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

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

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

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.

How should cardiac xenotransplantation be initiated in Japan?

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

Pretransplant Screening for Prevention of Hyperacute Graft Loss in Pig-to-primate Kidney Xenotransplantation

BACKGROUND

Xenotransplantation using pig organs is now a clinical reality. However, the process for xenograft recipient screening lacks clarity and scientific rigor: no established thresholds exist to determine which levels of preformed antipig natural antibodies (Nabs) will be safe for clinical xenograft transplantation, and hyperacute rejection (HAR) or acute humoral xenograft rejection (AHXR), which still impacts pig-to-primate kidney xenograft survivals, may impede broader application of pig-to-human clinical xenograft transplantation.

METHODS

We retrospectively examined 28 cases of pig-to-baboon kidney xenotransplantation using GalTKO±human complement regulatory protein (hCRP)-transgenic (Tg) pig donors, as well as 6 cases of triple-KO multi-Tg (10GE) pig donors, and developed screening algorithms to predict risk of HAR/AHXR based on recipient antipig Nab levels. Preformed Nabs were evaluated using both complement-dependent cytotoxicity and antibody (IgM and IgG) binding flow-cytometry assays.

RESULTS

High complement-dependent cytotoxicity was associated with HAR/AHXR as expected. However, we also found that high levels of IgG were independently associated with HAR/AHXR, and we developed 2 indices to interpret and predict the risk of IgG-mediated HAR/AHXR.

CONCLUSIONS

Based on the data in this study, we have established a new 2-step screening, which will be used for future clinical kidney xenotransplantation trials.

Iscalimab Combined With Transient Tesidolumab Prolongs Survival in Pig-to-Rhesus Monkey Renal Xenografts

OBJECTIVE

To evaluate the clinically relevant anti-CD40 antibody iscalimab for baseline immunosuppression in a preclinical pig-to-rhesus renal xenograft model.

SUMMARY BACKGROUND DATA

CD40/CD40L co-stimulation blockade-based immunosuppression has been more successful than calcineurin-based protocols in prolonging xenograft survival in preclinical models.

METHODS

GGTA1 knockout/CD55 transgenic pig kidneys were transplanted into rhesus monkeys (n = 6) receiving an iscalimab-based immunosuppressive regimen.

RESULTS

Two grafts were lost early (22 and 26 days) because of ectatic donor ureters with otherwise normal histology. The other recipients survived 171, 315, 422, and 439 days with good renal function throughout the posttransplant course. None of the recipients experienced serious infectious morbidity.

CONCLUSIONS

It may be reasonable to evaluate an iscalimab-based immunosuppressive regimen in clinical renal xenotransplantation.

Prolonged xenokidney graft survival in sensitized NHP recipients by expression of multiple human transgenes in a triple knockout pig

Genetic modification of porcine donors, combined with optimized immunosuppression, has been shown to improve outcomes of experimental xenotransplant. However, little is known about outcomes in sensitized recipients, a population that could potentially benefit the most from the clinical implementation of xenotransplantation. Here, five highly allosensitized rhesus macaques received a porcine kidney from GGTA1 (α1,3-galactosyltransferase) knockout pigs expressing the human CD55 transgene (1KO.1TG) and were maintained on an anti-CD154 monoclonal antibody (mAb)-based immunosuppressive regimen. These recipients developed de novo xenoreactive antibodies and experienced xenograft rejection with evidence of thrombotic microangiopathy and antibody-mediated rejection (AMR). In comparison, three highly allosensitized rhesus macaques receiving a kidney from GGTA1, CMAH (cytidine monophospho-N-acetylneuraminic acid hydroxylase), and b4GNT2/b4GALNT2 (β-1,4-N-acetyl-galactosaminyltransferase 2) knockout pigs expressing seven human transgenes including human CD46, CD55, CD47, THBD (thrombomodulin), PROCR (protein C receptor), TNFAIP3 (tumor necrosis factor-α-induced protein 3), and HMOX1 (heme oxygenase 1) (3KO.7TG) experienced significantly prolonged graft survival and reduced AMR, associated with dampened post-transplant humoral responses, early monocyte and neutrophil activation, and T cell repopulation. After withdrawal of all immunosuppression, recipients who received kidneys from 3KO.7TG pigs rejected the xenografts via AMR. These data suggest that allosensitized recipients may be suitable candidates for xenografts from genetically modified porcine donors and could benefit from an optimized immunosuppression regimen designed to target the post-transplant humoral response, thereby avoiding AMR.

