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

Meeting report: Xenotransplantation Development Conference in Neijiang, China

A conference on progress in the development of xenotransplantation in China was held in Neijiang, Sichuan, in May 2023, and was attended by approximately 100 established researchers and trainees. Progress in xenotransplantation research was reviewed by both Chinese and foreign experts. The topics discussed ranged from genetic engineering of pigs and the results of pig-to-nonhuman primate organ transplantation to the requirements for designated pathogen-free (DPF) pig facilities and regulation of xenotransplantation. This conference served as an opportunity to collectively advance the development of xenotransplantation in China and pave the way for its clinical application.

Genetic engineering of pigs for xenotransplantation to overcome immune rejection and physiological incompatibilities: The first clinical steps

Xenotransplantation has the potential to solve the shortfall of human organ donors. Genetically modified pigs have been considered as potential animal donors for human xenotransplantation and have been widely used in preclinical research. The genetic modifications aim to prevent the major species-specific barriers, which include humoral and cellular immune responses, and physiological incompatibilities such as complement and coagulation dysfunctions. Genetically modified pigs can be created by deleting several pig genes related to the synthesis of various pig specific antigens or by inserting human complement- and coagulation-regulatory transgenes. Finally, in order to reduce the risk of infection, genes related to porcine endogenous retroviruses can be knocked down. In this review, we focus on genetically modified pigs and comprehensively summarize the immunological mechanism of xenograft rejection and recent progress in preclinical and clinical studies. Overall, both genetically engineered pig-based xenografts and technological breakthroughs in the biomedical field provide a promising foundation for pig-to-human xenotransplantation in the future.

Current status of xenotransplantation research and the strategies for preventing xenograft rejection

Transplantation is often the last resort for end-stage organ failures, e.g., kidney, liver, heart, lung, and pancreas. The shortage of donor organs is the main limiting factor for successful transplantation in humans. Except living donations, other alternatives are needed, e.g., xenotransplantation of pig organs. However, immune rejection remains the major challenge to overcome in xenotransplantation. There are three different xenogeneic types of rejections, based on the responses and mechanisms involved. It includes hyperacute rejection (HAR), delayed xenograft rejection (DXR) and chronic rejection. DXR, sometimes involves acute humoral xenograft rejection (AHR) and cellular xenograft rejection (CXR), which cannot be strictly distinguished from each other in pathological process. In this review, we comprehensively discussed the mechanism of these immunological rejections and summarized the strategies for preventing them, such as generation of gene knock out donors by different genome editing tools and the use of immunosuppressive regimens. We also addressed organ-specific barriers and challenges needed to pave the way for clinical xenotransplantation. Taken together, this information will benefit the current immunological research in the field of xenotransplantation.

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.

Decellularized xenografts in regenerative medicine: From processing to clinical application

Decellularized xenografts are an inherent component of regenerative medicine. Their preserved structure, mechanical integrity and biofunctional composition have well established them in reparative medicine for a diverse range of clinical indications. Nonetheless, their performance is highly influenced by their source (ie species, age, tissue) and processing (ie decellularization, crosslinking, sterilization and preservation), which govern their final characteristics and determine their success or failure for a specific clinical target. In this review, we provide an overview of the different sources and processing methods used in decellularized xenografts fabrication and discuss their effect on the clinical performance of commercially available decellularized xenografts.

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.

Incompatibility between recipient CD47 and donor SIRPα is not a key risk factor for thrombocytopenia or anemia following rat liver xenotransplantation in mice

