Protective effects of recombinant human antithrombin III in pig-to-primate renal xenotransplantation

Characterisation of transgenic pigs expressing a human T cell-depleting anti-CD2 monoclonal antibody

BACKGROUND

Pig islet xenotransplantation is a potential treatment for type 1 diabetes. We have shown that maintenance immunosuppression is required to protect genetically modified (GM) porcine islet xenografts from T cell-mediated rejection in baboons. Local expression of a depleting anti-CD2 monoclonal antibody (mAb) by the xenograft may provide an alternative solution. We have previously reported the generation of GGTA1 knock-in transgenic pigs expressing the chimeric anti-CD2 mAb diliximab under an MHC class I promoter (MHCIP). In this study, we generated GGTA1 knock-in pigs in which MHCIP was replaced by the β-cell-specific porcine insulin promoter (PIP), and compared the pattern of diliximab expression in the two lines.

METHODS

A PIP-diliximab knock-in construct was prepared and validated by transfection of NIT-1 mouse insulinoma cells. The construct was knocked into GGTA1 in wild type (WT) porcine fetal fibroblasts using CRISPR, and knock-in cells were used to generate pigs by somatic cell nuclear transfer (SCNT). Expression of the transgene in MHCIP-diliximab and PIP-diliximab knock-in pigs was characterised at the mRNA and protein levels using RT-qPCR, flow cytometry, ELISA and immunohistochemistry. Islets from MHCIP-diliximab and control GGTA1 KO neonatal pigs were transplanted under the kidney capsule of streptozotocin-diabetic SCID mice.

RESULTS

NIT-1 cells stably transfected with the PIP-diliximab knock-in construct secreted diliximab into the culture supernatant, confirming correct expression and processing of the mAb in β cells. PIP-diliximab knock-in pigs showed a precise integration of the transgene within GGTA1. Diliximab mRNA was detected in all tissues tested (spleen, kidney, heart, liver, lung, pancreas) in MHCIP-diliximab pigs, but was not detectable in PIP-diliximab pigs. Likewise, diliximab was present in the serum of MHCIP-diliximab pigs, at a mean concentration of 1.8 μg/mL, but was not detected in PIP-diliximab pig serum. An immunohistochemical survey revealed staining for diliximab in all organs of MHCIP-diliximab pigs but not of PIP-diliximab pigs. Whole genome sequencing (WGS) of a PIP-diliximab pig identified a missense mutation in the coding region for the dixilimab light chain. This mutation was also found to be present in the fibroblast knock-in clone used to generate the PIP-diliximab pigs. Islet xenografts from neonatal MHCIP-diliximab pigs restored normoglycemia in diabetic immunodeficient mice, indicating no overt effect of the transgene on islet function, and demonstrated expression of diliximab in situ.

CONCLUSION

Diliximab was widely expressed in MHCIP-diliximab pigs, including in islets, consistent with the endogenous expression pattern of MHC class I. Further investigation is required to determine whether the level of expression in islets from the MHCIP-diliximab pigs is sufficient to prevent T cell-mediated islet xenograft rejection. The unexpected absence of diliximab expression in the islets of PIP-diliximab pigs was probably due to a mutation in the transgene arising during the generation of the knock-in cells used for SCNT.

Cardiac xenografts show reduced survival in the absence of transgenic human thrombomodulin expression in donor pigs

