Comparative histopathology of hepatic allografts and xenografts in the nonhuman primate

Bridging to Allotransplantation-Is Pig Liver Xenotransplantation the Best Option?

In the past 20 y, the number of patients in the United States who died while waiting for a human donor liver totaled >52 000. The median national wait time for patients with acute liver failure and the most urgent liver transplant listing was 7 d in 2018. The need for a clinical "bridge" to allotransplantation is clear. Current options for supporting patients with acute liver failure include artificial liver support devices, extracorporeal liver perfusion, and hepatocyte transplantation, all of which have shown mixed results with regard to survival benefit and are largely experimental. Progress in the transplantation of genetically engineered pig liver grafts in nonhuman primates has grown steadily, with survival of the pig graft extended to almost 1 mo in 2017. Further advances may justify consideration of a pig liver transplant as a clinical bridge to allotransplantation. We provide a brief history of pig liver xenotransplantation, summarize the most recent progress in pig-to-nonhuman primate liver transplantation models, and suggest criteria that may be considered for patient selection for a clinical trial of bridging by genetically engineered pig liver xenotransplantation to liver allotransplantation.

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

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

Liver xenotransplantation

PURPOSE OF REVIEW

There continues to be an inadequate organ supply and lack of effective temporary support, for patients with liver failure. The purpose of this review is to discuss recent progress in the field of orthotopic pig-to-nonhuman primate (NHP) liver xenotransplantation (LXT).

RECENT FINDINGS

From 1968 to 2012, survival in pig-to-NHP LXT was limited to 9 days, initially due to hyperacute rejection which has been ameliorated through use of genetically engineered donor organs, but ultimately because of profound thrombocytopenia, thrombotic microangiopathy, and bleeding. Most recently, however, demise secondary to lethal coagulopathy has been avoided with LXT of α(1,3)-galactosyltransferase knockouts and cytomegalovirus-negative porcine xenografts into baboons receiving exogenous administration of coagulation factors and co-stimulation blockade, establishing that a porcine liver is capable of supporting NHP life for nearly a month.

SUMMARY

Continued consistent achievement of pig-to-NHP LXT survival beyond 2 weeks justifies consideration of a clinical application as a bridge to allotransplantation for patients with acute hepatic failure. Further genetic modifications to the donor, as well as additional studies, are required in order to apply LXT as destination therapy.

Use and Importance of Nonhuman Primates in Metabolic Disease Research: Current State of the Field

Obesity and its multiple metabolic sequelae, including type 2 diabetes, cardiovascular disease, and fatty liver disease, are becoming increasingly widespread in both the developed and developing world. There is an urgent need to identify new approaches for the prevention and treatment of these costly and prevalent metabolic conditions. Accomplishing this will require the use of appropriate animal models for preclinical and translational investigations in metabolic disease research. Although studies in rodent models are often useful for target/pathway identification and testing hypotheses, there are important differences in metabolic physiology between rodents and primates, and experimental findings in rodent models have often failed to be successfully translated into new, clinically useful therapeutic modalities in humans. Nonhuman primates represent a valuable and physiologically relevant model that serve as a critical translational bridge between basic studies performed in rodent models and clinical studies in humans. The purpose of this review is to evaluate the evidence, including a number of specific examples, in support of the use of nonhuman primate models in metabolic disease research, as well as some of the disadvantages and limitations involved in the use of nonhuman primates. The evidence taken as a whole indicates that nonhuman primates are and will remain an indispensable resource for evaluating the efficacy and safety of novel therapeutic strategies targeting clinically important metabolic diseases, including dyslipidemia and atherosclerosis, type 2 diabetes, hepatic steatosis, steatohepatitis, and hepatic fibrosis, and potentially the cognitive decline and dementia associated with metabolic dysfunction, prior to taking these therapies into clinical trials in humans.

Pig Liver Xenotransplantation: A Review of Progress Toward the Clinic

Experience with clinical liver xenotransplantation has largely involved the transplantation of livers from nonhuman primates. Experience with pig livers has been scarce. This brief review will be restricted to assessing the potential therapeutic impact of pig liver xenotransplantation in acute liver failure and the remaining barriers that currently do not justify clinical trials. A relatively new surgical technique of heterotopic pig liver xenotransplantation is described that might play a role in bridging a patient with acute liver failure until either the native liver recovers or a suitable liver allograft is obtained. Other topics discussed include the possible mechanisms for the development of the thrombocytopenis that rapidly occurs after pig liver xenotransplantation in a primate, the impact of pig complement on graft injury, the potential infectious risks, and potential physiologic incompatibilities between pig and human. There is cautious optimism that all of these problems can be overcome by judicious genetic manipulation of the pig. If liver graft survival could be achieved in the absence of thrombocytopenia or rejection for a period of even a few days, there may be a role for pig liver transplantation as a bridge to allotransplantation in carefully selected patients.

