Physiologic Aspects of Pig Kidney Transplantation in Nonhuman Primates

Hypothermic machine perfusion prevents hyperacute graft loss in pig-to-primate kidney xenotransplantation after 5-hours of cold Ischemia

BACKGROUND

Xenotransplantation (XTx) is a promising strategy to address the organ shortage. Clinical application will likely require off-site procurement from designated pathogen-free (DPF) facilities, introducing unavoidable cold ischemic time (CIT). The impact of CIT and organ preservation method on graft function in XTx remains unclear.

METHODS

We evaluated eight cases of pig-to-baboon kidney xenotransplantation performed after five hours of CIT, comparing static cold storage (SCS) to hypothermic machine perfusion (HMP) preservation. Outcomes were assessed relative to six additional pig-to-baboon transplants performed with minimal CIT.

RESULTS

All grafts preserved with SCS experience hyperacute rejection within 90 min of reperfusion, even in recipients with low levels of preformed anti-pig antibodies. In contrast, all HMP-preserved grafts reperfuse without clinical evidence of injury and maintain function for more than 14 days. Grafts transplanted with minimal CIT show similarly favorable outcomes.

CONCLUSIONS

Porcine kidneys are highly sensitive to ischemia-reperfusion injury after cold preservation across xenogeneic barriers. Routine SCS leads to early graft failure, while HMP mitigates ischemic injury and may enable successful clinical XTx despite prolonged CIT.

Current Scenario and Future Perspectives of Porcine Corneal Xenotransplantation

ABSTRACT

Corneal diseases represent a significant cause of blindness worldwide, with corneal transplantation being an effective treatment to prevent vision loss. Despite substantial advances in transplantation techniques, the demand for donor corneas exceeds the available supply, particularly in developing countries. Cornea xenotransplantation has emerged as a promising strategy to address the worldwide scarcity, notably using porcine corneas. In addition to the inherent immune privilege of the cornea, the low cost of porcine breeding and the anatomical and physiological similarities between humans and pigs have made porcine corneas a viable alternative. Nonetheless, ethical concerns, specifically the risk of xenozoonotic transmission and the necessity for stringent biosafety measures, remain significant obstacles. Moreover, the success of xenotransplantation is compromised by innate and adaptive immune responses, which requires meticulous consideration and further studies. Despite these challenges, recent breakthroughs have further contributed to reducing immunogenicity while preserving the corneal architecture. Advances in genetic engineering, such as the use of CRISPR-Cas9 to eliminate critical porcine antigens, have shown promise for mitigating immune reactions. Additionally, new immunosuppressive protocols, such as have techniques like decellularization and the use of porcine-derived acellular matrices, have greatly increased graft survival in preclinical models. Future research must focus on refining immunomodulatory strategies and improving graft preparation techniques to ensure the long-term survival and safety of porcine corneal xenotransplantation in clinical trials in humans.

Progress in Porcine Kidney Transplantation to Non-Human Primates

Renal xenotransplantation has recently made considerable progress in overcoming the barrier to its use in humans. This progress has been made possible owing to the use of preclinical pig-to-primate models. Overall, renal xenotransplantation has long been associated with lower survival rates than that of porcine hearts (mainly due to its life-sustaining nature). However, the use of the latest strains of genetically modified porcine donors, combined with progress in the control of the anti-porcine immune response and coagulation, has now enabled survival of up to 2 years. Although the pig-to-primate combination has long been considered a perfect reflection of the human situation, it has several limitations, particularly in terms of different natural anti-porcine antibodies. This fact, in association with survival prolongation, which is considered a prerequisite, has led some pioneering teams to cross the line of human application. However, use in humans will remain anecdotal, and further progress in renal xenotransplantation will be difficult to achieve without the use of non-human primates, which will remain complementary, particularly with regard to major innovations that have never been tested in humans.

Routine cold storage leads to hyperacute graft loss in pig-to-primate kidney xenotransplantation; hypothermic machine perfusion may be preferred preservation modality in xenotransplantation

Xenotransplantation (XTx) is an increasingly realistic solution to the organ shortage. Clinical XTx may require off-site procurement in a designated pathogen free (DPF) facility necessitating a period of cold ischemic time during transportation. This study evaluates the impact of different kidney preservation strategies on early graft function in pig-to-baboon XTx in a series of eight cases of pig-to-baboon xenotransplantation performed after five hours of cold ischemic time and compares these results to six cases of pig-to-baboon xenotransplantation performed with minimal ischemic time. Our data indicates that porcine kidneys appear to be particularly sensitive to IRI after cold preservation, especially across xenogeneic barriers, and routine static cold storage leads to hyperacute graft loss even in recipients with low levels of preformed antibodies. Hypothermic machine perfusion minimizes IRI and may prevent early xenograft loss.

Research prospects for kidney xenotransplantation: a bibliometric analysis

BACKGROUND

Xenograft kidney transplantation has been receiving increasing attention. The purpose of this study is to use bibliometric analysis to identify papers in this research field and explore their current status and development trends.

METHODS

Using the data in the Web of Science core database from Clarivate Analytics as the object of study, we used 'TS = Kidney OR Renal AND xenotransplantation' as the search term to find all literature from 1980 to 2 November 2022.

