Isolation and characterization of a heptaglycosylceramide from bovine erythrocyte membranes

Porcine intestinal glycosphingolipids recognized by F6-fimbriated enterotoxigenic Escherichia coli

One important virulence factor of enterotoxigenic Escherichia coli is their ability to adhere via fimbrial adhesins to specific receptors located on the intestinal mucosa. Here, the potential glycosphingolipid receptors of enterotoxigenic F6-fimbriated E. coli were examined by binding of purified F6 fimbriae, and F6-expressing bacteria, to glycosphingolipids on thin-layer chromatograms. When intestinal mucosal non-acid glycosphingolipids from single pigs were assayed for F6 binding capacity, a selective interaction with two glycosphingolipids was observed. The binding-active glycosphingolipids were isolated and characterized as lactotriaosylceramide (GlcNAcβ3Galβ4Glcβ1Cer) and lactotetraosylceramide (Galβ3GlcNAcβ3Galβ4Glcβ1Cer). Further binding assays using a panel of reference glycosphingolipids showed a specific interaction between the F6 fimbriae and a number of neolacto core chain (Galβ4GlcNAc) glycosphingolipids. In addition, an occasional binding of the F6 fimbriae to sulfatide, galactosylceramide, lactosylceramide with phytosphingosine and/or hydroxy fatty acids, isoglobotriaosylceramide, gangliotriaosylceramide, and gangliotetraosylceramide was obtained. From the results we conclude that lactotriaosylceramide and lactotetraosylceramide are major porcine intestinal receptors for F6-fimbriated E. coli.

Characterization of acid and non-acid glycosphingolipids of porcine heart valve cusps as potential immune targets in biological heart valve grafts

BACKGROUND

Although xenotransplantation of vascularized organs/cells has not yet reached the clinic, glutaraldehyde-treated bioprosthetic heart valves (BHV), derived from porcine or bovine tissues, are today used for clinical replacement of diseased heart valves. However, the durability of these valve cusps is limited partly due to the onset of immune responses to the grafts. The xenoantigen-determinant Galα3Gal- and corresponding anti-Gal antibodies have been postulated to in part contribute to BHV damage. However, the presence of other non-Gal carbohydrate antigen determinants as well as the immune response to these non-Gal antigens and the inflammatory response generated by their interaction with the immune system has not been studied. In this study, we have isolated and structurally characterized both non-acid and acid glycosphingolipids from naïve porcine aortic and pulmonary valve cusps.

METHODS

Total non-acid and acid glycosphingolipids were isolated from porcine aortic and pulmonalis valve cusps of 20 animals. Glycosphingolipid components were structurally characterized by thin-layer chromatography, liquid chromatography-mass spectrometry and binding of monoclonal antibodies and lectins.

RESULTS

The non-acid glycosphingolipids were characterized as globotetraosylceramide, H-type 2 pentaosylceramide, fucosyl-gangliotetraosylceramide, and Galα3neolactotetraosylceramide. The acid glycosphingolipid fractions had both sulfatide and gangliosides (GM3, GM2, GM1, fucosyl-GM1, GD3 and GD1a), and all gangliosides contained N-acetyl-neuraminic acid. Significantly, the N-glycolyl-neuraminic acid (NeuGc) variant, a major component in many pig organs and to which humans can develop antibodies, was not detected among the gangliosides.

CONCLUSIONS

Pig valve cusps contain several complex lipid-bound carbohydrate structures that may be targets for the human immune system. Notable, the NeuGc determinant was absent in the cusp gangliosides. This work forms a platform for further characterizing the antibody reactivity of patients with porcine-derived BHV.

The Galalpha1,3Galbeta1,4GlcNAc-R (alpha-Gal) epitope: a carbohydrate of unique evolution and clinical relevance

In 1985, we reported that a naturally occurring human antibody (anti-Gal), produced as the most abundant antibody (1% of immunoglobulins) throughout the life of all individuals, recognizes a carbohydrate epitope Galalpha1-3Galbeta1-4GlcNAc-R (the alpha-gal epitope). Since that time, an extensive literature has developed on discoveries related to the alpha-gal epitope and the anti-Gal antibody, including the barrier they form in xenotransplantation and their reciprocity in mammalian evolution. This review covers these topics and new avenues of clinical importance related to this unique antigen/antibody system (alpha-gal epitope/anti-Gal) in improving the efficacy of viral vaccines and in immunotherapy against cancer.

