Mice infected with Schistosoma mansoni generate antibodies to LacdiNAc (GalNAc beta 1-->4GlcNAc) determinants

Unique repertoire of anti-carbohydrate antibodies in individual human serum

Humoral immunity to pathogens and other environmental challenges is paramount to maintain normal health, and individuals lacking or unable to make antibodies are at risk. Recent studies indicate that many human protective antibodies are against carbohydrate antigens; however, little is known about repertoires and individual variation of anti-carbohydrate antibodies in healthy individuals. Here we analyzed anti-carbohydrate antibody repertoires (ACARs) of 105 healthy individual adult donors, aged 20-60+ from different ethnic backgrounds to explore variations in antibodies, as defined by binding to glycan microarrays and by affinity purification. Using microarrays that contained > 1,000 glycans, including antigens from animal cells and microbes, we profiled the IgG and IgM ACARs from all donors. Each donor expressed many ACAs, but had a relatively unique ACAR, which included unanticipated antibodies to carbohydrate antigens not well studied, such as chitin oligosaccharides, Forssman-related antigens, globo-type antigens, and bacterial glycans. We also saw some expected antibodies to ABO(H) blood group and α-Gal-type antigens, although these also varied among individuals. Analysis suggests differences in ACARs are associated with ethnicity and age. Thus, each individual ACAR is relatively unique, suggesting that individualized information could be useful in precision medicine for predicting and monitoring immune health and resistance to disease.

Recombinant mucin-type proteins carrying LacdiNAc on different O-glycan core chains fail to support H. pylori binding

The β4-N-acetylgalactosaminyltransferase 3 (B4GALNT3) transfers GalNAc in a β1,4-linkage to GlcNAc forming the LacdiNAc (LDN) determinant on oligosaccharides. The LacdiNAc-binding adhesin (LabA) has been suggested to mediate attachment of Helicobacter pylori to the gastric mucosa via binding to the LDN determinant. The O-glycan core chain specificity of B4GALNT3 is poorly defined. We investigated the specificity of B4GALNT3 on GlcNAc residues carried by O-glycan core 2, core 3 and extended core 1 precursors using transient transfection of CHO-K1 cells and a mucin-type immunoglobulin fusion protein as reporter protein. Binding of the LabA-positive H. pylori J99 and 26695 strains to mucin fusion proteins carrying the LDN determinant on different O-glycan core chains and human gastric mucins with and without LDN was assessed in a microtiter well-based binding assay, while the binding of ¹²⁵I-LDN-BSA to various clinical H. pylori isolates was assessed in solution. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) and western blotting confirmed the requirement of a terminal GlcNAc for B4GALNT3 activity. B4GALNT3 added a β1,4-linked GalNAc to GlcNAc irrespective of whether the latter was carried by a core 2, core 3 or extended core 1 chain. No LDN-mediated adhesion of H. pylori strains 26 695 and J99 to LDN determinants on gastric mucins or a mucin-type fusion protein carrying core 2, 3 and extended core 1 O-glycans were detected in a microtiter well-based adhesion assay and no binding of a ¹²⁵I-labelled LDN-BSA neoglycoconjugate to clinical H. pylori isolates was identified.

"Stuck on sugars - how carbohydrates regulate cell adhesion, recognition, and signaling"

We have explored the fundamental biological processes by which complex carbohydrates expressed on cellular glycoproteins and glycolipids and in secretions of cells promote cell adhesion and signaling. We have also explored processes by which animal pathogens, such as viruses, bacteria, and parasites adhere to glycans of animal cells and initiate disease. Glycans important in cell signaling and adhesion, such as key O-glycans, are essential for proper animal development and cellular differentiation, but they are also involved in many pathogenic processes, including inflammation, tumorigenesis and metastasis, and microbial and parasitic pathogenesis. The overall hypothesis guiding these studies is that glycoconjugates are recognized and bound by a growing class of proteins called glycan-binding proteins (GBPs or lectins) expressed by all types of cells. There is an incredible variety and diversity of GBPs in animal cells involved in binding N- and O-glycans, glycosphingolipids, and proteoglycan/glycosaminoglycans. We have specifically studied such molecular determinants recognized by selectins, galectins, and many other C-type lectins, involved in leukocyte recruitment to sites of inflammation in human tissues, lymphocyte trafficking, adhesion of human viruses to human cells, structure and immunogenicity of glycoproteins on the surfaces of human parasites. We have also explored the molecular basis of glycoconjugate biosynthesis by exploring the enzymes and molecular chaperones required for correct protein glycosylation. From these studies opportunities for translational biology have arisen, involving production of function-blocking antibodies, anti-glycan specific antibodies, and synthetic glycoconjugates, e.g. glycosulfopeptides, that specifically are recognized by GBPs. This invited short review is based in part on my presentation for the IGO Award 2019 given by the International Glycoconjugate Organization in Milan.

The directed migration of gonadal distal tip cells in Caenorhabditis elegans requires NGAT-1, a ß1,4-N-acetylgalactosaminyltransferase enzyme

Glycoproteins such as growth factor receptors and extracellular matrix have well-known functions in development and cancer progression, however, the glycans at sites of modification are often heterogeneous molecular populations which makes their functional characterization challenging. Here we provide evidence for a specific, discrete, well-defined glycan modification and regulation of a stage-specific cell migration in Caenorhabditis elegans. We show that a chain-terminating, putative null mutation in the gene encoding a predicted β1,4-N-acetylgalactosaminyltransferase, named ngat-1, causes a maternally rescued temperature sensitive (ts) defect in the second phase of the three phase migration pattern of the posterior, but not the anterior, hermaphrodite Distal Tip Cell (DTC). An amino-terminal partial deletion of ngat-1 causes a similar but lower penetrance ts phenotype. The existence of multiple ts alleles with distinctly different molecular DNA lesions, neither of which is likely to encode a ts protein, indicates that NGAT-1 normally prevents innate temperature sensitivity for phase 2 DTC pathfinding. Temperature shift analyses indicate that the ts period for the ngat-1 mutant defect ends by the beginning of post-embryonic development-nearly 3 full larval stages prior to the defective phase 2 migration affected by ngat-1 mutations. NGAT-1 homologs generate glycan-terminal GalNAc-β1-4GlcNAc, referred to as LacdiNAc modifications, on glycoproteins and glycolipids. We also found that the absence of the GnT1/Mgat1 activity [UDP-N-acetyl-D-glucosamine:α-3-D-mannoside β-1,2-N-acetylglucosaminyltransferase 1 (encoded by C. elegans gly-12, gly-13, and gly-14 and homologous to vertebrate GnT1/Mgat1)], causes a similar spectrum of DTC phenotypes as ngat-1 mutations-primarily affecting posterior DTC phase 2 migration and preventing manifestation of the same innate ts period as ngat-1. GnT1/Mgat1 is a medial Golgi enzyme known to modify mannose residues and initiate N-glycan branching, an essential step in the biosynthesis of hybrid, paucimannose and complex-type N-glycans. Quadruple mutant animals bearing putative null mutations in ngat-1 and the three GnT genes (gly-12, gly-13, gly-14) were not enhanced for DTC migration defects, suggesting NGAT-1 and GnT1 act in the same pathway. These findings suggest that GnTI generates an N-glycan substrate for NGAT-1 modification, which is required at restrictive temperature (25°C) to prevent, stabilize, reverse or compensate a perinatal thermo-labile process (or structure) causing late larval stage DTC phase 2 migration errors.

Glycans and glycan-specific IgE in clinical and molecular allergology: Sensitization, diagnostics, and clinical symptoms

Glycan-specific IgE antibodies cross-react with highly similar or even identical carbohydrate structures on a variety of different natural allergens, the so-called cross-reactive carbohydrate determinants (CCDs). In clinical practice CCDs often interfere with the specificity of in vitro allergy diagnostics, thus impairing allergy therapy decisions for individual patients. Strikingly, these IgE antibodies directed against CCDs often do not cause clinically relevant allergy symptoms. On the other hand, the IgE-binding glycan allergen galactose-α-(1,3)-galactose (α-Gal) is associated with IgE-mediated delayed anaphylaxis in meat allergy. The reason for this discrepancy is not known. The discovery of α-Gal stimulated new discussions and investigations regarding the relevance of anti-glycan IgE for allergic diseases. In this review the effect of glycans and glycan-specific IgE on sensitization to allergens and allergy diagnosis is described. Because parasite infections elicit a similar immunologic environment as allergic diseases, the association of glycan-specific antibodies against parasite glycoproteins with glycan structures on allergens is discussed.

