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

Allosensitisation in NHP results in cross-reactive anti-SLA antibodies not detected by a lymphocyte-based flow cytometry crossmatch

Xenotransplantation is a potential option for individuals for whom an acceptable human allograft is unavailable. Individuals with broadly reactive HLA antibodies due to prior exposure to foreign HLA are potential candidates for a clinical xenotransplant trial. It remains controversial if allosensitisation results in the development of cross-reactive antibodies against SLA. This may require increased histocompatibility scrutiny for highly sensitised individuals prior to enrollment in a clinical trial. Serum samples were obtained from non-human primates sensitised via serial skin transplantation from maximally MHC-mismatched donor, as reported. Sera from pre- and post-allosensitisation timepoints were assessed in a flow crossmatch (FXM) for IgM and IgG binding to pig splenocytes with or without red blood cell adsorption. Xenoreactive antibodies were eluted from pig splenocytes and screened on a single antigen HLA bead assay. A MHC Matchmaker algorithm was developed to predict potential conserved amino acid motifs among the pig, NHP, and human. Our sensitised NHP model was used to demonstrate that allosensitisation does not result in an appreciable difference in xenoreactive antibody binding in a cell-based FXM. However, antibody elution and screening on single antigen HLA beads suggest the existence of potential cross-reactive antibodies against SLA. The cross-reactive IgG after allosensitisation were predicted by comparing the recipient Mamu alleles against its previous allograft donor Mamu alleles and the donor pig SLA alleles. Our study suggests that allosensitisation could elevate cross-reactive antibodies, but a more sensitive assay than a cell-based FXM is required to detect them. The MHC Matchmaker algorithm was developed as a potential tool to help determine amino acid motif conservation and reactivity pattern.

Current Status of Cardiac Xenotransplantation: Report of a Workshop of the German Heart Transplant Centers, Martinsried, March 3, 2023

This report comprises the contents of the presentations and following discussions of a workshop of the German Heart Transplant Centers in Martinsried, Germany on cardiac xenotransplantation. The production and current availability of genetically modified donor pigs, preservation techniques during organ harvesting, and immunosuppressive regimens in the recipient are described. Selection criteria for suitable patients and possible solutions to the problem of overgrowth of the xenotransplant are discussed. Obviously microbiological safety for the recipient and close contacts is essential, and ethical considerations to gain public acceptance for clinical applications are addressed. The first clinical trial will be regulated and supervised by the Paul-Ehrlich-Institute as the National Competent Authority for Germany, and the German Heart Transplant Centers agreed to cooperatively select the first patients for cardiac xenotransplantation.

Late graft failure of pig-to-rhesus renal xenografts has features of glomerulopathy and recipients have anti-swine leukocyte antigen class I and class II antibodies

Prolonged survival in preclinical renal xenotransplantation demonstrates that early antibody mediated rejection (AMR) can be overcome. It is now critical to evaluate and understand the pathobiology of late graft failure and devise new means to improve post xenograft outcomes. In renal allotransplantation the most common cause of late renal graft failure is transplant glomerulopathy-largely due to anti-donor MHC antibodies, particularly anti-HLA DQ antibodies. We evaluated the pig renal xenograft pathology of four long-surviving (>300 days) rhesus monkeys. We also evaluated the terminal serum for the presence of anti-SLA class I and specifically anti-SLA DQ antibodies. All four recipients had transplant glomerulopathy and expressed anti-SLA DQ antibodies. In one recipient tested for anti-SLA I antibodies, the recipient had antibodies specifically reacting with two of three SLA I alleles tested. These results suggest that similar to allotransplantation, anti-MHC antibodies, particularly anti-SLA DQ, may be a barrier to improved long-term xenograft outcomes.

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.

