HLA-E/human beta2-microglobulin transgenic pigs: protection against xenogeneic human anti-pig natural killer cell cytotoxicity

Differences between xenotransplantation and allogeneic kidney transplantation: the current situation and future challenges in Japan

Kidney transplantation is the only curative option for patients with chronic renal failure, significantly improving their survival and quality of life. However, this treatment remains limited by the shortage of organ donors. The shortage of kidney donors remains a serious problem all over the world, and is particularly severe in Japan. While advancements in immunosuppressive therapies and histocompatibility testing have improved outcomes in allogeneic kidney transplantation, the rising number of dialysis patients has worsened the gap between the demand for and supply of suitable donor organs. In response to this pressing need, xenotransplantation has gained attention as a promising alternative solution. Recent progress driven by gene-editing technologies, including CRISPR-Cas9, has facilitated the development of genetically modified pigs suitable for potential human transplantation. This review provides an overview of the key differences in immune response and infection risks between xenogeneic and allogeneic kidney transplants. In addition, it comprehensively examines the challenges and potential of xenogeneic kidney transplantation from multiple perspectives, including differences in immunosuppressive therapies between allogeneic and xenogeneic transplantation. We also discuss the feasibility of xenogeneic kidney transplantation as a solution to the organ shortage in Japan and present directions for addressing challenges toward clinical application. We hope this review will provide valuable insights into the potential of xenogeneic kidney transplantation as a new treatment option for chronic renal failure and contribute to efforts to address the donor shortage problem in Japan.

Addressing glycan and hematological barriers in pig-to-nonhuman primate liver xenotransplantation: challenges and future directions

Achieving long-term survival in pig-to-primate liver xenotransplantation has proven highly challenging due to significant hematological issues. This paper investigates the primary obstacles from a hematological perspective, focusing on coagulation disorders caused by molecular incompatibility between species. It also examines the mismatched glycan structures on the surfaces of platelets and red blood cells, which lead to sequestration and phagocytosis by recipient macrophages. These mismatches underscore the need for improved glycan and molecular compatibility to overcome immunological and physiological barriers. Moreover, the liver's unique role in synthesizing a wide array of proteins, especially those involved in blood coagulation, introduces additional challenges of molecular incompatibility compared to other organs, such as the heart and kidneys. This study highlights the importance of addressing these challenges to improve the outcomes of liver xenotransplantation and suggests the necessity of strategies like glycan matching and the development of gene-edited pigs specifically tailored for liver transplantation.

Cytotoxic Responses Mediated by NK Cells and Cytotoxic T Lymphocytes in Xenotransplantation

Xenotransplantation represents a potential solution to the shortage of organs for transplantation. The recent advancements in porcine genetic modification have addressed hyperacute and acute vascular rejection; however, challenges persist with regard to delayed xenograft rejection. Porcine endothelial cells (pECs) represent a crucial target in the context of xenograft rejection, which is mediated by cytotoxic lymphocytes. It is crucial to comprehend the manner in which human natural killer (NK) cells and cytotoxic CD8+ T lymphocytes (CTL) recognize and target pECs in order to develop efficacious prophylactic strategies against rejection. The objective of the present review is to synthesize the existing knowledge regarding the mechanisms and techniques employed to modulate xenogeneic responses mediated by human NK cells and CTL. We will elucidate recent methodological advancements, debate potential novel strategies, and emphasize the imperative necessity for further research and innovative approaches to enhance graft survival.

What Genetic Modifications of Source Pigs Are Essential and Sufficient for Cell, Tissue, and Organ Xenotransplantation?

Xenotransplantation of porcine organs has made remarkable progress towards clinical application. A key factor has been the generation of genetically multi-modified source pigs for xenotransplants, protected against immune rejection and coagulation dysregulation. While efficient gene editing tools and multi-cistronic expression cassettes facilitate sophisticated and complex genetic modifications with multiple gene knockouts and protective transgenes, an increasing number of independently segregating genetic units complicates the breeding of the source pigs. Therefore, an optimal combination of essential genetic modifications may be preferable to extensive editing of the source pigs. Here, we discuss the prioritization of genetic modifications to achieve long-term survival and function of xenotransplants and summarise the genotypes that have been most successful for xenogeneic heart, kidney, and islet transplantation. Specific emphasis is given to the choice of the breed/genetic background of the source pigs. Moreover, multimodal deep phenotyping of porcine organs after xenotransplantation into human decedents will be discussed as a strategy for selecting essential genetic modifications of the source pigs. In addition to germ-line gene editing, some of these modifications may also be induced during organ preservation/perfusion, as demonstrated recently by the successful knockdown of swine leukocyte antigens in porcine lungs during ex vivo perfusion.

Pig Xenotransplantation in Beta Cell Replacement: Addressing Challenges and Harnessing Potential for Type 1 Diabetes Therapy

This opinion paper evaluates the potential of porcine islets as a promising alternative in beta cell replacement therapy for Type 1 Diabetes (T1D), juxtaposed with the current limitations of human donor islets. It analyzes the compatibility of pig islets with human glucose metabolism, their prospects as a limitless and high-quality source of beta cells, and the unique immunogenic challenges they present in xenotransplantation. Additionally, the paper discusses the regulatory and ethical considerations pertinent to the use of porcine islets. By synthesizing current research and expert perspectives, the paper highlights both the opportunities and significant barriers that need addressing to advance pig islets as a viable therapeutic option. The findings advocate for a balanced and forward-looking approach to the integration of pig islets in T1D treatment, underscoring the need for continued research and dialogue in this evolving field.

How should cardiac xenotransplantation be initiated in Japan?

The world's first clinical cardiac xenotransplantation, using a genetically engineered pig heart with 10 gene modifications, prolonged the life of a 57-year-old man with no other life-saving options, by 60 days. It is foreseeable that xenotransplantation will be introduced in clinical practice in the United States. However, little clinical or regulatory progress has been made in the field of xenotransplantation in Japan in recent years. Japan seems to be heading toward a "device lag", and the over-importation of medical devices and technology in the medical field is becoming problematic. In this review, we discuss the concept of pig-heart xenotransplantation, including the pathobiological aspects related to immune rejection, coagulation dysregulation, and detrimental heart overgrowth, as well as genetic modification strategies in pigs to prevent or minimize these problems. Moreover, we summarize the necessity for and current status of xenotransplantation worldwide, and future prospects in Japan, with the aim of initiating xenotransplantation in Japan using genetically modified pigs without a global delay. It is imperative that this study prompts the initiation of preclinical xenotransplantation research using non-human primates and leads to clinical studies.