Protective immunity against Schistosoma japonicum infection can be provided by IgG antibodies towards periodate-sensitive or periodate-resistant glycans

BACKGROUND

It has been well accepted that glycans present in schistosomes are highly antigenic. However, it is not clear what kind of worm glycans can affect the infected host to mount IgG responses and whether mounted anti-glycan IgG responses are protective.

METHODS

The contribution of antigenicity by glycans was measured by using competitive ELISA assay in sera from infected mice and humans. Monoclonal antibodies towards soluble Schistosoma japonicum egg antigens (SjEA) were generated from SjEA immunizated mice. The expression of glycans on surfaces of cercaria or young worm and their distributions were examined by immunofluorescence assay. The protective roles of glycans-specific mAbs were assayed by determination of the worm and egg burden in infected mice.

RESULTS

Both periodate-resistant glycans and periodate-sensitive glycans are antigenic in schistosome infections. When monoclonal antibodies against either periodate-sensitive or periodate-resistant glycans were administered prior to schistosome infections in mice, both kinds of anti-glycan antibodies were found to successfully provide protective immunity to infected mice.

CONCLUSIONS

Both periodate-resistant and periodate-sensitive glycans are antigenic, and dominant anti-glycan IgG responses can play important roles in protective immunity in schistosome infected hosts.

Glycomic Analysis of Life Stages of the Human Parasite Schistosoma mansoni Reveals Developmental Expression Profiles of Functional and Antigenic Glycan Motifs

Glycans present on glycoproteins and glycolipids of the major human parasite Schistosoma mansoni induce innate as well as adaptive immune responses in the host. To be able to study the molecular characteristics of schistosome infections it is therefore required to determine the expression profiles of glycans and antigenic glycan-motifs during a range of critical stages of the complex schistosome lifecycle. We performed a longitudinal profiling study covering schistosome glycosylation throughout worm- and egg-development using a mass spectrometry-based glycomics approach. Our study revealed that during worm development N-glycans with Galβ1–4(Fucα1–3)GlcNAc (LeX) and core-xylose motifs were rapidly lost after cercariae to schistosomula transformation, whereas GalNAcβ1–4GlcNAc (LDN)-motifs gradually became abundant and predominated in adult worms. LeX-motifs were present on glycolipids up to 2 weeks of schistosomula development, whereas glycolipids with mono- and multifucosylated LDN-motifs remained present up to the adult worm stage. In contrast, expression of complex O-glycans diminished to undetectable levels within days after transformation. During egg development, a rich diversity of N-glycans with fucosylated motifs was expressed, but with α3-core fucose and a high degree of multifucosylated antennae only in mature eggs and miracidia. N-glycan antennae were exclusively LDN-based in miracidia. O-glycans in the mature eggs were also diverse and contained LeX- and multifucosylated LDN, but none of these were associated with miracidia in which we detected only the Galβ1–3(Galβ1–6)GalNAc core glycan. Immature eggs also exhibited short O-glycan core structures only, suggesting that complex fucosylated O-glycans of schistosome eggs are derived primarily from glycoproteins produced by the subshell envelope in the developed egg. Lipid glycans with multifucosylated GlcNAc repeats were present throughout egg development, but with the longer highly fucosylated stretches enriched in mature eggs and miracidia. This global analysis of the developing schistosome's glycome provides new insights into how stage-specifically expressed glycans may contribute to different aspects of schistosome-host interactions.

Trefoil factor family domains represent highly efficient conformational determinants for N-linked N,N'-di-N-acetyllactosediamine (LacdiNAc) synthesis

The disaccharide N,N'-di-N-acetyllactose diamine (LacdiNAc, GalNAcβ1-4GlcNAcβ) is found in a limited number of extracellular matrix glycoproteins and neuropeptide hormones indicating a protein-specific transfer of GalNAc by the glycosyltransferases β4GalNAc-T3/T4. Whereas previous studies have revealed evidence for peptide determinants as controlling elements in LacdiNAc biosynthesis, we report here on an entirely independent conformational control of GalNAc transfer by single TFF (Trefoil factor) domains as high stringency determinants. Human TFF2 was recombinantly expressed in HEK-293 cells as a wild type full-length probe (TFF2-Fl, containing TFF domains P1 and P2), as single P1 or P2 domain probes, as a series of Cys/Gly mutant forms with aberrant domain structures, and as a double point-mutated probe (T68Q/F59Q) lacking aromatic residues within a hydrophobic patch. The N-glycosylation probes were analyzed by mass spectrometry for their glycoprofiles. In agreement with natural gastric TFF2, the recombinant full-length and single domain probes expressed nearly exclusively fucosylated LacdiNAc on bi-antennary complex-type chains indicating that a single TFF domain was sufficient to induce transfer of this modification. Contrasting to this, the Cys/Gly mutants showed strongly reduced LacdiNAc levels and instead preponderant LacNAc expression. The probe with point mutations of two highly conserved aromatic residues in loop 3 (T68Q/F59Q) revealed that these are essential determinant components, as the probe lacked LacdiNAc expression. The structural features of the LacdiNAc-inducing determinant on human TFF2 are discussed on the basis of crystal structures of porcine TFF2, and a series of extracellular matrix-related LacdiNAc-positive glycoproteins detected as novel candidate proteins in the secretome of HEK-293 cells.

Differential expression of anti-glycan antibodies in schistosome-infected humans, rhesus monkeys and mice

Schistosomiasis is a debilitating parasitic disease of humans, endemic in tropical areas, for which no vaccine is available. Evidence points to glycan antigens as being important in immune responses to infection. Here we describe our studies on the comparative humoral immune responses to defined schistosome-type glycan epitopes in Schistosoma mansoni-infected humans, rhesus monkeys and mice. Rhesus anti-glycan responses over the course of infection were screened on a defined glycan microarray comprising semi-synthetic glycopeptides terminating with schistosome-associated or control mammalian-type glycan epitopes, as well as a defined glycan microarray of mammalian-type glycans representing over 400 glycan structures. Infected rhesus monkeys generated a high immunoglobulin G (IgG) antibody response to the core xylose/core α3 fucose epitope of N-glycans, which peaked at 8-11 weeks post infection, coinciding with maximal ability to kill schistosomula in vitro. By contrast, infected humans generated low antibody levels to this epitope. At 18 months following praziquantel therapy to eliminate the parasite, antibody levels were negligible. Mice chronically infected with S. mansoni generated high levels of anti-fucosylated LacdiNAc (GalNAcβ1, 4(Fucα1, 3)GlcNAc) IgM antibodies, but lacked a robust response to the core xylose/core α3 fucose N-glycan antigens compared with other species studied, and their sera demonstrated an intermediate level of schistosomula killing in vitro. These differential responses to parasite glycan antigens may be related to the ability of rhesus monkeys to self-cure in contrast to the chronic infection seen in humans and mice. Our results validate defined glycan microarrays as a useful technology to evaluate diagnostic and vaccine antigens for schistosomiasis and perhaps other infections.

Glycoconjugates in host-helminth interactions

Helminths are multicellular parasitic worms that comprise a major class of human pathogens and cause an immense amount of suffering worldwide. Helminths possess an abundance of complex and unique glycoconjugates that interact with both the innate and adaptive arms of immunity in definitive and intermediate hosts. These glycoconjugates represent a major untapped reservoir of immunomodulatory compounds, which have the potential to treat autoimmune and inflammatory disorders, and antigenic glycans, which could be exploited as vaccines and diagnostics. This review will survey current knowledge of the interactions between helminth glycans and host immunity and highlight the gaps in our understanding which are relevant to advancing therapeutics, vaccine development, and diagnostics.

