Life-supporting human complement regulator decay accelerating factor transgenic pig liver xenograft maintains the metabolic function and coagulation in the nonhuman primate for up to 8 days

Immunometabolism of Liver Xenotransplantation and Prospective Solutions

End-stage liver diseases, such as hepatocellular carcinoma or acute liver failure, critically necessitate liver transplantation. However, the shortage of available organ donors fails to meet the rapidly growing transplantation demand. Due to the high similarity of liver tissue structure and metabolism between miniature pigs and humans, xenotransplantation of pig livers is considered as a potentially viable solution to organ scarcity. In the 2024, teams from China first time have successfully transplanted a genetically modified Bama miniature pig liver into a clinically brain-dead man lasting for 10 days. This milestone in human xenotransplantation research not only confirms the feasibility of clinical application of xenotransplantation, but also underscores the daunting and protracted nature of this pathway. Despite advanced gene-editing technologies theoretically circumventing the occurrence of most transplant rejection reactions, patients still face challenges such as chronic immune rejection, coagulation disorders, and thrombotic microangiopathy after receiving xenografts. Moreover, prolonged use of immunosuppressive drugs may induce irreversible immune dysfunction, leading to opportunistic infections and metabolic disorders. This article compares the similarities and differences in livers between humans and pigs, summarizes the immunometabolism of xenotransplantation based on current findings, and provides research perspectives on pre-transplantation and post-transplantation strategies for prolonging the survival time of xenografts.

Liver Xenotransplantation: A Path to Clinical Reality

Liver xenotransplantation has emerged as a potential solution to the shortage of deceased human donor organs and is now becoming a reality due to recent developments in genetic engineering and immunosuppressive therapy. Early efforts using non-human primates and genetically modified pigs faced significant challenges such as thrombocytopenia and graft rejection. Understanding the mechanism behind those challenges and using novel genetically engineered pigs enabled researchers to overcome some of the hurdles, but more research is needed. However, new advances might allow pig liver xenotransplantation to potentially serve as a bridge to liver allotransplantation or allow native liver regeneration in the near future.

Systematic Review and Comparative Outcomes Analysis of NHP Liver Allotransplants and Xenotransplants

Patients with fulminant liver failure ineligible for transplantation have a high mortality rate. With recent progress in genetic modifications and clinical achievements, using pig livers as a bridge-to-transplant has regained popularity. Preclinical testing has been done in small cohorts of nonhuman primates (NHP), and maximum survival is limited to 1-month. We conducted a systematic review and comparative outcomes analysis of NHP-liver xenotransplantation and gathered 203 pig-to-NHP and NHP-to-NHP transplants reported in 23 studies. Overall, NHP survival after pig-liver xenotransplantation was limited (1, 3, 4 weeks: 18.0%, 5.6%, 1.1%), compared to NHPs after allotransplantation (1, 3, 4 weeks: 60.6%, 47.4%, 45.4%). A focus on pigs with genetic modifications evidenced some short-term survival benefits (1, 3, 4 weeks: 29.1%, 9.1%, 1.8%). The use of the auxiliary transplant technique was also associated with better short-term results (1, 3, 4 weeks: 40.9%, 9.1%, 4.5%). Causes of graft and animal loss were mostly rejection and liver failure in allotransplants, while bleeding, liver, and respiratory failure predominated in xenotransplants. Notably, the 1-month survival rate for NHP-allotransplants was significantly lower than the national > 98% rate for human liver transplants. This data confirms the short-term improvements brought by genetic modifications and auxiliary implantation in the NHP model, which remains imperfect.

Genetic engineering drives the breakthrough of pig models in liver disease research

Compared with the widely used rodents, pigs are anatomically, physiologically, and genetically more similar to humans, making them high-quality models for the study of liver diseases. Here, we review the latest research progress on pigs as a model of human liver disease, including methods for establishing them and their advantages in studying cystic fibrosis liver disease, acute liver failure, liver regeneration, non-alcoholic fatty liver disease, liver tumors, and xenotransplantation. We also emphasize the importance of genetic engineering techniques, mainly the CRISPR/Cas9 system, which has greatly enhanced the utility of porcine models as a tool for substantially advancing liver disease research. Genetic engineering is expected to propel the pig as one of the irreplaceable animal models for future biomedical research.

HDAC inhibitors support long-term expansion of porcine hepatocytes in vitro

Hepatocyte transplantation is an effective treatment for end-stage liver disease. However, due to the limited supply of human hepatocytes, porcine hepatocytes have garnered attention as a potential alternative source. Nonetheless, traditional primary porcine hepatocytes exhibit certain limitations in function maintenance and in vitro proliferation. This study has discovered that by using histone deacetylase inhibitors (HDACi), primary porcine hepatocytes can be successfully reprogrammed into liver progenitor cells with high proliferative potential. This method enables porcine hepatocytes to proliferate over an extended period in vitro and exhibit increased susceptibility in lentivirus-mediated gene modification. These liver progenitor cells can readily differentiate into mature hepatocytes and, upon microencapsulation transplantation into mice with acute liver failure, significantly improve the survival rate. This research provides new possibilities for the application of porcine hepatocytes in the treatment of end-stage liver disease.

A new paradigm in transplant immunology: At the crossroad of synthetic biology and biomaterials

Solid organ transplant (SOT) recipients require meticulously tailored immunosuppressive regimens to minimize graft loss and mortality. Traditional approaches focus on inhibiting effector T cells, while the intricate and dynamic immune responses mediated by other components remain unsolved. Emerging advances in synthetic biology and material science have provided novel treatment modalities with increased diversity and precision to the transplantation community. This review investigates the active interface between these two fields, highlights how living and non-living structures can be engineered and integrated for immunomodulation, and discusses their potential application in addressing the challenges in SOT clinical practice.

The Influence of Interdisciplinary Work towards Advancing Knowledge on Human Liver Physiology

The knowledge accumulated throughout the years about liver regeneration has allowed a better understanding of normal liver physiology, by reconstructing the sequence of steps that this organ follows when it must rebuild itself after being injured. The scientific community has used several interdisciplinary approaches searching to improve liver regeneration and, therefore, human health. Here, we provide a brief history of the milestones that have advanced liver surgery, and review some of the new insights offered by the interdisciplinary work using animals, in vitro models, tissue engineering, or mathematical models to help advance the knowledge on liver regeneration. We also present several of the main approaches currently available aiming at providing liver support and overcoming organ shortage and we conclude with some of the challenges found in clinical practice and the ethical issues that have concomitantly emerged with the use of those approaches.

