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

Compulsive methamphetamine self-administration in the presence of adverse consequences is associated with increased hippocampal mRNA expression of cellular adhesion molecules

Methamphetamine (METH) is a popular but harmful psychostimulant. METH use disorder (MUD) is characterized by compulsive and continued use despite adverse life consequences. METH users experience impairments in learning and memory functions that are thought to be secondary to METH-induced abnormalities in the hippocampus. Recent studies have reported that about 50% of METH users develop MUD, suggesting that there may be differential molecular effects of METH between the brains of individuals who met criteria for addiction and those who did not after being exposed to the drug. The present study aimed at identifying potential transcriptional differences between compulsive and non-compulsive METH self-administering male rats by measuring global gene expression changes in the hippocampus using RNA sequencing. Herein, we used a model of METH self-administration (SA) accompanied by contingent foot-shock punishment. This approach led to the separation of animals into shock-resistant rats (compulsive) that continued to take METH and shock-sensitive rats (non-compulsive) that suppressed their METH intake in the presence of punished METH taking. Rats were euthanized 2 h after the last METH SA plus foot-shock session. Their hippocampi were immediately removed, frozen, and used later for RNA sequencing and qRT-PCR analyses. RNA sequencing analyses revealed differential expression of mRNAs encoding cell adhesion molecules (CAMs) between the two rat phenotypes. qRT-PCR analyses showed significant higher levels of Cdh1, Glycam1, and Mpzl2 mRNAs in the compulsive rats in comparison to non-compulsive rats. The present results implicate altered CAM expression in the hippocampus in the behavioral manifestations of continuous compulsive METH taking in the presence of adverse consequences. Our results raise the novel possibility that altered CAM expression might play a role in compulsive METH taking and the cognitive impairments observed in MUD patients.

Single-Cell Transcriptional Heterogeneity of Lymphatic Endothelial Cells in Normal and Inflamed Murine Lymph Nodes

The lymphatic system plays a crucial role in immunity and lymph nodes (LNs) undergo drastic remodeling during inflammation. Here, we used single-cell RNA sequencing to investigate transcriptional changes in lymphatic endothelial cells (LECs) in LNs draining naïve and inflamed skin. We found that subsets of LECs lining the different LN sinuses responded individually to skin inflammation, suggesting that they exert distinct functions under pathological conditions. Among the genes dysregulated during inflammation, we confirmed an up-regulation of CD200 in the LECs lining the subcapsular sinus floor with a possible function in immune regulation. Furthermore, by in silico analysis, we predicted numerous possible interactions of LECs with diverse immune cells in the LNs and found similarities in the transcriptional changes of LN LECs in different skin inflammation settings. In summary, we provide an in-depth analysis of the transcriptional landscape of LN LECs in the naïve state and in skin inflammation.

A Single-Cell Transcriptional Roadmap of the Mouse and Human Lymph Node Lymphatic Vasculature

Single-cell transcriptomics promise to revolutionize our understanding of the vasculature. Emerging computational methods applied to high-dimensional single-cell data allow integration of results between samples and species and illuminate the diversity and underlying developmental and architectural organization of cell populations. Here, we illustrate these methods in the analysis of mouse lymph node (LN) lymphatic endothelial cells (LEC) at single-cell resolution. Clustering identifies five well-delineated subsets, including two medullary sinus subsets not previously recognized as distinct. Nearest neighbor alignments in trajectory space position the major subsets in a sequence that recapitulates the known features and suggests novel features of LN lymphatic organization, providing a transcriptional map of the lymphatic endothelial niches and of the transitions between them. Differences in gene expression reveal specialized programs for (1) subcapsular ceiling endothelial interactions with the capsule connective tissue and cells; (2) subcapsular floor regulation of lymph borne cell entry into the LN parenchyma and antigen presentation; and (3) pathogen interactions and (4) LN remodeling in distinct medullary subsets. LEC of the subcapsular sinus floor and medulla, which represent major sites of cell entry and exit from the LN parenchyma respectively, respond robustly to oxazolone inflammation challenge with enriched signaling pathways that converge on both innate and adaptive immune responses. Integration of mouse and human single-cell profiles reveals a conserved cross-species pattern of lymphatic vascular niches and gene expression, as well as specialized human subsets and genes unique to each species. The examples provided demonstrate the power of single-cell analysis in elucidating endothelial cell heterogeneity, vascular organization, and endothelial cell responses. We discuss the findings from the perspective of LEC functions in relation to niche formations in the unique stromal and highly immunological environment of the LN.

Single-cell mapping reveals new markers and functions of lymphatic endothelial cells in lymph nodes

Lymph nodes (LNs) are highly organized secondary lymphoid organs that mediate adaptive immune responses to antigens delivered via afferent lymphatic vessels. Lymphatic endothelial cells (LECs) line intranodal lymphatic sinuses and organize lymph and antigen distribution. LECs also directly regulate T cells, mediating peripheral tolerance to self-antigens, and play a major role in many diseases, including cancer metastasis. However, little is known about the phenotypic and functional heterogeneity of LN LECs. Using single-cell RNA sequencing, we comprehensively defined the transcriptome of LECs in murine skin-draining LNs and identified new markers and functions of distinct LEC subpopulations. We found that LECs residing in the subcapsular sinus (SCS) have an unanticipated function in scavenging of modified low-density lipoprotein (LDL) and also identified a specific cortical LEC subtype implicated in rapid lymphocyte egress from LNs. Our data provide new, to our knowledge, insights into the diversity of LECs in murine LNs and a rich resource for future studies into the regulation of immune responses by LN LECs.

L-selectin: A Major Regulator of Leukocyte Adhesion, Migration and Signaling

L-selectin (CD62L) is a type-I transmembrane glycoprotein and cell adhesion molecule that is expressed on most circulating leukocytes. Since its identification in 1983, L-selectin has been extensively characterized as a tethering/rolling receptor. There is now mounting evidence in the literature to suggest that L-selectin plays a role in regulating monocyte protrusion during transendothelial migration (TEM). The N-terminal calcium-dependent (C-type) lectin domain of L-selectin interacts with numerous glycans, including sialyl Lewis X (sLe^x) for tethering/rolling and proteoglycans for TEM. Although the signals downstream of L-selectin-dependent adhesion are poorly understood, they will invariably involve the short 17 amino acid cytoplasmic tail. In this review we will detail the expression of L-selectin in different immune cell subsets, and its influence on cell behavior. We will list some of the diverse glycans known to support L-selectin-dependent adhesion, within luminal and abluminal regions of the vessel wall. We will describe how each domain within L-selectin contributes to adhesion, migration and signal transduction. A significant focus on the L-selectin cytoplasmic tail and its proposed contribution to signaling via the ezrin-radixin-moesin (ERM) family of proteins will be outlined. Finally, we will discuss how ectodomain shedding of L-selectin during monocyte TEM is essential for the establishment of front-back cell polarity, bestowing emigrated cells the capacity to chemotax toward sites of damage.