Development and characterization of a specific IgG monoclonal antibody toward the Lewis x antigen using splenocytes of Schistosoma mansoni-infected mice

The parasitic blood fluke Schistosoma mansoni synthesizes immunogenic glycans containing the human Lewis x antigen (Le(x); Galactose-β1-4(Fucα1-3)N-acetylglucosamine-β-R, also called CD15), but the biological role(s) of this antigen in the parasites and in humans is poorly understood. To develop IgG-based monoclonal antibodies (mAbs) specific for Le(x), we harvested splenocytes from S. mansoni-infected Swiss Webster mice at Week 10 postinfection, when peak IgG responses to glycan antigens occur, and generated a panel of hybridomas secreting anti-glycan IgG that recognize periodate-sensitive epitopes in soluble egg antigens of the parasites, and also recognizes a neoglycoprotein containing a pentasaccharide with the Le(x) sequence. One murine mAb, an IgG3 designated F8A1.1, bound to glycoproteins and glycolipids from schistosome adults and human promyelocytic leukemic HL-60 cells that express Le(x) antigens, as assessed by a wide variety of approaches including immunofluorescence staining, confocal microscopy, flow cytometry and western blotting, as well as overlay assays of glycolipids after thin-layer chromatography. In contrast, F8A1.1 bound weakly to cercariae, 3-h schistosomula and human Jurkat cells. We also directly compared the glycan specificity of F8A1.1 with commercially available anti-CD15 IgG1 (clone W6D3) using a defined glycan microarray. The results demonstrated that F8A1.1 recognized glycans expressing Le(x) epitopes in a terminal nonreducing position, whereas anti-CD15 bound to glycans with multiple repeats of Le(x) epitopes, but not to glycans with a single, terminal Le(x) epitope. Our results show that F8A1.1 recognizes terminal Le(x) epitopes and can be used for identification, immunolocalization, immunoprecipitation and purification of Le(x)-containing glycoconjugates from schistosomes and mammalian cells.

Parasite glycans and antibody-mediated immune responses in Schistosoma infection

Schistosome infections in humans are characterized by the development of chronic disease and high re-infection rates after treatment due to the slow development of immunity. It appears that anti-schistosome antibodies are at least partially mediating protective mechanisms. Efforts to develop a vaccine based on immunization with surface-exposed or secreted larval or worm proteins are ongoing. Schistosomes also express a large number of glycans as part of their glycoprotein and glycolipid repertoire, and antibody responses to those glycans are mounted by the infected host. This observation raises the question if glycans might also form novel vaccine targets for immune intervention in schistosomiasis. This review summarizes current knowledge of antibody responses and immunity in experimental and natural infections with Schistosoma, the expression profiles of schistosome glycans (the glycome), and antibody responses to individual antigenic glycan motifs. Future directions to study anti-glycan responses in schistosomiasis in more detail in order to address more precisely the possible role of glycans in antibody-mediated immunity are discussed.

Glycan microarray profiling of parasite infection sera identifies the LDNF glycan as a potential antigen for serodiagnosis of trichinellosis

Diagnostic methods for parasite infections still highly depend on the identification of the parasites by direct methods such as microscopic examination of blood, stool and tissue biopsies. Serodiagnosis is often carried out to complement the direct methods; however, few synthetic antigens with sufficient sensitivity and specificity are available. Here we evaluated a glycan microarray approach to select for synthetic glycan antigens that could be used for serodiagnosis of parasitic infections. Using a glycan array containing over 250 different glycan antigens, we identified GalNAcβ1-4(Fucα1-3)GlcNAc-R (LDNF) as a glycan antigen that is recognized by antibodies from Trichinella-infected individuals. We synthesized a neoglycoconjugate, consisting of five LDNF molecules covalently coupled to bovine serum albumin (BSA), and used this neoglycoconjugate as an antigen to develop a highly sensitive total-Ig ELISA for serological screening of trichinellosis. The results indicate that glycan microarrays constitute a promising technology for fast and specific identification of parasite glycan antigens to improve serodiagnosis of different parasitic infections, either using an ELISA format, or parasite-specific glycan arrays.

Targeted glycoproteomic analysis reveals that kappa-5 is a major, uniquely glycosylated component of Schistosoma mansoni egg antigens

Glycans present on glycoproteins from the eggs of the parasite Schistosoma mansoni are mediators of various immune responses of the human host, including T-cell modulation and granuloma formation, and they are the target of glycan-specific antibodies. Here we have analyzed the glycosylation of kappa-5, a major glycoprotein antigen from S. mansoni eggs using a targeted approach of lectin purification followed by mass spectrometry of glycopeptides as well as released glycans. We demonstrate that kappa-5 has four fully occupied N-glycosylation sites carrying unique triantennary glycans composed of a difucosylated and xylosylated core region, and immunogenic GalNAcβ1-4GlcNAc (LDN) termini. Furthermore, we show that the kappa-5 specific IgE antibodies in sera of S. mansoni-infected individuals are directed against the core region of the kappa-5 glycans. Whereas two previously analyzed immunomodulatory egg glycoproteins, IPSE/alpha-1 and omega-1, both express diantennary N-glycans with a difucosylated core and one or two Galβ1-4(Fucα1-3)GlcNAc (Lewis X) antennae, the kappa-5 glycosylation appears unique among the major soluble egg antigens of S. mansoni. The distinct structural and antigenic properties of kappa-5 glycans suggest a specific role for kappa-5 in schistosome egg immunogenicity.

Surface membrane proteins of Biomphalaria glabrata embryonic cells bind fucosyl determinants on the tegumental surface of Schistosoma mansoni primary sporocysts

Previous observations that in vitro adherence of Biomphalaria glabrata embryonic (Bge) cells to sporocyst larval stages of Schistosoma mansoni was strongly inhibited by fucoidan, a sulfated polymer of L-fucose, suggested a role for lectinlike Bge cell receptors in sporocyst binding interactions. In the present investigation, monoclonal antibodies with specificities to 3 major glycan determinants found on schistosomes, LacdiNAc, fucosylated LacdiNAc (LDNF), and the Lewis X antigen, were used in adhesion blocking studies to further analyze the molecular interactions at the host-parasite interface. Results showed that only the anti-LDNF antibody significantly reduced snail Bge cell adhesion to the surface of sporocysts, suggesting that fucosyl determinants may be important in larval-host cell interactions. Affinity chromatographic separation of fucosyl-reactive Bge cell proteins from fucoidan-bound Sepharose 4B revealed the presence of polypeptides ranging from 6 to 200 kDa after elution with fucoidan-containing buffer. Pre-elution of the Bge protein-bound affinity column with dextran (Dex) and dextran sulfate (DexS) before introduction of the fucoidan buffer served as controls for protein binding based on nonspecific sugar or negative charge interactions. A subset of polypeptides (approximately 35-150 kDa) released by fucoidan elution was identified as Bge surface membrane proteins, representing putative fucosyl-binding proteins. Far-western blot analysis also demonstrated binding reactivity between Bge cell and sporocyst tegumental proteins. The finding that several of these parasite-binding Bge cell proteins were also fucoidan-reactive suggests the possible involvement of these molecules in mediating cellular interactions with sporocyst tegumental carbohydrates. It is concluded that Bge cells have surface protein(s) that may be playing a role in facilitating host cell adhesion to the surface of schistosome primary sporocysts through larval fucosylated glycoconjugates.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

The asialoglycoprotein receptor regulates levels of plasma glycoproteins terminating with sialic acid alpha2,6-galactose

The asialoglycoprotein receptor (ASGP-R) is an abundant, carbohydrate-specific, endocytic receptor expressed by parenchymal cells of the liver. We recently demonstrated that the ASGP-R mediates the clearance of glycoproteins bearing Siaalpha2,6GalNAc as well as those bearing terminal Gal or GalNAc. We now report that glycoproteins such as haptoglobin, serum amyloid protein (SAP), and carboxylesterase that bear oligosaccharides with terminal Siaalpha2,6Gal are elevated in the plasma of ASGP-R-deficient mice. Because of their abundance in plasma, glycoproteins bearing terminal Siaalpha2,6Gal will saturate the ASGP-R and compete with each other on the basis of their relative affinities for the ASGP-R and their relative abundance. We propose that the ASGP-R mediates the clearance of glycoproteins that bear oligosaccharides terminating with Siaalpha2,6Gal and thereby helps maintain the relative concentrations of these glycoproteins in the blood.

