Anti-Galalpha1-3Gal antibody levels in organ transplant recipients receiving immunosuppressive therapy

Modifying the sugar icing on the transplantation cake

As a transplant surgeon, my interest in glycobiology began through my research into ABO-incompatible allotransplantation, and grew when my goal became overcoming the shortage of organs from deceased human donors by the transplantation of pig organs into patients with terminal organ failure (xenotransplantation/cross-species transplantation). The major target for human "natural" (preformed) anti-pig antibodies is galactose-α(1,3)-galactose (the "Gal" epitope), which is expressed on many pig cells, including the vascular endothelium. The binding of human IgM and IgG antibodies to Gal antigens initiates the process of hyperacute rejection, resulting in destruction of the pig graft within minutes or hours. This major barrier has been overcome by the production of pigs in which the gene for the enzyme α(1,3)-galactosyltransferase (GT) has been deleted by genetic engineering, resulting in GT knockout (GTKO) pigs. The two other known carbohydrate antigenic targets on pig cells for human anti-pig antibodies are (i) the product of the cytidine monophosphate-N-acetylneuraminic acid hydroxylase (CMAH) gene, i.e., N-glycolylneuraminic acid, and (ii) the product of the β1,4 N-acetylgalactosaminyltransferase gene, i.e., the Sd(a) antigen. Expression of these two has also been deleted in pigs. These genetic manipulations, together with others directed to overcoming primate complement and coagulation activation (the latter of which also relates to glycobiology) have contributed to the prolongation of pig graft survival in nonhuman primate recipients to many months rather than a few minutes. Clinical trials of the transplantation of pig cells are already underway and transplantation of pig organs may be expected within the relatively near future.

Anti-alphaGal-dependent complement-mediated cytotoxicity in metastatic melanoma

Antibodies to the cell surface disaccharide galactose(alpha1,3)galactose (alphaGal) are the most prevalent natural antibodies in human serum. The anti-alphaGal immunoglobulin M-dependent activation of complement causes hyperacute rejection of organ transplants from discordant species by human recipients. It has been shown in vitro that human tumour cells transduced with the gene that synthesizes alphaGal become sensitive to human serum. A prerequisite for anti-alphaGal antibody-based therapeutic strategies is that patients with cancer have adequate serum levels of anti-alphaGal immunoglobulins and complement. The objective of this work was to measure the levels and function of anti-alphaGal immunoglobulins and complement in the serum of patients with metastatic melanoma and healthy volunteers. Serum complement levels were assayed by radial immunodiffusion. Anti-alphaGal immunoglobulin G and immunoglobulin M titres were measured by enzyme-linked immunosorbent assay. Disaccharide sugar blocking was used to investigate antibody specificity. The functional integrity of anti-alphaGal antibodies and complement was investigated in cell lysis assays. It was found that the levels of the complement components C1q, C3 and C4 and the function of the classical complement pathway were normal in metastatic melanoma patients. Anti-alphaGal antibody titres were as variable in metastatic melanoma patients as in healthy controls, and the lysis of alphaGal-expressing cells correlated with anti-alphaGal immunoglobulin M titre (P < 0.0001). Anti-alphaGal antibody titres, complement levels and overall cytolytic function in the serum of patients with metastatic melanoma were indistinguishable from those of healthy controls. There is thus nothing intrinsic to the disease that will limit anti-alphaGal-based therapeutic strategies for enhanced antigen presentation or induced cell lysis, including the mimicry of hyperacute rejection.

Carbohydrates in xenotransplantation

The success of allotransplantation has led to an increasing shortage of human organs from deceased donors. This crisis could be resolved by the use of organs from an anatomically suitable animal, such as the pig. The pig and human have, however, been evolving differently for approximately 80 million years, and numerous immunological and physiological barriers have developed that need to be overcome. Differences in carbohydrate epitopes on pig and human cells have been found to play a major role in some of the immunological barriers that have been identified to date. The rejection caused by the presence of galactose-alpha1,3-galactose (Gal) on the pig vascular endothelium and of natural anti-Gal antibodies in humans has recently been prevented by the breeding of pigs that do not express Gal, achieved by knocking out the gene for the enzyme alpha1,3-galactosyltransferase, which was made possible by the introduction of nuclear transfer/embryo transfer techniques. N-glycolylneuraminic acid (the so-called Hanganutziu-Deicher antigen) has been identified as another carbohydrate antigen present in pigs that may need to be deleted if xenotransplantation is to be successful, although some doubt remains regarding its importance. There remain other antipig antibodies against hitherto unidentified antigenic targets that may well be involved in graft destruction; their possible carbohydrate target epitopes are discussed.

