Targeted insertion of an anti-CD2 monoclonal antibody transgene into the GGTA1 locus in pigs using FokI-dCas9

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.

Characterisation of transgenic pigs expressing a human T cell-depleting anti-CD2 monoclonal antibody

BACKGROUND

Pig islet xenotransplantation is a potential treatment for type 1 diabetes. We have shown that maintenance immunosuppression is required to protect genetically modified (GM) porcine islet xenografts from T cell-mediated rejection in baboons. Local expression of a depleting anti-CD2 monoclonal antibody (mAb) by the xenograft may provide an alternative solution. We have previously reported the generation of GGTA1 knock-in transgenic pigs expressing the chimeric anti-CD2 mAb diliximab under an MHC class I promoter (MHCIP). In this study, we generated GGTA1 knock-in pigs in which MHCIP was replaced by the β-cell-specific porcine insulin promoter (PIP), and compared the pattern of diliximab expression in the two lines.

METHODS

A PIP-diliximab knock-in construct was prepared and validated by transfection of NIT-1 mouse insulinoma cells. The construct was knocked into GGTA1 in wild type (WT) porcine fetal fibroblasts using CRISPR, and knock-in cells were used to generate pigs by somatic cell nuclear transfer (SCNT). Expression of the transgene in MHCIP-diliximab and PIP-diliximab knock-in pigs was characterised at the mRNA and protein levels using RT-qPCR, flow cytometry, ELISA and immunohistochemistry. Islets from MHCIP-diliximab and control GGTA1 KO neonatal pigs were transplanted under the kidney capsule of streptozotocin-diabetic SCID mice.

RESULTS

NIT-1 cells stably transfected with the PIP-diliximab knock-in construct secreted diliximab into the culture supernatant, confirming correct expression and processing of the mAb in β cells. PIP-diliximab knock-in pigs showed a precise integration of the transgene within GGTA1. Diliximab mRNA was detected in all tissues tested (spleen, kidney, heart, liver, lung, pancreas) in MHCIP-diliximab pigs, but was not detectable in PIP-diliximab pigs. Likewise, diliximab was present in the serum of MHCIP-diliximab pigs, at a mean concentration of 1.8 μg/mL, but was not detected in PIP-diliximab pig serum. An immunohistochemical survey revealed staining for diliximab in all organs of MHCIP-diliximab pigs but not of PIP-diliximab pigs. Whole genome sequencing (WGS) of a PIP-diliximab pig identified a missense mutation in the coding region for the dixilimab light chain. This mutation was also found to be present in the fibroblast knock-in clone used to generate the PIP-diliximab pigs. Islet xenografts from neonatal MHCIP-diliximab pigs restored normoglycemia in diabetic immunodeficient mice, indicating no overt effect of the transgene on islet function, and demonstrated expression of diliximab in situ.

CONCLUSION

Diliximab was widely expressed in MHCIP-diliximab pigs, including in islets, consistent with the endogenous expression pattern of MHC class I. Further investigation is required to determine whether the level of expression in islets from the MHCIP-diliximab pigs is sufficient to prevent T cell-mediated islet xenograft rejection. The unexpected absence of diliximab expression in the islets of PIP-diliximab pigs was probably due to a mutation in the transgene arising during the generation of the knock-in cells used for SCNT.

Modeling human anti-pig xenoimmune responses in a pig artery tissue grafted humanized mouse model

BACKGROUND

Blood vessels that contain endothelial cells (ECs) on the surface are in direct contact with host blood and are the first target of xenograft rejection. Currently, our understanding of human anti-pig vessel immune responses is primarily based on in vitro assays using pig ECs. Therefore, it is necessary to develop an animal model that permits in vivo study of human immunological rejection of pig vessels.

METHODS

Pig artery tissues (PAT) were transplanted into human immune system (HIS) mice or immunodeficient NSG mice (as controls). Intragraft human immune cell infiltration and antibody deposition were quantified using histology and immunohistochemistry. Donor antigen-specific immune responses were quantified using a mixed lymphocyte reaction and a complement-dependent killing assay.