Generation of novel chimeric LacdiNAcS by gene fusion of alpha-lactalbumin and beta1,4-galactosyltransferase 1

Novel chimeric lacdiNAc (GalNAc(beta1-4)GlcNAc) synthase (c-LacdiNAcS) was generated by gene fusion of alpha-lactalbumin (alpha-LA) and beta1,4-galactosyltransferase 1 (beta1,4-GalT1). c-LacdiNAcS was expressed in Lec8 Chinese hamster ovary (Lec8 CHO) cells and exhibited N-acetylgalactosaminyltransferase (GalNAcT) activity in the absence of exogenous alpha-LA as well as other glycosyltransferase activities including lactose synthase (LacS), and beta1,4-GalT. These glycosyltransferase activities of c-LacdiNAcS were compared to those activities induced in LacS system under the co-presence of bovine beta1,4-GalT1 and alpha-LA, indicating that each domain of alpha-LA and beta1,4-GalT1 on c-LacdiNAcS is not only folding correctly, but also interacting together. Furthermore, c-LacdiNAcS was found to be auto-lacdiNAcylated and can synthesize lacdiNAc structures on cellular glycoproteins, demonstrating that GalNAcT activity of c-LacdiNAcS is functional in Lec8 CHO cells.

The C-type lectin L-SIGN differentially recognizes glycan antigens on egg glycosphingolipids and soluble egg glycoproteins from Schistosoma mansoni

Recognition of pathogen-derived carbohydrate constituents by antigen presenting cells is an important step in the induction of protective immunity. Here we investigated the interaction of L-SIGN (liver/lymph node specific ICAM-3-grabbing nonintegrin), a C-type lectin that functions as antigen receptor on human liver sinusoidal endothelial cells, with egg-derived glycan antigens of the parasitic trematode Schistosoma mansoni. Our data demonstrate that L-SIGN binds both schistosomal soluble egg antigens (SEA) and egg glycosphingolipids, and can mediate internalization of SEA by L-SIGN expressing cells. Binding and internalization of SEA was strongly reduced after treatment of SEA with endoglycosidase H, whereas defucosylation affected neither binding nor internalization. These data indicate that L-SIGN predominantly interacts with oligomannosidic N-glycans of SEA. In contrast, binding to egg glycosphingolipids was completely abolished after defucosylation. Our data show that L-SIGN binds to a glycosphingolipid fraction containing fucosylated species with compositions of Hex(1)HexNAc(5-7)dHex(3-6)Cer, as evidenced by mass spectrometry. The L-SIGN "gain of function" mutant Ser363Val, which binds fucosylated Lewis antigens, did not bind to this fucosylated egg glycosphingolipid fraction, suggesting that L-SIGN displays different modes in binding fucoses of egg glycosphingolipids and Lewis antigens, respectively. Molecular modeling studies indicate that the preferred binding mode of L-SIGN to the respective fucosylated egg glycosphingolipid oligosaccharides involves a Fucalpha1-3GalNAcbeta1-4(Fucalpha1-3)GlcNAc tetrasaccharide at the nonreducing end. In conclusion, our data indicate that L-SIGN recognizes both oligomannosidic N-glycans and multiply fucosylated carbohydrate motifs within Schistosoma egg antigens, which demonstrates that L-SIGN has a broad but specific glycan recognition profile.

Molecular cloning and characterization of the Caenorhabditis elegans alpha1,3-fucosyltransferase family

The genome of Caenorhabditis elegans encodes five genes with homology to known alpha1,3 fucosyltransferases (alpha1,3FTs), but their expression and functions are poorly understood. Here we report the molecular cloning and characterization of these C. elegans alpha1,3FTs (CEFT-1 through -5). The open-reading frame for each enzyme predicts a type II transmembrane protein and multiple potential N-glycosylation sites. We prepared recombinant epitope-tagged forms of each CEFT and found that they had unusual acceptor specificity, cation requirements, and temperature sensitivity. CEFT-1 acted on the N-glycan pentasaccharide core acceptor to generate Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-Asn. In contrast, CEFT-2 did not act on the pentasaccharide acceptor, but instead utilized a LacdiNAc acceptor to generate GalNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc, which is a novel activity. CEFT-3 utilized a LacNAc acceptor to generate Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc without requiring cations. CEFT-4 was similar to CEFT-3, but its activity was enhanced by some divalent cations. Recombinant CEFT-5 was well expressed, but did not act on available acceptors. Each CEFT was optimally active at room temperature and rapidly lost activity at 37 degrees C. Promoter analysis showed that CEFT-1 is expressed in C. elegans eggs and adults, but its expression was restricted to a few neuronal cells at the head and tail. We prepared deletion mutants for each enzyme for phenotypic analysis. While loss of CEFT-1 correlated with loss of pentasaccharide core activity and core alpha1,3-fucosylated glycans in worms, loss of other enzymes did not correlate with any phenotypic changes. These results suggest that each of the alpha1,3FTs in C. elegans has unique specificity and expression patterns.

O-methylated glycans from Toxocara are specific targets for antibody binding in human and animal infections

The parasitic helminth Toxocara canis is a widely distributed nematode of mammals. Larval parasites, which infect a wide range of hosts including mice and humans, export glycosylated macromolecules bearing novel methylated oligosaccharide structures, similar to the mammalian blood group antigen H but bearing one or two O-methylated substitutions on the terminal fucose and subterminal galactose residues. We have studied the reactivity of synthetic forms of these glycans to parasite-specific antibodies and mammalian immune system lectins. Murine antibodies, generated to T. canis infection, predominantly recognise the mono-O-methylated form with the beta-configuration of the GalNAc residue (MoMbeta), and antibodies are entirely IgM isotype. The mAb Tcn-2 reproduces this pattern, and shows little reactivity to either the alpha isomer (MoMalpha) or the di-O-methylated form (DiM). Antibodies generated to helminth infections other than T. canis were unreactive with the glycans, except antibodies to other members of the Toxocara genus. Hence, the carbohydrate structures represent immunogenic, genus-specific antigens. Antibodies from human toxocariasis patients are reactive with the same sugars, although preferentially towards DiM. Sera from unrelated helminth infections do not react, confirming the status of these structures as Toxocara-specific glycans. The human dendritic cell lectin, DC-SIGN, was found to bind both Toxocara excretory/secretory products and mammalian blood group antigen H3. However, DC-SIGN did not bind the synthetic glycans, indicating additional non-methylated carbohydrates may also play a role in the interaction between T. canis and its host.

Fucosylation in prokaryotes and eukaryotes

Fucosylated carbohydrate structures are involved in a variety of biological and pathological processes in eukaryotic organisms including tissue development, angiogenesis, fertilization, cell adhesion, inflammation, and tumor metastasis. In contrast, fucosylation appears less common in prokaryotic organisms and has been suggested to be involved in molecular mimicry, adhesion, colonization, and modulating the host immune response. Fucosyltransferases (FucTs), present in both eukaryotic and prokaryotic organisms, are the enzymes responsible for the catalysis of fucose transfer from donor guanosine-diphosphate fucose to various acceptor molecules including oligosaccharides, glycoproteins, and glycolipids. To date, several subfamilies of mammalian FucTs have been well characterized; these enzymes are therefore delineated and used as models. Non-mammalian FucTs that possess different domain construction or display distinctive acceptor substrate specificity are highlighted. It is noteworthy that the glycoconjugates from plants and schistosomes contain some unusual fucose linkages, suggesting the presence of novel FucT subfamilies as yet to be characterized. Despite the very low sequence homology, striking functional similarity is exhibited between mammalian and Helicobacter pylori alpha1,3/4 FucTs, implying that these enzymes likely share a conserved mechanistic and structural basis for fucose transfer; such conserved functional features might also exist when comparing other FucT subfamilies from different origins. Fucosyltranferases are promising tools used in synthesis of fucosylated oligosaccharides and glycoconjugates, which show great potential in the treatment of infectious and inflammatory diseases and tumor metastasis.

