The problem of anti-pig antibodies in pig-to-primate xenografting: current and novel methods of depletion and/or suppression of production of anti-pig antibodies

The first clinical pig heart transplant: Was IVIg or pig cytomegalovirus detrimental to the outcome?

The clinical course of the first patient to receive a gene-edited pig heart transplant was recently reported by the University of Maryland team. Although the pig heart functioned well for >40 days, serum anti-pig antibodies then increased, and the patient sadly died after 60 days. Because of his debilitated pre-transplant state, the patient never thrived despite excellent graft function for several weeks, and the cause of his demise continues to be uncertain. A few days before an increase in anti-pig antibodies was observed, the patient had received intravenous human immunoglobulin (IVIg), and whether this played a role in his cardiac deterioration has been discussed. Furthermore, mcfDNA testing indicated an increase in pig cytomegalovirus (CMV), and its possible role in the development of cardiac dysfunction has also been considered. On the basis of the limited data provided in the publication and on our previous investigations into whether IVIg contains anti-TKO pig antibodies and therefore might be deleterious to TKO pig organ xenografts, we suggest that the steady rise in anti-pig antibody titer was more consistent with the failure of the immunosuppressive regimen to prevent elicited anti-TKO pig antibody production, rather than from the passive transfusion of IVIg or the presence of pig CMV in the graft. Although the outcome of the Maryland experience was disappointing, valuable lessons were learned. Our attention was drawn to the potential risks of heart transplantation in a "deconditioned" patient, the administration of IVIg, the transmission of pig CMV, and of the difficulties in interpreting myocardial biopsy findings.

Investigation of the influence of xenoreactive antibodies on activation of complement and coagulation in an ex vivo perfusion animal study using porcine kidneys

During pig-to-primate xenotransplantation or perfusion of porcine organs with human blood, a xenogeneic coagulopathy with consecutive development of thrombotic microangiopathy (TMA) can be observed. The aim of this study was to elucidate the influence of the reduction of xenoreactive natural antibodies on the coagulopathy using an ex vivo perfusion system. Thirteen perfusion experiments using landrace wild-type porcine kidneys were performed in three different experimental groups: autologous, xenogeneic, and immunoadsorption. During and after perfusion, blood and tissue samples were collected to assess markers of coagulation, complement, inflammation, and endothelial activation. Immunoadsorption prior to perfusion did not prolong perfusion time (174 min ±28) compared to xenogeneic (182 min ±22) experiments, whereas autologous perfusion was possible for maximum of 240 min in all experiments. Activation of coagulation was similar comparing perfusions after immunoadsorption (D-Dimer 24 186 μg/l ±5813; TAT 566 μg/l ±34) to xenogeneic (D-Dimer 22 175 μg/l ±7826, TAT 600 μg/l ±0) experiments. But antibody-mediated complement activation was reduced in the immunoadsorption group. TNF-alpha and markers of endothelial cell activation were lower in the immunoadsorption group compared to the xenogeneic experiments. In this ex vivo perfusion model, we observed that marked removal of xenogeneic antibodies can reduce complement activation via the classical pathway as well as endothelial cell activation and inflammation. Immunoadsorption cannot prevent the activation of the terminal complement cascade and coagulation.

The pathobiology of pig-to-primate xenotransplantation: a historical review

The immunologic barriers to successful xenotransplantation are related to the presence of natural anti-pig antibodies in humans and non-human primates that bind to antigens expressed on the transplanted pig organ (the most important of which is galactose-α1,3-galactose [Gal]), and activate the complement cascade, which results in rapid destruction of the graft, a process known as hyperacute rejection. High levels of elicited anti-pig IgG may develop if the adaptive immune response is not prevented by adequate immunosuppressive therapy, resulting in activation and injury of the vascular endothelium. The transplantation of organs and cells from pigs that do not express the important Gal antigen (α1,3-galactosyltransferase gene-knockout [GTKO] pigs) and express one or more human complement-regulatory proteins (hCRP, e.g., CD46, CD55), when combined with an effective costimulation blockade-based immunosuppressive regimen, prevents early antibody-mediated and cellular rejection. However, low levels of anti-non-Gal antibody and innate immune cells and/or platelets may initiate the development of a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. This pathogenic process is accentuated by the dysregulation of the coagulation-anticoagulation systems between pigs and primates. The expression in GTKO/hCRP pigs of a human coagulation-regulatory protein, for example, thrombomodulin, is increasingly being associated with prolonged pig graft survival in non-human primates. Initial clinical trials of islet and corneal xenotransplantation are already underway, and trials of pig kidney or heart transplantation are anticipated within the next few years.

IgG response to intracerebral xenotransplantation: specificity and role in the rejection of porcine neurons

Xenogenic fetal neuroblasts are considered as a potential source of transplantable cells for the treatment of neurodegenerative diseases, but immunological barriers limit their use in the clinic. While considerable work has been performed to decipher the role of the cellular immune response in the rejection of intracerebral xenotransplants, there is much still to learn about the humoral reaction. To this end, the IgG response to the transplantation of fetal porcine neural cells (PNC) into the rat brain was analyzed. Rat sera did not contain preformed antibodies against PNC, but elicited anti-porcine IgG was clearly detected in the host blood once the graft was rejected. Only the IgG1 and IgG2a subclasses were up-regulated, suggesting a T-helper 2 immune response. The main target of these elicited IgG antibodies was porcine neurons, as determined by double labeling in vitro and in vivo. Complement and anti-porcine IgG were present in the rejecting grafts, suggesting an active role of the host humoral response in graft rejection. This hypothesis was confirmed by the prolonged survival of fetal porcine neurons in the striatum of immunoglobulin-deficient rats. These data suggest that the prolonged survival of intracerebral xenotransplants relies on the control of both cell-mediated and humoral immune responses.

Pathologic characteristics of transplanted kidney xenografts

For xenotransplantation to become a clinical reality, we need to better understand the mechanisms of graft rejection or acceptance. We examined pathologic changes in α1,3-galactosyltransferase gene-knockout pig kidneys transplanted into baboons that were treated with a protocol designed to induce immunotolerance through thymic transplantation (n=4) or were treated with long-term immunosuppressants (n=3). Hyperacute rejection did not occur in α1,3-galactosyltransferase gene-knockout kidney xenografts. By 34 days, acute humoral rejection led to xenograft loss in all three xenografts in the long-term immunosuppression group. The failing grafts exhibited thrombotic microangiopathic glomerulopathy with multiple platelet-fibrin microthrombi, focal interstitial hemorrhage, and acute cellular xenograft rejection. Damaged glomeruli showed IgM, IgG, C4d, and C5b-9 deposition. They also demonstrated endothelial cell death, diffuse endothelial procoagulant activation with high expression of tissue factor and vWF, and low expression of the ectonucleotidase CD39. In contrast, in the immunotolerance group, two of four grafts had normal graft function and no pathologic findings of acute or chronic rejection at 56 and 83 days. One of the remaining kidneys had mild but transient graft dysfunction with reversible, mild microangiopathic glomerulopathy, probably associated with preformed antibodies. The other kidney in the immunotolerance group developed unstable graft function at 81 days and developed chronic xenograft glomerulopathy. In summary, the success of pig-to-primate xenotransplantation may necessitate immune tolerance to inhibit acute humoral and cellular xenograft rejection.

The Baboon in Xenotransplant Research

If cross-species transplantation is ever to become a reasonable therapeutic modality for human beings, it will be because the potential for success has been demonstrated in a nonhuman primate model. The imperative has always been to select a primate research subject from a species that is plentiful, is not endangered, readily procreates in a managed environment, and mimics the human response (immunologic homology) to both organ transplantation and potential transfer of infectious disease. Several Papio subspecies of baboons, including Papio hamadryas anubis (olive baboon), meet these important criteria. These animals remain ubiquitous throughout sub-Saharan Africa and have adapted well to the managed environments of major primate centers worldwide. A list of United States-based primate centers housing breeding colonies of baboons can be found in Table 19.1. The Surgical Research Laboratory at Loma Linda University, for instance, has maintained a salutary relationship with the Southwest National Primate Research Center in San Antonio, Texas, for the procurement of juvenile baboon research subjects.

