Hematopoietic defects in mice lacking the sialomucin CD34

Although the pluripotent hematopoietic stem cell can only be definitively identified by its ability to reconstitute the various mature blood lineages, a diversity of cell surface antigens have also been specifically recognized on this subset of hematopoietic progenitors. One such stem cell-associated antigen is the sialomucin CD34, a highly O-glycosylated cell surface glycoprotein that has also been shown to be expressed on all vascular endothelial cells throughout murine embryogenesis as well as in the adult. The functional significance of CD34 expression on hematopoietic progenitor cells and developing blood vessels is unknown. To analyze the involvement of CD34 in hematopoiesis, we have produced both embryonic stem (ES) cells and mice that are null for the expression of this mucin. Analysis of yolk saclike hematopoietic development in embryoid bodies derived from CD34-null ES cells showed a significant delay in both erythroid and myeloid differentiation that could be reversed by transfection of the mutant ES cells with CD34 constructs expressing either a complete or truncated cytoplasmic domain. Measurements of colony-forming activity of hematopoietic progenitor cells derived from yolk sacs or fetal livers isolated from CD34-null embryos also showed a decreased number of these precursor cells. In spite of these diminished embryonic hematopoietic progenitor numbers, the CD34-null mice developed normally, and the hematopoietic profile of adult blood appeared typical. However, the colony-forming activity of hematopoietic progenitors derived from both bone marrow and spleen is significantly reduced in adult CD34-deficient animals, and these CD34-deficient progenitors also appear to be unable to expand in liquid cultures in response to hematopoietic growth factors. Even with these apparent progenitor cell deficiencies, CD34-null animals showed kinetics of erythroid, myeloid, and platelet recovery after sublethal irradiation that are indistinguishable from wild-type mice. These data strongly suggest that CD34 plays an important role in the formation of progenitor cells during both embryonic and adult hematopoiesis. However, the hematopoietic sites of adult CD34-deficient mice may still have a significant reservoir of progenitor cells that allows for normal recovery after nonmyeloablative peripheral cell depletion.

CD34+ endothelial cell lines derived from murine yolk sac induce the proliferation and differentiation of yolk sac CD34+ hematopoietic progenitors

Embryonic hematopoiesis is initiated in part in the blood islands of the yolk sac. Previous confocal microscopic analysis has shown that the CD34 antigen, a mucin-like cell surface glycoprotein that is expressed by hematopoietic progenitors and all endothelial cells of the adult and embryo, is also found on a subset of luminal hematopoietic-like cells in the yolk sac blood islands as well as on the vascular endothelium lining these early hematopoietic locations. We show here that, as in all other hematopoietic sites thus far examined, immunoaffinity-purified CD34+ nonadherent cells from murine yolk sacs contain the vast majority of erythroid and myeloid progenitor cell colony forming activity. To examine the developmental interactions between these CD34+ hematopoietic progenitor cells of the yolk sac and the CD34+ yolk sac endothelium, we have immunaffinity-purified adherent endothelial cells from day 10.5 yolk sacs using CD34 antiserum and produced cell lines by transformation with a retrovirus expressing the polyoma middle T antigen. Analysis of these cell lines for CD34, von Willebrand's factor, FLK 1 and FLT 1 expression, and capillary growth in Matrigel indicates that they appear to be endothelial cells, consistent with their original phenotype in vivo. Coculture of yolk sac CD34+ hematopoietic cells on these endothelial cell lines results in up to a 60-fold increase in total hematopoietic cell number after approximately 8 days. Analysis of these expanded hematopoietic cells showed that the majority were of the monocyte/macrophage lineage. In addition, examination of the cultures showed the rapid formation of numerous cobblestone areas, a previously described morphologic entity thought to be representative of early pluripotential stem cells. Scrutiny of the ability of these endothelial cell lines to expand committed progenitor cells showed up to a sixfold increase in erythroid and myeloid colony-forming cells after 3 to 6 days in culture, consistent with the notion that these embryonic endothelial cells mediate the expansion of these precursor cells. Polymerase chain reaction analyses showed that most of the cell lines produce FLK-2/FLT-3 ligand, stem cell factor, macrophage colony-stimulating factor, leukemia-inhibitory factor, and interleukin-6 (IL-6), whereas there is a generally low or not measurable production of granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, IL-1, IL-3, transforming growth factor beta-1, erythropoietin, or thrombopoietin. The output of mature hematopoietic cells from these cocultures can be modified to include an erythroid population by the addition of exogenous erythropoietin. These data suggest that endothelial cell lines derived form the yolk sac provide an appropriate hematopoietic environment for the expansion and differentiation of yolk sac progenitor cells into at least the myeloid and erythroid lineages.