Progress in pig-to-non-human primate transplantation models (1998-2013): a comprehensive review of the literature

BACKGROUND

The pig-to-non-human primate model is the standard choice for in vivo studies of organ and cell xenotransplantation. In 1998, Lambrigts and his colleagues surveyed the entire world literature and reported all experimental studies in this model. With the increasing number of genetically engineered pigs that have become available during the past few years, this model is being utilized ever more frequently.

METHODS

We have now reviewed the literature again and have compiled the data we have been able to find for the period January 1, 1998 to December 31, 2013, a period of 16 yr.

RESULTS

The data are presented for transplants of the heart (heterotopic and orthotopic), kidney, liver, lung, islets, neuronal cells, hepatocytes, corneas, artery patches, and skin. Heart, kidney, and, particularly, islet xenograft survival have increased significantly since 1998.

DISCUSSION

The reasons for this are briefly discussed. A comment on the limitations of the model has been made, particularly with regard to those that will affect progression of xenotransplantation toward the clinic.

Immunobiology of liver xenotransplantation

Pigs are currently the preferred species for future organ xenotransplantation. With advances in the development of genetically modified pigs, clinical xenotransplantation is becoming closer to reality. In preclinical studies (pig-to-nonhuman primate), the xenotransplantation of livers from pigs transgenic for human CD55 or from α1,3-galactosyltransferase gene-knockout pigs+/- transgenic for human CD46, is associated with survival of approximately 7-9 days. Although hepatic function, including coagulation, has proved to be satisfactory, the immediate development of thrombocytopenia is very limiting for pig liver xenotransplantation even as a 'bridge' to allotransplantation. Current studies are directed to understand the immunobiology of platelet activation, aggregation and phagocytosis, in particular the interaction between platelets and liver sinusoidal endothelial cells, hepatocytes and Kupffer cells, toward identifying interventions that may enable clinical application.

Genetically-engineered pig-to-baboon liver xenotransplantation: histopathology of xenografts and native organs

Orthotopic liver transplantation was carried out in baboons using wild-type (WT, n = 1) or genetically-engineered pigs (α1,3-galactosyltransferase gene-knockout, GTKO), n = 1; GTKO pigs transgenic for human CD46, n = 7) and a clinically-acceptable immunosuppressive regimen. Biopsies were obtained from the WT pig liver pre-Tx and at 30 min, 1, 2, 3, 4 and 5 h post-transplantation. Biopsies of genetically-engineered livers were obtained pre-Tx, 2 h after reperfusion and at necropsy (4–7 days after transplantation). Tissues were examined by light, confocal, and electron microscopy. All major native organs were also examined. The WT pig liver underwent hyperacute rejection. After genetically-engineered pig liver transplantation, hyperacute rejection did not occur. Survival was limited to 4–7 days due to repeated spontaneous bleeding in the liver and native organs (as a result of profound thrombocytopenia) which necessitated euthanasia. At 2 h, graft histology was largely normal. At necropsy, genetically-engineered pig livers showed hemorrhagic necrosis, platelet aggregation, platelet-fibrin thrombi, monocyte/macrophage margination mainly in liver sinusoids, and vascular endothelial cell hypertrophy, confirmed by confocal and electron microscopy. Immunohistochemistry showed minimal deposition of IgM, and almost absence of IgG, C3, C4d, C5b-9, and of a cellular infiltrate, suggesting that neither antibody- nor cell-mediated rejection played a major role.

Pig liver xenotransplantation as a bridge to allotransplantation: which patients might benefit?

