Creating class I MHC-null pigs using guide RNA and the Cas9 endonuclease

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.

Genome editing: An insight into disease resistance, production efficiency, and biomedical applications in livestock

One of the primary concerns for the survival of the human species is the growing demand for food brought on by an increasing global population. New developments in genome-editing technology present promising opportunities for the growth of wholesome and prolific farm animals. Genome editing in large animals is used for a variety of purposes, including biotechnology to improve food production, animal health, and pest management, as well as the development of animal models for fundamental research and biomedicine. Genome editing entails modifying genetic material by removing, adding, or manipulating particular DNA sequences from a particular locus in a way that does not happen naturally. The three primary genome editors are CRISPR/Cas 9, TALENs, and ZFNs. Each of these enzymes is capable of precisely severing nuclear DNA at a predetermined location. One of the most effective inventions is base editing, which enables single base conversions without the requirement for a DNA double-strand break (DSB). As reliable methods for precise genome editing in studies involving animals, cytosine and adenine base editing are now well-established. Effective zygote editing with both cytosine and adenine base editors (ABE) has resulted in the production of animal models. Both base editors produced comparable outcomes for the precise editing of point mutations in somatic cells, advancing the field of gene therapy. This review focused on the principles, methods, recent developments, outstanding applications, the advantages and disadvantages of ZFNs, TALENs, and CRISPR/Cas9 base editors, and prime editing in diverse lab and farm animals. Additionally, we address the methodologies that can be used for gene regulation, base editing, and epigenetic alterations, as well as the significance of genome editing in animal models to better reflect real disease. We also look at methods designed to increase the effectiveness and precision of gene editing tools. Genome editing in large animals is used for a variety of purposes, including biotechnology to improve food production, animal health, and pest management, as well as the development of animal models for fundamental research and biomedicine. This review is an overview of the existing knowledge of the principles, methods, recent developments, outstanding applications, the advantages and disadvantages of zinc finger nucleases (ZFNs), transcription-activator-like endonucleases (TALENs), and clustered regularly interspaced short palindromic repeats associated protein 9 (CRISPR/Cas 9), base editors and prime editing in diverse lab and farm animals, which will offer better and healthier products for the entire human race.

Non-Classical Swine Leukocyte Antigens SLA-6, -7, and -8, Are Xenoantigens for Some Waitlisted Patients

Attack of donor tissues by pre-formed anti-pig antibodies is well known to cause graft failure in xenotransplantation. Genetic engineering of porcine donors to eliminate targets of these pre-formed antibodies coupled with advances in immunosuppressive medicines have now made it possible to achieve extended survival in the pre-clinical pig-to-non-human primate model. Despite these improvements, antibodies remain a risk over the lifetime of the transplant, and many patients continue to have pre-formed donor-specific antibodies even to highly engineered pigs. While therapeutics exist that can help mitigate the detrimental effects of antibodies, they act broadly potentially dampening beneficial immunity. Identifying additional xenoantigens may enable more targeted approaches, such as gene editing, to overcome these challenges by further eliminating antibody targets on donor tissue. Because we have found that classical class I swine leukocyte antigens are targets of human antibodies, we now examine whether related pig proteins may also be targeted by human antibodies. We show here that non-classical class I swine leukocyte proteins (SLA-6, -7, -8) can be expressed at the surface of mammalian cells and act as antibody targets.

Incidence of serum antibodies to xenoantigens on triple-knockout pig cells in different human groups

BACKGROUND

Xenoantigens other than Gal, Neu5Gc, and Sda may be playing a role in pig graft rejection. We investigated the incidence of antibodies to unknown pig xenoantigen in different human groups.

METHODS

We collected blood from TKO/hCD55 pigs (n = 3), and isolated PBMCs and RBCs. Serum samples were collected from (i) healthy human volunteers (n = 43), (ii) patients with end-stage renal disease (ESRD) (n = 87), (iii) the same patients after kidney allotransplantation (n = 50), and (iv) renal allotransplant recipients experiencing T cell-mediated rejection (allo-TCMR, n = 10). The sera were initially incubated with TKO/hCD55 pRBCs (1 × 108 cells) for 1 h to absorb anti-pig antibodies (except against SLA and possibly other antigens not expressed on pRBCs) and then the serum (absorbed or unabsorbed) was tested for antibody binding and complement-dependent cytotoxicity (CDC) to TKO/hCD55 pig PBMCs.

RESULTS

A significant reduction in IgM/IgG binding and CDC was observed in the absorbed sera. Serum obtained before and after renal allotransplantation showed no significant difference in IgM or IgG binding to, or in CDC of, TKO/hCD55 pig cells. IgM antibodies (but rarely IgG) against unknown xenoantigens expressed on TKO/hCD55 PBMCs, possibly against swine leukocyte antigens, were documented in healthy humans, patients with ESRD, and those with renal allografts undergoing acute T cell rejection. IgM (but not CDC) was higher in patients experiencing allo-TCMR.