A combination of genetic manipulations of donor organs and target-specific immunosuppression is instrumental in achieving long-term cardiac xenograft survival. Recently, results from our preclinical pig-to-baboon heterotopic cardiac xenotransplantation model suggest that a three-pronged approach is successful in extending xenograft survival: (a) α-1,3-galactosyl transferase (Gal) gene knockout in donor pigs (GTKO) to prevent Gal-specific antibody-mediated rejection; (b) transgenic expression of human complement regulatory proteins (hCRP; hCD46) and human thromboregulatory protein thrombomodulin (hTBM) to avoid complement activation and coagulation dysregulation; and (c) effective induction and maintenance of immunomodulation, particularly through co-stimulation blockade of CD40-CD40L pathways with anti-CD40 (2C10R4) monoclonal antibody (mAb). Using this combination of manipulations, we reported significant improvement in cardiac xenograft survival. In this study, we are reporting the survival of cardiac xenotransplantation recipients (n = 3) receiving xenografts from pigs without the expression of hTBM (GTKO.CD46). We observed that all grafts underwent rejection at an early time point (median 70 days) despite utilization of our previously reported successful immunosuppression regimen and effective control of non-Gal antibody response. These results support our hypothesis that transgenic expression of human thrombomodulin in donor pigs confers an independent protective effect for xenograft survival in the setting of a co-stimulation blockade-based immunomodulatory regimen.

SerpinC1/Antithrombin III in kidney-related diseases

The gene SerpinC1 encodes a serine protease inhibitor named antithrombin III (ATIII). This protease demonstrates both anticoagulant and anti-inflammatory action. ATIII is the most important coagulation factor inhibitor, and even minor changes in ATIII can significantly alter the risk of thromboembolism. ATIII can also suppress inflammation via a coagulation-dependent or -independent effect. Moreover, apart from ATIII deficiency, ATIII and its gene SerpinC1 may also be related to many diseases (e.g. hypertension, kidney diseases). The present review summarizes how ATIII affects the progress of kidney disease and its mechanism. Further studies are required to investigate how ATIII affects renal function and the treatment.

Current status of pig kidney xenotransplantation

Significant progress in life-supporting kidney xenograft survival in nonhuman primates (NHPs) has been associated largely with the increasing availability of pigs with genetic modifications that protect the pig tissues from the primate immune response and/or correct molecular incompatibilities between pig and primate. Blockade of the CD40/CD154 costimulation pathway with anti-CD154 mAb therapy has contributed to prolongation of kidney xenograft survival, although this agent may not be clinically available. An anti-CD40 mAb-based regimen is proving equally successful, but blockade of the CD28/B7 pathway is inadequate. Severe proteinuria were uniformly documented in the early studies of pig kidney xenotransplantation, but whether this resulted from immune injury or from physiological incompatibilities between the species, or both, remained uncertain. Recent experiments suggest it was related to a continuing immune response. Before 2014, the longest survival of a pig kidney graft in a NHP was 90 days, though graft survival >30 days was unusual. Recently this has been extended to >125 days, without features of a consumptive coagulopathy or a protein-losing nephropathy. In conclusion, overcoming the immune, coagulation, and inflammatory responses by the development of precise genetic modifications in donor pigs, along with effective immunosuppressive and anticoagulant/anti-inflammatory therapy is advancing the field towards clinical trials.

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.

Control of IBMIR in neonatal porcine islet xenotransplantation in baboons

The instant blood-mediated inflammatory reaction (IBMIR) is a major obstacle to the engraftment of intraportal pig islet xenografts in primates. Higher expression of the galactose-α1,3-galactose (αGal) xenoantigen on neonatal islet cell clusters (NICC) than on adult pig islets may provoke a stronger reaction, but this has not been tested in the baboon model. Here, we report that WT pig NICC xenografts triggered profound IBMIR in baboons, with intravascular clotting and graft destruction occurring within hours, which was not prevented by anti-thrombin treatment. In contrast, IBMIR was minimal when recipients were immunosuppressed with a clinically relevant protocol and transplanted with NICC from αGal-deficient pigs transgenic for the human complement regulators CD55 and CD59. These genetically modified (GM) NICC were less susceptible to humoral injury in vitro than WT NICC, inducing significantly less complement activation and thrombin generation when incubated with baboon platelet-poor plasma. Recipients of GM NICC developed a variable anti-pig antibody response, and examination of the grafts 1 month after transplant revealed significant cell-mediated rejection, although scattered insulin-positive cells were still present. Our results indicate that IBMIR can be attenuated in this model, but long-term graft survival may require more effective immunosuppression or further donor genetic modification.

