Further evidence for sustained systemic inflammation in xenograft recipients (SIXR)

Regulatory macrophages in solid organ xenotransplantation

Due to a critical organ shortage, pig organs are being explored for use in transplantation. Differences between species, particularly in cell surface glycans, can trigger elevated immune responses in xenotransplantation. To mitigate the risk of hyperacute rejection, genetically modified pigs have been developed that lack certain glycans and express human complement inhibitors. Nevertheless, organs from these pigs may still provoke stronger inflammatory and innate immune reactions than allotransplants. Dysregulation of coagulation and persistent inflammation remain obstacles in the transplantation of pig organs into primates. Regulatory macrophages (Mregs), known for their anti-inflammatory properties, could offer a potential solution. Mregs secrete interleukin 10 and transforming growth factor beta, thereby suppressing immune responses and promoting the development of regulatory T cells. These Mregs are typically induced via the stimulation of monocytes or macrophages with macrophage colony-stimulating factor and interferon gamma, and they conspicuously express the stable marker dehydrogenase/reductase 9. Consequently, understanding the precise mechanisms governing Mreg generation, stability, and immunomodulation could pave the way for the therapeutic use of Mregs generated in vitro. This approach has the potential to reduce the required dosages and durations of anti-inflammatory and immunosuppressive medications in preclinical and clinical settings.

How, where and when to screen for porcine cytomegalovirus (PCMV) in donor pigs for xenotransplantation

Porcine cytomegalovirus (PCMV), that is actually a porcine roseolovirus (PRV), is a common herpesvirus in domestic pigs and wild boars. In xenotransplantation, PCMV/PRV has been shown to significantly reduce the survival time of pig kidneys and hearts in preclinical trials with different non-human primates. Furthermore, PCMV/PRV has been transmitted in the first pig to human heart xenotransplantation and contributed to the death of the patient. Although transmitted to the recipient, there is no evidence that PCMV/PRV can infect primate cells including human cells. PCMV/PRV is closely related to the human herpesviruses 6 and 7, and only distantly related to the human CMV (HCMV). Antiviral drugs used for the treatment of HCMV are less effective against PCMV/PRV. However, there are well described strategies to eliminate the virus from pig facilities. In order to detect the virus and to eliminate it, highly sensitive detection methods and the knowledge of how, where and when to screen the donor pigs is required. Here, a comparative testing of organs from pigs of different ages using polymerase chain reaction (PCR)-based and immunological methods was performed. Testing young piglets, PCMV/PRV was detected effectively by PCR in blood, bronchoalveolar lavage fluid, tonsils and heart. In adult animals, detection by PCR was not successful in most cases, because the virus load was below the detection limit or the virus was in its latent stage. Therefore, detection of antibodies against selected recombinant proteins corresponding to epitopes detected by nearly all infected animals in a Western blot assay is advantageous. By contrast, immunological testing is not beneficial in young animals as piglets might have PCMV/PRV-specific antibodies obtained from their infected mother via the colostrum. Using a thoughtful combination of PCR-based and immunological methods, detection of PCMV/PRV in donor pigs for xenotransplantation is feasible and a controlled elimination of the virus by early weaning or other methods is possible.

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.

Pig-to-baboon lung xenotransplantation: Extended survival with targeted genetic modifications and pharmacologic treatments

Galactosyl transferase knock-out pig lungs fail rapidly in baboons. Based on previously identified lung xenograft injury mechanisms, additional expression of human complement and coagulation pathway regulatory proteins, anti-inflammatory enzymes and self-recognition receptors, and knock-down of the β4Gal xenoantigen were tested in various combinations. Transient life-supporting GalTKO.hCD46 lung function was consistently observed in association with either hEPCR (n = 15), hTBM (n = 4), or hEPCR.hTFPI (n = 11), but the loss of vascular barrier function in the xenograft and systemic inflammation in the recipient typically occurred within 24 h. Co-expression of hEPCR and hTBM (n = 11) and additionally blocking multiple pro-inflammatory innate and adaptive immune mechanisms was more consistently associated with survival >1 day, with one recipient surviving for 31 days. Combining targeted genetic modifications to the lung xenograft with selective innate and adaptive immune suppression enables prolonged initial life-supporting lung function and extends lung xenograft recipient survival, and illustrates residual barriers and candidate treatment strategies that may enable the clinical application of other organ xenografts.

The Role of Interleukin-6 (IL-6) in the Systemic Inflammatory Response in Xenograft Recipients and in Pig Kidney Xenograft Failure

Background

In pig-to-baboon transplantation models, there is increasing evidence of systemic inflammation in xenograft recipients (SIXR) associated with pig xenograft failure. We evaluated the relationship between systemic inflammatory factors and pig kidney xenograft failure.

Methods

Baboons received kidney transplants from genetically engineered pigs (n=9), and received an anti-CD40mAb-based (n=4) or conventional (n=5) immunosuppressive regimen. The pig kidney grafts were monitored by measurements of serum creatinine, serum amyloid A (SAA), white blood cell (WBC) and platelet counts, plasma fibrinogen, and pro-inflammatory cytokines (baboon and pig IL-6, TNF-α, IL-1β).

Results

Six baboons were euthanized or died from rejection, and 3 were euthanized for infection. Changes in serum creatinine correlated with those of SAA (r=0.56, p<0.01). Serum baboon IL-6 was increased significantly on day 1 after transplantation and at euthanasia (both p<0.05) and correlated with serum creatinine and SAA (r=0.59, p<0.001, r=0.58, p<0.01; respectively). but no difference was observed between rejection and infection. Levels of serum pig IL-6, TNF-α, IL-1β were also significantly increased on day 1 and at euthanasia, and serum pig IL-6 and IL-1β correlated with serum creatinine and SAA. The level of serum baboon IL-6 correlated with the expression of IL-6 and amyloid A in the baboon liver (r=0.93, p<0.01, r=0.79, p<0.05; respectively).