Advance of genetically modified pigs in xeno-transplantation

As the standard of living improves, chronic diseases and end-stage organ failure have been a regular occurrence in human beings. Organ transplantation has become one of the hopes in the fight against chronic diseases and end-stage organ failure. However, organs available for transplantation are far from sufficient to meet the demand, leading to a major organ shortage crisis. To solve this problem, researchers have turned to pigs as their target since pigs have many advantages as xenograft donors. Pigs are considered the ideal organ donor for human xenotransplantation, but direct transplantation of porcine organs to humans faces many obstacles, such as hyperacute rejection, acute humoral xenograft rejection, coagulation dysregulation, inflammatory response, coagulation dysregulation, and endogenous porcine retroviral infection. Many transgenic strategies have been developed to overcome these obstacles. This review provides an overview of current advances in genetically modified pigs for xenotransplantation. Future genetic engineering-based delivery of safe and effective organs and tissues for xenotransplantation remains our goal.

Genetically modified immunomodulatory cell-based biomaterials in tissue regeneration and engineering

To date, the wide application of cell-based biomaterials in tissue engineering and regeneration is remarkably hampered by immune rejection. Reducing the immunogenicity of cell-based biomaterials has become the latest direction in biomaterial research. Recently, genetically modified cell-based biomaterials with immunomodulatory genes have become a feasible solution to the immunogenicity problem. In this review, recent advances and future challenges of genetically modified immunomodulatory cell-based biomaterials are elaborated, including fabrication approaches, mechanisms of common immunomodulatory genes, application and, more importantly, current preclinical and clinical advances. The fabrication approaches can be categorized into commonly used (e.g., virus transfection) and newly developed approaches. The immunomodulatory mechanisms of representative genes involve complicated cell signaling pathways and metabolic activities. Wide application in curing multiple end-term diseases and replacing lifelong immunosuppressive therapy in multiple cell and organ transplantation models is demonstrated. Most significantly, practices of genetically modified organ transplantation have been conducted on brain-dead human decedent and even on living patients after a series of experiments on nonhuman primates. Nevertheless, uncertain biosecurity, nonspecific effects and overlooked personalization of current genetically modified immunomodulatory cell-based biomaterials are shortcomings that remain to be overcome.

Liver and Hepatocyte Transplantation: What Can Pigs Contribute?

About one-fifth of the population suffers from liver diseases in China, meaning that liver disorders are prominent causative factors relating to the Chinese mortality rate. For patients with end-stage liver diseases such as hepatocellular carcinoma or acute liver diseases with life-threatening liver dysfunction, allogeneic liver transplantation is the only life-saving treatment. Hepatocyte transplantation is a promising alternative for patients with acute liver failure or those considered high risk for major surgery, particularly for the bridge-to-transplant period. However, the lack of donors has become a serious global problem. The clinical application of porcine xenogeneic livers and hepatocytes remains a potential solution to alleviate the donor shortage. Pig grafts of xenotransplantation play roles in providing liver support in recipients, together with the occurrence of rejection, thrombocytopenia, and blood coagulation dysfunction. In this review, we present an overview of the development, potential therapeutic impact, and remaining barriers in the clinical application of pig liver and hepatocyte xenotransplantation to humans and non-human primates. Donor pigs with optimized genetic modification combinations and highly effective immunosuppressive regimens should be further explored to improve the outcomes of xenogeneic liver and hepatocyte transplantation.

Current Barriers to Clinical Liver Xenotransplantation

Preclinical trials of pig-to-nonhuman primate liver xenotransplantation have recently achieved longer survival times. However, life-threatening thrombocytopenia and coagulation dysregulation continue to limit preclinical liver xenograft survival times to less than one month despite various genetic modifications in pigs and intensive pharmacological support. Transfusion of human coagulation factors and complex immunosuppressive regimens have resulted in substantial improvements in recipient survival. The fundamental biological mechanisms of thrombocytopenia and coagulation dysregulation remain incompletely understood. Current studies demonstrate that porcine von Willebrand Factor binds more tightly to human platelet GPIb receptors due to increased O-linked glycosylation, resulting in increased human platelet activation. Porcine liver sinusoidal endothelial cells and Kupffer cells phagocytose human platelets in an asialoglycoprotein receptor 1-dependent and CD40/CD154-dependent manner, respectively. Porcine Kupffer cells phagocytose human platelets via a species-incompatible SIRPα/CD47 axis. Key drivers of coagulation dysregulation include constitutive activation of the extrinsic clotting cascade due to failure of porcine tissue factor pathway inhibitor to repress recipient tissue factor. Additionally, porcine thrombomodulin fails to activate human protein C when bound by human thrombin, leading to a hypercoagulable state. Combined genetic modification of these key genes may mitigate liver xenotransplantation-induced thrombocytopenia and coagulation dysregulation, leading to greater recipient survival in pig-to-nonhuman primate liver xenotransplantation and, potentially, the first pig-to-human clinical trial.

Bridging to Allotransplantation-Is Pig Liver Xenotransplantation the Best Option?

In the past 20 y, the number of patients in the United States who died while waiting for a human donor liver totaled >52 000. The median national wait time for patients with acute liver failure and the most urgent liver transplant listing was 7 d in 2018. The need for a clinical "bridge" to allotransplantation is clear. Current options for supporting patients with acute liver failure include artificial liver support devices, extracorporeal liver perfusion, and hepatocyte transplantation, all of which have shown mixed results with regard to survival benefit and are largely experimental. Progress in the transplantation of genetically engineered pig liver grafts in nonhuman primates has grown steadily, with survival of the pig graft extended to almost 1 mo in 2017. Further advances may justify consideration of a pig liver transplant as a clinical bridge to allotransplantation. We provide a brief history of pig liver xenotransplantation, summarize the most recent progress in pig-to-nonhuman primate liver transplantation models, and suggest criteria that may be considered for patient selection for a clinical trial of bridging by genetically engineered pig liver xenotransplantation to liver allotransplantation.

Profiling Human CD55 Transgene Performance Assist in Selecting Best Suited Specimens and Tissues for Swine Organ Xenotransplantation

Simple Summary

The unbalance between availability and needs of human organs has drawn researchers’ attention to xenotransplantation as an option to cope with this shortage. Pig organs have received substantial attention for being comparable to human’s; nevertheless, compatibility constrains still block clinical applications. Transgenesis of human complement regulatory proteins, including the CD55 gene and its product the decay-accelerating factor (DAF), has been proposed to overcome xenorejection. This line of research has obtained interesting results along the years; however, most works assessing the impact of this strategy for xenotransplantation are limited to analyzing gene expression and assessing resistance to conventional serum challenge hemolysis assays, which provide somewhat reduced information prior to surgery. In this work, we tried to expand the analysis of the hCD55 transgene performance beyond common practice and into a better molecular understanding of its impact in xenotransplantation. We determined hCD55 gene expression, as well as hDAF protein presence, in different organs from five transgenic pigs, comparing readings from organs worthy for transplantation and other non-valuable organs and tissues. We also assessed the ability of transgenic cells, compared to non-transgenic, to withstand hemolysis and cytolysis. Finally, we made an effort to establish potential correlations between the hCD55 mRNA and hDAF protein levels detected.