RESULTS

Pig CD31+ ECs were detected and increased 2-fold from weeks 3 to 5 in PAT xenografts from immunodeficient NSG mice. However, compared with NSG mice, PAT xenografts in HIS mice had significantly lower numbers of porcine CD31+ ECs and showed a marked reduction from week 3 to week 5. PAT xenograft rejection in HIS mice is associated with intensive infiltration of human immune cells, deposition of human IgM and IgG antibodies, and the formation of a tertiary lymphoid structure. Robust donor pig antigen-specific human T cells and antibody responses were detected in PAT-transplanted HIS mice.

CONCLUSION

We have developed a humanized mouse model to evaluate human anti-pig xenoimmune responses by PAT transplantation in vivo. This model is expected to facilitate the refinement of pig gene-editing strategies (the expression on EC surface) and the testing of local immunosuppressive strategies for clinical pig organ xenotransplantation.

Activation of immune signals during organ transplantation

The activation of host's innate and adaptive immune systems can lead to acute and chronic graft rejection, which seriously impacts graft survival. Thus, it is particularly significant to clarify the immune signals, which are critical to the initiation and maintenance of rejection generated after transplantation. The initiation of response to graft is dependent on sensing of danger and stranger molecules. The ischemia and reperfusion of grafts lead to cell stress or death, followed by releasing a variety of damage-associated molecular patterns (DAMPs), which are recognized by pattern recognition receptors (PRRs) of host immune cells to activate intracellular immune signals and induce sterile inflammation. In addition to DAMPs, the graft exposed to 'non-self' antigens (stranger molecules) are recognized by the host immune system, stimulating a more intense immune response and further aggravating the graft damage. The polymorphism of MHC genes between different individuals is the key for host or donor immune cells to identify heterologous 'non-self' components in allogeneic and xenogeneic organ transplantation. The recognition of 'non-self' antigen by immune cells mediates the activation of immune signals between donor and host, resulting in adaptive memory immunity and innate trained immunity to the graft, which poses a challenge to the long-term survival of the graft. This review focuses on innate and adaptive immune cells receptor recognition of damage-associated molecular patterns, alloantigens and xenoantigens, which is described as danger model and stranger model. In this review, we also discuss the innate trained immunity in organ transplantation.

Genome Editing of Pig

Pigs have anatomical and physiological characteristics similar to humans; therefore, genetically modified pigs have the potential to become a valuable bioresource in biomedical research. In fact, considering the increasing need for translational research, pigs are useful for studying intractable diseases, organ transplantation, and regenerative medicine as large-scale experimental animals with excellent potential for extrapolation to humans. With the advent of zinc finger nucleases (ZFNs), breakthroughs in genome editing tools such as transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR associated protein 9 (Cas9) have facilitated the efficient generation of genetically modified pigs. Genome editing has been used in pigs for more than 10 years; now, along with knockout pigs, knock-in pigs are also gaining increasing importance. In this chapter, we describe the establishment of gene-modified cells (nuclear donor cells), which are necessary for gene knockout and production of knock-in pigs via somatic cell nuclear transplantation, as well as the production of gene knockout pigs using a simple cytoplasmic injection method.

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.

The birth of Dolly and xenotransplantation 25 years on

A number of reviews have been written recently celebrating the 25th anniversary of the birth of Dolly the cloned sheep and the effect this breakthrough has had on various fields of research. However, arguably the biggest impact Dolly has had is on the field of xenotransplantation, described here based on our own experience and that of others.

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.

Cellular Immune Responses in Islet Xenograft Rejection

Porcine islets surviving the acute injury caused by humoral rejection and IBMIR will be subjected to cellular xenograft rejection, which is predominately mediated by CD4⁺ T cells and is characterised by significant infiltration of macrophages, B cells and T cells (CD4⁺ and CD8⁺). Overall, the response is different compared to the alloimmune response and more difficult to suppress. Activation of CD4⁺ T cells is both by direct and indirect antigen presentation. After activation they recruit macrophages and direct B cell responses. Although they are less important than CD4⁺ T cells in islet xenograft rejection, macrophages are believed to be a major effector cell in this response. Rodent studies have shown that xenoantigen-primed and CD4⁺ T cell-activated macrophages were capable of recognition and rejection of pancreatic islet xenografts, and they destroyed a graft via the secretion of various proinflammatory mediators, including TNF-α, reactive oxygen and nitrogen species, and complement factors. B cells are an important mediator of islet xenograft rejection via xenoantigen presentation, priming effector T cells and producing xenospecific antibodies. Depletion and/or inhibition of B cells combined with suppressing T cells has been suggested as a promising strategy for induction of xeno-donor-specific T- and B-cell tolerance in islet xenotransplantation. Thus, strategies that expand the influence of regulatory T cells and inhibit and/or reduce macrophage and B cell responses are required for use in combination with clinical applicable immunosuppressive agents to achieve effective suppression of the T cell-initiated xenograft response.