Conclusion

Early upregulation of SAA and serum IL-6 may indicate the development of rejection or infection, and are associated with impaired kidney graft function. Detection and prevention of systemic inflammation may be required to prevent pig kidney xenograft failure after xenotransplantation.

Early Interferon-Gamma Response in Nonhuman Primate Recipients of Solid-Organ Xenotransplantation

BACKGROUND

To understand changes in biological responses in nonhuman primate (NHP) recipients of xenotransplantation (XTP), we retrospectively investigated chronological changes in cytokine profiles of NHP recipients after solid-organ XTP.

METHODS

Plasma samples were collected from 7 NHP recipients of pig heart or kidney XTP with α-1,3-galactosyltransferase gene knockout (GTKO) under anti-CD154-based immune suppression at the following time points: immediately before; 2 hours, 3 days, and 7 days after XTP; and weekly thereafter until the graft failed. The plasma levels of the following cytokines were measured: interleukin (IL)-1α, IL-1β, IL-6, IL-12p70, IL-8, IL-10, IL-15, tumor necrosis factor, interferon gamma (IFN-γ), D-dimer, C3a, and histone-complexed DNA fragments. For in vitro experiments, human natural killer (NK) cells were cocultured with wild-type porcine endothelial cells (PECs), GTKO-PECs, and human umbilical vein endothelial cells, with or without anti-CD154 antibody. IFN-γ levels in the culture supernatants were compared.

RESULTS

IFN-γ levels peaked on day 7 or 10 of XTP and then decreased to basal levels, whereas proinflammatory cytokine levels increased along with the elevation of histone-complexed DNA fragments and were sustained until xenograft failure. In vitro, human NK cells produced more IFN-γ when in contact with wild-type PECs than with human umbilical vein endothelial cells, which was not reduced by the use of GTKO-PECs or addition of anti-CD154 antibody to the mixture.

CONCLUSIONS

In NHP recipients of XTP, the early peak of IFN-γ priming subsequent inflammatory responses may be attributed to NK cell activation in response to xenografts.

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.

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.

The Genetically Engineered Heart as a Bridge to Allotransplantation in Infants Just Around the Corner?

BACKGROUND

Mortality for infants on the heart transplant wait list remains unacceptably high, and available mechanical circulatory support is suboptimal. Our goal is to demonstrate the feasibility of utilizing genetically engineered pig (GEP) heart as a bridge to allotransplantation by transplantation of a GEP heart in a baboon.

METHODS

Four baboons underwent orthotopic cardiac transplantation from GEP donors. All donor pigs had galactosyl-1,3-galactose knocked out. Two donor pigs had human complement regulatory CD55 transgene and the other 2 had human complement regulatory CD46 and thrombomodulin. Induction immunosuppression included thymoglobulin, and Anti-CD20. Maintenance immunosuppression was Rapamycin, AntiCD-40 and methylprednisolone. One donor heart was preserved with University of Wisconsin (UW) solution and the other three with del Nido solution.

RESULTS

All baboons weaned from cardiopulmonary bypass. B217 received a donor heart preserved with UW. Ventricular arrhythmias and depressed cardiac function resulted in early death. All recipients of del Nido preserved hearts easily weaned from cardiopulmonary bypass with minimal inotropic support. B15416 and B1917 survived for 90 days and 241 days respectively. Histopathology in B15416 revealed no significant myocardial rejection but cellular infiltrate around Purkinje fibers. Histopathology in B1917 was consistent with severe rejection. B37367 had uneventful transplant but developed significant respiratory distress with a cardiac arrest.

CONCLUSIONS

Survival of B15416 and B1917 demonstrates the feasibility of pursuing additional research to document the ability to bridge an infant to cardiac allotransplant with a GEP heart.

Stable expression of the human thrombomodulin transgene in pig endothelial cells is associated with a reduction in the inflammatory response

BACKGROUND

Xenotransplantation is associated with an inflammatory response. The proinflammatory cytokine, TNF-α, downregulates the expression of thrombomodulin (TBM), and induces coagulation dysfunction. Although human (h) TBM-transgenic pigs (p) have been developed to reduce coagulation dysfunction, the effect of TNF-α on the expression of hTBM and its functional activity has not been fully investigated. The aims of this study were to investigate (i) whether the expression of hTBM on pig (p) cells is down-regulated during TNF-α stimulation, and (ii) whether cells from hTBM pigs regulate the inflammatory response.

METHODS

TNF-α-producing T, B, and natural killer cells in blood from baboons with pig heart or kidney xenografts were investigated by flow cytometry. TNF-α staining in the grafts was detected by immunohistochemistry. Aortic endothelial cells (AECs) from GTKO/CD46 and GTKO/CD46/hTBM pigs were stimulated by hTNF-α, and the expression of the inflammatory/coagulation regulatory protein, TBM, was investigated.

RESULTS

After pig organ xenotransplantation, there was a trend to increases in TNF-α-producing T and natural killer cells in the blood of baboons. In vitro observations demonstrated that after hTNF-α stimulation, there was a significant reduction in the expression of endogenous pTBM on pAECs, and a significant increase in the expression of inflammatory molecules. Blocking of NF-κB signaling significantly up-regulated pTBM expression, and suppressed the inflammatory response induced by hTNF-α in pAECs. Whereas the expression of pTBM mRNA was significantly reduced by hTNF-α stimulation, hTBM expression on the GTKO/CD46/hTBM pAECs was not affected. Furthermore, after hTNF-α stimulation, there was significant suppression of expression of inflammatory molecules on GTKO/CD46/hTBM pAECs compared to GTKO/CD46 pAECs.

