© Springer International Publishing Switzerland 2016
Ralf J. Jox, Galia Assadi and Georg Marckmann (eds.)Organ Transplantation in Times of Donor ShortageInternational Library of Ethics, Law, and the New Medicine5910.1007/978-3-319-16441-0_1919. Discordant Cellular and Organ Xenotransplantation—From Bench to Bedside
Bruno Reichart1 , Sonja Guethoff1 , Tanja Mayr1 , Stefan Buchholz2 , Jan-Michael Abicht3 , Alexander J. Kind4 and Paolo Brenner2
(1)
Walter Brendel Centre of Experimental Medicine, Ludwig-Maximilians-Universität München, Munich, Germany
(2)
Department of Cardiac Surgery, Ludwig-Maximilians-Universität München, Munich, Germany
(3)
Department of Anaesthesiology, Ludwig-Maximilians-Universität München, Munich, Germany
(4)
Chair of Livestock Biotechnology, School of Life Sciences, Weihenstephan, Technische Universität München, Freising, Germany
Bruno Reichart
is a German heart surgeon and former Director of the Department of Cardiovascular Surgery, Ludwig-Maximilians-Universität München (LMU). For many years, he has worked in the various fields of thoracic transplantations, both clinical and experimental. Since 2012, he is the Speaker of the Collaborative Research Center (SFB/Transregio) 127: “Biology of xenogeneic cell, tissue and organ transplantation—from bench to bedside” of the German Research Foundation (DFG). Within the Consortium, he covers legal and ethical aspects as well as cardiac transplantation.
Sonja Guethoff
was a resident physician at the Department of Cardiac Surgery, Ludwig-Maximilians- Universität München (LMU, Director Prof. B. Reichart/Prof. C. Hagl) from 2010 to 2014, and started her fellowship at the Department of Vascular and Endovascular Surgery, Technische Universität München (TUM, Director Prof. H.-H. Eckstein) in 2015. Since 2012, she is a member of the Collaborative Research Center 127: “Biology of xenogeneic cell, tissue and organ transplantation –from bench to bedside.” (Speaker Prof. B. Reichart). Her current research interests are in the field of experimental cardiac transplantation and xenotransplantation using small and large animal models at the Walter Brendel Centre of Experimental Medicine (Director Prof. U. Pohl), as well as clinical trials in human heart transplantation and aortic diseases.
Tanja Mayr
is a veterinarian and works as an associate research fellow at the Walter Brendel Centre of Experimental Medicine, Ludwig-Maximilians-Universität München (LMU, Director Prof. U. Pohl). Her research interests are in the field of heart transplantation. She is a member of the Transregio Collaborative Research Centre 127: “Biology of xenogeneic cell, tissue and organ transplantation – from bench to bedside.” (Speaker Prof. B. Reichart).
Stefan Buchholz
is a resident at the Department of Cardiac Surgery, Ludwig-Maximilians- Universität München (LMU, Director Prof. C. Hagl). His research interest lies in xenogenic and allogenic rejection after heart and lung transplantation in rodents.
Jan-Michael Abicht
is a consultant at the Department of Anaesthesiology, Ludwig-Maximilians- Universität München (LMU, Director Prof. B. Zwißler). He is also the Principal Investigator at the Transregio Collaborative Research Centre 127: “Biology of xenogeneic cell, tissue and organ transplantation—from bench to bedside” (Speaker Prof. B. Reichart). His current research interests are ex-vivo cardiac perfusions.
Alexander J. Kind
is with the Chair of Livestock Biotechnology, School of Life Sciences Weihenstephan, Technische Universität München. Dr. Kind received his PhD from Cambridge University and has worked on diverse aspects of transgenic animal technology, including embryonic stem cells and the development of nuclear transfer to generate cloned and genetically modified large animals. His current research interests are in the development of large animals to model human disease and as xenotransplantation donors.
Paolo Brenner
has been an assistant professor for cardiac surgery, Department of Cardiac Surgery, Ludwig-Maximilians-Universität München (LMU, Director Prof. C. Hagl) since 2011. He was previously a specialist for cardiac surgery, Dept. of Cardiac Surgery, at the LMU Munich under the supervision of Prof. Bruno Reichart. In 2004, Dr. Brenner completed his PhD fellowship at the Bavarian Research Foundation with the Xenotransplantation (XT) Project, which began in 1997. In addition, he has been a Principal Investigator in the DFG Transregio research group FOR 535 Xenotransplantation since 2004. Since 2012, he is the Head Principal Investigator of the cardiac xenotransplantation group at the Transregio Collaborative Research Centre 127 „Xenotransplantation“ at the LMU. Additionally, he is a specialist for cardiac assist devices, as well as heart and lung transplantation.