Evidence for Galalpha(1-3)Gal expression on primary porcine hepatocytes: implications for bioartificial liver systems

BACKGROUND/AIMS

To bridge acute liver failure (ALF) patients to orthotopic liver transplantation, several bioartificial liver (BAL) systems have been developed. The bio-component of most BAL systems consists mainly of porcine hepatocytes. Plasma or blood of ALF patients is perfused through the BAL thereby contacting porcine hepatocytes. Xenogeneic BAL systems may suffer from hyperacute rejection similar to whole-organ xenotransplants. Hyperacute rejection is mediated by antibodies directed against Galalpha(1-3)Gal, a carbohydrate structure present on most mammalian cells. Galalpha(1-3)Gal is produced by the enzyme alpha1,3-galactosyltansferase (alphaGal-T). Conflicting data have been published concerning Galalpha(1-3)Gal expression on hepatocytes in intact porcine liver. We investigated whether isolated porcine hepatocytes express Galalpha(1-3)Gal.

METHODS

Immunofluorescence, flow cytometry, RT-PCR and enzyme activity assays were performed on freshly isolated and cultured porcine hepatocytes and liver biopsies. Anti-Galalpha(1-3)Gal antibodies were measured in plasma from patients treated with BAL by ELISA.

RESULTS

Isolated porcine hepatocytes express (alphaGal-T) at low levels and Galalpha(1-3)Gal is present in low quantities on these cells, in contrast to hepatocytes in situ. Furthermore, IgG and IgM anti-Galalpha(1-3)Gal are depleted from the plasma of ALF patients during BAL treatment.

CONCLUSIONS

Isolation and culture of porcine hepatocytes induce Galalpha(1-3)Gal expression, which may elicit immunological responses potentially compromising BAL functionality.

Removal of anti-Galalpha1,3Gal xenoantibodies with an injectable polymer

Preformed and elicited Ab's against the Galalpha1,3Gal terminating carbohydrate chains (alphaGal Ab's) are the primary cause of hyperacute and acute vascular xenograft rejection in pig-to-primate transplantation. alphaGal Ab's are produced by long-lived Ab-producing cells that are not susceptible to pharmacological immunosuppression. We reasoned that antigen-specific elimination of alphaGal Ab's might be achieved in vivo by systemic administration of nonimmunogenic polyvalent alphaGal structures with high avidity for alphaGal Ab's. We devised GAS914, a soluble trisaccharide-polylysine conjugate of approximately 500 kDa that effectively competes for alphaGal binding by alphaGal IgM (IC(50), 43 nM) and IgG (IC(50), 28 nM) in vitro. Injections of GAS914 in cynomolgus monkeys, at the dose of 1 mg/kg, resulted in the immediate decrease of more than 90% of circulating alphaGal Ab's and serum anti-pig cytotoxicity. In baboons, repeated injections of GAS914 effectively reduced both circulating alphaGal Ab's and cytotoxicity over several months. Studies with [(14)C]GAS914 in rhesus monkeys and Gal(-/-) mice indicate that GAS914 binds to circulating alphaGal Ab's and that the complex is quickly metabolized by the liver and excreted by the kidney. Remarkably, posttreatment alphaGal Ab titers never exceeded pretreatment levels and no sensitization to either alphaGal or the polylysine backbone has been observed. Furthermore there was no apparent acute or chronic toxicity associated with GAS914 treatment in primates. We conclude that GAS914 may be used therapeutically for the specific removal of alphaGal Ab's.