Rejection of cardiac xenografts transplanted from alpha1,3-galactosyltransferase gene-knockout (GalT-KO) pigs to baboons

The use of alpha1,3-galactosyltransferase gene-knockout (GalT-KO) swine donors in discordant xenotransplantation has extended the survival of cardiac xenografts in baboons following transplantation. Eight baboons received heterotopic cardiac xenografts from GalT-KO swine and were treated with a chronic immunosuppressive regimen. The pathologic features of acute humoral xenograft rejection (AHXR), acute cellular xenograft rejection (ACXR) and chronic rejection were assessed in the grafts. No hyperacute rejection developed and one graft survived up to 6 months after transplantation. However, all GalT-KO heart grafts underwent graft failure with AHXR, ACXR and/or chronic rejection. AHXR was characterized by interstitial hemorrhage and multiple thrombi in vessels of various sizes. ACXR was characterized by TUNEL(+) graft cell injury with the infiltration of T cells (including CD3 and TIA-1(+) cytotoxic T cells), CD4(+) cells, CD8(+) cells, macrophages and a small number of B and NK cells. Chronic xenograft vasculopathy, a manifestation of chronic rejection, was characterized by arterial intimal thickening with TUNEL(+) dead cells, antibody and complement deposition, and/or cytotoxic T-cell infiltration. In conclusion, despite the absence of the Gal epitope, acute and chronic antibody and cell-mediated rejection developed in grafts, maintained by chronic immunosupression, presumably due to de novo responses to non-Gal antigens.

Expeditious chemoenzymatic synthesis of homogeneous N-glycoproteins carrying defined oligosaccharide ligands

An efficient chemoenzymatic method for the construction of homogeneous N-glycoproteins was described that explores the transglycosylation activity of the endo-beta-N-acetylglucosaminidase from Arthrobacter protophormiae (Endo-A) with synthetic sugar oxazolines as the donor substrates. First, an array of large oligosaccharide oxazolines were synthesized and evaluated as substrates for the Endo-A-catalyzed transglycosylation by use of ribonuclease B as a model system. The experimental results showed that Endo-A could tolerate modifications at the outer mannose residues of the Man3GlcNAc-oxazoline core, thus allowing introduction of large oligosaccharide ligands into a protein and meanwhile preserving the natural, core N-pentasaccharide (Man3GlcNAc2) structure in the resulting glycoprotein upon transglycosylation. In addition to ligands for galectins and mannose-binding lectins, azido functionality could be readily introduced at the N-pentasaccharide (Man3GlcNAc2) core by use of azido-containing Man3GlcNAc oxazoline as the donor substrate. The introduction of azido functionality permits further site-specific modifications of the resulting glycoproteins, as demonstrated by the successful attachment of two copies of alphaGal epitopes to ribonuclease B. This study reveals a broad substrate specificity of Endo-A for transglycosylation, and the chemoenzymatic method described here points to a new avenue for quick access to various homogeneous N-glycoproteins for structure-activity relationship studies and for biomedical applications.

Thrombotic microangiopathy associated with humoral rejection of cardiac xenografts from alpha1,3-galactosyltransferase gene-knockout pigs in baboons

Heterotopic cardiac xenotransplantation from alpha1,3-galactosyltransferase gene-knockout (GalT-KO) swine to baboons was performed to characterize immunological reaction to the xenograft in the absence of anti-Gal antibody-mediated rejection. Eight baboons received heterotopic cardiac xenografts from GalT-KO porcine donors. All baboons were treated with chronic immunosuppressive therapy. Both histological and immunohistochemical studies were performed on biopsy and graftectomy samples. No hyperacute rejection was observed. Three baboons were euthanized or died 16 to 56 days after transplantation. The other five grafts ceased beating between days 59 and 179 (median, 78 days). All failing grafts exhibited thrombotic microangiopathy (TM) with platelet-rich fibrin thrombi in the microvasculature, myocardial ischemia and necrosis, and focal interstitial hemorrhage. TM developed in parallel with increases in immunoglobulin (IgM and IgG) and complement (C3, C4d, and C5b-9) deposition, as well as with subsequent increases in both TUNEL(+) endothelial cell death and procoagulant activation (increased expression of both tissue factor and von Willebrand factor and decreased expression of CD39). CD3(+) T-cell infiltration occurred in all grafts and weakly correlated with the development of TM. In conclusion, although the use of GalT-KO swine donors prevented hyperacute rejection and prolonged graft survival, slowly progressive humoral rejection--probably associated with non-Gal antibodies to the xenograft--and disordered thromboregulation represent major immunological barriers to long-term xenograft survival.

Identification of an anti-idiotypic antibody that defines a B-cell subset(s) producing xenoantibodies in primates

Synthetic anti-idiotypic antibodies represent a potentially valuable tool for the isolation and characterization of B cells that produce xenoantibodies. An anti-idiotypic antibody that binds to a subset of B cells producing antibodies encoded by the variable-region heavy chain 3 (V(H)3) germline genes DP35 [immunoglobulin variable-region heavy chain 3-11 (IGHV3-11)], DP-53 and DP-54 plus a small number of V(H)4 gene-encoded antibodies in humans has recently been identified. These germline progenitors also encode xenoantibodies in humans. We tested whether the small, clearly defined group of B cells identified with this anti-idiotypic antibody produce xenoantibodies in non-human primates mounting active immune responses to porcine xenografts. Peripheral blood B cells were sorted by flow cytometry on the basis of phenotype, and cDNA libraries were prepared from each of these sorted groups of cells. Immunoglobulin V(H) gene libraries were prepared from the sorted cells, and the V(H) genes expressed in each of the sorted groups were identified by nucleic acid sequencing. Our results indicate that xenoantibody-producing peripheral blood B cells, defined on the basis of binding to fluorescein isothiocyanate (FITC)-conjugated galactose alpha(1,3) galactose-bovine serum albumin (Gal-BSA) and the anti-idiotypic antibody 2G10, used the IGHV3-11 germline gene to encode xenoantibodies and were phenotypically CD11b+ (Mac-1+) and CD5-. This novel reagent may be used in numerous applications including definition of xenoantibody-producing B-cell subsets in humans and non-human primates and immunosuppression by depletion of B cells producing anti-Gal xenoantibodies.

Soluble Galalpha(1,3)Gal conjugate combined with hDAF preserves morphology and improves function of cardiac xenografts

BACKGROUND

Cytotoxic anti-Galalpha(1,3)Gal antibodies play a key role in the rejection of pig organs transplanted into primates. Regimens reducing anti-Galalpha(1,3)Gal antibodies were associated with severe side effects unable to prevent antibody rebound until soluble synthetic oligosaccharides with terminal Galalpha(1,3)Gal inhibiting antigen binding became available. We displayed kinetics of anti-pig and anti-Galalpha(1,3)Gal IgM and IgG antibody levels using GAS914, a Galalpha(1,3)Gal trisaccharide conjugated to poly-l-lysine, and investigated corresponding changes of parameters of heart function.

METHODS

Using a working heart model, hDAF pig hearts were perfused with human blood containing GAS914 (group 1). As controls hDAF pig hearts (group 2) and landrace pig hearts (group 3) were perfused with human blood only. Levels of anti-Galalpha(1,3)Gal (IgM, IgG) and anti-pig antibodies were assessed to prove the effectiveness of GAS914. As parameters of heart function, cardiac output (CO), stroke work index (SWI), coronary blood flow (CBF) and coronary resistance were measured. Creatine phosphokinases, lactate dehydrogenase and aspartate aminotransferase were evaluated as markers of myocardial damage. Histological and immunohistochemical investigations were performed at the end of perfusion.

RESULTS

In group 1 an immediate and extensive reduction in both IgM and IgG anti-Galalpha(1,3)Gal was found. Anti-pig antibodies were eliminated accordingly. Antibody binding to GAS914 was complete before the start of organ perfusion. Corresponding to rapid antibody elimination in group 1 GAS914 not only was able to significantly prolong the beating time of the heart in hDAF pigs, but also to clearly improve functional parameters. When switching to the working heart mode hDAF pig hearts perfused with human blood containing GAS914 (group 1) revealed a CO starting at a significantly higher level than hDAF (group 2) and non-transgenic pig hearts (group 3) perfused with human blood only. Similarly, in group 1 SWI was significantly increased at the beginning of perfusion compared to that of group 2 and group 3. The increase in CBF during perfusion and the corresponding fall of coronary resistance occurred without significant differences between the groups revealing the independence of hDAF and GAS914.

CONCLUSIONS

Due to an immediate and profound reduction in Galalpha(1,3)Gal-specific antibodies, soluble Galalpha(1,3)Gal conjugates not only prolong survival, but also improve the hemodynamic performance of the heart in DAF pigs.

