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

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.

Bridging Therapies-Ex Vivo Liver Xenoperfusion and the Role of Machine Perfusion: An Update

Advancements in xenotransplantation intersecting with modern machine perfusion technology offer promising solutions to patients with liver failure providing a valuable bridge to transplantation and extending graft viability beyond current limitations. Patients facing acute or acute chronic liver failure, post-hepatectomy liver failure, or fulminant hepatic failure often require urgent liver transplants which are severely limited by organ shortage, emphasizing the importance of effective bridging approaches. Machine perfusion is now increasingly used to test and use genetically engineered porcine livers in translational studies, addressing the limitations and costs of non-human primate models. Current reports about artificial and bioartificial liver support combined with xenografts showcase the potential in ex vivo xenogeneic perfusion. Breakthroughs, such as the perfusion of genetically modified porcine liver with FDA-approved machine perfusion systems connected to human blood circulation, underscore the interest and potential feasibility of a "liver dialysis" bridge to allotransplantation or recovery. This review provides an overview of the past and current research in the field of ex vivo pig liver xenoperfusion.

Complement networks in gene-edited pig xenotransplantation: enhancing transplant success and addressing organ shortage

The shortage of organs for transplantation emphasizes the urgent need for alternative solutions. Xenotransplantation has emerged as a promising option due to the greater availability of donor organs. However, significant hurdles such as hyperacute rejection and organ ischemia-reperfusion injury pose major challenges, largely orchestrated by the complement system, and activated immune responses. The complement system, a pivotal component of innate immunity, acts as a natural barrier for xenotransplantation. To address the challenges of immune rejection, gene-edited pigs have become a focal point, aiming to shield donor organs from human immune responses and enhance the overall success of xenotransplantation. This comprehensive review aims to illuminate strategies for regulating complement networks to optimize the efficacy of gene-edited pig xenotransplantation. We begin by exploring the impact of the complement system on the effectiveness of xenotransplantation. Subsequently, we delve into the evaluation of key complement regulators specific to gene-edited pigs. To further understand the status of xenotransplantation, we discuss preclinical studies that utilize gene-edited pigs as a viable source of organs. These investigations provide valuable insights into the feasibility and potential success of xenotransplantation, offering a bridge between scientific advancements and clinical application.

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.

Progress in xenotransplantation: overcoming immune barriers

A major limitation of organ allotransplantation is the insufficient supply of donor organs. Consequently, thousands of patients die every year while waiting for a transplant. Progress in xenotransplantation that has permitted pig organ graft survivals of years in non-human primates has led to renewed excitement about the potential of this approach to alleviate the organ shortage. In 2022, the first pig-to-human heart transplant was performed on a compassionate use basis, and xenotransplantation experiments using pig kidneys in deceased human recipients provided encouraging data. Many advances in xenotransplantation have resulted from improvements in the ability to genetically modify pigs using CRISPR-Cas9 and other methodologies. Gene editing has the capacity to generate pig organs that more closely resemble those of humans and are hence more physiologically compatible and less prone to rejection. Despite such modifications, immune responses to xenografts remain powerful and multi-faceted, involving innate immune components that do not attack allografts. Thus, the induction of innate and adaptive immune tolerance to prevent rejection while preserving the capacity of the immune system to protect the recipient and the graft from infection is desirable to enable clinical xenotransplantation.

hEPCR.hTBM.hCD47.hHO-1 with donor clodronate and DDAVP treatment improves perfusion and function of GalTKO.hCD46 porcine livers perfused with human blood

INTRODUCTION

Platelet sequestration, inflammation, and inappropriate coagulation cascade activation are prominent in liver xenotransplant models and are associated with poor outcomes. Here, we evaluate a cassette of six additional genetic modifications to reduce anti-pig antibody binding (α-1,3-galactosyl transferase knockout [GalTKO]) and target coagulation dysregulation (human endothelial protein C receptor [hEPRC] and thrombomodulin [hTBM]), complement pathway regulation (human membrane cofactor protein, hCD46), inflammation heme oxygenase 1 [hHO-1]), and a self-recognition receptor (integrin-associated protein [hCD47]), as well as donor pharmacologic treatments designed to blunt these phenomena.