Repeats of LacdiNAc and fucosylated LacdiNAc on N-glycans of the human parasite Schistosoma mansoni

N-Glycans from glycoproteins of the worm stage of the human parasite Schistosoma mansoni were enzymatically released, fluorescently labelled and analysed using various mass spectrometric and chromatographic methods. A family of 28 mainly core-alpha1-6-fucosylated, diantennary N-glycans of composition Hex(3-4)HexNAc(6-12)Fuc(1-6) was found to carry dimers of N,N'-diacetyllactosediamine [LacdiNAc or LDN; GalNAc(beta1-4)GlcNAc(beta1-] with or without fucose alpha1-3-linked to the N-acetylglucosamine residues in the antennae {GalNAc(beta1-4)[+/-Fuc(alpha1-3)]GlcNAc(beta1-3)GalNAc(beta1-4)[+/-Fuc(alpha1-3)]GlcNAc(beta1-}. To date, oligomeric LDN and oligomeric fucosylated LDN (LDNF) have been found only on N-glycans from mammalian cells engineered to express Caenorhabditis elegansbeta4-GalNAc transferase and human alpha3-fucosyltransferase IX [Z. S. Kawar et al. (2005) J Biol Chem280, 12810-12819]. It now appears that LDN(F) repeats can also occur in a natural system such as the schistosome parasite. Like monomeric LDN and LDNF, the dimeric LDN(F) moieties found here are expected to be targets of humoral and cellular immune responses during schistosome infection.

LacdiNAc- and LacNAc-containing glycans induce granulomas in an in vivo model for schistosome egg-induced hepatic granuloma formation

Schistosomes, major parasitic helminths, express numerous glycoconjugates that provoke humoral and cellular immune responses in the infected human host. The main pathology in schistosomiasis is due to the formation of granulomas around tissue-trapped eggs and the resulting organ damage. By using a mouse model of induction of granulomas by hepatic implantation of antigen-coated beads, it has been determined that the glycan part of schistosomal soluble egg antigens (SEA) initiates granulomogenesis. To identify which individual glycan elements in this complex SEA mixture are granulomogenic, we have tested in the same mouse model conjugates of various synthetic oligosaccharides characteristic for schistosome eggs, including GalNAcbeta1-4GlcNAc (LacdiNAc, LDN), Galbeta1-4(Fucalpha1-3)GlcNAc (Lewisx), Fucalpha1-2Fucalpha1-3GlcNAc (DF-Gn), and Fucalpha1-3GalNAcbeta1-4(Fucalpha1-3)GlcNAc (F-LDN-F). Ribonuclease (RNase) A and B, and different fetuin glycoforms were included as controls. Only beads that carry glycoconjugates with terminal LacdiNAc or Galbeta1-4GlcNAc (LacNAc, LN) elements gave rise to granulomas, with macrophage, lymphocyte, and eosinophil levels similar to the granulomatous lesions caused by schistosome eggs in a natural infection. Uncoated beads, and beads coated with fucosylated glycoconjugates or glycoconjugates lacking terminally exposed Gal or GalNAc, only attracted a monolayer of macrophages. These results indicate that the formation of hepatic granulomas is triggered specifically by glycoconjugates which carry terminal LacNAc or LacdiNAc, both constituents of the schistosome egg.

Versatile fluorescent derivatization of glycans for glycomic analysis

The new field of functional glycomics encompasses information about both glycan structure and recognition by carbohydrate-binding proteins (CBPs) and is now being explored through glycan array technology. Glycan array construction, however, is limited by the complexity of efficiently generating derivatives of free, reducing glycans with primary amines for conjugation. Here we describe a straightforward method to derivatize glycans with 2,6-diaminopyridine (DAP) to generate fluorescently labeled glycans (glycan-DAP conjugates or GDAPs) that contain a primary amine for further conjugation. We converted a wide variety of glycans, including milk sugars, N-glycans, glycosaminoglycans and chitin-derived glycans, to GDAPs, as verified by HPLC and mass spectrometry. We covalently conjugated GDAPs to N-hydroxysuccinimide (NHS)-activated glass slides, maleimide-activated protein, carboxylated microspheres and NHS-biotin to provide quantifiable fluorescent derivatives. All types of conjugated glycans were well-recognized by appropriate CBPs. Thus, GDAP derivatives provide versatile new tools for biologists to quantify and covalently capture minute quantities of glycans for exploring their structures and functions and generating new glycan arrays from naturally occurring glycans.

Activation of human endothelial cells by tumor necrosis factor-α results in profound changes in the expression of glycosylation-related genes

The endothelium plays a central role in the logistics of the immune system by allowing the selective transmigration of leukocytes, as well as the maintenance of the circulation and coagulation homeostasis. Evidence is increasing that the carbohydrate composition of the endothelial cell surface is critical for the cells to exert their physiological function. The major aim of this study is to unravel the mechanisms underlying the expression of carbohydrate structures by endothelial cells, which are involved in leukocyte adhesion and migration. Using quantitative real-time PCR, the expression profile of a selected group of 74 glycosylation-related genes has been determined in human umbilical vein endothelial cells (HUVEC) and human foreskin microvascular endothelial cells (FMVEC) under control and TNFalpha-induced conditions. The set of genes comprised 59 glycosyltransferases, 6 mannosidases and 9 sulfotransferases. In parallel, the overall cell surface glycan profile has been assessed by the use of glycan-specific lectins and monoclonal antibodies. The results demonstrate that HUVEC and FMVEC differ substantially in the expression of glycosylation-related genes and, accordingly, also in the presence of different glycan epitopes on the cell membrane. Induction of an inflamed phenotype of the cells by treatment with TNFalpha differentially modulates a set of these genes in HUVEC and FMVEC resulting in a change in the cell membrane associated glycans that are of importance in inflammation-related endothelial cell-surface processes.

Novel Poly-GalNAcβ1–4GlcNAc (LacdiNAc) and Fucosylated Poly-LacdiNAc N-Glycans from Mammalian Cells Expressing β1,4-N-Acetylgalactosaminyltransferase and α1,3-Fucosyltransferase

Glycans containing the GalNAcbeta1-4GlcNAc (LacdiNAc or LDN) motif are expressed by many invertebrates, but this motif also occurs in vertebrates and is found on several mammalian glycoprotein hormones. This motif contrasts with the more commonly occurring Galbeta1-4GlcNAc (LacNAc or LN) motif. To better understand LDN biosynthesis and regulation, we stably expressed the cDNA encoding the Caenorhabditis elegans beta1,4-N-acetylgalactosaminyltransferase (GalNAcT), which generates LDN in vitro, in Chinese hamster ovary (CHO) Lec8 cells, to establish L8-GalNAcT CHO cells. The glycan structures from these cells were determined by mass spectrometry and linkage analysis. The L8-GalNAcT cell line produces complex-type N-glycans quantitatively bearing LDN structures on their antennae. Unexpectedly, most of these complex-type N-glycans contain novel "poly-LDN" structures consisting of repeating LDN motifs (-3GalNAcbeta1-4GlcNAcbeta1-)n. These novel structures are in contrast to the well known poly-LN structures consisting of repeating LN motifs (-3Galbeta1-4GlcNAcbeta1-)n. We also stably expressed human alpha1,3-fucosyltransferase IX in the L8-GalNAcT cells to establish a new cell line, L8-GalNAcT-FucT. These cells produce complex-type N-glycans with alpha1,3-fucosylated LDN (LDNF) GalNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-R as well as novel "poly-LDNF" structures (-3GalNAcbeta1-4(Fucalpha 1-3)GlcNAcbeta1-)n. The ability of these cell lines to generate glycoprotein hormones with LDN-containing N-glycans was studied by expressing a recombinant form of the common alpha-subunit in L8-GalNAcT cells. The alpha-subunit N-glycans carried LDN structures, which were further modified by co-expression of the human GalNAc 4-sulfotransferase I, which generates SO4-4GalNAcbeta1-4GlcNAc-R. Thus, the generation of these stable mammalian cells will facilitate future studies on the biological activities and properties of LDN-related structures in glycoproteins.

Carbohydrate profiling reveals a distinctive role for the C-type lectin MGL in the recognition of helminth parasites and tumor antigens by dendritic cells

Dendritic cells (DCs) are key to the maintenance of peripheral tolerance to self-antigens and the orchestration of an immune reaction to foreign antigens. C-type lectins, expressed by DCs, recognize carbohydrate moieties on antigens that can be internalized for processing and presentation. Little is known about the exact glycan structures on self-antigens and pathogens that are specifically recognized by the different C-type lectins and how this interaction influences DC function. We have analyzed the carbohydrate specificity of the human C-type lectin macrophage galactose-type lectin (MGL) using glycan microarray profiling and identified an exclusive specificity for terminal alpha- and beta-linked GalNAc residues that naturally occur as parts of glycoproteins or glycosphingolipids. Specific glycan structures containing terminal GalNAc moieties, expressed by the human helminth parasite Schistosoma mansoni as well as tumor antigens and a subset of gangliosides, were identified as ligands for MGL. Our results indicate an endogenous function for DC-expressed MGL in the clearance and tolerance to self-gangliosides, and in the pattern recognition of tumor antigens and foreign glycoproteins derived from helminth parasites.