Abstract

Xenotransplantation of pig organs receives substantial attention for being comparable to human’s. However, compatibility constraints involving hyper-acute rejection (HAR) still block clinical applications. Transgenesis of human complement regulatory proteins has been proposed to overcome xenorejection. Pigs expressing human-CD55 have been widely tested in experimental surgery. Still, no standardized method has been developed to determine tissue expression of human decay-accelerating factor (DAF), hCD55’s product, or to predict the ability to overpass HAR. Here we describe objective procedures addressing this need. Organs and tissues from five hCD55 transgenic pigs were collected and classified according to their xenotransplantation value. The ability to overcome HAR was assessed by classical complement pathway hemolysis assays. Quantitative PCR mRNA expression and Western blot protein level studies were performed. Real-time cytotoxicity assays (RTCA) on fibroblast cultures exposed to baboon and human sera informed on longer-term rejection dynamics. While greater hCD55/DAF expression correlated with better performance, the results obtained varied among specimens. Interestingly, the individual with highest mRNA and protein levels showed positive feedback for hCD55 transcript after challenge with human and baboon sera. Moreover, hCD55 expression correlated to DAF levels in the liver, lung and intestine, but not in the heart. Moreover, we found significant correlations among valuable and non-valuable tissues. In sum, the methodology proposed allows us to characterize the hCD55 transgene functioning and performance. Moreover, the correlations found could allow us to predict hCD55/DAF expression in surrogate tissues, thus eliminating the need for direct biopsies, resulting in preservation of organ integrity before xenotransplantation.

What Are the Immune Obstacles to Liver Xenotransplantation Which Is Promising for Patients with Hepatocellular Carcinoma?

PURPOSE

Liver transplantation is the most important achievement in the twentieth and twenty-first century. It is the gold standard treatment for hepatocellular carcinoma. However, it provides the best results when performed under strict selection criteria. Nevertheless, organ supply is overwhelmed by the number of patients on the waiting list. There are certain strategies to expand the donor pool such as split liver transplantation, use of extended criteria donors, and living donor liver transplantation. Xenotransplantation can also be a strategy in decreasing the organ shortage. We reviewed the current status of xenotransplantation.

METHODS

We evaluated the historical attempts of xenotransplantation to humans and also made a summary of the preclinical studies in the field.

RESULTS

Molecular biology and genetic engineering are developing with an incredible speed. There are great achievements made in cell therapy, 3D bioprinting of the organs, and ultimately xenotransplantation. There is a vast amount of problems to be handled before evaluating the efficacy of xenotransplantation in the treatment of hepatocellular carcinoma. Major problems include antibody-mediated rejection to antigens such as galactose ⍺1-3 galactose, N- glycolylneuraminic acid, β1,4-N-acetylgalactosaminyltransferase, lethal thrombocytopenia, and erythrocyte sequestration. Antibody mediated rejection to these specific antigens are addressed using gene editing technology including CRISPR Cas9, TALEN and other recombination methods. Although hyperacute rejection is reduced, long-term survival could not be achieved in experimental models.

CONCLUSION

The future is yet to come, there are developments made in the field of genetic editing, immunosuppressive medication, and pretransplant desensitization techniques. Therefore, we believe that xenotransplantation will be in clinical practice, at least for treatment of critically ill patients.

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.

The complex functioning of the complement system in xenotransplantation

The role of complement in xenotransplantation is well-known and is a topic that has been reviewed previously. However, our understanding of the immense complexity of its interaction with other constituents of the innate immune response and of the coagulation, adaptive immune, and inflammatory responses to a xenograft is steadily increasing. In addition, the complement system plays a function in metabolism and homeostasis. New reviews at intervals are therefore clearly warranted. The pathways of complement activation, the function of the complement system, and the interaction between complement and coagulation, inflammation, and the adaptive immune system in relation to xenotransplantation are reviewed. Through several different mechanisms, complement activation is a major factor in contributing to xenograft failure. In the organ-source pig, the detrimental influence of the complement system is seen during organ harvest and preservation, for example, in ischemia-reperfusion injury. In the recipient, the effect of complement can be seen through its interaction with the immune, coagulation, and inflammatory responses. Genetic-engineering and other therapeutic methods by which the xenograft can be protected from the effects of complement activation are discussed. The review provides an updated source of reference to this increasingly complex subject.

A review of pig liver xenotransplantation: Current problems and recent progress

Pig liver xenotransplantation appears to be more perplexing when compared to heart or kidney xenotransplantation, even though great progress has been achieved. The relevant molecular mechanisms involved in xenogeneic rejection, including coagulopathy, and particularly thrombocytopenia, are complex, and need to be systematically investigated. The deletion of expression of Gal antigens in the liver graft highlights the injurious impact of nonGal antigens, which continue to induce humoral rejection. Innate immunity, particularly mediated by macrophages and natural killer cells, interplays with inflammation and coagulation disorders. Kupffer cells and liver sinusoidal endothelial cells (LSECs) together mediate leukocyte, erythrocyte, and platelet sequestration and phagocytosis, which can be exacerbated by increased cytokine production, cell desialylation, and interspecies incompatibilities. The coagulation cascade is activated by release of tissue factor which can be dependent or independent of the xenoreactive immune response. Depletion of endothelial anticoagulants and anti-platelet capacity amplify coagulation activation, and interspecies incompatibilities of coagulation-regulatory proteins facilitate dysregulation. LSECs involved in platelet phagocytosis and transcytosis, coupled with hepatocyte-mediated degradation, are responsible for thrombocytopenia. Adaptive immunity could also be problematic in long-term liver graft survival. Currently, relevant evidence and study results of various genetic modifications to the pig donor need to be fully determined, with the aim of identifying the ideal transgene combination for pig liver xenotransplantation. We believe that clinical trials of pig liver xenotransplantation should initially be considered as a bridge to allotransplantation.