Acute liver failure is a potentially devastating clinical syndrome that, without liver transplantation (Tx), is associated with high mortality. Rapid deterioration in clinical status and a shortage of deceased human organs prohibits liver Tx in many patients. Bridging to liver Tx has been attempted by various approaches, for example, bioartificial liver support, extracorporeal pig liver perfusion, and hepatocyte Tx, but none of these approaches has convincingly improved patient survival. The orthotopic Tx of a genetically engineered pig liver could theoretically provide successful bridging. Immediate availability, perfect metabolic condition, adequate size-match and hepatocyte mass, and freedom from potentially pathogenic microorganisms could be assured. The advantages and disadvantages of bridging by pig liver Tx compared with other approaches are discussed. The selection of patients for an initial clinical trial of pig liver Tx would be similar to that of various prior trials in patients experiencing rapid and severe deterioration in liver function. The ability to give truly informed consent for a pig bridging procedure at the time of listing for liver Tx renders the patient with acute-on-chronic liver failure or primary allograft failure is a preferable candidate for this procedure than a patient who is admitted urgently with acute (fulminant) liver failure in whom consent may not be possible. Although several barriers to successful pig organ xenoTx remain, for example, coagulation dysfunction between pig and primate, if these can be resolved by further genetic engineering of the organ-source pigs, a pig liver may prove life saving to patients dying rapidly of liver failure.

Liver xenografts for the treatment of acute liver failure: clinical and experimental experience and remaining immunologic barriers

A critical element restricting the application of liver transplantation is the shortage of human deceased donor organs. Xenotransplantation using pig organs might be a solution to this shortage. Although the problems that still require resolution include the immunologic barrier, the potential risk of transferring infectious agents with the transplanted organ, and uncertainty about whether the transplanted organ will function satisfactorily in the human environment, recent progress in the genetic manipulation of pigs has led to the prospect that clinical xenografting, at least as a bridge to allotransplantation, may be possible in the foreseeable future. Experience with clinical auxiliary and orthotopic liver xenotransplantation and experimental liver xenotransplantation in nonhuman primate and other large animal models is reviewed, and the remaining immunologic problems are discussed. Evidence suggests that, in patients with hepatic failure, the pig liver may be less susceptible to antibody-mediated injury than other pig organs, such as the heart or kidney. Pig Kupffer cells and other macrophages will recognize and phagocytose primate red blood cells, but this problem should be overcome by pretransplant depletion of macrophages from the organ-source pig. From the evidence currently available, it does not seem unduly optimistic to anticipate that a liver from an alpha1,3-galactosyltransferase gene-knockout pig would survive at least long enough to function as a successful bridge to allotransplantation.

HDAF transgenic pig livers are protected from hyperacute rejection during ex vivo perfusion with human blood

The aim of this study was to determine if human decay-accelerating factor (hDAF) protects against hyperacute rejection in an ex vivo liver perfusion system using human blood. Pig livers were perfused ex vivo via the portal vein for an average of 5-6 h using a membrane oxygenator. Three groups were studied. Group I: Wild-type pig livers were alloperfused with fresh pig blood (n = 5). Group II: Wild-type pig livers were xenoperfused with fresh human blood (n = 5). Group III: hDAF transgenic pig livers were xenoperfused with fresh human blood (n = 5). The graft ischemic time, ratio of perfusate volume to liver weight, flow rate, and perfusate hematocrit were similar in each group. The hDAF livers perfused with human blood (Group III) had a lower ALT level, less protein and albumin losses, lower bilirubin levels in the perfusate, less weight gain, and greater bile production than the wild-type livers perfused with human blood. Histology showed classic features of hyperacute rejection in Group II, including massive hemorrhage, severe vasculitits, fibrin and C5b-9 deposition, and endothelial damage within 1 h of perfusion, whereas liver histology studies in Groups I and III were near normal. IgG and IgM deposits were seen in the xenoperfused livers. Electron microscopy (EM) and immuno-EM showed loss of endothelial cells, trapping of white blood cells and platelets, and diffuse fibrin deposits in Group II only. hDAF pig livers perfused with human blood showed superior function and histology when compared with wild-type pig livers. These data suggest that (1) hyperacute rejection may contribute to the inconsistent results using wild-type pig livers for extracorporeal liver support and (2) genetically modified pigs that express hDAF may provide a better donor source than wild-type pigs for extracorporeal liver support.

Life-supporting human complement regulator decay accelerating factor transgenic pig liver xenograft maintains the metabolic function and coagulation in the nonhuman primate for up to 8 days

BACKGROUND

It is not known whether the pig liver is capable of functioning efficiently when transplanted into a primate, neither is there experience in transplanting a liver from a transgenic pigs expressing the human complement regulator human complement regulator decay accelerating factor (h-DAF) into a baboon. The objective of this study was to determine whether the porcine liver would support the metabolic functions of non-human primates and to establish the effect of hDAF expression in the prevention of hyperacute rejection of porcine livers transplanted into primates.