Lack of evidence for PERV expression after apoptosis-mediated horizontal gene transfer between porcine and human cells

Evidence for porcine endogenous retrovirus (PERV) infection of human cells has provoked a public health debate over the proposed use of porcine xenografts to alleviate the worldwide shortage of human allografts. Nevertheless, the potential relevance of PERV transmission by apoptosis-mediated horizontal DNA transfer, a documented means of infection-independent retrovirus delivery, appears to have been overlooked in this discussion. To examine the hypothesis that apoptotic cell death during porcine xenograft rejection is capable of fostering horizontal DNA transfer, we have now assessed in vitro cocultures, consisting of phagocytic human fibroblasts and apoptotic or necrotic porcine B-lymphoblastoid cells, for evidence of cross-species PERV exchange and eventual replication. Using real-time polymerase chain reaction (PCR) assays, designed to differentiate nuclear and cytoplasmic DNA derived from either porcine or human cells, we now report evidence for the presence of porcine DNA, including PERV, in the nucleus of human fibroblasts exposed to apoptotic porcine cells. This novel demonstration of apoptosis-mediated horizontal PERV transfer is characterized by a low efficiency of transfer and a transient nature, being present in only 0.22% of the cocultured human cells and disappearing to undetectable levels within 4 weeks of exposure to apoptotic porcine cells. In contrast, using PERV-specific real-time reverse-transcriptase PCR (RT-PCR) and ultra-sensitive product-enhanced reverse transcriptase (PERT) assays, we find no evidence for human fibroblast-derived cellular PERV RNA or coculture supernatant-based RT-activity, indicating a lack of subsequent PERV replication. Together, these results suggest that apoptosis-mediated horizontal PERV transfer does not present an overt hazard within the framework of porcine xenotransplantation. However, we also present arguments against extrapolation of these in vitro observations directly to clinical circumstances.

Xenotransplantation: an ethical dilemma

PURPOSE OF REVIEW

Xenotransplantation is an attractive alternative to allogeneic transplantation because of the intractable shortage of donor organs. The latest published developments in this area are discussed as well as recent publications regarding the continuing debates on the ethics of xenotransplantation and the safeguards that should be imposed to assure the minimization of risk to the patient and to human societies.

RECENT FINDINGS

Several laboratories are concentrating on the development of genetically manipulated donor pigs in order to bypass the immunological barriers against transplantation of porcine organs and cells into humans. Preliminary experiments with these new pig strains suggest that this strategy has a high likelihood of success with respect to hyperacute rejection. Ethical considerations have been extended to the establishment of regulatory, legal, and ethical frameworks that will be necessary to minimize the risk of xenotransplantation to the recipients, their families, and to the worldwide community.

SUMMARY

Xenotransplantation, because of the complexity of the medical, ethical and legal issues, will likely remain a controversial issue. If the scientific problems are solved, the decision to proceed with clinical application of this technique will depend on a collective decision guided by ethical, regulatory, and legal frameworks established by consensus.

Potential of an Injectable Polymer to Prevent Hyperacute Rejection of Ex Vivo Perfused Porcine Lungs

BACKGROUND

Removal of xenoreactive antibodies in pig-to-human lung transplantation by columns or organ perfusions proofed to be unsatisfactory and associated with adverse effects. In an ex-vivo lung perfusion model, we evaluated the potential of a soluble trisaccharide polymer (GAS914) to bind alpha-Gal antibodies and to protect a pulmonary xenograft from hyperacute rejection (HAR) and pulmonary xenograft dysfunction.

METHODS

Porcine lungs were perfused with fresh human blood for 240 min. In the GAS914 treated group (n=6) the polymer was applied in three different concentrations. The control group (n=6) received no GAS914. Survival and function of perfused xenografts were monitored, and alpha-Gal antibodies as well as cytolytic anti-porcine antibodies analyzed.

RESULTS

In the GAS-treated group survival of lungs was significantly prolonged, pulmonary vascular resistance reduced, pulmonary edema prevented, and oxygenation improved. On histopathological evaluation application of GAS resulted in minimal graft injury and significantly less deposition of the terminal complement complex C5b-9. Following application of GAS914, up to 89.8% of IgG alpha-Gal, 79.5% of IgM and 73.6% of anti-porcine antibodies in the human blood were bound by the polymer. Subsequent perfusion of porcine lungs resulted in absorption of only 3% of the baseline IgG alpha-Gal antibodies in the GAS914 group, compared to 87% in the controls.

CONCLUSIONS

In this ex-vivo lung perfusion model, a trisaccharide polymer prevented immediate HAR, due to effective removal of alpha-Gal antibodies. In combination with additional strategies GAS914 may be a valuable tool in overcoming HAR and dysfunction of pulmonary xenografts.

Pathology of renal xenograft rejection in pig to non-human primate transplantation

Xenotransplantation has the potential to alleviate the critical shortage of organs for transplantation in humans. Miniature swine are a promising donor species for xenotransplantation. However, when swine organs are transplanted into primates, hyperacute rejection (HAR), acute humoral xenograft rejection (AHXR), acute cellular xenograft rejection (ACXR), and chronic xenograft rejection prevent successful engraftment. Developing a suitable regimen for preventing xenograft rejection requires the ability to accurately diagnosis the severity and type of rejection in the graft. For this purpose, histopathology remains the most definitive and reliable tool. We discuss here the characteristic features of xenograft rejection in a preclinical pig-to-non-human primate transplantation model. In miniature swine to baboon xenotransplantation, marked interstitial hemorrhage develops in HAR, and renal microvascular injury develops with multiple platelet-fibrin microthrombi in both HAR and AHXR. T-cell-mediated cellular immunity plays an important role in ACXR. Chronic humoral and cellular rejection may induce chronic xenograft rejection, and will be a major cause of graft loss in discordant xenotransplantation.

Decay-Accelerating Factor Prevents Acute Humoral Rejection Induced by Low Levels of Anti-αGal Natural Antibodies

BACKGROUND

Hyperacute and delayed vascular rejection due to natural antibodies (NAb) present major obstacles in pig-to-primate xenotransplantation. Although "supraphysiologic" expression of human complement regulatory proteins (CRPs) can prevent hyperacute rejection in discordant xenogenic recipients, their physiologic role in the homologous setting is undefined. We have evaluated the effect of the absence of decay-accelerating factor (DAF) on cardiac allograft rejection in the presence of different levels of antidonor antibodies (Ab).

METHODS

DAF1-deficient (DAF KO; B6129F2 H-2) mice were used as heart graft donors to alpha1,3-galactosyltransferase deficient (GalT KO; B6, H-2) recipients. Heterotopic heart grafting was performed with or without presensitization. Graft survival, histology, and anti-alphaGal Ab levels were monitored.

RESULTS

DAF knockout (KO) but not wild-type (WT) grafts showed hyperacute or acute humoral rejection in nonsensitized GalT KO mice with low levels of anti-alphaGal IgM NAb. However, humoral rejection of both DAF KO and DAF WT donor grafts occurred in presensitized GalT KO recipients.

CONCLUSIONS

The expression of DAF prevents hyperacute rejection in mice with low titers of anti-alphaGal antibody. These studies demonstrate the physiologic role of DAF in preventing humoral rejection in the presence of low levels of NAb and have implications for transplantation of discordant vascularized xenografts.

Characteristics of protein-carbohydrate interactions as a basis for developing novel carbohydrate-based antirejection therapies

The relative shortage of human organs for transplantation is today the major barrier to a broader use of transplantation as a means of treating patients with end-stage organ failure. This barrier could be partly overcome by an increased use of blood group ABO-incompatible live donors, and such trials are currently underway at several transplant centres. If xenotransplantation can be used clinically in the future, the human organ shortage will, in principle, be eradicated. In both these cases, carbohydrate antigens and the corresponding anti-carbohydrate antibodies are the major primary immunological barriers to overcome. Refined carbohydrate-based therapeutics may permit an increased number of ABO-incompatible transplantations to be carried out, and may remove the initial barriers to clinical xenotransplantation. Here, we will discuss the chemical characteristics of protein-carbohydrate interactions and outline carbohydrate-based antirejection therapies as used today in experimental as well as in clinical settings. Novel mucin-based adsorbers of natural anti-carbohydrate antibodies will also be described.

Reducing Gal expression on the pig organ - a retrospective review

The rejection caused by the presence of Galalpha1,3Gal (Gal) on the pig vascular endothelium and of natural anti-Gal antibodies in human blood 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. However, prior to the introduction of nuclear transfer/embryo transfer techniques, a major effort was directed towards reducing Gal expression on pig cells by other methods, such as by cleaving Gal from the underlying substrate, or replacing Gal with an alternative, innocuous oligosaccharide by a process that has been termed 'competitive glycosylation'. Gal has been cleaved by alpha-galactosidase or endo-beta-galactosidase C. Competitive glycosylation has largely targeted replacement of Gal by insertion of a gene for a fucosyltransferase or a sialyltransferase, or by insertions of the gene for N-acetylglucosaminyltransferase III to reduce cell-surface expression of several oligosaccharides. The results of these approaches to render the pig cells less immunogenic to the human immune system are summarized. With regard to the problem provided by Gal expression, the above approaches may be considered by some to be largely obsolete, but the principles underlying them may prove valuable when other antigen targets for human antibodies are definitively identified, if these prove to be carbohydrates.