The role of platelets in coagulation dysfunction in xenotransplantation, and therapeutic options

Xenotransplantation could resolve the increasing discrepancy between the availability of deceased human donor organs and the demand for transplantation. Most advances in this field have resulted from the introduction of genetically engineered pigs, e.g., α1,3-galactosyltransferase gene-knockout (GTKO) pigs transgenic for one or more human complement-regulatory proteins (e.g., CD55, CD46, CD59). Failure of these grafts has not been associated with the classical features of acute humoral xenograft rejection, but with the development of thrombotic microangiopathy in the graft and/or consumptive coagulopathy in the recipient. Although the precise mechanisms of coagulation dysregulation remain unclear, molecular incompatibilities between primate coagulation factors and pig natural anticoagulants exacerbate the thrombotic state within the xenograft vasculature. Platelets play a crucial role in thrombosis and contribute to the coagulation disorder in xenotransplantation. They are therefore important targets if this barrier is to be overcome. Further genetic manipulation of the organ-source pigs, such as pigs that express one or more coagulation-regulatory genes (e.g., thrombomodulin, endothelial protein C receptor, tissue factor pathway inhibitor, CD39), is anticipated to inhibit platelet activation and the generation of thrombus. In addition, adjunctive pharmacologic anti-platelet therapy may be required. The genetic manipulations that are currently being tested are reviewed, as are the potential pharmacologic agents that may prove beneficial.

Kidney xenotransplantation

Xenotransplantation using pigs as donors offers the possibility of eliminating the chronic shortage of donor kidneys, but there are several obstacles to be overcome before this goal can be achieved. Preclinical studies have shown that, while porcine renal xenografts are broadly compatible physiologically, they provoke a complex rejection process involving preformed and elicited antibodies, heightened innate immune cell reactivity, dysregulated coagulation, and a strong T cell-mediated adaptive response. Furthermore, the susceptibility of the xenograft to proinflammatory and procoagulant stimuli is probably increased by cross-species molecular defects in regulatory pathways. To balance these disadvantages, xenotransplantation has at its disposal a unique tool to address particular rejection mechanisms and incompatibilities: genetic modification of the donor. This review focuses on the pathophysiology of porcine renal xenograft rejection, and on the significant genetic, pharmacological, and technical progress that has been made to prolong xenograft survival.

Xeno-kidney transplantation: from idea to reality

Although kidney transplantation is a widely used therapy for chronic renal failure, not all patients can be transplanted due to the limited numbers of organ donations. A possible solution could be xenogenic kidney transplantation. Herein we have described the present state, problems and possible solutions using xenograft treatments.

Current status of immunomodulatory and cellular therapies in preclinical and clinical islet transplantation

Clinical islet transplantation is a β-cell replacement strategy that represents a possible definitive intervention for patients with type 1 diabetes, offering substantial benefits in terms of lowering daily insulin requirements and reducing incidences of debilitating hypoglycemic episodes and unawareness. Despite impressive advances in this field, a limiting supply of islets, inadequate means for preventing islet rejection, and the deleterious diabetogenic and nephrotoxic side effects associated with chronic immunosuppressive therapy preclude its wide-spread applicability. Islet transplantation however allows a window of opportunity for attempting various therapeutic manipulations of islets prior to transplantation aimed at achieving superior transplant outcomes. In this paper, we will focus on the current status of various immunosuppressive and cellular therapies that promote graft function and survival in preclinical and clinical islet transplantation with special emphasis on the tolerance-inducing capacity of regulatory T cells as well as the β-cells regenerative capacity of stem cells.

Potential value of human thrombomodulin and DAF expression for coagulation control in pig-to-human xenotransplantation

BACKGROUND

Problems of coagulation disorder remain to be resolved in pig-to-primate xenotransplantation. Molecular incompatibilities in the coagulation systems between pigs and humans, such as the thrombomodulin (TM)-protein C system or direct prothrombinase activity, have been suggested as possible causes. Coagulation and complement activation are closely related to each other. The purpose of this study was to elucidate the protective effects on the coagulation system of the expression of human TM and decay accelerating factor (hDAF) (for inhibition of complement activation) in pig endothelial cells.

