The role of natural anti-Gal alpha 1-3Gal antibodies in hyperacute rejection of pig-to-baboon cardiac xenotransplants

Xenotransplantation in transition

The application of xenotransplantation of porcine organs and tissues for treatment of disease, sought for more than a century, might soon be realized. Until now, the immune response of recipients against xenogeneic organs and tissues posed the main obstacle to clinical application. However, decades of research into this immune response and identification of other molecular barriers together with advances in genetic engineering and cloning of large animals and immune therapeutics coalesced to support prolonged survival and function of porcine organ grafts in nonhuman primates. This experimental progress in turn sparks consideration of clinical trials. The papers in this special section provide authoritative views concerning the immune hurdles that still limit and potentially still preclude clinical application of xenotransplantation. Xenoreactive antibodies elicited in T cell-dependent B cell-responses constitute the most important hurdle and control of these responses impels use of intense regimens of immunosuppression. These antibodies pose a danger to xenografts and potentially compromise subsequent allografts. However, new insights into the specificity of these antibodies, the pathways and kinetics of production and genetic determinants of pathogenicity offer novel opportunities for intervention. Likewise, the rapid ability to propose and test new strategies in nonhuman primate models hastens needed advances. However further progress will depend on development and validation of laboratory methods for identification and assay of pathogenic immune responses and evaluation of the response to therapy.

Accommodation in allogeneic and xenogeneic organ transplantation: Prevalence, impact, and implications for monitoring and for therapeutics

Accommodation refers to acquired resistance of organs or tissues to immune or inflammatory reactions that might otherwise cause severe injury or rejection. As first observed in ABO-incompatible kidney transplants and heterotopic cardiac xenografts, accommodation was identified when organ transplants continued to function despite the presence of anti-graft antibodies and/or other reactants in the blood of recipients. Recent evidence suggests many and perhaps most organ transplants have accommodation, as most recipients mount B cell responses specific for the graft. Wide interest in the impact of graft-specific antibodies on the outcomes of transplants prompts questions about which mechanisms confer protection against such antibodies, how accommodation might be detected and whether and how rejection could be superimposed on accommodation. Xenotransplantation offers a unique opportunity to address these questions because immune responses to xenografts are easily detected and the pathogenic impact of immune responses is so severe. Xenotransplantation also provides a compelling need to apply these and other insights to decrease the intensity and toxicity of immunosuppression that otherwise could limit clinical application.

AGI-134: a fully synthetic α-Gal glycolipid that converts tumors into in situ autologous vaccines, induces anti-tumor immunity and is synergistic with an anti-PD-1 antibody in mouse melanoma models

Background

Treatments that generate T cell-mediated immunity to a patient’s unique neoantigens are the current holy grail of cancer immunotherapy. In particular, treatments that do not require cumbersome and individualized ex vivo processing or manufacturing processes are especially sought after. Here we report that AGI-134, a glycolipid-like small molecule, can be used for coating tumor cells with the xenoantigen Galα1-3Galβ1-4GlcNAc (α-Gal) in situ leading to opsonization with pre-existing natural anti-α-Gal antibodies (in short anti-Gal), which triggers immune cascades resulting in T cell mediated anti-tumor immunity.

Methods

Various immunological effects of coating tumor cells with α-Gal via AGI-134 in vitro were measured by flow cytometry: (1) opsonization with anti-Gal and complement, (2) antibody-dependent cell-mediated cytotoxicity (ADCC) by NK cells, and (3) phagocytosis and antigen cross-presentation by antigen presenting cells (APCs). A viability kit was used to test AGI-134 mediated complement dependent cytotoxicity (CDC) in cancer cells. The anti-tumoral activity of AGI-134 alone or in combination with an anti-programmed death-1 (anti-PD-1) antibody was tested in melanoma models in anti-Gal expressing galactosyltransferase knockout (α1,3GT^−/−) mice. CDC and phagocytosis data were analyzed by one-way ANOVA, ADCC results by paired t-test, distal tumor growth by Mantel–Cox test, C5a data by Mann–Whitney test, and single tumor regression by repeated measures analysis.

Results

In vitro, α-Gal labelling of tumor cells via AGI-134 incorporation into the cell membrane leads to anti-Gal binding and complement activation. Through the effects of complement and ADCC, tumor cells are lysed and tumor antigen uptake by APCs increased. Antigen associated with lysed cells is cross-presented by CD8α+ dendritic cells leading to activation of antigen-specific CD8+ T cells. In B16-F10 or JB/RH melanoma models in α1,3GT^−/− mice, intratumoral AGI-134 administration leads to primary tumor regression and has a robust abscopal effect, i.e., it protects from the development of distal, uninjected lesions. Combinations of AGI-134 and anti-PD-1 antibody shows a synergistic benefit in protection from secondary tumor growth.

Conclusions

We have identified AGI-134 as an immunotherapeutic drug candidate, which could be an excellent combination partner for anti-PD-1 therapy, by facilitating tumor antigen processing and increasing the repertoire of tumor-specific T cells prior to anti-PD-1 treatment.

Xenotransplantation: Progress Along Paths Uncertain from Models to Application

For more than a century, transplantation of tissues and organs from animals into man, xenotransplantation, has been viewed as a potential way to treat disease. Ironically, interest in xenotransplantation was fueled especially by successful application of allotransplantation, that is, transplantation of human tissue and organs, as a treatment for a variety of diseases, especially organ failure because scarcity of human tissues limited allotransplantation to a fraction of those who could benefit. In principle, use of animals such as pigs as a source of transplants would allow transplantation to exert a vastly greater impact than allotransplantation on medicine and public health. However, biological barriers to xenotransplantation, including immunity of the recipient, incompatibility of biological systems, and transmission of novel infectious agents, are believed to exceed the barriers to allotransplantation and presently to hinder clinical applications. One way potentially to address the barriers to xenotransplantation is by genetic engineering animal sources. The last 2 decades have brought progressive advances in approaches that can be applied to genetic modification of large animals. Application of these approaches to genetic engineering of pigs has contributed to dramatic improvement in the outcome of experimental xenografts in nonhuman primates and have encouraged the development of a new type of xenograft, a reverse xenograft, in which human stem cells are introduced into pigs under conditions that support differentiation and expansion into functional tissues and potentially organs. These advances make it appropriate to consider the potential limitation of genetic engineering and of current models for advancing the clinical applications of xenotransplantation and reverse xenotransplantation.