Current challenges in xenotransplantation

PURPOSE OF REVIEW

In recent years, the xenotransplantation science has advanced tremendously, with significant strides in both preclinical and clinical research. This review intends to describe the latest cutting-edge progress in knowledge and methodologies developed to overcome potential obstacles that may preclude the translation and successful application of clinical xenotransplantation.

RECENT FINDINGS

Preclinical studies have demonstrated that it is now possible to extend beyond two years survival of primate recipients of life saving xenografts. This has been accomplished thanks to the utilization of genetic engineering methodologies that have allowed the generation of specifically designed gene-edited pigs, a careful donor and recipient selection, and appropriate immunosuppressive strategies.In this light, the compassionate use of genetically modified pig hearts has been authorized in two human recipients and xenotransplants have also been achieved in human decedents. Although encouraging the preliminary results suggest that several challenges have yet to be fully addressed for a successful clinical translation of xenotransplantation. These challenges include immunologic, physiologic and biosafety aspects.

SUMMARY

Recent progress has paved the way for the initial compassionate use of pig organs in humans and sets the scene for a wider application of clinical 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.

Spatiotemporal immune atlas of a clinical-grade gene-edited pig-to-human kidney xenotransplant

Pig-to-human xenotransplantation is rapidly approaching the clinical arena; however, it is unclear which immunomodulatory regimens will effectively control human immune responses to pig xenografts. Here, we transplant a gene-edited pig kidney into a brain-dead human recipient on pharmacologic immunosuppression and study the human immune response to the xenograft using spatial transcriptomics and single-cell RNA sequencing. Human immune cells are uncommon in the porcine kidney cortex early after xenotransplantation and consist of primarily myeloid cells. Both the porcine resident macrophages and human infiltrating macrophages express genes consistent with an alternatively activated, anti-inflammatory phenotype. No significant infiltration of human B or T cells into the porcine kidney xenograft is detectable. Altogether, these findings provide proof of concept that conventional pharmacologic immunosuppression may be able to restrict infiltration of human immune cells into the xenograft early after compatible pig-to-human kidney xenotransplantation.

Xenotransplantation experiments in brain-dead human subjects-A critical appraisal

Brain-dead human subjects (decedents) were recently introduced as a potential preclinical experimental model in xenotransplantation. Brain death is associated with major pathophysiological changes, eg, structural injury and cell infiltration in vital organs, and major hormonal, metabolic, inflammatory, and hemodynamic changes. In 2 of the 3 initial experiments, the design of the experiments resulted in little or no new information becoming available. In the third, the experiment was unfortunately unsuccessful as neither of the 2 pig kidneys transplanted into the decedent functioned adequately. Failure may well have been associated with the effects of brain death, but an immune/inflammatory response to the xenograft could not be excluded. Subsequently, 2 further pig kidney transplants and 2 pig heart transplants have been carried out in human decedents, but again the data obtained do not add much to what is already known. In view of the profound changes that take place during and after brain death, it may prove difficult to determine whether graft failure or dysfunction results from the effects of brain death or from an immune/inflammatory response to the xenograft. A major concern is that, if the results are confusing, they may impact decisions relating to the introduction of clinical xenotransplantation.

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.

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.

Combined islet and kidney xenotransplantation for diabetic nephropathy: an update in ongoing research for a clinically relevant application of porcine islet transplantation

Combined islet and kidney xenotransplantation for the treatment of diabetic nephropathy represents a compelling and increasingly relevant therapeutic possibility for an ever-growing number of patients who would benefit from both durable renal replacement and cure of the underlying cause of their renal insufficiency: diabetes. Here we briefly review immune barriers to islet transplantation, highlight preclinical progress in the field, and summarize our experience with combined islet and kidney xenotransplantation, including both challenges with islet-kidney composite grafts as well as our recent success with sequential kidney followed by islet xenotransplantation in a pig-to-baboon model.

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.