METHODS

Livers from GaltKO.hCD46 pigs ("2-gene," n = 3) and GalTKO.hCD46 pigs also transgenic for hEPRC, hTBM, hCD47, and hHO-1 ("6-gene," n = 4) were perfused ex vivo with whole human blood. Six-gene pigs were additionally pretreated with desmopressin (DDAVP) and clodronate liposomes to deplete vWF and kupffer cells, respectively.

RESULTS

The average perfusion times increased from 304 (±148) min in the 2-gene group to 856 (±61) min in the 6-gene group (p = .010). The average heparin administration was decreased from 8837 U/h in the 2-gene to 1354 U/h in the 6-gene group (p = .047). Platelet sequestration tended to be delayed in the 6-gene group (p = .070), while thromboxane B2 (TXB2, a platelet activation marker) levels were lower over the first hour (p = .044) (401 ± 124 vs. 2048 ± 712 at 60 min). Thrombin production as measured by F1+2 levels tended to be lower in the 6-gene group (p = .058).

CONCLUSIONS

The combination of the hEPCR.hTBM.hCD47.hHO-1 cassette along with donor pig DDAVP and clodronate liposome pretreatment was associated with prolonged function of xenoperfused livers, reduced coagulation pathway perturbations, and decreased TXB2 elaboration, and reflects significant progress to modulate liver xenograft injury in a pig to human model.

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.

Interleukin-27 in liver xenotransplantation: A rational target to mitigate ischemia reperfusion injury and increase xenograft survival

Transplantation of xenogeneic organs is an attractive solution to the existing organ shortage dilemma, thus, securing a clinically acceptable prolongation of xenograft survival is an important goal. In preclinical transplantation models, recipients of liver, kidney, heart, or lung xenotransplants demonstrate significant graft damages through the release of pro-inflammatory molecules, including the C-reactive protein, cytokines, and histone-DNA complexes that all foster graft rejection. Recent studies have demonstrated that mitigation of ischemia reperfusion injury (IRI) greatly improves xenograft survival. Organ IRI develops primarily on a complex network of cytokines and chemokines responding to molecular cues from the graft milieu. Among these, interleukin 27 (IL-27) plays an immunomodulatory role in IRI onset due to graft environment-dependent pro- and anti- inflammatory activities. This review focuses on the impact of IL-27 on IRI of liver xenotransplants and provides insights on the function of IL-27 that could potentially guide genetic engineering strategies of donor pigs and/or conditioning of organs prior to transplantation.

Transplanting organs from pigs to humans

The success of organ transplantation is limited by the complications of immunosuppression, by chronic rejection, and by the insufficient organ supply, and thousands of patients die every year while waiting for a transplant. With recent progress in xenotransplantation permitting porcine organ graft survival of months or even years in nonhuman primates, there is renewed interest in its potential to alleviate the organ shortage. Many of these advances are the result of our heightened capacity to modify pigs genetically, particularly with the development of CRISPR-Cas9-based gene editing methodologies. Although this approach allows the engineering of pig organs that are less prone to rejection, the clinical application of xenotransplantation will require the ability to avoid the ravages of a multifaceted attack on the immune system while preserving the capacity to protect both the recipient and the graft from infectious microorganisms. In this review, we will discuss the potential and limitations of these modifications and how the engineering of the graft can be leveraged to alter the host immune response so that all types of immune attack are avoided.

Novel alternative transplantation therapy for orthotopic liver transplantation in liver failure: A systematic review

BACKGROUND

Orthotopic liver transplantation (OLT) is the only treatment for end-stage liver failure; however, graft shortage impedes its applicability. Therefore, studies investigating alternative therapies are plenty. Nevertheless, no study has comprehensively analyzed these therapies from different perspectives.

AIM

To summarize the current status of alternative transplantation therapies for OLT and to support future research.