CONCLUSION

Human sera contain IgM antibodies against unknown pig xenoantigens expressed on TKO/hCD55 pPBMCs. Although not confirmed in the present study, the targets for these antibodies may include swine leukocyte antigens.

A brief review of the current status of pig islet xenotransplantation

An estimated 1.5 million Americans suffer from Type I diabetes mellitus, and its incidence is increasing worldwide. Islet allotransplantation offers a treatment, but the availability of deceased human donor pancreases is limited. The transplantation of islets from gene-edited pigs, if successful, would resolve this problem. Pigs are now available in which the expression of the three known xenoantigens against which humans have natural (preformed) antibodies has been deleted, and in which several human 'protective' genes have been introduced. The transplantation of neonatal pig islets has some advantages over that of adult pig islets. Transplantation into the portal vein of the recipient results in loss of many islets from the instant blood-mediated inflammatory reaction (IBMIR) and so the search for an alternative site continues. The adaptive immune response can be largely suppressed by an immunosuppressive regimen based on blockade of the CD40/CD154 T cell co-stimulation pathway, whereas conventional therapy (e.g., based on tacrolimus) is less successful. We suggest that, despite the need for effective immunosuppressive therapy, the transplantation of 'free' islets will prove more successful than that of encapsulated islets. There are data to suggest that, in the absence of rejection, the function of pig islets, though less efficient than human islets, will be sufficient to maintain normoglycemia in diabetic recipients. Pig islets transplanted into immunosuppressed nonhuman primates have maintained normoglycemia for periods extending more than two years, illustrating the potential of this novel form of therapy.

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.

Patients on the Transplant Waiting List Have Anti-Swine Leukocyte Antigen Class I Antibodies

Organ supply remains inadequate to meet the needs of many patients who could benefit from allotransplantation. Xenotransplantation, the use of animals as organ donors, provides an opportunity to alleviate this challenge. Pigs are widely accepted as the ideal organ donor, but humans and nonhuman primates have strong humoral immune responses to porcine tissue. Although carbohydrate xenoantigens have been studied intensively, the primate Ab response also targets class I and class II swine leukocyte Ags (SLAs). Human Abs that recognize HLAs can cross-react with SLA molecules because epitopes can be shared across species. However, ∼15% of people may also exhibit Abs toward class II SLAs despite lacking Abs that also recognize class II HLAs. Here, we extend these studies to better understand human Ab responses toward class I SLAs. When tested against a panel of 18 unique class I SLA proteins, 14 of 52 sera samples collected from patients in need of an organ transplant contained Abs that bound class I SLAs. Class I SLA-reactive sera may contain IgM only, IgG, only, or IgM and IgG capable of recognizing the pig proteins. The presence of class I HLA-reactive Abs was not essential to generating anti-class I SLA Ig. Last, anti-class I SLA reactivity varied by serum; some recognized a single SLA allele, whereas others recognized multiple class I SLA proteins.

Downregulation of Swine Leukocyte Antigen Expression Decreases the Strength of Xenogeneic Immune Responses towards Renal Proximal Tubular Epithelial Cells

Xenotransplantation reemerged as a promising alternative to conventional transplantation enlarging the available organ pool. However, success of xenotransplantation depends on the design and selection of specific genetic modifications and on the development of robust assays allowing for a precise assessment of tissue-specific immune responses. Nevertheless, cell-based assays are often compromised by low proliferative capacity of primary cells. Proximal tubular epithelial cells (PTECs) play a crucial role in kidney function. Here, we generated immortalized PTECs (imPTECs) by overexpression of simian virus 40 T large antigen. ImPTECs not only showed typical morphology and phenotype, but, in contrast to primary PTECs, they maintained steady cell cycling rates and functionality. Furthermore, swine leukocyte antigen (SLA) class I and class II transcript levels were reduced by up to 85% after transduction with lentiviral vectors encoding for short hairpin RNAs targeting β2-microglobulin and the class II transactivator. This contributed to reducing xenogeneic T-cell cytotoxicity (p < 0.01) and decreasing secretion of pro-inflammatory cytokines such as IL-6 and IFN-γ. This study showed the feasibility of generating highly proliferative PTECs and the development of tissue-specific immunomonitoring assays. Silencing SLA expression on PTECs was demonstrated to be an effective strategy to prevent xenogeneic cellular immune responses and may strongly support graft survival after xenotransplantation.