Biological functions of fucose in mammals

Fucose is a 6-deoxy hexose in the l-configuration found in a large variety of different organisms. In mammals, fucose is incorporated into N-glycans, O-glycans and glycolipids by 13 fucosyltransferases, all of which utilize the nucleotide-charged form, GDP-fucose, to modify targets. Three of the fucosyltransferases, FUT8, FUT12/POFUT1 and FUT13/POFUT2, are essential for proper development in mice. Fucose modifications have also been implicated in many other biological functions including immunity and cancer. Congenital mutations of a Golgi apparatus localized GDP-fucose transporter causes leukocyte adhesion deficiency type II, which results in severe developmental and immune deficiencies, highlighting the important role fucose plays in these processes. Additionally, changes in levels of fucosylated proteins have proven as useful tools for determining cancer diagnosis and prognosis. Chemically modified fucose analogs can be used to alter many of these fucose dependent processes or as tools to better understand them. In this review, we summarize the known roles of fucose in mammalian physiology and pathophysiology. Additionally, we discuss recent therapeutic advances for cancer and other diseases that are a direct result of our improved understanding of the role that fucose plays in these systems.

Structure, Function and Gene Expression of Epithelial Mucins

In this review the main characteristics, i.e., structure, function and gene expression, of the different mucins are discussed. Mucin-type molecules consist of a core protein moiety (apomucin) where a number of carbohydrate chains are attached to serines and threonines by glycosidic bonds. O-linked carbohydrates form up to 80% of the molecule and the length of the glucidic side chains varies from one to more than 20 residues. At least eight mucin-like genes have been isolated so far, and the main characteristic is the presence of a central domain composed of a variable number of “tandem repeats”. The sequence homology of the central domain among the different members of the mucin-type family is limited, indicating that this internal domain is unique for each mucin. Thanks to the integrated results of genetic, immunological and biochemical studies, it is now possible to identify eight apomucin genes, namely MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, MUC6 and MUC7. MUC1 is the best characterized mucin and it is expressed on the apical surface of most polarized epithelial cells. The MUC1 gene has been cloned and sequenced. The MUC2 gene encodes a typical secretory gel-forming mucin which represents the predominant form in human intestinal and colon tissues. Another intestinal mucin is MUC3. The MUC4, MUC5AC and MUC5B genes have been isolated from a bronchial tissue cDNA library. The MUC4 and MUC5AC genes are mainly expressed in the respiratory tract, in gastric and reproductive mucosa, while MUC5B is highly detectable only in the bronchial glands. The MUC6 gene is expressed by gastric tissue and, recently, MUC7 has been cloned and sequenced using a salivary cDNA library.

Fulfilling Koch's postulates in glycoscience: HCELL, GPS and translational glycobiology

Glycoscience-based research that is performed expressly to address medical necessity and improve patient outcomes is called "translational glycobiology". In the 19th century, Robert Koch proposed a set of postulates to rigorously establish causality in microbial pathogenesis, and these postulates can be reshaped to guide knowledge into how naturally-expressed glycoconjugates direct molecular processes critical to human well-being. Studies in the 1990s indicated that E-selectin, an endothelial lectin that binds sialofucosylated carbohydrate determinants, is constitutively expressed on marrow microvessels, and investigations in my laboratory indicated that human hematopoietic stem cells (HSCs) uniquely express high levels of a specialized glycoform of CD44 called "hematopoietic cell E-/L-selectin ligand" (HCELL) that functions as a highly potent E-selectin ligand. To assess the role of HCELL in directing HSC migration to marrow, a method called "glycosyltransferase-programmed stereosubstitution" (GPS) was developed to custom-modify CD44 glycans to enforce HCELL expression on viable cell surfaces. Human mesenchymal stem cells (MSCs) are devoid of E-selectin ligands, but GPS-based glycoengineering of CD44 on MSCs licenses homing of these cells to marrow in vivo, providing direct evidence that HCELL serves as a "bone marrow homing receptor". This review will discuss the molecular basis of cell migration in historical context, will describe the discovery of HCELL and its function as the bone marrow homing receptor, and will inform on how glycoengineering of CD44 serves as a model for adapting Koch's postulates to elucidate the key roles that glycoconjugates play in human biology and for realizing the immense impact of translational glycobiology in clinical medicine.

Understanding cell homing-based tissue regeneration from the perspective of materials

The triad of cell homing-based tissue engineering.

Glycoengineering of HCELL, the human bone marrow homing receptor: sweetly programming cell migration

The successful clinical implementation of adoptive cell therapeutics, including bone marrow transplantation and other stem cell-based treatments, depends critically on the ability to deliver cells to sites where they are needed. E-selectin, an endothelial C-type lectin, binds sialofucosylated carbohydrate determinants on its pertinent ligands. This molecule is expressed in a constitutive manner on bone marrow and dermal microvascular endothelium, and inducibly on post-capillary venules at all sites of tissue injury. Engagement of E-selectin with relevant ligand(s) expressed on circulating cells mediates initial "tethering/rolling" endothelial adhesive interactions prerequisite for extravasation of blood-borne cells at any target tissue. Most mammalian cells express high levels of a transmembrane glycoprotein known as CD44. A specialized glycoform of CD44 called "Hematopoietic Cell E-/L-selectin Ligand" (HCELL) is a potent E-selectin ligand expressed on human cells. Under native conditions, HCELL expression is restricted to human hematopoietic stem/progenitor cells. We have developed a technology called "Glycosyltransferase-Programmed Stereosubstitution" (GPS) for custom-modifying CD44 glycans to create HCELL on the surface of living cells. GPS-based glycoengineering of HCELL endows cell migration to endothelial beds expressing E-selectin. Enforced HCELL expression targets human mesenchymal stem cell homing to marrow, licensing transendothelial migration without chemokine signaling via a VLA-4/VCAM-1-dependent "Step 2-bypass pathway." This review presents an historical framework of the homing receptor concept, and will describe the discovery of HCELL, its function as the bone marrow homing receptor, and how enforced expression of this molecule via chemical engineering of CD44 glycans could enable stem cell-based regenerative medicine and other adoptive cell therapeutics.

Sulfate migration in oligosaccharides induced by negative ion mode ion trap collision-induced dissociation

Migration of sulfate groups between hydroxyl groups was identified after collision-induced dissociation (CID) of sulfated oligosaccharides in an ion trap mass spectrometer in negative ion mode. Analysis of various sulfated oligosaccharides showed that this was a common phenomenon and was particularly prominent in sulfated oligosaccharides also containing sialic acid. It was also shown that the level of migration was increased when the sulfate was positioned on the flexible areas of the oligosaccharides not involved in the pyranose ring, such as the extra-cyclic C-6 carbon of hexoses or N-acetylhexosamines, or on reduced oligosaccharide. This suggested that migration is dependent on the spatial availability of the sulfate in the ion trap during collision. It is proposed that the migration is initiated when the negatively charged -SO3 (-) residue attached to the oligosaccharide precursor becomes protonated by a CID-induced proton transfer. This is supported by the CID fragmentation of precursor ions depleted of acidic protons such as doubly charged [M - 2H](2-) ions or the sodiated [M + Na - 2H](-) ions of oligosaccharides containing one sulfate and one sialic acid in the same molecule. Compared to the CID fragmentation of their monocharged [M - H](-) ions, no migration was observed in CID of proton depleted precursors. Alternative fragmentation parameters to suppress migration of sulfated oligosaccharides also showed that it was not present when sulfated oligosaccharides were fragmented by HCD (High-Energy C-trap Dissociation) in an Orbitrap mass spectrometer.