Glycans of Schistosoma mansoni and keyhole limpet haemocyanin induce hepatic granulomas in vivo

Schistosoma mansoni eggs trapped in the liver of an infected host cause the major pathological manifestations of schistosomiasis. Miracidia within the deposited eggs secrete soluble egg antigens (SEA) that induce periovular granuloma formation, which may lead to severe hepatic fibrosis. Several reports have highlighted the immunomodulatory capacities of carbohydrate determinants present in the glycoproteins of SEA. These glycans contain among others the immunogenic Galbeta1-4(Fucalpha1-3)GlcNAc (LewisX) and GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAc (LDN-DF) elements. Due to cross-reactivity with schistosomal glycan antigens, keyhole limpet haemocyanin (KLH) has been used extensively for diagnostic and therapeutic studies on schistosomiasis. In the present study, a granulomatous response with numerous eosinophils towards SEA- and KLH-coated beads implanted in the liver by mesenteric injection was observed. Immunophenotyping of these experimentally induced granulomas for cellular recruitment, chemokines, adhesion and extracellular matrix proteins revealed very close resemblance with hepatic lesions evoked by native schistosome eggs, hence demonstrating the usefulness of the bead model, in general, as well as of KLH as a model antigen to study the immunopathological mechanisms of schistosome infections. While trypsin digestion of KLH did not alter its antigenic characteristics, beads coated with SEA or KLH treated with sodium periodate to destroy the immunological properties of their carbohydrate chains, yielded only a monolayer of macrophages similar to negative control beads. Up-regulation of ICAM-1, LFA-1 and fibronectin in SEA-induced granulomas and in native and trypsinised KLH-induced granulomas indicates a major role of the carbohydrate elements of SEA and KLH in the initiation and homeostasis of the inflammatory response. These data provide new insights in the complex and multifactorial carbohydrate-dependent host-parasite immunological interactions.

LacdiNAc-Glycans Constitute a Parasite Pattern for Galectin-3-Mediated Immune Recognition

Although Gal beta 1-4GlcNAc (LacNAc) moieties are the most common constituents of N-linked glycans on vertebrate proteins, GalNAc beta 1-4GlcNAc (LacdiNAc, LDN)-containing glycans are widespread in invertebrates, such as helminths. We postulated that LDN might be a molecular pattern for recognition of helminth parasites by the immune system. Using LDN-based affinity chromatography and mass spectrometry, we have identified galectin-3 as the major LDN-binding protein in macrophages. By contrast, LDN binding was not observed with galectin-1. Surface plasmon resonance (SPR) analysis and a solid phase binding assay demonstrated that galectin-3 binds directly to neoglycoconjugates carrying LDN glycans. In addition, galectin-3 bound to Schistosoma mansoni soluble egg Ags and a mAb against the LDN glycan inhibited this binding, suggesting that LDN glycans within S. mansoni soluble egg Ags contribute to galectin-3 binding. Immunocytochemistry demonstrated high levels of galectin-3 in liver granulomas of S. mansoni-infected hamsters, and a colocalization of galectin-3 and LDN glycans was observed on the parasite eggshells. Finally, we demonstrate that galectin-3 can mediate recognition and phagocytosis of LDN-coated particles by macrophages. These findings provide evidence that LDN-glycans constitute a parasite pattern for galectin-3-mediated immune recognition.

Enzymatic N-glycan analysis of 31 kDa molecule in plerocercoid of Spirometra mansoni (sparganum) and its antigenicity after chemical oxidation

A highly specific antigenic protein of 31 kDa from plerocercoid of Spirometra mansoni (sparganum) was obtained by gelatin affinity and Mono Q anion-exchange column chromatography. The purified 31 kDa protein was subjected to N-glycan enzymatic digestion for structural analysis. The relative electrophoretic mobility was analyzed by SDS-PAGE, before and after digestion. On SDS-PAGE after enzymatic digestion, the 31 kDa protein showed a molecular shift of approximately 2 kDa, which indicated the possession of complex N-linked oligosaccharides (N-glycosidase F sensitive) but not of high-mannose oligosaccharides (endo-beta-N-acetylglucosaminidase H, non-sensitive). Chemically periodated 31 kDa protein showed statistically non-significant changes with human sparganosis sera by enzyme linked immunosorbent assay (ELISA). Therefore, the dominant epitopes of the 31 kDa molecule in human sparganosis were found to be mainly polypeptide, while N-glycans of the antigenic molecule in sparganum was minimal in anti-carbohydrate antibody production.

Molecular basis of the differences in binding properties of the highly related C-type lectins DC-SIGN and L-SIGN to Lewis X trisaccharide and Schistosoma mansoni egg antigens

The dendritic cell-specific C-type lectin DC-SIGN functions as a pathogen receptor that recognizes Schistosoma mansoni egg antigens through its major glycan epitope Galbeta1,4(Fucalpha1,3)GlcNAc (Lex). Here we report that L-SIGN, a highly related homologue of DC-SIGN found on liver sinusoidal endothelial cells, binds to S. mansoni egg antigens but not to the Lex epitope. L-SIGN does bind the Lewis antigens Lea, Leb, and Ley, similar as DC-SIGN. A specific mutation in the carbohydrate recognition domain of DC-SIGN (V351G) abrogates binding to all Lewis antigens. In L-SIGN Ser363 is present at the corresponding position of Val351 in DC-SIGN. Replacement of this Ser into Val resulted in a "gain of function" L-SIGN mutant that binds to Lex, and shows increased binding to the other Lewis antigens. These data indicate that Val351 is important for the fucose specificity of DC-SIGN. Molecular modeling and docking of the different Lewis antigens in the carbohydrate recognition domains of L-SIGN, DC-SIGN, and their mutant forms, demonstrate that Val351 in DC-SIGN creates a hydrophobic pocket that strongly interacts with the Fucalpha1,3/4-GlcNAc moiety of the Lewis antigens. The equivalent amino acid residue Ser363 in L-SIGN creates a hydrophilic pocket that prevents interaction with Fucalpha1,3-GlcNAc in Lex but supports interactions with the Fucalpha1,4-GlcNAc moiety in Lea and Leb antigens. These data demonstrate for the first time that DC-SIGN and L-SIGN differ in their carbohydrate binding profiles and will contribute to our understanding of the functional roles of these C-type lectin receptors, both in recognition of pathogen and self-glycan antigens.

Antigenic glycans in parasitic infections: implications for vaccines and diagnostics

Infections by parasitic protozoans and helminths are a major world-wide health concern, but no vaccines exist to the major human parasitic diseases, such as malaria, African trypanosomiasis, amebiasis, leishmaniasis, schistosomiasis, and lymphatic filariasis. Recent studies on a number of parasites indicate that immune responses to parasites in infected animals and humans are directed to glycan determinants within cell surface and secreted glycoconjugates and that glycoconjugates are important in host-parasite interactions. Because of the tremendous success achieved recently in generating carbohydrate-protein conjugate vaccines toward microbial infections, such as Haemophilus influenzae type b, there is renewed interest in defining parasite-derived glycans in the prospect of developing conjugate vaccines and new diagnostics for parasitic infections. Parasite-derived glycans are compelling vaccine targets because they have structural features that distinguish them from mammalian glycans. There have been exciting new developments in techniques for glycan analysis and the methods for synthesizing oligosaccharides by chemical or combined chemo-enzymatic approaches that now make it feasible to generate parasite glycans to test as vaccine candidates. Here, we highlight recent progress made in elucidating the immunogenicity of glycans from some of the major human and animal parasites, the potential for developing conjugate vaccines for parasitic infections, and the possible utilization of these novel glycans in diagnostics.