The history of cardiac xenotransplantation: early attempts, major advances, and current progress

In light of ongoing shortage of donor organs for transplantation, alternative sources for donor organ sources have been examined to address this supply-demand mismatch. Of these, xenotransplantation, or the transplantation of organs across species, has been considered, with early applications dating back to the 1600s. The purpose of this review is to summarize the early experiences of xenotransplantation, with special focus on heart xenotransplantation. It aims to highlight the important ethical concerns of animal-to-human heart xenotransplantation, identify the key immunological barriers to successful long-term xenograft survival, as well as summarize the progress made in terms of development of pharmacological and genetic engineering strategies to address these barriers. Lastly, we discuss more recent attempts of porcine-to-human heart xenotransplantation, as well as provide some commentary on the current concerns and possible applications for future clinical heart xenotransplantation.

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.

Current Status of Renal Xenotransplantation and Next Steps

Renal transplantation is the preferred treatment of ESKD, but the shortage of suitable donor kidneys from the cadaver pool means that many patients with ESKD will not receive a kidney transplant. Xenotransplantation has long represented a solution to the kidney shortage, but the occurrence of antibody-mediated rejection has precluded its clinical development. Developments in somatic cell nuclear transfer in pigs and gene editing tools have led to the creation of new donor pigs with greatly improved crossmatches to patients. In addition, improvements in preclinical kidney xenotransplant survival using new anti-CD40/CD154-based immunosuppression have pushed xenotransplantation to the point where it is reasonable to consider initiating a clinical trial to evaluate this potential therapy in patients.

The respective relevance of sensitization to alloantigens and xenoantigens in pig organ xenotransplantation

BACKGROUND

Antibody-mediated rejection is a major cause of graft injury and contributes to failure of pig xenografts in nonhuman primates (NHPs). Most 'natural' or elicited antibodies found in humans and NHPs are directed against pig glycan antigens, but antibodies binding to swine leukocyte antigens (SLA) have also been detected. Of clinical importance is (i) whether the presence of high levels of antibodies directed towards human leukocyte antigens (HLA) (i.e., high panel-reactive antibodies) would be detrimental to the outcome of a pig organ xenograft; and (ii) whether, in the event of sensitization to pig antigens, a subsequent allotransplant would be at increased risk of graft failure due to elicited anti-pig antibodies that cross-react with human HLA or other antigens.

SUMMARY

A literature review of pig-to-primate studies indicates that relatively few highly-HLA-sensitized humans have antibodies that cross-react with pigs, predicting that most would not be at increased risk of rejecting an organ xenograft. Furthermore, the existing evidence indicates that sensitization to pig antigens will probably not elicit increased alloantibody titers; if so, 'bridging' with a pig organ could be carried out without increased risk of subsequent antibody-mediated allograft failure.

KEY MESSAGE

These issues have important implications for the design and conduct of clinical xenotransplantation trials.

Advance of genetically modified pigs in xeno-transplantation

As the standard of living improves, chronic diseases and end-stage organ failure have been a regular occurrence in human beings. Organ transplantation has become one of the hopes in the fight against chronic diseases and end-stage organ failure. However, organs available for transplantation are far from sufficient to meet the demand, leading to a major organ shortage crisis. To solve this problem, researchers have turned to pigs as their target since pigs have many advantages as xenograft donors. Pigs are considered the ideal organ donor for human xenotransplantation, but direct transplantation of porcine organs to humans faces many obstacles, such as hyperacute rejection, acute humoral xenograft rejection, coagulation dysregulation, inflammatory response, coagulation dysregulation, and endogenous porcine retroviral infection. Many transgenic strategies have been developed to overcome these obstacles. This review provides an overview of current advances in genetically modified pigs for xenotransplantation. Future genetic engineering-based delivery of safe and effective organs and tissues for xenotransplantation remains our goal.

Recent advances in porcine cardiac xenotransplantation: from aortic valve replacement to heart transplantation

INTRODUCTION

Cardiac xenotransplantation presents significant potential to the field of heart failure by addressing the high demand for donor organs. The availability of xenograft hearts would substantially augment the number of life-saving organs available to patients and may ultimately liberalize eligibility criteria for transplantation.

AREAS COVERED

In this review, we will discuss the need for cardiac xenotransplantation and the history of research and clinical practice in this field. Specifically, we address immunologic concepts and clinical lessons learned from heart valve replacement using xenogeneic tissues, the advancement of xenotransplantation using organs from genetically modified animals, and the progression of this research to the first-in-man pig-to-human heart transplantation.

EXPERT OPINION

Cardiac xenotransplantation holds tremendous promise, but the indications for this new treatment will need to be clearly defined because mechanical support with ventricular assist devices and total artificial hearts are increasingly successful alternatives for adults in heart failure. Cardiac xenotransplantation will also serve as temporary bridge to allotransplantation in babies with complex congenital heart disease who are too small for the currently available mechanical assist devices. Moreover, xenotransplantation of the part of the heart containing a heart valve could deliver growing heart valve implants for babies with severe heart valve dysfunction.