METHODS

A systematic literature search was performed using PubMed, Cochrane Library and EMBASE for articles published between January 2010 and 2018, using the following MeSH terms: [(liver transplantation) AND cell] OR [(liver transplantation) AND differentiation] OR [(liver transplantation) AND organoid] OR [(liver transplantation) AND xenotransplantation]. Various types of studies describing therapies to replace OLT were retrieved for full-text evaluation. Among them, we selected articles including in vivo transplantation.

RESULTS

A total of 89 studies were selected. There are three principle forms of treatment for liver failure: Xeno-organ transplantation, scaffold-based transplantation, and cell transplantation. Xeno-organ transplantation was covered in 14 articles, scaffold-based transplantation was discussed in 22 articles, and cell transplantation was discussed in 53 articles. Various types of alternative therapies were discussed: Organ liver, 25 articles; adult hepatocytes, 31 articles; fetal hepatocytes, three articles; mesenchymal stem cells (MSCs), 25 articles; embryonic stem cells, one article; and induced pluripotent stem cells, three articles and other sources. Clinical applications were discussed in 12 studies: Cell transplantation using hepatocytes in four studies, five studies using umbilical cord-derived MSCs, three studies using bone marrow-derived MSCs, and two studies using hematopoietic stem cells.

CONCLUSION

The clinical applications are present only for cell transplantation. Scaffold-based transplantation is a comprehensive treatment combining organ and cell transplantations, which warrants future research to find relevant clinical applications.

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.

Xenogeneic Heterotopic Auxiliary Liver transplantation (XHALT) promotes native liver regeneration in a Post-Hepatectomy Liver failure model

The liver’s regenerative capacity is unique, but too small a segment can overwhelm its ability to simultaneously regenerate and support the host, resulting in liver dysfunction and death. Here we tested a temporary Xenogeneic Heterotopic Auxiliary Liver Transplant (XHALT) from Gal-KO miniature swine in a baboon model of Post-Hepatectomy Liver Failure (PHLF) by 90%- hepatectomy. Immunosuppression consisted of CVF, ATG, FK 506 and steroids. 90%-hepatectomized animals died within 4–5 days with the clinical picture of PHLF, (high LFTs and bilirubin, ascites, encephalopathy and coagulopathy). The 10% remnants had macroscopic and histological evidence of severe steatosis and absence of hepatocyte replication. In contrast, the addition of XHALT prolonged survival up to 11 days, with the cause of death being sepsis, rather than liver failure. The remnant liver appeared grossly normal, and on histology, there was no evidence of fatty infiltration, but there was pronounced Ki-67 staining. In conclusion, temporary auxiliary xenografts have the potential to support a small for size liver graft while it grows to adequate size or provide an opportunity for organ recovery in acute liver failure.

Innate networking: Thrombotic microangiopathy, the activation of coagulation and complement in the sensitized kidney transplant recipient

Thrombotic microangiopathy (TMA) is a histological feature of antibody-mediated rejection and has the potential to cause problematic graft dysfunction, particularly for highly sensitized cross-match positive kidney transplant recipients. Prompt recognition of pertinent histopathological and systemic features of TMA in kidney transplantation is necessary. Underlying mechanisms of this process involve the activation of both complement and coagulation systems as a response to HLA antibody. As serine proteases, coagulation and complement cascades exhibit similar characteristics with respect to homeostatic function. Increasing evidence now exists for the interaction between these innate defenses in both activation and regulation, lending scope for intervention. Understanding the complexities of these interactions remains a challenge. This review provides an overview of the current understanding, particularly with respect to the activation of coagulation and complement by HLA antibody in the setting of highly sensitized kidney transplantation.

Xenotransplantation: past, present, and future

PURPOSE OF REVIEW

To review the progress in the field of xenotransplantation with special attention to most recent encouraging findings which will eventually bring xenotransplantation to the clinic in the near future.