Immunoprotection Strategies in β-Cell Replacement Therapy: A Closer Look at Porcine Islet Xenotransplantation

Type 1 diabetes mellitus (T1DM) is characterized by absolute insulin deficiency primarily due to autoimmune destruction of pancreatic β-cells. The prevailing treatment for T1DM involves daily subcutaneous insulin injections, but a substantial proportion of patients face challenges such as severe hypoglycemic episodes and poorly controlled hyperglycemia. For T1DM patients, a more effective therapeutic option involves the replacement of β-cells through allogeneic transplantation of either the entire pancreas or isolated pancreatic islets. Unfortunately, the scarcity of transplantable human organs has led to a growing list of patients waiting for an islet transplant. One potential alternative is xenotransplantation of porcine pancreatic islets. However, due to inter-species molecular incompatibilities, porcine tissues trigger a robust immune response in humans, leading to xenograft rejection. Several promising strategies aim to overcome this challenge and enhance the long-term survival and functionality of xenogeneic islet grafts. These strategies include the use of islets derived from genetically modified pigs, immunoisolation of islets by encapsulation in biocompatible materials, and the creation of an immunomodulatory microenvironment by co-transplanting islets with accessory cells or utilizing immunomodulatory biomaterials. This review concentrates on delineating the primary obstacles in islet xenotransplantation and elucidates the fundamental principles and recent breakthroughs aimed at addressing these challenges.

Physiological basis for xenotransplantation from genetically modified pigs to humans

The collective efforts of scientists over multiple decades have led to advancements in molecular and cellular biology-based technologies including genetic engineering and animal cloning that are now being harnessed to enhance the suitability of pig organs for xenotransplantation into humans. Using organs sourced from pigs with multiple gene deletions and human transgene insertions, investigators have overcome formidable immunological and physiological barriers in pig-to-nonhuman primate (NHP) xenotransplantation and achieved prolonged pig xenograft survival. These studies informed the design of Revivicor's (Revivicor Inc, Blacksburg, VA) genetically engineered pigs with 10 genetic modifications (10 GE) (including the inactivation of 4 endogenous porcine genes and insertion of 6 human transgenes), whose hearts and kidneys have now been studied in preclinical human xenotransplantation models with brain-dead recipients. Additionally, the first two clinical cases of pig-to-human heart xenotransplantation were recently performed with hearts from this 10 GE pig at the University of Maryland. Although this review focuses on xenotransplantation of hearts and kidneys, multiple organs, tissues, and cell types from genetically engineered pigs will provide much-needed therapeutic interventions in the future.

IFN-β Pretreatment Alleviates Allogeneic Renal Tubular Epithelial Cell-Induced NK Cell Responses via the IRF7/HLA-E/NKG2A Axis

Immune checkpoint molecules are promising targets for suppressing the immune response but have received little attention in immune tolerance induction in organ transplantation. In this study, we found that IFN-β could induce the expression of HLA-E as well as PD-L1 on human renal tubular epithelial cell line HK-2 and renal tissue of the C57BL/6 mouse. The JAK/STAT2 pathway was necessary for this process. Upregulation of both HLA-E and PD-L1 was fully abrogated by the JAK1/2 inhibitor ruxolitinib. Signaling pathway molecules, including STAT1, STAT2, mTOR, Tyk2, and p38 MAPK, were involved in HLA-E and PD-L1 upregulation. IRF7 is the key transcription factor responsible for the activation of HLA-E and PD-L1 promoters. Through screening an epigenetic regulation library, we found a natural compound, bisdemethoxycurcumin, enhanced IFN-β-induced HLA-E and PD-L1 expression in vitro and in vivo. In PBMC-derived CD56+ NK cells, we found that NKG2A but not PD1 was constitutively expressed, indicating HLA-E/NKG2A as a more potent target to induce tolerance to innate immune cells. Pretreating HK-2 cells by IFN-β significantly attenuated the degranulation of their coincubated NK cells and protected cells from NK-mediated lysis. In conclusion, IFN-β pretreatment could activate HLA-E and PD-L1 transcription through the JAK/STAT/IRF7 pathway and then could protect renal tubular epithelial cells from allogeneic immune attack mediated by NK cells.

Advances in Organ and Tissue Xenotransplantation

End-stage organ failure can result from various preexisting conditions and occurs in patients of all ages, and organ transplantation remains its only treatment. In recent years, extensive research has been done to explore the possibility of transplanting animal organs into humans, a process referred to as xenotransplantation. Due to their matching organ sizes and other anatomical and physiological similarities with humans, pigs are the preferred organ donor species. Organ rejection due to host immune response and possible interspecies infectious pathogen transmission have been the biggest hurdles to xenotransplantation's success. Use of genetically engineered pigs as tissue and organ donors for xenotransplantation has helped to address these hurdles. Although several preclinical trials have been conducted in nonhuman primates, some barriers still exist and demand further efforts. This review focuses on the recent advances and remaining challenges in organ and tissue xenotransplantation.

Porcine UL-16 Binding Protein 1 Is Not a Functional Ligand for the Human Natural Killer Cell Activating Receptor NKG2D

Natural killer (NK) cells play a vital role in xenotransplantation rejection. One approach to induce NK cell immune tolerance is to prevent the NK cell-mediated direct killing of porcine cells by targeting the interaction of the activating receptor NKG2D and its ligands. However, the identity of porcine ligands for the human NKG2D receptor has remained elusive. Previous studies on porcine UL-16 binding protein 1 (pULBP-1) as a ligand for human NKG2D have yielded contradictory results. The goal of the present study was to clarify the role of pULBP-1 in the immune response and its interaction with human NKG2D receptor. To accomplish this, the CRISPR/Cas9 gene editing tool was employed to disrupt the porcine ULBP-1 gene in a 5-gene knockout porcine endothelial cell line (GGTA1, CMAH, β4galNT2, SLA-I α chain, and β-2 microglobulin, 5GKO). A colony with two allele mutations in pULBP-1 was established as a 6-gene knockout pig cell line (6GKO). We found that pULBP-1-deficient pig cells exhibited a reduced binding capacity to human NKG2D-Fc, a recombinant chimera protein. However, the removal of ULBP-1 from porcine endothelial cells did not significantly impact human NK cell degranulation or cytotoxicity upon stimulation with the pig cells. These findings conclusively demonstrate that pULBP-1 is not a crucial ligand for initiating xenogeneic human NK cell activation.