Xenotransplantation: A New Era

Organ allotransplantation has now reached an impassable ceiling inherent to the limited supply of human donor organs. In the United States, there are currently over 100,000 individuals on the national transplant waiting list awaiting a kidney, heart, and/or liver transplant. This is in contrast with only a fraction of them receiving a living or deceased donor allograft. Given the morbidity, mortality, costs, or absence of supportive treatments, xenotransplant has the potential to address the critical shortage in organ grafts. Last decade research efforts focused on creation of donor organs from pigs with various genes edited out using CRISPR technologies and utilizing non-human primates for trial. Three groups in the United States have recently moved forward with trials in human subjects and obtained initial successful results with pig-to-human heart and kidney xenotransplantation. This review serves as a brief discussion of the recent progress in xenotransplantation research, particularly as it concerns utilization of porcine heart, renal, and liver xenografts in clinical practice.

Advances in pig models of human diseases

Animal models of human diseases play a critical role in medical research. Pigs are anatomically and physiologically more like humans than are small rodents such as mice, making pigs an attractive option for modeling human diseases. Advances in recent years in genetic engineering have facilitated the rapid rise of pig models for use in studies of human disease. In the present review, we summarize the current status of pig models for human cardiovascular, metabolic, neurodegenerative, and various genetic diseases. We also discuss areas that need to be improved. Animal models of human diseases play a critical role in medical research. Advances in recent years in genetic engineering have facilitated the rapid rise of pig models for use in studies of human disease. In the present review, we summarize the current status of pig models for human cardiovascular, metabolic, neurodegenerative, various genetic diseases and xenotransplantation.

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.

Anti-C5 Antibody Tesidolumab Reduces Early Antibody-mediated Rejection and Prolongs Survival in Renal Xenotransplantation

OBJECTIVE

Pig-to-primate renal xenotransplantation is plagued by early antibody-mediated graft loss which precludes clinical application of renal xenotransplantation. We evaluated whether temporary complement inhibition with anti-C5 antibody Tesidolumab could minimize the impact of early antibody-mediated rejection in rhesus monkeys receiving pig kidneys receiving costimulatory blockade-based immunosuppression.

METHODS

Double (Gal and Sda) and triple xenoantigen (Gal, Sda, and SLA I) pigs were created using CRISPR/Cas. Kidneys from DKO and TKO pigs were transplanted into rhesus monkeys that had the least reactive crossmatches. Recipients received anti-C5 antibody weekly for 70 days, and T cell depletion, anti-CD154, mycophenolic acid, and steroids as baseline immunosuppression (n = 7). Control recipients did not receive anti-C5 therapy (n = 10).

RESULTS

Temporary anti-C5 therapy reduced early graft loss secondary to antibody-mediated rejection and improved graft survival (P < 0.01). Deleting class I MHC (SLA I) in donor pigs did not ameliorate early antibody-mediated rejection (table). Anti-C5 therapy did not allow for the use of tacrolimus instead of anti-CD154 (table), prolonging survival to a maximum of 62 days.

CONCLUSION

Inhibition of the C5 complement subunit prolongs renal xenotransplant survival in a pig to non-human primate model.

Advances and Perspectives in the Application of CRISPR-Cas9 in Livestock

The sophistication and revolution in genome editing and manipulation have revolutionized livestock by harvesting essential biotechnological products such as drugs, proteins, and serum. It laid down areas for the large production of transgenic food, resistance against certain diseases such as mastitis, and large production of milk and leaner meat. Nowadays, the increasing demand for animal food and protein is fulfilled using genome-editing technologies. The recent genome-editing techniques have overcome the earlier methods of animal reproduction, such as cloning and artificial embryo transfer. The genome of animals now is modified using the recent alteration techniques such as ZFNs, TALENS technique, and clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR-Cas9) system. The literature was illustrated for identifying the researchers to address the advances and perspectives in the application of Cas9 in Livestock. Cas9 is considered better than the previously identified techniques in livestock because of the production of resilience against diseases, improvement of reproductive traits, and animal production to act as a model biomedical research.

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.

Oligonucleotide-Based Therapies for Renal Diseases

The global burden of chronic kidney disease (CKD) is increasing every year and represents a great cost for public healthcare systems, as the majority of these diseases are progressive. Therefore, there is an urgent need to develop new therapies. Oligonucleotide-based drugs are emerging as novel and promising alternatives to traditional drugs. Their expansion corresponds with new knowledge regarding the molecular basis underlying CKD, and they are already showing encouraging preclinical results, with two candidates being evaluated in clinical trials. However, despite recent technological advances, efficient kidney delivery remains challenging, and the presence of off-targets and side-effects precludes development and translation to the clinic. In this review, we provide an overview of the various oligotherapeutic strategies used preclinically, emphasizing the most recent findings in the field, together with the different strategies employed to achieve proper kidney delivery. The use of different nanotechnological platforms, including nanocarriers, nanoparticles, viral vectors or aptamers, and their potential for the development of more specific and effective treatments is also outlined.