Removal of anti-Galalpha1,3Gal xenoantibodies with an injectable polymer

Preformed and elicited Ab's against the Galalpha1,3Gal terminating carbohydrate chains (alphaGal Ab's) are the primary cause of hyperacute and acute vascular xenograft rejection in pig-to-primate transplantation. alphaGal Ab's are produced by long-lived Ab-producing cells that are not susceptible to pharmacological immunosuppression. We reasoned that antigen-specific elimination of alphaGal Ab's might be achieved in vivo by systemic administration of nonimmunogenic polyvalent alphaGal structures with high avidity for alphaGal Ab's. We devised GAS914, a soluble trisaccharide-polylysine conjugate of approximately 500 kDa that effectively competes for alphaGal binding by alphaGal IgM (IC(50), 43 nM) and IgG (IC(50), 28 nM) in vitro. Injections of GAS914 in cynomolgus monkeys, at the dose of 1 mg/kg, resulted in the immediate decrease of more than 90% of circulating alphaGal Ab's and serum anti-pig cytotoxicity. In baboons, repeated injections of GAS914 effectively reduced both circulating alphaGal Ab's and cytotoxicity over several months. Studies with [(14)C]GAS914 in rhesus monkeys and Gal(-/-) mice indicate that GAS914 binds to circulating alphaGal Ab's and that the complex is quickly metabolized by the liver and excreted by the kidney. Remarkably, posttreatment alphaGal Ab titers never exceeded pretreatment levels and no sensitization to either alphaGal or the polylysine backbone has been observed. Furthermore there was no apparent acute or chronic toxicity associated with GAS914 treatment in primates. We conclude that GAS914 may be used therapeutically for the specific removal of alphaGal Ab's.

The potential role of xenotransplantation in treating endstage cardiac disease: a summary of the report of the Xenotransplantation Advisory Committee of the International Society for Heart and Lung Transplantation

There is an increasing shortage of human donor hearts for transplantation. One potential solution is using hearts from a suitable animal source, such as the pig. A committee of the International Society for Heart and Lung Transplantation has reviewed the current status of research into xenotransplantation. Furthermore, the committee considered what results in the pig-to-nonhuman primate experimental model would justify a clinical trial of xenotransplantation, and the criteria for selecting patients to be entered in the trial. This review emphasizes initial patient selection issues, although the committee's overall conclusions and recommendations are summarized. Although the current experimental results do not presently justify initiating a clinical trial, the committee concluded that xenotransplantation theoretically has immense potential, and that research in this field should be encouraged and supported. Human cadaveric organ donation will still be of the highest priority for the foreseeable future.

Report of the Xenotransplantation Advisory Committee of the International Society for Heart and Lung Transplantation: the present status of xenotransplantation and its potential role in the treatment of end-stage cardiac and pulmonary diseases

An urgent and steadily increasing need exists world-wide for a greater supply of donor thoracic organs. Xenotransplantation offers the possibility of an unlimited supply of hearts and lungs that could be available electively when required. However, anti-body- mediated mechanisms cause the rejection of pig organs transplanted into non-human primates, and these mechanisms provide major immunologic barriers that have not yet been overcome. Having reviewed the literature on xenotransplantation, we present a number of conclusions on its present status with regard to thoracic organs, and we make a number of recommendations relating to eventual clinical trials. Although pig hearts have functioned in heterotopic sites in non-human primates for periods of several weeks, median survival of orthotopically transplanted hearts is currently ,1 month. No transplanted pig lung has functioned for even 24 hours. Current experimental results indicate that a clinical trial would be premature. A potential risk exists, hitherto undetermined, of transferring infectious organisms along with the donor pig organ to the recipient, and possibly to other members of the community. A clinical trial of xeno-transplantation should not be undertaken until experts in microbiology and the relevant regulatory authorities consider this risk to be minimal. A clinical trial should be considered when approximately 60% survival of life-supporting pig organs in non-human primates has been achieved for a minimum of 3 months, with at least 10 animals surviving for this minimum period. Furthermore, evidence should suggest that longer survival (.6 months) can be achieved. These results should be achieved in the absence of life-threatening complications caused by the immunosuppressive regimen used. The relationship between the presence of anti-HLA antibody and anti-pig antibody and their cross-reactivity, and the outcome of pig-organ xenotransplantation in recipients previously sensitized to HLA antigens require further investigation. We recommend that the patients who initially enter into a clinical trial of cardiac xenotransplantation be unacceptable for allotransplantation, or acceptable for allotransplantation but unlikely to survive until a human cadaveric organ becomes available, and in whom mechanical assist-device bridging is not possible. National bodies that have wide-reaching government-backed control over all aspects of the trials should regulate the initial clinical trial and all subsequent clinical xenotransplantation procedures for the foreseeable future. We recommend coordination and monitoring of these trials through an international body, such as the International Society for Heart and Lung Transplantation, and setting up a registry to record and widely disperse the results of these trials. Xenotransplantation has the potential to solve the problem of donor-organ supply, and therefore research in this field should be actively encouraged and supported.