Genetically Modified Pigs as Organ Donors for Xenotransplantation

The growing shortage of available organs is a major problem in transplantology. Thus, new and alternative sources of organs need to be found. One promising solution could be xenotransplantation, i.e., the use of animal cells, tissues and organs. The domestic pig is the optimum donor for such transplants. However, xenogeneic transplantation from pigs to humans involves high immune incompatibility and a complex rejection process. The rapid development of genetic engineering techniques enables genome modifications in pigs that reduce the cross-species immune barrier.

Overcoming Coagulation Dysregulation in Pig Solid Organ Transplantation in Nonhuman Primates: Recent Progress

There has recently been considerable progress in the results of pig organ transplantation in nonhuman primates, largely associated with the availability of (i) pigs genetically engineered to overcome coagulation dysregulation, and (ii) novel immunosuppressive agents. The barriers of thrombotic microangiopathy and/or consumptive coagulation were believed to be associated with (i) activation of the graft vascular endothelial cells by a low level of antipig antibody binding and/or complement deposition and/or innate immune cell activity, and (ii) molecular incompatibilities between the nonhuman primate and pig coagulation-anticoagulation systems. The introduction of a human coagulation-regulatory transgene, for example, thrombomodulin, endothelial protein C receptor, into the pig vascular endothelial cells has contributed to preventing a procoagulant state from developing, resulting in a considerable increase in graft survival. In the heterotopic (non-life-supporting) heart transplant model, graft survival has increased from a maximum of 179 days in 2005 to 945 days. After life-supporting kidney transplantation, survival has been extended from 90 days in 2004 to 499 days. In view of the more complex coagulation dysfunction seen after pig liver and, particularly, lung transplantation, progress has been less dramatic, but the maximum survival of a pig liver has been increased from 7 days in 2010 to 29 days, and of a pig lung from 4 days in 2007 to 9 days. There is a realistic prospect that the transplantation of a kidney or heart, in combination with a conventional immunosuppressive regimen, will enable long-term recipient survival.

Liver xenotransplantation

PURPOSE OF REVIEW

There continues to be an inadequate organ supply and lack of effective temporary support, for patients with liver failure. The purpose of this review is to discuss recent progress in the field of orthotopic pig-to-nonhuman primate (NHP) liver xenotransplantation (LXT).

RECENT FINDINGS

From 1968 to 2012, survival in pig-to-NHP LXT was limited to 9 days, initially due to hyperacute rejection which has been ameliorated through use of genetically engineered donor organs, but ultimately because of profound thrombocytopenia, thrombotic microangiopathy, and bleeding. Most recently, however, demise secondary to lethal coagulopathy has been avoided with LXT of α(1,3)-galactosyltransferase knockouts and cytomegalovirus-negative porcine xenografts into baboons receiving exogenous administration of coagulation factors and co-stimulation blockade, establishing that a porcine liver is capable of supporting NHP life for nearly a month.

SUMMARY

Continued consistent achievement of pig-to-NHP LXT survival beyond 2 weeks justifies consideration of a clinical application as a bridge to allotransplantation for patients with acute hepatic failure. Further genetic modifications to the donor, as well as additional studies, are required in order to apply LXT as destination therapy.

Presence of Pig IgG and IgM in Sera Samples From Baboons After an Orthotopic Liver Xenotransplantation

INTRODUCTION

The immunorejection in xenotransplantation has mostly been studied from the host's immune system activation point of view and there is very little information about the graft-vs-host reaction.

OBJECTIVES

To validate an enzyme-linked immunosorbent assay (ELISA) test for porcine IgM and IgG quantitation, the assessment of porcine IgG and IgM in sera samples from baboons after liver orthotopic xenotransplantation or in human plasma after xenotransfusion through pig organs, and to assess the presence of porcine immunoglobulin in a baboon after plasmapheresis to a complete change of plasma after 4 passages through pig liver.

MATERIALS AND METHODS

Two commercial ELISA kits for pig IgG and IgM quantitation were evaluated for cross reactivity with samples from baboons, Rhesus monkeys, squirrel monkeys, and humans. Then, samples from 18 baboons after orthotopic liver xenotransplantation were studied for porcine IgG and IgM. To understand the phenomenon, human plasma samples after xenotransfusion 1, 2, 3, or 4 times through liver or kidney were assessed for porcine IgG presence and finally, the porcine IgG were quantified in sera samples obtained during more than 4 years from a baboon after plasmapheresis with baboon plasma after xenotransfusion 4 times through a pig liver.

RESULTS

Porcine IgG and IgM were found in samples from xenotransplanted baboon during all survival. The quantity of porcine IgG in plasma after xenotransfusion correlated with the number of passages through the pig liver, and the IgG were completely cleared from the baboon 16 days after plasmapheresis and complete substitution of plasma after 4 xenotransfusions through a pig liver.

Cytokine profiles in Tibetan macaques following α-1,3-galactosyltransferase-knockout pig liver xenotransplantation

BACKGROUND

Pig-to-nonhuman primate orthotopic liver xenotransplantation is often accompanied by thrombocytopenia and coagulation disorders. Furthermore, the release of cytokines can trigger cascade reactions of coagulation and immune attacks within transplant recipients. To better elucidate the process of inflammation in liver xenograft recipients, we utilized a modified heterotopic auxiliary liver xenotransplantation model for xeno-immunological research. We studied the cytokine profiles and the relationship between cytokine levels and xenograft function after liver xenotransplantation.

METHODS

Appropriate donor and recipient matches were screened using complement-dependent cytotoxicity assays. Donor liver grafts from α1,3-galactosyltransferase gene-knockout (GTKO) pigs or GTKO pigs additionally transgenic for human CD47 (GTKO/CD47) were transplanted into Tibetan macaques via two different heterotrophic auxiliary liver xenotransplantation procedures. The cytokine profiles, hepatic function, and coagulation parameters were monitored during the clinical course of xenotransplantation.

RESULTS

Xenograft blood flow was stable in recipients after heterotopic auxiliary transplantation. A Doppler examination indicated that the blood flow speed was faster in the hepatic artery (HA) and hepatic vein (HV) of xenografts subjected to the modified Sur II (HA-abdominal aorta+HV-inferior vena cava) procedure than in those subjected to our previously reported Sur I (HA-splenic artery+HV-left renal vein) procedure. Tibetan macaques receiving liver xenografts did not exhibit severe coagulation disorders or immune rejection. Although the recipients did suffer from a rapid loss of platelets, this loss was mild. In blood samples dynamically collected after xenotransplantation (post-Tx), dramatic increases in the levels of monocyte chemoattractant protein 1, interleukin (IL)-8, granulocyte-macrophage colony-stimulating factor, IL-6, and interferon gamma-induced protein 10 were observed at 1 hour post-Tx, even under immunosuppression. We further confirmed that the elevation in individual cytokine levels was correlated with the onset of graft damage. Finally, the release of cytokines might contribute to leukocyte infiltration in the xenografts.