In vitro and in vivo immune assessments of genetically-engineered pig skin grafts in New World (squirrel) monkeys

Half a million patients in the USA alone require treatment for burns annually. Following an extensive burn, it may not be possible to provide sufficient autografts in a single setting. Genetic manipulations (GM) of pigs offer the possibility of reducing primate humoral and cellular rejection of pig skin xenografts and thus extending graft survival. We compared the survival of skin grafts from pigs with 9-GM with that of autografts and allografts in squirrel monkeys. Monitoring for rejection was by (1) macroscopic examination, (2) histopathological examination of skin biopsies, and (3) measurement of anti-monkey and anti-pig IgM and IgG antibodies. Autografts (n = 5) survived throughout the 28 days of follow-up without histopathological features of rejection. Median survival of allografts (n = 6) was 14 days and of pig xenografts (n = 12) 21 days. Allotransplantation was associated with an increase in anti-monkey IgM, but the anticipated subsequent rise in IgG had not yet occurred at the time of euthanasia. Pig grafts were associated with increases in anti-pig IgM and IgG. In all cases, histopathologic features of rejection were similar. 9-GM pig skin xenografts survive at least as long as monkey skin allografts (and trended to survive longer), suggesting that they are a realistic clinical option for the temporary treatment of burns. Although monkeys with pig skin grafts developed anti-pig IgM and IgG antibodies, these did not cross-react with monkey antigens, indicating that a primary 9-GM pig skin graft would not be detrimental to a subsequent monkey skin allograft.

Expression of viral CD45 ligand E3/49K on porcine cells reduces human anti-pig immune responses

Transgenic expression of protective molecules in porcine cells and tissues is a promising approach to prevent xenograft rejection. Viruses have developed various strategies to escape the host's immune system. We generated porcine B cells (B cell line L23) expressing the human adenovirus protein E3/49K or the human cytomegalovirus protein pUL11 and investigated how human T, NK and B cell responses are affected by the expression of the viral proteins. Binding studies revealed that E3/49K and pUL11 interact with CD45 on human but not porcine peripheral blood mononuclear cells. T cell proliferation in response to L23-E3/49K cells was significantly reduced and accompanied by development of an anti-inflammatory cytokine milieu (low: TNF-alpha, IFN-gamma, IL-6; high: IL-4, IL-10). Human peripheral blood mononuclear cells which had been primed for four weeks by L23-E3/49K cells included an extended population of regulatory T cells. Cytotoxicity of effector T and natural killer cells against L23 cells was significantly reduced (40 to 50%) by E3/49K expression. B cell activation and antibody production to E3/49K expressing cells was also diminished. Surprisingly, pUL11 expression showed no effects. Reduction of human anti-pig immune responses by transgenic expression of selected viral genes may be a novel approach for protection of porcine xenografts.

Immune response after pig-to-human kidney xenotransplantation: a multimodal phenotyping study

BACKGROUND

Cross-species immunological incompatibilities have hampered pig-to-human xenotransplantation, but porcine genome engineering recently enabled the first successful experiments. However, little is known about the immune response after the transplantation of pig kidneys to human recipients. We aimed to precisely characterise the early immune responses to the xenotransplantation using a multimodal deep phenotyping approach.

METHODS

We did a complete phenotyping of two pig kidney xenografts transplanted to decedent humans. We used a multimodal strategy combining morphological evaluation, immunophenotyping (IgM, IgG, C4d, CD68, CD15, NKp46, CD3, CD20, and von Willebrand factor), gene expression profiling, and whole-transcriptome digital spatial profiling and cell deconvolution. Xenografts before implantation, wild-type pig kidney autografts, as well as wild-type, non-transplanted pig kidneys with and without ischaemia-reperfusion were used as controls.

FINDINGS

The data collected from xenografts suggested early signs of antibody-mediated rejection, characterised by microvascular inflammation with immune deposits, endothelial cell activation, and positive xenoreactive crossmatches. Capillary inflammation was mainly composed of intravascular CD68+ and CD15+ innate immune cells, as well as NKp46+ cells. Both xenografts showed increased expression of genes biologically related to a humoral response, including monocyte and macrophage activation, natural killer cell burden, endothelial activation, complement activation, and T-cell development. Whole-transcriptome digital spatial profiling showed that antibody-mediated injury was mainly located in the glomeruli of the xenografts, with significant enrichment of transcripts associated with monocytes, macrophages, neutrophils, and natural killer cells. This phenotype was not observed in control pig kidney autografts or in ischaemia-reperfusion models.

INTERPRETATION

Despite favourable short-term outcomes and absence of hyperacute injuries, our findings suggest that antibody-mediated rejection in pig-to-human kidney xenografts might be occurring. Our results suggest specific therapeutic targets towards the humoral arm of rejection to improve xenotransplantation results.

FUNDING

OrganX and MSD Avenir.

Co-expression of HLA-E and HLA-G on genetically modified porcine endothelial cells attenuates human NK cell-mediated degranulation

Natural killer (NK) cells play an important role in immune rejection in solid organ transplantation. To mitigate human NK cell activation in xenotransplantation, introducing inhibitory ligands on xenografts via genetic engineering of pigs may protect the graft from human NK cell-mediated cytotoxicity and ultimately improve xenograft survival. In this study, non-classical HLA class I molecules HLA-E and HLA-G were introduced in an immortalized porcine liver endothelial cell line with disruption of five genes (GGTA1, CMAH, β4galNT2, SLA-I α chain, and β-2 microglobulin) encoding three major carbohydrate xenoantigens (αGal, Neu5Gc, and Sda) and swine leukocyte antigen class I (SLA-I) molecules. Expression of HLA-E and/or HLA-G on pig cells were confirmed by flow cytometry. Endogenous HLA-G molecules as well as exogenous HLA-G VL9 peptide could dramatically enhance HLA-E expression on transfected pig cells. We found that co-expression of HLA-E and HLA-G on porcine cells led to a significant reduction in human NK cell activation compared to the cells expressing HLA-E or HLA-G alone and the parental cell line. NK cell activation was assessed by analysis of CD107a expression in CD3-CD56+ population gated from human peripheral blood mononuclear cells. CD107a is a sensitive marker of NK cell activation and correlates with NK cell degranulation and cytotoxicity. HLA-E and/or HLA-G on pig cells did not show reactivity to human sera IgG and IgM antibodies. This in vitro study demonstrated that co-expression of HLA-E and HLA-G on genetically modified porcine endothelial cells provided a superior inhibition in human xenoreactive NK cells, which may guide further genetic engineering of pigs to prevent human NK cell mediated rejection.