RESULTS

In total, 1005 articles were included. The United States has the highest number of publications and has made significant contributions in this field. Harvard University was at the forefront of this study. Professor Cooper has published 114 articles in this field. Xenotransplantation has the largest number of relevant articles. Transplantation was the most cited journal. High-frequency keywords illustrated the current state of development and future trends in xenotransplantation. The use of transgenic pigs and the development of coordinated co-stimulatory blockers have greatly facilitated progress in xenotransplantation research. We found that 'co-stimulation blockade', 'xenograft survival', 'pluripotent stem cell', 'translational research', and 'genetic engineering' were likely to be the focus of attention in the coming years.

CONCLUSIONS

This study screened global publications related to xenogeneic kidney transplantation; analyzed their literature metrology characteristics; identified the most cited articles in the research field; understood the current situation, hot spots, and trends of global research; and provided future development directions for researchers and practitioners.

Genetically Modified Porcine Kidneys Have Sufficient Tissue Integrity for Use in Pig-to-Human Xenotransplantation

OBJECTIVE

We sought to determine if genetically modified porcine kidneys used for xenotransplantation had sufficient tissue integrity to support long-term function in a human recipient.

BACKGROUND

Kidney transplantation remains the best available treatment for patients with end-stage kidney disease. However, a shortage of available donor human kidneys prevents many patients from achieving the benefits of transplantation. Xenotransplantation is a potential solution to this shortage. Recent pre-clinical human studies have demonstrated kidneys from genetically modified pig donors can be transplanted without hyperacute rejection and are capable of providing creatinine and other solute clearance. It is unknown whether the porcine kidneys would tolerate the relatively higher resting blood pressure in an adult human recipient compared with the pig donor or non-human primate (NHP) recipients used in translational studies. Furthermore, previous experience in NHPs raised concerns about the tissue integrity of the porcine ureter and post-xenotransplant growth of the porcine kidney.

METHODS

Kidneys recovered from porcine donors with 10 gene edits were transplanted into decedent brain-dead recipients who were not eligible for organ donation. Decedents underwent bilateral native nephrectomy before transplant and were followed for 3 to 7 days. Standard induction and maintenance immunosuppression was used as previously reported. Vital signs, including blood pressure, were recorded frequently. Kidney xenografts were assessed daily, serially biopsied, and were measured at implantation and study completion.

RESULTS

Three decedents underwent successful xenotransplantation. Subcapsular hematomas developed, requiring incision of the xenograft capsules to prevent Page kidney. Blood pressures were maintained in a physiologic range for adult humans (median arterial pressures (MAP) 108.5 mm Hg (Interquartile Range (IQR): 97-114 mm Hg), 74 mm Hg (IQR: 71-78 mm Hg), and 95 mm Hg (IQR: 88-99 mm Hg, respectively) and no bleeding complications or aneurysm formation was observed. Serial biopsies were taken from the xenografts without apparent loss of tissue integrity despite the lack of a capsule. Ureteroneocystotomies remained intact without evidence of urine leak. Xenograft growth was observed, but plateaued, in 1 decedent with increased volume of the left and right xenografts by 25% and 26%, respectively, and in the context of human growth hormone levels consistently less <0.1 ng/ml and insulin-like growth factor 1 levels ranging from 34-50 ng/ml.

CONCLUSIONS

The findings of this study suggest kidneys from 10-gene edited porcine donors have sufficient tissue integrity to tolerate xenotransplantation into a living human recipient. There was no evidence of anastomotic complications, and the xenografts tolerated needle biopsy without issue. Xenograft growth occurred but plateaued by the study end; further observation and investigation will be required to confirm this finding and elucidate underlying mechanisms.

[Xenotransplantation of solid organs]

Transplantation of genetically modified porcine hearts and kidneys could become a solution to the persistent shortage of human organ donors. Progress has been made in genetic engineering of donor pigs, preservation techniques after organ harvesting and immunosuppression using co-stimulation blockade with anti-CD40/CD40L monoclonal antibodies. Progress has also been made in in the development of methods that detect pathogenic porcine viruses and prevent their transmission to the recipient. As normal land breed pig organs continue to grow in the recipient to their original size, different pig breeds (such as Auckland Island pigs) are now used which reach a final size suitable for humans. Alternatively, a knock-out of the growth hormone receptor gene has been established, e.g., in the 10GM genetically modified pigs from Revivicor/United Therapeutics, USA. The first clinical pilot studies including patients suffering from terminal heart failure are expected to start in Germany in about 2 years.

Physiological basis for xenotransplantation from genetically modified pigs to humans

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

Cross-Species Insights into Autosomal Dominant Polycystic Kidney Disease: Provide an Alternative View on Research Advancement

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a prevalent hereditary disorder that affects the kidneys, characterized by the development of an excessive number of fluid-filled cysts of varying sizes in both kidneys. Along with the progression of ADPKD, these enlarged cysts displace normal kidney tissue, often accompanied by interstitial fibrosis and inflammation, and significantly impair renal function, leading to end-stage renal disease. Currently, the precise mechanisms underlying ADPKD remain elusive, and a definitive cure has yet to be discovered. This review delineates the epidemiology, pathological features, and clinical diagnostics of ADPKD or ADPKD-like disease across human populations, as well as companion animals and other domesticated species. A light has been shed on pivotal genes and biological pathways essential for preventing and managing ADPKD, which underscores the importance of cross-species research in addressing this complex condition. Treatment options are currently limited to Tolvaptan, dialysis, or surgical excision of large cysts. However, comparative studies of ADPKD across different species hold promise for unveiling novel insights and therapeutic strategies to combat this disease.

