Infections of humans and animals by parasitic helminths share key features with atopic diseases, such as allergic asthma. Both diseases lead to the induction of high levels of Th2- type cytokines associated with abundant IgE production and eosinophilia. This immunological association has raised strong interest in the nature of the molecules that promote Th2 and regulatory T cell responses, and the molecular mechanism. Complex carbohydrates are potent inducers of Th2 responses, and carbohydrate antigens (Ags) can stimulate the production of different classes of glycan-specific antibodies (Abs), including Th2 associated IgG but also non-specific IgE. In this review we focus on the immunological responses towards glycan Ags derived from allergens and parasitic helminths, especially schistosomes. Biological effects of carbohydrate Ags are dependent on recognition of these Ags by carbohydrate- binding proteins (lectins). Cell-surface C-type lectin receptors (CLRs), such as DCSIGN, L-SIGN, the mannose receptor, macrophage galactose binding lectin, and other lectins, such as the soluble collectins and galectin-3, recognize particular glycan Ags of schistosomes and allergens, which may contribute to orchestrate Th2 associated adaptive responses. Remarkably, schistosomes express 'self glycan' Ags that are recognized by CLRs on DCs, whose principal function is thought to capture self-glycan Ags and generate regulatory T-cells to induce tolerance to these Ags. By expressing such self-glycan Ags, schistosomes may deceive the host immune system to their own benefit. The host protects itself against too much damage by down-regulating helminth-induced Th2 immune responses, and may thus simultaneously be protected against excessive Th2 cell-mediated allergic responses.
Anaplasma phagocytophilum, the aetiologic agent of human granulocytic anaplasmosis, is an obligate intracellular bacterium that colonizes neutrophils and neutrophil precursors. The granulocytotropic bacterium uses multiple adhesins that cooperatively bind to the N-terminal region of P-selectin glycoprotein ligand-1 (PSGL-1) and to sialyl Lewis x (sLe(x)) expressed on myeloid cell surfaces. Recognition of sLe(x) occurs through interactions with alpha2,3-sialic acid and alpha1,3-fucose. It is unknown whether other bacteria-host cell interactions are involved. In this study, we have enriched for A. phagocytophilum organisms that do not rely on sialic acid for cellular adhesion and entry by maintaining strain NCH-1 in HL-60 cells that are severely undersialylated. The selected bacteria, termed NCH-1A, also exhibit lessened dependencies on PSGL-1 and alpha1,3-fucose. Optimal adhesion and invasion by NCH-1A require interactions with the known determinants (sialic acid, PSGL-1 and alpha1,3-fucose), but none of them is absolutely necessary. NCH-1A binding to sLe(x)-modified PSGL-1 requires recognition of the known determinants in the same manners as other A. phagocytophilum strains. These data suggest that A. phagocytophilum expresses a separate adhesin from those targeting sialic acid, alpha1,3-fucose and the N-terminal region of PSGL-1. We propose that NCH-1A upregulates expression of this adhesin.
The common O-glycan core structure in animal glycoproteins is the core 1 disaccharide Galbeta1-3GalNAcalpha1-Ser/Thr, which is generated by the addition of Gal to GalNAcalpha1-Ser/Thr by core 1 UDP-alpha-galactose (UDP-Gal):GalNAcalpha1-Ser/Thr beta1,3-galactosyltransferase (core 1 beta3-Gal-T or T-synthase, EC18.104.22.168). Although O-glycans play important roles in vertebrates, much remains to be learned from model organisms such as the free-living nematode Caenorhabditis elegans, which offer many advantages in exploring O-glycan structure/function. Here, we report the cloning and enzymatic characterization of T-synthase from C. elegans (Ce-T-synthase). A putative C. elegans gene for T-synthase, C38H2.2, was identified in GenBank by a BlastP search using the human T-synthase protein sequence. The full-length cDNA for Ce-T-synthase, which was generated by polymerase chain reaction using a C. elegans cDNA library as the template, contains 1170 bp including the stop TAA. The cDNA encodes a protein of 389 amino acids with typical type II membrane topology and a remarkable 42.7% identity to the human T-synthase. Ce-T-synthase has seven Cys residues in the lumenal domain including six conserved Cys residues in all orthologs. The Ce-T-synthase has four potential N-glycosylation sequons, whereas the mammalian orthologs lack N-glycosylation sequons. Only one gene for Ce-T-synthase was identified in the genome-wide search, and it contains eight exons. Promoter analysis of the Ce-T-synthase using green fluorescent protein (GFP) constructs shows that the gene is expressed at all developmental stages and appears to be in all cells. Unexpectedly, only minimal activity was recovered in the recombinant, soluble Ce-T-synthase secreted from a wide variety of mammalian cell lines, whereas robust enzyme activity was recovered in the soluble Ce-T-synthase expressed in Hi-5 insect cells. Vertebrate T-synthase requires the molecular chaperone Cosmc, but our results show that Ce-T-synthase does not require Cosmc and might require invertebrate-specific factors for the formation of the optimally active enzyme. These results show that the Ce-T-synthase is a functional ortholog to the human T-synthase in generating core 1 O-glycans and open new avenues to explore O-glycan function in this model organism.