Cardiac xenotransplantation: from concept to clinic

For many patients with terminal/advanced cardiac failure, heart transplantation is the most effective, durable treatment option, and offers the best prospects for a high quality of life. The number of potentially life-saving donated human organs is far fewer than the population who could benefit from a new heart, resulting in increasing numbers of patients awaiting replacement of their failing heart, high waitlist mortality, and frequent reliance on interim mechanical support for many of those deemed among the best candidates but who are deteriorating as they wait. Currently, mechanical assist devices supporting left ventricular or biventricular heart function are the only alternative to heart transplant that is in clinical use. Unfortunately, the complication rate with mechanical assistance remains high despite advances in device design and patient selection and management, and the quality of life of the patients even with good outcomes is only moderately improved. Cardiac xenotransplantation from genetically multi-modified (GM) organ-source pigs is an emerging new option as demonstrated by the consistent long-term success of heterotopic (non-life-supporting) abdominal and life-supporting orthotopic porcine heart transplantation in baboons, and by a recent 'compassionate use' transplant of the heart from a GM pig with 10 modifications into a terminally ill patient who survived for 2 months. In this review, we discuss pig heart xenotransplantation as a concept, including pathobiological aspects related to immune rejection, coagulation dysregulation, and detrimental overgrowth of the heart, as well as GM strategies in pigs to prevent or minimize these problems. Additional topics discussed include relevant results of heterotopic and orthotopic heart transplantation experiments in the pig-to-baboon model, microbiological and virologic safety concepts, and efficacy requirements for initiating formal clinical trials. An adequate regulatory and ethical framework as well as stringent criteria for the selection of patients will be critical for the safe clinical development of cardiac xenotransplantation, which we expect will be clinically tested during the next few years.

Genetically engineered pigs for xenotransplantation: Hopes and challenges

The shortage of donor resources has greatly limited the application of clinical xenotransplantation. As such, genetically engineered pigs are expected to be an ideal organ source for xenotransplantation. Most current studies mainly focus on genetically modifying organs or tissues from donor pigs to reduce or prevent attack by the human immune system. Another potential organ source is interspecies chimeras. In this paper, we reviewed the progress of the genetically engineered pigs from the view of immunologic barriers and strategies, and discussed the possibility and challenges of the interspecies chimeras.

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.

Mechanisms and strategies to promote cardiac xenotransplantation

End stage heart failure is a terminal disease, and the only curative therapy is orthotopic heart transplantation. Due to limited organ availability, alternative strategies have received intense interest for treatment of patients with advanced heart failure. Recent studies using gene-edited porcine organs suggest that cardiac xenotransplantation may provide a future source of organs. In this review, we highlight the historical milestones for cardiac xenotransplantation and the gene editing strategies designed to overcome immunological barriers, which have culminated in a recent cardiac pig-to-human xenotransplant. We also discuss recent results of studies on the engineering of human-porcine chimeric organs that may provide an alternative and complementary strategy to overcome some of the major immunological barriers to producing a new source of transplantable organs.

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.

The Relative Abundances of Human Leukocyte Antigen-E, α-Galactosidase A and α-Gal Antigenic Determinants Are Biased by Trichostatin A-Dependent Epigenetic Transformation of Triple-Transgenic Pig-Derived Dermal Fibroblast Cells

The present study sought to establish the mitotically stable adult cutaneous fibroblast cell (ACFC) lines stemming from hFUT2×hGLA×HLA-E triple-transgenic pigs followed by trichostatin A (TSA)-assisted epigenetically modulating the reprogrammability of the transgenes permanently incorporated into the host genome and subsequent comprehensive analysis of molecular signatures related to proteomically profiling the generated ACFC lines. The results of Western blot and immunofluorescence analyses have proved that the profiles of relative abundance (RA) noticed for both recombinant human α-galactosidase A (rhα-Gal A) and human leukocyte antigen-E (HLA-E) underwent significant upregulations in tri-transgenic (3×TG) ACFCs subjected to TSA-mediated epigenetic transformation as compared to not only their TSA-unexposed counterparts but also TSA-treated and untreated non-transgenic (nTG) cells. The RT-qPCR-based analysis of porcine tri-genetically engineered ACFCs revealed stable expression of mRNA fractions transcribed from hFUT2, hGLA and HLA-E transgenes as compared to a lack of such transcriptional activities in non-transgenic ACFC variants. Furthermore, although TSA-based epigenomic modulation has given rise to a remarkable increase in the expression levels of Galα1→3Gal (α-Gal) epitopes that have been determined by lectin blotting analysis, their semi-quantitative profiles have dwindled profoundly in both TSA-exposed and unexposed 3×TG ACFCs as compared to their nTG counterparts. In conclusion, thoroughly exploring proteomic signatures in such epigenetically modulated ex vivo models devised on hFUT2×hGLA×HLA-E triple-transgenic ACFCs that display augmented reprogrammability of translational activities of two mRNA transcripts coding for rhα-Gal A and HLA-E proteins might provide a completely novel and powerful research tool for the panel of further studies. The objective of these future studies should be to multiply the tri-transgenic pigs with the aid of somatic cell nuclear transfer (SCNT)-based cloning for the purposes of both xenografting the porcine cutaneous bioprostheses and dermoplasty-mediated surgical treatments in human patients.

Strategies to induce natural killer cell tolerance in xenotransplantation

Eliminating major xenoantigens in pig cells has drastically reduced human antibody-mediated hyperacute xenograft rejection (HXR). Despite these advancements, acute xenograft rejection (AXR) remains one of the major obstacles to clinical xenotransplantation, mediated by innate immune cells, including macrophages, neutrophils, and natural killer (NK) cells. NK cells play an 'effector' role by releasing cytotoxicity granules against xenogeneic cells and an 'affecter' role on other immune cells through cytokine secretion. We highlight the key receptor-ligand interactions that determine the NK cell response to target cells, focusing on the regulation of NK cell activating receptor (NKG2D, DNAM1) and inhibitory receptor (KIR2DL1-4, NKG2A, and LIR-1) signaling pathways. Inhibition of NK cell activity may protect xenografts from cytotoxicity. Recent successful approaches to reducing NK cell-mediated HXR and AXR are reviewed, including genetic modifications of porcine xenografts aimed at improving pig-to-human compatibility. Future directions to promote xenograft acceptance are discussed, including NK cell tolerance in pregnancy and NK cell evasion in viral infection.

Xenotransplantation becoming reality

To bridge the gap between organ demand and supply, xenotransplantation has long been considered as a realistic option for end-stage organ failure. Early this year this promise became reality for David Bennett Sr., the first patient whose own failing heart was replaced with a xeno-pig heart. To get here has been a rollercoaster ride of physiological hurdles seemingly impossible to overcome, technological breakthroughs and ethical and safety concerns. It started in 1984, with Stephanie Fae Beauclair, also known as baby Fae, receiving a baboon heart, which allowed her to survive for another 30 days. For ethical reasons primate work was soon abandoned in favour of the pig. But increased phylogenetic distance also brought with it an increased immunological incompatibility. It has been the development of ever more sophisticated genetic engineering tools, which brought down the physiological barriers, enabled humanisation of porcine organs and helped addressing safety concerns. This renewed the confidence in xenotransplantation, brought new funding opportunities and resulted finally in the first in human trial.