METHODS

Human aortic endothelial cells (HAEC), porcine aortic endothelial cells (PAEC), hDAF-expressing PAEC (hDAF-PAEC), hDAF/Endo-beta-galactosidase C-expressing PAEC (hDAF/EndoGalC-PAEC), hTM-expressing PAEC (hTM-PAEC), hDAF/hTM expressing-PAEC (hDAF/hTM-PAEC), and hDAF/EndoGalC/hTM-expressing PAEC (hDAF/EndoGalC/hTM-PAEC) were used in this study. Coagulation activity was examined by clotting, activated protein C (APC), and thrombin generation assay.

RESULTS

A large difference was observed in clotting time of human plasma when exposed to PAEC (170 s) and HAEC (1020 s). hTM expression on PAEC was proven to produce a comparable level of APC to that produced by HAEC, which prolonged the clotting time, though not to the level of HAEC. Pretreatment with human sera considerably shortened the clotting time in PAEC (80 s). hDAF-PAEC significantly inhibited such a shortening of clotting time by reductions in tissue factor expression and thrombin generation. Thrombin generation through direct prothrombinase activity, which was detected only in PAEC, could be suppressed by hTM expression. Suppression of antibody binding and complement activation improved clotting time not in PAEC, but in PAEC expressing hTM.

CONCLUSIONS

In addition to effective suppression of antibody-induced complement activation, hTM expression in PAEC may be essential for regulating procoagulant activity in xenotransplantation.

Overcoming the barriers to xenotransplantation: prospects for the future

Cross-species transplantation (xenotransplantation) has immense potential to solve the critical need for organs, tissues and cells for clinical transplantation. The increasing availability of genetically engineered pigs is enabling progress to be made in pig-to-nonhuman primate experimental models. Potent pharmacologic immunosuppressive regimens have largely prevented T-cell rejection and a T-cell-dependent elicited antibody response. However, coagulation dysfunction between the pig and primate is proving to be a major problem, and this can result in life-threatening consumptive coagulopathy. This complication is unlikely to be overcome until pigs expressing a human 'antithrombotic' or 'anticoagulant' gene, such as thrombomodulin, tissue factor pathway inhibitor or CD39, become available. Progress in islet xenotransplantation has been more encouraging, and diabetes has been controlled in nonhuman primates for periods in excess of 6 months, although this has usually been achieved using immunosuppressive protocols that might not be clinically applicable. Further advances are required to overcome the remaining barriers.

Anti-inflammatory and anticoagulant effects of transgenic expression of human thrombomodulin in mice

Thrombomodulin (TBM) is an important vascular anticoagulant that has species specific effects. When expressed as a transgene in pigs, human (h)TBM might abrogate thrombotic manifestations of acute vascular rejection (AVR) that occur when GalT-KO and/or complement regulator transgenic pig organs are transplanted to primates. hTBM transgenic mice were generated and characterized to determine whether this approach might show benefit without the development of deleterious hemorrhagic phenotypes. hTBM mice are viable and are not subject to spontaneous hemorrhage, although they have a prolonged bleeding time. They are resistant to intravenous collagen-induced pulmonary thromboembolism, stasis-induced venous thrombosis and pulmonary embolism. Cardiac grafts from hTBM mice to rats treated with cyclosporine in a model of AVR have prolonged survival compared to controls. hTBM reduced the inflammatory reaction in the vein wall in the stasis-induced thrombosis and mouse-to-rat xenograft models and reduced HMGB1 levels in LPS-treated mice. These results indicate that transgenic expression of hTBM has anticoagulant and antiinflammatory effects that are graft-protective in murine models.