19.1 Introduction
19.1.1 The Clinical Need for Solid Organ Transplantation—the Pig as Preferred Donor
Human allotransplantation has been very successful over the past six decades. Heart and kidney transplantations remain the therapy of choice for end-stage organ failure. Although surgical competence is available in many medical centres around the world, the demand for organs far exceeds the supply from human donors. The consequences for patients waiting for transplants are severe, as can be seen by the following two examples. In Germany, the annual mortality for waiting heart transplant candidates is 18 %.1 The average waiting time for a cadaveric kidney is five years, which significantly reduces the prospects for patients eventually receiving a donated kidney, because graft survival drops substantially after extended dialysis .2
Several alternatives have been suggested to overcome the grave shortage of organs. One possible solution would be clinical xenotransplantation using non-human primates as concordant donors and triple drug immunosuppression , as applied in human allotransplants.3 However, ethical and logistical considerations preclude this. Apes are endangered species and their use is out of the question, other non-human primates are too small and their growth too slow.
In contrast, discordant species, notably pigs, offer an abundant new source of organs (and cells) for various reasons:
Similarities in size, anatomy, nutrition and physiology to man
Short generation intervals (12 months) and high fertility (10–14 offspring per litter)
Well-established and economic housing and breeding conditions with high hygienic standards
Availability of advanced reproductive biotechnologies and genetic engineering techniques
Minor concerns regarding the slaughtering of pigs, at least in western countries, because they are raised for meat production on an industrial scale
Nevertheless, despite these obvious advantages, serious ethical concerns do exist in society regarding the use of pigs as donors, and these have to be allayed.4
19.1.2 The Clinical Need for Discordant Cellular Transplantation
There is also clinical need for a huge variety of cell types, some of which are already being investigated as possible xenotransplants, such as liver cells,5 neurons6 and corneas7. At the moment, there is a particular focus on pancreatic islets.
An epidemic of obesity in Western populations has led to an increasing threat of diabetes mellitus, with the number of patients set to double within the next two decades.8 Although anti-diabetic therapy is successful for most patients, hypoglycaemia is a life-threatening complication in 5–10 % of cases. At present, allogeneic pancreatic islet cell transplantation offers a solution for type 1 diabetes only and delivers a greatly improved quality of life with relatively low operative risk. Unfortunately, however, a shortage of suitable donors and the extraordinarily high number of islet cells required for each patient severely restricts the availability of treatment. A total of 1400 islet allotransplantations9 performed worldwide to date stands in striking contrast to the dramatically increasing number of diabetes patients.
Discordant xenogeneic islet transplantation would therefore offer a practical solution. This is supported by the impressive results of porcine islet transplantation into diabetic primate models, using islets from wild-type pigs and immunosuppression of the recipient,10 encapsulated islets from wild-type pigs,11 or islets from genetically engineered donor pigs.12
19.1.3 The Need for Biological Valve Prostheses for Younger Patients
Another focus of our consortium is on the replacement of heart valves. Approximately 300,000 patients worldwide now carry prosthetic heart valve implants. Current valves are however less than ideal. Mechanical devices necessitate lifelong anticoagulation therapy, incurring serious bleeding side effects with a mortality of one percent per patient year. Biological valves (porcine, bovine) do not need anticoagulation if the patient is in sinus rhythm, but have restricted durability, degenerating quickly in children , adolescents and young adults.13 Promising new prostheses have been made from decellularised biological heart valve matrices that are revitalised in vivo by cells from the recipient forming a functional epithelium and live interstitium. A decellularised heart valve matrix from wild-type pigs does however attract inflammatory cells and induces platelet activation.14 Pigs genetically modified to overcome these immune mechanisms may provide a superior source of such materials.