Pigs in Transplantation Research and Their Potential as Sources of Organs in Clinical Xenotransplantation

The pig has long been used as a research animal and has now gained importance as a potential source of organs for clinical xenotransplantation. When an organ from a wild-type (i. e., genetically unmodified) pig is transplanted into an immunosuppressed nonhuman primate, a vigorous host immune response causes hyperacute rejection (within minutes or hours). This response has been largely overcome by 1) extensive gene editing of the organ-source pig and 2) the administration to the recipient of novel immunosuppressive therapy based on blockade of the CD40/CD154 T cell costimulation pathway. Gene editing has consisted of 1) deletion of expression of the 3 known carbohydrate xenoantigens against which humans have natural (preformed) antibodies and 2) the introduction of human 'protective' genes. The combination of gene editing and novel immunosuppressive therapy has extended life-supporting pig kidney graft survival to greater than 1 y and of pig heart survival to up to 9 mo. This review briefly describes the techniques of gene editing, the potential risks of transfer of porcine endogenous retroviruses with the organ, and the need for breeding and housing of donor pigs under biosecure conditions.

Developments in kidney xenotransplantation

The search for kidney xenografts that are appropriate for patients with end-stage renal disease has been ongoing since the beginning of the last century. The major cause of xenograft loss is hyperacute and acute rejection, and this has almost been overcome via scientific progress. The success of two pre-clinical trials of α1,3-galactosyltransferase gene-knockout porcine kidneys in brain-dead patients in 2021 triggered research enthusiasm for kidney xenotransplantation. This minireview summarizes key issues from an immunological perspective: the discovery of key xenoantigens, investigations into key co-stimulatory signal inhibition, gene-editing technology, and immune tolerance induction. Further developments in immunology, particularly immunometabolism, might help promote the long-term outcomes of kidney xenografts.

American Society of Transplant Surgeons-American Society of Transplantation report of FDA meeting on regulatory expectations for xenotransplantation products

In June 2022, the US Food and Drug Administration Center for Biologics Evaluation and Research held the 73rd meeting of the Cellular, Tissue, and Gene Therapies Advisory Committee for public discussion of regulatory expectations for xenotransplantation products. The members of a joint American Society of Transplant Surgeons/American Society of Transplantation committee on xenotransplantation compiled a meeting summary focusing on 7 topics believed to be key by the committee: (1) preclinical evidence supporting progression to a clinical trial, (2) porcine kidney function, (3) ethical aspects, (4) design of initial clinical trials, (5) infectious disease issues, (6) industry perspectives, and (7) regulatory oversight.

Transitioning of renal transplant pathology from allograft to xenograft and tissue engineering pathology: Are we prepared?

Currently, the most feasible and widely practiced option for patients with end-stage organ failure is the transplantation of part of or whole organs, either from deceased or living donors. However, organ shortage has posed and is still posing a big challenge in this field. Newer options being explored are xenografts and engineered/bioengineered tissues/organs. Already small steps have been taken in this direction and sooner or later, these will become a norm in this field. However, these developments will pose different challenges for the diagnosis and management of problems as compared with traditional allografts. The approach to pathologic diagnosis of dysfunction in these settings will likely be significantly different. Thus, there is a need to increase awareness and prepare transplant diagnosticians to meet this future challenge in the field of xenotransplantation/ regenerative medicine. This review will focus on the current status of transplant pathology and how it will be changed in the future with the emerging scenario of routine xenotransplantation.

Renin-angiotensin-aldosterone system function in the pig-to-baboon kidney xenotransplantation model

After pig-to-baboon kidney transplantation, episodes of hypovolemia and hypotension from an unexplained mechanism have been reported. This study evaluated the renin-angiotensin-aldosterone system post-kidney xenotransplantation. Kidneys from genetically-engineered pigs were transplanted into 5 immunosuppressed baboons after the excision of the native kidneys. Immunosuppressive therapy was based on the blockade of the CD40/CD154 costimulation pathway. Plasma renin, angiotensinogen (AGT), angiotensin II (Ang II), aldosterone levels, and urine osmolality and electrolytes were measured in healthy pigs, healthy nonimmunosuppressed baboons, and immunosuppressed baboons with life-supporting pig kidney grafts. After pig kidney transplantation, plasma renin and Ang II levels were not significantly different, although Ang II trended lower, even though plasma AGT and potassium were increased. Plasma aldosterone levels were unchanged. Urine osmolality and sodium concentration were decreased. Even in the presence of increasing AGT and potassium levels, lower plasma Ang II concentrations may be because of reduced, albeit not absent, the reactivity of pig renin to cleave baboon AGT, suggesting an impaired response of the renin-angiotensin-aldosterone system to hypovolemic and hypotensive episodes. The maintenance of aldosterone may be protective. The reduced urine osmolality and sodium concentration reflect the decreased ability of the pig kidney to concentrate urine. These considerations should not prohibit successful clinical pig kidney xenotransplantation.