CONCLUSION

Here, we established a modified auxiliary liver xenotransplantation model resulting in near-normal hepatic function. Inflammatory cytokines might contribute to early damage in liver xenografts. Controlling the systemic inflammatory response of recipients might prevent early post-Tx graft dysfunction.

Pig Liver Xenotransplantation: A Review of Progress Toward the Clinic

Experience with clinical liver xenotransplantation has largely involved the transplantation of livers from nonhuman primates. Experience with pig livers has been scarce. This brief review will be restricted to assessing the potential therapeutic impact of pig liver xenotransplantation in acute liver failure and the remaining barriers that currently do not justify clinical trials. A relatively new surgical technique of heterotopic pig liver xenotransplantation is described that might play a role in bridging a patient with acute liver failure until either the native liver recovers or a suitable liver allograft is obtained. Other topics discussed include the possible mechanisms for the development of the thrombocytopenis that rapidly occurs after pig liver xenotransplantation in a primate, the impact of pig complement on graft injury, the potential infectious risks, and potential physiologic incompatibilities between pig and human. There is cautious optimism that all of these problems can be overcome by judicious genetic manipulation of the pig. If liver graft survival could be achieved in the absence of thrombocytopenia or rejection for a period of even a few days, there may be a role for pig liver transplantation as a bridge to allotransplantation in carefully selected patients.

Clinical xenotransplantation, a closer reality: Literature review

Xenotransplantation could provide an unlimited supply of organs and solve the current shortage of organs for transplantation. To become a reality in clinical practice, the immunological and physiological barriers and the risk of xenozoonosis that they possess should be resolved. From the immunological point of view, in the last 30 years a significant progress in the production of transgenic pigs has prevented the hyperacute rejection. About xenozoonosis, attention has been focused on the risk of transmission of porcine endogenous retroviruses; however, today, it is considered that the risk is very low and the inevitable transmission should not prevent the clinical xenotransplantation. Regarding the physiological barriers, encouraging results have been obtained and it's expected that the barriers that still need to be corrected can be solved in the future through genetic modifications.

The Effects of Exogenous Administration of Human Coagulation Factors Following Pig-to-Baboon Liver Xenotransplantation

We sought to determine the effects of exogenous administration of human coagulation factors following pig-to-baboon liver xenotransplantation (LXT) using GalT-KO swine donors. After LXT, baboons received no coagulation factors (historical control, n = 1), bolus administration of a human prothrombin concentrate complex (hPCC; 2.5 mL/kg, n = 2), continuous infusion of hPCC (1.0 mL/h, n = 1) or continuous infusion of human recombinant factor VIIa (1 µg/kg per hour, n = 3). The historical control recipient demonstrated persistent thrombocytopenia despite platelet administration after transplant, along with widespread thrombotic microangiopathy (TMA). In contrast, platelet levels were maintained in bolus hPCC recipients; however, these animals quickly developed large-vessel thrombosis and TMA, leading to graft failure with shortened survival. Recipients of continuous coagulation factor administration experienced either stabilization or an increase in their circulating platelets with escalating doses. Furthermore, transfusion requirements were decreased, and hepatic TMA was noticeably absent in recipients of continuous coagulation factor infusions compared with the historical control and bolus hPCC recipients. This effect was most profound with a continuous, escalating dose of factor VIIa. Further studies are warranted because this regimen may allow for prolonged survival following LXT.

A hemodynamic, metabolic and histopathological study of a heterotopic auxiliary swine liver graft with portal vein arterialization

BACKGROUND

Auxiliary heterotopic liver transplantation with portal vein arterialization (AHLT-PVA) is a model that has been hardly studied, despite its therapeutic potential.

METHODS

Hemodynamic and biochemical characterization was carried out during graft implantation, in a pig-to-pig model (n=15 AHLT-PVA). Furthermore a histopathological study was performed to establish microscopic alterations due to PVA.

RESULTS

Reperfusion of the arterialized graft produced an increase in heart rate (HR) vs. baseline (P=.004) and vs. inferior vena cava clamping phase (P=.004); and a decrease in systemic vascular resistance vs. cava clamping phase (P=.021). At the end of implantation, cardiac output remained elevated (P=.001), likewise HR remained increased vs. baseline phase (P=.002). Mean arterial pressure decreased with cava clamping, but was not affected by the reperfusion of the graft, nor the skin closure. The histopathological study at 3, 10, and 21 days post-PVA revealed that functional liver structure was maintained although it is common to find foci of perilobular necrosis on day 3 (P=.049), and perilobular connective tissue proliferation at day 10 (P=.007), vs. native liver.

CONCLUSIONS

The described arterialized liver graft model minimizes the number of vascular anastomoses vs. previously described models. It is hemodynamically and metabolically well tolerated and the double arterial vascularization of the graft does not cause significant changes in liver histology.

Current status of pig liver xenotransplantation

The shortage of organs from deceased human donors is a major problem limiting the number of organs transplanted each year and results in the death of thousands of patients on the waiting list. Pigs are currently the preferred species for clinical organ xenotransplantation. Progress in genetically-engineered (GE) pig liver xenotransplantation increased graft and recipient survival from hours with unmodified pig livers to up to 9 days with normal to near-normal liver function. Deletion of genes such as GGTA1 (Gal-knockout pigs) or adding genes such as human complement regulatory proteins (hCD55, hCD46 expressing pigs) enabled hyperacute rejection to be overcome. Although survival up to 9 days was recorded, extended pig graft survival was not achieved due to lethal thrombocytopenia. The current status of GE pig liver xenotransplantation with world experience, potential factors causing thrombocytopenia, new targets on pig endothelial cells, and novel GE pigs with more genes deletion to avoid remaining antibody response, such as beta1,4-N-acetyl galactosaminyl transferase 2 (β4GalNT2), are discussed.