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.

FDA/PI flow cytometry assay of complement-mediated cytotoxicity of antibodies generated during xenotransplantation

BACKGROUND

A flow cytometry complement-mediated cytotoxicity assay (FCCA) using fluorescein diacetate (FDA) and propidium iodide (PI) to measure antibody-dependent toxicity is useful to determine the success of xenotransplant organs. We evaluated the validity of different mathematical models as a measure of cytotoxicity in FCCA.

METHODS

Sera from untreated baboons (n = 7) and from immunosuppressed animals (n = 5) undergoing different xenotransplantation protocols with pig organs were tested by endogenous FCCA and a similar assay also using exogenous complement, and the results were compared with those of a complement-dependent hemolytic assay to detect anti-pig antibodies (APHA). The influence of PI/FDA staining and the use of several mathematical models were analyzed.

RESULTS

For both groups of animals, we observed high correlations between the endogenous and exogenous FCCA pathways and between calculations based on PI and FDA staining. Of the four mathematical models tested--the Von Krogh equation, two exponential models, and area under the curve--the Von Krogh equation was the most appropriate in terms of goodness of fit and concordance with APHA.

CONCLUSIONS

FDA/PI FCCA is useful to measure endogenous and exogenous complement-mediated cytotoxicities, and it has advantages related to identification of potential new xenoantibodies. Although all four mathematical models produced acceptable solutions, the Von Krogh equation was the best option.

Administration of GAS914 in an orthotopic pig-to-baboon heart transplantation model

BACKGROUND

Long-term survival of transgenic cardiac xenografts is currently limited by a form of humoral rejection named acute vascular rejection. Preformed and elicited cytotoxic antibodies against Galalpha(1,3)Gal terminating carbohydrate chains, known as the primary cause of hyperacute rejection, are crucial for this process. We investigated whether GAS914, a soluble, polymeric form of a Galalpha(1,3)Gal trisaccharide would sufficiently minimize xenograft rejection of hDAF-transgenic pig hearts orthotopically transplanted into baboons.

METHODS

Orthotopic heart transplantations were performed using hDAF transgenic piglets as donors and four non-splenectomized baboons as recipients. Baseline immunosuppression consisted of tacrolimus, sirolimus, ATG, steroids. In addition two animals received low-dose GAS914, and two animals high-dose GAS914. One of these baboons received high dose GAS914 and cyclophosphamide induction therapy. Serum levels of anti-Galalpha(1,3)Gal IgM and IgG antibodies, and anti-pig antibodies were controlled daily by anti-Galalpha(1,3)Gal enzyme-linked immunosorbant assay and anti-pig hemolytic assays. Histomorphological (hematoxylin and eosin, elastic van Gieson) and immunohistochemical (IgM, IgG) evaluations were performed on tissue specimens.

RESULTS

Following low-dose GAS914 therapy survival time was 1 and 9 days, respectively. In baboons treated with high dosages of GAS914 a survival of 30 h and 25 days could be obtained. GAS914 caused an immediate and significant reduction of both anti-Galalpha(1,3)Gal IgM and IgG antibodies. However, sufficient antibody reduction was independent of dosage and form of application of GAS914. A pre-transplant GAS914 treatment was not necessary to effectively reduce antibody levels and prevent hyperacute rejection. In the early postoperative period preformed anti-pig antibodies corresponded predominantly to anti-Galalpha(1,3)Gal antibodies making them susceptible to GAS914. Subsequently, while anti-Galalpha(1,3)Gal antibodies remained low, anti-pig antibodies increased despite of GAS914 application. Corresponding to increased anti-pig antibody titers depositions of IgM and IgG immunoglobulins were detected, which were possibly non-Galalpha(1,3)Gal-specific.

CONCLUSIONS

Following orthotopic transplantation of hDAF-transgenic pig hearts into baboons, GAS914 is able to maintain a sufficient reduction of Galalpha(1,3)Gal-specific cytotoxicity to the graft. GAS914 therefore is able to prevent not only hyperacute rejection, but also acute vascular rejection at its beginning, when serum cytotoxicity to the pig heart appears to be predominantly Galalpha(1,3)Gal-specific. A sustained prevention of acute vascular rejection, however, still requires the identification of antibody specificities other than to Galalpha(1,3)Gal.

Porcine Hematopoietic Progenitor Cell Transplantation in Nonhuman Primates: A Review of Progress

The critical shortage of human donor organs for transplantation would be overcome if a suitable animal, e.g., the pig, could be used as an organ source. There are, however, several immune barriers that have to date resulted in limited function of pig organs transplanted into nonhuman primates. It would be beneficial, and indeed may be essential, to induce a state of tolerance in the primate recipient to the pig organ. In allotransplantation, the successful transplantation of hematopoietic progenitor cells with the development of mixed chimerism is associated with the induction of tolerance toward a donor-specific organ. For some years, this approach has been explored in the pig-to-nonhuman primate model. This experience is briefly reviewed. The problems of natural and elicited anti-pig antibodies, recipient platelet adhesion to pig hematopietic progenitor cells, and the rapid removal of these cells by the host macrophage-phagocytic system are highlighted. Recent experience with the use of hematopoietic cells from pigs homozygous for alpha1,3-galactosyltransferase gene-knockout is reported.

Does exposure to swine leukocyte antigens after pig-to-nonhuman primate xenotransplantation provoke antibodies that cross-react with human leukocyte antigens?

BACKGROUND

A potential concern of using pig kidney xenografts for human transplantation is that antibodies produced to swine leukocyte antigens (SLA) may cross-react with human leukocyte antigens (HLA) and thereby limit the scope for a subsequent human organ donor transplant. We therefore investigated whether exposure to SLA after pig-to-nonhuman primate kidney xenotransplantation gives rise to HLA cross-reactive antibodies.

METHODS

Serum samples were obtained from 52 cynomolgus monkeys that received kidney transplants from human decay-accelerating factor (hDAF) transgenic pigs. Samples were collected pre-transplant and at time of autopsy (mean 20 days post-transplantation, range 1 to 53 days) and analyzed for IgG HLA class I and HLA class II specific antibodies by enzyme-linked immunosorbent assay (ELISA) against pooled purified HLA antigens. To ensure the ability of the HLA ELISA to detect cynomolgus monkey IgG binding, parallel experiments were performed to detect IgG Gal-alpha-1,3-Gal-specific antibodies known to be present in cynomolgus monkey serum.

RESULTS

Analysis of both pre- and post-transplantation serum samples by ELISA demonstrated no detectable IgG antibody binding to HLA class I or class II antigens. Using the same ELISA antibody detection reagents, IgG Gal-alpha-1,3-Gal-specific antibodies were identified in 13 of 38 (34%) sera obtained before transplantation and 21 of 52 (40%) sera collected post-transplantation, confirming that the negativeHLA ELISA results were not due to a technical aspect of the assay.

CONCLUSION

This study suggests that exposure to SLA following transplantation of porcine kidneys in nonhuman primates does not give rise to antibodies that cross-react with HLA.

In vitro and in vivo investigation of a novel monoclonal antibody to plasma cells (W5 mAb)

Natural antibodies (Abs), predominantly anti-Gal alpha 1-3Gal (Gal) Abs, in non-human primates and human beings present a major hurdle to successful pig-to-primate xenotransplantation. Attempts to inhibit anti-Gal Ab production in naïve baboons using non-specific immunosuppressive or B cell-specific reagents have failed. A new rat monoclonal antibody (W5 mAb) has been generated, which binds to all B cells, including memory cells, and to the majority of plasma cells, but not to T cells. It has been tested in vitro and in vivo. By immunoprecipitation, W5 mAb bound a human leukocyte antigen class II (HLA-DR) determinant. Sorting splenic or bone marrow W5+ cells resulted in a highly enriched anti-Gal Ab and total immunoglobulin (Ig)-secretory population. In vivo studies in baboons demonstrated that W5 mAb was safe but, despite the concomitant administration of an anti-CD154 mAb to inhibit sensitization, anti-rat Abs were detected within 10 days and inhibited the effect of the W5 mAb. High levels of W5 mAb were able to completely deplete B cells in the blood, but not in lymphoid tissues. Enzyme-linked spot-forming assay (ELISPOT) demonstrated that only 50 to 60% of secreting cells (SC) were depleted in the bone marrow. No reduction in the serum levels of anti-Gal Ab was observed. W5 mAb did not cause complete inhibition of anti-Gal Ab production, probably as a result of its inability to completely deplete B and plasma cells from all lymphoid compartments.