CONCLUSIONS

The stable expression of hTBM in pig cells may locally regulate the inflammatory response. This will help suppress the inflammatory response and prevent coagulation dysregulation after xenotransplantation.

Porcine Xenograft and Epidermal Fully Synthetic Skin Substitutes in the Treatment of Partial-Thickness Burns: A Literature Review

Background and Objectives: Porcine xenografts have been used successfully in partial thickness burn treatment for many years. Their disappearance from the market led to the search for effective and efficient alternatives. In this article, we examine the synthetic epidermal skin substitute Suprathel^® as a substitute in the treatment of partial thickness burns. Materials and Methods: A systematic review following the PRISMA guidelines has been performed. Sixteen Suprathel^® and 12 porcine xenograft studies could be included. Advantages and disadvantages between the treatments and the studies’ primary endpoints have been investigated qualitatively and quantitatively. Results: Although Suprathel had a nearly six times larger TBSA in their studies (p < 0.001), it showed a significantly lower necessity for skin grafts (p < 0.001), and we found a significantly lower infection rate (p < 0.001) than in Porcine Xenografts. Nonetheless, no significant differences in the healing time (p = 0.67) and the number of dressing changes until complete wound healing (p = 0.139) could be found. Both products reduced pain to various degrees with the impression of a better performance of Suprathel^® on a qualitative level. Porcine xenograft was not recommended for donor sites or coverage of sheet-transplanted keratinocytes, while Suprathel^® was used successfully in both indications. Conclusion: The investigated parameters indicate that Suprathel^® to be an effective replacement for porcine xenografts with even lower subsequent treatment rates. Suprathel^® appears to be usable in an extended range of indications compared to porcine xenograft. Data heterogeneity limited conclusions from the results.

Immunological selection and monitoring of patients undergoing pig kidney transplantation

Pig kidney xenotransplantation has the potential to alleviate the current shortage of deceased and living human organs and provide patients with end-stage renal disease with a greater opportunity for long-term survival and a better quality of life. In recent decades, advances in the genetic engineering of pigs and in immunosuppressive therapy have permitted the resolution of many historical obstacles to the success of pig kidney transplantation in nonhuman primates. Pig kidney xenotransplantation may soon be translated to the clinic. Given the potential risks of kidney xenotransplantation, particularly of immunologic rejection of the graft, potential patients must be carefully screened for inclusion in the initial clinical trials and immunologically monitored diligently post-transplantation. We provide an overview of the immunological methods we believe should be used to (i) screen potential patients for the first clinical trials to exclude those with a higher risk of rejection, and (ii) monitor patients with a pig kidney graft to determine their immunological response to the graft.

A perspective on the potential detrimental role of inflammation in pig orthotopic heart xenotransplantation

There is a critical shortage of deceased human donor organs for transplantation. The need is perhaps most acute in neonates and infants with life-threatening congenital heart disease, in whom mechanical support devices are largely unsuccessful. If orthotopic (life-supporting) heart transplantation (OHTx) were consistently successful in the genetically engineered pig-to-nonhuman primate (NHP) model, a clinical trial of bridging with a pig heart in such patients might be justified. However, the results of pig OHTx in NHPs have been mixed and largely poor. We hypothesise that a factor is the detrimental effects of the inflammatory response that is known to develop (a) during any surgical procedure that requires cardiopulmonary bypass, and (b) immediately after an NHP recipient is exposed to a pig xenograft. We suggest that the combination of these two inflammatory responses has a direct detrimental effect on pig heart graft function, but also, and possibly of more importance, on recipient baboon pulmonary function, which further impacts survival of the pig heart graft. In addition, the inflammatory response almost certainly adversely impacts the immune response to the graft. If our hypothesis is correct, the potential steps that could be taken to reduce the inflammatory response or its effects (with varying degrees of efficacy) include (a) white blood cell filtration, (b) complement depletion or inactivation, (c) immunosuppressive therapy, (d) high-dose corticosteroid therapy, (e) cytokine/chemokine-targeted therapy, (f) ultrafiltration or CytoSorb hemoperfusion, (g) reduction in the levels of endogenous catecholamines, (h) triiodothyronine therapy and (i) genetic engineering of the organ-source pig. Prevention of the inflammatory response, or attenuation of its effects, by judicious anti-inflammatory therapy may contribute not only to early survival of the recipient of a genetically engineered pig OHTx, but also to improved long-term pig heart graft survival. This would open the possibility of initiating a clinical trial of genetically engineered pig OHTx as a bridge to allotransplantation.

Porcine genome engineering for xenotransplantation

The extreme shortage of human donor organs for treatment of patients with end-stage organ failures is well known. Xenotransplantation, which might provide unlimited organ supply, is a most promising strategy to solve this problem. Domestic pigs are regarded as ideal organ-source animals owing to similarity in anatomy, physiology and organ size to humans as well as high reproductive capacity and low maintenance cost. However, several barriers, which include immune rejection, inflammation and coagulative dysfunctions, as well as the cross-species transmission risk of porcine endogenous retrovirus, blocked the pig-to-human xenotransplantation. With the rapid development of genome engineering technologies and the potent immunosuppressive medications in recent years, these barriers could be eliminated through genetic modification of pig genome together with the administration of effective immunosuppressants. A number of candidate genes involved in the regulation of immune response, inflammation and coagulation have been explored to optimize porcine xenograft survival in non-human primate recipients. PERV inactivation in pigs has also been accomplished to firmly address the safety issue in pig-to-human xenotransplantation. Many encouraging preclinical milestones have been achieved with some organs surviving for years. Therefore, the clinical trials of some promising organs, such as islet, kidney and heart, are aimed to be launched in the near future.