Current status of xenotransplantation and prospects for clinical application

Xenotransplantation is one promising approach to bridge the gap between available human cells, tissues, and organs and the needs of patients with diabetes or end-stage organ failure. Based on recent progress using genetically modified source pigs, improving results with conventional and experimental immunosuppression, and expanded understanding of residual physiologic hurdles, xenotransplantation appears likely to be evaluated in clinical trials in the near future for some select applications. This review offers a comprehensive overview of known mechanisms of xenograft injury, a contemporary assessment of preclinical progress and residual barriers, and our opinions regarding where breakthroughs are likely to occur.

The coagulation barrier in xenotransplantation: incompatibilities and strategies to overcome them

PURPOSE OF REVIEW

Dysregulated coagulation is now recognized as a major contributor to graft loss in xenotransplantation. This review summarizes recent data on putative mechanisms of pathogenic coagulation in xenotransplantation and discusses progress on strategies to overcome them.

RECENT FINDINGS

Evidence continues to grow that the primary cause of failure of pig cardiac and renal xenografts is probably antibody-mediated injury to the endothelium, leading to development of microvascular thrombosis. Several factors that may exacerbate the problem will remain, even in the absence of a humoral response. These include molecular incompatibilities that affect the control of coagulation - in particular the failure of pig thrombomodulin to activate the primate protein C pathway - and platelet reactivity. Expression of anticoagulant and antiplatelet molecules within the graft is a potential solution that has been successfully tested in rodent models and will soon be applied to the pig-to-primate model. This strategy, in parallel with physical methods such as encasing islets in a protective layer, also holds promise for reducing the thrombogenicity of pig islet xenografts.

SUMMARY

Thrombosis is a barrier to long-term survival and function of porcine xenografts, which may eventually be overcome by various combinations of genetic and physical manipulation.

The perspectives for porcine-to-human xenografts

The shortage of donated human organs for transplantation continues to be a life threatening problem for patients suffering from complete organ failure. Although this gap is increasing due to the demographic changes in aging Western populations, it is generally accepted that international trading in human organ is not an ethical solution. Alternatives to the use of human organs for transplantation must be developed and these alternatives include stem cell therapy, artificial organs and organs from other species, i.e. xenografts. For practical reasons but most importantly because of its physiological similarity with humans, the pig is generally accepted as the species of choice for xenotransplantation. Nevertheless, before porcine organs can be used in human xenotransplantation, it is necessary to make a series of precise genetic modifications to the porcine genome, including the addition of genes for factors which suppress the rejection of transplanted porcine tissues and the inactivation or removal of undesirable genes which can only be accomplished at this time by targeted recombination and somatic nuclear transfer. This review will give an insight into the advances in transgenic manipulation and cloning in pigs--in the context of porcine-to-human xenotransplantation.

Xenotransplantation: current status and a perspective on the future

Xenotransplantation using pigs as the transplant source has the potential to resolve the severe shortage of human organ donors. Although the development of relatively non-toxic immunosuppressive or tolerance-inducing regimens will be required to justify clinical trials using pig organs, recent advances in our understanding of the biology of xenograft rejection and zoonotic infections, and the generation of alpha1,3-galactosyltransferase-deficient pigs have moved this approach closer to clinical application. This Review highlights the major obstacles impeding the translation of xenotransplantation into clinical therapies and the potential solutions, providing a perspective on the future of clinical xenotransplantation.

Increased immunosuppression, not anticoagulation, extends cardiac xenograft survival

BACKGROUND

Cardiac xenograft function is lost due to delayed xenograft rejection (DXR) characterized by microvascular thrombosis and myocardial necrosis. The cause of DXR is unknown but may result from thrombosis induced by antibody-mediated activation of endothelial cells and/or by incompatibilities in thromboregulatory interactions.

METHODS

To examine these issues, a series (Groups 1-6) of previous transgenic CD46 pig-to-baboon heterotopic cardiac transplants were reanalyzed for baseline immunosuppressive levels, graft survival and infectious complications with and without systemic anticoagulation. Groups 1-4 received low dose tacrolimus and sirolimus maintenance therapy, with splenectomy, anti-CD20 and daily alpha-Gal polymer. Group 1 recipients received no anticoagulation. Groups 2-4 were anticoagulated with aspirin and Plavix, Lovenox, or Coumadin, respectively. Group 5 was treated with Lovenox and high dose tacrolimus and sirolimus maintenance therapy. Group 6 recipients received no postoperative anticoagulation but the same immunosuppression as group 5.