The alpha-gal epitope and the anti-Gal antibody in xenotransplantation and in cancer immunotherapy

The alpha-gal epitope (Galalpha1-3Galbeta1-(3)4GlcNAc-R) is abundantly synthesized on glycolipids and glycoproteins of non-primate mammals and New World monkeys by the glycosylation enzyme alpha1,3galactosyltransferase (alpha1,3GT). In humans, apes and Old World monkeys, this epitope is absent because the alpha1,3GT gene was inactivated in ancestral Old World primates. Instead, humans, apes and Old World monkeys produce the anti-Gal antibody, which specifically interacts with alpha-gal epitopes and which constitutes approximately 1% of circulating immunoglobulins. Anti-Gal has functioned as an immunological barrier, preventing the transplantation of pig organs into humans, because anti-Gal binds to the alpha-gal epitopes expressed on pig cells. The recent generation of alpha1,3GT knockout pigs that lack alpha-gal epitopes has resulted in the elimination of this immunological barrier. Anti-Gal can be exploited for clinical use in cancer immunotherapy by targeting autologous tumour vaccines to APC, thereby increasing their immunogenicity. Autologous intact tumour cells from haematological malignancies, or autologous tumour cell membranes from solid tumours are processed to express alpha-gal epitopes by incubation with neuraminidase, recombinant alpha1,3GT and with uridine diphosphate galactose. Subsequent immunization with such autologous tumour vaccines results in in vivo opsonization by anti-Gal IgG binding to these alpha-gal epitopes. The interaction of the Fc portion of the vaccine-bound anti-Gal with Fcgamma receptors of APC induces effective uptake of the vaccinating tumour cell membranes by the APC, followed by effective transport of the vaccinating tumour membranes to the regional lymph nodes, and processing and presentation of the tumour-associated antigen (TAA) peptides. Activation of tumour-specific T cells within the lymph nodes by autologous TAA peptides may elicit an immune response that in some patients will be potent enough to eradicate the residual tumour cells that remain after completion of standard therapy. A similar expression of alpha-gal epitopes can be achieved by transduction of tumour cells with an adenovirus vector (or other vectors) containing the alpha1,3GT gene, thus enabling anti-Gal-mediated targeting of the vaccinating transduced cells to APC. Intratumoral delivery of the alpha1,3GT gene by various vectors results in the expression of alpha-gal epitopes. Such expression of the xenograft carbohydrate phenotype is likely to induce anti-Gal-mediated destruction of the tumour lesion, similar to rejection of xenografts by this antibody. Opsonization of the destroyed tumour cell membranes by anti-Gal IgG further targets them to APC, thus converting the tumour lesion, treated by the alpha1,3GT gene, into an in situ autologous tumour vaccine.

The α-Gal epitope (Galα1-3Galβ1-4GlcNAc-R) in xenotransplantation

Many patients with failing organs (e.g., heart, liver or kidneys), do not receive the needed organ because of an insufficient number of organ donors. Pig xenografts have been considered as an alternative source of organs for transplantation. The major obstacle currently known to prevent pig to human xenotransplantation is the interaction between the human natural anti-Gal antibody and the α-gal epitope (Galα1-3Galβ1-4GlcNAc-R), abundantly expressed on pig cells. This short review describes the characteristics of anti-Gal and of the alpha-gal epitope, their role in inducing xenograft rejection and some experimental approaches for preventing this rejection.

alpha-Gal epitopes in animal tissue glycoproteins and glycolipids

alpha-Gal terminated saccharides are present on the cell surface both as glycolipids and glycoproteins in all mammals except Old World monkeys and humans. The structural diversity among identified saccharides terminated by this epitope in animal tissues is steadily increasing. The majority of these saccharides have the alpha-Gal linked to lactosamine but other core saccharides exist. The alpha-Gal terminated saccharides are recognized by the immune system as a specific antigen and antibodies directed to the alpha-Gal, which do not cross-react with the classic blood group B trisaccharide, are found in man and Old World monkeys. Similar to other complex carbohydrate cell surface antigens, the alpha-Gal epitope is heterogeneously distributed in different organs and in different cells within an organ. It is present on the vascular endothelium and it is the primary target for human naturally occurring antibodies following pig to primate/man xenotransplantation leading to hyperacute rejection of the graft. Important for the future will be to further structurally characterize this antigen system, its cellular/subcellular distribution, and to identify possible of additional glycosyltransferases, related to the already described alpha 1,3galactosyltransferase that may explain the structural diversity. Such information will be of importance in the studies of, for example, the pathogenesis of autoimmune diseases and for the production of genetically modified pigs to prevent xenograft rejection.