19.2 Safety Issues in Pig-to-Primate Xenotransplantation
The possible transfer of infectious agents to a graft recipient is a major problem in allotransplantation, and risks might be exacerbated with tissue from non-human species. On the other hand, xenotransplantation offers the opportunity to systematically examine the donor for infectious agents before transplantation.15 To control the infectious burden, donor animals should be raised in a clean environment (designated pathogen free, DPF) and xenograft recipients should be monitored post-operatively. Among the numerous infectious agents, porcine endogenous retroviruses (PERV-A, B, C) have received the most attention, because they are integrated in the germ line and transmitted vertically to offspring, and thus cannot be eliminated by raising pigs in a DPF facility. Initial studies showed the human-tropic potential of PERV in vitro16 and revealed their predisposition for retroviral recombination.17 Recombined PERV-A/C has higher infectious potential than PERV-C.18 But most importantly, there was no evidence of cross-species transmission in the first clinical trials of islet xenotransplantation.19 Regarding safety issues, the International Xenotransplantation Association (IXA) and the WHO have defined and regularly update their conditions for xenotransplantation20 (current update on the 2nd International Conference on Clinical Islet Xenotransplantation, August 2014, San Francisco, USA, current unpublished).
19.3 Immunological Barriers and Strategies to Overcome them
Xenotransplantation would undoubtedly provide substantial advantages for human regenerative medicine. The main problems arise from disparities between swine and primates resulting from approximately 90 million years of evolutionary divergence, which can affect important protein-protein and other biochemical interactions. Considerable immunological and physiological incompatibilities must therefore be overcome before xenogeneic grafts can be clinically effective. Fortunately, our understanding of these barriers is increasing rapidly and rational strategies are being developed to overcome them. Humoral rejection from preformed antibodies and the blood coagulation system present the immediate obstacles, in the longer term the greatest challenge comes from the adaptive immune response.
19.4 Humoral Responses in Vascularised Organs
An unmodified porcine organ transplanted into a human or primate recipient is confronted with a series of rejection responses. The first is hyperacute rejection (HAR), followed by acute humoral xenograft rejection (AHXR), also known as acute vascular or delayed xenograft reaction. Both HAR and AHXR are ultimately the result of antibodies binding to cell surface antigens on the graft endothelium.
19.4.1 Hyperacute Rejection
Pre-existing antibodies21 in human blood against the α1,3-galactosyl-galactose (α-Gal) epitope on porcine vessel walls cause the rapid formation of an antigen-antibody complex that immediately activates the host complement system. This causes cell disruption and lysis of donor endothelium, which in turn activates the blood coagulation cascade. The result is extensive haemorrhage, oedema and thrombosis of small blood vessels, leading to death of the graft within hours. Old-world primates (including humans) lack α-Gal epitopes, due to an inactive α-1,3-galactosyltransferase (GGTA1) gene. Therefore, a suitable solution would be to disable or knock out the orthologous porcine gene. Homozygous GGTA1 deficient pigs have been generated22 and several independent herds established. Organs from these pigs have been tested in numerous pig-to-baboon organ transplantation studies23 and revealed a maximum survival of three month for kidneys 24 and of over 800 days for beating but non-working hearts25.
19.4.2 Acute (Delayed) Humoral Rejection, Thrombotic Microangiopathy
Once HAR is overcome, AHXR presents the next immunological obstacle. Hearts from GGTA1-KO pigs transplanted into baboons were found to exhibit widespread thrombotic microangiopathy, ischemia, focal haemorrhage and necrosis as a consequence of progressive humoral rejection and disordered thromboregulation.26 The underlying mechanisms are not completely understood, but are thought to involve changes to the porcine endothelium following transplantation that lead to a procoagulant state. Antibodies to antigens other than α-Gal (non-Gal) epitopes also seem to play a major role in AHXR.27 However, the number and diversity of non-Gal antigens precludes their removal by gene targeting. The preferred strategy is thus to prevent a complement-mediated destruction of the xenograft. Various transgenic pigs expressing human complement regulators28 on the vascular endothelium have been generated, combined with GGTA1-KO animals, and tested in pig-to-baboon transplantation experiments. Transgenes that modulate endothelial activation, such as heme oxygenase 1 (HO-1), are also thought to be beneficial.29
In addition to the antibody-mediated activation of the xenograft endothelium, incompatibilities in the coagulation components in the human blood stream and the porcine vessel wall might also contribute to the formation of microthrombi.30 One example: Porcine thrombomodulin binds weakly to primate thrombin, leading to insufficient levels of activated protein C to interrupt coagulation.31 This effect might be overcome by expressing human thrombomodulin in the porcine donor.32
Related posts:
The Ethics of Organ Donation: Foundations and Challenges
Challenges to the Dead Donor Rule: Configuring a Biopolitical Response
Legal Consequences of Organ Transplantation Malpractice
Is an Absolute Prohibition of Living Kidney Donation by Minors Appropriate? A Discussion of the Arguments in Favor and Against
India’s Kidney Belt and Medical Tourism: A Double-edged Sword
Organ Donation and Transplantation in Germany
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