Kidney xenotransplantation: Recent progress in preclinical research

Kidney transplantation is the most effective treatment for end-stage renal disease, but is limited by the increasing shortage of deceased and living human donor kidneys. Xenotransplantation using pig organs provides the possibility to resolve the issue of organ supply shortage and is regarded as the next great medical revolution. In the past five years, there have been sequential advances toward the prolongation of life-supporting pig kidney xenograft survival in non-human primates, with the longest survival being 499 days. This progress is due to the growing availability of pigs with multi-layered genetic modifications to overcome the pathobiological barriers and the application of a costimulation blockade-based immunosuppressive regimen. These encouraging results bring the hope to initiate the clinical trials of pig kidney transplantation in the near future. In this review, we summarized the latest advances regarding pig kidney xenotransplantation in preclinical models to provide a basis for future investigation and potential clinical translation.

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.

Impact of porcine cytomegalovirus on long-term orthotopic cardiac xenotransplant survival

Xenotransplantation using pig organs has achieved survival times up to 195 days in pig orthotopic heart transplantation into baboons. Here we demonstrate that in addition to an improved immunosuppressive regimen, non-ischaemic preservation with continuous perfusion and control of post-transplantation growth of the transplant, prevention of transmission of the porcine cytomegalovirus (PCMV) plays an important role in achieving long survival times. For the first time we demonstrate that PCMV transmission in orthotopic pig heart xenotransplantation was associated with a reduced survival time of the transplant and increased levels of IL-6 and TNFα were found in the transplanted baboon. Furthermore, high levels of tPA-PAI-1 complexes were found, suggesting a complete loss of the pro-fibrinolytic properties of the endothelial cells. These data show that PCMV has an important impact on transplant survival and call for elimination of PCMV from donor pigs.

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.

Macrophages in xenotransplantation

Xenotransplantation refers to organ transplantation across species. Immune rejection of xenografts is stronger and faster than that of allografts because of significant molecular differences between species. Recent studies have revealed the involvement of macrophages in xenograft and allograft rejections. Macrophages have been shown to play a critical role in inflammation, coagulation, and phagocytosis in xenograft rejection. This review presents a recent understanding of the role of macrophages in xenograft rejection and possible strategies to control macrophage-mediated xenograft rejection.

Evidence for the important role of inflammation in xenotransplantation

There is increasing evidence of a sustained state of systemic inflammation after pig-to-nonhuman primate (NHP) xenotransplantation (that has been termed systemic inflammation in xenograft recipients [SIXR]). Increases in inflammatory markers, e.g., C-reactive protein, histones, serum amyloid A, D-dimer, cytokines, chemokines, and a decrease in free triiodothyronine, have been demonstrated in the recipient NHPs. The complex interactions between inflammation, coagulation, and the immune response are well-recognized, but the role of inflammation in xenograft recipients is not fully understood. The evidence suggests that inflammation can promote the activation of coagulation and the adaptive immune response, but the exact mechanisms remain uncertain. If prolonged xenograft survival is to be achieved, anti-inflammatory strategies (e.g., the administration of anti-inflammatory agents, and/or the generation of genetically-engineered organ-source pigs that are protected from the effect of inflammation) may be necessary to prevent, control, or negate the effect of the systemic inflammation that develops in xenograft recipients. This may allow for a reduction in the intensity of exogenous immunosuppressive therapy. If immunological tolerance to a xenograft is to be obtained, then control of inflammation may be essential.

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.

Potential pathological role of pro-inflammatory cytokines (IL-6, TNF-α, and IL-17) in xenotransplantation

The major limitation of organ transplantation is the shortage of available organs from deceased human donors which leads to the deaths of thousands of patients each year. Xenotransplantation is considered to be an effective way to resolve the problem. Immune rejection and coagulation dysfunction are two major hurdles for the successful survival of pig xenografts in primate recipients. Pro-inflammatory cytokines, such as IL-6, TNF-α, and IL-17, play important roles in many diseases and in allotransplantation. However, the pathological roles of these pro-inflammatory cytokines in xenotransplantation remain unclear. Here, we briefly review the signaling transduction and expression regulation of IL-6, TNF-α, and IL-17 and evaluate their potential pathological roles in in vitro and in vivo models of xenotransplantation. We found that IL-6, TNF-α, and IL-17 were induced in most in vitro or in vivo xenotransplantation model. Blockade of these cytokines using gene modification, antibody, or inhibitor had different effects in xenotransplantation. Inhibition of IL-6 signaling with tocilizumab decreased CRP but did not increase xenograft survival. The one possible reason is that tocilizumab can not suppress IL-6 signaling in porcine cells or organs. Other drugs which inhibit IL-6 signaling need to be investigated in xenotransplantation model. Inhibition of TNF-α was beneficial for the survival of xenografts in pig-to-mouse, rat, or NHP models. Blockade of IL-17 using a neutralizing antibody also increased xenograft survival in several animal models. However, the role of IL-17 in the pig-to-NHP xenotransplantation model remains unclear and needs to be further investigated. Moreover, blockade of TNF-α and IL-6 together has got a better effect in pig-to-baboon kidney xenotransplantation. Blockade two or even more cytokines together might get better effect in suppressing xenograft rejection. Better understanding the role of these cytokines in xenotransplantation will be beneficial for choosing better immunosuppressive strategy or producing genetic modification pig.

Serum amyloid a as an indicator of impending xenograft failure: Experimental studies

BACKGROUND

There is well-documented systemic inflammatory response in xenograft recipients to the presence of a pig graft. Serum amyloid A (SAA) is an inflammatory marker that is elevated in various pathological states. The assay used to measure it is (i) simple, (ii) relatively inexpensive, and (iii) provides an answer within minutes.

METHOD

The levels of SAA (n = 11) and C-reactive protein (C-RP) (n = 8) were measured retrospectively in the serum of baboons with pig kidney transplants, who received therapy with an IL-6R inhibitor and a TNF-α antagonist. Immunohistochemistry (IHC) was used to identify amyloid A and C-RP expression in the native livers and deposition in the pig kidney grafts.