Genetic Engineering of Donor Pig for the First Human Cardiac Xenotransplantation: Combatting Rejection, Coagulopathy, Inflammation, and Excessive Growth

PURPOSE OF REVIEW

The first successful pig to human cardiac xenotransplantation in January 2022 represented a major step forward in the fields of heart failure, immunology, and applied genetic engineering, using a 10-gene edited (GE) pig. This review summarizes the evolution of preclinical modelling data which informed the use of each of the 10 genes modified in the 10-GE pig: GGTA1, Β4GalNT2, CMAH, CD46, CD55, TBM, EPCR, CD47, HO-1, and growth hormone receptor.

RECENT FINDINGS

The translation of the 10-GE pig from preclinical modelling to clinical compassionate xenotransplant use was the culmination of decades of research combating rejection, coagulopathy, inflammation, and excessive xenograft growth. Understanding these 10 genes with a view to their combinatorial effects will be useful in anticipated xenotransplant clinical trials.

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.

Milestones on the path to clinical pig organ xenotransplantation

Progress in pig organ xenotransplantation has been made largely through (1) genetic engineering of the organ-source pig to protect its tissues from the human innate immune response, and (2) development of an immunosuppressive regimen based on blockade of the CD40/CD154 costimulation pathway to prevent the adaptive immune response. In the 1980s, after transplantation into nonhuman primates (NHPs), wild-type (genetically unmodified) pig organs were rejected within minutes or hours. In the 1990s, organs from pigs expressing a human complement-regulatory protein (CD55) transplanted into NHPs receiving intensive conventional immunosuppressive therapy functioned for days or weeks. When costimulation blockade was introduced in 2000, the adaptive immune response was suppressed more readily. The identification of galactose-α1,3-galactose as the major antigen target for human and NHP anti-pig antibodies in 1991 allowed for deletion of expression of galactose-α1,3-galactose in 2003, extending pig graft survival for up to 6 months. Subsequent gene editing to overcome molecular incompatibilities between the pig and primate coagulation systems proved additionally beneficial. The identification of 2 further pig carbohydrate xenoantigens allowed the production of 'triple-knockout' pigs that are preferred for clinical organ transplantation. These combined advances enabled the first clinical pig heart transplant to be performed and opened the door to formal clinical trials.

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.

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

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

Progress in xenotransplantation: overcoming immune barriers

A major limitation of organ allotransplantation is the insufficient supply of donor organs. Consequently, thousands of patients die every year while waiting for a transplant. Progress in xenotransplantation that has permitted pig organ graft survivals of years in non-human primates has led to renewed excitement about the potential of this approach to alleviate the organ shortage. In 2022, the first pig-to-human heart transplant was performed on a compassionate use basis, and xenotransplantation experiments using pig kidneys in deceased human recipients provided encouraging data. Many advances in xenotransplantation have resulted from improvements in the ability to genetically modify pigs using CRISPR-Cas9 and other methodologies. Gene editing has the capacity to generate pig organs that more closely resemble those of humans and are hence more physiologically compatible and less prone to rejection. Despite such modifications, immune responses to xenografts remain powerful and multi-faceted, involving innate immune components that do not attack allografts. Thus, the induction of innate and adaptive immune tolerance to prevent rejection while preserving the capacity of the immune system to protect the recipient and the graft from infection is desirable to enable clinical xenotransplantation.

The 2021 IXA Keith Reemtsma Lecture: Moving xenotransplantation to the clinic

Keith Reemtsma was a pioneer in xenotransplantation, the Honorary Founding President of the International Xenotransplantation Association (in 1998), and a wonderful personality. It is a privilege to be invited to give this lecture in his memory. If he were alive today, he would be delighted to see the progress that has been made in pig organ transplantation into nonhuman primate recipients. This progress has largely resulted from two major advances: (i) the increasing availability of pigs with multiple genetic manipulations aimed at protecting the cells of the organ from the primate immune response and (ii) the introduction of novel immunosuppressive agents that block the CD40/CD154 costimulation pathway. There is strong evidence from numerous in vitro studies that the transplantation of a triple-knockout pig organ, particularly if expressing several human protective proteins, into a patient is likely to be significantly more successful than if that same organ is transplanted into a nonhuman primate recipient. With this fact in mind, and in view of the advances currently being made, the time has surely come when we need to consider moving from the laboratory to the clinic. However, there are still questions we need to definitively resolve: (i) What exact genetic modifications do we need in the organ-source pig? (ii) What exact immunosuppressive regimen will we choose? (iii) How will we monitor the immune response and diagnose and treat rejection? and (iv) How do we plan to prevent or treat potential infectious complications? Furthermore, when these matters have been resolved, which patients will be offered a pig organ in the first trial? We have suggested that patients who are very unlikely to survive until a suitable deceased human donor kidney becomes available are those who should be considered for the initial trials. Assessing public attitudes to xenotransplantation is also important before embarking on a clinical trial. I suggest that progress is much more likely to be made from a small clinical trial than if we persist in carrying out experiments in an animal model that no longer mimics the clinical situation.