Lack of Galactose-α-1,3-Galactose Expression on Porcine Endothelial Cells Prevents Complement-Induced Lysis but Not Direct Xenogeneic NK Cytotoxicity

The galactose-alpha-1,3-galactose (alphaGal) carbohydrate epitope is expressed on porcine, but not human cells, and therefore represents a major target for preformed human anti-pig natural Abs (NAb). Based on results from pig-to-primate animal models, NAb binding to porcine endothelial cells will likely induce complement activation, lysis, and hyperacute rejection in pig-to-human xenotransplantation. Human NK cells may also contribute to innate immune responses against xenografts, either by direct recognition of activating molecules on target cells or by FcgammaRIII-mediated xenogeneic Ab-dependent cellular cytotoxicity (ADCC). The present study addressed the question as to whether the lack of alphaGal protects porcine endothelial cells from NAb/complement-induced lysis, direct xenogeneic NK lysis, NAb-dependent ADCC, and adhesion of human NK cells under shear stress. Homologous recombination, panning, and limiting dilution cloning were used to generate an alphaGal-negative porcine endothelial cell line, PED2*3.51. NAb/complement-induced xenogeneic lysis of PED2*3.51 was reduced by an average of 86% compared with the alphaGal-positive phenotype. PED2*3.51 resisted NK cell-mediated ADCC with a reduction of lysis ranging from 30 to 70%. However, direct xenogeneic lysis of PED2*3.51, mediated either by freshly isolated or IL-2-activated human NK cells or the NK cell line NK92, was not reduced. Furthermore, adhesion of IL-2-activated human NK cells did not rely on alphaGal expression. In conclusion, removal of alphaGal leads to a clear reduction in complement-induced lysis and ADCC, but does not resolve adhesion of NK cells and direct anti-porcine NK cytotoxicity, indicating that alphaGal is not a dominant target for direct human NK cytotoxicity against porcine cells.

Expression of CTLA-4 in nonhuman primate lymphocytes and its use as a potential target for specific immunotoxin-mediated apoptosis: results of in vitro studies

T-cell-mediated immunoregulation is one of the main mechanisms implicated in induction and maintenance of transplantation tolerance. In this regard, deletion or modulation of xeno/alloantigen-specific T cells, as well as blocking of their interactions with other cell populations, are currently being pursued for tolerance induction in humans as well as nonhuman primates. In order to investigate whether cytotoxic T-lymphocyte antigen-4 (CTLA-4) may represent a suitable target for a T cell depletion approach in nonhuman primate models, we analysed CTLA-4 expression in peripheral blood mononuclear cells (PBMCs) from nonhuman primates and the potential role of two anti-CTLA-4 saporin-conjugated immunotoxins. The analysis was performed in PBMCs from 8 cynomolgus monkeys from Philippines and from Mauritius both at protein level by flow cytometry and at transcriptional level by RT-PCR. In addition, the apoptotic role of the immunotoxins was investigated. The results showed that CTLA-4 was expressed at variable levels depending on the origin of the cynomolgus monkeys and the resting or activated cell condition. CTLA-4 was not expressed on resting Mauritius PBMCs and showed a lower up-regulation upon PMA/PHA activation compared to the Philippines PBMCs that expressed CTLA-4 also before activation. Two CTLA-4 RNA transcripts (672 and 550 bp) were detected with levels variations after cell stimulation. Two anti-CTLA-4 immunotoxins induced in vitro apoptosis of activated PBMCs from both sources of cynomolgus monkeys. This is the first report that documents CTLA-4 expression both at protein and transcriptional level by nonhuman primate PBMCs and provides novel perspectives of xeno/allograft rejection immunotherapy based on CTLA-4 targeting.

Suppression of natural and elicited antibodies in pig-to-baboon heart transplantation using a human anti-human CD154 mAb-based regimen

Natural and elicited antipig antibodies (Abs) lead to acute humoral xenograft rejection (AHXR). Ten baboons underwent heterotopic heart transplantation (Tx) from human decay-accelerating factor (hDAF) pigs. Depletion of anti-Galalpha1, 3Gal (Gal) Abs was achieved by the infusion of a Gal glycoconjugate from day-1. Immunosuppression included induction of antithymocyte globulin, thymic irradiation, and cobra venom factor, and maintenance with a human antihuman CD154 mAb, mycophenolate mofetil, and methylprednisolone; heparin and prophylactic ganciclovir were also administered. Pig heart survival ranged from 4 to 139 (mean 37, median 27) days, with three functioning for >50 days. Graft failure (n = 8) was from classical AHXR [4], thrombotic microangiopathy [3], or intragraft thrombosis [1], with death (n = 2) from pneumonia [1], or possible drug toxicity (with features of thrombotic microangiopathy) [1]. Anti-Gal Abs (in microg/mL) were depleted by Gal glycoconjugate before graft implantation from means of 41.3 to 6.3 (IgM) and 12.4-4.6 (IgG), respectively, and at graft excision were 6.3 and 1.7 microg/mL, respectively. No elicited Abs developed, and no cellular infiltration was seen. The treatment regimen was effective in maintaining low anti-Gal Ab levels and in delaying or preventing AHXR. The combination of costimulatory blockade and heparin with Tx of a Gal-negative pig organ may prolong graft survival further.

Feasibility of xeno-transplantation

Within a relatively short time span, a significant number of barriers to xeno-transplantation have been identified and potential solutions generated; however, the survival rates for pig-to-primate heart transplantation remain modest at best, with the longest functioning heterotopic heart transplant surviving only 99 days and the longest functioning orthotopic heart transplant surviving only 39 days. A great deal of improvement in immunological strategies will be needed to make xeno-transplantation a clinical reality. The most exciting prospect in the near term is the use of organs from homozygous alphaGal knockout pigs. The diversity of the biological pathways involved in the total spectrum of xenograft rejection, however, makes it highly likely that the clinical feasibility of xeno-transplantation will depend on a multipronged approach that incorporates the advantages of genetically eliminating the alphaGal epitope on hyperacute and acute xenograft rejection and the advantages of tolerance induction on cellular and chronic xenograft rejection.

Xenotransplantation: a tool for reproductive biology and animal conservation?

The transplantation of reproductive organs, including ovaries and ovarian tissue, was pioneered over 100 years ago. In the 1960s, ovarian grafting was used as a tool to investigate ovarian function, but with the recent development of more effective cryopreservation protocols for ovarian tissue, germline preservation and propagation have now become realistic goals. This review describes progress in ovarian banking and ovarian tissue transplantation, with emphasis on how fresh and frozen ovarian tissue can be used in assisted reproduction for both humans and animals. This paper focuses most closely on the potential value of xenotransplantation, the transplantation of gonads from one species to another, to conserve rare and endangered species. Specific attention is drawn to the use of xenotransplantation as a strategy for generating viable gametes that can be used to produce live fertile offspring. Other upcoming xenogeneic technologies that may be of potential significance in animal conservation, such as transplantation of whole ovaries or isolated growing follicles, and even male germ cells, are discussed.

Design and synthesis of αGal-conjugated peptide T20 as novel antiviral agent for HIV-immunotargeting

An efficient chemo-enzymatic synthesis of alpha Gal-conjugated peptide T20 as novel HIV-immuno-targeting agent is described. The synthesis involves chemo-enzymatic preparation of maleimide-functionalized alpha Gal epitope and its chemoselective ligation with the peptide T20. The title compound contains two functional domains: the trisaccharide alpha Gal epitope that binds to human natural anti-Gal antibodies and the 36-amino acid gp41 peptide (T20) that recognizes the gp41 N-terminal ectodomain of the HIV envelope. Biological assays demonstrated that the synthetic conjugate could readily bind to natural anti-Gal antibodies (both IgG and IgM type) in normal human serum and exhibited potent anti-HIV activity even in the absence of human antibodies and complement system. The experimental data suggest that the synthetic alpha Gal-T20 might be valuable for in vivo HIV-immuno-targeting via antibody-mediated cytotoxicity and/or antibody-dependent, complement-mediated lysis of HIV particles and HIV-infected cells, thus providing an additional dimension of HIV intervention.