RESULTS

One kidney graft underwent hyperacute rejection, 6 (55%) underwent acute antibody-mediated rejection, 3 baboons (27%) were euthanized for serious systemic infections, and one was euthanized for acute gastric dilatation. The SAA increased temporarily after kidney transplantation, and increased again by the day of euthanasia, indicating moderate (n = 3) or significant (severe) (n = 8) inflammation. In contrast, as the baboons were receiving tocilizumab, C-RP did not increase. There was greater expression of amyloid A in baboon livers (by IHC) than of C-RP (mean OD 53 vs 1, p < 0.01), and greater deposition of amyloid A than C-RP in the pig kidney grafts (mean OD 24 vs 2, p < 0.001). Plasma fibrinogen negatively correlated with the expression of amyloid A in the liver (r = -0.72, p < 0.05). The results of the SAA assay correlated with amyloid A expression in the liver and deposition in the kidney grafts.

CONCLUSIONS

SAA is a sensitive, but non-specific, marker for inflammation in baboons with pig kidney grafts, and is not affected by therapy that suppresses the response of C-RP. The SAA assay is a rapid, reliable, and relatively inexpensive method of following the inflammatory state of baboons with pig xenografts.

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.

B cell phenotypes in baboons with pig artery patch grafts receiving conventional immunosuppressive therapy

BACKGROUND

In the pig-to-baboon artery patch model with no immunosuppressive therapy, a graft from an α1,3-galactosyltransferase gene-knockout (GTKO) pig elicits a significant anti-nonGal IgG response, indicating sensitization to the graft. A costimulation blockade-based regimen, e.g., anti-CD154mAb or anti-CD40mAb, prevents sensitization. However, neither of these agents is currently FDA-approved. The aim of the present study was to determine the efficacy of FDA-approved agents on the T and B cell responses.

METHODS

Artery patch xenotransplantation in baboons was carried out using GTKO/CD46 pigs with (n = 2) or without (n = 1) the mutant transgene for CIITA-knockdown. Immunosuppressive therapy consisted of induction with ATG and anti-CD20mAb, and maintenance with different combinations of CTLA4-Ig, tacrolimus, and rapamycin. In addition, all 3 baboons received daily corticosteroids, the IL-6R blocker, tocilizumab, at regular intervals, and the TNF-α blocker, etanercept, for the first 2 weeks. Recipient blood was monitored for anti-nonGal antibody levels by flow cytometry (using GTKO/CD46 pig aortic endothelial cells), and mixed lymphocyte reaction (MLR). CD22+B cell profiles (naïve [IgD+/CD27-], non-switched memory [IgD+/CD27+], and switched memory [IgD-/CD27+] B cell subsets) were measured by flow cytometry. At 6 months, the baboons were euthanized and the grafts were examined histologically.

RESULTS

No elicited anti-pig antibodies developed in any baboon. The frequency of naïve memory B cells increased significantly (from 34% to 90%, p = 0.0015), but there was a significant decrease in switched memory B cells (from 17% to 0.5%, p = 0.015). MLR showed no increase in the proliferative T cell response in those baboons that had received CTLA4-Ig (n = 2). Histological examination showed few or no features of rejection in any graft.

CONCLUSIONS

The data suggest that immunosuppressive therapy with only FDA-approved agents may be adequate to prevent an adaptive immune response to a genetically-engineered pig graft, particularly if CTLA4-Ig is included in the regimen, in part because the development of donor-specific memory B cells is inhibited.

An approach to induction of tolerance to pig cardiac xenografts in neonates

There is a continuing need for donor hearts for infants with complex congenital heart defects. The transplantation of hearts from neonatal pigs would be an alternative to human organs, particularly if donor-specific immunological tolerance could be achieved. The great majority of infant humans do not make natural (preformed) antibodies against triple-knockout (TKO) pigs (that do not express any of the three known pig antigens against which humans have natural anti-pig antibodies). The transplantation of a heart from a TKO pig into an infant would therefore minimize any risk of early antibody-mediated rejection, and, with adequate immunosuppressive therapy, prolonged graft survival may well be achieved. Total host thymectomy (commonly carried out at the time of orthotopic heart transplantation in this age group) ± residual T-cell depletion and donor-specific pig thymus tissue transplantation might induce T-cell tolerance and allow immunosuppressive therapy to be discontinued (if there is in vitro evidence of T-cell and B-cell nonresponsiveness to donor-specific pig cells). Even if tolerance were not achieved, with continuing immunosuppressive therapy, the graft would likely "bridge" the patient until a suitable allograft became available or be associated with prolonged xenograft function.

Serum amyloid A as a marker of inflammation in xenotransplantation

Increasing evidence indicates that inflammation plays a role in pig-to-baboon organ xenotransplantation. We have evaluated serum amyloid A (SAA) as a marker of inflammation in baboons with various pig xenografts. We measured SAA levels in recipient baboons with pig artery patch (n = 5), life-supporting kidney (n = 5), heterotopic heart (n = 2), or hepatocyte (n = 1) grafts and using an OmniChek-SAA for Inflammation & Infection kit. C-reactive protein (CRP), another marker of inflammation (e.g. D-dimer), was also measured.SAA indicated increased inflammation when baboons developed consumptive coagulopathy (CC; e.g. thrombocytopenia) or infection. SAA also indicated that treatment of the recipient with tocilizumab reduced inflammation. There was significant positive correlation between SAA with changes in CRP (r = 0.6, P < 0.05) and with D-dimer (r = 0.8, P < 0.01), but SAA appeared at times to more accurately reflect the clinical state of the baboon. In sum, measurement of SAA proved simple and quick, and indicated (1) significant inflammation when CC or infection was present, and (2) reduced inflammation when treatment with tocilizumab was administered.

