Alpha 1,3galactosyltransferase: a target for in vivo genetic manipulation in xenotransplantation

Investigation of ABO Gene Variants across More Than 60 Pig Breeds and Populations and Other Suidae Species Using Whole-Genome Sequencing Datasets

Polymorphisms in the human ABO gene determine the major blood classification system based on the three well-known forms: A; B; and O. In pigs that carry only two main alleles in this gene (A and O), we still need to obtain a more comprehensive distribution of variants, which could also impact its function. In this study, we mined more than 500 whole-genome sequencing datasets to obtain information on the ABO gene in different Suidae species, pig breeds, and populations and provide (i) a comprehensive distribution of the A and O alleles, (ii) evolutionary relationships of ABO gene sequences across Suidae species, and (iii) an exploratory evaluation of the effect of the different ABO gene variants on production traits and blood-related parameters in Italian Large White pigs. We confirmed that allele O is likely under balancing selection, present in all Sus species investigated, without being fixed in any of them. We reported a novel structural variant in perfect linkage disequilibrium with allele O that made it possible to estimate the evolutionary time window of occurrence of this functional allele. We also identified two single nucleotide polymorphisms that were suggestively associated with plasma magnesium levels in pigs. Other studies can also be constructed over our results to further evaluate the effect of this gene on economically relevant traits and basic biological functions.

Virus recognition by specific natural antibodies and complement results in MHC I cross-presentation

Natural antibodies (NAb) and complement (C’) are important regulators of immune system activation. We have shown previously that the galactosyl‐α1,3‐galactosyl (Galα1,3Gal) xenoantigen and the similar ABO histo‐blood group antigens are transferred onto virus from the producer cell, resulting in sensitisation of the virus to the respective NAb in a C’‐dependent manner. Here we show that measles virus (Mv) that expresses Galα1,3Gal termini can drive the proliferation of human T cells in the presence of serum and autologous DC, whereas without such targets, measles, as expected, suppress T cell reactivity. The use of affinity‐purified NAb to Galα1,3Gal and rabbit C’ demonstrated the components in human serum responsible for this effect. Proteasome inhibition and blocking of antigen presentation showed that the increased T cell proliferation was mediated by MHC class I cross‐presentation of immune complexes. These results lend further support to the idea that polymorphic carbohydrates of the Galα1,3Gal/ABO type serve as important targets for NAb and C’ and that their expression on virus has influenced their evolution by contributing to protection against viral transmission within as well as between species. The adjuvance effect of this recognition, acting as a bridge between the natural innate and adaptive immune systems, also has important implications for vaccine development.

Anti-pig antibody adsorption efficacy of {alpha}-Gal carrying recombinant P-selectin glycoprotein ligand-1/immunoglobulin chimeras increases with core 2 {beta}1, 6-N-acetylglucosaminyltransferase expression

We have previously described the construction of a P-selectin glycoprotein ligand-1-mouse immunoglobulin Fc fusion protein, which when transiently coexpressed with the porcine alpha1,3 galactosyltransferase in COS cells becomes a very efficient adsorber of xenoreactive, anti-pig antibodies. To relate the adsorption capacity with the glycan expression of individual fusion proteins produced in different cell lines, stable CHO-K1, COS, and 293T cells producing this fusion protein have been engineered. On alpha1,3 galactosyltransferase coexpression, high-affinity adsorbers were produced by both COS and 293T cells, whereas an adsorber of lower affinity was derived from CHO-K1 cells. Stable coexpression of a core 2 beta1,6 N-acetylglucosaminyltransferase in CHO-K1 cells led to increased alpha-Gal epitope density and improved anti-pig antibody adsorption efficacy. ESI-MS/MS of O-glycans released from PSGL-1/mIgG(2b) produced in an alpha1,3 galactosyl- and core 2 beta1,6 N-acetylglucosaminyltransferase expressing CHO-K1 cell clone revealed a number of structures with carbohydrate sequences consistent with terminal Gal-Gal. In contrast, no O-glycan structures with terminal Gal-Gal were identified on the fusion protein when expressed alone or in combination with the alpha1,3 galactosyltransferase in CHO-K1 cells. In conclusion, the density of alpha-Gal epitopes on PSGL-1/mIgG(2b) was dependent on the expression of O-linked glycans with core 2 structures and lactosamine extensions. The structural complexity of the terminal Gal-Gal expressing O-glycans with both neutral as well as sialic acid-containing structures is likely to contribute to the high adsorption efficacy.