TNFα enhances the motility and invasiveness of prostatic cancer cells by stimulating the expression of selective glycosyl- and sulfotransferase genes involved in the synthesis of selectin ligands

Sialyl Lewis x (sLe(x)) plays an important role in cancer metastasis. But, the mechanism for its production in metastatic cancers remains unclear. The objective of current study was to examine the effects of a proinflammatory cytokine on the expression of glycosyltransferase and sulfotransferase genes involved in the synthesis of selectin ligands in a prostate cancer cell line. Androgen-independent human lymph node-derived metastatic prostate cancer cells (C-81 LNCaP), which express functional androgen receptor and mimic the castration-resistant advanced prostate cancer, were used. TNFα treatment of these cells increased their binding to P-, E- and L-selectins, anti-sLe(x) antibody, and anti-6-sulfo-sialyl Lewis x antibody by 12%, 240%, 43%, 248% and 21%, respectively. Also, the expression of C2GnT-1, B4GalT1, GlcNAc6ST3, and ST3Gal3 genes was significantly upregulated. Further treatment of TNFα-treated cells with either anti-sLe(x) antibody or E-selectin significantly suppressed their in vitro migration (81% and 52%, respectively) and invasion (45% and 56%, respectively). Our data indicate that TNFα treatment enhances the motility and invasion properties of LNCaP C-81 cells by increasing the formation of selectin ligands through stimulation of the expression of selective glycosyl- and sulfotransferase genes. These results support the hypothesis that inflammation contributes to cancer metastasis.

Role of c-Abl in L-selectin shedding from the neutrophil surface

L-selectin is a key molecule that participates in neutrophil tethering and subsequent rolling. It is cleaved from the surface of neutrophils activated in the presence of lipopolysaccharides, N-formyl-methionine-leucine-phenylalanine (fMLP), or Interleukin-8 (IL-8). We previously showed that L-selectin is also shed from the neutrophil surface during rolling on sialyl Lewis-x coated surfaces in a force-, ADAM-17 sheddase-, and p38 MAP kinase-dependent manner under flow. c-Abl tyrosine kinase is phosphorylated when L-selectin on the surface of neutrophils is cross-linked with anti-L-selectin antibodies. Here, we study the effect of c-Abl inhibition on L-selectin shedding from primary human neutrophils in static conditions following exposure to fMLP, IL-8, and hypotonic buffer and under flow through sialyl Lewis-x coated microtubes. Results indicate that c-Abl inhibition by STI571 significantly affects neutrophil adhesion via L-selectin, by decreasing the average rolling velocity and increasing the flux of rolling cells. The change in surface receptor expression was verified by flow cytometry. Interestingly, other forms of L-selectin shedding induced by fMLP, IL-8 or osmotic swelling were unaffected by STI571 treatment. These findings implicate the c-Abl signaling molecule in regulating L-selectin mechanical shedding in response to shear stress, setting this type of signaling apart from those triggered by the presence of a hypotonic environment, fMLP, or IL-8. This study sheds light on the role of c-Abl in neutrophil adhesion not previously reported in the literature.

Expression of α2,6-sialic acid-containing and Lewis-active glycolipids in several types of human ovarian carcinomas

To identify glycolipid antigens associated with histologically defined types of ovarian carcinomas, we determined the amounts of α2,6-sialyl and Lewis-active glycolipids, the specific activities of the α2,3- and α2,6-sialyltransferases, and the gene expression of sugar transferases in mucinous and serous cystadenocarcinoma, clear cell adenocarcinoma and endometrioid carcinoma tissues and cell lines derived from them. α2,6-sialyl glycolipid IV(6)NeuAcα-nLc(4)Cer detected with a newly developed monoclonal antibody, Y916, was present in 5/7 serous cystadenocarcinoma cases in relatively higher amounts than those in the other carcinoma tissues. On the other hand, the amounts of Lewis-active glycolipids in serous cystadenocarcinoma tissues were lower than those in the other carcinoma tissues. No correlation was observed between the structures of Lewis glycolipids and the histological classification. The gene expression of α2,3- and α2,6-sialyltransferases and α1,3/4-fucosyltransferase for the synthesis of Lewis-active glycolipids was not positively correlated with the amounts of the respective glycolipids, probably due to the epigenetic regulation of transferases in the overall metabolic pathways for lacto-series glycolipids. However, the amounts of GM3 and GD3 with short carbohydrate chains correlated with the relative intensities of GM3 and GD3 synthase gene expression, respectively. Among ovarian carcinoma-derived cell lines, the serous cystadenocarcinoma-derived ones exhibited a lower frequency of Lewis-active glycolipid expression than the other carcinoma-derived ones, which was similar to that in the respective tissues. Thus, malignancy-related Lewis-active glycolipids were shown to be regulated in different modes in ovarian serous cystadenocarcinomas and the other carcinomas.

Human L-selectin preferentially binds synthetic glycosulfopeptides modeled after endoglycan and containing tyrosine sulfate residues and sialyl Lewis x in core 2 O-glycans

Endoglycan is a mucin-like glycoprotein expressed by endothelial cells and some leukocytes and is recognized by L-selectin, a C-type lectin important in leukocyte trafficking and extravasation during inflammation. Here, we show that recombinant L-selectin and human T lymphocytes expressing L-selectin bind to synthetic glycosulfopeptides (GSPs). These synthetic glycosulfopeptides contain 37 amino acid residues modeled after the N-terminus of human endoglycan and contain one or two tyrosine sulfates (TyrSO(3)) along with a nearby core-2-based Thr-linked O-glycan with sialyl Lewis x (C2-SLe(x)). TyrSO(3) at position Y118 was more critical for binding than at Y97. C2-SLe(x) at T124 was required for L-selectin recognition. Interestingly, under similar conditions, neither L-selectin nor T lymphocytes showed appreciable binding to the sulfated carbohydrate epitope 6-sulfo-SLe(x). P-selectin also bound to endoglycan-based GSPs but with lower affinity than toward GSPs modeled after PSGL-1, the physiological ligand for P- and L-selectin that is expressed on leukocytes. These results demonstrate that TyrSO(3) residues in association with a C2-SLe(x) moiety within endoglycan and PSGL-1 are preferentially recognized by L-selectin.

Sequential enrichment of sulfated glycans by strong anion-exchange chromatography prior to mass spectrometric measurements

Structural characterization of sulfated glycans through mass spectrometry (MS) has been often limited by their low abundance in biological materials and inefficient ionization in the positive-ion mode. Here, we describe a microscale method for sequentially enriching sulfated glycans according to their degree of sulfation. This method is based on modifying the binding ability of strong anion-exchange material through the use of different sodium acetate concentrations, thus enabling fairly selective binding and a subsequent elution of different glycans according to their degree of sulfation. Before this enrichment, the negative charge on the sialic acid, which is commonly associated with such glycans, was eliminated through permethylation that is used to enhance the positive-ion mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS) signal for all glycans. This enrichment approach minimizes competitive ionization between sulfated and neutral glycans, as well as that between sulfated species with a different degree of sulfation. The described method was initially optimized using sulfated oligosaccharide standards, while its potential has been verified for the sulfated N-glycans originated from the bovine thyroid-stimulating hormone (bTSH), a glycoprotein possessing mono- and disulfated N-glycans. This enhancement of the MALDI-MS signal facilitates analysis of some otherwise undetected components.

Bioluminescence-based visualization of CD4 T cell dynamics using a T lineage-specific luciferase transgenic model

BACKGROUND

Rapid clonal expansion of T cells occurs in response to antigenic challenges. The kinetics of the T cell response has previously been described using tissue-based studies performed at defined time points. Luciferase bioluminescence has recently been utilized for non-invasive analysis of in vivo biologic processes in real-time.