Heart transplantation: advances in expanding the donor pool and xenotransplantation

Approximately 65 million adults globally have heart failure, and the prevalence is expected to increase substantially with ageing populations. Despite advances in pharmacological and device therapy of heart failure, long-term morbidity and mortality remain high. Many patients progress to advanced heart failure and develop persistently severe symptoms. Heart transplantation remains the gold-standard therapy to improve the quality of life, functional status and survival of these patients. However, there is a large imbalance between the supply of organs and the demand for heart transplants. Therefore, expanding the donor pool is essential to reduce mortality while on the waiting list and improve clinical outcomes in this patient population. A shift has occurred to consider the use of organs from donors with hepatitis C virus, HIV or SARS-CoV-2 infection. Other advances in this field have also expanded the donor pool, including opt-out donation policies, organ donation after circulatory death and xenotransplantation. We provide a comprehensive overview of these various novel strategies, provide objective data on their safety and efficacy, and discuss some of the unresolved issues and controversies of each approach.

A study of knowledge and acceptance of kidney xenotransplantation among Chinese kidney transplant recipients and candidates

BACKGROUND

In recent years, the implementation of the first case of pig-to-human heart xenotransplantation and the report of three cases of pig-to-brain-dead human recipient kidney transplantation indicate that xenotransplantation is getting closer to clinical application. In the near future, China may also launch clinical trials of kidney xenotransplantation. Therefore, it is necessary to investigate the level of knowledge and acceptance of xenotransplantation among kidney transplant recipients and candidates in China. This study aims to investigate the level of comprehension and acceptance of kidney xenotransplantation in kidney transplant recipients and explore related factors, providing a reference for promoting the application and clinical trials of xenotransplantation in the near future.

METHODS

A questionnaire was completed by 211 kidney transplant recipients and 21 candidates. Answers to the questionnaires were self-administered by the respondents. Scores were compared using nonparametric tests, as well as using Chi-square test or Fisher's exact test to compare differences in answers.

RESULTS

Respondents demonstrated a high score of 75 (out of 100) on knowledge and acceptance of kidney xenotransplantation. The sector "Knowledge and Attitude" received the overall highest score from respondents (85.0 out of 100), while "Risks and Concerns" received the lowest score (50 out of 100). Interestingly, respondents paid more attention to infection risks but showed less concern about rejection or unknown risks. Furthermore, 191 respondents (82.3%) expected that pig kidney xenografts could function for at least 5 years or more. The scores were statistically significant in terms of age, gender, level of education, level of knowledge on the case of xenotransplantation clinical trial, and willingness to donate organs. cognition CONCLUSIONS: The awareness rates of xenotransplantation are high among kidney transplant recipients and candidates, for which the majority showed a generally favorable attitude towards this procedure. Respondents did not have a comprehensive understanding of the specific knowledge of xenotransplantation and expressed more concern about the risk of infection compared to the risk of rejection and other unknown risks, while also expecting long-term survival similar to allograft transplantation for pig kidney xenografts.

Design and testing of a humanized porcine donor for xenotransplantation

Recent human decedent model studies1,2 and compassionate xenograft use3 have explored the promise of porcine organs for human transplantation. To proceed to human studies, a clinically ready porcine donor must be engineered and its xenograft successfully tested in nonhuman primates. Here we describe the design, creation and long-term life-supporting function of kidney grafts from a genetically engineered porcine donor transplanted into a cynomolgus monkey model. The porcine donor was engineered to carry 69 genomic edits, eliminating glycan antigens, overexpressing human transgenes and inactivating porcine endogenous retroviruses. In vitro functional analyses showed that the edited kidney endothelial cells modulated inflammation to an extent that was indistinguishable from that of human endothelial cells, suggesting that these edited cells acquired a high level of human immune compatibility. When transplanted into cynomolgus monkeys, the kidneys with three glycan antigen knockouts alone experienced poor graft survival, whereas those with glycan antigen knockouts and human transgene expression demonstrated significantly longer survival time, suggesting the benefit of human transgene expression in vivo. These results show that preclinical studies of renal xenotransplantation could be successfully conducted in nonhuman primates and bring us closer to clinical trials of genetically engineered porcine renal grafts.

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.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

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.