RESULTS

Median survival (15-22 days) within groups 1-4 was not significantly different. At rejection all tissues exhibited microvascular thrombosis, coagulative necrosis and similar levels of platelet and fibrin deposition. Groups 5 and 6 median survival (76 days) was significantly increased compared to groups 1-4. There was no significant difference in median survival between Lovenox treated recipients (68 days) and anticoagulant free recipients (96 days). Rejected tissues showed vascular antibody deposition, microvascular thrombosis, and myocyte necrosis.

CONCLUSION

Significant prolongation in xenograft survival is achieved by improved immunosuppression. These results suggest that ongoing immune responses remain the major stimulus for DXR.

Coagulation cascade activation triggers early failure of pig hearts expressing human complement regulatory genes

BACKGROUND

Hyperacute rejection (HAR) and early graft failure (EGF) have been described in a minority of pig-to-baboon heart transplants using organs transgenic for human complement regulatory proteins (hCRP). Here we investigate the role of coagulation cascade activation in the pathogenesis of HAR and EGF in a consecutive series where a high incidence of these outcomes was observed.

METHODS

Twenty-eight naïve wild-caught Papio anubis baboons received heterotopic heart transplants from pigs transgenic for hDAF (n = 23) or hMCP (n = 5). Immunosuppression consisted of cyclosporine A, cyclophosphamide and MMF (n = 18) or anti-CD154 mAb (IDEC-131) and ATG (n = 10). Eleven received anti-Gal carbohydrates (GAS914, n = 8, or NEX1285, n = 3), of which four also underwent extracorporeal immunoadsorption (EIA), and 12 also received pharmacologic complement inhibitors (C1 INH, n = 9, or APT070, n = 3).

RESULTS

Excluding one technical failure, 14 of 27 transplants (11 hDAF, 3 hMCP) exhibited either HAR (n = 10) or EGF (n = 4). Surprisingly, neither complement inhibition (with C1 INH or APT070) nor anti-Gal antibody depletion with GAS914, NEX1285, or additional EIA consistently prevented HAR or EGF despite low or undetectable complement deposition. Strikingly, most grafts with HAR/EGF exhibited prominent fibrinogen and platelet deposition associated with systemic coagulation cascade activation, consistent with non-physiologic intravascular coagulation, in many instances despite little evidence for antibody-mediated complement activation.

CONCLUSION

We conclude that dysregulated coagulation correlates closely with and probably causes primary failure of pig hearts transgenic for hCRP. These data support efforts to define effective strategies to prevent dysregulated coagulation in pig organ xenografts.

Coagulation and the xenograft endothelium

Acute humoral rejection remains the major barrier to long-term pig-to-primate xenograft survival, and microvascular thrombosis is a critical element of the rejection process. It appears that persistent endothelial cell activation and injury, by even low levels of anti-graft antibodies, eventually overwhelm the cellular anticoagulant defences and promote the development of thrombotic microangiopathy. Porcine endothelium may be particularly vulnerable because of cross-species molecular incompatibilities affecting the function of thrombomodulin and possibly TFPI. Recent data from small animal models suggest that transgenic overexpression of anti-thrombotic molecules on xenograft endothelium is capable of inhibiting intravascular thrombosis and preventing acute humoral rejection. In conjunction with existing genetic modifications (e.g. Gal KO, hDAF), this is a promising strategy to move xenotransplantation to the clinic.