HLA-G1+ Expression in GGTA1KO Pigs Suppresses Human and Monkey Anti-Pig T, B and NK Cell Responses

The human leukocyte antigen G1 (HLA-G1), a non-classical class I major histocompatibility complex (MHC-I) protein, is a potent immunomodulatory molecule at the maternal/fetal interface and other environments to regulate the cellular immune response. We created GGTA1-/HLAG1+ pigs to explore their use as organ and cell donors that may extend xenograft survival and function in both preclinical nonhuman primate (NHP) models and future clinical trials. In the present study, HLA-G1 was expressed from the porcine ROSA26 locus by homology directed repair (HDR) mediated knock-in (KI) with simultaneous deletion of α-1-3-galactotransferase gene (GGTA1; GTKO) using the clustered regularly interspersed palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9) (CRISPR/Cas9) gene-editing system. GTKO/HLAG1+ pigs showing immune inhibitory functions were generated through somatic cell nuclear transfer (SCNT). The presence of HLA-G1 at the ROSA26 locus and the deletion of GGTA1 were confirmed by next generation sequencing (NGS) and Sanger's sequencing. Fibroblasts from piglets, biopsies from transplantable organs, and islets were positive for HLA-G1 expression by confocal microscopy, flow cytometry, or q-PCR. The expression of cell surface HLA-G1 molecule associated with endogenous β2-microglobulin (β2m) was confirmed by staining genetically engineered cells with fluorescently labeled recombinant ILT2 protein. Fibroblasts obtained from GTKO/HLAG1+ pigs were shown to modulate the immune response by lowering IFN-γ production by T cells and proliferation of CD4+ and CD8+ T cells, B cells and natural killer (NK) cells, as well as by augmenting phosphorylation of Src homology region 2 domain-containing phosphatase-2 (SHP-2), which plays a central role in immune suppression. Islets isolated from GTKO/HLA-G1+ genetically engineered pigs and transplanted into streptozotocin-diabetic nude mice restored normoglycemia, suggesting that the expression of HLA-G1 did not interfere with their ability to reverse diabetes. The findings presented here suggest that the HLA-G1+ transgene can be stably expressed from the ROSA26 locus of non-fetal maternal tissue at the cell surface. By providing an immunomodulatory signal, expression of HLA-G1+ may extend survival of porcine pancreatic islet and organ xenografts.

Pathways to Clinical Cardiac Xenotransplantation

Heart transplantation is the only long-lasting lifesaving option for patients with terminal cardiac failure. The number of available human organs is however far below the actual need, resulting in substantial mortality of patients while waiting for a human heart. Mechanical assist devices are used to support cardiac function but are associated with a high risk of severe complications and poor quality of life for the patients. Consistent success in orthotopic transplantation of genetically modified pig hearts into baboons indicates that cardiac xenotransplantation may become a clinically applicable option for heart failure patients who cannot get a human heart transplant. In this overview, we project potential paths to clinical cardiac xenotransplantation, including the choice of genetically modified source pigs; associated requirements of microbiological, including virological, safety; optimized matching of source pig and recipient; and specific treatments of the donor heart after explantation and of the recipients. Moreover, selection of patients and the regulatory framework will be discussed.

HLA Class I Molecules as Immune Checkpoints for NK Cell Alloreactivity and Anti-Viral Immunity in Kidney Transplantation

Natural killer (NK) cells are innate lymphocytes that can kill diseased- or virally-infected cells, mediate antibody dependent cytotoxicity and produce type I immune-associated cytokines upon activation. NK cells also contribute to the allo-immune response upon kidney transplantation either by promoting allograft rejection through lysis of cells of the transplanted organ or by promoting alloreactive T cells. In addition, they protect against viral infections upon transplantation which may be especially relevant in patients receiving high dose immune suppression. NK cell activation is tightly regulated through the integrated balance of signaling via inhibitory- and activating receptors. HLA class I molecules are critical regulators of NK cell activation through the interaction with inhibitory- as well as activating NK cell receptors, hence, HLA molecules act as critical immune checkpoints for NK cells. In the current review, we evaluate how NK cell alloreactivity and anti-viral immunity are regulated by NK cell receptors belonging to the KIR family and interacting with classical HLA class I molecules, or by NKG2A/C and LILRB1/KIR2DL4 engaging non-classical HLA-E or -G. In addition, we provide an overview of the methods to determine genetic variation in these receptors and their HLA ligands.

The Role of SLAs in Xenotransplantation

Advances in genetic engineering, particularly CRISPR/Cas9, have resulted in the development of a triple glycan-knockout (TKO) pig. There is minimal human antipig antibody binding to TKO pig cells. The TKO background has decreased antibody binding to a sufficiently low level that any additional xenoantigens expressed on the cells can now be more easily detected. One of these xenoantigens is the swine major histocompatibility complex, termed swine leukocyte antigens (SLA). SLA are the homolog to HLAs, a protein complex expressed on human tissue capable of stimulating the development of new antibodies in allotransplantation. These antibodies can result in graft failure through hyperacute, acute, or chronic rejection. Our knowledge of SLA, particularly in the last 5 years, has grown considerably. The presence, cause, and methods to detect anti-SLA antibodies will need to be carefully considered for the first clinical trial of xenotransplantation. The focus of this review is to summarize the role of SLA in xenotransplantation and consider whether it will prove to be a major barrier. Techniques are now available to mutate target SLA amino acids to ensure that cross-reactive anti-HLA antibodies no longer bind to SLA on the cells of the organ-source pigs. While deletion of SLA expression is possible, it would render the pig at risk for infectious complications. The ideal organ-source pig for HLA highly sensitized recipients may therefore be 1 with site-specific mutations to eliminate cross-reactive binding.

Trichostatin A-Assisted Epigenomic Modulation Affects the Expression Profiles of Not Only Recombinant Human α1,2-Fucosyltransferase and α-Galactosidase A Enzymes But Also Galα1→3Gal Epitopes in Porcine Bi-Transgenic Adult Cutaneous Fibroblast Cells