Vascularized Islet Transplantation as Composite Islet-Kidney Grafts with Nanoparticle-Labeled Islets in Large Animal Preclinical Transplant Models

Although there are many patients with diabetes and end-stage renal failure (DM/ESRD) who would benefit from a transplantation strategy that addresses both their ESRD and its underlying cause, current methods of islet and kidney transplantation using live donors have had only limited success. The first major obstacle is that the number of islets obtained from a live donor partial pancreatectomy is generally insufficient to cure diabetes in recipients, as large numbers of intraportally administered islets are lost due to ischemia before they are engrafted and vascularized in the recipient liver. To overcome this hurdle, we have developed a strategy to transplant islets as a vascularized graft. Autologous prevascularization of donor islets under the donor's own renal capsule prior to transplantation preserves islets and thus achieves normal glycemic control in diabetic recipients in our preclinical transplant models with a limited donor pancreas resection. In addition, from an immunological perspective, the innate tolerogenic qualities of the kidney provide immunoprotection for the engrafted, vascularized islets when they are transplanted as part of the composite islet-kidney (I-K) grafts. This "Trojan Horse" approach of transplanting a composite I-K eliminates the lengthy time which is otherwise required for vascularization of intraportally administered free islets, minimizing loss of islets to ischemic damage and facilitating the induction of tolerance. We have also recently developed a strategy to further minimize the required size of resected donor pancreas to prepare composite I-K graft using a novel, synthesized, small interfering RNA (siRNA)-nanoparticle probe. In this chapter, we introduce our living donor transplantation strategy to cure diabetic nephropathy using composite I-K graft.

Practical ethical concerns in allocation of pig kidneys to humans

The fundamental ethical question of whether pig organs should be transplanted into humans has been settled, as recent surgeries demonstrating proof of concept demonstrate. Other issues need to be considered and reconciled before xenotransplantation of pig kidneys becomes a solution to the organ shortage for people waiting for a kidney transplant or as a viable alternative to the deceased donor or living donor human kidneys. Human trials will be needed beyond brain-dead individuals to show that xenotransplantation is safe from immunologic and infectious standpoints. Transplant centers will need to show that xenotransplantation provides a long-term benefit to recipients and is financially viable. If trials are successful and receive regulatory approval, pig xenotransplants may become another option for people waiting for a kidney. Before patients are discharged with a functioning xenograft, practical issues with ethical implications remain.

What is the clinical relevance of deviant serum calcium and phosphate levels after pig-to-primate kidney xenotransplantation?

Experience from human renal allotransplantation informs us that disturbances in serum calcium and phosphate levels are relatively common. Post-transplant hypercalcemia is associated with an increased risk of recipient mortality, but not of graft loss or nephropathy, and post-transplant hyperphosphatemia with an increased risk of both recipient mortality and death-censored graft failure, but neither post-transplant hypocalcemia nor hypophosphatemia is associated with adverse outcome. Studies after pig-to-nonhuman primate kidney xenotransplantation have demonstrated consistent supranormal serum calcium and subnormal serum phosphate levels. If these trends in serum electrolyte levels were to occur following pig-to-human kidney xenotransplantation, the data from allotransplant studies would indicate an increased risk of recipient mortality (associated with hypercalcemia) but not of graft loss or nephropathy, and no adverse outcome from hypophosphatemia. Furthermore, some nonhuman primates are now surviving in a healthy state for longer than a year after life-supporting pig kidney transplantation, suggesting that chronic hypercalcemia and/or hypophosphatemia are not detrimental to long-term survival, and should not prevent clinical trials of pig kidney transplantation from being undertaken.

Wearable and implantable artificial kidney devices for end-stage kidney disease treatment-Current status and review

BACKGROUND

Chronic kidney disease (CKD) is a major cause of early death worldwide. By 2030, 14.5 million people will have end-stage kidney disease (ESKD, or CKD stage 5), yet only 5.4 million will receive kidney replacement therapy (KRT) due to economic, social, and political factors. Even for those who are offered KRT by various means of dialysis, the life expectancy remains far too low.

OBSERVATION

Researchers from different fields of artificial organs collaborate to overcome the challenges of creating products such as Wearable and/or Implantable Artificial Kidneys capable of providing long-term effective physiologic kidney functions such as removal of uremic toxins, electrolyte homeostasis, and fluid regulation. A focus should be to develop easily accessible, safe, and inexpensive KRT options that enable a good quality of life and will also be available for patients in less-developed regions of the world.

CONCLUSIONS

Hence, it is required to discuss some of the limits and burdens of transplantation and different techniques of dialysis, including those performed at home. Furthermore, hurdles must be considered and overcome to develop wearable and implantable artificial kidney devices that can help to improve the quality of life and life expectancy of patients with CKD.

[Research on the development of genetically engineered xenogenic porcine skin and its application in the treatment of burn wounds]

In the recent years, the shortage of allo-skin sources has resulted in great challenges for salvage of patients with large area severe burns. Although being similar to human skin in construction and function, the clinical application of xenogenic porcine skin in burn wound management is limited due to factors including immuno-rejection, porcine endogenous retroviruses infection, etc. With the development of gene editing technology, especially the emerge of clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein-9 system, multiple target genes could be possibly edited at the same time, which will bring broad prospect for the application of xenogenic porcine skin in the treatment of burn wounds. The paper mainly discusses the development, the existed barrier, the strategies of gene modification/editing, and the applications and research of xenogenic porcine skin xenografts in the clinical treatment of burn wound.