Intravascular Thrombosis in Discordant Xenotransplantation

A series of immunological and physiological barriers must be overcome for the successful clinical application of xenotransplantation. The acute phases of xenograft rejection have been prevented or at least attenuated by a variety of interventions including treatment of the recipient and genetic modification of the donor. However, recent data suggest that xenografts have a heightened susceptibility to intravascular thrombosis, a process that is emerging as a major contributor to xenograft loss. Current data strongly suggest that thrombosis is primarily a direct consequence of the rejection process, but it may also be facilitated by the failure of porcine regulators of coagulation to efficiently regulate the primate coagulation cascade. Systemic anticoagulant therapy has met with limited success and poses significant risks. Genetic strategies to express antithrombotic agents on xenograft endothelium appear to be more promising and achievable, with candidate molecules including human and leech anticoagulants and the antiplatelet enzyme CD39. Deletion of porcine procoagulants may also prove to be a useful approach.

The transgenic expression of human CD39 on murine islets inhibits clotting of human blood

Platelet activation is believed to play an important role in the triggering of thrombosis of human blood by pig islets. We used a transgenic mouse model to investigate whether overexpression of CD39 (ecto nucleoside triphosphate diphosphohydrolase 1 [ENTPD1], EC 3.6.1.5), an ectonucleotidase that degrades the platelet agonists ATP, could interfere with this process. Islets isolated from CD39 transgenic mice showed 2.4-fold higher NTPDase activity than wild-type controls. When incubated with human blood, these islets significantly delayed clotting time compared to wild type islets (7.9 +/- 0.89 min versus 4.3 +/- 0.77 min, P = 0.007). Importantly, expression of human CD39 in the islets of transgenic mice had no deleterious effect on glucose metabolism. These results suggest that transgenic expression of human CD39 does not interfere with islet function and may be a useful strategy to inhibit thrombosis induced by intraportal administration of islet xenografts.

Single-shot antithrombin in human pancreas-kidney transplantation: reduction of reperfusion pancreatitis and prevention of graft thrombosis*

Reperfusion pancreatitis and graft thrombosis often induce early graft loss in simultaneous pancreas-kidney (SPK) transplantation. Antithrombin (AT) is a coagulatory inhibitor with pleiotropic activities that reduces experimental ischemia/reperfusion injury. This study retrospectively analyses prophylactic high-dose AT application in patients with first SPK. In an university transplantation center, 53 consecutive patients with SPK were studied without randomization. In one group, 3000 IU of AT was given intravenously before pancreatic reperfusion (AT, n = 24). Patients receiving standard therapy including postoperative AT supplementation (controls, n = 29) served as controls. Daily blood sampling was performed as a part of the clinical routine during four postoperative days. There were no differences in demographic and laboratory parameters [donor/recipient age, ischemia time, perfusion solution, body weight, mismatches] between both groups. Baseline creatinine values were lower in the control group versus AT group (P < 0.05). Coagulatory parameters and bleeding incidence were not influenced by AT, while incidence of graft thrombosis was reduced (control: 7/29; AT: 4/24; relative reduction of risk: -33%; P < 0.05). Single-shot AT application during SPK modulated serum lipase activity on postoperative days 2 and 3, and minimized risk for graft thromboses without increasing perioperative bleeding. This new concept should deserve testing in a prospective clinical trial.

Current status of animal-to-human transplantation

The transplantation of animal organs into humans as a way of treating organ failure has been pursued for 100 years. Clinical xenotransplantation, as such, has always failed because the transplanted organ is rejected by the recipient. Recent advances in transplant immunology have revealed some mechanisms underlying the rejection of xenografts, and these discoveries have sparked efforts to use genetic engineering of animals and therapeutics directed at the recipient to overcome this problem. This paper reviews the current understanding of the mechanisms of xenograft rejection and efforts to overcome rejection and other hurdles.

alpha1,3-Galactosyltransferase gene-knockout pig heart transplantation in baboons with survival approaching 6 months

BACKGROUND

The recent generation of alpha1,3-galactosyltransferase gene-knockout (GalT-KO) pigs has allowed investigation of the survival of GalT-KO pig organs in nonhuman primates.