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.

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.

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.

Generation of myostatin edited horse embryos using CRISPR/Cas9 technology and somatic cell nuclear transfer

The application of new technologies for gene editing in horses may allow the generation of improved sportive individuals. Here, we aimed to knock out the myostatin gene (MSTN), a negative regulator of muscle mass development, using CRISPR/Cas9 and to generate edited embryos for the first time in horses. We nucleofected horse fetal fibroblasts with 1, 2 or 5 µg of 2 different gRNA/Cas9 plasmids targeting the first exon of MSTN. We observed that increasing plasmid concentrations improved mutation efficiency. The average efficiency was 63.6% for gRNA1 (14/22 edited clonal cell lines) and 96.2% for gRNA2 (25/26 edited clonal cell lines). Three clonal cell lines were chosen for embryo generation by somatic cell nuclear transfer: one with a monoallelic edition, one with biallelic heterozygous editions and one with a biallelic homozygous edition, which rendered edited blastocysts in each case. Both MSTN editions and off-targets were analyzed in the embryos. In conclusion, CRISPR/Cas9 proved an efficient method to edit the horse genome in a dose dependent manner with high specificity. Adapting this technology sport advantageous alleles could be generated, and a precision breeding program could be developed.

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.

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: Current Status in Preclinical Research

The increasing life expectancy of humans has led to a growing numbers of patients with chronic diseases and end-stage organ failure. Transplantation is an effective approach for the treatment of end-stage organ failure; however, the imbalance between organ supply and the demand for human organs is a bottleneck for clinical transplantation. Therefore, xenotransplantation might be a promising alternative approach to bridge the gap between the supply and demand of organs, tissues, and cells; however, immunological barriers are limiting factors in clinical xenotransplantation. Thanks to advances in gene-editing tools and immunosuppressive therapy as well as the prolonged xenograft survival time in pig-to-non-human primate models, clinical xenotransplantation has become more viable. In this review, we focus on the evolution and current status of xenotransplantation research, including our current understanding of the immunological mechanisms involved in xenograft rejection, genetically modified pigs used for xenotransplantation, and progress that has been made in developing pig-to-pig-to-non-human primate models. Three main types of rejection can occur after xenotransplantation, which we discuss in detail: (1) hyperacute xenograft rejection, (2) acute humoral xenograft rejection, and (3) acute cellular rejection. Furthermore, in studies on immunological rejection, genetically modified pigs have been generated to bridge cross-species molecular incompatibilities; in the last decade, most advances made in the field of xenotransplantation have resulted from the production of genetically engineered pigs; accordingly, we summarize the genetically modified pigs that are currently available for xenotransplantation. Next, we summarize the longest survival time of solid organs in preclinical models in recent years, including heart, liver, kidney, and lung xenotransplantation. Overall, we conclude that recent achievements and the accumulation of experience in xenotransplantation mean that the first-in-human clinical trial could be possible in the near future. Furthermore, we hope that xenotransplantation and various approaches will be able to collectively solve the problem of human organ shortage.

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.

Introduction: The Present Status of Xenotransplantation Research

There is a well-known worldwide shortage of deceased human donor organs for clinical transplantation. The transplantation of organs from genetically engineered pigs may prove an alternative solution. In the past 5 years, there have been sequential advances that have significantly increased pig graft survival in nonhuman primates. This progress has been associated with (1) the availability of increasingly sophisticated genetically engineered pigs; (2) the introduction of novel immunosuppressive agents, particularly those that block the second T-cell signal (costimulation blockade); (3) a better understanding of the inflammatory response to pig xenografts; and (4) increasing experience in the management of nonhuman primates with pig organ or cell grafts. The range of investigations required in experimental studies has increased. The standard immunologic assays are still carried out, but increasingly investigations aimed toward other pathobiologic barriers (e.g., coagulation dysregulation and inflammation) have become more important in determining injury to the graft.Now that prolonged graft survival, extending to months or even years, is increasingly being obtained, the function of the grafts can be more reliably assessed. If the source pigs are bred and housed under biosecure isolation conditions, and weaned early from the sow, most microorganisms can be eradicated from the herd. The potential risk of porcine endogenous retrovirus (PERV) infection remains unknown, but is probably small. Attention is being directed toward the selection of patients for the first clinical trials of xenotransplantation.