RECENT FINDINGS

Starting from early 2000, with the introduction of galactose-α1,3-galactose (Gal)-knockout pigs, prolonged survival especially in heart and kidney xenotransplantation was recorded. However, remaining antibody barriers to non-Gal antigens continue to be the hurdle to overcome. The production of genetically engineered pigs was difficult requiring prolonged time. However, advances in gene editing, such as zinc finger nucleases, transcription activator-like effector nucleases, and most recently clustered regularly interspaced short palindromic repeats (CRISPR) technology made the production of genetically engineered pigs easier and available to more researchers. Today, the survival of pig-to-nonhuman primate heterotopic heart, kidney, and islet xenotransplantation reached more than 900, more than 400, and more than 600 days, respectively. The availability of multiple-gene pigs (five or six genetic modifications) and/or newer costimulation blockade agents significantly contributed to this success. Now, the field is getting ready for clinical trials with an international consensus.

SUMMARY

Clinical trials in cellular or solid organ xenotransplantation are getting closer with convincing preclinical data from many centers. The next decade will show us new achievements and additional barriers in clinical xenotransplantation.

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.

Xenotransplantation: back to the future?

The field of xenotransplantation has fluctuated between great optimism and doubts over the last 50 years. The initial clinical attempts were extremely ambitious but faced technical and ethical issues that prompted the research community to go back to preclinical studies. Important players left the field due to perceived xenozoonotic risks and the lack of progress in pig-to-nonhuman-primate transplant models. Initial apparently unsurmountable issues appear now to be possible to overcome due to progress of genetic engineering, allowing the generation of multiple-xenoantigen knockout pigs that express human transgenes and the genomewide inactivation of porcine endogenous retroviruses. These important steps forward were made possible by new genome editing technologies, such as CRISPR/Cas9, allowing researchers to precisely remove or insert genes anywhere in the genome. An additional emerging perspective is the possibility of growing humanized organs in pigs using blastocyst complementation. This article summarizes the current advances in xenotransplantation research in nonhuman primates, and it describes the newly developed genome editing technology tools and interspecific organ generation.

Effects of carbon monoxide on early dysfunction and microangiopathy following GalT-KO porcine pulmonary xenotransplantation in cynomolgus monkeys

BACKGROUND

Despite progress in the current genetic manipulation of donor pigs, most non-human primates were lost within a day of receiving porcine lung transplants. We previously reported that carbon monoxide (CO) treatment improved pulmonary function in an allogeneic lung transplant (LTx) model using miniature swine. In this study, we evaluated whether the perioperative treatment with low-dose inhalation of CO has beneficial effects on porcine lung xenografts in cynomolgus monkeys (cynos).

METHODS

Eight cynos received orthotopic left LTx using either α-1,3-galactosyltransferase knockout (GalT-KO; n = 2) or GalT-KO with human decay accelerating factor (hDAF) (GalT-KO/hDAF; n = 6) swine donors. These eight animals were divided into three groups. In Group 1 (n = 2), neither donor nor recipients received CO therapy. In Group 2 (n = 4), donors were treated with inhaled CO for 180-minute. In Group 3 (n = 2), both donors and recipients were treated with CO (donor: 180-minute; recipient: 360-minute). Concentration of inhaled CO was adjusted based on measured levels of carboxyhemoglobin in the blood (15%-20%).

RESULTS

Two recipients survived for 3 days; 75 hours (no-CO) and 80 hours (CO in both the donor and the recipient), respectively. Histology showed less inflammatory cell infiltrates, intravascular thrombi, and hemorrhage in the 80-hour survivor with the CO treatment than the 75-hours non-CO treatment. Anti-non-Gal cytotoxicity levels did not affect the early loss of the grafts. Although CO treatment did not prolong overall xeno lung graft survival, the recipient/donor CO treatment helped to maintain platelet counts and inhibit TNF-α and IL-6 secretion at 2 hours after revascularization of grafts. In addition, lung xenografts that were received recipient/donor CO therapy demonstrated fewer macrophage and neutrophil infiltrates. Infiltrating macrophages as well as alveolar epithelial cells in the CO-treated graft expressed heme oxygenase-1.

CONCLUSION

Although further investigation is required, CO treatment may provide a beneficial strategy for pulmonary xenografts.