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.

Current status of the application of gene editing in pigs

Genetically modified animals, especially rodents, are widely used in biomedical research. However, non-rodent models are required for efficient translational medicine and preclinical studies. Owing to the similarity in the physiological traits of pigs and humans, genetically modified pigs may be a valuable resource for biomedical research. Somatic cell nuclear transfer (SCNT) using genetically modified somatic cells has been the primary method for the generation of genetically modified pigs. However, site-specific gene modification in porcine cells is inefficient and requires laborious and time-consuming processes. Recent improvements in gene-editing systems, such as zinc finger nucleases, transcription activator-like effector nucleases, and the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (CRISPR/Cas) system, represent major advances. The efficient introduction of site-specific modifications into cells via gene editors dramatically reduces the effort and time required to generate genetically modified pigs. Furthermore, gene editors enable direct gene modification during embryogenesis, bypassing the SCNT procedure. The application of gene editors has progressively expanded, and a range of strategies is now available for porcine gene engineering. This review provides an overview of approaches for the generation of genetically modified pigs using gene editors, and highlights the current trends, as well as the limitations, of gene editing in pigs.

CRISPR/Cas Technology in Pig-to-Human Xenotransplantation Research

CRISPR/Cas (clustered regularly interspaced short palindromic repeats linked to Cas nuclease) technology has revolutionized many aspects of genetic engineering research. Thanks to it, it became possible to study the functions and mechanisms of biology with greater precision, as well as to obtain genetically modified organisms, both prokaryotic and eukaryotic. The changes introduced by the CRISPR/Cas system are based on the repair paths of the single or double strand DNA breaks that cause insertions, deletions, or precise integrations of donor DNA. These changes are crucial for many fields of science, one of which is the use of animals (pigs) as a reservoir of tissues and organs for xenotransplantation into humans. Non-genetically modified animals cannot be used to save human life and health due to acute immunological reactions resulting from the phylogenetic distance of these two species. This review is intended to collect and summarize the advantages as well as achievements of the CRISPR/Cas system in pig-to-human xenotransplantation research. In addition, it demonstrates barriers and limitations that require careful evaluation before attempting to experiment with this technology.

Improvements in Gene Editing Technology Boost Its Applications in Livestock

Accelerated development of novel CRISPR/Cas9-based genome editing techniques provides a feasible approach to introduce a variety of precise modifications in the mammalian genome, including introduction of multiple edits simultaneously, efficient insertion of long DNA sequences into specific targeted loci as well as performing nucleotide transitions and transversions. Thus, the CRISPR/Cas9 tool has become the method of choice for introducing genome alterations in livestock species. The list of new CRISPR/Cas9-based genome editing tools is constantly expanding. Here, we discuss the methods developed to improve efficiency and specificity of gene editing tools as well as approaches that can be employed for gene regulation, base editing, and epigenetic modifications. Additionally, advantages and disadvantages of two primary methods used for the production of gene-edited farm animals: somatic cell nuclear transfer (SCNT or cloning) and zygote manipulations will be discussed. Furthermore, we will review agricultural and biomedical applications of gene editing technology.

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.

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.

CRISPR FokI Dead Cas9 System: Principles and Applications in Genome Engineering

The identification of the robust clustered regularly interspersed short palindromic repeats (CRISPR) associated endonuclease (Cas9) system gene-editing tool has opened up a wide range of potential therapeutic applications that were restricted by more complex tools, including zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs). Nevertheless, the high frequency of CRISPR system off-target activity still limits its applications, and, thus, advanced strategies for highly specific CRISPR/Cas9-mediated genome editing are continuously under development including CRISPR–FokI dead Cas9 (fdCas9). fdCas9 system is derived from linking a FokI endonuclease catalytic domain to an inactive Cas9 protein and requires a pair of guide sgRNAs that bind to the sense and antisense strands of the DNA in a protospacer adjacent motif (PAM)-out orientation, with a defined spacer sequence range around the target site. The dimerization of FokI domains generates DNA double-strand breaks, which activates the DNA repair machinery and results in genomic edit. So far, all the engineered fdCas9 variants have shown promising gene-editing activities in human cells when compared to other platforms. Herein, we review the advantages of all published variants of fdCas9 and their current applications in genome engineering.