Liver xenotransplantation (LXT) is greatly impeded by severe thrombocytopenia, anemia, and coagulopathy. Hepatic phagocytic cells are thought to play an important role in LXT-induced thrombocytopenia and anemia. In this study, we investigated whether the lack of recipient CD47-donor SIRPα interaction, which is known to induce xenograft rejection by macrophages, exacerbates platelet and RBC depletion following LXT. We first addressed this question in the absence of anti-donor immune responses using a syngeneic mouse liver transplantation (LT) model. Neither wild-type (WT) nor CD47KO B6 mice developed thrombocytopenia following LT from WT B6 donors. Although a moderate decline in RBCs was detected following LT, there was no significant difference in RBC counts between WT and CD47KO recipients. Because mouse CD47 is cross-reactive with rat SIRPα, we then compared thrombocytopenia and anemia between WT and CD47KO mice following rat LXT. Unlike syngeneic mouse LT, significant thrombocytopenia and anemia were detected following rat LXT. However, the severities of both platelet and RBC depletions were comparable between WT and CD47KO recipients. Furthermore, WT and CD47KO recipients showed a similar extent of early platelet activation. Our results indicate that CD47-SIRPα signaling does not significantly affect the loss of platelets or RBCs following LXT, suggesting that the limited cross-reactivity between recipient CD47 and donor SIRPα is not a significant risk factor for LXT-induced thrombocytopenia and anemia.

The resurgent landscape of xenotransplantation of pig organs in nonhuman primates

Organ shortage is a major bottleneck in allotransplantation and causes many wait-listed patients to die or become too sick for transplantation. Genetically engineered pigs have been discussed as a potential alternative to allogeneic donor organs. Although xenotransplantation of pig-derived organs in nonhuman primates (NHPs) has shown sequential advances in recent years, there are still underlying problems that need to be completely addressed before clinical applications, including (i) acute humoral xenograft rejection; (ii) acute cellular rejection; (iii) dysregulation of coagulation and inflammation; (iv) physiological incompatibility; and (v) cross-species infection. Moreover, various genetic modifications to the pig donor need to be fully characterized, with the aim of identifying the ideal transgene combination for upcoming clinical trials. In addition, suitable pretransplant screening methods need to be confirmed for optimal donor-recipient matching, ensuring a good outcome from xenotransplantation. Herein, we summarize the understanding of organ xenotransplantation in pigs-to-NHPs and highlight the current status and recent progress in extending the survival time of pig xenografts and recipients. We also discuss practical strategies for overcoming the obstacles to xenotransplantation mentioned above to further advance transplantation of pig organs in the clinic.

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.

Progress in multiple genetically modified minipigs for xenotransplantation in China

Pig-to-human organ transplantation provides an alternative for critical shortage of human organs worldwide. Genetically modified pigs are promising donors for xenotransplantation as they show many anatomical and physiological similarities to humans. However, immunological rejection including hyperacute rejection (HAR), acute humoral xenograft rejection (AHXR), immune cell-mediated rejection, and other barriers associated with xenotransplantation must be overcome with various strategies for the genetic modification of pigs. In this review, we summarize the outcomes of genetically modified and cloned pigs achieved by Chinese scientists to resolve the above-mentioned problems in xenotransplantation. It is now possible to knockout several porcine genes associated with the expression of sugar residues, antigens for (naturally) existing antibodies in humans, including GGTA1, CMAH, and β4GalNT2, and thereby preventing the antigen-antibody response. Moreover, insertion of human complement- and coagulation-regulatory transgenes, such as CD46, CD55, CD59, and hTBM, can further overcome effects of the humoral immune response and coagulation dysfunction, while expression of regulatory factors of immune responses can inhibit the adaptive immune rejection. Furthermore, transgenic strategies have been developed by Chinese scientists to reduce the potential risk of infections by endogenous porcine retroviruses (PERVs). Breeding of multi-gene low-immunogenicity pigs in China is also presented in this review. Lastly, we will briefly mention the preclinical studies on pig-to-non-human primate xenotransplantation conducted in several centers in China.