Ex vivo and extracorporeal perfusion with hDAF pig kidneys

The present study was undertaken to determine whether human decay accelerating factor (hDAF) transgene would prevent hyperacute rejection (HAR) while perfused with human blood or extracorporeally in baboons. Four hDAF pig kidneys and three non-hDAF pig kidneys were perfused ex vivo with fresh human blood for 6 h. Additionally four hDAF pig kidneys and four non-hDAF pig kidneys were extracorporeally perfused in baboons and pigs, respectively, for 3 h. In ex vivo perfusion, the color of hDAF pig kidneys remained pink at the end of 6-h perfusion and they had normal histology, while non-hDAF kidneys developed HAR. HDAF pig kidneys had superior function over non-transgenic pig kidneys. Urine output was 17.31 +/- 3.70 ml/h for hDAF pig kidneys, and only 5.81 +/- 0.26 ml/h for non-hDAF kidneys (P < 0.05). Creatinine clearance was 1.16 +/- 1.24 ml/min for hDAF kidneys and 0.22 +/- 0.15 ml/min for non-hDAF kidneys (P < 0.05). Other functional data including potassium, urine specific density, and osmolality were normal in the hDAF kidneys, while in non-hDAF kidneys, serum potassium was elevated to over 9 mmol/l by the end of perfusion (P < 0.01). Non-hDAF kidneys also lost more sodium through urine than hDAF kidneys (173.67 +/- 14.05 mmol/l vs. 109 +/- 31 mmol/l, P < 0.05). In the extracorporeal perfusion, all the baboons tolerated the procedure well with normal hemodynamic and hemotologic profiles. These baboons were well until killed 42 to 56 days after perfusion, although their antiporcine antibodies were greatly elevated. We conclude that hDAF transgene protects against HAR, allowing the pig kidney to function normally while perfused with human blood, and that extracorporeal perfusion using hDAF pig kidneys is a safe procedure in baboons.

Depletion of anti-Gal antibodies by the intravenous infusion of Gal type 2 and 6 glycoconjugates in baboons

BACKGROUND

Natural anti-Gal antibodies (NAb) to Gal epitopes play a key role in the rejection of pig cells or organs transplanted into primates. We have investigated the effect on NAb return after extracorporeal immunoadsorption (EIA) of the continuous intravenous (i.v.) infusion of (i) bovine serum albumin conjugated to Gal type 6 oligosaccharides (BSA-Gal) or (ii) a poly l-lysine backbone conjugated to Gal type 2 or 6 oligosaccharides (PLL-Gal).

METHODS

Porcine mobilized peripheral blood progenitor cells (PBPC) obtained by leukapheresis from MHC-inbred miniature swine (n = 9) were infused intravenously (i.v.) into baboons: Group 1 baboons (n = 4) received whole body and thymic irradiation, splenectomy, antithymocyte globulin, cobra venom factor, cyclosporine, mycophenolate mofetil, anti-CD154mAb, porcine hematopoietic growth factors, and EIA before transplantation of high doses (2 to 4 x 1010 cells/kg) of PBPC; Group 2 baboons (n = 3) received the Group 1 regimen plus a continuous i.v. infusion of BSA-Gal for up to 30 days; Group 3 baboons (n = 5) received the Group 1 regimen plus a continuous i.v. infusion of PLL-Gal type 2 (n = 2) or both PLL-Gal types 2 and 6 (n = 3) for up to 30 days.

RESULTS

Group 1: NAb returned to pre-PBPC levels within 20-30 days, but there was no induction of antibody to Gal or non-Gal determinants; Group 2: NAb was undetectable or at very low level during BSA-Gal therapy. In one baboon, however, IgG to Gal type 2, but not to type 6, returned during BSA-Gal therapy; Group 3: NAb was undetectable or at very low level during PLL-Gal therapy. In two baboons that received PLL-Gal type 2, NAb to Gal type 6, but not to type 2, returned during PLL-Gal treatment. Two of five baboons, however, developed systemic infection. Four of five baboons died within 14 days; autopsy revealed focal hemorrhagic injury to their hearts, lungs, and small intestines, with histologic abnormalities that varied between animals from hemorrhage and/or thrombosis in some organs (heart, lungs, or intestine) to signs of infections (bacteria in intestine, cytomegalovirus in liver).

CONCLUSIONS

(i) BSA-Gal and PLL-Gal therapy maintained depletion of NAb. (ii) Some heterogeneity in specificity of NAb was identified, indicating that the infusion of a combination of Gal type 2 and 6 glycoconjugates may be required. (iii) The addition of PLL-Gal to the immunosuppressive regimen was associated with a high incidence of morbidity and mortality without a clear histopathologic entity underlying the cause of death.

Potential implications of ABO blood group for vascular rejection in pig to human kidney xenotransplantation

BACKGROUND

A substantial hurdle for successful xenotransplantation is to negate the effect of xenoreactive natural antibodies [mainly Galalpha1-3Galbeta1-4GlcNAc (alpha-Gal) specific] that cause hyperacute xenograft rejection. Galalpha1-3Gal molecules (alpha-Gal) have close structural homology with human ABO blood groups and therefore an individual's blood group might influence the formation of alpha-Gal specific antibodies. Genetic heterogeneity controlling alpha-Gal specific antibody formation could have important implications for future pig to human xenotransplantation clinical trials. We have investigated the relationship between ABO blood group and immunoglobulin M (IgM) and immunoglobulin G (IgG) alpha-Gal specific antibody titres in sera obtained from renal dialysis patients and healthy blood donors.

METHODS

Serially diluted sera (n = 166) obtained from renal dialysis patients awaiting kidney transplantation (n = 116) and healthy blood donors (n = 50) were tested for IgM and IgG alpha-Gal antibodies using an enzyme-linked immunosorbent assay (ELISA) specific for alpha-Gal. The study cohort comprised 62, 48, 36 and 20 sera obtained from blood group O, A, B and AB individuals, respectively. Reciprocal alpha-Gal specific antibody titres were calculated from ELISA titration curves and stratified by individual blood group.

RESULTS

No significant heterogeneity was found in IgM alpha-Gal specific antibody titres across ABO blood groups. In contrast, marked heterogeneity was observed in IgG alpha-Gal specific antibody titres when stratified by blood group. IgG alpha-Gal specific antibody titres were higher in sera obtained from blood group O renal dialysis patients [median titre 40, interquartile range (IQR) 14 to 72], compared with blood group A (median titre 18, IQR 7 to 54, P = 0.05), blood group B (median titre 6, IQR 0 to 15, P < 0.001) and blood group AB patients (median titre 3.5, IQR 0 to 16, P = 0.002). A similar correlation was found for IgG alpha-Gal specific antibody titres in sera obtained from healthy blood donors with median titres of 20 (IQR 12 to 34), 37 (10 to 91), 9 (0 to 20), and 5.5 (0 to 12) in blood groups O, A, B and AB individuals, respectively. There was a strong interrelationship between alpha-Gal specific antibody class and blood group, with both IgM and IgG alpha-Gal specific antibodies found in 84% of the blood group O sera, 73% of blood group A sera, 50% of blood group B sera and 40% of blood group AB sera (P < 0.001). In a subgroup of 39 renal dialysis patients, IgM and IgG alpha-Gal specific antibody titres were measured in two serum samples obtained at different time-points (median time interval 581 days, range 42 to 4414), and showed a high degree of stability (correlation coefficient 0.88 and 0.90 for IgM and IgG, respectively).

CONCLUSION

IgG alpha-Gal specific antibody titres are significantly higher in the sera of blood group O and A renal dialysis patients and healthy individuals compared with blood groups B and AB. These data indicate that future clinical trials of pig to human xenotransplantation may be more problematic for non-blood group B patients who are likely to have high levels of IgG alpha-Gal specific antibodies that are associated with acute vascular rejection.

Human natural killer cell activity against porcine targets: modulation by control of the oxidation-reduction environment and role of adhesion molecule interactions

Xenotransplantation, especially using porcine sources, has been proposed as a means to alleviate the shortage of human organs for transplantation. NK cells appear to be important mediators of the xenogeneic immune responses, including the human anti-pig response. Having previously established the redox regulation of NK cell activity against tumor target cells, we now report that the interaction of human NK cells with porcine target cells is also regulated by redox. Thiol-deprivation strongly diminished the capacity of IL-2-activated human NK cells to kill porcine endothelial cells. This inhibition correlated with reduced proliferation and interferon (IFN)-gamma production by IL-2-activated NK cells. For fresh NK cells, pretreatment with diethyl maleate (DEM), which was used to deplete intracellular thiols, reduced lysis of porcine and human targets. Because many adhesion molecules exhibit interspecies recognition, we further investigated whether changes in expression of adhesion molecules might explain our observations. DEM treatment reduced the expression of CD11b and CD29 on fresh NK cells. Monoclonal antibody blocking studies showed that the combination of mAb to CD11b and CD18 reduced lytic activity against both PAEC as well as K562, although other qualitative differences were observed between the porcine and human target cells. These findings suggest that the oxidative stress-induced downregulation of CD18 may be important in modulating cytotoxic activity of fresh NK cells against PAEC and K562 targets through reduced formation of the CD11b/CD18 heterodimer. Thus, the appropriate manipulation of redox status may provide a means to enhance survival of non-human animal tissues in humans through modulation of adhesion molecule expression/interactions.