Heterologous Antibodies Adsorption in Xenotransplantation of a Landrace Piglet Kidney Into a Rhesus Monkey

BACKGROUND

To explore the adsorption of heterologous antibodies in 6 xenotransplants of Landrace piglet kidneys into rhesus monkeys.

METHODS

The Landrace piglets and rhesus monkeys were used as donors and recipients, respectively. The donor kidney was the left kidney excised from each Landrace piglet and lavaged with University of Wisconsin solution through the renal artery and vein ex vivo. The renal arteriovenous end of the recipient was preserved. After anastomosis of the renal artery and vein with the arteriovenous end of the recipient for reperfusion, a cross-lymphocyte cytotoxicity test of the heterogeneous kidney was performed.

RESULTS

All 6 Landrace piglet kidneys absorbed heterologous antibodies that were pre-existing in the rhesus macaques' kidneys. The cross-lymphocyte toxicity test was performed after the kidney were completely blackened. The cross-lymphocyte toxicity in all each heterogeneous kidney changed from strong positive to weak positive.

CONCLUSIONS

Heterologous antibodies were adsorbed in xenotransplants of Landrace piglet kidneys into rhesus monkeys. Xenotransplanted kidney can adsorb heterologous antibodies and consume relevant complements, which is a good model for research of hyperacute rejection in xenotransplantation.

The role of sialic acids in the immune recognition of xenografts

Presentation of sialic acid (Sia) varies among different tissues and organs within each species, and between species. This diversity has biologically important consequences regarding the recognition of cells by "xeno" antibodies (Neu5Gc vs Neu5Ac). Sia also plays a central role in inflammation by influencing binding of the asialoglycoprotein receptor 1 (ASGR-1), Siglec-1 (Sialoadhesin), and cellular interactions mediated by the selectin, integrin, and galectin receptor families. This review will focus on what is known about basic Sia structure and function in association with xenotransplantation, how changes in sialylation may occur in this context (through desialylation or changes in sialyltransferases), and how this fundamental pathway modulates adhesive and cell activation pathways that appear to be particularly crucial to homeostasis and inflammation for xenografts.

Four-dimensional characterization of thrombosis in a live-cell, shear-flow assay: development and application to xenotransplantation

Background

Porcine xenografts are a promising source of scarce transplantable organs, but stimulate intense thrombosis of human blood despite targeted genetic and pharmacologic interventions. Current experimental models do not enable study of the blood/endothelial interface to investigate adhesive interactions and thrombosis at the cellular level under physiologic conditions. The purpose of this study was to develop and validate a live-cell, shear-flow based thrombosis assay relevant to general thrombosis research, and demonstrate its potential in xenotransplantation applications.

Methodology/Principal Findings

Confluent wild-type (WT, n = 48) and Gal transferase knock-out (GalTKO, which resist hyperacute rejection; n = 11) porcine endothelia were cultured in microfluidic channels. To mimic microcirculatory flow, channels were perfused at 5 dynes/cm² and 37°C with human blood stained to fluorescently label platelets. Serial fluorescent imaging visualized percent surface area coverage (SA, for adhesion of labeled cells) and total fluorescence (a metric of clot volume). Aggregation was calculated by the fluorescence/SA ratio (FR). WT endothelia stimulated diffuse platelet adhesion (SA 65 ± 2%) and aggregation (FR 120 ± 1 a.u.), indicating high-grade thrombosis consistent with the rapid platelet activation and consumption seen in whole-organ lung xenotransplantation models. Experiments with antibody blockade of platelet aggregation, and perfusion of syngeneic and allo-incompatible endothelium was used to verify the biologic specificity and validity of the assay. Finally, with GalTKO endothelia thrombus volume decreased by 60%, due primarily to a 58% reduction in adhesion (P < 0.0001 each); importantly, aggregation was only marginally affected (11% reduction, P < 0.0001).

Conclusions/Significance

This novel, high-throughput assay enabled dynamic modeling of whole-blood thrombosis on intact endothelium under physiologic conditions, and allowed mechanistic characterization of endothelial and platelet interactions. Applied to xenogeneic thrombosis, it enables future studies regarding the effect of modifying the porcine genotype on sheer-stress-dependent events that characterize xenograft injury. This in-vitro platform is likely to prove broadly useful to study thrombosis and endothelial interactions under dynamic physiologic conditions.

Requirements for the welfare of baboons and pigs used in animal-to-animal xenotransplantation experiments

OBJECTIVE

Review the welfare requirements of pigs and baboons used for xenotransplantation in research laboratories. Because of the requirements to maintain optimum health status, these animals are often kept in barren enclosures with little or no enrichment. They may also be exposed to procedures causing stress and discomfort. Although animal-to-human xenotransplantation is, at the present time, not approved in Australia, research is currently being performed to develop laboratory procedures, using the pig-to-baboon model.

RESULTS AND CONCLUSION

We make recommendations for the husbandry of baboons and pigs used for xenotransplantation, to increase their welfare and minimise stress during experimental procedures, while attempting to preserve the health status required. It is proposed that novel standards should be devised and implemented for baboons, whereas existing pig welfare appraisal schemes could, with minor changes, be suitable for assessing the welfare of pigs used for xenotransplantation.

Study of the microcirculation in hDAF transgenic rat livers xenoperfused with human blood

BACKGROUND

The microcirculation was assessed in the livers of human decay accelerating factors (hDAF) and wild-type transgenic rats by fluorescent intravital microscopy, histology and histomorphology to determine the benefits of hDAF expression for the microcirculation of a rat liver xenograft perfused with human blood.

METHODS

Male hDAF transgenic rats (group A; n = 20) and wild-type Sprague-Dawley rats (group B; n = 20) were xenoperfused with human blood, while other male wild-type Sprague-Dawley rats (group C; n = 10) were perfused with allogeneic blood. Following plasma and leukocyte staining with fluorescein sodium, and platelet staining with rhodamine, the right lobe of the liver was assessed by intravital microscopy, counting the numbers of perfused sinusoids and leukocytes adhering to the endothelium per mm(2), and calculating the acinar perfusion index (Pi). The liver underwent histological assessment at the end of each experiment. Mean +/- SEM values were calculated and the Mann-Whitney U-test was used for statistical analysis.