Experimental hepatocyte xenotransplantation--a comprehensive review of the literature

Hepatocyte transplantation (Tx) is a potential therapy for certain diseases of the liver, including hepatic failure. However, there is a limited supply of human livers as a source of cells and, after isolation, human hepatocytes can be difficult to expand in culture, limiting the number available for Tx. Hepatocytes from other species, for example, the pig, have therefore emerged as a potential alternative source. We searched the literature through the end of 2014 to assess the current status of experimental research into hepatocyte xenoTx. The literature search identified 51 reports of in vivo cross-species Tx of hepatocytes in a variety of experimental models. Most studies investigated the Tx of human (n = 23) or pig (n = 19) hepatocytes. No studies explored hepatocytes from genetically engineered pigs. The spleen was the most common site of Tx (n = 23), followed by the liver (through the portal vein [n = 6]) and peritoneal cavity (n = 19). In 47 studies (92%), there was evidence of hepatocyte engraftment and function across a species barrier. The data provided by this literature search strengthen the hypothesis that xenoTx of hepatocytes is feasible and potentially successful as a clinical therapy for certain liver diseases, including hepatic failure. By excluding vascular structures, hepatocytes isolated from genetically engineered pig livers may address some of the immunological problems of xenoTx.

Generation of a felinized swine endothelial cell line by expression of feline decay-accelerating factor

Embryonic stem cell research has facilitated the generation of many cell types for the production of tissues and organs for both humans and companion animals. Because ≥30% of pet cats suffer from chronic kidney disease (CKD), xenotransplantation between pigs and cats has been studied. For a successful pig to cat xenotransplant, the immune reaction must be overcome, especially hyperacute rejection. In this study, we isolated the gene for feline decay-accelerating factor (fDAF), an inhibitor of complement proteins, and transfected a swine endothelial cell line with fDAF to "felinize" the pig cells. These fDAF-expressing cells were resistant to feline serum containing anti-pig antibodies, suggesting that felinized pig cells were resistant to hyperacute rejection. Our results suggest that a "felinized" pig kidney can be generated for the treatment of CKD in cats in the future.

Xenotransplantation of Cells, Tissues, Organs and the German Research Foundation Transregio Collaborative Research Centre 127

Human organ transplantation is the therapy of choice for end-stage organ failure. However, the demand for organs far exceeds the donation rate, and many patients die while waiting for a donor. Clinical xenotransplantation using discordant species, particularly pigs, offers a possible solution to this critical shortfall. Xenotransplantation can also increase the availability of cells, such as neurons, and tissues such as cornea, insulin producing pancreatic islets and heart valves. However, the immunological barriers and biochemical disparities between pigs and primates (human) lead to rejection reactions despite the use of common immunosuppressive drugs. These result in graft vessel destruction, haemorrhage, oedema, thrombus formation, and transplant loss. Our consortium is pursuing a broad range of strategies to overcome these obstacles. These include genetic modification of the donor animals to knock out genes responsible for xenoreactive surface epitopes and to express multiple xenoprotective molecules such as the human complement regulators CD46, 55, 59, thrombomodulin and others. We are using (new) drugs including complement inhibitors (e.g. to inhibit C3 binding), anti-CD20, 40, 40L, and also employing physical protection methods such as macro-encapsulation of pancreatic islets. Regarding safety, a major objective is to assure that possible infections are not transmitted to recipients. While the aims are ambitious, recent successes in preclinical studies suggest that xenotransplantation is soon to become a clinical reality.

Current progress in xenotransplantation and organ bioengineering

Organ transplantation represents a unique method of treatment to cure people with end-stage organ failure. Since the first successful organ transplant in 1954, the field of transplantation has made great strides forward. However, despite the ability to transform and save lives, transplant surgery is still faced with a fundamental problem the number of people requiring organ transplants is simply higher than the number of organs available. To put this in stark perspective, because of this critical organ shortage 18 people every day in the United States alone die on a transplant waiting list (U.S. Department of Health & Human Services, http://organdonor.gov/about/data.html). To address this problem, attempts have been made to increase the organ supply through xenotransplantation and more recently, bioengineering. Here we trace the development of both fields, discuss their current status and highlight limitations going forward. Ultimately, lessons learned in each field may prove widely applicable and lead to the successful development of xenografts, bioengineered constructs, and bioengineered xeno-organs, thereby increasing the supply of organs for transplantation.

Progress in pig-to-non-human primate transplantation models (1998-2013): a comprehensive review of the literature

BACKGROUND

The pig-to-non-human primate model is the standard choice for in vivo studies of organ and cell xenotransplantation. In 1998, Lambrigts and his colleagues surveyed the entire world literature and reported all experimental studies in this model. With the increasing number of genetically engineered pigs that have become available during the past few years, this model is being utilized ever more frequently.

METHODS

We have now reviewed the literature again and have compiled the data we have been able to find for the period January 1, 1998 to December 31, 2013, a period of 16 yr.

RESULTS

The data are presented for transplants of the heart (heterotopic and orthotopic), kidney, liver, lung, islets, neuronal cells, hepatocytes, corneas, artery patches, and skin. Heart, kidney, and, particularly, islet xenograft survival have increased significantly since 1998.

DISCUSSION

The reasons for this are briefly discussed. A comment on the limitations of the model has been made, particularly with regard to those that will affect progression of xenotransplantation toward the clinic.

Increased transfusion-free survival following auxiliary pig liver xenotransplantation

BACKGROUND

Pig to baboon liver xenotransplantation typically results in severe thrombocytopenia and coagulation disturbances, culminating in death from hemorrhage within 9 days, in spite of continuous transfusions. We studied the contribution of anticoagulant production and clotting pathway deficiencies to fatal bleeding in baboon recipients of porcine livers.

METHODS

By transplanting liver xenografts from α1,3-galactosyltransferase gene-knockout (GalT-KO) miniature swine donors into baboons as auxiliary organs, leaving the native liver in place, we provided the full spectrum of primate clotting factors and allowed in vivo mixing of porcine and primate coagulation systems.

RESULTS

Recipients of auxiliary liver xenografts develop severe thrombocytopenia, comparable to recipients of conventional orthotopic liver xenografts and consistent with hepatic xenograft sequestration. However, baboons with both pig and native livers do not exhibit clinical signs of bleeding and maintain stable blood counts without transfusion for up to 8 consecutive days post-transplantation. Instead, recipients of auxiliary liver xenografts undergo graft failure or die of sepsis, associated with thrombotic microangiopathy in the xenograft, but not the native liver.

CONCLUSION

Our data indicate that massive hemorrhage in the setting of liver xenotransplantation might be avoided by supplementation with primate clotting components. However, coagulation competent hepatic xenograft recipients may be predisposed to graft loss related to small vessel thrombosis and ischemic necrosis.

