucosa and was not significantly enhanced or altered in distribution in mucosa of patients with inflammatory bowel disease regardless of the activity of the inflammatory process. In contrast, ELAM-1 was not detected by these techniques in normal colonic mucosa (n = 11) or in colonic mucosa of patients with inflammatory bowel disease which was either uninvolved or quiescent (n = 30). However, high levels of ELAM-1 were consistently found on endothelial surfaces in association with active inflammation in affected areas of colonic mucosa in patients with either ulcerative colitis (n = 27) or Crohn's colitis (n = 9). In addition, ELAM-1 appeared to be present within neutrophils which had migrated into crypt abscesses in affected mucosa. Similar analysis was carried out in the cotton-top tamarin (CTT), a primate that experiences an idiopathic chronic diffuse colitis resembling human ulcerative colitis. Although anti-human VCAM-1 antibodies did not react with the CTT, anti-human ELAM-1 stained endothelial surfaces in mucosal biopsies from CTT with active colitis. No ELAM-1 was identified in mucosa of CTT in which colitis activity was quiescent. Thus ELAM-1 is expressed on colonic endothelial surfaces in association with inflammation and may play an important role in facilitating leukocyte migration into sites of active IBD involvement.

Evidence that mannose recognition by splenic sinusoidal cells plays a role in the splenic entry of lymphocytes

Previous studies demonstrated that mannan is a potent inhibitor of splenic entry of lymphocytes and mediates its inhibitory effect at an unidentified site in the spleen rather than acting directly on lymphocytes. This report describes the in vivo site of action of mannan. In vivo localization studies with fluoresceinated preparations of mannan (Fl-mannan) and a mannose-6-phosphate-containing yeast phosphomannan monoester core from P. holstii exopolysaccharide (Fl-PPME) demonstrated that the polysaccharide specifically localize in the splenic marginal sinuses in cells with a dendritic morphology termed splenic sinusoidal cells (SSC). Uptake of the polysaccharides by SSC was mediated by a mannan-specific receptor which was saturable and of high avidity. Several lines of evidence suggested that mannan uptake by SSC inhibited splenic entry of lymphocytes. First, the ability of SSC to bind Fl-mannan and Fl-PPME closely paralleled the ability of these polysaccharides to inhibit splenic entry of lymphocytes. In fact, doses of mannan and PPME which would saturate SSC mannan receptors completely blocked splenic entry of lymphocytes. Second, SSC are situated at the initial entry point of lymphocytes into spleen and passage of lymphocytes through the SSC region of spleen was profoundly inhibited by mannan. Finally, direct evidence for adhesion between lymphocytes and SSC was obtained with spleen cell suspensions where clustering between Fl-mannan labeled SSC and lymphocytes was observed. Collectively, these data indicate that mannan (and PPME) inhibit splenic entry of lymphocytes by interacting with SSC, cell which play a critical role in the entry of lymphocytes into the spleen. Whether mannan-specific receptors on SSC directly mediate lymphocyte-SSC adhesion or play on indirect role in modifying lymphocyte migration requires further investigation.

Identification of a specific glycoprotein ligand for P-selectin (CD62) on myeloid cells

P-selectin (CD62, GMP-140, PADGEM), a Ca(2+)-dependent lectin on activated platelets and endothelium, functions as a receptor for myeloid cells by interacting with sialylated, fucosylated lactosaminoglycans. P-selectin binds to a limited number of protease-sensitive sites on myeloid cells, but the protein(s) that carry the glycans recognized by P-selectin are unknown. Blotting of neutrophil or HL-60 cell membrane extracts with [125I]P-selectin and affinity chromatography of [3H]glucosamine-labeled HL-60 cell extracts were used to identify P-selectin ligands. A major ligand was identified with an approximately 250,000 M(r) under nonreducing conditions and approximately 120,000 under reducing conditions. Binding of P-selectin to the ligand was Ca2+ dependent and was blocked by mAbs to P-selectin. Brief sialidase digestion of the ligand increased its apparent molecular weight; however, prolonged digestion abolished binding of P-selectin. Peptide:N-glycosidase F treatment reduced the apparent molecular weight of the ligand by approximately 3,000 but did not affect P-selectin binding. Western blot and immunodepletion experiments indicated that the ligand was not lamp-1, lamp-2, or L-selectin, which carry sialyl Le(x), nor was it leukosialin, a heavily sialylated glycoprotein of similar molecular weight. The preferential interaction of the ligand with P-selectin suggests that it may play a role in adhesion of myeloid cells to activated platelets and endothelial cells.