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

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

Cardiac and Pulmonary Histopathology in Baboons Following Genetically-Engineered Pig Orthotopic Heart Transplantation

BACKGROUND Although improving, survival after pig orthotopic heart transplantation (OHTx) in baboons has been mixed and largely poor. The causes for the high incidence of early failure remain uncertain. MATERIAL AND METHODS We have carried out pig OHTx in 4 baboons. Two died or were euthanized within hours, and 2 survived for 3 and 8 months, respectively. There was evidence of a significant 'cytokine storm' in the immediate post-OHTx period with the elevations in IL-6 correlating closely with the final outcome. RESULTS All 4 baboons demonstrated features suggestive of respiratory dysfunction, including increased airway resistance, hypoxia, and tachypnea. Histopathological observations of pulmonary infiltration by neutrophils and, notably, eosinophils within vessels and in the perivascular and peribronchiolar space, with minimal cardiac pathology, suggested a role for early lung acute inflammation. In one, features suggestive of transfusion-related acute lung injury were present. The 2 longer-term survivors died of (i) a cardiac dysrhythmia with cellular infiltration around the conducting tissue (at 3 months), and (ii) mixed cellular and antibody-mediated rejection (at 8 months). CONCLUSIONS These initial findings indicate a potential role of acute lung injury early after OHTx. If this response can be prevented, increased survival may result, providing an opportunity to evaluate the factors affecting long-term survival.

The science of xenotransplantation for nephrologists

PURPOSE OF REVIEW

The field of xenotransplantation has seen remarkable progress since its inception with recent preclinical trials in human recipients pushing kidney xenotransplantation one-step closer to clinical reality. In this review, we update practicing clinicians on recent advances in kidney xenotransplantation given the proximity of clinical trials in humans.

RECENT FINDINGS

Early studies in the field established the physiologic basis of xenotransplantation and suggested that the pig kidney will support human physiology. Genetic engineering of source pigs has greatly reduced the immunogenicity of kidney grafts, and studies in nonhuman primates have demonstrated the viability of kidney xenotransplants for months after transplantation. Finally, a recent study in a novel preclinical human model demonstrated that key findings in NHP experiments are generalizable to humans, namely, the absence of hyperacute rejection.

SUMMARY

Overall, it appears that critical physiologic, immunologic and technical barriers to implementation of clinical trials in humans have been overcome.

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.

Generation of functional chimeric kidney containing exogenous progenitor-derived stroma and nephron via a conditional empty niche

Generation of new kidneys can be useful in various research fields, including organ transplantation. However, generating renal stroma, an important component tissue for structural support, endocrine function, and kidney development, remains difficult. Organ generation using an animal developmental niche can provide an appropriate in vivo environment for renal stroma differentiation. Here, we generate rat renal stroma with endocrine capacity by removing mouse stromal progenitor cells (SPCs) from the host developmental niche and transplanting rat SPCs. Furthermore, we develop a method to replace both nephron progenitor cells (NPCs) and SPCs, called the interspecies dual replacement of the progenitor (i-DROP) system, and successfully generate functional chimeric kidneys containing rat nephrons and stroma. This method can generate renal tissue from progenitors and reduce xenotransplant rejection. Moreover, it is a safe method, as donor cells do not stray into nontarget organs, thus accelerating research on stem cells, chimeras, and xenotransplantation.

Physiological aspects of pig kidney xenotransplantation and implications for management following transplant

Successful organ transplantation between species is now possible, using genetic modifications. This article aims to provide a comprehensive overview of the differences and similarities in kidney function between humans, primates, and pigs, in preparation for pig-allograft to human xenotransplantation. The kidney, as the principal defender of body homeostasis, acts as a sensor, effector, and regulator of physiologic feedback systems. Considerations are made for anticipated effects on each system when a pig kidney is placed into a human recipient. Discussion topics include anatomy, global kidney function, sodium and water handling, kidney hormone production and response to circulating hormones, acid-base balance, and calcium and phosphorus handling. Based on available data, pig kidneys are anticipated to be compatible with human physiology, despite a few barriers.

Serum Antibody Binding and Cytotoxicity to Pig Cells in Chinese Subjects: Relevance to Clinical Renal Xenotransplantation