Future prospects for the clinical transfusion of pig red blood cells

Transfusion of allogeneic human red blood cell (hRBCs) is limited by supply and compatibility between individual donors and recipients. In situations where the blood supply is constrained or when no compatible RBCs are available, patients suffer. As a result, alternatives to hRBCs that complement existing RBC transfusion strategies are needed. Pig RBCs (pRBCs) could provide an alternative because of their abundant supply, and functional similarities to hRBCs. The ability to genetically modify pigs to limit pRBC immunogenicity and augment expression of human 'protective' proteins has provided major boosts to this research and opens up new therapeutic avenues. Although deletion of expression of xenoantigens has been achieved in genetically-engineered pigs, novel genetic methods are needed to introduce human 'protective' transgenes into pRBCs at the high levels required to prevent hemolysis and extend RBC survival in vivo. This review addresses recent progress and examines future prospects for clinical xenogeneic pRBC transfusion.

Antibody-mediated rejection in xenotransplantation: Can it be prevented or reversed?

Antibody-mediated rejection (AMR) is the commonest cause of failure of a pig graft after transplantation into an immunosuppressed nonhuman primate (NHP). The incidence of AMR compared to acute cellular rejection is much higher in xenotransplantation (46% vs. 7%) than in allotransplantation (3% vs. 63%) in NHPs. Although AMR in an allograft can often be reversed, to our knowledge there is no report of its successful reversal in a pig xenograft. As there is less experience in preventing or reversing AMR in models of xenotransplantation, the results of studies in patients with allografts provide more information. These include (i) depletion or neutralization of serum anti-donor antibodies, (ii) inhibition of complement activation, (iii) therapies targeting B or plasma cells, and (iv) anti-inflammatory therapy. Depletion or neutralization of anti-pig antibody, for example, by plasmapheresis, is effective in depleting antibodies, but they recover within days. IgG-degrading enzymes do not deplete IgM. Despite the expression of human complement-regulatory proteins on the pig graft, inhibition of systemic complement activation may be necessary, particularly if AMR is to be reversed. Potential therapies include (i) inhibition of complement activation (e.g., by IVIg, C1 INH, or an anti-C5 antibody), but some complement inhibitors are not effective in NHPs, for example, eculizumab. Possible B cell-targeted therapies include (i) B cell depletion, (ii) plasma cell depletion, (iii) modulation of B cell activation, and (iv) enhancing the generation of regulatory B and/or T cells. Among anti-inflammatory agents, anti-IL6R mAb and TNF blockers are increasingly being tested in xenotransplantation models, but with no definitive evidence that they reverse AMR. Increasing attention should be directed toward testing combinations of the above therapies. We suggest that treatment with a systemic complement inhibitor is likely to be most effective, possibly combined with anti-inflammatory agents (if these are not already being administered). Ultimately, it may require further genetic engineering of the organ-source pig to resolve the problem entirely, for example, knockout or knockdown of SLA, and/or expression of PD-L1, HLA E, and/or HLA-G.

Clinical and molecular correlation defines activity of physiological pathways in life-sustaining kidney xenotransplantation

Porcine kidney xenotransplantation is accelerating towards clinical translation. However, despite the demonstrated ability of porcine kidneys to remove metabolic waste products, questions remain about their ability to faithfully recapitulate renal endocrine functions after transplantation. Here we analyze xenograft growth and function of two kidney dependent endocrine pathways in seventeen cynomolgus macaques after kidney xenotransplantation from gene edited Yucatan minipigs. Xenograft growth, the renin-angiotensinogen aldosterone-system, and the calcium-vitamin D-parathyroid hormone axis are assessed using clinical chemistries data, renin activity and beta-C-terminal-telopeptide assays, kidney graft RNA-sequencing and serial ultrasonography. We demonstrate that xenografts transplanted from minipigs show only modest growth and do not substantially contribute to recipient RAAS pathway activity. However, parathyroid hormone-independent hypercalcemia and hypophosphatemia are observed, suggesting a need for close monitoring and timely intervention during human testing. Further study of these phenotypes is warranted in designing prospective clinical trials.

What Have We Learned From In Vitro Studies About Pig-to-primate Organ Transplantation?