Simultaneous expression by porcine aorta endothelial cells of glycosphingolipids bearing the major epitope for human xenoreactive antibodies (Gal alpha 1-3Gal), blood group H determinant and N-glycolylneuraminic acid

Glycosphingolipids were isolated from primary cultures of porcine endothelial cells labelled with 14C-galactose or 14C-glucosamine. They were characterized by their mobility on thin layer chromatogram, their sensitivity to exoglycosidases, and their labelling with antibodies. In addition to the major glycosphingolipids, globotetra- and globotriaosylceramide, minor ones were identified as penta- and heptaglycosylceramide of the neolactoseries terminated by either Gal alpha 1-3Gal- (xenoreactive epitope) or Fuc alpha 1-2Gal- (H determinant). Two gangliosides were found, GM3 and GD3, and N-glycolylneuraminic acid was their major sialic acid. Therefore, porcine endothelial cells differ from human endothelial cells by expression of glycosphingolipids that are absent in man: two Gal alpha 1-3Gal-terminated glycolipids recognized by human natural antibodies, and two N-glycolylneuraminic acid-terminated gangliosides which are potent immunogens.

Oligosaccharides and discordant xenotransplantation

The initiating factor in the hyperacute rejection of pig organs by human or non-human primates is believed to be related to the presence of preformed "natural" antibodies in the host. In 1991, we demonstrated that human anti-pig antibodies were IgG, IgM and IgA and bound most strongly to oligosaccharides with an alpha galactose (alpha Gal) terminal residue. These included (i) alpha Gal-R (alpha galactose), (ii) alpha Gall-3 beta Gal-R (B disaccharide), (iii) alpha Gall-3 beta Gall-4 beta GlcNAc-R (linear B type 2 trisaccharide) and (iv) alpha Gall-3 beta Gall-4 beta Glc-R (linear B type 6 trisaccharide) where R is (CH2) 8COOCH3. In vitro studies using both the chromium release assay and a live/dead staining technique demonstrated that the cytotoxicity of human sera towards pig cells can be significantly reduced or abolished by immunoadsorption of the serum with immunoaffinity columns of an alpha Gal structure, particularly those with an alpha 1-3 linkage, and not by a large selection of other carbohydrates. Similarly, human anti-pig antibodies can be largely inhibited or "neutralized" by the addition of an alpha 1-3Gal di- or trisaccharide to the serum. Staining of pig vascular endothelium utilizing a panel of carbohydrate-specific lectins and immunoaffinity antibodies demonstrated the presence of three different carbohydrate epitopes, namely (i) alpha Gall-3 beta Gall-4 beta GlcNAc-R (linear B type 2 trisaccharide (ii) alpha NeuAc2-3 beta Gall-4 beta GlcNAc-R (sialyl-N-acetyllactosamine), and (iii) beta Gall-4 beta GlcNAc-R (N-acetyllactosamine). We have investigated organs from several breeds of pig and have concluded that the alpha Gal epitope is either monomorphic or at least has a high incidence in porcine species, since we have not found any pig negative for this antigen. Human vascular endothelial cells have at their surface the same lactosamine-ended precursor and sialylated chains as pigs, but instead of terminal alpha Gal they express the fucosylated polymorphic ABH histo-blood group epitopes. As we have found no evidence that human or baboon plasma contain antibodies directed against sialic acid or lactosamine, and as human tissues contain both of these carbohydrates, it seems unlikely that either of these epitopes plays a role in the vascular rejection that takes place when pig organs are transplanted into primates. Unfortunately, the alpha Gal disaccharide and trisaccharides were not available to us in the large quantities required for extracorporeal immunoadsorption or continuous intravenous infusion in adult baboons.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural determination of three neutral oligosaccharides in bovine (Holstein-Friesian) colostrum, including the novel trisaccharide; GalNAc alpha 1-3Gal beta 1-4Glc

Two trisaccharides, and a pentasaccharide were obtained from bovine colostrum. Their chemical structures were determined by using methylation and 13C-NMR analyses as follows: GalNac alpha 1-3Gal beta 1-4Glc, Gal alpha-1-3Gal beta 1-4Glc, GaL beta 1-3[Gal beta 1-4GlcNAc beta 1-6]Gal beta 1-4Glc. GalNAc alpha 1-3Gal beta 1-4Glc, which was identified in this study, is a novel oligosaccharide from natural sources. Gal alpha 1-3Gal beta 1-4Glc and Gal beta 1-3[Gal beta 1-4GlcNAc beta 1-6]Gal beta 1-4Glc (lacto-N-novopentaose) have been already found in ovine colostrum, and in horse colostrum and marsupial milk, respectively.