Drug repurposing: Ibrutinib exhibits immunosuppressive potential in organ transplantation

Long-term administration of classic immunosuppressants can induce severe adverse effects. The development of novel immunosuppressants confronts great challenges and opportunities. Ibrutinib, an approved drug for B-cell lineages and chronic graft versus host disease (cGVHD), exhibits immunosuppressive efficacy in autoimmune diseases. Ibrutinib's potential as an immunosuppressant in organ transplantation has not been investigated to date. In a xeno-artery patch model ex vivo, ibrutinib inhibited the proliferation of PBMCs (POD 14-42), mainly CD3⁺CD4⁺ and CD3⁺CD8⁺ T cells ex vivo. The secretion of cytokines (IL-6, IL-2 and IFN-γ) was suppressed in response to ibrutinib. In allo-skin transplantation models, ibrutinib delayed the rejection of grafted skins. Ibrutinib decreased the amount of T/B cells and lymphocyte infiltration. Altogether, ibrutinib exhibited immunosuppressive potential through cytokine regulation and T cell inhibition ex vivo and in vitro. Repositioning of ibrutinib as an immunosuppressant will greatly facilitate novel immunosuppressant development.

Delayed revascularization of islets after transplantation by IL-6 blockade in pig to non-human primate islet xenotransplantation model

BACKGROUND

Pancreatic islet transplantation is currently proven as a promising treatment for type 1 diabetes patients with labile glycemic control and severe hypoglycemia unawareness. Upon islet transplantation, revascularization is essential for proper functioning of the transplanted islets. As IL-6 is important for endothelial cell survival and systemic inflammation related to xenograft, the effect of IL-6 receptor antagonist, tocilizumab, on revascularization of the transplanted islets was examined in pig to non-human primate islet xenotransplantation model. Also, the endothelial cell origin in a new vessel of the transplanted pig islets was determined.

METHODS

Pig islets were isolated from designated pathogen-free (DPF) SNU miniature pigs and transplanted via portal vein into five streptozotocin-induced diabetic monkeys. One group (n = 2, basal group) was treated with anti-thymoglobulin (ATG), anti-CD40 antibody (2C10R4), sirolimus, and tacrolimus, and the other group was additionally given tocilizumab on top of basal immunosuppression (n = 3, Tocilizumab group). To confirm IL-6 blocking effect, C-reactive protein (CRP) levels and serum IL-6 concentration were measured. Scheduled biopsy of the margin of the posterior segment right lobe inferior of the liver was performed at 3 weeks after transplantation to assess the degree of revascularization of the transplanted islets. Immunohistochemical staining using anti-insulin, anti-CD31 antibodies, and lectin IB4 was conducted to find the origin of endothelial cells in the islet graft.

RESULTS

CRP significantly increased at 1~2 days after transplantation in Basal group, but not in Tocilizumab group, and higher serum IL-6 concentration was measured in latter group, showing the biological potency of tocilizumab. In Basal group, well-developed endothelial cells were observed on the peri- and intraislet area, whereas the number of CD31⁺ cells in the intraislet space was significantly reduced in Tocilizumab group. Finally, new endothelial cells in the pig islet graft were positive for CD31, but not for lectin IB4, suggesting that they are originated from the recipient monkey.

CONCLUSIONS

Our results demonstrated that tocilizumab can delay revascularization of the transplanted islet, although this effect had no significant correlation to the overall islet graft survival. In the pig to NHP islet xenotransplantation model, the endothelial cells from recipient monkey form new blood vessels in and around pig islets.

An Investigation of Extracellular Histones in Pig-To-Baboon Organ Xenotransplantation

BACKGROUND

Serum (extracellular) histone levels are increased in inflammatory states and in the presence of coagulation dysfunction, for example, trauma, chemical/ischemic injury, infection. There is increasing evidence of a systemic inflammatory response associated with the presence of a pig xenograft in a nonhuman primate. We evaluated extracellular histone levels in baboons with various pig xenografts.

METHODS

We measured serum histones in baboons with pig heterotopic heart (n = 8), life-supporting kidney (n = 5), orthotopic liver (n = 4), and artery patch (n = 9) grafts by enzyme-linked immunosorbent assay. C-reactive protein (CRP), free triiodothyronine (fT3), serum amyloid A (SAA), and platelet counts were also measured, all of which may provide an indication of an inflammatory state. We investigated the effect of histones on platelet aggregation and on cytotoxicity of pig cells in vitro.

RESULTS

Serum histones increased when baboons developed consumptive coagulopathy (eg, thrombocytopenia) or infection. CRP levels tended to be higher and fT3 levels lower when consumptive coagulopathy developed. Measurement of SAA correlated fairly well with CRP and indicated the state of inflammation. Treatment of the recipient with tocilizumab reduced the level of serum histones, CRP, and SAA, and increased the level of fT3 and platelet counts. In vitro, histone-induced platelet aggregation and endothelial cell apoptosis were both significantly reduced by the NF-κB pathway inhibitor, parthenolide.

CONCLUSIONS

These noninvasive assays may be useful for monitoring the health status of nonhuman primate recipients of pig organ grafts and may help in management after xenotransplantation. Tocilizumab and NF-κB inhibitors might prove valuable in reducing the inflammatory response to a pig xenograft.

Cytokine profiles in Tibetan macaques following α-1,3-galactosyltransferase-knockout pig liver xenotransplantation

BACKGROUND

Pig-to-nonhuman primate orthotopic liver xenotransplantation is often accompanied by thrombocytopenia and coagulation disorders. Furthermore, the release of cytokines can trigger cascade reactions of coagulation and immune attacks within transplant recipients. To better elucidate the process of inflammation in liver xenograft recipients, we utilized a modified heterotopic auxiliary liver xenotransplantation model for xeno-immunological research. We studied the cytokine profiles and the relationship between cytokine levels and xenograft function after liver xenotransplantation.