Kidney xenotransplantation is expected to contribute to resolving the shortage of kidneys from deceased human donors. Although progress in experimental life-supporting pig renal xenotransplantation has been encouraging, there are still issues to be considered before a clinical trial can be initiated. We attempted to clarify some of these by an in vitro study. Blood was drawn from healthy volunteers (Volunteers, n=20), patients with end-stage renal disease (ESRD, n=20) pre-operation (Pre), and on Day 1 (POD 1) and Day 14 (POD 14) after renal allotransplantation, brain-dead organ donors (DBD, n=20), and renal allotransplant recipients who were currently experiencing T cell-mediated rejection (Allo-TCMR, n=20). Serum IgM/IgG binding to, and complement-dependent cytotoxicity (CDC) of, PBMCs and RBCs from (a) wild-type (WT), (b) α1,3-galactosyltransferase gene-knockout (GTKO), (c) GTKO/beta-1,4-N-acety1 galactosaminyltransferase 2-knockout (GTKO/β4GalNT2KO), (d) GTKO/cytidine monophosphate-N-acetylneuraminic acid hydroxylase-knockout (GTKO/CMAHKO), and (e) GTKO/β4GalNT2KO/CMAHKO/hCD55 (TKO/hCD55) pigs were measured by flow cytometry. We obtained the following results: (i) Serum IgM/IgG binding and CDC in Volunteers were significantly greater to WT, GTKO, and GTKO/β4GalNT2KO PBMCs or RBCs than to GTKO/CMAHKO and TKO/hCD55 cells; (ii) ESRD, DBD, and Allo-TCMR serum antibody binding and CDC to WT pig PBMCs were significantly greater than to GTKO, GTKO/β4GalNT2KO, GTKO/CMAHKO, and TKO/hCD55 cells; (iii) antibody binding to GTKO/CMAHKO pig cells was significantly lower in hemodialysis than peritoneal dialysis patients. (iv) Two of twenty allotransplantation recipients' serum IgG binding to GTKO pig PBMCs increased on POD14 compared with Pre, but IgG binding to GTKO pig RBCs did not; (v) In all sera, the lowest antibody binding and CDC were to GTKO/CMAHKO and TKO/CD55 pig cells. We conclude (i) CMAHKO in the pig may be critical to the success of clinical pig kidney xenotransplantation, and may be the most important after GTKO, at least in Chinese patients; (ii) subjects with ESRD, or who are immunosuppressed after kidney allotransplantation, and DBD, have lower levels of antibody binding and CDC to genetically-engineered pig cells than do volunteers; (iii) TKO pigs with selected human 'protective' transgenes, e.g., CD55, are likely to prove to be the optimal sources of kidneys for clinical xenotransplantation.

Histopathology of pig kidney grafts with/without expression of the carbohydrate Neu5Gc in immunosuppressed baboons

INTRODUCTION

Pigs deficient in three glycosyltransferase enzymes (triple-knockout [TKO] pigs, that is, not expressing the three known carbohydrate xenoantigens) and expressing 'protective' human transgenes are considered a likely source of organs for transplantation into human recipients. Some human sera have no or minimal natural antibody binding to red blood cells (RBCs) and peripheral blood mononuclear cells (PBMCs) from TKO pigs. However, all Old World monkeys exhibit natural antibody binding to TKO pig cells. The xenoantigen targets of Old World monkey natural antibodies are postulated to be carbohydrate moieties exposed when the expression of the carbohydrate N-glycolylneuraminic acid (Neu5Gc) is deleted. The aim of this study was to compare the survival in baboons and histopathology of renal grafts from pigs that either (a) expressed Neu5Gc (GTKO pigs; Group 1) or (b) did not express Neu5Gc (GTKO/CMAHKO [DKO] or TKO pigs; Group 2).

METHODS

Life-supporting renal transplants were carried out using GTKO (n = 5) or DKO/TKO (n = 5) pig kidneys under an anti-CD40mAb-based immunosuppressive regimen.

RESULTS

Group 1 baboons survived longer than Group 2 baboons (median 237 vs. 35 days; mean 196 vs. 57 days; p < 0.07) and exhibited histopathological features of antibody-mediated rejection in only two kidneys. Group 2 exhibited histopathological features of antibody-mediated rejection in all five grafts, with IgM and IgG binding to renal interstitial arteries and peritubular capillaries. Rejection-free survival was significantly longer in Group 1 (p < 0.05).

CONCLUSIONS

The absence of expression of Neu5Gc on pig kidney grafts is associated with increased binding of baboon antibodies to pig endothelium and reduced graft survival.

"You cannot stay in the laboratory forever"*: Taking pig kidney xenotransplantation from the laboratory to the clinic

Progress in life-supporting kidney transplantation in the genetically-engineered pig-to-nonhuman primate model has been encouraging, with pig kidneys sometimes supporting life for > 1 year. What steps need to be taken by (i) the laboratory team, and (ii) the clinical team to prepare for the first clinical trial? The major topics include (i) what currently-available genetic modifications are optimal to reduce the possibility of graft rejection, (ii) what immunosuppressive therapeutic regimen is optimal, and (iii) what steps need to be taken to minimize the risk of transfer of an infectious microorganism with the graft. We suggest that patients who are unlikely to live long enough to receive a kidney from a deceased human donor would benefit from the opportunity of a period of dialysis-free support by a pig kidney, and the experience gained would enable xenotransplantation to progress much more rapidly than if we remain in the laboratory.

What Therapeutic Regimen Will Be Optimal for Initial Clinical Trials of Pig Organ Transplantation?

We discuss what therapeutic regimen might be acceptable/successful in the first clinical trial of genetically engineered pig kidney or heart transplantation. As regimens based on a calcineurin inhibitor or CTLA4-Ig have proved unsuccessful, the regimen we administer to baboons is based on induction therapy with antithymocyte globulin, an anti-CD20 mAb (Rituximab), and cobra venom factor, with maintenance therapy based on blockade of the CD40/CD154 costimulation pathway (with an anti-CD40 mAb), with rapamycin, and a corticosteroid. An anti-inflammatory agent (etanercept) is administered for the first 2 wk, and adjuvant therapy includes prophylaxis against thrombotic complications, anemia, cytomegalovirus, and pneumocystis. Using this regimen, although antibody-mediated rejection certainly can occur, we have documented no definite evidence of an adaptive immune response to the pig xenograft. This regimen could also form the basis for the first clinical trial, except that cobra venom factor will be replaced by a clinically approved agent, for example, a C1-esterase inhibitor. However, none of the agents that block the CD40/CD154 pathway are yet approved for clinical use, and so this hurdle remains to be overcome. The role of anti-inflammatory agents remains unproven. The major difference between this suggested regimen and those used in allotransplantation is the replacement of a calcineurin inhibitor with a costimulation blockade agent, but this does not appear to increase the complications of the regimen.