RESULTS

The number of perfused sinusoids was higher in the group of hDAF rat livers (group A) and controls (group C) than in the group of non-transgenic rat livers perfused with human blood (group B) (P < 0.05), although only group C still had a significantly more perfused sinusoids than the other groups after 90 min of perfusion (P < 0.05). The acinar perfusion index was higher in groups A and C than in group B (P < 0.05); here again, only group C still had a significantly higher Pi than group B after 90 min of perfusion (P < 0.05). There was a massive accumulation of leukocytes that peaked after 5 min and persisted throughout the perfusion in all three groups. Histology showed portal and subendothelial hepatic vein hemorrhage, necrosis and inflammatory reaction, which were particularly evident in group B.

CONCLUSION

In our study, rat livers transgenic for hDAF were better protected against early tissue damage by perfusion with human blood, but this did not result in a longer xenograft survival.

Proteomic identification of non-Gal antibody targets after pig-to-primate cardiac xenotransplantation

BACKGROUND

Experience with non-antigenic galactose alpha1,3 galactose (alphaGal) polymers and development of alphaGal deficient pigs has reduced or eliminated the significance of this antigen in xenograft rejection. Despite these advances, delayed xenograft rejection (DXR) continues to occur most likely due to antibody responses to non-Gal endothelial cell (EC) antigens.

METHODS

To gauge the diversity of the non-Gal antibody response we used antibody derived from CD46 transgenic heterotopic cardiac xenografts performed without T-cell immunosuppression, Group A (n = 4) and Gal knockout (GT-KO) heart transplants under tacrolimus and sirolimus immunosuppression, Group B (n = 8). Non-Gal antibody was measured by flow cytometry and by western blots using GT-KO EC membrane antigens. A nanoLC/MS/MS analysis of proteins recovered from 2D gels was used to identify target antigens.

RESULTS

Group A recipients exhibited a mixed cellular and humoral rejection. Group B recipients mainly exhibited classical DXR. Western blot analysis showed a non-Gal antibody response induced by GT+ and GT-KO hearts to an overlapping set of pig aortic EC membrane antigens. Proteomic analysis identified 14 potential target antigens but failed to define several immunodominant targets.

CONCLUSIONS

These experiments indicate that the non-Gal antibody response is directed to a number of stress response and inflammation related pig EC antigens and a few undefined targets. Further analysis of these antibody specificities using alternative methods is required to more fully define the repertoire of non-Gal antibody responses.

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.

Xenotransplantation: role of natural immunity

Hyperacute rejection, mediated by natural anti-Galalpha1,3Galbeta1,4GlcNAc (alphaGal) antibodies and the classically activated complement pathway, was identified as the first major barrier to the survival of porcine organs in humans. Subsequently, discordant pig-to-nonhuman primate and concordant rodent models revealed key roles for T and B lymphocytes in the second form of rejection, acute vascular rejection (AVR) or delayed xenograft rejection (DXR). As significant progress was made in strategies to circumvent or suppress xenoreactivity of the adaptive immune system, it became clear that, apart from natural antibodies, other innate immune system elements actively participate in AVR/DXR and represent a barrier to xenograft acceptance that may be particularly difficult to overcome. Observations in pig-to-primate and semi-discordant and concordant rodent models indicate that Natural Killer (NK) cells play a more prominent role in xenograft than in allograft rejection. Several mechanisms through which human NK cells recognize porcine endothelial cells have been elucidated and these appear to be more diverse than those involved in NK cell alloreactivity. Further, it has been demonstrated that human macrophages and neutrophils can directly recognize pig derived cells and can mediate direct xenograft damage. Here, we review the recent progress in the understanding of the xenoreactivity of the natural immune system, focussing on preclinical pig-to-(non)human primate systems, and discuss the proposed strategies to overcome these barriers.

Ig knock-in mice producing anti-carbohydrate antibodies: breakthrough of B cells producing low affinity anti-self antibodies

Natural Abs specific for the carbohydrate Ag Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) play an important role in providing protective host immunity to various pathogens; yet little is known about how production of these or other anti-carbohydrate natural Abs is regulated. In this study, we describe the generation of Ig knock-in mice carrying functionally rearranged H chain and L chain variable region genes isolated from a B cell hybridoma producing alphaGal-specific IgM Ab that make it possible to examine the development of B cells producing anti-carbohydrate natural Abs in the presence or absence of alphaGal as a self-Ag. Knock-in mice on a alphaGal-deficient background spontaneously developed alphaGal-specific IgM Abs of a sufficiently high titer to mediate rejection of alphaGal expressing cardiac transplants. In the spleen of these mice, B cells expressing alphaGal-specific IgM are located in the marginal zone. In knock-in mice that express alphaGal, B cells expressing the knocked in BCR undergo negative selection via receptor editing. Interestingly, production of low affinity alphaGal-specific Ab was observed in mice that express alphaGal that carry two copies of the knocked in H chain. We suggest that in these mice, receptor editing functioned to lower the affinity for self-Ag below a threshold that would result in overt pathology, while allowing development of low affinity anti-self Abs.

Somatic cell nuclear transfer in pigs: recent achievements and future possibilities

During the past 6 years, considerable advancement has been achieved in experimental embryology of pigs. This process was mainly generated by the rapidly increasing need for transgenic pigs for biomedical research purposes, both for future xenotransplantation to replace damaged human organs or tissues, and for creating authentic animal models for human diseases to study aetiology, pathogenesis and possible therapy. Theoretically, among various possibilities, an established somatic cell nuclear transfer system with genetically engineered donor cells seems to be an efficient and reliable approach to achieve this goal. However, as the result of unfortunate coincidence of known and unknown factors, porcine embryology had been a handicapped branch of reproductive research in domestic animals and a very intensive and focused research was required to eliminate or minimise this handicap. This review summarises recent achievements both in the background technologies (maturation, activation, embryo culture) and the actual performance of the nuclear replacement. Recent simplified methods for in vivo development after embryo transfer are also discussed. Finally, several fields of potential application for human medical purposes are discussed. The authors conclude that although in this early phase of research no direct evidence can be provided about the practical use of transgenic pigs produced by somatic cell nuclear transfer as organ donors or disease models, the future chances even in medium term are good, and at least proportional with the efforts and sums that are invested into this research area worldwide.