This study was conducted to explore whether trichostatin A-assisted epigenomic modulation (TSA-EM) can affect the expression of not only recombinant human α1,2-fucosyltransferase (rhα1,2-FT) and α-galactosidase A (rhα-Gal A) immune system enzymes but also Galα1→3Gal epitopes in ex vivo proliferating adult cutaneous fibroblast cells (ACFCs) derived from hFUT2×hGLA bi-transgenic pigs that had been produced for the needs of future xenotransplantation efforts. The ACFC lines were treated with 50 nM TSA for 24 h and then the expression profiles of rhα1,2-FT and rhα-Gal A enzymes were analyzed by Western blot and immunofluorescence. The expression profiles of the Galα1→3Gal epitope were determined by lectin blotting and lectin fluorescence. The ACFCs derived from non-transgenic (nTG) pigs were served as the negative (TSA-) and positive (TSA+) control groups. For both hFUT2×hGLA and nTG samples, the expression levels of α1,2-FT and α-Gal A proteins in TSA+ cells were more than twofold higher in comparison to TSA- cells. Moreover, a much lower expression of the Galα1→3Gal epitopes was shown in TSA- hFUT2×hGLA cells as compared to the TSA- nTG group. Interestingly, the levels of Galα1→3Gal expression in TSA-treated hFUT2×hGLA and nTG ACFCs were significantly higher than those noticed for their TSA-untreated counterparts. Summing up, ex vivo protection of effectively selected bi-transgenic ACFC lines, in which TSA-dependent epigenetic transformation triggered the enhancements in reprogrammability and subsequent expression of hFUT2 and hGLA transgenes and their corresponding transcripts, allows for cryopreservation of nuclear donor cells, nuclear-transferred female gametes, and resultant porcine cloned embryos. The latter can be used as a cryogenically conserved genetic resource of biological materials suitable for generation of bi-transgenic cloned offspring in pigs that is targeted at biomedical research in the field of cell/tissue xenotransplantation.

Immunogenetics of xenotransplantation

Xenotransplantation may become the highly desired solution to close the gap between the availability of donated organs and number of patients on the waiting list. In recent years, enormous progress has been made in the development of genetically engineered donor pigs. The introduced genetic modifications showed to be efficient in prolonging xenograft survival. In this review, we focus on the type of immune responses that may target xeno-organs after transplantation and promising immunogenetic modifications that show a beneficial effect in ameliorating or eliminating harmful xenogeneic immune responses. Increasing histocompatibility of xenografts by eliminating genetic discrepancies between species will pave their way into clinical application.

Utilizing Xenogeneic Cells As a Therapeutic Agent for Treating Diseases

The utilization of biologically produced cells to treat diseases is a revolutionary invention in modern medicine after chemically synthesized small molecule drugs and biochemically made protein drugs. Cells are basic units of life with diverse functions in mature and developing organs, which biological properties could be utilized as a promising therapeutic approach for currently intractable and incurable diseases. Xenogeneic cell therapy utilizing animal cells other than human for medicinal purpose has been studied as a new way of treating diseases. Xenogeneic cell therapy is considered as a potential regenerative approach to fulfill current unmet medical needs because xenogeneic cells could be isolated from different animal organs and expanded ex vivo as well as maintain the characteristics of original organs, providing a versatile and plenty cell source for cell-based therapeutics beside autologous and allogeneic sources. The swine species is considered the most suitable source because of the similarity with humans in size and physiology of many organs in addition to the economic and ethical reasons plus the possibility of genetic modification. This review discusses the old proposed uses of xenogeneic cells such as xenogeneic pancreatic islet cells, hepatocytes and neuronal cells as a living drug for the treatment of degenerative and organ failure diseases. Novel applications of xenogeneic mesenchymal stroma cells and urothelial cells are also discussed. There are formidable immunological barriers toward successful cellular xenotransplantation in clinic despite major progress in the development of novel immunosuppression regimens and genetically multimodified donor pigs. However, immunological barriers could be turn into immune boosters by using xenogeneic cells of specific tissue types as a novel immunotherapeutic agent to elicit bystander antitumor immunity due to rejection immune responses. Xenogeneic cells have the potential to become a safe and efficacious option for intractable diseases and hard-to-treat cancers, adding a new class of cellular medicine in our drug armamentarium.

Porcine genome engineering for xenotransplantation

The extreme shortage of human donor organs for treatment of patients with end-stage organ failures is well known. Xenotransplantation, which might provide unlimited organ supply, is a most promising strategy to solve this problem. Domestic pigs are regarded as ideal organ-source animals owing to similarity in anatomy, physiology and organ size to humans as well as high reproductive capacity and low maintenance cost. However, several barriers, which include immune rejection, inflammation and coagulative dysfunctions, as well as the cross-species transmission risk of porcine endogenous retrovirus, blocked the pig-to-human xenotransplantation. With the rapid development of genome engineering technologies and the potent immunosuppressive medications in recent years, these barriers could be eliminated through genetic modification of pig genome together with the administration of effective immunosuppressants. A number of candidate genes involved in the regulation of immune response, inflammation and coagulation have been explored to optimize porcine xenograft survival in non-human primate recipients. PERV inactivation in pigs has also been accomplished to firmly address the safety issue in pig-to-human xenotransplantation. Many encouraging preclinical milestones have been achieved with some organs surviving for years. Therefore, the clinical trials of some promising organs, such as islet, kidney and heart, are aimed to be launched in the near future.

[Novel platelet pharming using human induced pluripotent stem cells]

La production in vitro de plaquettes offre une opportunité de résoudre les problèmes liés aux limitations d’approvisionnement et à la sécurité des dons de produits dérivés du sang. Les cellules souches pluripotentes induites – ou iPSC – sont une source idéale pour la production de cellules à des fins de thérapies régénératives. Nous avons précédemment établi avec succès une lignée mégacaryocytaire immortalisée à partir d’iPSC. Celle-ci possède une capacité de prolifération fiable. Par ailleurs, il est possible de les cryoconserver. Elle est donc une source adaptée de cellules primaires pour la production de plaquettes suivant les Bonnes Pratiques de Fabrication (BPF). Dans le même temps, la capacité améliorée des bioréacteurs à reproduire certaines conditions physiologiques, telle que la turbulence, de pair avec la découverte de molécules favorisant la thrombopoïèse, a contribué à l’accomplissement de la production de plaquettes en quantité et qualité suffisantes pour répondre aux besoins cliniques. La production de plaquettes à partir de cellules iPS s’étend aussi aux patients en état de réfraction allo-immune, par la production de plaquettes autologues ou dont on a génétiquement manipulé l’expression des Antigènes des Leucocytes Humains (HLA) et des Antigènes Plaquettaires Humain (HPA). Considérant ces avancées fondamentales, les plaquettes iPSC avec expression des HLA modifiées se présentent comme un potentiel produit de transfusion universel. Dans cette revue, nous souhaitons apporter une vue d’ensemble de la production in vitro de plaquettes à partir de cellules iPS, et de son possible potentiel transformatif, d’importance capitale dans le domaine de la transfusion des produits sanguins.

Skin xenotransplantation: technological advances and future directions

PURPOSE OF REVIEW

To summarize the evolution of skin xenotransplantation and contextualize technological advances and the status of clinically applicable large animal research as well as prospects for translation of this work as a viable future treatment option.