Toxin A from Clostridium difficile binds to rabbit erythrocyte glycolipids with terminal Gal alpha 1-3Gal beta 1-4GlcNAc sequences

The binding of Toxin A isolated from Clostridium difficile to rabbit erythrocyte glycolipids has been studied. Total lipid extracts from rabbit erythrocytes were subjected to thin-layer chromatography and toxin-binding glycolipids detected by using 125I-labeled Toxin A in a direct binding overlay technique. Two major and several minor toxin-binding glycolipids were detected in rabbit erythrocytes by this method. The results of structural analyses of the major toxin-binding glycolipids were consistent with a pentasaccharide-ceramide (Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4Glc-Cer) and a branched decasaccharide-ceramide (Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3[Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-6]Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4Glc-Cer) previously identified as the two most abundant glycolipids in rabbit erythrocytes. 125I-Toxin A binding to these glycolipids could be inhibited by bovine thyroglobulin, monospecific antiserum to the toxin, or by treatment of the glycolipids with alpha-galactosidase. The absence of toxin interaction with isoglobotriaosylceramide (Gal alpha 1-3Gal beta 1-4Glc-Cer) isolated from canine intestine suggested that the GlcNAc residue present in the terminal Gal alpha 1-3Gal beta 1-4GLcNAc sequence common to all known toxin binding glycoconjugates is required for carbohydrate-specific recognition by Toxin A. These observations are consistent with the proposed carbohydrate binding specificity of Toxin A for the nonreducing terminal sequence, Gal alpha 1-3Gal beta 1-4GlcNAc.

A unique glycosphingolipid-splitting enzyme (ceramide-glycanase from leech) cleaves the linkage between the oligosaccharide and the ceramide

A novel type of enzyme which hydrolyzes the linkage between the ceramide and the sugar chain in various glycosphingolipids has been found in the leech, Hirudo medicinalis. This enzyme releases the intact oligosaccharide from LacCer, GbOse3Cer, GbOse4Cer, GbOse5Cer, nLcOse4Cer, GM3, GM2, GM1, GD1a and GT1 with the concurrent release of ceramides. By using tritium-labeled GM1 as substrate we found the optimum pH of this enzyme to be between pH 4 and 5. Since the enzyme cleaves the linkage between the ceramide and the sugar chain in various glycosphingolipids with no apparent preference toward the sugar chain, we propose to call this enzyme ceramide-glycanase.

Identification and characterization of an UDP-Gal: N-acetyllactosaminide alpha-1,3-D-galactosyltransferase in calf thymus

Calf thymus was found to contain a high activity of a galactosyltransferase, which transfers galactose from UDP-galactose to asialo-alpha 1-acid glycoprotein and N-acetyllactosamine. By means of competition and acceptor-specificity studies the enzyme could be distinguished from an N-acetylglucosaminide beta-1,4-galactosyltransferase and an N-acetylgalactosamine-protein beta-1,3-galactosyltransferase, which in addition occur in calf thymus, as well as from the blood-group-B-associated alpha-galactosyltransferase. Identification of the products revealed that the enzyme accomplishes an alpha 1 leads to 3 linkage resulting in a terminal Gal(alpha 1 leads to 3)Gal(beta 1 leads to 4)GlcNAc sequence. The enzyme is membrane-bound and is activated by Triton X-100. It shows optimal activity over a broad pH range (5.5-7.0) and has a pronounced requirement for Mn2+ ions (Km = 6.1 mM) for its action. It is suggested that the alpha-1,3-galactosyltransferase functions in the biosynthesis of calf thymocyte cell-surface glycoconjugates including glycoproteins.

Glycosphingolipids of guinea pig gastric mucosa

Glycosphingolipids have beenn isolated from guinea pig gastric mucosa and their composition and content determined. The neutral glycospingolipids were found to consist of mono-, di-, tri- and pentaglycosylceramide. The acidic glycosphingolipids wee represented by galactosyl and lactosyl sulfatides, and GM4, GM3 and GD3 gangliosides. None of the analyzed glycolipids contained N-acetylglucosamine and fucose.