Selectin-carbohydrate interactions and the initiation of the inflammatory response

The orderly migration of various white blood cell types to inflammatory sites is a highly regulated process that involves a diversity of adhesion and signaling molecules. This cellular influx is initiated by relatively low affinity interactions that allow for leukocytes to roll along the vascular surface. This rolling phenomenon is mediated by adhesive interactions between lectin containing adhesion molecules, termed selectins, on both the vascular endothelium and leukocytes, and carbohydrate ligands immobilized on mucin-like scaffolds. This adhesion allows for a rapid recognition of various cell types under the conditions of vascular flow, with the result that inflammatory cells are specifically decelerated adjacent to sites of inflammation. This review focuses on the various biochemical aspects of the interactions between the selectins and their cognate carbohydrate ligands, with an emphasis on the importance of these adhesive events to the inflammatory response.

The sialomucin CD34 is expressed on hematopoietic cells and blood vessels during murine development

The processes of angiogenesis and hematopoiesis require a high degree of coordination during embryogenesis. Whereas much is understood about the development of the vascular system in avian embryos, little information has been attained in mammals, predominantly because there are no specific markers for either blood vessels or hematopoietic cells in any developing mammalian system. We have recently shown that murine CD34 (mCD34) is expressed on the vascular endothelium in all organs and tissues of the adult mouse as well as on a small percentage of presumably hematopoietic stem cells in the bone marrow and fetal liver. Here we show that mCD34 is also expressed on the endothelium of blood vessels and on a subset of hematopoietic-like cells throughout murine development. mCD34 is first observed on the yolk sac endothelium of day 7.5 embryos and on a subset of hematopoietic cells within these yolk sacs. mCD34 expression is maintained on vessels and hematopoietic cells in all organs and tissues throughout embryogenesis. In addition, mCD34 is localized on growth conelike filopodial processes that appear at the budding edge of newly sprouted capillaries. Double staining of capillaries for mCD34 and laminin shows that these growth conelike processes seem to be free of laminin, whereas the formed capillaries seem to be coated with this extracellular matrix protein. Analysis of vessels in developing brain shows that these filopodial processes seem to be directed toward the ventricular epithelium, a previously described site of vascular endothelial growth factor synthesis. Finally, we show that the vascular structures of developing murine embryoid bodies also express mCD34. These data suggest that mCD34 is a useful marker for the analysis of the development of the blood vascular system in murine embryos.

Global vascular expression of murine CD34, a sialomucin-like endothelial ligand for L-selectin

Extravasation of leukocytes into organized lymphoid tissues and into sites of inflammation is critical to immune surveillance. Leukocyte migration to peripheral lymph nodes (PLN), mesenteric lymph nodes (MLN) and Peyer's patches (PP) depends on L-selectin, which recognizes carbohydrate-bearing, sialomucin-like endothelial cell surface glycoproteins. Two of these ligands have been identified at the molecular level. One is the potentially soluble mucin, GlyCAM 1, which is almost exclusively produced by high endothelial venules (HEV) of PLN and MLN. The second HEV ligand for L-selectin is the membrane-bound sialomucin CD34. Historically, this molecule has been successfully used to purify human pluripotent bone marrow stem cells, and limited data suggest that human CD34 is present on the vascular endothelium of several organs. Here we describe a comprehensive analysis of the vascular expression of CD34 in murine tissues using a highly specific antimurine CD34 polyclonal antibody. CD34 was detected on vessels in all organs examined and was expressed during pancreatic and skin inflammatory episodes. A subset of HEV-like vessels in the inflamed pancreas of nonobese diabetic (NOD) mice are positive for both CD34 and GlyCAM 1, and bind to an L-selectin/immunoglobulin G (IgG) chimeric probe. Finally, we found that CD34 is present on vessels of deafferentiated PLN, despite the fact that these vessels are no longer able to interact with L-selectin or support lymphocyte binding in vitro or trafficking in vivo. Our data suggest that the regulation of posttranslational carbohydrate modifications of CD34 is critical in determining its capability to act as an L-selectin ligand. Based on its ubiquitous expression, we propose that an appropriately glycosylated form of vascular CD34 may act as a ligand for L-selectin-mediated leukocyte trafficking to both lymphoid and nonlymphoid sites.