Expression of ABO or related antigenic carbohydrates on viral envelopes leads to neutralization in the presence of serum containing specific natural antibodies and complement

No definitive biologic function has been associated with the human ABO histo-blood group polymorphism, or any other terminal carbohydrate differences within or between closely related species. We have experimentally addressed the question of whether viral particles can become glycosylated as determined by the glycosylation (eg, ABO) status of the producer cell and as a result be affected by human serum containing specific natural antibodies (NAbs). Measles virus was produced in cells transfected with cDNA encoding, either human A-transferase, B-transferase, an inactive "O-transferase," or a pig alpha1-3galactosyltransferase (alpha1-3GT) synthesizing the Galalpha1-3Gal structure. The viruses were shown to carry the same ABO structures as the cells; that is, A but not B if produced in A-type cells, and B but not A if produced in B-type cells. Only O was detected on the virus produced from O-type cells, whereas reduced amounts of O appeared on the A- and B-type viral particles. In addition, the Galalpha1-3Gal structure was transferred onto measles only when grown in human cells expressing this structure. When subjected to human preimmune sera, the A-type, the B-type, and the Galalpha1-3Gal viral particles were partially neutralized in a complement-dependent manner. However, the O-type or the Galalpha1-3Gal-negative viral particles were not neutralized. The neutralization appeared to be mediated by specific NAb, as judged by specific inhibition using synthetic A and Galalpha1-3Gal oligosaccharides. Such viral glycosylation may thus partly explain why the ABO antigens and other similar intraspecies as well as interspecies polymorphic carbohydrates have evolved and been maintained over long evolutionary periods.

Xenotransplantation: the importance of the Galalpha1,3Gal epitope in hyperacute vascular rejection

The transplantation of organs from other species into humans is considered to be a potential solution to the shortage of human donor organs. Organ transplantation from pig to human, however, results in hyperacute rejection, initiated by the binding of human natural antidonor antibody and complement. The major target antigen of this natural antibody is the terminal disaccharide Galalphal,3Gal, which is synthesized by Galbeta1,4GlcNAc alpha1,3-galactosyltransferase. Here we review our current knowledge of this key enzyme. A better understanding of structure, enzyme properties, and expression pattern of alpha1,3-galactosyltransferase has opened up several novel therapeutic approaches to prevent hyperacute vascular rejection. Cloning, and expression in vitro of the corresponding cDNA, has allowed to develop strategies to induce immune tolerance, and deplete or neutralize the natural xenoreactive antibody. Elucidation of the genomic structure has led to the production of transgenic animals that are lacking alpha1,3-galactosyltransferase activity. A detailed knowledge of the enzyme properties has formed the basis of approaches to modify donor organ glycosylation by intracellular competition. Study of the expression pattern of alpha1,3-galactosyltransferase has helped to understand the mechanism of hyperacute rejection in discordant xenotransplantation, and that of complement-mediated, natural immunity against interspecies transmission of retroviruses.

Acceptor substrate-based selective inhibition of galactosyltransferases

This paper describes the discovery of glycosyl acceptor analogs as potent and selective inhibitors of alpha-1,3- and beta-1,4-galactosyltransferases. Incorporation of an appropriate aromatic group to the aglycon position of the enzyme's acceptors results in a strong inhibition, representing the first and most potent small uncharged molecules as selective inhibitors of these two enzymes and thus providing a new strategy for the development of selective glycosyltransferase inhibitors.

On the preparative use of recombinant pig alpha(1-3)galactosyl-transferase

A series of non-natural N-acyl derivatives of lactosamine is incubated with recombinant alpha(1-3)galactosyl-transferase and UDP-galactose. The enzyme shows a high promiscuity towards the non-natural acceptors. It selectively transfers a galactose unit onto the 3-OH group of the terminal beta-linked galactose in an alpha-mode to give an array of linear-B trisaccharides.