RESULTS

We have created a novel transgenic mouse model (T-Lux) using a human CD2 mini-gene to direct luciferase expression specifically to the T cell compartment. T-Lux T cells demonstrated normal homing patterns within the intact mouse and following adoptive transfer. Bioluminescent signal correlated with T cell numbers in the whole body images as well as within specific organ regions of interest. Following transfer into lymphopenic (RAG2-/-) recipients, homeostatic proliferation of T-Lux T cells was visualized using bioluminescent imaging. Real-time bioluminescent analysis of CD4+ T cell antigen-specific responses enabled real-time comparison of the kinetics and magnitude of clonal expansion and contraction in the inductive lymph node and tissue site of antigen injection. T cell expansion was dose-dependent despite the presence of supraphysiologic numbers of OVA-specific OT-II transgenic TCR T-Lux T cells. CD4+ T cells subsequently underwent a rapid (3-4 day) contraction phase in the draining lymph node, with a delayed contraction in the antigen delivery site, with bioluminescent signal diminished below initial levels, representing TCR clonal frequency control.

CONCLUSION

The T-Lux mouse provides a novel, efficient model for tracking in vivo aspects of the CD4+ T cell response to antigen, providing an attractive approach for studies directed at immunotherapy or vaccine design.

Immunotargeting and cloning of two CD34 variants exhibiting restricted expression in adult rat endothelia in vivo

Mapping protein expression of endothelial cells (EC) in vivo is fundamental to understanding cellular function and may yield new tissue-selective targets. We have developed a monoclonal antibody, MAb J120, to a protein expressed primarily in rat lung and heart endothelium. The antigen was identified as CD34, a marker of hematopoietic stem cells and global marker of endothelial cells in human and mouse tissues. PCR-based cloning identified two CD34 variant proteins, full length and truncated, both of which are expressed on luminal endothelial cell plasma membranes (P) isolated from lung. Truncated CD34 predominated in heart P, and neither variant was detected in P from kidney or liver. CD34 in lung was readily accessible to (125)I-J120 inoculated intravenously, and immunohistochemistry showed strong CD34 expression in lung EC. Few microvessels stained in heart and kidney, and no CD34 was detected in vessels of other organs or in lymphatics. We present herein the first complete sequence of a rat CD34 variant and show for the first time that the encoded truncated variant is endogenously expressed on EC in vivo. We also demonstrate that CD34 expression in rat EC, unlike mouse and human, is restricted in its distribution enabling quite specific lung targeting in vivo.

Glycoforms of human endothelial CD34 that bind L-selectin carry sulfated sialyl Lewis x capped O- and N-glycans

Endothelial sialomucin CD34 functions as an L-selectin ligand mediating lymphocyte extravasation only when properly glycosylated to express a sulfated carbohydrate epitope, 6-sulfo sialyl Lewis x (6-sulfo SLe(x)). It is thought that multivalent 6-sulfo SLe(x) expression promotes high-affinity binding to L-selectin by enhancing avidity. However, the reported low amount of 6-sulfo SLe(x) in total human CD34 is inconsistent with this model and prompted us to re-evaluate CD34 glycosylation. We separated CD34 into 2 glycoforms, the L-selectin-binding and nonbinding glycoforms, L-B-CD34 and L-NB-CD34, respectively, and analyzed released O- and N-glycans from both forms. L-B-CD34 is relatively minor compared with L-NB-CD34 and represented less than 10% of total tonsillar CD34. MECA-79, a mAb to sulfated core-1 O-glycans, bound exclusively to L-B-CD34 and this form contained all sulfated and fucosylated O-glycans. 6-Sulfo SLe(x) epitopes occur on core-2 and extended core-1 O-glycans with approximately 20% of total L-B-CD34 O-glycans expressing 6-sulfo SLe(x). N-glycans containing potential 6-sulfo SLe(x) epitopes were also present in L-B-CD34, but their removal did not abolish binding to L-selectin. Thus, a minor glycoform of CD34 carries relatively abundant 6-sulfo SLe(x) epitopes on O-glycans that are important for its recognition by L-selectin.

Elevated expression of L-selectin ligand in lymph node-derived human prostate cancer cells correlates with increased tumorigenicity

Human prostate cancer LNCaP cells including C-33 and C-81 cells were originally derived from the lymph nodes of a patient with metastatic prostate cancer. These two cells were employed for characterization of L-selectin ligand and in vitro tumorigenicity, because they mimic the clinical conditions of early and late-stage human prostate cancer. C-81 cells exhibit higher in vitro migratory and invasive properties as compared with C-33 cells. We find that the L-selectin ligand and mucin glycan-associated MECA-79 epitope were elevated in C-81 cells. An increase of these glycotopes positively correlates with elevated tumorigenicity and expression of key glycosyl- and sulfotransferase genes. These results suggest that modulated expression of selective glycogenes correlates with altered tumorigenicity of cancer cells.

Mechanical shedding of L-selectin from the neutrophil surface during rolling on sialyl Lewis x under flow

The interaction of L-selectin expressed on leukocytes with endothelial cells leads to capture and rolling and is critical for the recruitment of leukocytes into sites of inflammation. It is known that leukocyte activation by chemoattractants, the change of osmotic pressure in cell media, or cross-linking of L-selectin all result in rapid shedding of L-selectin. Here we present a novel mechanism for surface cleavage of L-selectin on neutrophils during rolling on a sialyl Lewis x-coated surface that involves mechanical force. Flow cytometry and rolling of neutrophils labeled with Qdot(R)-L-selectin antibodies in an in vitro flow chamber showed that the mechanical shedding of L-selectin occurs during rolling and depends on the amount of shear applied. In addition, the mechanical L-selectin shedding causes an increase in cell rolling velocity with rolling duration, suggesting a gradual loss of L-selectin and is mediated by p38 mitogen-activated protein kinase activation. Thus, these data show that mechanical force induces the cleavage of L-selectin from the neutrophil surface during rolling and therefore decreases the adhesion of cells to a ligand-presenting surface in flow.

Hematopoietic stem cell antigen CD34: role in adhesion or homing

CD34 is highly glycosylated surface antigen of enormous clinical utility in the identification, enumeration, and purification of engraftable lymphohematopoietic progenitors for transplantation. However, recently its importance in the specific marking of most immature hematopoietic stem/progenitor cells have been questioned by addressing long-term reconstitution capability of CD34(-) hematopoietic cellular fractions. These controversies have stimulated a demand for elucidation of the structure, function, and molecular interactions of CD34 to define exactly its biological significance in clinical regimens. There is accumulating data showing the participation of CD34 in adhesion or perhaps homing of lymphohematopoietic progenitors. On the other hand, CD34 has been demonstrated to down-regulate cytokine-induced differentiation and proliferation of CD34(+) cells. Studies in CD34 knockout mice revealed normal hematopoiesis but a profound delay in hematopoietic reconstitution after sublethal irradiation of the mice. In short, CD34 expression is likely to represent a specific state of hematopoietic development that may have altered adhering properties with expanding and differentiating capabilities in both in vitro and in vivo conditions. This article focuses on the adhesive properties of CD34 and its potential role in homing, which are likely to mimic lymphocyte homing to the inflammatory sites.