Importance of the Major Histocompatibility Complex (Swine Leukocyte Antigen) in Swine Health and Biomedical Research

In pigs, the major histocompatibility complex (MHC), or swine leukocyte antigen (SLA) complex, maps to Sus scrofa chromosome 7. It consists of three regions, the class I and class III regions mapping to 7p1.1 and the class II region mapping to 7q1.1. The swine MHC is divided by the centromere, which is unique among mammals studied to date. The SLA complexspans between 2.4 and 2.7 Mb, depending on haplotype, and encodes approximately 150 loci, with at least 120 genes predicted to be functional. Here we update the whole SLA complex based on the Sscrofa11.1 build and annotate the organization for all recognized SLA genes and their allelic sequences. We present SLA nomenclature and typing methods and discuss the expression of SLA proteins, as well as their role in antigen presentation and immune, disease, and vaccine responses. Finally, we explore the role of SLA genes in transplantation and xenotransplantation and their importance in swine biomedical models.

Clinical Pig Kidney Xenotransplantation: How Close Are We?

Patients with ESKD who would benefit from a kidney transplant face a critical and continuing shortage of kidneys from deceased human donors. As a result, such patients wait a median of 3.9 years to receive a donor kidney, by which time approximately 35% of transplant candidates have died while waiting or have been removed from the waiting list. Those of blood group B or O may experience a significantly longer waiting period. This problem could be resolved if kidneys from genetically engineered pigs offered an alternative with an acceptable clinical outcome. Attempts to accomplish this have followed two major paths: deletion of pig xenoantigens, as well as insertion of "protective" human transgenes to counter the human immune response. Pigs with up to nine genetic manipulations are now available. In nonhuman primates, administering novel agents that block the CD40/CD154 costimulation pathway, such as an anti-CD40 mAb, suppresses the adaptive immune response, leading to pig kidney graft survival of many months without features of rejection (experiments were terminated for infectious complications). In the absence of innate and adaptive immune responses, the transplanted pig kidneys have generally displayed excellent function. A clinical trial is anticipated within 2 years. We suggest that it would be ethical to offer a pig kidney transplant to selected patients who have a life expectancy shorter than the time it would take for them to obtain a kidney from a deceased human donor. In the future, the pigs will also be genetically engineered to control the adaptive immune response, thus enabling exogenous immunosuppressive therapy to be significantly reduced or eliminated.

Viable pigs after simultaneous inactivation of porcine MHC class I and three xenoreactive antigen genes GGTA1, CMAH and B4GALNT2

BACKGROUND

Cell surface carbohydrate antigens play a major role in the rejection of porcine xenografts. The most important for human recipients are α-1,3 Gal (Galactose-alpha-1,3-galactose) causing hyperacute rejection, also Neu5Gc (N-glycolylneuraminic acid) and Sd(a) blood group antigens both of which are likely to elicit acute vascular rejection given the known human immune status. Porcine cells with knockouts of the three genes responsible, GGTA1, CMAH and B4GALNT2, revealed minimal xenoreactive antibody binding after incubation with human serum. However, human leucocyte antigen (HLA) antibodies cross-reacted with swine leucocyte antigen class I (SLA-I). We previously demonstrated efficient generation of pigs with multiple xeno-transgenes placed at a single genomic locus. Here we wished to assess whether key xenoreactive antigen genes can be simultaneously inactivated and if combination with the multi-transgenic background further reduces antibody deposition and complement activation.

METHODS

Multiplex CRISPR/Cas9 gene editing and somatic cell nuclear transfer were used to generate pigs carrying functional knockouts of GGTA1, CMAH, B4GALNT2 and SLA class I. Fibroblasts derived from one- to four-fold knockout animals, and from multi-transgenic cells (human CD46, CD55, CD59, HO1 and A20) with the four-fold knockout were used to examine the effects on human IgG and IgM binding or complement activation in vitro.

RESULTS

Pigs were generated carrying four-fold knockouts of important xenoreactive genes. In vitro assays revealed that combination of all four gene knockouts reduced human IgG and IgM binding to porcine kidney cells more effectively than single or double knockouts. The multi-transgenic background combined with GGTA1 knockout alone reduced C3b/c and C4b/c complement activation to such an extent that further knockouts had no significant additional effect.

CONCLUSION

We showed that pigs carrying several xenoprotective transgenes and knockouts of xenoreactive antigens can be readily generated and these modifications will have significant effects on xenograft survival.

Examining epitope mutagenesis as a strategy to reduce and eliminate human antibody binding to class II swine leukocyte antigens