Immunoaffinity removal of xenoreactive antibodies using modified dialysis or microfiltration membranes

Hyperacute rejection following xenogeneic transplantation in primates is mediated by naturally occurring IgM antibodies, which are specifically directed to alpha-Galactosyl residues on many nonprimate mammalian cells. Current approaches to remove these anti-alphaGal IgM include plasmapheresis followed by immunoaffinity adsorption on bead columns using synthetic Gal epitopes, which requires two pieces of complex equipment. In this study, we explored the use of immunoaffinity adsorption with hollow fiber microporous or dialysis membranes to which a synthetic alphaGal trisaccharide ligand is bound. Covalent attachment of ligand directly to the surface produced negligible binding, but use of long-chain polyamines as reactive spacers yielded binding densities for anti-alphaGal IgM as high as 89 mg/mL membrane volume in breakthrough curve experiments with microporous nylon membranes having an internal surface area of 4.2 m(2)/mL membrane volume. A crossflow microfilter fabricated from the membranes described in this study and having about 0.4 m(2) luminal surface area would be able to carry out plasma separation and immunoadsorption in a single device with a large excess of binding capacity to ensure that all plasma that filters across the device and is returned to a human patient is essentially free of anti-alphaGal IgM. We conclude that immunoaffinity removal of xenoreactive antibodies using microfiltration hollow fiber membranes is feasible and has potential advantages of efficiency and simplicity for clinical application.

Improvement in human decay accelerating factor transgenic porcine kidney xenograft rejection with intravenous administration of gas914, a polymeric form of alphaGAL

BACKGROUND

The present study was undertaken to determine whether intravenous administration of GAS914, a polymeric form of alphaGal, would minimize porcine kidney xenograft rejection in baboons. Human decay accelerating factor renal xenografts were transplanted into 16 baboon recipients.

METHODS

Baseline immunosuppression for all groups included cyclosporine A, cyclophosphamide, SDZ-RAD, and methylprednisolone. Group 1 received only baseline immunosuppression; group 2 animals received low-dose GAS914 with baseline immunosuppression; group 3 animals received high dose GAS914 with high-dose baseline immunosuppression; and animals from group 4 received high-dose GAS914 and low-dose baseline immunosuppression.

RESULTS

None of the animals in this study developed hyperacute rejection. Intravenous administration of GAS914 significantly reduced xenoreactive antibodies as measured by antiporcine hemolytic assays and anti-Gal (immunoglobulin [Ig] G and IgM) antibody assays. Rejection was less severe in the GAS914-treated group. Only 25% (3 of 12) of GAS914-treated animals were killed as a result of rejection, whereas 75% (three of four) of non-GAS914-treated animals were killed because of terminal rejection (P<0.01). Protocol biopsies demonstrated that the degree of acute humoral xenograft rejection (AHXR) was reduced in the GAS914-treated animals compared with non-GAS914-treated animals.

CONCLUSION

The intravenous administration of GAS914 reduces xenoreactive antibody levels and reduces the degree of porcine kidney xenograft rejection, but does not improve survival. AHXR and drug toxicity remain major barriers to the long-term success of xenotransplantation.

An exploratory investigation of the effect of arsenic trioxide on anti-Gal antibody production in baboons

BACKGROUND

Arsenic trioxide (As2O3) is an anticancer drug that has been reported to induce apoptosis and inhibit differentiation in human plasmacytoma and normal plasma/B cells without significant myelosuppression. We assessed the ability of As2O3 as single therapy or in combination with an anti-CD20 monoclonal antibody (mAb) and whole body irradiation (WBI) to deplete B and plasma cells, both in vitro and in vivo, and to reduce the level of anti-alphaGal1-3Gal antibody (anti-Gal Ab) in baboons.

METHODS

In vitro the effect of As2O3 on antibody secretion (anti-Gal IgM, total IgG and IgM) was measured by enzyme-linked immunospot assay (ELISPOT). Its inhibition of proliferation of baboon splenocytes and the NCI-H929 human plasmacytoma cell line was measured by tritiated thymidine uptake. In vivo: all baboons (n=7) had undergone splenectomy. The effects of As2O3 (0.18 to 0.36 mg/kg) on B/plasma cell depletion and anti-Gal Ab production were assessed in three baboons. For comparison, three baboons received either WBI (2 x 150 cGy) or anti-CD20 mAb (20 mg/kg x 4 doses), or both WBI and anti-CD20 mAb. A final baboon received As2O3 + WBI (150 cGy) + anti-CD20 mAb. Anti-Gal Ab levels were measured daily by ELISA. Depletion of B cells from blood and bone marrow (BM) was monitored by flow cytometry and by histology of lymph nodes (LN). Autopsy was performed in three baboons.

RESULTS

In vitro: As2O3 (at 5 x 10-6 mol/l) reduced anti-Gal IgM and total IgM secretors by 76% (P=0.53) and 95% (P < 0.001), respectively, but did not reduce total IgG secretors. As2O3 inhibited in a dose-dependent manner the proliferation of activated splenocytes and of the NCI-H929 plasmacytoma cell line; complete inhibition was achieved at a dose of 1 x 10-5 mol/l. In vivo: As2O3 was found to be toxic at the doses given and was associated with the deaths of two of the four baboons that received it. Daily intravenous therapy with As2O3 alone reduced B cells (CD20+) in the blood (by 50 to 90%), BM (40%) and LN (20 to 30%), but anti-Gal Ab levels were not significantly decreased. Anti-CD20 mAb therapy alone or WBI alone depleted B cells by 100% in the blood and BM, and 80 to 100% in the LN. The combination of anti-CD20 mAb + WBI led to depletion of B cells in blood, BM and LN for 3 months, but reduction of anti-Gal Ab remained marginal. The combination of As2O3 + anti-CD20 mAb + WBI did not reduce anti-Gal Ab levels further. At autopsy in the latter baboon, B cells remained present in Peyer's patches and tonsils.

CONCLUSIONS

In vitro: As2O3 reduced B/plasma cell numbers and suppressed IgM secretors, but not IgG secretors. In vivo: As2O3 was not as effective as either anti-CD20 mAb or WBI in depleting B/plasma cells, and was largely ineffective in reducing anti-Gal Ab levels. Its administration was associated with considerable toxicity. Autopsy in one baboon suggested that B cells in Peyer's patches and tonsils may be resistant to therapy and remain a source of continuing production of anti-Gal Ab.

Delayed rejection of porcine cartilage is averted by transgenic expression of alpha1,2-fucosyltransferase

The use of xenogeneic cells or tissues for tissue engineering applications may lead to advances in biomedical research. Hyperacute and delayed rejection are immunologic hurdles that must be addressed to achieve xenograft survival in the pig-to-primate setting. Expression of human alpha1,2-fucosyltransferase (HT) in the donor cell or tissue protects from hyperacute rejection (HAR) by reducing expression of Galalpha1,3-Gal epitope, the major xenoantigen recognized by human natural antibodies. We hypothesized that Galalpha1,3-Gal antigen contributes to delayed tissue rejection. To test this hypothesis, we transplanted control or HT-transgenic engineered porcine cartilage s.c. into alpha1,3-galactosyltransferase knockout (Gal KO) mice. Control porcine cartilage grafted in Gal KO mice was not susceptible to HAR but was rejected in several wk by a prominent cellular immune infiltrate and elevated antibody titers. In contrast, Gal KO mice receiving the HT engineered cartilage showed a markedly reduced anti-pig antibody response and no anti-Galalpha1,3-Gal-elicited antibody response. The HT implants had a mild cellular infiltrate that was confined to the graft periphery. Our study demonstrates that a marked reduction of Galalpha1,3-Gal antigen in HT-transgenic porcine cartilage confers resistance to a process of delayed rejection. Further development of tissue engineering applications that use genetically modified porcine tissues is encouraged.