L-selectin: adhesion, signalling and its importance in pathologic posttraumatic endotoxemia and non-septic inflammation

The leucocyte expressed surface-bound L-selectin belongs to the selectin family of adhesion molecules. It exhibits adhesive as well as signalling functions. Mainly, it is of importance in lymphocyte homing and in the extravasation of leucocytes into the surrounding tissue during inflammation. Acting in the initial step of the cell adhesion cascade, L-selectin is responsible for the rolling of leucocytes on endothelial layers. Therefore, L-selectin is thought to be an adequate target for pharmacological interventions. Beneath the discussion of the molecules' general features like molecule structure and its regulation, the review focuses firstly on L-selectin in the context of posttraumatic inflammatory disorders, and secondly on the importance of L-selectin specific signalling events.

A comparative study of endothelial cell markers expressed in chronically inflamed human tissues: MECA-79, Duffy antigen receptor for chemokines, von Willebrand factor, CD31, CD34, CD105 and CD146

Endothelial cells play a central role in chronic inflammation: for example, they express adhesion molecules and present chemokines leading to enhanced leukocyte recruitment into tissues. Numerous markers of endothelial cells have been reported but there has been a lack of comparative data on their specificity. The present study compared the specificity of seven endothelial cell markers in the rheumatoid synovium and the colon of patients with Crohn's disease. These markers were: the sulphated epitope MECA-79, the Duffy antigen receptor for chemokines (DARC), von Willebrand factor, CD31 (PECAM-1), CD34, CD105 (endoglin) and CD146. MECA-79, DARC and von Willebrand factor showed a specific endothelial cell distribution. MECA-79, which recognizes sulphated ligands for leukocyte adhesion receptor L-selectin (CD62L), was selective for a subset of venules in highly inflamed tissue and was present in rheumatoid but not control osteoarthritic synovia. DARC was also specific for venules but had a more widespread distribution than MECA-79, and was present in rheumatoid and control synovia. The other markers all labelled endothelial cells in venules, arterioles and capillaries. However, they also localized to other cell types. For example, CD34 stained fibroblasts, CD146 was expressed by the pericytes and smooth muscle cells of vessel walls and CD31 and CD105 labelled a broad range of cell types.

Highly glycosylated human salivary molecules present oligosaccharides that mediate adhesion of leukocytes and Helicobacter pylori

Glycoproteins display carbohydrate facets that serve as adhesion receptors for cells including leukocytes and bacterial cells. Our aim was to understand the role of the specialized carbohydrate motifs carried by highly glycosylated human salivary proteins in regulating the oral ecology. To date, our structural studies suggest that these molecules display a wide array of oligosaccharide structures, including many species with highly charged and/or fucosylated termini. Here, we used an immunoblot approach to gain additional information about the nature of these oligosaccharides. The results showed that MG1 and the salivary agglutinin express the MECA-79 epitope, an unusual sulfated carbohydrate structure that belongs to an important class of high-affinity (endothelial) L-selectin ligands. Unexpectedly, we discovered that in many women the expression of this epitope is hormonally regulated. Additional experiments revealed that MG1, MG2, and the salivary agglutinin also present Lewis blood group antigens, the exact repertoire varying on an individual basis. In parallel, we explored the functions of these carbohydrate motifs. Using an assay that detects L-selectin ligands, we found that the subset of MECA-79-reactive oligosaccharides displayed on salivary molecules specifically bind an L-selectin/Fc chimera. In contrast, the Lewis blood group structures are receptors for many strains of Helicobacter pylori, an organism that is implicated in the development of gastric ulcers and cancer. Together, these results suggest that MG1, MG2, and the salivary agglutinin play important roles in governing leukocyte and bacterial adhesion. Our findings suggest novel strategies, based on the relevant carbohydrate structures, for promoting or inhibiting these processes.

L-Selectin ligands in lymphoid tissues and models of inflammation

Both lymphocyte recirculation through the lymphoid tissues and leukocyte recruitment to sites of inflammation are essential components of immune surveillance, and are necessary for sustained protection against pathogens. This process is mediated by the leukocyte-endothelial adhesion cascade of which the interaction of leukocyte L-Selectin with its endothelial ligand initiates the first critical tethering and rolling step. As well as discussing the constitutive L-Selectin ligands in lymphoid tissues, this review examines the literature regarding their induction in inflammation, and draws attention to recent findings regarding soluble L-Selectin ligands that suggest an emerging multifunctional role in leukocyte recirculation and inflammation.

Metabolic Functionalization of Recombinant Glycoproteins

Glycoproteins are essential for cellular communication and are the most rapidly growing class of therapeutic agents. Chemical modification of glycoproteins has been employed to improve their in vivo efficacy or to label them for detection. Methods for the controlled derivatization of glycoproteins are presently limited by the repertoire of natural amino acid side chain and carbohydrate functionalities. Here, we use metabolic oligosaccharide engineering to introduce a bioorthogonal functional group, the azide, into cellular and recombinant glycoproteins for subsequent chemical elaboration via Staudinger ligation. As most therapeutic glycoproteins are sialylated and require this saccharide for optimal pharmacokinetics, we targeted sialic acid as a host for azides using N-azidoacetylmannosamine (ManNAz) as a biosynthetic precursor. Metabolic conversion of ManNAz to N-azidoacetylsialic acid (SiaNAz) within membrane-bound and secreted glycoproteins was quantified in a variety of cell types. SiaNAz was found to comprise between 4% and 41% of total sialosides, depending on the system. Metabolic labeling of recombinant interferon-beta and GlyCAM-Ig was achieved, demonstrating the utility of the method for functionalizing N-linked and O-linked glycoproteins of therapeutic interest. More generally, the generation of recombinant glycoproteins containing chemical handles within their glycans provides a means for studying their behavior and for improving their in vivo efficacy.

Ligands for L-selectin: homing, inflammation, and beyond

Understanding the molecular basis of lymphocyte homing to lymphoid organs was originally a problem of concern only to immunologists. With the discovery of L-selectin and its ligands, interested scientists have expanded to include glycobiologists, immunopathologists, cancer biologists, and developmental biologists. Going beyond its first discovered role in homing to lymph nodes, the L-selectin system is implicated in such diverse processes as inflammatory leukocyte trafficking in both acute and chronic settings, hematogenous metastasis of carcinoma cells, effector mechanisms for inflammatory demyelination of axons, and implantation of the early mammalian embryo. This review focuses on the ligands for L-selectin that are found on vascular endothelium, leukocytes, carcinoma cells, and at various extravascular sites. The discovery of selectins and their ligands has validated the long-predicted hypothesis that carbohydrate-directed cell adhesion is relevant in eukaryotic systems. Emphasis will be given to the carbohydrate and sulfation modifications of the ligands, which enable recognition by L-selectin. The rapid "homing" of labeled cells into the lymph nodes presumably had its basis in the special affinity of small lymphocytes for the endothelium of the postcapillary venules.