Recent progress in the pig-to-nonhuman primate kidney transplantation model: Report of a symposium

Three excellent presentations at an industry-sponsored symposium at the (virtual) congress of the combined IXA/CTRMS (September 23-25, 2021) were directed to the value and limitations of the pig-to-nonhuman primate (NHP) kidney transplantation model. Daniel Firl and James Markmann provided a meta-analysis comparing the results of kidney allotransplantation and xenotransplantation in NHPs during the past 25 years. Remarkably, the authors had identified 73 published reports that included 910 individual experiments. Although recipient survival after allotransplantation was longer, the superiority over the survival of xenografts was less than anticipated. Given the excellent short- and medium-term results of clinical kidney allotransplantation today, these data provide hope that the results of clinical pig kidney xenotransplantation may prove significantly better than in NHPs. The authors identified several factors that were shown to statistically influence the success or failure of xenotransplantation. Jean Kwun provided valuable information relating to the longstanding question of whether the survival of a pig organ would be jeopardized if transplanted into an allosensitized recipient. He demonstrated that pig kidney transplantation in an HLA-sensitized patient may be at a disadvantage, although multiple genetic engineering of the organ-source pig significantly delayed rejection. In the initial clinical trials, therefore, it would seem wise to exclude any patient with evidence of anti-HLA antibodies. Andrew Adams reported longer survival (>1 year) of Rhesus monkeys with life-supporting pig kidney grafts than has been achieved previously. Although not consistently achieved, these excellent results were obtained with an anti-CD154mAb-based regimen after CD4⁺ T cell and partial CD20⁺ B cell depletion. Factors that might have contributed to this success, including the phenotype of the pig, the species of the recipient, the recipient's anti-pig antibody level, and the immunosuppressive regimen, were discussed. Importantly, pig kidney function appeared to be normal in long-term surviving monkeys. Each study contributed to our goal of introducing xenotransplantation into the clinic.

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.

"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.

Aspects of histocompatibility testing in xenotransplantation

Xenotransplantation, using genetically-modified pigs for clinical organ transplantation, is a solution to the organ shortage. The biggest barrier to clinical implementation is the antigenicity of pig cells. Humans possess preformed antibody to pig cells that initiate antibody-mediated rejection of pig organs in primates. Advances in genetic engineering have led to the development of a pig lacking the three known glycan xenoantigens (triple-knockout [TKO] pigs). A significant number of human sera demonstrate no antibody binding to TKO pig cells. As a result of the TKO pig's low antigen expression, survival of life-supporting pig organs in immunosuppressed nonhuman primates has significantly increased, and hope has been renewed for clinical trials of xenotransplantation. It is important to understand the context in which xenotransplantation's predecessor, allotransplantation, has been successful, and the steps needed for the success of xenotransplantation. Successful allotransplantation has been based on two main immunological approaches - (i) adequate immunosuppressive therapy, and (ii) careful histocompatibility matching. In vivo studies suggest that the available immunosuppressive regimens are adequate to suppress the human anti-pig cellular response. Methods to evaluate and screen patients for the first clinical xenotransplantation trial are the next challenge. The goal of this review is to summarize the history of histocompatibility testing, and the available tools that can be utilized to determine xenograft histocompatibility.

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.