RECENT FINDINGS

Porcine xenografts at the start of the millennium were merely biologic dressings subject to rapid rejection. Since then, numerous important advances in swine to nonhuman primate models have yielded xenotransplant products at the point of clinical translation. Critical genetic modifications in swine from a designated pathogen-free donor herd have allowed xenograft survival reaching 30 days without preconditioning or maintenance immunosuppression. Further, xenograft coverage appears not to sensitize the recipient to subsequent allograft placement and vice versa, allowing for temporary coverage times to be doubled using both xeno and allografts.

SUMMARY

Studies in large animal models have led to significant progress in the creation of living, functional skin xenotransplants with clinically relevant shelf-lives to improve the management of patients with extensive burns.

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.

Application of Genetically Engineered Pigs in Biomedical Research

Progress in genetic engineering over the past few decades has made it possible to develop methods that have led to the production of transgenic animals. The development of transgenesis has created new directions in research and possibilities for its practical application. Generating transgenic animal species is not only aimed towards accelerating traditional breeding programs and improving animal health and the quality of animal products for consumption but can also be used in biomedicine. Animal studies are conducted to develop models used in gene function and regulation research and the genetic determinants of certain human diseases. Another direction of research, described in this review, focuses on the use of transgenic animals as a source of high-quality biopharmaceuticals, such as recombinant proteins. The further aspect discussed is the use of genetically modified animals as a source of cells, tissues, and organs for transplantation into human recipients, i.e., xenotransplantation. Numerous studies have shown that the pig (Sus scrofa domestica) is the most suitable species both as a research model for human diseases and as an optimal organ donor for xenotransplantation. Short pregnancy, short generation interval, and high litter size make the production of transgenic pigs less time-consuming in comparison with other livestock species This review describes genetically modified pigs used for biomedical research and the future challenges and perspectives for the use of the swine animal models.

Triple (GGTA1, CMAH, B2M) modified pigs expressing an SLA class Ilow phenotype-Effects on immune status and susceptibility to human immune responses

Porcine xenografts lacking swine leukocyte antigen (SLA) class I are thought to be protected from human T cell responses. We have previously shown that SLA class I deficiency can be achieved in pigs by CRISPR/Cas9-mediated deletion of β₂ -microglobulin (B2M). Here, we characterized another line of genetically modified pigs in which targeting of the B2M locus did not result in complete absence of B2M and SLA class I but rather in significantly reduced expression levels of both molecules. Residual SLA class I was functionally inert, because no proper differentiation of the CD8⁺ T cell subset was observed in B2M^low pigs. Cells from B2M^low pigs were less capable in triggering proliferation of human peripheral blood mononuclear cells in vitro, which was mainly due to the nonresponsiveness of CD8⁺ T cells. Nevertheless, cytotoxic effector cells developing from unaffected cell populations (eg, CD4⁺ T cells, natural killer cells) lysed targets from both SLA class I⁺ wildtype and SLA class I^low pigs with similar efficiency. These data indicate that the absence of SLA class I is an effective approach to prevent the activation of human CD8⁺ T cells during the induction phase of an anti-xenograft response. However, cytotoxic activity of cells during the effector phase cannot be controlled by this approach.

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.

Role of human and porcine MHC DRB1 alleles in determining the intensity of individual human anti-pig T-cell responses

BACKGROUND

Differences in quality and strength of immune responses between individuals are mainly due to polymorphisms in major histocompatibility complex (MHC) molecules. Focusing on MHC class-II, we asked whether the intensity of human anti-pig T-cell responses is influenced by genetic variability in the human HLA-DRB1 and/or the porcine SLA-DRB1 locus.

METHODS

ELISpot assays were performed using peripheral blood mononuclear cells (PBMCs) from 62 HLA-DRB1-typed blood donors as responder and the porcine B cell line L23 as stimulator cells. Based on the frequency of IFN-γ-secreting cells, groups of weak, medium, and strong responder individuals were defined. Mixed lymphocyte reaction (MLR) assays were performed to study the stimulatory capacity of porcine PBMCs expressing different SLA-DRB1 alleles.

RESULTS

Concerning the MHC class-II configuration of human cells, we found a significant overrepresentation of HLA-DRB1*01 alleles in the medium/strong responder group as compared to individuals showing weak responses to stimulation with L23 cells. Evaluation of the role of MHC class-II variability in porcine stimulators revealed that cells expressing SLA-DRB1*06 alleles triggered strong proliferation in approximately 70% of humans. Comparison of amino acid sequences indicated that strong human anti-pig reactivity may be associated with a high rate of similarity between human and pig HLA/SLA-DRB1 alleles.

CONCLUSION

Variability in human and porcine MHC determines the intensity of individual human anti-pig T-cell responses. MHC typing and cross-matching of prospective recipients of xenografts and donor pigs could be relevant to select for donor-recipient combinations with minimal anti-porcine immunity.

Using genome editing to engineer universal platelets

Genome editing technologies such as zinc finger nucleases, TALENs and CRISPR/Cas9 have recently emerged as tools with the potential to revolutionise cellular therapy. This is particularly exciting for the field of regenerative medicine, where the large-scale, quality-controlled editing of large numbers of cells could generate essential cellular products ready to move towards the clinic. This review details recent progress towards generating HLA Class I null platelets using genome editing technologies for β2-microglobulin deletion, generating a universally transfusable cellular product. In addition, we discuss various methods for megakaryocyte (MK) production from human pluripotent stem cells and subsequent platelet production from the MKs. As well as simply producing platelets, differentiating MK cultures can enable us to understand megakaryopoiesis in vivo and take steps towards ameliorating bleeding disorders or deficiencies in MK maturation in patients. Thus by intersecting both these areas of research, we can produce optimised differentiation systems for the production of universal platelets, thus offering a stable supply of platelets for difficult-to-match patients and providing areas with transmissible disease concerns or an unpredictable supply of platelets with a steady supply of quality-controlled platelet units.

Genetically-engineered pigs as sources for clinical red blood cell transfusion: What pathobiological barriers need to be overcome?

An alternative to human red blood cells (RBCs) for clinical transfusion would be advantageous, particularly in situations of massive acute blood loss (where availability and compatibility are limited) or chronic hematologic diseases requiring frequent transfusions (resulting in alloimmunization). Ideally, any alternative must be neither immunogenic nor pathogenic, but readily available, inexpensive, and physiologically effective. Pig RBCs (pRBCs) provide a promising alternative due to their several similarities with human RBCs, and our increasing ability to genetically-modify pigs to reduce cellular immunogenicity. We briefly summarize the history of xenotransfusion, the progress that has been made in recent years, and the remaining barriers. These barriers include prevention of (i) human natural antibody binding to pRBCs, (ii) their phagocytosis by macrophages, and (iii) the T cell adaptive immune response (in the absence of exogenous immunosuppressive therapy). Although techniques of genetic engineering have advanced in recent years, novel methods to introduce human transgenes into pRBCs (which do not have nuclei) will need to be developed before clinical trials can be initiated.