Evidence suggesting that deletion of expression of N-glycolylneuraminic acid (Neu5Gc) in the organ-source pig is associated with increased antibody-mediated rejection of kidney transplants in baboons

Pigs deficient in three glycosyltransferase enzymes (triple-knockout [TKO] pigs) and expressing "protective" human transgenes are likely sources of organs for transplantation into human recipients. Testing of human sera against red blood cells (RBCs) and peripheral blood mononuclear cells (PBMCs) from TKO pigs has revealed minimal evidence of natural antibody binding. However, unlike humans, baboons exhibit natural antibody binding to TKO pig cells. The xenoantigen specificities of these natural antibodies are postulated to be one or more carbohydrate moieties exposed when N-glycolylneuraminic acid (Neu5Gc) is deleted. The aim of this study was to compare the survival of renal grafts in baboons from pigs that either expressed Neu5Gc (GTKO pigs; Group1, n = 5) or did not express Neu5Gc (GTKO/CMAHKO [DKO] or TKO pigs; Group2, n = 5). An anti-CD40mAb-based immunosuppressive regimen was administered in both groups. Group1 kidneys functioned for 90-260 days (median 237, mean 196 days), with histopathological features of antibody-mediated rejection in two kidneys. Group2 kidneys functioned for 0-183 days (median 35, mean 57), with all of the grafts exhibiting histologic features of antibody-mediated rejection. These findings suggest that the absence of expression of Neu5Gc on pig kidneys impacts graft survival in baboon recipients.

Recent progress and remaining hurdles toward clinical xenotransplantation

BACKGROUND

Xenotransplantation has made tremendous progress over the last decade.

METHODS

We discuss kidney and heart xenotransplantation, which are nearing initial clinical trials.

RESULTS

Life sustaining genetically modified kidney xenografts can now last for approximately 500 days and orthotopic heart xenografts for 200 days in non-human primates. Anti-swine specific antibody screening, preemptive desensitization protocols, complement inhibition and targeted immunosuppression are currently being adapted to xenotransplantation with the hope to achieve better control of antibody-mediated rejection (AMR) and improve xenograft longevity. These newest advances could probably facilitate future clinical trials, a significant step for the medical community, given that dialysis remains difficult for many patients and can have prohibitive costs. Performing a successful pig-to-human clinical kidney xenograft, that could last for more than a year after transplant, seems feasible but it still has significant potential hurdles to overcome. The risk/benefit balance is progressively reaching an acceptable equilibrium for future human recipients, e.g. those with a life expectancy inferior to two years. The ultimate question at this stage would be to determine if a "proof of concept" in humans is desirable, or whether further experimental/pre-clinical advances are still needed to demonstrate longer xenograft survival in non-human primates.

CONCLUSION

In this review, we discuss the most recent advances in kidney and heart xenotransplantation, with a focus on the prevention and treatment of AMR and on the recipient's selection, two aspects that will likely be the major points of discussion in the first pig organ xenotransplantation clinical trials.

Pig kidney xenotransplantation: Progress toward clinical trials

Pig organ xenotransplantation offers a solution to the shortage of deceased human organs for transplantation. The pathobiological response to a pig xenograft is complex, involving antibody, complement, coagulation, inflammatory, and cellular responses. To overcome these barriers, genetic manipulation of the organ-source pigs has largely been directed to two major aims-(a) deletion of expression of the known carbohydrate xenoantigens against which humans have natural (preformed) antibodies, and (b) transgenic expression of human protective proteins, for example, complement- and coagulation-regulatory proteins. Conventional (FDA-approved) immunosuppressive therapy is unsuccessful in preventing an adaptive immune response to pig cells, but blockade of the CD40:CD154 costimulation pathway is successful. Survival of genetically engineered pig kidneys in immunosuppressed nonhuman primates can now be measured in months. Non-immunological aspects, for example, pig renal function, a hypovolemia syndrome, and rapid growth of the pig kidney after transplantation, are briefly discussed. We suggest that patients on the wait-list for a deceased human kidney graft who are unlikely to receive one due to long waiting times are those for whom kidney xenotransplantation might first be considered. The potential risk of infection, public attitudes to xenotransplantation, and ethical, regulatory, and financial aspects are briefly addressed.

Growth hormone receptor knockout: Relevance to xenotransplantation

Xenotransplantation research has made considerable progress in recent years, largely through the increasing availability of pigs with multiple genetic modifications, effective immunosuppressive therapy, and anti-inflammatory therapy to protect pig tissues from the primate immune and inflammatory responses and correct molecular incompatibilities. Further study is required regarding identification and investigation of physiological incompatibilities. Although the exact cause remains uncertain, we and others have observed relatively rapid growth of kidney xenografts after transplantation into nonhuman primates (NHPs). There has also been some evidence of growth, or at least ventricular hypertrophy, of the pig heart after orthotopic transplantation into NHPs. Rapid growth could be problematic, particularly with regard to the heart within the relatively restricted confines of the chest. It has been suggested that the problem of rapid growth of the pig organ after transplantation could be resolved by growth hormone receptor (GHR) gene knockout in the pig. The GHR, although most well-known for regulating growth, has many other biological functions, including regulating metabolism and controlling physiological processes. Genetically modified GHRKO pigs have recently become available. We provide data on their growth compared to comparable pigs that do not include GHRKO, and we have reviewed the literature regarding the effect of GHRKO, and its relevance to xenotransplantation.