Progress towards xenogenic tolerance

PURPOSE OF REVIEW

To describe the most recent progress towards tolerance in xenotransplantation.

RECENT FINDINGS

Mixed chimerism and thymic transplantation have been used to promote tolerance in xenotransplantation models. Intra-bone bone marrow transplantation is a recent advance for mixed chimerism, which promotes longer lasting chimerism and early graft function of subsequent organ transplantation. The hybrid thymus, an advancement to the vascularized thymokidney and vascularized thymic lobe, is being developed to allow for both donor and recipient T-cell selection in the chimeric thymus, encouraging tolerance to self and donor while maintaining appropriate immune function. Regulatory T cells show promise to promote tolerance by suppressing effector T cells and by supporting mixed chimerism. Monoclonal antibodies such as anti-CD2 may promote tolerance through suppression of CD2+ effector and memory T cells whereas Tregs, which express lower numbers of CD2, are relatively spared and might be used to promote tolerance.

SUMMARY

These findings contribute major advances to tolerance in xenotransplantation. A combination of many of these mechanisms will likely be needed to have long-term tolerance maintained without the use of immunosuppression.

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.

Xenogeneic cross-circulation for extracorporeal recovery of injured human lungs

Patients awaiting lung transplantation face high wait-list mortality, as injury precludes the use of most donor lungs. Although ex vivo lung perfusion (EVLP) is able to recover marginal quality donor lungs, extension of normothermic support beyond 6 h has been challenging. Here we demonstrate that acutely injured human lungs declined for transplantation, including a lung that failed to recover on EVLP, can be recovered by cross-circulation of whole blood between explanted human lungs and a Yorkshire swine. This xenogeneic platform provided explanted human lungs a supportive, physiologic milieu and systemic regulation that resulted in functional and histological recovery after 24 h of normothermic support. Our findings suggest that cross-circulation can serve as a complementary approach to clinical EVLP to recover injured donor lungs that could not otherwise be utilized for transplantation, as well as a translational research platform for immunomodulation and advanced organ bioengineering.

Xeno-organ donor pigs with multiple genetic modifications - the more the better?

The number of donated human organs and tissues for patients with terminal organ failure falls far short of the need. Alternative sources, such as organs and tissues from animals, are therefore urgently required. During the past few years, major progress has been made in the development of genetically multi-modified donor pigs, and their organs have been shown to be safe and efficacious in life-supporting transplantation models into non-human primates, paving the way to clinical xenotransplantation studies. Here, we summarize recent developments in pig genome engineering and discuss efforts to develop the optimum donor pig for xenotransplantation. In addition, we speculate on how many genetic modifications may be required for initial xenotransplantation clinical trials.

Xenogeneic transplantation and tolerance in the era of CRISPR-Cas9

PURPOSE OF REVIEW

The use of genetically modified donor pigs has been integral to recent major advances in xenograft survival in preclinical nonhuman primate models. CRISPR-Cas9 gene editing technology has dramatically accelerated the development of multimodified pigs. This review examines the current and projected impact of CRISPR-Cas9-mediated donor modification on preventing rejection and potentially promoting tolerance of porcine xenografts.

RECENT FINDINGS

CRISPR-Cas9 has been used to engineer several genetic modifications relevant to xenotransplantation into pigs, including glycosyltransferase knockouts (GGTA1, CMAH, β4GALNT2, A3GALT2 and combinations thereof), other knockouts (SLA-I, ULBP1, PERV and GHR), and one knock-in (anti-CD2 monoclonal antibody transgene knocked into GGTA1). Although the use of these pigs as donors in preclinical nonhuman primate models has been limited to a single study to date, in-vitro analysis of their cells has provided invaluable information. For example, deletion of three of the glycosyltransferases progressively decreased the binding and cytotoxicity of preexisting immunoglobulin G and immunoglobulin M in human sera, suggesting that this 'triple-KO' pig could be a platform for clinical xenotransplantation.