Immunity to schistosomiasis: glycans are potential antigenic targets for immune intervention

The major humoral immune responses in animals infected with Schistosoma mansoni are directed toward carbohydrate antigens. Among these antigens are complex-type N-glycans expressing LDN [GalNAcbeta1-4GlcNAc-R], LDNF [GalNAcbeta1-4(Fucalpha1-3)GlcNAc-R], and polymeric Lewis x (Lex) [Galbeta1-4(Fucalpha1-3)GlcNAc]n-R epitopes. We have now evaluated the potential of the three glycan antigens as targets for immune-mediated intervention of infections and serodiagnosis. A variety of approaches were employed, including ELISA, Western blot, immunohistology, and in vitro complement lysis assays, to determine the immunogenicity of the glycans in infected humans, their localization on the parasites and their efficacy as targets for parasite lysis. Our results show that S. mansoni-infected patients, with either intestinal or hepatosplenic disease, generate predominantly IgM, but also IgG and IgA, antibodies to LDN, LDNF, and Lex. However, immune responses to Lex are generally lower than responses to LDN and LDNF and less specific to schistosome infections. Western blot analysis with monoclonal antibodies (mAb) to LDN, LDNF, and Lex determinants show that the glycan antigens occur on multiple glycoproteins from cercariae, 3-h, 48-h, and lung stage schistosomula, as well as adults and eggs. Immunohistological studies demonstrate that LDN, LDNF, and Lex are expressed on the parasite surface at all stages of development in the vertebrate host. Importantly, a mAb to LDN in the presence of complement efficiently kills schistosomula in vitro, as demonstrated by flow-cytometric assays that quantify cytolysis by propidium iodide uptake into damaged parasites. These findings raise the possibility that LDN and LDNF may be targets for vaccination and/or serodiagnosis of chronic schistosomiasis in humans.

Differential expression of LacdiNAc, fucosylated LacdiNAc, and Lewis x glycan antigens in intramolluscan stages of Schistosoma mansoni

We report the expression of 3 well-characterized adult Schistosoma mansoni glycan antigens among molluscan stages of the parasite. These antigens are LacdiNAc (LDN; GalNAcbeta1-4GlcNAc-R), fucosylated LacdiNAc (LDNF; GalNAc[Fucal-3]beta1-4GlcNAc-R), and Lewis x (Le(x); Gal[Fucalpha1-3]beta1-4GlcNAc-R). The presence of the glycans was determined by both immunoblot and immunohistological methods using monoclonal antibodies that specifically recognize each glycan epitope. Immunoblot analyses reveal that LDN and LDNF epitopes are expressed on many different glycoproteins, including eggs, mother sporocysts, daughter sporocysts, and cercariae, although LDN expression among daughter sporocysts is greatly reduced. LDN and LDNF epitopes are localized on the tegument and in the intrasporocyst cell masses of both in vitro-derived and in vivo-derived mother sporocysts and in the daughter sporocysts derived on day 16 after infection. Unexpectedly, high levels of LDN and LDNF glycans were detected in the infected, but not in the uninfected, snail hemolymph, suggesting that the infecting larvae secrete LDN and LDNF glycoconjugates into the snail hosts. In contrast, the expression of Le(x) antigen among the molluscan stages is highly restricted. Le(x) is present on a few high-molecular weight glycoproteins in eggs and cercariae but is undetectable in mother and daughter sporocysts. Taken together with our earlier studies on vertebrate stages of S. mansoni, these results show that LDN and LDNF glycans are conserved during schistosome development. The study further extends the evidence that Le(x) is a developmentally regulated antigen in schistosomes.

Molecular cloning and enzymatic characterization of a UDP-GalNAc:GlcNAc(beta)-R beta1,4-N-acetylgalactosaminyltransferase from Caenorhabditis elegans

A common terminal structure in glycans from animal glycoproteins and glycolipids is the lactosamine sequence Gal(beta)4GlcNAc-R (LacNAc or LN). An alternative sequence that occurs in vertebrate as well as in invertebrate glycoconjugates is GalNAc(beta)4GlcNAc-R (LacdiNAc or LDN). Whereas genes encoding beta4GalTs responsible for LN synthesis have been reported, the beta4GalNAcT(s) responsible for LDN synthesis has not been identified. Here we report the identification of a gene from Caenorhabditis elegans encoding a UDP-GalNAc:GlcNAc(beta)-R beta1,4-N-acetylgalactosaminyltransferase (Ce(beta)4GalNAcT) that synthesizes the LDN structure. Ce(beta)4GalNAcT is a member of the beta4GalT family, and its cDNA is predicted to encode a 383-amino acid type 2 membrane glycoprotein. A soluble, epitope-tagged recombinant form of Ce(beta)4GalNAcT expressed in CHO-Lec8 cells was active using UDP-GalNAc, but not UDP-Gal, as a donor toward a variety of acceptor substrates containing terminal beta-linked GlcNAc in both N- and O-glycan type structures. The LDN structure of the product was verified by co-chromatography with authentic standards and (1)H NMR spectroscopy. Moreover, Chinese hamster ovary CHO-Lec8 and CHO-Lec2 cells expressing Ce(beta)4GalNAcT acquired LDN determinants on endogenous glycoprotein N-glycans, demonstrating that the enzyme is active in mammalian cells as an authentic beta4GalNAcT. The identification and availability of this novel enzyme should enhance our understanding of the structure and function of LDN-containing glycoconjugates.

Several polylactosamine-modifying glycosyltransferases also use internal GalNAcbeta1-4GlcNAc units of synthetic saccharides as acceptors

The GalNAcbeta1-4GlcNAc determinant (LdN) occurs in some human and bovine glycoconjugates and also in lower vertebrates and invertebrates. It has been found in unsubstituted as well as terminally substituted forms at the distal end of conjugated glycans, but it has not been reported previously at truly internal positions of polylactosamine chains. Here, we describe enzyme-assisted conversion of LdNbeta1-OR oligosaccharides into GlcNAcbeta1-3GalNAcbeta1-4GlcNAcbeta1-OR. The extension reactions, catalyzed by human serum, were modeled after analogous beta3-GlcNAc transfer processes that generate GlcNAcbeta1-3Galbeta1-4GlcNAcbeta1-OR. The newly synthesized GlcNAcbeta1-3GalNAc linkages were unambiguously identified by nuclear magnetic resonance data, including the appropriate long-range correlations in heteronuclear multiple bond correlation spectra. The novel GlcNAcbeta1-3'LdN determinant proved to be a functional acceptor for several mammalian glycosyltransferases, suggesting that human polylactosamines may contain internal LdN units in many distinct forms. The GlcNAcbeta1-3'LdN determinant was unusually resistant toward jackbean beta-N-acetylhexosaminidase; the slow degradation should lead to a convenient method for the search of putative internal LdN determinants in natural polylactosamine chains.

Schistosome glycoconjugates in host-parasite interplay

Schistosomes are digenetic trematodes which cause schistosomiasis, also known as bilharzia, one of the main parasitic infections in man. In tropical and subtropical areas an estimated 200 million people are infected and suffer from the debilitating effects of this chronic disease. Schistosomes live in the blood vessels and strongly modulate the immune response of their host to be able to survive the hostile environment that they are exposed to. It has become increasingly clear that glycoconjugates of schistosome larvae, adult worms and eggs play an important role in the evasion mechanisms that schistosomes utilise to withstand the immunological measures of the host. Upon infection, the host mounts innate as well as adaptive immune responses to antigenic glycan elements, setting the immunological scene characteristic for schistosomiasis. In this review we summarise the structural data now available on schistosome glycans and provide data and ideas regarding the role that these glycans play in the various aspects of the glycobiology and immunology of schistosomiasis.

Profiles of immunoglobulin M (IgM) and IgG antibodies against defined carbohydrate epitopes in sera of Schistosoma-infected individuals determined by surface plasmon resonance

We report here that sera of children and adults infected with Schistosoma mansoni, S. haematobium, or S. japonicum contain antibodies against GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAc (LDN-DF) and to a lesser extent to Galbeta1-4(Fucalpha1-3)GlcNAc (Lewis(x)) and GalNAcbeta1-4GlcNAc (LDN). Surface plasmon resonance (SPR) spectroscopy was used to monitor the presence of serum antibodies to neoglycoconjugates containing these carbohydrate epitopes and to define the immunoglobulin M (IgM) and IgG subclass distribution of the antibodies. The serum levels of antibodies to LDN-DF are high related to LDN and Lewis(x) for all examined groups of Schistosoma-infected individuals. A higher antibody response to the LDN-DF epitope was found in sera of infected children than in sera of infected adults regardless of the schistosome species. With respect to the subclasses, we found surprisingly that individuals infected with S. japonicum have predominantly IgG antibodies, while individuals infected with S. mansoni mainly show an IgM response; high levels of both isotypes were measured in sera of individuals infected with S. haematobium. These data provide new insights in the human humoral immune response to schistosome-derived glycans.