IXA Honorary Member Lecture, 2017: The long and winding road to tolerance

The last 15 years or so have seen exciting progress in xenotransplantation, with porcine organ grafts surviving months or even years in non-human primates. These advances reflect the application of new scientific knowledge, improved immunosuppressive agents, and genetic engineering. The field has recently enjoyed a renaissance of interest and hope, largely due to the exponential increase in our capacity to genetically engineer porcine source animals. However, immune responses to xenografts are very powerful and widespread clinical application of xenotransplantation will depend on the ability to suppress these immune responses while preserving the capacity to protect both the recipient and the graft from infectious microorganisms. Our work over the last three decades has aimed to engineer the immune system of the recipient in a manner that achieves specific tolerance to the xenogeneic donor while preserving otherwise normal immune function. Important proofs of principle have been obtained, first in rodents, and later in human immune systems in "humanized mice" and finally in non-human primates, demonstrating the capacity and potential synergy of mixed xenogeneic chimerism and xenogeneic thymic transplantation in tolerizing multiple arms of the immune system. Considering the fact that clinical tolerance has recently been achieved for allografts and the even greater importance of avoiding excessive immunosuppression for xenografts, it is my belief that it is both possible and imperative that we likewise achieve xenograft tolerance. I expect this to be accomplished through the availability of targeted approaches to recipient immune conditioning, understanding of immunological mechanisms of tolerance, advanced knowledge of physiological incompatibilities, and the availability of inbred miniature swine with optimized use of genetic engineering.

Characterization of Three Generations of Transgenic Pigs Expressing the HLA-E Gene

The use of pigs as a source of organs and tissues for xenotransplantation can overcome the growing shortage of human donors. Human NK cells play an important role in the cell-mediated rejection of pig-to-human xenografts. In this paper we report the generation and extensive characterization of three generations of transgenic pigs with HLA-E gene encoding the antigen which can inhibit the human NK cell-mediated response. The gene construct pHLAE-GFPBsd containing the human gene encoding the human leukocyte antigen under the promoter of the EF-1α elongation factor ensuring systemic expression was introduced by microinjection into a pronucleus of the fertilized porcine oocyte. PCR analysis revealed and FISH analysis confirmed that the pHLAE-GFPBsd gene construct was present in the genome of the founder female pig. As a result of inter-breeding, an additional 7 transgenic animals were obtained (one individual from F1 generation and six individuals from F2 generation). The transgene expression was shown by RT-PCR and flow cytometry. Real Time PCR analysis estimated the approximate number of transgene copies at 16–34. Karyotype analysis did not show any changes in the structure or the number of chromosomes. The expression level of the transgene was stable in the next generation of genetically modified pigs. An NK cell-mediated cytotoxicity assay showed the increased viability of the transgenic cells in comparison with the wild-type, which confirmed the protective influence of HLA-E expression.

Will Genetic Engineering Carry Xenotransplantation of Pig Islets to the Clinic?

PURPOSE OF REVIEW

Porcine islets represent a potentially attractive beta-cell source for xenotransplantation into patients with type 1 diabetes, who are not eligible to islet allo-transplantation due to a lack of suitable human donor organs. Recent progress in genetic engineering/gene editing of donor pigs provides new opportunities to overcome rejection of xeno-islets, to improve their engraftment and insulin secretion capacity, and to reduce the risk for transmission of porcine endogenous retroviruses. This review summarizes the current issues and progress in islet xenotransplantation with special emphasis on genetically modified/gene edited donor pigs.

RECENT FINDINGS

Attempts to overcome acute rejection of xeno-islets, especially after intraportal transplantation into the liver, include the genetic elimination of specific carbohydrate antigens such as αGal, Neu5Gc, and Sd(a) for which humans and-in part-non-human primates have natural antibodies that bind to these targets leading to activation of complement and coagulation. A complementary approach is the expression of one or more human complement regulatory proteins (hCD46, hCD55, hCD59). Transgenic attempts to overcome cellular rejection of islet xenotransplants include the expression of proteins that inhibit co-stimulation of T cells. Expression of glucagon-like peptide-1 and M3 muscarinic receptors has been shown to increase the insulin secretion of virally transduced porcine islets in vitro and it will be interesting to see the effects of these modifications in transgenic pigs and islet products derived from them. Genome-wide inactivation of porcine endogenous retrovirus (PERV) integrants by mutating their pol genes using CRISPR/Cas9 is a recent approach to reduce the risk for PERV transmission by xeno-islets. Genetic engineering/gene editing of xeno-islet donor pigs facilitated major progress towards clinical islet xenotransplantation. The required set of genetic modifications will depend on the source of islets (fetal/neonatal vs. adult), the mode of delivery (encapsulated vs. free), and the transplantation site.

Clustered Regularly Interspaced Short Palindromic Repeats/Cas9 Gene Editing Technique in Xenotransplantation

Genetically modified pigs have been considered favorable resources in xenotransplantation. Microinjection of randomly integrating transgenes into zygotes, somatic cell nuclear transfer, homologous recombination, zinc finger nucleases, transcription activator-like effector nucleases, and most recently, clustered regularly interspaced short palindromic repeats-cas9 (CRISPR/Cas9) are the techniques that have been used to generate these animals. Here, we provide an overview of the CRISPR approaches that have been used to modify genes which are vital in improving xenograft survival rate, including cytidine monophosphate-N-acetylneuraminic acid hydroxylase, B1,4N-acetylgalactosaminyltransferase, isoglobotrihexosylceramide synthase, class I MHC, von Willebrand factor, C3, and porcine endogenous retroviruses. In addition, we will mention the importance of potential candidate genes which could be targeted using CRISPR/Cas9.

Skin xenotransplantation: Historical review and clinical potential

Half a million patients in the USA alone require treatment for burns annually. Following an extensive burn, it may not be possible to provide sufficient autografts in a single setting. Pig skin xenografts may provide temporary coverage. However, preformed xenoreactive antibodies in the human recipient activate complement, and thus result in rapid rejection of the graft. Because burn patients usually have some degree of immune dysfunction and are therefore at increased risk of infection, immunosuppressive therapy is undesirable. Genetic engineering of the pig has increased the survival of pig heart, kidney, islet, and corneal grafts in immunosuppressed non-human primates from minutes to months or occasionally years. We summarize the current status of research into skin xenotransplantation for burns, with special emphasis on developments in genetic engineering of pigs to protect the graft from immunological injury. A genetically-engineered pig skin graft now survives as long as an allograft and, importantly, rejection of a skin xenograft is not detrimental to a subsequent allograft. Nevertheless, currently, systemic immunosuppressive therapy would still be required to inhibit a cellular response, and so we discuss what further genetic manipulations could be carried out to inhibit the cellular response.