The Role of Costimulation Blockade in Solid Organ and Islet Xenotransplantation

Pig-to-human xenotransplantation offers a potential bridge to the growing disparity between patients with end-stage organ failure and graft availability. Early studies attempting to overcome cross-species barriers demonstrated robust humoral immune responses to discordant xenoantigens. Recent advances have led to highly efficient and targeted genomic editing, drastically altering the playing field towards rapid production of less immunogenic porcine tissues and even the discussion of human xenotransplantation trials. However, as these humoral immune barriers to cross-species transplantation are overcome with advanced transgenics, cellular immunity to these novel xenografts remains an outstanding issue. Therefore, understanding and optimizing immunomodulation will be paramount for successful clinical xenotransplantation. Costimulation blockade agents have been introduced in xenotransplantation research in 2000 with anti-CD154mAb. Most recently, prolonged survival has been achieved in solid organ (kidney xenograft survival > 400 days with anti-CD154mAb, heart xenograft survival > 900 days, and liver xenograft survival 29 days with anti-CD40mAb) and islet xenotransplantation (>600 days with anti-CD154mAb) with the use of these potent experimental agents. As the development of novel genetic modifications and costimulation blocking agents converges, we review their impact thus far on preclinical xenotransplantation and the potential for future application.

The Resurgence of Xenotransplantation

There has been an upsurge of interest in xenotransplantation in recent years. This resurgence can attributed to a combination of factors. First, there has been a dramatic improvement in efficacy in several preclinical models, with maximum xenograft survival times increasing to 950 days for islets, 945 days for hearts, and 310 days for kidneys. Second, the rapid development of genome editing technology (particularly the advent of clustered regularly interspaced short palindromic repeats/Cas9) has revolutionized the capacity to generate new donor pigs with multiple protective genetic modifications; what once took many years to achieve can now be performed in months, with much greater precision and scope. Third, the specter of porcine endogenous retrovirus (PERV) has receded significantly. There has been no evidence of PERV transmission in clinical trials and preclinical models, and improved screening methods and new options for the treatment or even elimination of PERV are now available. Balancing these positive developments are several remaining challenges, notably the heavy and often clinically inapplicable immunosuppression required to prevent xenograft rejection. Nonetheless, the potential for xenotransplantation as a solution to the shortage of human organs and tissues for transplantation continues to grow.

N-glycolylneuraminic acid knockout reduces erythrocyte sequestration and thromboxane elaboration in an ex vivo pig-to-human xenoperfusion model

BACKGROUND

Wild-type pigs express several carbohydrate moieties on their cell surfaces that differ from those expressed by humans. This difference in profile leads to pig tissue cell recognition of human blood cells causing sequestration, in addition to antibody-mediated xenograft injury. One such carbohydrate is N-glycolylneuraminic acid (Neu5Gc), a sialic acid molecule synthesized in pigs but not in humans. Here, we evaluate livers with and without Neu5Gc in an ex vivo liver xeno perfusion model.

METHODS

Livers from pigs with an α1,3-galactosyl transferase gene knockout (GalTKO) and transgenic for human membrane cofactor (hCD46) with (n = 5) or without (n = 7) an additional Neu5Gc gene knock out (Neu5GcKO) were perfused ex vivo with heparinized whole human blood. A drug regimen consisting of a histamine inhibitor, thromboxane synthase inhibitor, and a murine anti-human GPIb-blocking antibody fragment was given to half of the experiments in each group.

RESULTS

Liver function tests (AST and ALT) were not significantly different between livers with and without the Neu5GcKO. GalTKO.hCD46.Neu5GcKO livers had less erythrocyte sequestration as evidenced by a higher mean hematocrit over time compared to GalTKO.hCD46 livers (P = .0003). The addition of Neu5GcKO did not ameliorate profound thrombocytopenia seen within the first 15 minutes of perfusion. TXB2 was significantly less with the added drug regimen (P = .006) or the presence of Neu5GcKO (P = .017).

CONCLUSIONS

The lack of Neu5Gc expression attenuated erythrocyte loss but did not prevent profound early onset thrombocytopenia or platelet activation, although TXB2 levels were decreased in the presence of Neu5GcKO.