Cell adhesion in the stromal regulation of haemopoiesis

Haemopoiesis occurs in close physical contact with elements of the bone marrow stroma. This review examines some adhesion molecules which are expressed in haemopoietic environments. These molecules may act to maintain cell contact between haemopoietic progenitors and the cellular and extracellular matrix elements of the marrow stroma. A review is undertaken of several families of adhesion molecules and their ligands, including integrins, CD44, LEC-CAMs, CAMs and variant glycosylation patterns expressed by haemopoietic cells. Both CD44 and integrin receptors have been implicated in mediating important cellular interactions within the haemopoietic system and the significance of these and other molecules in haemopoiesis is discussed.

An endothelial ligand for L-selectin is a novel mucin-like molecule

The adhesive interaction between circulating lymphocytes and the high endothelial venules (HEV) of lymph nodes (LN) is mediated by lymphocyte L-selectin, a member of the selectin family of cell adhesion proteins. Previous work has identified a sulfated 50 kd glycoprotein (Sgp50) as an HEV ligand for L-selectin. We now report the purification of this glycoprotein and the utilization of the derived N-terminal amino acid sequence to clone a cDNA. The predicted sequence reveals a novel, mucin-like molecule containing two serine/threonine-rich domains. The mRNA encoding this glycoprotein is preferentially expressed in LN. Antibodies against predicted peptides immunoprecipitate Sgp50 and stain the apical surface of LN HEV. These results thus define a tissue-specific mucin-like endothelial glycoprotein that appears to function as a scaffold that presents carbohydrates to the L-selectin lectin domain.

Selectins: cell surface lectins which mediate the binding of leukocytes to endothelial cells

The selectins are the most recently identified family of cell adhesion molecules. The three known members of this family (L-, E- and P-selectin) mediate the binding of leukocytes to endothelial cells and are involved in the homing of lymphocytes to lymph nodes, as well as the extravasation of neutrophilic granulocytes into inflamed tissues. The lectin character of these cell adhesion molecules (CAMs) makes the selectin protein family unique among all known CAM families. The review will summarize present knowledge about the structural organization, the ligands identified (carbohydrates and glycoproteins) and the different regulation mechanisms of the cell surface activity of the three selectins.

The three members of the selectin receptor family recognize a common carbohydrate epitope, the sialyl Lewis(x) oligosaccharide

The selectins (lectin-EGF-complement binding-cell adhesion molecules [LEC-CAMs]) are a family of mammalian receptors implicated in the initial interactions between leukocytes and vascular endothelia, leading to lymphocyte homing, platelet binding, and neutrophil extravasation. The three known selectins, L-selectin (leukocyte adhesion molecule-1 [LECAM-1]), E-selectin (endothelial-leukocyte adhesion molecule-1 [ELAM-1]), and P-selectin (GMP-140) share structural features that include a calcium-dependent lectin domain. The sialyl Lewis(x) carbohydrate epitope has been reported as a ligand for both E- and P-selectins. Although L-selectin has been demonstrated to bind to carbohydrates, structural features of potential mammalian carbohydrate ligand(s) have not been well defined. Using an ELISA developed with a sialyl Lewis(x)-containing glycolipid and an E-selectin-IgG chimera, we have demonstrated the direct binding of the L-selectin-IgG chimera to sialyl Lewis(x). This recognition was calcium dependent, and could be blocked by Mel-14 antibody but not by other antibodies. Recognition was confirmed by the ability of cells expressing the native L-selectin to adhere to immobilized sialyl Lewis(x). These data suggest that the sialyl Lewis(x) oligosaccharide may form the basis of a recognition domain common to all three selectins.