Accommodation of grafts: implications for health and disease

Accommodation refers to the acquired resistance of a graft to immune-mediated injury. It is typically observed after antibodies that would cause rejection of a graft are removed from a recipient and then later return. In addition to being induced in this manner, accommodation can occur spontaneously, without depleting antibodies. Indeed, we postulate spontaneous accommodation may be the most common outcome of clinical organ transplantation. The paper reviews the current understanding of accommodation, emphasizing recent advances and important questions. Among the recent advances are the discoveries of potentially broader relevance of accommodation for biology and immunology and pathways by which accommodation may be achieved. To investigate these pathways and to understand how accommodation begins and how it evolves, clinical organ transplants might offer a useful and incisive model.

Clinical xenotransplantation of organs: why aren't we there yet?

Mohiuddin discusses the lessons learned from large animal xenograft models and why the immunological barrier is still the most important hurdle preventing clinical xenotransplantation of organs.

T-cell responses during pig-to-primate xenotransplantation

Xenotransplantation using porcine organs may resolve a chronic shortage of donor organs for clinical transplantation if significant immunological barriers can be overcome. To determine the potential role of T lymphocytes in Xenograft (Xg) rejection, we transplanted transgenic hCD46 porcine hearts heterotopically into baboon recipients.

METHODS

Recipients were treated to deplete anti-Gal antibody with a non-antigenic alpha-Gal polyethylene glycol polymer (TPC) (n = 2), TPC plus rituximab (anti-CD20) (n = 1) or were untreated (n = 1). None of the recipients received T-cell immunosuppression.

RESULTS

All Xgs failed within 7 days and showed evidence of a mixed humoral and cellular rejection process. Cellular infiltration consisting primarily of CD4+ T cells and few CD8+ T cells. Proliferation and cytotoxicity assays showed sensitization of CD4+ and CD8+ T cells that reacted with porcine IFN-gamma (pIFN-gamma)-stimulated porcine aortic endothelial cells (PAEC). The CD4+ lymphocytes displayed greater cytotoxicity than CD8+ cells. An increased frequency of PAEC-specific interleukin (IL) 2 and IFN-gamma-secreting T cells was observed, suggesting a Th1 cytokine bias. An increase in the percentage of circulating CD4+CD28- cells was observed at the time of rejection and over 50% of the CD4+ cells recovered from residual pig tissue at necropsy lacked CD28 expression.

CONCLUSIONS

These findings show that lymphocytes are efficiently stimulated by PAEC antigens and can mediate direct tissue destruction. These studies (1) provide an insight into the potential of cellular-mediated cardiac Xg rejection, (2) show for the first time the induction of cytotoxic pig-specific CD4+CD28- lymphocytes and (3) provide a rational basis for determining different modes of immunosuppression to treat Xg rejection.

Sequential analysis of anti-alpha Gal natural antibody-producing B cells in GalT knockout mice in cyclophosphamide-induced tolerance

Previously, we have shown that cyclophosphamide (CP)-induced tolerance, marked by permanent acceptance of donor skin graft and establishment of donor mixed chimerism, was readily induced with treatment with donor spleen cells (SC), CP, busulfan (BU) and donor bone marrow cells (BMC). Here, we investigated the mechanism of anti-donor natural antibody (nAb) producing B-cell tolerance in our CP-induced tolerance systems in alpha1,3-galactosyltransferase-deficient knockout mice (GalT KO; GalT-/-, H-2(b/d)). After induction of tolerance using donor AKR SC and BMC, survival of donor heart and skin grafts and production of anti-Galalpha1-3Galbeta1-4GlcNAc (anti-alphaGal) Ab in recipient GalT KO mice were analyzed. In addition, the production of anti-alphaGal Ab and the presence of Gal-BSA binding B cells in GalT KO mice were analyzed by flow cytometry (FCM) after treatments with rabbit red blood cells (RRBC) and CP. Permanent acceptance of donor skin and heart grafts and abrogation of anti-alphaGal Ab were achieved in GalT KO mice treated with donor SC + CP/BU + BMC. However, in the GalT KO mice treated with donor SC and CP, donor skin grafts were acutely rejected, even though anti-alphaGal Ab was undetectable. Similarly, anti-alphaGal Ab was undetectable in GalT KO mice treated with RRBC and CP. Our data strongly indicated the following mechanisms: the clonal destruction in the early stage and the clonal anergy or ignorance in the late stage after conventional conditioning with RRBC and CP. In conclusion, our drug-induced tolerance protocols are effective to induce tolerance in recipients that produce anti-donor nAb.

Current status of animal-to-human transplantation

The transplantation of animal organs into humans as a way of treating organ failure has been pursued for 100 years. Clinical xenotransplantation, as such, has always failed because the transplanted organ is rejected by the recipient. Recent advances in transplant immunology have revealed some mechanisms underlying the rejection of xenografts, and these discoveries have sparked efforts to use genetic engineering of animals and therapeutics directed at the recipient to overcome this problem. This paper reviews the current understanding of the mechanisms of xenograft rejection and efforts to overcome rejection and other hurdles.

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.

Foundation Review: Transgenic animals and their impact on the drug discovery industry

The ability to direct genetic changes at the molecular level has resulted in a revolution in biology. Nowhere has this been more apparent than in the production of transgenic animals. Transgenic technology lies at the junction of several enabling techniques in such diverse fields as embryology, cell biology and molecular genetics. A host of techniques have been used to effect change in gene expression and develop new pharmaceutical and nutraceutical compounds cost-effectively. Scientific advances gained by transgenic capabilities enable further understanding of basic biological pathways and yield insights into how changes in fundamental processes can perturb programmed development or culminate in disease pathogenesis.