Production of multiple transgenic Yucatan miniature pigs expressing human complement regulatory factors, human CD55, CD59, and H-transferase genes

The present study was conducted to generate transgenic pigs coexpressing human CD55, CD59, and H-transferase (HT) using an IRES-mediated polycistronic vector. The study focused on hyperacute rejection (HAR) when considering clinical xenotransplantation as an alternative source for human organ transplants. In total, 35 transgenic cloned piglets were produced by somatic cell nuclear transfer (SCNT) and were confirmed for genomic integration of the transgenes from umbilical cord samples by PCR analysis. Eighteen swine umbilical vein endothelial cells (SUVEC) were isolated from umbilical cord veins freshly obtained from the piglets. We observed a higher expression of transgenes in the transgenic SUVEC (Tg SUVEC) compared with the human umbilical vein endothelial cells (HUVEC). Among these genes, HT and hCD59 were expressed at a higher level in the tested Tg organs compared with non-Tg control organs, but there was no difference in hCD55 expression between them. The transgenes in various organs of the Tg clones revealed organ-specific and spatial expression patterns. Using from 0 to 50% human serum solutions, we performed human complement-mediated cytolysis assays. The results showed that, overall, the Tg SUVEC tested had greater survival rates than did the non-Tg SUVEC, and the Tg SUVEC with higher HT expression levels tended to have more down-regulated α-Gal epitope expression, resulting in greater protection against cytotoxicity. By contrast, several Tg SUVEC with low CD55 expression exhibited a decreased resistance response to cytolysis. These results indicated that the levels of HT expression were inversely correlated with the levels of α-Gal epitope expression and that the combined expression of hCD55, hCD59, and HT proteins in SUVECs markedly enhances a protective response to human serum-mediated cytolysis. Taken together, these results suggest that combining a polycistronic vector system with SCNT methods provides a fast and efficient alternative for the generation of transgenic large animals with multiple genetic modifications.

Nucleotide metabolic mismatches in mammalian hearts: implications for transplantation

Introduction

Human donor organ shortages have led surgeons and scientists to explore the use of animals as alternative organ sources. Acute thrombovascular rejection (AVR) is the main hurdle in xenotransplantation. Disparities in nucleotide metabolism in the vessels of different species may contribute significantly to the microvascular component of AVR.

Methods

We evaluated the extent of nucleotide metabolism mismatch in selected organs and endothelial cells of different mammals with particular focus on the changes in activity of ecto-5’-nucleotidase (E5’N) elicited by exposure of porcine hearts or endothelial cells to human blood (ex vivo) or human plasma (in vitro).

Results

E5’N activity in the rat heart was significantly higher than in other species. We noted a significant difference (p<0.001) in E5’N activity between human and pig endothelial cell lines. Initial pig aortic endothelial E5’N activity decreased in vitro after a three-hour exposure to human and porcine plasma while remaining constant in controls. Ex vivo perfusion with fresh human blood for four hours resulted in a significant decrease of E5’N activity in both wild type and transgenic pig hearts overexpressing human decay accelerating factor (p<0.001).

Conclusions

This study provides evidence that mismatches in basal mammalian metabolic pathways and humoral immunity interact in a xenogeneic environment. Understanding the role of nucleotide metabolism and signalling in xenotransplantation may identify new targets for genetic modifications and may lead to the development of new therapies extending graft survival.

Up to 9-day survival and control of thrombocytopenia following alpha1,3-galactosyl transferase knockout swine liver xenotransplantation in baboons

BACKGROUND

With standard miniature swine donors, survivals of only 3 days have been achieved in primate liver-transplant recipients. The recent production of alpha1,3-galactosyl transferase knockout (GalT-KO) miniature swine has made it possible to evaluate xenotransplantation of pig organs in clinically relevant pig-to-non-human primate models in the absence of the effects of natural anti-Gal antibodies. We are reporting our results using GalT-KO liver grafts.

METHODS

We performed GalT-KO liver transplants in baboons using an immunosuppressive regimen previously used by our group in xeno heart and kidney transplantation. Post-operative liver function was assessed by laboratory function tests, coagulation parameters and histology.

RESULTS

In two hepatectomized recipients of GalT-KO grafts, post-transplant liver function returned rapidly to normal. Over the first few days, the synthetic products of the donor swine graft appeared to replace those of the baboon. The first recipient survived for 6 days and showed no histopathological evidence of rejection at the time of death from uncontrolled bleeding, probably caused by transfusion-refractory thrombocytopenia. Amicar treatment of the second and third recipients led to maintenance of platelet counts of over 40 000 per μl throughout their 9- and 8-day survivals, which represents the longest reported survival of pig-to-primate liver transplants to date. Both of the last two animals nevertheless succumbed to bleeding and enterococcal infection, without evidence of rejection.

CONCLUSIONS

These observations suggest that thrombocytopenia after liver xenotransplantation may be overcome by Amicar therapy. The coagulopathy and sepsis that nevertheless occurred suggest that additional causes of coagulation disturbance must be addressed, along with better prevention of infection, to achieve long-term survival.

Immunobiology of liver xenotransplantation

Pigs are currently the preferred species for future organ xenotransplantation. With advances in the development of genetically modified pigs, clinical xenotransplantation is becoming closer to reality. In preclinical studies (pig-to-nonhuman primate), the xenotransplantation of livers from pigs transgenic for human CD55 or from α1,3-galactosyltransferase gene-knockout pigs+/- transgenic for human CD46, is associated with survival of approximately 7-9 days. Although hepatic function, including coagulation, has proved to be satisfactory, the immediate development of thrombocytopenia is very limiting for pig liver xenotransplantation even as a 'bridge' to allotransplantation. Current studies are directed to understand the immunobiology of platelet activation, aggregation and phagocytosis, in particular the interaction between platelets and liver sinusoidal endothelial cells, hepatocytes and Kupffer cells, toward identifying interventions that may enable clinical application.

Porcine alanine transaminase after liver allo-and xenotransplantation

Aspartate transaminase (AST) and alanine transaminase (ALT) are measured following liver transplantation as indicators of hepatocellular injury. During a series of orthotopic liver allo-and xenotransplants, we observed that there was an increase in AST in all cases. The anticipated concomitant rise in ALT did not occur when a wild-type (WT) pig was the source of the liver graft, but did occur when a baboon or a genetically engineered (α1,3-galactosyltransferase gene-knockout [GTKO]) pig was the source of the graft. We hypothesized that the cience of Galα1,3Gal in GTKO pig livers may render pig hepatocytes similar to human and baboon hepatocytes in their response to hepatocellular injury. Reviewing the literature, after WT pig liver allotransplantation or xenotransplantation, in the majority of reports, although changes in AST were reported, no mention was made of changes in ALT, suggesting that there was no change in ALT. However, Ramirez et al. reported two cases of liver xenotransplants from hCD55 pigs, following which there were increases in both AST and ALT, suggesting that it is not simply the cience of expression of Galα1,3Gal that is the cause. We acknowledge that our observation is based on a small number of experiments, but we believe it is worth recording.