SUMMARY

CRISPR-Cas9 enables the rapid generation of gene-edited pigs containing multiple tailored genetic modifications that are anticipated to have a positive impact on the efficacy and safety of pig-to-human xenotransplantation.

LIVER TRANSPLANTATION: WILL XENOTRANSPLANTATION BE THE ANSWER TO THE DONOR ORGAN SHORTAGE?

Since the first report of a successful liver transplant in 1968, access to this operation has dramatically improved. In 2018, 8,250 patients underwent liver transplantation in the United States. Despite this remarkable advance, a persistent shortage of donor organs remains the primary obstacle to optimal utilization of this life-saving operation. Over the past two decades, transplant professionals have pursued two broad strategies to overcome this roadblock: increasing the number of donor organs and decreasing the number of patients requiring transplantation through advances in medical interventions. Despite these efforts, more than 13,500 patients remained on liver transplant waiting lists at the end of 2018. Almost 1,200 died while waiting, and 1,350 were removed from wait lists because they had become too sick to survive the operation. Clearly, a dramatic new approach to the donor organ shortage is needed. One effort, first attempted by surgeons in the 1960s, was to utilize donor organs from other species (xenotransplantation). The major obstacle to xenotransplantation acceptance has been the fear of transmitting new infectious diseases from animals to humans. As the twentieth century came to a close, national moratoria on xenotransplantation ended both research and clinical activities in this field. The recent discoveries that modern gene-editing techniques can be used to eliminate the retrovirus that is ubiquitous in pigs and that retrovirus-free pigs can be cloned has reopened the possibility that xenotransplantation may be a potentially game-changing approach to eliminating the donor shortage for liver and other solid organ transplant recipients. In response to these advances, the FDA has released comprehensive industry guidelines regarding all aspects of xenotransplantation. This release has resulted in numerous preclinical studies in which organs from genetically modified pigs are transplanted into various nonhuman primates (NHPs). Use of a variety of gene-editing and immunosuppressive techniques has greatly increased the survival of recipient animals in the past few years. Survival of NHP renal transplant recipients has been extended to 435 days, functional cardiac transplant recipients to 195 days, and liver transplant recipients to 29 days. Current research studies using various gene modification strategies combined with newer immunosuppressive protocols are attempting to further extend the survival of these experimental animals. These encouraging results have raised the possibility that clinical xenotransplantation in humans is just beyond the horizon. The most likely candidates for initial clinical studies probably will be kidney transplant recipients who are difficult to crossmatch for human organs, neonates with severe congenital heart disease, and liver transplant candidates with acute liver failure.

Xenoantigen Deletion and Chemical Immunosuppression Can Prolong Renal Xenograft Survival

OBJECTIVE

Xenotransplantation using pig organs could end the donor organ shortage for transplantation, but humans have xenoreactive antibodies that cause early graft rejection. Genome editing can eliminate xenoantigens in donor pigs to minimize the impact of these xenoantibodies. Here we determine whether an improved cross-match and chemical immunosuppression could result in prolonged kidney xenograft survival in a pig-to-rhesus preclinical model.

METHODS

Double xenoantigen (Gal and Sda) knockout (DKO) pigs were created using CRISPR/Cas. Serum from rhesus monkeys (n = 43) was cross-matched with cells from the DKO pigs. Kidneys from the DKO pigs were transplanted into rhesus monkeys (n = 6) that had the least reactive cross-matches. The rhesus recipients were immunosuppressed with anti-CD4 and anti-CD8 T-cell depletion, anti-CD154, mycophenolic acid, and steroids.

RESULTS

Rhesus antibody binding to DKO cells is reduced, but all still have positive CDC and flow cross-match. Three grafts were rejected early at 5, 6, and 6 days. Longer survival was achieved in recipients with survival to 35, 100, and 435 days. Each of the 3 early graft losses was secondary to IgM antibody-mediated rejection. The 435-day graft loss occurred secondary to IgG antibody-mediated rejection.

CONCLUSIONS

Reducing xenoantigens in donor pigs and chemical immunosuppression can be used to achieve prolonged renal xenograft survival in a preclinical model, suggesting that if a negative cross-match can be obtained for humans then prolonged survival could be achieved.