METHODS

Appropriate donor and recipient matches were screened using complement-dependent cytotoxicity assays. Donor liver grafts from α1,3-galactosyltransferase gene-knockout (GTKO) pigs or GTKO pigs additionally transgenic for human CD47 (GTKO/CD47) were transplanted into Tibetan macaques via two different heterotrophic auxiliary liver xenotransplantation procedures. The cytokine profiles, hepatic function, and coagulation parameters were monitored during the clinical course of xenotransplantation.

RESULTS

Xenograft blood flow was stable in recipients after heterotopic auxiliary transplantation. A Doppler examination indicated that the blood flow speed was faster in the hepatic artery (HA) and hepatic vein (HV) of xenografts subjected to the modified Sur II (HA-abdominal aorta+HV-inferior vena cava) procedure than in those subjected to our previously reported Sur I (HA-splenic artery+HV-left renal vein) procedure. Tibetan macaques receiving liver xenografts did not exhibit severe coagulation disorders or immune rejection. Although the recipients did suffer from a rapid loss of platelets, this loss was mild. In blood samples dynamically collected after xenotransplantation (post-Tx), dramatic increases in the levels of monocyte chemoattractant protein 1, interleukin (IL)-8, granulocyte-macrophage colony-stimulating factor, IL-6, and interferon gamma-induced protein 10 were observed at 1 hour post-Tx, even under immunosuppression. We further confirmed that the elevation in individual cytokine levels was correlated with the onset of graft damage. Finally, the release of cytokines might contribute to leukocyte infiltration in the xenografts.

CONCLUSION

Here, we established a modified auxiliary liver xenotransplantation model resulting in near-normal hepatic function. Inflammatory cytokines might contribute to early damage in liver xenografts. Controlling the systemic inflammatory response of recipients might prevent early post-Tx graft dysfunction.

Therapeutic regulation of systemic inflammation in xenograft recipients

Inflammation is known to preclude tolerance after transplantation. We have previously shown that systemic inflammation in xenograft recipients (SIXR) precedes activation of coagulation in the absence of T cell responses. Accordingly, SIXR may amplify innate and adaptive immune responses against xenografts after pig-to-primate xenotransplantation, even with efficient immunosuppressive therapy. We evaluated the impact of anti-inflammatory agents on pro-inflammatory cytokines and chemokines in pig artery patch and heart xenograft recipients. Baboons received an artery patch (Group1, n=8) or heart (Group2, n=4) from genetically engineered pigs. All baboons received lymphodepletion with thymoglobulin (ATG) and costimulation blockade-based immunosuppression (anti-CD40 and/or CTLA4Ig). In Group1, baboons received either (i) no anti-inflammatory agents (n=2), (ii) cobra venom factor (CVF, n=2), (iii) α1-antitrypsin (AAT, n=2), or (iv) interleukin (IL)-6 receptor antagonist (IL-6RA, n=2). In Group2, all baboon received corticosteroids, either without (n=2) or with (n=2) IL-6RA. Serum IFN-γ, TNF-α, IL-1β, IL-17, IL-6, IL-8, MCP-1, and sCD40L levels were measured by Luminex. Fibrinogen, D-dimers, and C-reactive protein (C-RP) were also measured. Recipient baboon T cell proliferation was evaluated by mixed lymphocyte reaction (MLR) before and after transplantation. Pig and baboon tissue factor (TF) mRNA levels in heart xenografts were measured by RT-PCR. In no recipient was a marked increase in T cell response to pig cells observed after transplantation. In Groups 1 and 2, post-transplantation levels of IFN-γ, TNF-α, IL-1β, and IL-17 remained comparable to or lower than pre-transplant levels, except in one heart recipient that succumbed to CMV infection. In Group1, when no anti-inflammatory agent was administered, post-transplant levels of IL-6, IL-8, and MCP-1 were elevated. After CVF, IL-6, IL-8, and MCP-1 remained low. After IL-6RA, IL-6 and MCP-1 were elevated. After AAT, IL-8 was elevated. sCD40L became elevated intermittently in most recipients irrespective of the administered anti-inflammatory agent. In Group2, IL-6 was transiently elevated, particularly after IL-6RA administration. MCP-1 gradually increased by 2 months in Group2 recipients. sCD40L generally remained low except in one recipient. In Group1 and Group2 recipients, C-RP levels were elevated except after IL-6RA administration, while D-dimers were elevated regardless of administration of anti-inflammatory agent. In Group2, pig TF mRNA levels were increased in heart xenografts compared to naive pig hearts, irrespective of IL-6 receptor antagonist administration. Additionally, baboon TF mRNA levels were detectable in heart xenografts, but not in naive pig hearts. Some pro-inflammatory cytokines and chemokines are elevated in xenograft recipients, even with efficient T cell-directed immunosuppressive therapy. Persistent elevation of D-dimers, and individual cytokines and chemokines suggest a continuous inflammatory response, despite administration of anti-inflammatory agents. Systemic administration of combined anti-inflammatory agents as well as complement regulation may be essential to prevent SIXR after xenotransplantation.

Immunological and physiological observations in baboons with life-supporting genetically engineered pig kidney grafts

BACKGROUND

Genetically engineered pigs could provide a source of kidneys for clinical transplantation. The two longest kidney graft survivals reported to date have been 136 and 310 days, but graft survival >30 days has been unusual until recently.