Warfarin or low-molecular-weight heparin therapy does not prolong pig-to-primate cardiac xenograft function

Microvascular thrombosis is a prominent feature in cardiac delayed xenograft rejection (DXR). We investigated the impact of warfarin or low-molecular-weight heparin (LMWH) anti-coagulation on xenograft function using a heterotopic pig-to-primate model. Donor hearts were from CD46 transgenic pigs and baboon immunosuppression included tacrolimus, sirolimus, anti-CD20 and TPC, an alpha-galactosyl-polyethylene glycol conjugate. Three groups of animals were studied. Group 1 (n = 9) was treated with warfarin, Group 2 (n = 13) with LMWH and Group 3, received no anti-coagulant drugs. The median duration of xenograft function was 20 days (range 3-62 days), 18 days (range 5-109 days) and 15 days (range 4-53 days) in Groups 1 to 3 respectively. Anti-coagulation achieved the targeted international normalized prothrombin ratio (INR) and anti-factor Xa levels consistent with effective in vivo therapy yet, no significant impact on median xenograft function was observed. At rejection, a similar histology of thrombosis and ischemia was apparent in each group and the levels of fibrin deposition and platelet thrombi in rejected tissue was the same. Anti-coagulation with warfarin or LMWH did not have a significant impact on the onset of DXR and microvascular thrombosis. However, a role for specific anti-coagulant strategies to achieve long-term xenograft function cannot be excluded.

The bacterial flora of α-Gal knockout mice express the α-Gal epitope comparable to wild type mice

The human genome possesses pseudogenes for the enzyme alpha1,3 galactosyltransferase and hence, human cells and tissues do not express the Galalpha terminated trisaccharide structure Galalpha1-3Galbeta1-4GlcNAc, the so-called alpha-Gal epitope. Circulating antibodies specific for this carbohydrate epitope are, however, present in high amounts in humans. It has previously been hypothesized that the antibody production is induced by the presence of the alpha-Gal epitope in the cell walls of the enteric flora, especially Enterobacteriaceae spp. However, in mice, in which the epitope has been deleted by targeted mutation of the gal-transferase gene, alpha-Gal antibodies do not appear without prior immunization, although the mice through their growth probably have been exposed to a normal bacterial flora of e.g. Enterobacteriaceae spp. It is unknown whether there are different types of immune reactions to antigenic carbohydrate expressing bacteria and whether there are discrepancies in the enteric flora between these knockout mice and their wild type litter mates. In this study the enteric flora of alpha-Gal knockout and wild type mice was compared both in relation to the prevalence of different types of bacteria in the two groups of mice, as well as in relation to the expression of the epitope on the surface of Enterobacteriaceae spp. Our results showed that the enteric flora did not differ significantly between knockout and wild type mice and that it was comparable to the flora known to be present in the intestines of other mice. All Enterobacteriaceae spp. examined expressed the alpha-Gal epitope no matter whether they were isolated from knockout or wild type mice. It is, therefore, discussed whether it is more reasonable to assume that alpha-Gal antibodies in mammals that do not produce alpha1,3 galactosyltransferase such as in the knock mice and in humans are the result of another antigen stimulant than these common representatives of the enteric flora, that we isolated from the two types of mice. Possible candidates for a carrier in humans could be bacteria or viruses not isolated from barrier-bred mice.

Progress in xenotransplantation

Organ transplantation is considered the most effective treatment for end-stage organ failure; currently it is limited by a severe worldwide shortage of human donor organs. This has led to investigation of the potential use of animals as organ donors. For a number of reasons, the pig represents the most likely organ donor candidate. Transplantation of a vascularised porcine organ into a human or non-human primate results in an immediate and dramatic rejection process, known as hyperacute rejection, which is mediated by the binding of pre-existing antibody to the porcine graft and the subsequent activation of host complement. Strategies aimed at preventing this initial rejection have been largely successful in experimental models. This has allowed attention to turn towards an understanding of the immunological barriers comprising the next phase of xenograft rejection, termed acute vascular rejection. This delayed rejection process appears to be a humoral event, and it is likely that the control of antibody synthesis will play a pivotal role in overcoming the current barrier to successful xenotransplantation.

Cardiac Xenotransplantation: Progress Toward the Clinic

BACKGROUND

Animal organs could satisfy the demand for solid organ transplants, which currently exceeds the limited human donor supply. Hyperacute rejection, the initial immune barrier to successful xenotransplantation, has been overcome with pig donors transgenic for human complement regulatory proteins. Delayed xenograft rejection, thought to be mediated by anti-pig antibodies predominantly to Gal antigens, is currently regarded as the major barrier to successful xenotransplantation. A median graft survival of 90 days in the life-supporting position is considered a reasonable initial standard for consideration of entry to the clinic.

METHODS

A series of 10 heterotopic heart transplants from CD46 transgenic pigs to baboons was completed. Immunosuppression consisted of splenectomy, Rituximab (Anti-CD20), tacrolimus, sirolimus, corticosteroids, and TPC. Thymoglobulin (Rabbit Anti-Thymocyte Globulin) was used to treat putative rejection episodes.

RESULTS

Median graft survival was 76 days (range 56-113 days, n = 9). Only three grafts were lost to rejection. The remaining grafts lost were due to recipient mortality with baboon cytomegalovirus (BCMV) being the major cause (n = 4). No cellular infiltrates were present as a manifestation of rejection. Three hearts showed chronic graft vasculopathy.

CONCLUSIONS

The median survival of 76 days in this group of heterotopic porcine-to-baboon cardiac xenografts represents a major advance over the median 27-day survival reported in the literature. Cellular rejection may not constitute a direct major barrier to xenotransplantation. A median survival of 90 days may be achievable with better control of BCMV infection. If further studies in the orthotopic position replicate these outcomes, criteria considered appropriate for clinical application of cardiac xenotransplantation would be approached.