In Vitro Validation of a CRISPR-Mediated CFTR Correction Strategy for Preclinical Translation in Pigs

Early efforts in cystic fibrosis (CF) gene therapy faced major challenges in delivery efficiency and sustained therapeutic gene expression. Recent advancements in engineered site-specific endonucleases such as clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 make permanent CF transmembrane conductance regulator (CFTR) gene correction possible. However, because of safety concerns of the CRISPR/Cas9 system and challenges in in vivo delivery to inflamed CF airway, CRISPR-based gene correction strategies need to be tested in proper animal models. In this study, we aimed at creating vectors for testing CFTR gene correction in pig models. We constructed helper-dependent adenoviral (HD-Ad) vectors to deliver CRISPR/Cas9 and a donor template (a 6 kb LacZ or 8.7 kb human CFTR expression cassette) into cultured pig cells. We demonstrated precise integration of each donor into the GGTA1 safe harbor through Cas9-induced homology directed repair with 3 kb homology arms. In addition, we showed that both LacZ and hCFTR were persistently expressed in transduced cells. Furthermore, we created a CFTR-deficient cell line for testing CFTR correction. We detected hCFTR mRNA and protein expression in cells transduced with the hCFTR vector. We also demonstrated CFTR function in the CF cells transduced with the HD-Ad delivering the CRISPR-Cas9 system and hCFTR donor at late cellular passages using the membrane potential sensitive dye-based assay (FLIPR^®). Combined with our previous report on gene delivery to pig airway basal cells, these data provide the feasibility of testing CRISPR/Cas9-mediated permanent human CFTR correction through HD-Ad vector delivery in pigs.

Reducing porcine corneal graft rejection, with an emphasis on porcine endogenous retrovirus transmission safety: a review

Donor cornea shortage is a primary hurdle in the development of corneal transplantation. Of all species, porcine corneas are the ideal transplantation material for humans. However, the xenoimmune rejection induced by porcine corneal xenotransplantation compromises surgical efficacy. Although the binding of IgM/IgG in human serum to a genetically modified porcine cornea is significantly weaker than that of the wild type (WT), genetically modified porcine corneas do not display a prolonged graft survival time in vivo. Conversely, costimulatory blockade drugs, such as anti-CD40 antibodies, can reduce the xenoimmune response and prolong graft survival time in animal experiments. Moreover, porcine endothelial grafts can survive for more than 6mo with only the subconjunctival injection of a steroid-based immunosuppressants regime; therefore, they show great value for treating corneal endothelial disease. In addition, zoonotic transmission is a primary concern of xenotransplantation. Porcine endogenous retrovirus (PERV) is the most significant virus assessed by ophthalmologists. PERV integrates into the porcine genome and infects human cells in vitro. Fortunately, no evidence from in vivo studies has yet shown that PERV can be transmitted to hosts.

Gene Editing in Primary Cells of Cattle and Pig

Gene Editing by CRISPR/Cas has revolutionized many aspects of biotechnology within a short period of time. This is also true for the genetic manipulation of livestock species, but their specific challenges such as the lack of stem cells, the limited proliferative capacity of primary cells, and the genetic diversity of the pig and cattle populations need consideration when CRISPR/Cas is applied. Here we present guidelines for CRISPRing primary cells in pig and cattle, with a specific focus on testing gRNA in vitro, on generating single cell clones, and on identifying modifications in single cell clones.

Assembling multiple xenoprotective transgenes in pigs

This review gives a brief overview of the genetic modifications necessary for grafted porcine tissues and organs to overcome rejection in human recipients. It then focuses on the problem of generating and breeding herds of donor pigs carrying modified endogenous genes and multiple xenoprotective transgenes. A xenodonor pig optimised for human clinical use could well require the addition of ten or more xenoprotective transgenes. It is impractical to produce the required combination of transgene by cross-breeding animals bearing individual transgenes at unlinked genetic loci, because independent segregation means that huge numbers of pigs would be required to produce relatively few donor animals. A better approach is to colocate groups of transgenes at a single genomic locus. We outline current methods to assemble transgene arrays and consider their pros and cons. These include polycistronic expression systems, in vitro recombination of large DNA fragments in PAC and BAC vectors, transposon vectors, classical gene targeting by homologous recombination at permissive loci such as ROSA26, targeted transgene placement aided by gene editing systems such as CRISPR/Cas9, and transgene placement by site-specific recombination such as Min-tagging using the Bxb1recombinase.