A liquid-phase binding analysis for L-selectin. A strong dependency on highly clustered sulfate groups

L-selectin, together with E- and P-selectins, forms a newer group of cell adhesion molecules which are believed to interact with carbohydrate ligands [Lasky, L. A., Singer, M. S., Yednoch, T. A., Dowbenko, D., Fennie, C., Rodriguez, H., Nguyen, T., Stachel, S. & Rosen, S. D. (1989) Cell 56, 1045-1055]. Using radiolabeled fucoidan as a reference ligand, we have developed a new liquid-phase microcentrifugation assay where fine differences in binding affinity can be compared accurately. We found that glucan sulfates strongly inhibited the binding of fucoidan by murine L-selectin-IgG chimera. The efficacy of inhibition is extremely dependent on the size (up to 12 kDa) and the sulfate density (up to two sulfate groups/glucose molecule) of the glucan sulfates. The nature of the inter-glucose linkages is also important. These data suggest that the binding by L-selectin prefers certain clustering and proper spatial arrangement of the anionic groups.

Expression of GlyCAM-1, an endothelial ligand for L-selectin, is affected by afferent lymphatic flow

The interaction of naive, L-selectin-bearing lymphocytes with counterreceptors on the surface of high endothelial venules (HEV) is the initial step in the extravasation of these cells from the bloodstream into the peripheral lymph node. Recently, two sulfated glycoprotein ligands, 50 and 90 kDa, respectively, have been identified as ligands for L-selectin using an L-selectin-IgG chimera. cDNA cloning of one of these molecules, the 50-kDa sulfated glycoprotein (glycosylation-dependent cell adhesion molecule 1 [GlyCAM-1]), has shown it to be a mucinlike scaffold that presents a carbohydrate ligand(s) to the lectin domain of L-selectin. Herein, we analyze the factors that might regulate the expression of these ligands. Ligation of afferent lymphatics results in a complete loss of the mRNA for GlyCAM-1. In addition, L-selectin-mediated adhesion, as inferred by binding of an L-selectin-IgG chimera, is also lost on interruption of afferent flow. It thus appears that a soluble and/or cellular component(s) of afferent lymph regulates the expression of GlyCAM-1 mRNA and the resultant HEV adhesiveness for lymphocytes.

Expression of GlyCAM-1, an endothelial ligand for L-selectin, is affected by afferent lymphatic flow

The interaction of naive, L-selectin-bearing lymphocytes with counterreceptors on the surface of high endothelial venules (HEV) is the initial step in the extravasation of these cells from the bloodstream into the peripheral lymph node. Recently, two sulfated glycoprotein ligands, 50 and 90 kDa, respectively, have been identified as ligands for L-selectin using an L-selectin-IgG chimera. cDNA cloning of one of these molecules, the 50-kDa sulfated glycoprotein (glycosylation-dependent cell adhesion molecule 1 [GlyCAM-1]), has shown it to be a mucinlike scaffold that presents a carbohydrate ligand(s) to the lectin domain of L-selectin. Herein, we analyze the factors that might regulate the expression of these ligands. Ligation of afferent lymphatics results in a complete loss of the mRNA for GlyCAM-1. In addition, L-selectin-mediated adhesion, as inferred by binding of an L-selectin-IgG chimera, is also lost on interruption of afferent flow. It thus appears that a soluble and/or cellular component(s) of afferent lymph regulates the expression of GlyCAM-1 mRNA and the resultant HEV adhesiveness for lymphocytes.

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.

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.