Natural preformed and de novo antibodies against pig antigens are a major cause of pig xenograft rejection in nonhuman primates (NHPs). In vivo studies in pig-to-NHP models are time consuming. In vitro assays, for example, antibody binding to pig cells, complement-dependent cytotoxicity assays, provide valuable information quickly and inexpensively. Using in vitro assays for several years, it has been documented that (1) during the first year of life, humans and NHPs develop anti-wild-type pig antibodies, but humans develop no or minimal antibody to triple-knockout (TKO) pig cells. (2) Some adult humans have no or minimal antibodies to TKO pig cells and are therefore unlikely to rapidly reject a TKO organ, particularly if the organ also expresses human "protective" proteins. (3) There is good correlation between immunoglobulin (Ig)M (but no t IgG) binding and complement injury. (4) All Old World NHPs develop antibodies to TKO pig cells and are not optimal recipients of TKO organs. (5) galactosyltransferase gene-knockout/β4GalNT2KO pigs are preferred for Old World NHPs. (6) Humans develop anti-pig IgE and IgA antibodies against pig cells, but their role remains uncertain. (7) In a small percentage of allosensitized humans, antibodies that cross-react with swine leukocyte antigens may be detrimental to a pig organ xenograft. (8) Prior sensitization to pig antigens is unlikely to be detrimental to a subsequent allograft. (9) Deletion of expression of Gal and Neu5Gc is associated with a reduction in the T-cell response to pig cells. All of these valuable observations have largely predicted the results of in vivo studies.

The road to xenotransplantation

PURPOSE OF REVIEW

The aim of this study was to highlight recent progress in xenotransplantation and discuss the remaining obstacles/steps before the FDA is likely to approve a clinical trial.

RECENT FINDINGS

Long-term survival of life-supporting xenografts in preclinical models has led to discussion of clinical trials of xenotransplantation. The reports of clinical cardiac xenotransplant based on compassionate use FDA approval and renal xenotransplants to brain-dead humans have led to further considerations of clinical trials. Discussions between the transplant community and the FDA have established critical next steps before a clinical trial of xenotransplants is likely to be approved. These steps include testing the clinical immunosuppression protocol and the organ from a genetically modified source animal in nonhuman primates with reproducible survival of at least 6 months. In addition, appropriate viral surveillance protocols and confirmation that the xenografts support appropriate human physiology are likely to be critical elements for FDA-approval. Finally, further studies in the human decedent model are likely to provide critical information about human immune and physiologic responses to xenografts.

SUMMARY

This review highlights the current progress in nonhuman primate models and recent reports of human xenotransplantation. It also describes the remaining hurdles and currently understood FDA requirements that remain to be achieved before a clinical trial of xenotransplantation can be approved.

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.

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.

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.

Complement-targeted therapies in kidney transplantation-insights from preclinical studies

Aberrant activation of the complement system contributes to solid-organ graft dysfunction and failure. In kidney transplantation, the complement system is implicated in the pathogenesis of antibody- and cell-mediated rejection, ischemia-reperfusion injury, and vascular injury. This has led to the evaluation of select complement inhibitors (e.g., C1 and C5 inhibitors) in clinical trials with mixed results. However, the complement system is highly complex: it is composed of more than 50 fluid-phase and surface-bound elements, including several complement-activated receptors-all potential therapeutic targets in kidney transplantation. Generation of targeted pharmaceuticals and use of gene editing tools have led to an improved understanding of the intricacies of the complement system in allo- and xeno-transplantation. This review summarizes our current knowledge of the role of the complement system as it relates to rejection in kidney transplantation, specifically reviewing evidence gained from pre-clinical models (rodent and nonhuman primate) that may potentially be translated to clinical trials.

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.

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.

Measuring success in pig to non-human-primate renal xenotransplantation: Systematic review and comparative outcomes analysis of 1051 life-sustaining NHP renal allo- and xeno-transplants

Facile gene editing has accelerated progress in pig to non-human-primate (NHP) renal xenotransplantation, however, outcomes are considered inferior to NHP-allotransplantation. This systematic review and outcomes analysis of life-sustaining NHP-renal transplantation aimed to benchmark "preclinical success" and aggregated 1051 NHP-to-NHP or pig-to-NHP transplants across 88 articles. Although protocols varied, NHP-allotransplantation survival (1, 3, 12months, 67.5%, 37.1%, 13.2%) was significantly greater than NHP-xenotransplantation (1, 3, 12 months, 38.8%, 14.0%, 4.4%; p < .001); a difference partially mitigated by gene-edited donors containing at least knockout of alpha-1,3-galactosyltransferase (1, 3, 12 months, 47.1%, 24.2%, 7.6%; p < .001). Pathological analysis demonstrated more cellular rejection in allotransplantation (62.8% vs. 3.1%, p < .001) and more antibody-mediated rejection in xenotransplantation (6.8% vs. 45.5%, p < .001). Nonrejection causes of graft loss between allotransplants and xenotransplants differed; infection and animal welfare (1.7% vs. 11.2% and 3.9% vs. 17.0%, respectively, p < .001 for both). Importantly, even among a subgroup of unsensitized rhesus macaques under long-term immunosuppression, NHP-allotransplant survival was significantly inferior to clinical allotransplantation (6 months, 36.1% vs. 94.0%; p < .001), which suggests clinical outcomes with renal xenografts may be better than predicted by current preclinical data.