Comparison of the binding properties of the mushroom Marasmius oreades lectin and Griffonia simplicifolia I-B isolectin to alphagalactosyl carbohydrate antigens in the surface phase

The binding of two alpha-galactophilic lectins, Marasmius oreades agglutinin (MOA), and Griffonia simplicifolia I isolectin B(4) (GS I-B(4)) to neoglycoproteins and natural glycoproteins were compared in a surface phase assay. Neoglycoproteins carrying various alpha-galactosylated glycans and laminin from basement membrane of mouse sarcoma that contains the xenogenic Galalpha1-3Gal1-4GlcNAc epitope were immobilized in microtiter plate wells and lectin binding determined with an enzyme-linked assay. After 24 h of incubation, MOA had higher affinity for the xenogenic pentasaccharide (Galalpha1-3Gal1-4GlcNAcbeta1-3Galbeta1-4Glc) than for the Galalpha-monosaccharide. The binding properties of MOA and GS I-B(4) to the xenogenic disaccharide (Galalpha1-3Galbeta1) were comparable while the binding of MOA to the xenogenic pentasaccharide was much stronger than the binding of GS I-B(4) to the same epitope. Non-xenogenic disaccharide-coupled neoglycoproteins having galactose end groups linked alpha1-2 or alpha1-4 to Gal or linked alpha1-3 to GalNAc bound very weakly to MOA, whereas GS I-B(4) recognized all of these disaccharides with similarly high affinity. MOA also showed high affinity for laminin. The results indicate that the Marasmius oreades lectin has nearly the same affinities as does GS I-B(4) for the simple xenogenic carbohydrate antigens, but MOA has greater affinity for the pentasaccharide and is far more specific in its binding preferences than the Griffonia lectin.

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.

Potential applications and prospects for cardiac xenotransplantation

Despite improvements in pharmacologic therapies, the outlook for patients with severe cardiac disease remains poor. At present, the only "cure" for end-stage heart failure is transplantation. However, fewer than 5% of those who need a cardiac transplant receive one in the United States each year. As an alternative, some propose using animals as a source of organs for transplantation (i.e., xenotransplantation). In this article we review the potential applications of xenotransplantation for the treatment of cardiac disease, and weigh xenotransplantation against other new technologies that might be used. We also consider the current status of addressing the hurdles to application of xenotransplantation.

Human cytomegalovirus UL18 alleviated human NK-mediated swine endothelial cell lysis

Human cytomegalovirus UL18, a MHC class I homologue, is known to serve as a natural killer cell (NK) decoy and to ligate NK inhibitory receptors to prevent lysis of an infected target cell. To explore whether the cell surface expression of UL18 represents a potential immune suppressive approach to evade NK-mediated cytotoxicity in the prevention of xenograft rejection, we examined the effect of the UL18 expression in vitro upon human NK-mediated cytotoxicity against swine endothelial cells (SECs). UL18 expression on SECs by a retroviral vector (PLNCX2) significantly suppressed NK-mediated SEC lysis by approximately 25-100%. The protective effect of UL18 could be mediated through ILT-2 inhibitory receptor on NKs. Additionally, the interaction between UL18 and NKs resulted in the significant reduction of IFN-gamma production. This study demonstrates that UL18 can serve as an effective tool for the evasion of NK-mediated cytotoxicity and for the inhibition of IFN-gamma production during xenograft rejection.

Xenogeneic transplantation of porcine islets: an overview

The extreme demand for human organs or tissues for transplantation has driven the search for viable alternatives. Pigs are considered a possible source of tissue for a number of reasons including shared physiology, plentiful supply, short gestation, and, more recently, the generation of transgenic animals. Porcine islets show promise as a source of islets for the treatment of type 1 diabetes mellitus. Porcine islets regulate glucose levels in the same physiologic range as humans, and porcine insulin has been used for years as an exogenous source of insulin for glucose control. In this review, we discuss the advantages and disadvantages of the use of adult or neonatal porcine islets, the immunologic challenges facing transplantation of xenogeneic islets, and the concerns regarding transmission of infectious agents between species. Porcine islets isolated from both adult and neonatal pigs are capable of restoring euglycemia in experimental animal models of diabetes. Adult islets are more difficult to isolate, whereas neonatal islets have great proliferation potential but require several weeks to function posttransplantation. Xenogeneic islets are susceptible to complement-mediated lysis after the binding of preformed natural antibodies and cellular immunity involving both macrophages and CD4+ T cells. In addition, the potential for transmission of porcine endogenous retroviruses, porcine cytomegalovirus, and porcine lymphotropic herpesvirus type 1 are all concerns that must be addressed. Despite the challenges facing xenotransplantation, the extreme need for donor organs and tissues continues to drive progress toward overcoming the unique issues associated with transplantation between species.

Dextran sulfate acts as an endothelial cell protectant and inhibits human complement and natural killer cell-mediated cytotoxicity against porcine cells

BACKGROUND

The innate immune system, including complement and natural killer (NK) cells, plays a critical role in activation and damage of endothelial cells (ECs) during xenograft rejection. The semisynthetic proteoglycan analog dextran sulfate (DXS, molecular weight 5,000) is known to inhibit the complement and coagulation cascades. We hypothesized that DXS may act as an "EC-protectant" preventing complement and NK lysis by functionally replacing heparan sulfate proteoglycans that are shed from the EC surface on activation of the endothelium.

METHODS

Binding of DXS to ECs, deposition of human complement, cytotoxicity, and heparan sulfate expression after exposure to normal human serum were analyzed by flow cytometry. The efficacy of DXS to protect ECs from xenogeneic NK cell-mediated cytotoxicity was tested in standard 51Cr-release assays.

RESULTS

DXS dose-dependently inhibited all three pathways of complement activation. Binding of DXS to porcine cells increased on treatment with human serum or heparinase I and correlated positively with the inhibition of human complement deposition. This cytoprotective effect of DXS was still present when the challenge with normal human serum was performed up to 48 hr after DXS treatment of the cells. DXS incubation of porcine ECs with and without prior tumor necrosis factor-alpha stimulation reduced xenogeneic cytotoxicity mediated by human NK cells by 47.3% and 25.3%, respectively.

CONCLUSIONS

DXS binds to porcine cells and protects them from complement- and NK cell-mediated injury in vitro. It might therefore be used as a novel therapeutic strategy to prevent xenograft rejection and has potential for clinical application as an "EC protectant."