Xenotransplantation of pig organs into people may help alleviate the critical shortage of donors which faces organ transplantation. Unfortunately, human antibodies vigorously attack pig tissues preventing the clinical application of xenotransplantation. The swine leukocyte antigens (SLA), homologs of human HLA molecules, can be xenoantigens. SLA molecules, encoded by genes in the pig major histocompatibility complex, contribute to protective immune responses in pig. Therefore, simply inactivating them through genome engineering could reduce the ability of the human immune system to surveil transplanted pig organs for infectious disease or the development of neoplasms. A potential solution to this problem is to identify and modify epitopes in SLA proteins to eliminate their contribution to humoral xenoantigenicity while retaining their biosynthetic competence and ability to contribute to protective immunity. We previously showed that class II SLA proteins were recognized as xenoantigens and mutating arginine at position 55 to proline, in an SLA-DQ beta chain, could reduce human antibody binding. Here, we extend these observations by creating several additional point mutants at position 55. Using a panel of monoclonal antibodies specific for class II SLA proteins, we show that these mutants remain biosynthetically competent. Examining antibody binding to these variants shows that point mutagenesis can reduce, eliminate, or increase antibody binding to class II SLA proteins. Individual mutations can have opposite effects on antibody binding when comparing samples from different people. We also performed a preliminary analysis of creating point mutants near to position 55 to demonstrate that manipulating additional residues also affects antibody reactivity.

CRISPR/Cas9 guided genome and epigenome engineering and its therapeutic applications in immune mediated diseases

Recent developments in the nucleic acid editing technologies have provided a powerful tool to precisely engineer the genome and epigenome for studying many aspects of immune cell differentiation and development as well as several immune mediated diseases (IMDs) including autoimmunity and cancer. Here, we discuss the recent technological achievements of the CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)-based RNA-guided genome and epigenome editing toolkit and provide an insight into how CRISPR/Cas9 (CRISPR Associated Protein 9) toolbox could be used to examine genetic and epigenetic mechanisms underlying IMDs. In addition, we will review the progress in CRISPR/Cas9-based genome-wide genome and epigenome screens in various cell types including immune cells. Finally, we will discuss the potential of CRISPR/Cas9 in defining the molecular function of disease associated SNPs overlapping gene regulatory elements.

Possible detrimental effects of beta-2-microglobulin knockout in pigs

BACKGROUND

Despite major improvements in pig-to-primate xenotransplantation, long-term survival of xenografts is still challenging. The major histocompatibility complex (MHC) class I, which is crucial in cellular immune response, is an important xenoantigen. Abrogating MHC class I expression on xenografts might be beneficial for extending graft survival beyond current limits.

METHODS

In this study, we employed the CRISPR/Cas9 system to target exon 2 of the porcine beta-2-microglobulin (B2M) gene to abrogate SLA class I expression on porcine cells. B2M-KO cells served as donor cells for somatic cell nuclear transfer, and cloned embryos were transferred to three recipient sows. The offspring were genotyped for mutations at the B2M locus, and blood samples were analyzed via flow cytometry for the absence of SLA class I molecules.

RESULTS

Pregnancies were successfully established and led to the birth of seven viable piglets. Genomic sequencing proved that all piglets carried biallelic modifications at the B2M locus leading to a frameshift, a premature stop codon, and ultimately a functional knockout. However, survival times of these animals did not exceed 4 weeks due to unexpected disease processes.

CONCLUSION

Here, we demonstrate the feasibility of generating SLA class I knockout pigs by targeting the porcine beta-2-microglobulin gene using the CRISPR/Cas9 system. Additionally, our findings indicate for the first time that this genetic modification might have a negative impact on the viability of the animals. These issues need to be solved to unveil the real value for xenotransplantation in the future.

The ethics of genome editing in non-human animals: a systematic review of reasons reported in the academic literature

In recent years, new genome editing technologies have emerged that can edit the genome of non-human animals with progressively increasing efficiency. Despite ongoing academic debate about the ethical implications of these technologies, no comprehensive overview of this debate exists. To address this gap in the literature, we conducted a systematic review of the reasons reported in the academic literature for and against the development and use of genome editing technologies in animals. Most included articles were written by academics from the biomedical or animal sciences. The reported reasons related to seven themes: human health, efficiency, risks and uncertainty, animal welfare, animal dignity, environmental considerations and public acceptability. Our findings illuminate several key considerations about the academic debate, including a low disciplinary diversity in the contributing academics, a scarcity of systematic comparisons of potential consequences of using these technologies, an underrepresentation of animal interests, and a disjunction between the public and academic debate on this topic. As such, this article can be considered a call for a broad range of academics to get increasingly involved in the discussion about genome editing, to incorporate animal interests and systematic comparisons, and to further discuss the aims and methods of public involvement. This article is part of a discussion meeting issue 'The ecology and evolution of prokaryotic CRISPR-Cas adaptive immune systems'.

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.