Cloning of a rat homologue of mouse GlyCAM 1 reveals conservation of structural domains

Recently we described the isolation of a mouse cDNA clone encoding a mucin-like endothelial glycoprotein that appears to function as an adhesive ligand for L selectin. This ligand has been named GlyCAM 1 (Gly-cosylation-dependent Cell Adhesion Molecule 1) because its adhesive interactions with the L selectin lectin domain require that the GlyCAM 1 polypeptide chain be appropriately modified with carbohydrates. These carbohydrate modifications include the addition of sialic acid as well as sulfate residues to O-linked carbohydrate side chains that are clustered in two serine/threonine-rich domains of the mucin. An additional interesting structure that may have relevance to the association of GlyCAM 1 with the lumenal surface of the endothelium was a potential amphipathic helix at the C terminus of the glycoprotein. In order to examine the importance of the postulated O-linked domains as well as the potential amphipathic helix, we have cloned the rat homologue of GlyCAM 1. The sequence of this clone reveals a serine/threonine-rich protein that is highly homologous with the mouse GlyCAM 1. As was found for the mouse GlyCAM 1, the rat homologue shows a clustering of these potential O-linked carbohydrate acceptors in two domains of the protein. Interestingly, many of the serines and threonines are found to be spaced identically in the two homologues, consistent with the possibility that both density and position of the O-linked side chains may be important for appropriate L selectin-mediated adhesion. In support of its postulated functional importance, the C-terminal potential amphipathic helix is conserved in the rat homologue. Finally, immunoprecipitation analysis of [35S]sulfate-labeled rat lymph nodes with either a mouse L selectin IgG chimera or a peptide antiserum directed against a relatively conserved portion of mouse GlyCAM 1 demonstrates a approximately 45-kDa sulfated ligand in rat lymph nodes that is analogous to that previously described for mouse lymph nodes.

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.

Structure and chromosomal localization of the murine gene encoding GLYCAM 1. A mucin-like endothelial ligand for L selectin

We recently described the molecular cloning of a murine cDNA encoding an endothelial cell surface ligand for the leukocyte adhesion molecule, L Selectin (Lasky, L. A., Singer, M., Dowbenko, D., Ima, Y., Henzel, W., Grimley, C., Gennie, C., Gillett, N., Watson, S., and Rosen, S. D (1992) Cell 69, 927-938). This glycoprotein ligand was found to resemble mucins in that it contained a large percentage of serine and threonine residues that were apparently O-glycosylated. At least one of the O-linked carbohydrates found on this endothelial ligand interacts with the lectin domain of L Selectin. These data suggest that this endothelial ligand is an adhesion molecule that accomplishes cell binding by presenting carbohydrate(s) to the lectin domain of L Selectin, and the name GLYCAM 1 (GLY-cosylation-dependent Cell Adhesion Molecule 1) has been proposed. In this paper we describe the genomic structure and chromosomal localization of this unique Selectin ligand. The gene has been found to be encoded on four separate exons, and it thus differs from the cell surface mucin leukosialin, whose coding region is contained on one exon, but is similar to glycophorin and CD34, other cell surface mucins whose genes are divided into multiple coding exons. While there is some correlation between exon division and protein domain structure, these relationships are not as clear as they are in other genes. The gene encoding GLYCAM 1 was found to map to murine chromosome 15.

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.

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.

Further characterization of the interaction between L-selectin and its endothelial ligands

L-Selectin is a lectin-like receptor on lymphocytes which mediates their attachment to high endothelial venules (HEV) within lymph nodes. Previous work has identified HEV-associated endothelial ligands for L-selectin as sialylated, fucosylated and sulphated glycoproteins of approximately 50 kDa and approximately 90 kDa (Sgp50 and Sgp90). The interaction of L-selectin with these ligands is carbohydrate directed, reflecting the involvement of its amino-terminal, calcium-type lectin domain. It has been reported, and we have confirmed, that anti-Ly22 blocks the adhesive function of L-selectin without reducing its binding to a carbohydrate- based ligand PPME (phosphomannan monoester core from Hansenula hostii). The epitope for this monoclonal antibody depends on the epidermal growth factor (EGF) domain of L-selectin. We demonstrate that anti-Ly22 inhibits the interaction of L-selectin with both of the Sgps, thus establishing that the interaction of L-selectin with HEV can be accounted for by the Sgps. Furthermore, the interaction of trypsin fragments of Sgp50 with L-selectin is inhibitable both by an antibody that maps to the lectin domain and by anti-Ly22. These findings raise the possibility that anti-Ly22 is affecting the function of the lectin domain of L-selectin rather than directly antagonizing the EGF domain. Toward a further characterization of L-selectin's carbohydrate specificity, we show that Sgp50 is partially inactivated by the linkage-specific Newcastle Disease virus sialidase (alpha 2,3 linkage). We additionally demonstrate that a sialyl Lewis x-related tetrasaccharide can interact with L-selectin, as has also been demonstrated for E-selectin and P-selectin.