Removal of xenoreactive human anti-pig antibodies by absorption on recombinant mucin-containing glycoproteins carrying the Gal alpha1,3Gal epitope

BACKGROUND

The hyperacute rejection caused by preformed natural antibodies in the recipient species reacting with donor species endothelial antigens is one of the major obstacles preventing routine use of clinical xenotransplantation. Based on the known structure and biosynthetic pathway of the major porcine xenoantigen, Gal alpha1,3Gal, we developed a novel strategy aimed at specific removal of human, natural anti-pig antibodies.

METHODS

Cotransfection of COS cells with expression plasmids encoding a secreted mucin/immunoglobulin chimera and the porcine alpha1,3-galactosyltransferase facilitated simple immunoaffinity purification of a highly Gal alpha1,3Gal-substituted mucin/immunoglobulin fusion protein from transfected cell supernatants.

RESULTS

Cotransfection of COS cells resulted in a mucin/Ig concentration in the supernatants ranging from 150 to 200 ng/ml. Approximately 300 ng of mucin/Ig chimeras absorbed onto 50 microl of packed anti-mouse IgG agarose beads could completely remove cytotoxic human anti-pig antibodies from 1 ml of human AB serum, as estimated in porcine endothelial cell cytotoxicity assays. Purified human IgG, IgM, and IgA all bound porcine endothelium, but only IgG and IgM were cytotoxic in the presence of rabbit complement. When the cytotoxicity of human IgG at 8 mg/ml and IgM at 1 mg/ml was completely removed by absorption on the mucin/Ig chimera, the binding to porcine endothelium was only partly reduced. Antibodies mediating antibody-dependent cellular cytotoxicity against porcine endothelium were also absorbed, indicating the importance of Gal alpha1,3Gal epitopes for this effect.

CONCLUSIONS

We describe the construction and production of a new and effective Gal alpha1,3Gal-substituted, mucin domain-containing absorber that can be used in a pretransplant extracorporeal immunoabsorption setting to remove anti-pig antibodies involved in antibody-dependent, complement- and cell-mediated cytotoxicity of pig endothelial cells.

Sensitization of cells and retroviruses to human serum by (alpha 1-3) galactosyltransferase

Mammalian C-type retroviruses are inactivated by human serum, following triggering of the classical complement cascade. This may have inhibited transmission to humans of C-type oncoviruses from other mammals. Indeed, the retroviruses human immunodeficiency virus and human T-cell leukaemia virus are resistant to human complement. Antibody-independent activation of human C1q, the first component of the classical pathway, by retroviral envelope proteins has been described. However, retroviruses produced from human cells are resistant to inactivation by human complement and human serum is known to contain antibodies directed against carbohydrates on retroviral envelopes. Gal(alpha 1-3)Gal terminal carbohydrates are expressed by most mammals but are absent in humans, which lack a functional (alpha 1-3)galactosyltransferase gene. Here, we demonstrate that anti-Gal(alpha 1-3)Gal antibodies in human serum inactivate retroviruses produced from animal cells. Expression of porcine (alpha 1-3)galactosyltransferase in human cells renders the cells and the retroviruses they produce sensitive to human serum.

cDNA sequence and chromosome localization of pig alpha 1,3 galactosyltransferase

Human serum contains natural antibodies (NAb), which can bind to endothelial cell surface antigens of other mammals. This is believed to be the major initiating event in the process of hyperacute rejection of pig to primate xenografts. Recent work has implicated galactosyl alpha 1,3 galactosyl beta 1,4 N-acetyl-glucosaminyl carbohydrate epitopes, on the surface of pig endothelial cells, as a major target of human natural antibodies. This epitope is made by a specific galactosyltransferase (alpha 1,3 GT) present in pigs but not in higher primates. We have now cloned and sequenced a full-length pig alpha 1,3 GT cDNA. The predicted 371 amino acid protein sequence shares 85% and 76% identity with previously characterized cattle and mouse alpha 1,3 GT protein sequences, respectively. By using fluorescence and isotopic in situ hybridization, the GGTA1 gene was mapped to the region q2.10-q2.11 of pig chromosome 1, providing further evidence of homology between the subterminal region of pig chromosome 1q and human chromosome 9q, which harbors the locus encoding the AB0 blood group system as well as a human pseudogene homologous to the pig GGTA1 gene.