Prolonged Survival Following Pig-to-Primate Liver Xenotransplantation Utilizing Exogenous Coagulation Factors and Costimulation Blockade

Since the first attempt of pig-to-primate liver xenotransplantation (LXT) in 1968, survival has been limited. We evaluated a model utilizing α-1,3-galactosyltransferase knockout donors, continuous posttransplant infusion of human prothrombin concentrate complex, and immunosuppression including anti-thymocyte globulin, FK-506, methylprednisone, and costimulation blockade (belatacept, n = 3 or anti-CD40 mAb, n = 1) to extend survival. Baboon 1 remained well until postoperative day (POD) 25, when euthanasia was required because of cholestasis and plantar ulcers. Baboon 2 was euthanized following a seizure on POD 5, despite normal liver function tests (LFTs) and no apparent pathology. Baboon 3 demonstrated initial stable liver function but was euthanized on POD 8 because of worsening LFTs. Pathology revealed C4d positivity, extensive hemorrhagic necrosis, and a focal cytomegalovirus inclusion. Baboon 4 was clinically well with stable LFTs until POD29, when euthanasia was again necessitated by plantar ulcerations and rising LFTs. Final pathology was C4d negative and without evidence of rejection, inflammation, or thrombotic microangiopathy. Thus, nearly 1-mo rejection-free survival has been achieved following LXT in two of four consecutive recipients, demonstrating that the porcine liver can support life in primates for several weeks and has encouraging potential for clinical application as a bridge to allotransplantation for patients with acute-on-chronic or fulminant hepatic failure.

Regulation of Clinical Xenotransplantation-Time for a Reappraisal

The continual critical shortage of organs and cells from deceased human donors has stimulated research in the field of cross-species transplantation (xenotransplantation), with the pig selected as the most suitable potential source of organs. Since the US Food and Drug Administration concluded a comprehensive review of xenotransplantation in 2003, considerable progress has been made in the experimental laboratory to improve cell and organ xenograft survival in several pig-to-nonhuman primate systems that offer the best available models to predict clinical outcomes. Survival of heart, kidney, and islet grafts in nonhuman primates is now being measured in months or even years. The potential risks associated with xenotransplantation, for example, the transfer of an infectious microorganism, that were highlighted in the 2003 Food and Drug Administration guidance and subsequent World Health Organization consensus documents have been carefully studied and shown to be either less likely than previously thought or readily manageable by donor selection or recipient management strategies. In this context, we suggest that the national regulatory authorities worldwide should re-examine their guidelines and regulations regarding xenotransplantation, so as to better enable design and conduct of safe and informative clinical trials of cell and organ xenotransplantation when and as supported by the preclinical data. We identify specific topics that we suggest require reconsideration.

Thrombalexin: Use of a Cytotopic Anticoagulant to Reduce Thrombotic Microangiopathy in a Highly Sensitized Model of Kidney Transplantation

Early activation of coagulation is an important factor in the initiation of innate immunity, as characterized by thrombotic microangiopathy (TMA). In transplantation, systemic anticoagulation is difficult due to bleeding. A novel "cytotopic" agent, thrombalexin (TLN), combines a cell-membrane-bound (myristoyl tail) anti-thrombin (hirudin-like peptide [HLL]), which can be perfused directly to the donor organ or cells. Thromboelastography was used to measure time to clot formation (r-time) in both rhesus and human blood, comparing TLN versus HLL (without cytotopic tail) versus negative control. Both TLN- and HLL-treated rhesus or human whole blood result in significantly prolonged r-time compared to kaolin controls. Only TLN-treated human endothelial cells and neonatal porcine islets prolonged time to clot formation. Detection of membrane-bound TLN was confirmed by immunohistochemistry and fluorescence activated cell sorter. In vivo, perfusion of a nonhuman primate kidney TLN-supplemented preservation solution in a sensitized model of transplantation demonstrated no evidence of TLN systemically. Histologically, TLN was shown to be present up to 4 days after transplantation. There was no platelet deposition, and TMA severity, as well as microvascular injury scores (glomerulitis + peritubular capillaritis), were less in the TLN-treated animals. Despite promising evidence of localized efficacy, no survival benefit was demonstrated.

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.