The role of antibodies and von Willebrand factor in discordant pulmonary xenotransplantation

Pulmonary xenotransplantation is one potential solution to the paucity of donors but is currently limited by rapid failure of the graft. Unlike cardiac and renal xenotransplants, pulmonary xenografts release large quantities of swine von Willebrand factor (vWF). Swine vWF binds xenoreactive antibodies and is capable of activating primate platelets. The contribution of swine vWF to lung xenograft dysfunction is not entirely clear. To probe the role vWF plays in xenograft dysfunction, we traced the fate of xenoantibodies in vWF+ and von Willebrand factor-deficient (vWFD) swine lungs. These studies showed that the vast majority of xenoantibodies bind the vWF released from the vWF+ swine lung, and thus do not remain bound on lung endothelium. The vWF complexed to xenoantibody remained capable of aggregating primate platelets. With this information, we performed swine-to-baboon lung transplants using vWF+ and vWFD donors. Without vWF present to complex xenoantibodies, a picture of hyperacute rejection more typical of heart and kidney xenografts, with antibody deposition along the graft endothelium, interstitial hemorrhage, and edema occurred. These findings suggest that porcine vWF plays a major role in the pathogenesis of pulmonary xenograft dysfunction, and suggests promising strategies to treat lung xenograft dysfunction.

Pig cells that lack the gene for alpha1-3 galactosyltransferase express low levels of the gal antigen

BACKGROUND

The major antigen recognized on pig tissue by primate antibodies is a terminal galalpha1-3gal carbohydrate structure (gal antigen) present on glycolipids and glycoproteins. The production of animals from somatic cells allows for the inactivation of specific genes. It is anticipated that the complete inactivation of the gene encoding alpha1-3 galactosyltransferase, the enzyme that synthesizes the galalpha1-3gal linkage, will result in loss of that antigen from pig organs and tissue and will provide a survival benefit in pig-to-primate xenotransplants.

METHODS

Positive-negative selection was used to produce fetal-pig fibroblasts that were a heterozygous knockout (+/-) of the alpha1-3 galactosyltransferase gene. Nuclear transfer of these cells generated pig embryos and live born pigs with the appropriate genotype. Using a novel selection method with cells from (+/-) embryos, we produced homozygous (-/-) fetal-pig fibroblast cells.

RESULTS

Southern blot analysis of the alpha1-3 galactosyltransferase gene showed that we had produced (+/-) pig embryos, (+/-) live born pigs, and (-/-) pig-fetal fibroblast cells. Fluorescence-activated cell sorter (FACS) analysis with some, but not all, mouse anti-gal monoclonal antibodies and sensitized human serum showed that (-/-) cells still synthesized the gal antigen at 1 to 2% of the level of control heterozygous cells.

CONCLUSIONS

Fetal-pig fibroblasts homozygous for the knockout of the alpha1-3 galactosyltransferase gene appear to express low but detectable levels of the gal antigen.

The in vitro and in vivo effects of anti-galactose antibodies on endothelial cell activation and xenograft rejection

We have previously produced a series of antigalactose (anti-Gal) hybridomas and characterized their heavy chain gene usage. Here we have quantified the affinity of these Abs for the alpha-Gal epitope and characterized their in vitro effects on endothelial cell activation and apoptosis. We report that anti-Gal mAbs derived from Gal(-/-) mice show a range of affinity for the alpha-Gal epitope, and that affinity was generally increased as the V(H) gene usage transitioned from germline sequences to sequences exhibiting somatic maturation. Despite an 85-fold range in affinity, all the anti-Gal mAbs examined induced alpha-Gal-specific endothelial cell activation, and after prolonged exposure induced endothelial cell apoptosis in a complement-independent manner. Only murine anti-Gal mAbs of the IgM or IgG3 subclass, but not IgG1, were effective at initiating complement-dependent cell lysis. Using a novel rat to mouse xenograft model, we examined the in vivo ability of these mAbs to induce xenograft rejection and characterized the rejection using histology and immunohistochemistry. Infusion of complement-fixing IgG3 mAbs resulted in either hyperacute rejection or acute vascular rejection of the xenograft. Surprisingly, infusion of an equal amount of a high affinity anti-Gal IgG1 mAb, that fixed complement poorly also induced a rapid xenograft rejection, which we have labeled very acute rejection. These studies emphasize the importance of in vivo assays, in addition to in vitro assays, in understanding the role of anti-Gal IgG-mediated tissue injury and xenograft rejection.

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.

Genetic modification of xenografts

For nearly a century, xenotransplantation has been seen as a potential approach to replacing organs and tissues damaged by disease. Until recently, however, the application of xenotransplantation has seemed only a remote possibility. What has changed this perspective is the advent of genetic engineering of large animals; that is, the ability to add genes to and remove genes from lines of animals that could provide an enduring source of tissues and organs for clinical application. Genetic engineering could address the immunologic, physiologic and infectious barriers to xenotransplantation, and could allow xenotransplantation to provide a source of cells with defined and even controlled expression of exogenous genes. This communication will consider one perspective on the application of genetic engineering in xenotransplantation.

Role of complement in xenotransplantation

The xenotransplantation research is driven by the increasing gap between the number of patients with end-stage organ failure on waiting lists for transplantation and the supply of allografts. The lack of success in developing suitable artificial organs for permanent treatment of organ failure has further strengthened the need for xenotransplantation research. Pigs are now generally accepted to be the source animal of choice. Transplantation of pig organs to humans faces several barriers which have to be overcome before it comes to clinical application: (1) anatomical and physiological conditions; (2) immunological rejection mechanisms; (3) molecular compatibility between signal molecules of the two species; (4) risk of transmission of microorganisms, particularly pig endogenous retroviruses; and (5) legal and ethical aspects both with respect to the animal and the recipient. Here we will focus on the role of the complement system in the rejection of immediately vascularized pig-to-primate xenografts. The hyperacute rejection occurring within minutes after transplantation is mediated by binding of natural antibodies to the Galalpha(l-3)Gal epitope on the endothelial cells with subsequent complement activation. Whereas inhibition of complement activation protects against hyperacute rejection, the role of complement in the later rejection phases is less clarified.

Apoptosis and cellular activation in the pathogenesis of acute vascular rejection

Acute vascular or humoral rejection, a vexing outcome of organ transplantation, has been attributed by some to activation and by others to apoptosis of endothelial cells in the graft. We asked which of these processes causes acute vascular rejection by tracing the processes during the development of acute vascular rejection in porcine cardiac xenografts performed in baboons. Apoptosis, assayed by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL), expression of activated caspase-3, and proapoptotic genes Bax and Bcl-x(L), was not detected until acute vascular rejection was well advanced, and even then, apoptosis was largely confined to myocytes. Activation of the endothelium, as evidenced by expansion of rough endoplasmic reticulum and increased ribosomal antigen and phospho-p70 S6 kinase, occurred early in the course of acute vascular rejection and progressed through the disease process. These findings suggest that acute vascular rejection is caused by an active metabolic process and not by apoptosis in the endothelium.