Endomucin, a sialomucin expressed in high endothelial venules, supports L-selectin-mediated rolling

Lymphocyte homing to lymph nodes is regulated by transient but specific interactions between lymphocytes and high endothelial venules (HEVs), the initial phase of which is mainly governed by the leukocyte adhesion molecule L-selectin, which recognizes sulfated and sialylated O-linked oligosaccharides displayed on sialomucin core proteins. One of the sialomucin proteins, endomucin, is predominantly expressed in vascular endothelial cells of a variety of tissues including the HEVs of lymph nodes; however, whether it functions as a ligand for L-selectin remains to be formally proven. Here we show that the endomucin splice isoform a is predominantly expressed in PNAd+ HEVs and MAdCAM-1+ HEVs, as seen in non-HEV-type vascular endothelial cells. Using affinity purification with soluble L-selectin, we found that HEV endomucin is specifically modified with L-selectin-reactive oligosaccharides and can bind L-selectin as well as an HEV-specific mAb, MECA-79. Our results also indicated that a 90-100 kDa endomucin species is preferentially decorated with L-selectin-reactive sugar chains, whereas an 80 kDa species represents conventional forms expressed in non-HEV-type vascular endothelial cells in lymph nodes. Furthermore, a CHO cell line expressing endomucin together with a specific combination of carbohydrate-modifying enzymes [core-2 beta-1,6-N-acetylglucosaminyltransferase (C2GnT), alpha-1,3-fucosyltransferase VII (FucTVII) and L-selectin ligand sulfotransferase (LSST)] showed L-selectin-dependent rolling under flow conditions in vitro. These results suggest that when endomucin is appropriately modified by a specific set of glycosyltransferases and a sulfotransferase, it can function as a ligand for L-selectin, and that the endomucin expressed in HEVs may represent another sialomucin ligand for L-selectin.

Selectins-an emerging target for drug delivery

Selectins are multifunctional adhesion molecules that mediate the initial interactions between circulating leukocytes and cells of the endothelium. First identified over a decade ago, selectins have provided insight into areas as diverse as normal lymphocyte homing, leukocyte recruitment during inflammatory responses, carbohydrate ligand biosynthesis and adhesion-mediated signalling. Of late, selectins were introduced as targets for drug delivery in the development of new anti-inflammatory therapeutics and in anti-cancer therapy. This review will examine the selectins and their ligands with a focus on recent findings on their role in physiology and pathophysiology as well as the emerging role of selectins as targets in controlled drug delivery.

Endoglycan, a member of the CD34 family, functions as an L-selectin ligand through modification with tyrosine sulfation and sialyl Lewis x

During lymphocyte homing to secondary lymphoid organs and instances of inflammatory trafficking, the rolling of leukocytes on vascular endothelium is mediated by transient interactions between L-selectin on leukocytes and several carbohydrate-modified ligands on the endothelium. Most L-selectin ligands such as CD34 and podocalyxin present sulfated carbohydrate structures (6-sulfated sialyl Lewis x or 6-sulfo-sLex) as a recognition determinant within their heavily glycosylated mucin domains. We recently identified endoglycan as a new member of the CD34 family. We report here that endoglycan, like the two other members of this family (CD34 and podocalyxin) can function as a L-selectin ligand. However, endoglycan employs a different binding mechanism, interacting with L-selectin through sulfation on two tyrosine residues and O-linked sLex structures that are presented within its highly acidic amino-terminal region. Our analysis establishes striking parallels with PSGL-1, a leukocyte ligand that interacts with all three selectins, mediating leukocyte-endothelial, leukocyte-leukocyte, and platelet-leukocyte interactions. Since the distribution of endoglycan includes hematopoietic precursors and leukocyte subpopulations, in addition to endothelial cells, our findings suggest several potential settings for endoglycan-mediated adhesion events.

Model glycosulfopeptides from P-selectin glycoprotein ligand-1 require tyrosine sulfation and a core 2-branched O-glycan to bind to L-selectin

L-selectin expressed on leukocytes is involved in lymphocyte homing to secondary lymphoid organs and leukocyte recruitment into inflamed tissue. L-selectin binds to the sulfated sialyl Lewis x (6-sulfo-sLex) epitope present on O-glycans of various glycoproteins in high endothelial venules. In addition, L-selectin interacts with the dimeric mucin P-selectin glycoprotein ligand-1 (PSGL-1) expressed on leukocytes. PSGL-1 lacks 6-sulfo-sLex but contains sulfated tyrosine residues (Tyr-SO3)at positions 46, 48, and 51 and sLex in a core 2-based O-glycan (C2-O-sLex) on Thr at position 57. The role of tyrosine sulfation and core 2 O-glycans in binding of PSGL-1 to L-selectin is not well defined. Here, we show that L-selectin binds to a glycosulfopeptide (GSP-6) modeled after the extreme N terminus of human PSGL-1, containing three Tyr-SO3 and a nearby Thr modified with C2-O-sLex. Leukocytes roll on immobilized GSP-6 in an L-selectin-dependent manner, and rolling is dependent on Tyr-SO3 and C2-O-sLex on GSP-6. The dissociation constant for binding of L-selectin to GSP-6, as measured by equilibrium gel filtration, is approximately 5 microm. Binding is dependent on Tyr-SO3 residues as well as the sialic acid and fucose residues of C2-O-sLex. Binding to an isomeric glycosulfopeptide containing three Tyr-SO3 residues and a core 1-based O-glycan expressing sLex was reduced by approximately 90%. All three Tyr-SO3 residues of GSP-6 are required for high affinity binding to L-selectin. Low affinity binding to mono- and disulfated GSPs is largely independent of the position of the Tyr-SO3 residues, except for some binding preference for an isomer sulfated on both Tyr-48 and -51. These results demonstrate that L-selectin binds with high affinity to the N-terminal region of PSGL-1 through cooperative interactions with three sulfated tyrosine residues and an appropriately positioned C2-O-sLex O-glycan.

Development of a simple homogeneous assay to screen for inhibitors of N-acetylglucosamine-6-sulfotransferases

L-selectin, a leukocyte adhesion molecule, plays a central role in lymphocyte homing to secondary lymphoid tissue and to certain sites of inflammation. Carbohydrate sulfation was implicated in this process, when it was demonstrated that carbohydrate sulfotransferase-mediated sulfation of N-acetylglucosamine (GlcNAc) within sialyl Lewis X of cognate endothelial ligands for L-selectin was an essential modification for L-selectin binding. The recently identified GlcNAc-6-sulfotransferases GlcNAc6ST-1 and -2, which facilitate GlcNAc sulfation by catalyzing the transfer of a sulfonyl group from 3(')-phosphoadenosine 5(')-phosphosulfate (PAPS) to the 6-hydroxy group of the acceptor GlcNAc moiety, contribute to the biosynthesis of the 6-sulfosialyl Lewis X motif. Due to their pivotal role in L-selectin ligand biosynthesis, this enzyme class has recently emerged as an important and relatively unexplored class of potential targets for anti-inflammatory therapy. However, no inhibitors have been reported to date and screening for lead inhibitors has been hampered by the lack of simple assay formats suitable for high-throughput screening. Here, we report the development of a simple homogeneous in vitro sulfotransferase assay using a newly synthesized biotinylated glycoside as a substrate. The assay is based on GlcNAc6ST-2-mediated [35S]sulfate transfer from [35S]PAPS to the biotinylated glycoside and subsequent detection using streptavidin-coated SPA beads. K(m) values with partially purified GlcNAc6ST-2 for PAPS and the biotinylated glycoside were estimated to be 8.4 and 34.5 microM, respectively. The sulfotransferase reaction could be inhibited by 3('),5(')-ADP with an IC(50) of 2.1 microM. The assay can be operated in 384-well format; is characterized by a high signal-to-noise ratio, low variation, and excellent Z factors; and is highly suitable for high-throughput screening.