Glycoconjugates from Parasitic Helminths: Structure Diversity and Immunobiological Implications

We have provided an account of the progress we and others have made over the last decade on the structural characterization of glycans from parasitic helminths. We hope to have illustrated a few principles and patterns governing helminth glycosylation, as well as the experimental approaches adopted and their associated strengths and limitations. Schistosomes remain the best studied systems but are still punctuated with gaps of knowledge. An important theme developed here is the regulated developmental stage-specific expression of various glycan epitopes and their interplay with immediate host environments for successful parasitism. It is anticipated that more novel or unusual structures will continuously be uncovered in the future and that despite many difficulties, current analytical techniques should be well up to meet the challenge in at least elucidating the major or key glycoconjugates from each of the diverse range of worms. The bottle neck will in fact reside in finding suitable experimental models to test their putative immunobiological functions from which the intricate host-parasite interactions can be delineated and rational vaccine design be achieved. The glycobiology of parasitic helminths is an area waiting to be more fully explored and the rewards should be sweet.

Antibody responses to the fucosylated LacdiNAc glycan antigen in Schistosoma mansoni-infected mice and expression of the glycan among schistosomes

Infections of animals with parasitic worms, such as Schistosoma mansoni, induce humoral immune responses to carbohydrate antigens, raising the possibility that such antigens might be useful targets for the development of vaccines and new diagnostic approaches. Here we describe the identification of fucosylated LacdiNAc (LDNF) [GalNAc beta 1-4(Fuc alpha 1-3)GlcNAc-R] as a new carbohydrate antigen in S. mansoni that induces humoral immune responses in infected mice. The presence of antibodies was determined by ELISA using a neoglycoconjugate synthesized to express LDNF sequences. Sera from S. mansoni-infected, but not uninfected, mice contain IgM, IgG, IgA, and IgE antibodies to LDNF. The IgG antibodies are primarily of the IgG1 and IgG3 subclasses, with no detectable levels of the complement-fixing IgG2a and IgG2b isotypes. An IgM monoclonal antibody, designated SMLDNF1, was generated from the spleens of S. mansoni-infected mice, and the antibody exhibits specific recognition of LDNF sequences, but not other fucosylated glycans tested. Immunocytochemical analysis demonstrates that LDNF antigens are localized on the tegumental surface of adult S. mansoni. Western blot analysis indicates that LDNF sequences are expressed on numerous high-molecular-weight glycoproteins from the three major human schistosome species, as well as the bird schistosome Trichobilharzia ocellata. The identification of LDNF antigen on the tegumental glycoproteins of schistosomes and the ability to synthesize LDNF conjugates should aid in the development of glycan-based vaccines and immunodiagnostic tests for schistosomiasis and in determining the role(s) of the glycans in worm development and pathogenesis.

Expression cloning of a new member of the ABO blood group glycosyltransferases, iGb3 synthase, that directs the synthesis of isoglobo-glycosphingolipids

The large array of different glycolipids described in mammalian tissues is a reflection, in part, of diverse glycosyltransferase expression. Herein, we describe the cloning of a UDP-galactose: beta-d-galactosyl-1,4-glucosylceramide alpha-1, 3-galactosyltransferase (iGb(3) synthase) from a rat placental cDNA expression library. iGb(3) synthase acts on lactosylceramide, LacCer (Galbeta1,4Glcbeta1Cer) to form iGb(3) (Galalpha1,3Galbeta1, 4Glcbeta1Cer) initiating the synthesis of the isoglobo-series of glycosphingolipids. The isolated cDNA encoded a predicted protein of 339 amino acids, which shows extensive homology (40-50% identity) to members of the ABO gene family that includes: murine alpha1, 3-galactosyltransferase, Forssman (Gb(5)) synthase, and the ABO glycosyltransferases. In contrast to the murine alpha1, 3-galactosyltransferase, iGb(3) synthase preferentially modifies glycolipids over glycoprotein substrates. Reverse transcriptase-polymerase chain reaction revealed a widespread tissue distribution of iGb(3) synthase RNA expression, with high levels observed in spleen, thymus, and skeletal muscle. As an indirect consequence of the expression cloning strategy used, we have been able to identify several potential glycolipid biosynthetic pathways where iGb(3) functions, including the globo- and isoglobo-series of glycolipids.

Cloning of Gb3 synthase, the key enzyme in globo-series glycosphingolipid synthesis, predicts a family of alpha 1, 4-glycosyltransferases conserved in plants, insects, and mammals

We have cloned Gb(3) synthase, the key alpha1, 4-galactosyltransferase in globo-series glycosphingolipid (GSL) synthesis, via a phenotypic screen, which previously yielded iGb(3) synthase, the alpha1,3-galactosyltransferase required in isoglobo-series GSL (Keusch, J. J., Manzella, S. M., Nyame, K. A., Cummings, R. D., and Baenziger, J. U. (2000) J. Biol. Chem. 33). Both transferases act on lactosylceramide, Galbeta1,4Glcbeta1Cer (LacCer), to produce Gb(3) (Galalpha1,4LacCer) or iGb(3) (Galalpha1, 3LacCer), respectively. GalNAc can be added sequentially to either Gb(3) or iGb(3) yielding globoside and Forssman from Gb(3), and isogloboside and isoForssman from iGb(3). Gb(3) synthase is not homologous to iGb(3) synthase but shows 43% identity to a human alpha1,4GlcNAc transferase that transfers a UDP-sugar in an alpha1, 4-linkage to a beta-linked Gal found in mucin. Extensive homology (35% identity) is also present between Gb(3) synthase and genes in Drosophila melanogaster and Arabidopsis thaliana, supporting conserved expression of an alpha1,4-glycosyltransferase, possibly Gb(3) synthase, throughout evolution. The isolated Gb(3) synthase cDNA encodes a type II transmembrane glycosyltransferase of 360 amino acids. The highest tissue expression of Gb(3) synthase RNA is found in the kidney, mesenteric lymph node, spleen, and brain. Gb(3) glycolipid, also called P(k) antigen or CD77, is a known receptor for verotoxins. CHO cells that do not express Gb(3) and are resistant to verotoxin become susceptible to the toxin following transfection with Gb(3) synthase cDNA.

Various stages of schistosoma express Lewis(x), LacdiNAc, GalNAcbeta1-4 (Fucalpha1-3)GlcNAc and GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAc carbohydrate epitopes: detection with monoclonal antibodies that are characterized by enzymatically synthesized neoglycoproteins

We report here that fucosylated epitopes such as Lewis(x), LacdiNAc, fucosylated LacdiNAc (LDN-F) and GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAc (LDN-DF) are expressed by schistosomes throughout their life cycle. These four epitopes were enzymatically synthesized and coupled to bovine serum albumin to yield neoglycoproteins. Subsequently these neoglycoproteins were used to probe a panel of 188 monoclonal antibodies obtained from infected or immunized mice, in ELISA and surface plasmon resonance analysis. Of these antibodies, 25 recognized one of the fucosylated structures synthesized, indicating that these structures are immunogenic during infection. The MAbs identified could be subdivided in four different groups based on the recognition of either the Lewis(x)-, the LacdiNAc-, the LDN-DF-, or both the LDN-F- and LDN-DF epitope. These monoclonal antibodies were then used to investigate the localization of the fucosylated epitopes in various stages of Schistosoma mansoni using indirect immunofluorescence. Lewis(x)epitopes were mainly found in the gut and on the tegument of adult worms, on egg shells, and on the oral sucker of cercariae. The LacdiNAc epitope was expressed on the tegument of adult worms, on miracidia, and on the oral sucker of cercariae. In contrast, LDN-DF epitopes were mainly present in the excretory system of adult worms, on miracidia and on whole cercariae. These also stained positive with the LDN-F/LDN-DF epitope antibodies, while whole parenchyma reacted characteristically only with the latter antibodies. The identification of different carbohydrate structures in various stages of schistosomes may lead to a better understanding of the function of glycans in the immune response during infection.