METHODS

Five orthotopic liver xenotransplants from pig to baboon were carried out: three from unmodified pigs and two using livers from h-DAF transgenic pigs.

FINDINGS

The three control animals transplanted with livers from unmodified pigs survived for less than 12 hr. Baboons transplanted with livers from h-DAF transgenic pigs survived for 4 and 8 days. Hyperacute rejection was not detected in the baboons transplanted with hDAF transgenic pig livers; however, it was demonstrated in the three transplants from unmodified pigs. Baboons transplanted with livers from h-DAF transgenic pigs were extubated at postoperative day 1 and were awake and able to eat and drink. In the recipients of hDAF transgenic pig livers the clotting parameters reached nearly normal levels at day 2 after transplantation and remained normal up to the end of the experiments. In these hDAF liver recipients, porcine fibrinogen was first detected in the baboon plasma 2 hr postreperfusion, and was present up to the end of the experiments. One animal was euthanized at day 8 after development of sepsis and coagulopathy, the other animal arrested at day 4, after an episode of vomiting and aspiration. The postmortem examination of the hDAF transgenic liver xenografts did not demonstrate rejection.

INTERPRETATION

The livers from h-DAF transgenic pigs did not undergo hyperacute rejection after orthotopic xenotransplantation in baboons. When HAR is abrogated, the porcine liver maintains sufficient coagulation and protein levels in the baboon up to 8 days after OLT.

Pig xenogeneic antigen modification with green coffee bean alpha-galactosidase

Green coffee bean alpha-galactosidase can cleave the terminal alpha-galactose (alphaGal) on oligosaccharides that form the major antigen on pig endothelial cells recognized by primate-specific antibodies. Studies have been made of the conditions under which it is functional (e.g. temperature, pH) and of its biochemical and immunologic effects. Pig-to-rhesus monkey vein transplants were studied to identify the efficiency of the enzyme in delaying hyperacute rejection. When a graft became occluded, biopsies were taken for light microscopy (hematoxylin and eosin), scanning electron microscopy (SEM) and immunostaining with Griffonia simplicifolia IB4 lectin (GSIB4), and for IgM, IgG and C3. alpha-Galactosidase was stable for 72-96 h and was effective at 4 degrees C and pH 6.9 (conditions of human liver graft storage), although better function was obtained at 20 degrees C and pH 6.5. Using the porcine PK15 cell assay, the cytotoxicity of human serum was reduced after treatment of the pig cells with the enzyme. In vitro studies demonstrated that porcine veins treated with alpha-galactosidase lost endothelial expression of the Gal epitope within 30 min. SEM, however, demonstrated endothelial damage beginning within 2 h, probably caused by the alpha-galactosidase, as no damage was found in phosphate-buffered saline-treated veins, where the Gal epitope was preserved for >3 h. No change was found in either group on light microscopy. In vivo studies demonstrated that patency of the alpha-galactosidase-treated veins (mean 2.5 h) was longer than that of untreated veins (0.23 h) (P < 0.01). Biopsies showed no GSIB4 lectin staining for alpha-Gal epitopes and much less IgM and C3 deposition in the treated group. Light microscopy and SEM demonstrated more severe endothelial damage, hemorrhage, and fibrin formation in the untreated group. Galactosidase is effective in removing the terminal alphaGal and delays the onset of hyperacute rejection of pig veins transplanted into monkeys. However, its effect is temporary and, on its own, its use is unlikely to prolong survival of pig organs transplanted into primates sufficiently to be of clinical value.

T cell response in xenorecognition and xenografts: a review

Xenotransplantation has recently become a subject of interest for the transplantation community due to the current organ shortage, which could be partially or even totally solved by the development of this strategy. The humoral response, which arises as a result of species disparities, is the major obstacle to the success of xenotransplantation. However, if the use of different strategies such as plasmapheresis, immunoadsorption, the utilization of organs from transgenic pigs for complement regulatory molecules and new immunosuppressive drugs, may allow to overcome or reduce the early antibody mediated rejections (hyperacute or acute vascular rejection), delayed responses based on cellular activations will still occur. In this review, despite the fact that different cell populations have been shown to be implicated in these phenomena (NK, granulocytes, macrophages), we will focus on recent published information concerning T cell response only, in xenorecognition.