Clinical lung xenotransplantation--what donor genetic modifications may be necessary?

Barriers to successful lung xenotransplantation appear to be even greater than for other organs. This difficulty may be related to several macro anatomic factors, such as the uniquely fragile lung parenchyma and associated blood supply that results in heightened vulnerability of graft function to segmental or lobar airway flooding caused by loss of vascular integrity (also applicable to allotransplants). There are also micro-anatomic considerations, such as the presence of large numbers of resident inflammatory cells, such as pulmonary intravascular macrophages and natural killer (NK) T cells, and the high levels of von Willebrand factor (vWF) associated with the microvasculature. We have considered what developments would be necessary to allow successful clinical lung xenotransplantation. We suggest this will only be achieved by multiple genetic modifications of the organ-source pig, in particular to render the vasculature resistant to thrombosis. The major problems that require to be overcome are multiple and include (i) the innate immune response (antibody, complement, donor pulmonary and recipient macrophages, monocytes, neutrophils, and NK cells), (ii) the adaptive immune response (T and B cells), (iii) coagulation dysregulation, and (iv) an inflammatory response (e.g., TNF-α, IL-6, HMGB1, C-reactive protein). We propose that the genetic manipulation required to provide normal thromboregulation alone may include the introduction of genes for human thrombomodulin/endothelial protein C-receptor, and/or tissue factor pathway inhibitor, and/or CD39/CD73; the problem of pig vWF may also need to be addressed. It would appear that exploration of every available therapeutic path will be required if lung xenotransplantation is to be successful. To initiate a clinical trial of lung xenotransplantation, even as a bridge to allotransplantation (with a realistic possibility of survival long enough for a human lung allograft to be obtained), significant advances and much experimental work will be required. Nevertheless, with the steadily increasing developments in techniques of genetic engineering of pigs, we are optimistic that the goal of successful clinical lung xenotransplantation can be achieved within the foreseeable future. The optimistic view would be that if experimental pig lung xenotransplantation could be successfully managed, it is likely that clinical application of this and all other forms of xenotransplantation would become more feasible.

Clinical xenotransplantation: the next medical revolution?

The shortage of organs and cells from deceased individuals continues to restrict allotransplantation. Pigs could provide an alternative source of tissue and cells but the immunological challenges and other barriers associated with xenotransplantation need to be overcome. Transplantation of organs from genetically modified pigs into non-human primates is now not substantially limited by hyperacute, acute antibody-mediated, or cellular rejection, but other issues have become more prominent, such as development of thrombotic microangiopathy in the graft or systemic consumptive coagulopathy in the recipient. To address these problems, pigs that express one or more human thromboregulatory or anti-inflammatory genes are being developed. The results of preclinical transplantation of pig cells--eg, islets, neuronal cells, hepatocytes, or corneas--are much more encouraging than they are for organ transplantation, with survival times greater than 1 year in all cases. Risk of transfer of an infectious microorganism to the recipient is small.

Genetically-engineered pig-to-baboon liver xenotransplantation: histopathology of xenografts and native organs

Orthotopic liver transplantation was carried out in baboons using wild-type (WT, n = 1) or genetically-engineered pigs (α1,3-galactosyltransferase gene-knockout, GTKO), n = 1; GTKO pigs transgenic for human CD46, n = 7) and a clinically-acceptable immunosuppressive regimen. Biopsies were obtained from the WT pig liver pre-Tx and at 30 min, 1, 2, 3, 4 and 5 h post-transplantation. Biopsies of genetically-engineered livers were obtained pre-Tx, 2 h after reperfusion and at necropsy (4–7 days after transplantation). Tissues were examined by light, confocal, and electron microscopy. All major native organs were also examined. The WT pig liver underwent hyperacute rejection. After genetically-engineered pig liver transplantation, hyperacute rejection did not occur. Survival was limited to 4–7 days due to repeated spontaneous bleeding in the liver and native organs (as a result of profound thrombocytopenia) which necessitated euthanasia. At 2 h, graft histology was largely normal. At necropsy, genetically-engineered pig livers showed hemorrhagic necrosis, platelet aggregation, platelet-fibrin thrombi, monocyte/macrophage margination mainly in liver sinusoids, and vascular endothelial cell hypertrophy, confirmed by confocal and electron microscopy. Immunohistochemistry showed minimal deposition of IgM, and almost absence of IgG, C3, C4d, C5b-9, and of a cellular infiltrate, suggesting that neither antibody- nor cell-mediated rejection played a major role.

Xenotransplantation: an overview of the field

Xenotransplantation, the transplantation of cells, tissues, or organs between different species, has the potential to overcome the current shortage of human organs and tissues for transplantation. In the last decade, the progress made in the field is remarkable, suggesting that clinical xenotransplantation procedures, particularly those involving cells, may become a reality in the not-too-distant future. However, several hurdles remain, mainly immunological barriers, physiological discrepancies, and safety issues, making xenotransplantion a complex and multidisciplinary discipline.

Validation of xCELLigence real-time cell analyzer to assess compatibility in xenotransplantation with pig-to-baboon model

OBJECTIVE

To validate the use of a microelectronic real-time cell analyzer system (RTCA) we developed a complement-mediated antibody cytotoxicity assay to investigate the compatibility of a graft and a recipient in pig-to-baboon xenotransplantation.

MATERIALS AND METHODS

Fibroblasts isolated from the skin of five hCD55, hCD59, and hCD46 transgenic pigs (TP) were cultured in 96 microelectronic well plates for 17 hours. Then, we added to each microwell 20 μL of normal sera from nine healthy adult olive baboons (Papio anubis)-three males and six females. The evolution of the cell culture was assessed every 3 minutes during the pretreatment period, at 11 hours postaddition, and every 30 minutes from 12 to 96 hours. Simultaneously, we performed a 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Fibroblasts from wild-type (WT) pigs were used as positive controls and microwells without serum addition from each TP as negative controls. The RTCA results were expressed as a normalized cellular index (NCI).

RESULTS

Differences were observed between the five TP fibroblasts and the WT fibroblasts, with greater cytotoxicity on WT cells. Among TP, a higher cytolytic level was observed in males than females. The MTT results correlated with NCI at different times, with the minimum NCI and with the time to for NCI recovery before serum addition. The correlation was lower than that previously reported in environmental toxicity assays.

CONCLUSIONS

RTCA allows a long-term assessment of the immunocytotoxic effect of baboon sera on pig cells, providing a suitable tool to perform compatibility tests for xenotransplantation.