Glycosylation in the control of selectin counter-receptor structure and function

Leukocyte trafficking is characterized by sequential cell adhesion and activation events that deliver specific leukocyte subsets to distinct extravascular locations under different pathophysiological circumstances. E-, P- and/or L-selectin-dependent leukocyte-endothelial cell adhesive interactions contribute essentially to this process. Selectin counter-receptor activity on leukocyte and high endothelial venules is borne by specific glycoproteins whose ability to support adhesion requires specific post-translational modifications. These modifications are typified by serine/threonine-linked oligosaccharides capped with the sialyl Lewis x moiety, an alpha2-3sialylated, alpha1-3ucosylated tetrasaccharide synthesized by specific glycosyltransferases. Recent advances in glycan structure analysis and in characterizing mice with targeted deletions of glycosyltransferase and sulfotransferase genes discloses an essential role for 6-O GlcNAc sulfate modification of the sialyl Lewis x tetrasaccharide in L-selectin counter-receptor activity. Related studies identify novel extended Core 1 type O-glycans bearing the 6-sulfosialyl Lewis x moiety, define the molecular nature of the MECA-79 epitope, and disclose a requirement for the alpha1-3fucosyltransferases FucT-IV and FucT-VII in the elaboration of L-selectin counter-receptor activities. Parallel studies also demonstrate that these 2 fucosyltransferases, a core 2 GlcNAc transferase, and core 2-type sialyl Lewis x determinants make essential contributions to leukocyte P-selectin counter-receptor activity, and figure prominently in the control of leukocyte E-selectin counter-receptor activity.

Human endomucin: distribution pattern, expression on high endothelial venules, and decoration with the MECA-79 epitope

Endomucin is a typical sialomucin that we recently identified on the surface of mouse endothelial cells and on putative hematopoetic clusters of the dorsal aorta in the embryo. We have generated a panel of monoclonal antibodies (mAbs) against the extracellular part of human endomucin and polyclonal antibodies against the cytoplasmic part. Using immunohistochemistry endomucin was specifically detected on endothelial cells of blood and lymphatic vessels of all analyzed human tissues. In addition, the polyclonal antibodies stained the epithelium of the epidermis as well as epithelial and myoepithelial cells of the eccrine and apocrine glands in the skin. This nonendothelial staining could only be seen with a subset of mAbs if the staining procedure was amplified. Although high endothelial venules (HEVs) were not significantly stained with mAbs against endomucin, the polyclonal antibodies clearly detected endomucin on HEVs in lymphatic organs of the mouse and human, suggesting HEV-specific glycosylation affecting recognition by the mAbs. Indeed, endomucin isolated from human and mouse lymphoid organs carried the MECA-79 epitope that defines a set of L-selectin ligands on HEVs called peripheral node addressins. We conclude that human and mouse endomucin are endothelial sialomucins with the potential to function as L-selectin ligands.

Identification and characterization of L-selectin ligands in porcine lymphoid tissues

A human L-selectin-ZZ fusion protein was used to screen porcine inguinal lymph nodes for the presence of monoclonal antibody (mAb) MECA 79-negative ligands. Fractionation of lymph node-conditioned medium by anion-exchange chromatography revealed two distinct L-selectin-binding fractions, one containing a MECA 79 non-reactive species and the second containing two MECA 79 reactive species of approximately 84 000 and 210 000 molecular weight. The MECA 79 non-reactive species exhibited Ca2+- and lectin-dependent binding to L-selectin-ZZ in a solid-phase capture enzyme-linked immunosorbent assay (ELISA), and this was specifically disrupted by the addition of EDTA, mannose-6-phosphate and the blocking anti-L-selectin mAb, DREG-56. Enzymatic characterization of the ligand by trypsin, O-sialoglycoprotease endopeptidase, heparinases I and III, or chondroitinase ABC lyase digestion indicated that L-selectin binding was predominantly dependent upon a chondroitin sulphate-modified glycoprotein determinant. Although Coomassie Blue staining of sodium dodecyl sulphate (SDS) polyacrylamide gels did not reveal a detectable protein species, carbohydrate-specific staining using GlycoTrack revealed a single, heavily glycosylated protein of high molecular weight (> 220 000). These studies have revealed the existence of a MECA 79 non-reactive, chondroitin sulphate glycosaminoglycan-modified ligand, termed csgp>220, which is secreted by peripheral lymph nodes and may play a role in leucocyte trafficking to the lymph node.

Homing of human hematopoietic stem and progenitor cells: new insights, new challenges?

In healthy adults, hematopoiesis takes place in the bone marrow, where the majority of hematopoietic progenitor cells (HPC) reside. In patients undergoing chemo- and/or radiotherapy, hematopoiesis is seriously disturbed. Reconstitution of bone-marrow function can be achieved by bone marrow transplantation or peripheral blood stem cell transplantation. The success of stem cell transplantation depends on the ability of intravenously infused stem cells to lodge in the bone marrow, a process referred to as homing. However, the molecular mechanisms that govern this process are poorly understood. It is hypothesized that homing is a multistep process, consisting of adhesion of the HPC to endothelial cells of the marrow sinusoids, followed by transendothelial migration directed by chemoattractants, and finally anchoring within the extravascular bone marrow spaces where proliferation and differentiation will occur. In this review, we discuss the factors that determine the engraftment potential of stem cells, and focus on various aspects of migration and homing of HPC, i.e., the role of the chemokine stromal cell-derived factor-1 (SDF-1) and its receptor CXCR-4, the involvement of adhesion molecules, and the induction of actin polymerization in the HPC. Defining the role of chemokines and adhesion molecules in human stem cell migration and engraftment will help us uncover the underlying mechanisms that regulate stem cell homing and will eventually advance clinical stem cell transplantation.

Synthesis of oxime-linked mucin mimics containing the tumor-related T(N) and sialyl T(N) antigens

[reaction--see text] The synthesis of oxime-linked mucin mimics was accomplished via the incorporation of multiple ketone residues into a peptide followed by reaction with aminooxy sugars corresponding to the tumor-related T(N) and sialyl T(N) (ST(N)) antigens.

Distinct ligand binding properties of Mac-2-binding protein and mouse cyclophilin [correction of mousephilin] C-associated protein

Human Mac-2-binding protein (Mac-2-BP) is a secreted glycoprotein that is widely expressed. It binds to the human macrophage-associated lectin Mac-2 and has been suggested to have a role in host defence. Mouse cyclophilin C-associated protein (mCyCAP) is also a secreted glycoprotein that binds with high affinity to cyclophilin C in the absence of the immunosuppressive drug cyclosporin A. The two proteins share a similar domain structure and considerable sequence identity, including a highly conserved scavenger receptor cysteine-rich domain, and both of them exert their function within the immune system. To elucidate whether these molecules are also functional homologues, we compared their ligand binding properties using cell lines which express Mac-2-BP or mCyCAP as well as transfected cell lines stably expressing mCyCAP or a mutant version lacking the scavenger domain. These experiments show that Mac-2-BP is unable to bind to either human or mouse cyclophilin C and thatmCyCAP cannot bind to Mac-2. The scavenger domain is not required for the interaction between mCyCAP and cyclophilin C. We conclude that these proteins may be part of a larger family of proteins of immunological importance in which closer functional homologues might exists.