METHODS

Donor pigs (n=4) were on an α1,3-galactosyltransferase gene-knockout (GTKO)/human complement regulatory protein (CD46) background (GTKO/CD46). In addition, the pigs were transgenic for at least one human coagulation regulatory protein. Two baboons received a kidney from a six-gene pig (GroupA) and two from a three-gene pig (GroupB). Immunosuppressive therapy was identical in all four cases and consisted of anti-thymoglobulin (ATG)+anti-CD20mAb (induction) and anti-CD40mAb+rapamycin+corticosteroids (maintenance). Anti-TNF-α and anti-IL-6R mAbs were administered to reduce the inflammatory response. Baboons were followed by clinical/laboratory monitoring of immune/coagulation/inflammatory/physiological parameters. At biopsy or euthanasia, the grafts were examined by microscopy.

RESULTS

The two GroupA baboons remained healthy with normal renal function >7 and >8 months, respectively, but then developed infectious complications. However, no features of a consumptive coagulopathy, eg, thrombocytopenia and reduction of fibrinogen, or of a protein-losing nephropathy were observed. There was no evidence of an elicited anti-pig antibody response, and histology of biopsies taken at approximately 4, 6, and 7 months and at necropsy showed no significant abnormalities. In contrast, both GroupB baboons developed features of a consumptive coagulopathy and required euthanasia on day 12.

CONCLUSIONS

The combination of (i) a graft from a specific six-gene genetically modified pig, (ii) an effective immunosuppressive regimen, and (iii) anti-inflammatory therapy prevented immune injury, a protein-losing nephropathy, and coagulation dysfunction for >7 months. Although the number of experiments is very limited, our impression is that expression of human endothelial protein C receptor (±CD55) in the graft is important if coagulation dysregulation is to be avoided.

Thyroid hormone: relevance to xenotransplantation

BACKGROUND

It has been well documented that the level of serum/plasma free triiodothyronine (fT3) falls rapidly following brain death or during certain surgical procedures, for example, heart surgery carried out on cardiopulmonary bypass. The level in patients following cardiopulmonary bypass usually recovers within 2 days.

METHODS

We have measured serum fT3 in healthy naïve baboons (n = 31), healthy naïve monkeys (n = 5), and after pig-to-baboon heterotopic heart xenotransplantation (xenoTx) (Group 1, n = 9), orthotopic liver xenoTx (Group 2, n = 10), artery patch xenoTx (Group 3, n = 9), and in monkey-to-monkey heterotopic heart alloTx (Group 4, n = 5).

RESULTS

The mean level of fT3 in healthy naïve baboons was 3.1 ± 0.9 pg/ml and in healthy naïve monkeys was 2.6 ± 0.3 pg/ml. Following pig heart, liver, and artery patch xenoTx and monkey heart alloTx, there was an immediate rapid fall in fT3 level. Recovery of fT3 was more rapid in Groups 3 and 4 than in Groups 1 and 2. In Group 1, within 4 days fT3 had recovered, but only to the lower limit of normal range, where it remained throughout follow-up (for up to 42 days). In Group 2, no recovery was seen during the 7 days of follow-up. In immunosuppressed baboons with pig patch grafts that received IL-6R blockade (n = 2), the fT3 tended to rise higher than in those that received no IL-6R blockade (n = 6).

CONCLUSIONS

Following operative procedures, there is a dramatic fall in serum fT3 levels. The persistent low level of fT3 after pig heart and liver xenoTx may be associated with a continuing inflammatory state. We suggest that consideration should be given to the replacement of T3 therapy to maintain normal fT3 levels, particularly in nonhuman primates undergoing orthotopic pig heart or liver xenoTx.

The pathobiology of pig-to-primate xenotransplantation: a historical review

The immunologic barriers to successful xenotransplantation are related to the presence of natural anti-pig antibodies in humans and non-human primates that bind to antigens expressed on the transplanted pig organ (the most important of which is galactose-α1,3-galactose [Gal]), and activate the complement cascade, which results in rapid destruction of the graft, a process known as hyperacute rejection. High levels of elicited anti-pig IgG may develop if the adaptive immune response is not prevented by adequate immunosuppressive therapy, resulting in activation and injury of the vascular endothelium. The transplantation of organs and cells from pigs that do not express the important Gal antigen (α1,3-galactosyltransferase gene-knockout [GTKO] pigs) and express one or more human complement-regulatory proteins (hCRP, e.g., CD46, CD55), when combined with an effective costimulation blockade-based immunosuppressive regimen, prevents early antibody-mediated and cellular rejection. However, low levels of anti-non-Gal antibody and innate immune cells and/or platelets may initiate the development of a thrombotic microangiopathy in the graft that may be associated with a consumptive coagulopathy in the recipient. This pathogenic process is accentuated by the dysregulation of the coagulation-anticoagulation systems between pigs and primates. The expression in GTKO/hCRP pigs of a human coagulation-regulatory protein, for example, thrombomodulin, is increasingly being associated with prolonged pig graft survival in non-human primates. Initial clinical trials of islet and corneal xenotransplantation are already underway, and trials of pig kidney or heart transplantation are anticipated within the next few years.

The role of genetically engineered pigs in xenotransplantation research

There is a critical shortage in the number of deceased human organs that become available for the purposes of clinical transplantation. This problem might be resolved by the transplantation of organs from pigs genetically engineered to protect them from the human immune response. The pathobiological barriers to successful pig organ transplantation in primates include activation of the innate and adaptive immune systems, coagulation dysregulation and inflammation. Genetic engineering of the pig as an organ source has increased the survival of the transplanted pig heart, kidney, islet and corneal graft in non-human primates (NHPs) from minutes to months or occasionally years. Genetic engineering may also contribute to any physiological barriers that might be identified, as well as to reducing the risks of transfer of a potentially infectious micro-organism with the organ. There are now an estimated 40 or more genetic alterations that have been carried out in pigs, with some pigs expressing five or six manipulations. With the new technology now available, it will become increasingly common for a pig to express even more genetic manipulations, and these could be tested in the pig-to-NHP models to assess their efficacy and benefit. It is therefore likely that clinical trials of pig kidney, heart and islet transplantation will become feasible in the near future.