Gal mismatch alone causes skin graft rejection in mice

BACKGROUND

Elimination of galactose-alpha1,3-galactose (Gal), the major xenoantigen between pig and human, may extend pig-to-human xenograft survival beyond the current barrier of acute vascular rejection. However, it has been suggested that Gal is an essential molecule in the pig and that the generation of a Gal-deleted (Gal KO) pig will not be possible. Should this be the case, understanding the Gal-mediated immune response will be crucial in developing strategies to overcome pig xenograft rejection in humans. There are no existing models of xenograft rejection in which the sole difference between donor and recipient is Gal. We describe a model of exclusively Gal-mismatched skin graft rejection.

METHODS

The survival of Gal skin grafts on Gal KO mice with the same genetic background was analyzed. To examine innate anti-Gal immunity, Gal KO recipients that were also deficient in T and B cells (RAG-1 KO) were used. To study the role of cognate immunity, recipients were sensitized with a primary Gal allograft before receiving a second Gal graft that was otherwise isogeneic. To test the role of anti-Gal antibodies in this model, recipients were passively immunized with a non-complement-fixing anti-Gal monoclonal antibody.

RESULTS

Gal KO mice chronically reject Gal skin grafts by 100 days at a rate of 48% (n=25) on a BALB/c background and 25% (n=8) on a C57BL/6 background. The grafts had an infiltrate that consisted predominantly of CD4 T cells and macrophages, whereas recipients deficient in T and B cells were incapable of rejection and survived for more than 120 days (n=5). Sensitization with a primary Gal allograft increased the incidence and the tempo of rejection of a second Gal-only mismatched skin graft with 99% rejection that ranged from 11 to 45 days (n=26). Passive transfer of mouse IgG anti-Gal monoclonal-antibody-induced rejection in Gal KO and RAG-1/Gal double-KO recipients at a rate of 92% (n=13).

CONCLUSIONS

We have established a model to study rejection based solely on a Gal mismatch. Our results indicate that non-complement-fixing anti-Gal antibody can cause rejection in the acute vascular rejection time frame and that T-cell-mediated chronic rejection will be a further barrier to overcome if Gal cannot be deleted from the pig.

Analysis of the control of the anti-gal immune response in a non-human primate by galactose alpha1-3 galactose trisaccharide-polyethylene glycol conjugate

BACKGROUND

The current limitation to the clinical application of xenotransplantation using pig organs is a rejection process that has been termed delayed xenograft rejection or acute vascular rejection. It is thought that acute vascular rejection may be mediated at least in part by both the continued synthesis, of preexisting, and the induction, posttransplantation, of antibodies against the carbohydrate moiety galalpha1-3gal that is present on glycoproteins and glycolipids of the pig endothelium. The synthesis of these antibodies has proven difficult to control with currently available immunosuppressive agents.

METHODS

We have synthesized galalpha1-3gal conjugated polyethylene glycol polymers that can bind to anti-galalpha1-3gal antibodies and tested their activity in non-human primates.

RESULTS

These conjugates when administered to non-human primates can substantially reduce the levels of preexisting and control the induction of anti-galalpha1-3gal antibodies. The level of circulating antibody-secreting cells that make anti-galalpha1-3gal antibodies is also reduced.

CONCLUSION

These alpha-gal polyethylene glycol conjugates may have the potential to control the anti-gal antibody response in a pig to primate organ transplant setting and may be a useful therapeutic agent in prolonging graft survival.

Production of human monoclonal antibodies against Galalpha(1-3)Gal

The hyperacute rejection observed in models of pig-to-human xenotransplantation is mainly because of the presence of natural antibodies in human blood with specificity for the Galalpha(1-3)Gal (Gal) carbohydrate moiety present on the surface of porcine endothelial cells. Human monoclonal anti-Gal antibodies could be of use both in the study of the basic mechanisms of hyperacute rejection as well as in its clinical prevention. In the present study we prepared 42 heterohybridomas (human-mouse) secreting antibodies with specificity for the Gal epitope. All of the antibodies produced were of the IgM isotype, according to a dot-blot assay. Twenty-seven antibodies were further characterized, and shown to be specific for Gal by different methods, including an enzyme-linked immunosorbent assay, in which the plates were sensitized with mouse laminin as a source of Gal. Specificity was also confirmed using purified Gal carbohydrate in a hemagglutination inhibition assay. The antibodies were shown to mediate lysis of Gal-expressing rabbit erythrocytes in the presence of complement. However, the heterohybridomas themselves were shown to express Gal, a result of the mouse P3x63Ag8.653 hybridoma cells used during hybridoma generation. The presence of this epitope on the surface of anti-Gal-producing cells, and on the antibody itself, represents a limitation to the production of high affinity anti-Gal antibodies.

Will the pig solve the transplantation backlog?

The increasing shortage of human cadaveric organs for purposes of transplantation has become the critical limiting factor in the number of transplants performed each year. Some of this deficit is being met by the use of organs or partial organs from living donors, but this source is insufficient. Xenotransplantation-the transplantation of organs between species, namely from the pig to human-could provide a solution if immunologic and other associated problems could be solved. When a pig organ is transplanted into a primate, hyperacute rejection, induced by anti-pig antibody and mediated by complement and the coagulation system, develops rapidly. This immediate problem can now be overcome, but the return or persistence of anti-pig antibody leads to a delayed form of humoral rejection, acute humoral xenograft rejection, which leads to destruction of the organ within days or weeks. We review the various approaches being investigated to overcome this barrier. Whether they will also prevent subsequent acute cellular rejection remains unknown. Brief mention is made of the potential physiologic incompatibilities between pig and human organs, as well as the microbiologic safety aspects of xenotransplantation. Finally, the question of patient and societal acceptance of xenotransplantation is discussed.

Enzymatic removal of alphaGal antigen in pig kidneys by ex vivo and in vivo administration of endo-beta-galactosidase C

Xenotransplantation using the pig as a donor species is considered to be a promising solution to the serious shortage of organ donors. Both hyperacute and acute vascular rejection (AVR) are believed to be associated with xenoreactive antibody binding to alphaGal epitopes on the pig vascular endothelial cells. Thus, suppression of this antigen-antibody reaction would appear essential for successful long-term xenograft survival. The purpose of this study was to examine the efficacy of ex vivo and in vivo administration of recombinant endo-beta-galactosidase C (EndoGalC which, in previous in vitro studies, has been proven to digest alphaGal antigens completely) on alphaGal epitopes expressed in pig kidneys. Excised pig kidneys were perfused with University of Wisconsin solution containing EndoGalC and preserved for 4 h. After cold storage, the pig kidney was transplanted into another pig. Ex vivo perfusion and cold storage with EndoGalC reduced alphaGal epitope expression on vascular endothelial cells to an undetectable level. However, alphaGal antigens began to be expressed again as early as 1 day after transplantation. The digestion of alphaGal epitopes by EndoGalC did not cause any damage to the kidney graft. EndoGalC was intravenously administered to two pigs (15 kg), without causing any serious adverse effect. Twelve hours later, >98% of alphaGal antigens on pig red blood cells (RBCs) had been digested. Immunohistochemical study revealed almost complete elimination of alphaGal expression on vascular endothelial cells of the kidney graft 4 and 8 h after in vivo administration, but reappearance within 24 h. EndoGalC was administered to a baboon after an interval of 2 months. The second administration did not result in any serious toxicity or reduction in efficacy. These results suggest that ex vivo and in vivo administration of EndoGalC is simple and useful in removing alphaGal epitopes from pig organs. As the effect of EndoGalC is temporary, multiple in vivo administrations of EndoGalC would be required to inhibit the reappearance of alphaGal epitopes. Alternatively, transgenic techniques of introducing the gene for EndoGalC into the donor organ might permanently prevent alphaGal expression.

Antibody-mediated activation of the classical complement pathway in xenograft rejection

Transplant rejection is a multifactorial process involving complex interactions between components of the innate and the acquired immune system. In view of the shortage of donor organs available for transplantation, xenotransplantation of pig organs into man has been considered as a potential solution. However, in comparison to allografts, xenografts are subject to extremely potent rejection processes that are currently incompletely defined. Consequently, an appropriate and safe treatment protocol ensuring long-term graft survival is not yet available. The first barrier that has to be taken for a xenograft is hyperacute rejection, a rapid process induced by the binding of pre-formed antibodies from the host to the graft endothelium, followed by activation of the classical complement pathway. The present review concentrates on the role of antibodies and complement in xenograft rejection as well as on the approaches for treatment that target these components. The first part focuses on porcine xenoantigens that are recognized by human xenoreactive antibodies and the different treatment strategies that aim on interference in antibody binding. The second part of the review deals with complement activation by xenoreactive antibodies, and summarizes the role of complement in the induction of endothelial cell damage and cell activation. Finally, various options that are currently under development for complement inhibition are discussed, with special reference to the specific inhibition of the classical complement pathway by soluble complement inhibitors.