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.

Non-anti-Gal alpha1-3Gal antibody mechanisms are sufficient to cause hyperacute lung dysfunction in pulmonary xenotransplantation

BACKGROUND

Hyperacute lung dysfunction, which is always associated with pulmonary pig-to-primate xenotransplantation is not well understood. The mechanisms associated with its occurrence seem to differ from mechanisms involved in hyperacute xenograft rejection seen in porcine hearts or kidneys transplanted into primates. To determine the contribution of anti-Gal alpha1-3Gal antibodies (alphaGAb) in such a process, we performed a set of orthotopic pig lung transplants into baboons depleted of alphaGAb and compared graft function and survival with those receiving only immunosuppression.

STUDY DESIGN

Pigs expressing human membrane cofactor protein served as donors. All baboons received triple immunosuppressive therapy. Depletion of alphaGAb in the experimental group (n = 4) was done by way of immunoadsorption using immunoaffinity membranes. Controls (n = 4) did not undergo immunoadsorption. Orthotopic lung transplants were performed through a left thoracotomy. Main pulmonary artery blood flow and pressure, left pulmonary artery blood flow, and left atrial pressure were recorded.

RESULTS

At 1 hour after reperfusion, pulmonary artery graft flows and pulmonary vascular resistances (PVR) were better in animals depleted of alphaGAb than in controls (605 +/- 325.2 mL/min versus 230 +/- 21 mL/min; 27.1 +/- 41.3 mmHg/L/min versus 63 +/- 1 mmHg/L/min). But at 3 hours after reperfusion average graft flows in baboons depleted of alphaGAb had decreased to 277.6 +/- 302.2 mL/min and PVRs had increased 58.3 +/- 42.0 mmHg/L/min. On the other hand, controls maintained stable flows and PVRs (223 +/- 23 mL/min; 61 +/- 3 mmHg/L/min). Survival was ultimately better in control baboons when compared with alphaGAb depleted ones (12.2 +/- 3.3 h versus 4.4 +/- 3.2 h).

CONCLUSION

Unlike heart and kidney xenograft transplants, hyperacute lung xenograft dysfunction seems to be mediated by factors other than alphaGAb.

Anti-pig antibody levels in naïve baboons and cynomolgus monkeys

Anti-pig antibodies (APA) were analysed in serum from 28 naïve wild-caught baboons (originating from Kenya) and 31 naïve captive-bred cynomolgus monkeys (13 from the Philippines and 18 from Mauritius), using a haemolytic assay with pig erythrocytes (APA), flow cytometry on the porcine lymphoma T-cell cell line L35, and enzyme linked immunosorbent assay (ELISA) using alpha-Gal type II and type VI antigen. This was extended in baboon samples by the evaluation in two laboratories (Imutran, Cambridge, UK and Immerge, Boston, USA), and by antibody absorption using either immobilized alpha-Gal type II or alpha-Gal type VI. Anti-porcine antibodies were demonstrated in all assays with substantial variability within and between the three non-human primate groups. Immunoglobulin (Ig)M antibody levels tended to be similar to or higher than those in a pooled normal human standard serum while IgG levels tended to be lower. Highest antibody levels were recorded in Mauritius cynomolgus monkeys. There were statistically significant correlations between assays for IgM or IgG class anti-Gal antibodies using either alpha-Gal type II or alpha-Gal type VI as antigen, both for different assays and two laboratories involved. Also, significant correlations were observed between the anti-Gal and L35 binding assays. Baboon sera before and after absorption to immobilized alpha-Gal type II or type VI were analysed for anti-Gal type VI or type II antibody: levels were almost undetectable indicating that most anti-Gal antibodies react to epitopes shared between alpha-Gal type II and type VI oligosaccharides. Finally, the relation between APA and outcome of porcine heart xenotransplantation in cynomolgus monkeys and baboons showed no apparent relation between pre-transplant APA levels and the occurrence of hyperacute rejection (HAR) when compared with non-immunological cause of organ/recipient dysfunction or acute humoral xenograft rejection during the first 4 days post-transplantation or survival exceeding 4 days post-transplantation.

CTLA-41g in combination with anti-CD40L prolongs xenograft survival and inhibits anti-gal ab production in GT-Ko mice

The generation of GT-Ko mice has provided unique opportunities to study allograft and xenograft rejection in the context of anti-alpha1,3-Gal antibody (anti-Gal Ab) responses. In this study we used the allotransplantation model of C3H hearts into galactosyltransferase-deficient (GT-Ko) mice and the xenotransplantation model of baby Lewis rat hearts into GT-Ko mice to investigate the ability of CTLA-41g in combination with anti-CD40L mAb to control graft rejection and anti-Gal Ab production. Murine CTLA-41g or anti-CD40L monotherapy prolonged allograft survival, and the combination of these reagents was most immunosuppressive. However short-term treatment with murine cytotoxic T lymphocyte associated antigen-4 (muCTLA-41g) and/or CD40 ligand (CD154) monoclonal antibodies (anti-CD40L mAbs) was unable to induce indefinite allograft survival. CTLA-4-immunoglobulin fusion protein (CTLA-41g) or anti-CD40L monotherapy only marginally prolonged xenograft survival; the combination of human CTLA-41g and anti-CD40L significantly prolonged xenograft survival (74days), while the combination of murine CTLA-41g and anti-CD40L resulted in graft survival of >120days. CTLA-41g or anti-CD40L monotherapy or the combination of these agents inhibited the production of alloAbs, including anti-Gal Abs. CTLA-41g or anti-CD40L monotherapy partially controlled xenoAb and anti-Gal Ab production, while the combination was more effective. These observations corroborate our previous observations that humoral, including anti-Gal Ab, responses and rejection following allograft or concordant xenograft transplantation in GT-Ko mice are T-cell dependent and can be controlled by costimulation blockade.