Adhesion molecule cascades direct lymphocyte recirculation and leukocyte migration during inflammation

Leukocyte interactions with vascular endothelium are highly orchestrated processes that include the capture of free-flowing leukocytes from the blood with subsequent leukocyte rolling, arrest, firm adhesion, and ensuing diapedesis. These interactions occur under high shear stresses within venules and depend on multiple families of adhesion molecules. Many of the adhesion molecules involved are now identified. In addition, precise mechanisms underlying their regulation and our understanding of how different families of adhesion molecules work together is becoming clearer. Specifically, leukocyte/endothelial cell interactions such as capture, rolling, and firm adhesion can no longer be viewed as occurring in discrete steps mediated by individual families of adhesion molecules, but rather as a series of overlapping synergistic interactions among adhesion molecules resulting in an adhesion cascade. Although long thought to be mediated by distinct adhesion pathways, overlapping adhesion cascades mediate normal lymphocyte recirculation to peripheral lymphoid tissues and inflammation-induced leukocyte migration. These cascades thereby direct leukocyte migration, which is essential for the generation of effective inflammatory responses and the development of rapid immune responses.

Heterogeneity of O-glycosylation in the hinge region of human IgA1

Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry was applied to studies of the molecular heterogeneity of desialylated human IgA1 hinge region glycopeptides released with two IgA1 proteases. Typically, the hinge region of an alpha1 chain contains three to five O-linked glycan chains. Variants of the hinge region peptides released from IgA1(Kni) myeloma protein carrying 0, 1, 2, or 3 GalNAc residues were observed in the mass spectra as well as the nonglycosylated peptide. Variable numbers of Gal residues indicated additional heterogeneity in O-glycosylation of IgA1. In the hinge region preparation from normal human serum IgA1, glycopeptides carrying 2, 3, 4, or 5 GalNAc residues with variable numbers of Gal residues were detected. In conclusion, our new approach using the site-specific cleavage with two IgA1 proteases allowed precise and sensitive MALDI-TOF mass spectrometric analysis of O-glycosylation heterogeneity in IgA1 hinge region.

Minimal sulfated carbohydrates for recognition by L-selectin and the MECA-79 antibody

Sulfated forms of sialyl-Le(X) containing Gal-6-SO(4) or GlcNAc-6-SO(4) have been implicated as potential recognition determinants on high endothelial venule ligands for L-selectin. The optimal configuration of sulfate esters on the N-acetyllactosamine (Galbeta1-->4GlcNAc) core of sulfosialyl-Le(X), however, remains unsettled. Using a panel of sulfated lactose (Galbeta1-->4Glc) neoglycolipids as substrates in direct binding assays, we found that 6',6-disulfolactose was the preferred structure for L-selectin, although significant binding to 6'- and 6-sulfolactose was observed as well. Binding was EDTA-sensitive and blocked by L-selectin-specific monoclonal antibodies. Surprisingly, 6', 6-disulfolactose was poorly recognized by MECA-79, a carbohydrate- and sulfate-dependent monoclonal antibody that binds competitively to L-selectin ligands. Instead, MECA-79 bound preferentially to 6-sulfolactose. The difference in preferred substrates between L-selectin and MECA-79 may explain the variable activity of MECA-79 as an inhibitor of lymphocyte adhesion to high endothelial venules in lymphoid organs. Our results suggest that both Gal-6-SO(4) and GlcNAc-6-SO(4) may contribute to L-selectin recognition, either as components of sulfosialyl-Le(X) capping groups or in internal structures. By contrast, only GlcNAc-6-SO(4) appears to contribute to MECA-79 binding.

A hematopoietic cell L-selectin ligand that is distinct from PSGL-1 and displays N-glycan–dependent binding activity

Human hematopoietic progenitor cells express L-selectin and also express PSGL-1, a ligand for all selectins. Using a shear-based adhesion assay, a hematopoietic cell L-selectin ligand (HCLL) that is expressed on the hematopoietic cell line KG1a and on normal human hematopoietic progenitors was previously identified. To characterize the structural biology of HCLL and to define its relationship to PSGL-1, the effects of chemical and enzymatic treatments on HCLL activity of KG1a cells and membrane preparations were analyzed. Protease digestions and chemical treatments of KG1a cells and membranes indicated that HCLL is an integral membrane glycoprotein. Glycosidase digestions of membrane protein preparations and metabolic treatments of KG1a cells with glycosylation processing modifiers revealed that L-selectin binding determinants on HCLL are sialofucosylated structures presented on complex-type N-glycans. Adhesion assays and biochemical studies showed that this glycoprotein is also expressed on circulating blasts in native acute leukemias. HCLL is distinguishable from PSGL-1: (1) KG1a cells sorted for PSGL-1 expression had equivalent HCLL activity; (2) anti–PSGL-1 blocking antibodies and proteases known to eliminate L-selectin binding to PSGL-1 had no effect on HCLL binding activity of KG1a cells; (3) blasts from native leukemias with low expression of PSGL-1 and CD34 display high HCLL activity; and (4) despite high level expression of PSGL-1, HCLL activity was absent on HL60 cells. These data provide first evidence of a naturally expressed membrane L-selectin ligand expressing binding determinant(s) on an N-linked glycoconjugate. This novel ligand may help mediate L-selectin–dependent cell-cell adhesive interactions within the cytoarchitecture of the bone marrow microenvironment.

A hematopoietic cell L-selectin ligand that is distinct from PSGL-1 and displays N-glycan–dependent binding activity

Human hematopoietic progenitor cells express L-selectin and also express PSGL-1, a ligand for all selectins. Using a shear-based adhesion assay, a hematopoietic cell L-selectin ligand (HCLL) that is expressed on the hematopoietic cell line KG1a and on normal human hematopoietic progenitors was previously identified. To characterize the structural biology of HCLL and to define its relationship to PSGL-1, the effects of chemical and enzymatic treatments on HCLL activity of KG1a cells and membrane preparations were analyzed. Protease digestions and chemical treatments of KG1a cells and membranes indicated that HCLL is an integral membrane glycoprotein. Glycosidase digestions of membrane protein preparations and metabolic treatments of KG1a cells with glycosylation processing modifiers revealed that L-selectin binding determinants on HCLL are sialofucosylated structures presented on complex-type N-glycans. Adhesion assays and biochemical studies showed that this glycoprotein is also expressed on circulating blasts in native acute leukemias. HCLL is distinguishable from PSGL-1: (1) KG1a cells sorted for PSGL-1 expression had equivalent HCLL activity; (2) anti–PSGL-1 blocking antibodies and proteases known to eliminate L-selectin binding to PSGL-1 had no effect on HCLL binding activity of KG1a cells; (3) blasts from native leukemias with low expression of PSGL-1 and CD34 display high HCLL activity; and (4) despite high level expression of PSGL-1, HCLL activity was absent on HL60 cells. These data provide first evidence of a naturally expressed membrane L-selectin ligand expressing binding determinant(s) on an N-linked glycoconjugate. This novel ligand may help mediate L-selectin–dependent cell-cell adhesive interactions within the cytoarchitecture of the bone marrow microenvironment.

Carbohydrate sulfotransferases in lymphocyte homing

Sulfation is a critical modification in many instances of biological recognition. Early work in lymphocyte homing indicated that the endothelial ligands for L-selectin depended upon sulfation modifications. Subsequent studies showed that the two specific modifications, Gal-6-SO4 and GlcNAc-6-SO4, were present on actual biological ligands. Recently, a family of carbohydrate sulfotransferases capable of generating these modifications has been identified at the molecular level. Reconstitution experiments implicate members of this family as critical participants in lymphocyte homing.