Both Natural and Induced Anti-Sda Antibodies Play Important Roles in GTKO Pig-to-Rhesus Monkey Xenotransplantation

Sda, produced by the B4GALNT2 enzyme, has been recognized as an important xenoantigen for pig-to-nonhuman primate xenotransplantation. However, little is known about Sda expression in pigs and its immunogenicity in xenotransplantation. In this study, peripheral blood mononuclear cells (PBMCs) were isolated from wildtype, GTKO (with high, moderate, and low Sda expression), GTKO/β4GalNT2KO, GTKO/CMAHKO, or GTKO/CMAHKO/β4GalNT2KO pigs. Anti-pig IgM/IgG binding and complement-dependent cytotoxicity (CDC) to pig PBMCs was measured by flow cytometry using pooled rhesus monkey sera (n=20) or human sera (n=20). As compared to wild-type pigs (n=12), GTKO pigs (n=17) had a significantly higher mean level of Sda expression on PBMCs and showed a greater individual difference in expression. Both the overall binding of monkey serum IgM/IgG antibody to GTKO pig PBMCs and CDC against these PBMCs decreased significantly with a progressive reduction in Sda expression, showing a clear dose-effect relationship. Both the monkey serum antibody binding and CDC decreased significantly after the additional deletion of Sda, whereas the binding of human serum antibody and CDC against the GTKO pig PBMCs were markedly reduced after the deletion of Neu5Gc in the pigs. In addition, anti-Sda antibody accounted for > 50% of the induced anti-non-Gal antibody at the time of rejection in two rhesus monkeys that received GTKO/hCD55 pig kidney xenotransplantation, and the anti-Sda antibody showed significant cytotoxic activity against GTKO pig cells. We conclude that both natural and induced anti-Sda antibodies play important roles in GTKO pig-to-rhesus monkey xenotransplantation, thus providing further evidence for GTKO/β4GalNT2KO pigs as the preferred organ source for rhesus monkeys as a preclinical model of xenotransplantation.

First clinical-grade porcine kidney xenotransplant using a human decedent model

A radical solution is needed for the organ supply crisis, and the domestic pig is a promising organ source. In preparation for a clinical trial of xenotransplantation, we developed an in vivo pre-clinical human model to test safety and feasibility tenets established in animal models. After performance of a novel, prospective compatible crossmatch, we performed bilateral native nephrectomies in a human brain-dead decedent and subsequently transplanted two kidneys from a pig genetically engineered for human xenotransplantation. The decedent was hemodynamically stable through reperfusion, and vascular integrity was maintained despite the exposure of the xenografts to human blood pressure. No hyperacute rejection was observed, and the kidneys remained viable until termination 74 h later. No chimerism or transmission of porcine retroviruses was detected. Longitudinal biopsies revealed thrombotic microangiopathy that did not progress in severity, without evidence of cellular rejection or deposition of antibody or complement proteins. Although the xenografts produced variable amounts of urine, creatinine clearance did not recover. Whether renal recovery was impacted by the milieu of brain death and/or microvascular injury remains unknown. In summary, our study suggests that major barriers to human xenotransplantation have been surmounted and identifies where new knowledge is needed to optimize xenotransplantation outcomes in humans.