The resurgent landscape of xenotransplantation of pig organs in nonhuman primates

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

Old World Monkeys are less than ideal transplantation models for testing pig organs lacking three carbohydrate antigens (Triple-Knockout)

Triple-knockout (TKO) pigs (with added protective human transgenes) are likely to be optimal sources of organs for clinical organ xenotransplantation because many humans have minimal or no natural antibody to TKO pig cells. However, Old World monkeys (OWMs) have naturally-existing antibodies directed to TKO cells. We measured anti-pig IgM/IgG binding, and complement-dependent cytotoxicity to wild-type (WT), α1,3-galactosyltransferase gene-knockout (GTKO), and TKO pig peripheral blood mononuclear cells (PBMCs) using sera from humans, several OWMs, and two New World monkeys (NWMs). Furthermore, we compared survival of GTKO (n = 5) and TKO (n = 3) pig kidneys in baboons. OWMs had significantly greater IgM binding and cytotoxicity to TKO PBMCs than humans or NWMs. Mean anti-TKO IgM was significantly higher in OWMs and significantly lower in NWMs than in humans. Cytotoxicity of OWM sera to TKO PBMCs was significantly greater than of human serum, but there was no significant difference between human and NWM sera. The median survival of TKO pig kidneys (4 days) in baboons was significantly shorter than that of GTKO kidneys (136 days) (p < 0.05). Even though considered ideal for clinical xenotransplantation, the presence of naturally-existing antibodies to TKO pig cells in OWMs complicates the transplantation of TKO pig kidneys in OWMs.

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.

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.

Life-supporting Kidney Xenotransplantation From Genetically Engineered Pigs in Baboons: A Comparison of Two Immunosuppressive Regimens

BACKGROUND

The aims of this study were to evaluate the efficacy of US Food and Drug Administration-approved drugs in genetically engineered pig-to-baboon kidney xenotransplantation and compare the results with those using an anti-CD40 monoclonal antibody (mAb)-based regimen.

METHODS

Ten life-supporting kidney transplants were carried out in baboons using α1,3-galactosyltransferase gene-knockout/CD46 pigs with various other genetic manipulations aimed at controlling coagulation dysregulation. Eight transplants resulted in informative data. Immunosuppressive therapy consisted of induction with antithymocyte globulin and anti-CD20mAb, and maintenance based on either (1) CTLA4-Ig and/or tacrolimus (+rapamycin or mycophenolate mofetil) (GroupA [US Food and Drug Administration-approved regimens], n = 4) or (2) anti-CD40mAb + rapamycin (GroupB, n = 4). All baboons received corticosteroids, interleukin-6R blockade, and tumor necrosis factor-α blockade. Baboons were followed by clinical and laboratory monitoring of kidney function, coagulation, and immune parameters. At euthanasia, morphological and immunohistochemical studies were performed on the kidney grafts.

RESULTS

The median survival in GroupB was 186 days (range 90-260), which was significantly longer than in GroupA; median 14 days (range 12-32) (P < 0.01). Only GroupA baboons developed consumptive coagulopathy and the histopathological features of thrombotic microangiopathic glomerulopathy and interstitial arterial vasculitis.

CONCLUSIONS

Recognizing that the pig donors in each group differed in some genetic modifications, these data indicate that maintenance immunosuppression including anti-CD40mAb may be important to prevent pig kidney graft failure.

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.

Perspectives on the Optimal Genetically Engineered Pig in 2018 for Initial Clinical Trials of Kidney or Heart Xenotransplantation

For a clinical trial today, what might realistically be the optimal pig among those currently available? Deletion of expression of the 3 pig carbohydrate antigens, against which humans have natural (preformed) antibodies (triple-knockout pigs), should form the basis of any clinical trial. However, because both complement and coagulation can be activated in the absence of antibody, the expression of human complement- and coagulation-regulatory proteins is likely to be important in protecting the graft further. Any genetic manipulation that might reduce inflammation of the graft, for example, expression of hemeoxygenase-1 or A20, may also be beneficial to the long-term survival of the graft. The transgene for human CD47 is likely to have a suppressive effect on monocyte/macrophage and T-cell activity. Furthermore, deletion of xenoantigen expression and expression of a human complement-regulatory protein are both associated with a reduced T-cell response. Although there are several other genetic manipulations that may reduce the T-cell response further, it seems likely that exogenous immunosuppressive therapy, particularly if it includes costimulation blockade, will be sufficient. We would therefore suggest that, with our present knowledge and capabilities, the optimal pig might be a triple-knockout pig that expressed 1 or more human complement-regulatory proteins, 1 or more human coagulation-regulatory proteins, a human anti-inflammatory transgene, and CD47. Absent or minimal antibody binding is important, but we suggest that the additional insertion of protective human transgenes will be beneficial, and may be essential.