Justification of specific genetic modifications in pigs for clinical organ xenotransplantation

Xenotransplantation research has made considerable progress in recent years, largely through the increasing availability of pigs with multiple genetic modifications. We suggest that a pig with nine genetic modifications (ie, currently available) will provide organs (initially kidneys and hearts) that would function for a clinically valuable period of time, for example, >12 months, after transplantation into patients with end-stage organ failure. The national regulatory authorities, however, will likely require evidence, based on in vitro and/or in vivo experimental data, to justify the inclusion of each individual genetic modification in the pig. We provide data both from our own experience and that of others on the advantages of pigs in which (a) all three known carbohydrate xenoantigens have been deleted (triple-knockout pigs), (b) two human complement-regulatory proteins (CD46, CD55) and two human coagulation-regulatory proteins (thrombomodulin, endothelial cell protein C receptor) are expressed, (c) the anti-apoptotic and "anti-inflammatory" molecule, human hemeoxygenase-1 is expressed, and (d) human CD47 is expressed to suppress elements of the macrophage and T-cell responses. Although many alternative genetic modifications could be made to an organ-source pig, we suggest that the genetic manipulations we identify above will all contribute to the success of the initial clinical pig kidney or heart transplants, and that the beneficial contribution of each individual manipulation is supported by considerable experimental evidence.

Incidence of Neoplasia in Pigs and Its Relevance to Clinical Organ Xenotransplantation

As clinical pig organ xenotransplantation draws closer, more attention is being paid to diseases that affect pigs and those that provide a potential risk to human recipients of pig organs. Neoplasia arising from the pig organ graft is one such concern. Various tumors and other neoplastic diseases are well known to show increased incidence in organ allotransplant recipients receiving immunosuppressive therapy. Whether this effect will prove to be the case after xenotransplantation has not yet been established. Malignant tumors in young pigs are rare, with lymphosarcoma, nephroblastoma, and melanoma being the most common. The combination of noninvasive techniques and intraoperative examination of the pig organ likely will readily confirm that a pig organ graft is tumor-free before xenotransplantation. Posttransplantion lymphoproliferative disorder (PTLD) is a concern after allotransplantation, but the incidence after solid organ allotransplantation is low when compared with hematopoietic cell allotransplantation (for example, bone marrow transplantation), unless immunosuppressive therapy is particularly intensive. Organ-source pigs used for clinical xenotransplantation will be bred and housed under designated pathogen-free conditions and will be free of the γ-herpesvirus that is a key factor in the development of PTLD in pigs. Therefore if a recipient of a pig xenograft develops PTLD, it will almost certainly be of recipient origin. The increasing availability of organs from pigs genetically-engineered to protect them from the human immune response likely will diminish the need for intensive immunosuppressive therapy. Considering the low incidence of malignant disease in young pigs, donor-derived malignancy is likely to be rare in patients who receive pig organ grafts. However, if the graft remains viable for many years, the incidence of graft malignancy may increase.

CRISPR-mediated gene editing for the surgeon scientist

Tremendous advances have occurred in gene editing during the past 20 years with the development of a number of systems. The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-associated protein 9 (Cas9) system represents an exciting area of research. This review examines both the relevant studies pertaining to the history, current status, and modifications of this system, in comparison with other gene-editing systems and future applications, and limitations of the CRISPR-Cas9 gene-editing system, with a focus on applications of relevance to the surgeon scientist. The CRISPR-Cas9 system was described initially in 2012 for gene editing in bacteria and then in human cells, and since then, a number of modifications have improved the efficiency and specificity of gene editing. Clinical studies have been limited because further research is required to verify its safety in patients. Some clinical trials in oncology have opened, and early studies have shown that gene editing may have a particular role in the field of organ transplantation and in the care of trauma patients. Gene editing is likely to play an important role in future research in many aspects of the surgery arena.

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.

A review of pig liver xenotransplantation: Current problems and recent progress

Pig liver xenotransplantation appears to be more perplexing when compared to heart or kidney xenotransplantation, even though great progress has been achieved. The relevant molecular mechanisms involved in xenogeneic rejection, including coagulopathy, and particularly thrombocytopenia, are complex, and need to be systematically investigated. The deletion of expression of Gal antigens in the liver graft highlights the injurious impact of nonGal antigens, which continue to induce humoral rejection. Innate immunity, particularly mediated by macrophages and natural killer cells, interplays with inflammation and coagulation disorders. Kupffer cells and liver sinusoidal endothelial cells (LSECs) together mediate leukocyte, erythrocyte, and platelet sequestration and phagocytosis, which can be exacerbated by increased cytokine production, cell desialylation, and interspecies incompatibilities. The coagulation cascade is activated by release of tissue factor which can be dependent or independent of the xenoreactive immune response. Depletion of endothelial anticoagulants and anti-platelet capacity amplify coagulation activation, and interspecies incompatibilities of coagulation-regulatory proteins facilitate dysregulation. LSECs involved in platelet phagocytosis and transcytosis, coupled with hepatocyte-mediated degradation, are responsible for thrombocytopenia. Adaptive immunity could also be problematic in long-term liver graft survival. Currently, relevant evidence and study results of various genetic modifications to the pig donor need to be fully determined, with the aim of identifying the ideal transgene combination for pig liver xenotransplantation. We believe that clinical trials of pig liver xenotransplantation should initially be considered as a bridge to allotransplantation.