Glycan-binding proteins (lectins) are widely expressed in many invertebrates, although the biosynthesis and functions of the lectins are not well understood. Here we report that Manila clam (Ruditapes philippinarum) synthesizes a lectin termed Manila clam lectin (MCL) upon infection with the protozoan parasite Perkinsus olseni. MCL is synthesized in hemocytes as a approximately 74-kDa precursor and secreted into hemolymph where it is converted to 30- and 34-kDa polypeptides. The synthesis of MCL in hemocytes is stimulated by one or more factors in Perkinsus-infected hemolymph, but not directly by Perkinsus itself. MCL can bind to the surfaces of purified hypnospores and zoospores of the parasite, and this binding is inhibitable by either EDTA or GalNAc. Fluorescent beads coated with purified MCL were actively phagocytosed by hemocytes from the clam. Immunohistochemistry showed that secreted MCL is concentrated within cyst-like structures. To define the glycan binding specificity of MCL we examined its binding to an array of biotinylated glycans. MCL recognizes terminal non-reducing beta-linked GalNAc as expressed within the LacdiNAc motif GalNAcbeta1-4GlcNAcbeta1-R and glycans with terminal, non-reducing beta-linked Gal residues. Our results show that the synthesis of MCL is specifically up-regulated upon parasite infection of the clams and may serve as an opsonin through recognition of terminal GalNAc/Gal residues on the parasites.
Galectin-4 and its homologue galectin-6 are members of the tandem-repeat subfamily of monomer divalent galectins. Expression of mouse galectin-4 and galectin-6 by RT-PCR using primers designed to distinguish both galectin transcripts indicates that both are expressed in the small intestine, colon, liver, kidney, spleen and heart and P19X1 cells while only galectin-4 is expressed in BW-5147 and 3T3 cell lines. In situ hybridization confirmed the presence of galectin-4/-6 transcripts in the liver and small intestine. Galectin-4 is expressed in spermatozoons and oocytes and its expression during early mouse emryogenesis appears in 8-cell embryos and remains in later stages, as tested by RT-PCR. To study the role of carbohydrate recognition domains (CRDs) in oligosaccharide binding and epitope recognition, we cloned mouse full-length galectin-4 and galectin-6 cDNA and constructed bacterial expression vectors producing histidin-tagged recombinant galectin-4 and its truncated CRD1 and CRD2 forms. Oligosaccharide binding profile for all recombinant forms was assessed using Glycan Array available through the Consortium for Functional Glycomics. Acquired data indicate that mGalectin-4 binds to alpha-GalNAc and alpha-Gal A and B type structures with or without fucose. While the CRD2 domain has a high specificity and affinity for A type-2 alpha-GalNAc structures, the CRD1 domain has a broader specificity in correlation to the total binding profile. These data suggest that CRD2 might be the dominant binding domain of mouse galectin-4. Mapping of epitopes reactive for biotinylated his-tagged CRD1, CRD2 and mGalectin-4 performed on mouse cryosections showed that all three forms bind to alveolar macrophages, macrophages of red pulp of the spleen and proximal tubuli of the kidney and this binding was inhibited by 5 mM lactose. Interestingly, mGalectin-4, but not CRD forms, binds to the suprabasal layer of squamous epithelium of the tongue, suggesting that the link region also plays an important role in ligand recognition.
Contrast-enhanced ultrasound imaging has shown promise in the field of molecular imaging. This technique relies upon the adhesion of ultrasound contrast agent (UCA) to targeted molecular markers of disease. This is accomplished by coating the surface of the contrast agent with a ligand that specifically binds to the intended molecular marker. Most UCA particles remain in the blood space, and their retention is influenced by the forces imposed by blood flow. For a UCA bound to a molecular target on the vascular endothelium, blood flow imposes a dislodging force that counteracts retention. Additionally, contrast agent adhesion to the molecular marker requires rapid binding kinetics, especially in rapid blood flow. The ability of a ligand:target bond complex to mediate fast adhesion and withstand dislodging force is necessary for efficient ultrasound-based molecular imaging. In the current study, we describe a flow-based adhesion assay which, combined with a novel automated tracking algorithm, enables quick determination of the ability of a targeting ligand to mediate effective contrast agent adhesion. This system was used to explore the adhesion of UCA targeted to the proinflammatory endothelial protein P-selectin via four targeting ligands, which revealed several interesting adhesive behaviors. Contrast agents targeted with glycoconjugate ligands modeled on P-selectin glycoprotein ligand 1 exhibited primarily unstable or transient adhesion, while UCA targeted with an anti-P-selectin monoclonal antibody exhibited primarily firm adhesion, although the efficiency with which these agents were recruited to the target surface was relatively low.
The dendritic cell specific C-type lectin dendritic cell specific ICAM-3 grabbing non-integrin (DC-SIGN) binds to "self" glycan ligands found on human cells and to "foreign" glycans of bacterial or parasitic pathogens. Here, we investigated the binding properties of DC-SIGN to a large array of potential ligands in a glycan array format. Our data indicate that DC-SIGN binds with K(d)<2muM to a neoglycoconjugate in which Galbeta1-4(Fucalpha1-3)GlcNAc (Le(x)) trisaccharides are expressed multivalently. A lower selective binding was observed to oligomannose-type N-glycans, diantennary N-glycans expressing Le(x) and GalNAcbeta1-4(Fucalpha1-3)GlcNAc (LacdiNAc-fucose), whereas no binding was observed to N-glycans expressing core-fucose linked either alpha1-6 or alpha1-3 to the Asn-linked GlcNAc of N-glycans. These results demonstrate that DC-SIGN is selective in its recognition of specific types of fucosylated glycans and subsets of oligomannose- and complex-type N-glycans.