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.

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.

The complement binding-like domains of the murine homing receptor facilitate lectin activity

The leukocyte homing receptor (HR), the endothelial leukocyte adhesion molecule, and gmp140/platelet activation-dependent granule membrane protein are members of a family of adhesion molecules, termed the lectin cell adhesion molecules (LEC-CAMS) which are unified by a multi-domain structure containing a lectin motif, an epidermal growth factor-like (egf) motif, and variable numbers of a complement binding-like (CB) motif. Previous data have indicated a predominant role for the lectin motif in cell adhesion directed by the LEC-CAMS, although the egf-like domain of the HR may also play a potential role in cell binding. While the role(s) of the CB domains in the LEC-CAMS is currently not understood, they have been hypothesized to act as rigid spacers or stalks for lectin and perhaps, egf domain presentation. In this paper, we analyze the functional characteristics of murine HR-IgG chimeras containing the lectin, lectin plus egf, and lectin plus egf plus CB domains. The Mel 14 mAb, an adhesion blocking antibody which recognizes a conformational determinant in the N-terminus of the HR lectin domain, shows a significantly decreased affinity for a HR construct which lacks the CB motifs, consistent with the possibility that the CB domains are involved with lectin domain structure. In agreement with this conjecture, HR mutants lacking the CB domains show a profound decrease in lectin-specific interaction with the carbohydrate polyphosphomannan ester, suggesting that the changes in Mel 14 affinity for the lectin domain are reflected in lectin functionality. Various assays investigating the interactions between the HR deletion mutants and the peripheral lymph node high endothelium, including cell blocking, immunohistochemical staining, and radioactively labeled ligand binding, all showed that removal of the CB domains results in a lack of HR adhesive function. These results imply that the CB domains of the HR, and, by analogy, the other members of the LEC-CAM family, may play important structural roles involving induction of lectin domain conformation and resultant functionality.

Lectin-like cell adhesion molecule 1 mediates leukocyte rolling in mesenteric venules in vivo

During the inflammatory response, granulocytes and other leukocytes adhere to and emigrate from small venules. Before firm attachment, leukocytes are observed rolling slowly along the endothelium in venules of most tissues accessible to intravital microscopy. The molecular mechanism underlying this early type of leukocyte-endothelial interaction is unknown. Leukocyte rolling was investigated in venules (diameter, 40 microns) of the exposed rat mesentery. Micro-infusion of a recombinant soluble chimera (LEC-IgG) of the murine homing receptor lectin-like cell adhesion molecule 1 (LEC-CAM 1; gp90MEL) into individual venules reduced the number of rolling leukocytes by 89% +/- 2% (mean +/- SEM, n = 20 venules), while a similar CD4 chimera (CD4-IgG) had no effect (inhibition 14% +/- 7%, n = 25). Rolling was also greatly reduced by a polyclonal serum against LEC-CAM 1 (inhibition 84% +/- 3%, n = 35); preimmune serum was ineffective (11% +/- 13% inhibition, n = 28). These findings indicate that LEC-CAM 1 mediates the adhesive interaction underlying leukocyte rolling and thus may play an important role in inflammation and in pathologic conditions involving leukocytes.

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

Lymphocyte attachment to high endothelial venules within lymph nodes is mediated by the peripheral lymph node homing receptor (pnHR), originally defined on mouse lymphocytes by the MEL-14 mAb. The pnHR is a calcium-dependent lectin-like receptor, a member of the LEC-CAM family of adhesion proteins. Here, using a soluble recombinant form of the homing receptor, we have identified an endothelial ligand for the pnHR as an approximately 50-kD sulfated, fucosylated, and sialylated glycoprotein, which we designate Sgp50 (sulfated glycoprotein of 50 kD). Recombinant receptor binding to this lymph node-specific glycoprotein requires calcium and is inhibitable by specific carbohydrates and by MEL-14 mAb. Sialylation of the component is required for binding. Additionally, the glycoprotein is precipitated by MECA-79, an adhesion-blocking mAb reactive with lymph node HEV. A related glycoprotein of approximately 90 kD (designated as Sgp90) is also identified.