Both Natural and Induced Anti-Sda Antibodies Play Important Roles in GTKO Pig-to-Rhesus Monkey Xenotransplantation

Sda, produced by the B4GALNT2 enzyme, has been recognized as an important xenoantigen for pig-to-nonhuman primate xenotransplantation. However, little is known about Sda expression in pigs and its immunogenicity in xenotransplantation. In this study, peripheral blood mononuclear cells (PBMCs) were isolated from wildtype, GTKO (with high, moderate, and low Sda expression), GTKO/β4GalNT2KO, GTKO/CMAHKO, or GTKO/CMAHKO/β4GalNT2KO pigs. Anti-pig IgM/IgG binding and complement-dependent cytotoxicity (CDC) to pig PBMCs was measured by flow cytometry using pooled rhesus monkey sera (n=20) or human sera (n=20). As compared to wild-type pigs (n=12), GTKO pigs (n=17) had a significantly higher mean level of Sda expression on PBMCs and showed a greater individual difference in expression. Both the overall binding of monkey serum IgM/IgG antibody to GTKO pig PBMCs and CDC against these PBMCs decreased significantly with a progressive reduction in Sda expression, showing a clear dose-effect relationship. Both the monkey serum antibody binding and CDC decreased significantly after the additional deletion of Sda, whereas the binding of human serum antibody and CDC against the GTKO pig PBMCs were markedly reduced after the deletion of Neu5Gc in the pigs. In addition, anti-Sda antibody accounted for &gt; 50% of the induced anti-non-Gal antibody at the time of rejection in two rhesus monkeys that received GTKO/hCD55 pig kidney xenotransplantation, and the anti-Sda antibody showed significant cytotoxic activity against GTKO pig cells. We conclude that both natural and induced anti-Sda antibodies play important roles in GTKO pig-to-rhesus monkey xenotransplantation, thus providing further evidence for GTKO/β4GalNT2KO pigs as the preferred organ source for rhesus monkeys as a preclinical model of xenotransplantation.

Role of Complement System in Kidney Transplantation: Stepping From Animal Models to Clinical Application

Kidney transplantation is a life-saving strategy for patients with end-stage renal diseases. Despite the advances in surgical techniques and immunosuppressive agents, the long-term graft survival remains a challenge. Growing evidence has shown that the complement system, part of the innate immune response, is involved in kidney transplantation. Novel insights highlighted the role of the locally produced and intracellular complement components in the development of inflammation and the alloreactive response in the kidney allograft. In the current review, we provide the updated understanding of the complement system in kidney transplantation. We will discuss the involvement of the different complement components in kidney ischemia-reperfusion injury, delayed graft function, allograft rejection, and chronic allograft injury. We will also introduce the existing and upcoming attempts to improve allograft outcomes in animal models and in the clinical setting by targeting the complement system.

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.

Both Natural and Induced Anti-Sda Antibodies Play Important Roles in GTKO Pig-to-Rhesus Monkey Xenotransplantation

Sda, produced by the B4GALNT2 enzyme, has been recognized as an important xenoantigen for pig-to-nonhuman primate xenotransplantation. However, little is known about Sda expression in pigs and its immunogenicity in xenotransplantation. In this study, peripheral blood mononuclear cells (PBMCs) were isolated from wildtype, GTKO (with high, moderate, and low Sda expression), GTKO/β4GalNT2KO, GTKO/CMAHKO, or GTKO/CMAHKO/β4GalNT2KO pigs. Anti-pig IgM/IgG binding and complement-dependent cytotoxicity (CDC) to pig PBMCs was measured by flow cytometry using pooled rhesus monkey sera (n=20) or human sera (n=20). As compared to wild-type pigs (n=12), GTKO pigs (n=17) had a significantly higher mean level of Sda expression on PBMCs and showed a greater individual difference in expression. Both the overall binding of monkey serum IgM/IgG antibody to GTKO pig PBMCs and CDC against these PBMCs decreased significantly with a progressive reduction in Sda expression, showing a clear dose-effect relationship. Both the monkey serum antibody binding and CDC decreased significantly after the additional deletion of Sda, whereas the binding of human serum antibody and CDC against the GTKO pig PBMCs were markedly reduced after the deletion of Neu5Gc in the pigs. In addition, anti-Sda antibody accounted for > 50% of the induced anti-non-Gal antibody at the time of rejection in two rhesus monkeys that received GTKO/hCD55 pig kidney xenotransplantation, and the anti-Sda antibody showed significant cytotoxic activity against GTKO pig cells. We conclude that both natural and induced anti-Sda antibodies play important roles in GTKO pig-to-rhesus monkey xenotransplantation, thus providing further evidence for GTKO/β4GalNT2KO pigs as the preferred organ source for rhesus monkeys as a preclinical model of xenotransplantation.