Potential pathological role of pro-inflammatory cytokines (IL-6, TNF-α, and IL-17) in xenotransplantation

The major limitation of organ transplantation is the shortage of available organs from deceased human donors which leads to the deaths of thousands of patients each year. Xenotransplantation is considered to be an effective way to resolve the problem. Immune rejection and coagulation dysfunction are two major hurdles for the successful survival of pig xenografts in primate recipients. Pro-inflammatory cytokines, such as IL-6, TNF-α, and IL-17, play important roles in many diseases and in allotransplantation. However, the pathological roles of these pro-inflammatory cytokines in xenotransplantation remain unclear. Here, we briefly review the signaling transduction and expression regulation of IL-6, TNF-α, and IL-17 and evaluate their potential pathological roles in in vitro and in vivo models of xenotransplantation. We found that IL-6, TNF-α, and IL-17 were induced in most in vitro or in vivo xenotransplantation model. Blockade of these cytokines using gene modification, antibody, or inhibitor had different effects in xenotransplantation. Inhibition of IL-6 signaling with tocilizumab decreased CRP but did not increase xenograft survival. The one possible reason is that tocilizumab can not suppress IL-6 signaling in porcine cells or organs. Other drugs which inhibit IL-6 signaling need to be investigated in xenotransplantation model. Inhibition of TNF-α was beneficial for the survival of xenografts in pig-to-mouse, rat, or NHP models. Blockade of IL-17 using a neutralizing antibody also increased xenograft survival in several animal models. However, the role of IL-17 in the pig-to-NHP xenotransplantation model remains unclear and needs to be further investigated. Moreover, blockade of TNF-α and IL-6 together has got a better effect in pig-to-baboon kidney xenotransplantation. Blockade two or even more cytokines together might get better effect in suppressing xenograft rejection. Better understanding the role of these cytokines in xenotransplantation will be beneficial for choosing better immunosuppressive strategy or producing genetic modification pig.

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.

Xenotransplantation in China: Present status

The main obstacle to organ transplantation is the shortage of organs from deceased individuals. Especially in China, the ratio of patients on the waiting list versus the transplant recipients is 30:1. Therefore, there is an urgent need for organ donors. Genetically modified pig organs have proved to be a new source for xenotransplantation, and Chinese scientists have made considerable progress in this area during recent years. In this paper, we review four important aspects of the xenotransplantation field in China. First, a large variety of genetically modified pigs have been generated by Chinese scientists: all these genetically modified pigs and the purpose of these modifications will be summarized. Second, the preclinical research in pig-to-nonhuman primate xenotransplantation is outlined. The survival time and major biochemical parameters for the xenografts are summarized. Third, regarding the bench-to-bed approach, more suitable organs have been developed for xenotransplantation in humans, and in particular, pig islet transplantation into diabetic patients as well as pig-to-human cornea and skin transplantation. Fourth, we briefly address the regulations and prospects for recruiting xenotransplantation experts in China. Based on recent progress, we anticipate that genetically modified pigs will offer suitable organs for the treatment of end-stage organ diseases in humans in the near future. Given the recent influx of world-renowned scientists in xenotransplantation to China, our country will definitely become one of the major centers of xenotransplantation research and development in the world.

Promising potentials of Tibetan macaques in xenotransplantation

Organ transplantation is a crucial medical procedure, as it is often the only treatment for patients suffering from end-stage organ failure. Unfortunately, the shortage of donor organs limits the number of patients whose lives can be saved. Carrying out research on xenotransplantation with the aim of eventually replacing human organ transplants with those of animals is very promising, as it could effectively bridge the shortfall in donor organs. Thanks to the success of cloned pigs and to the emergence of gene-editing techniques, genetically modified pigs have come to be considered ideal animal donors for human xenotransplantation and have been widely used in basic research. Such research focuses on pig-to-nonhuman primates transplantation, as the recipients are suitable for preclinical studies because both their genes and organ sizes are similar to those of humans. Chinese transplantation scientists have carried out several experiments on Tibetan macaques, including successful preclinical transplants of material from genetically modified pigs, as well as research on such topics as intraocular pressure, Parkinson's disease, advanced cancer, islet transplantation, and liver transplantation. This article reviews basic and applied research on Tibetan macaques in xenotransplantation, as well as the issues of immune rejection and ethical concerns. We aim to demonstrate the various advantages of Tibetan macaques as transplant recipients compared to other nonhuman primate species and to provide a perspective for the future establishment of Tibetan macaques as principal recipients in preclinical studies of xenotransplantation.