METHODS

Heterotopic heart transplantation from GalT-KO pigs was carried out in baboons (n=8) using a human antihuman CD154 monoclonal antibody-based immunosuppressive regimen.

RESULTS

In six of the eight cases, graft survival extended to between approximately 2 and 6 months. All grafts developed thrombotic microangiopathy (TM). In particular, the clinical course of one baboon in which the graft functioned for 179 days is summarized. This baboon received aspirin (40 mg on alternate days) from day 4 in addition to heparin, which may have been a factor in the delay of onset and progression of TM and in prolonged graft survival. Maintenance therapy with anti-CD154 mAb, mycophenolate mofetil, and methylprednisolone was associated with persistently low numbers of CD3CD4 and CD3CD8 cells. Despite persisting depletion of these cells, no infectious complications occurred.

CONCLUSIONS

It remains to be established whether TM is related to a very low level of natural preformed or T-cell-induced antibody deposition on the graft, inducing endothelial activation and injury, or to molecular incompatibilities in the coagulation mechanisms between pig and baboon, or to both. However, function of a pig organ in a baboon for a period approaching six months, which has not been reported previously, lends encouragement that the barriers to xenotransplantation will eventually be overcome.

Endothelial cell protection in xenotransplantation: looking after a key player in rejection

The endothelium, as an organ at the interface between the intra- and extravascular space, actively participates in maintaining an anti-inflammatory and anti-coagulant environment under physiological conditions. Severe humoral as well as cellular rejection responses, which accompany cross-species transplantation of vascularized organs as well as ischemia/reperfusion injury, primarily target the endothelium and disrupt this delicate balance. Activation of pro-inflammatory and pro-coagulant pathways often lead to irreversible injury not only of the endothelial layer but also of the entire graft, with ensuing rejection. This review focuses on strategies targeted at protecting the endothelium from such damaging effects, ranging from genetic manipulation of the donor organ to soluble, as well as membrane-targeted, protective strategies.

Effects of Long-term Administration of High-dose Recombinant Human Antithrombin in Immunosuppressed Primate Recipients of Porcine Xenografts

BACKGROUND

Fibrin deposition is central to the acute humoral rejection process occurring in the presence of consumptive coagulopathy when pig organs are transplanted into primates.

METHODS

To assess whether strategies aimed at preventing fibrin formation may extend xenograft survival, we administered high daily doses of recombinant human antithrombin (rhAT) (500 U/kg twice daily) to obtain both anticoagulant and anti-inflammatory effects in immunosuppressed primate recipients of porcine kidneys.

RESULTS

Some degree of consumptive coagulopathy developed in both rhAT-treated (n=3) and untreated (n=3) primates. No major differences in the coagulation parameters analyzed were observed between the 2 groups. Similarly, no difference in survival was seen between rhAT-treated (20.6+/-4 days; range: 15-23 days) and untreated animals (17.3+/-11.6 days; range: 7-30 days), although the rhAT-treated primates had a higher bleeding tendency. Despite the high daily dose of rhAT, considerable fibrin deposition was observed in the graft as early as 2 weeks after transplantation.

CONCLUSIONS

These results suggest that a high daily dose of rhAT fails to influence survival or prevent fibrin formation and deposition in the graft in our pig-to-primate model. However, the potential role of rhAT administered in combination with heparins or other clotting inhibitor concentrates in this model remains to be determined.

Prevention of hyperacute rejection in a model of orthotopic liver xenotransplantation from pig to baboon using polytransgenic pig livers (CD55, CD59, and H-transferase)

INTRODUCTION

The search for alternative sources for transplant organs leads us to the search for animals as an inexhaustible source of organs. The objective of this study was to analyze whether livers from polytransgenic pigs expressing the human complement regulatory proteins CD55 (hDAF), CD59, and alfa alpha1,2-fucosyltransferase (H-transferase), protected against hyperacute rejection after orthotopic liver xenotransplantation to a baboon and also to study pig liver function in a nonhuman primate.

MATERIALS AND METHODS

Nine liver transplants from pig to baboon were divided into two groups: a control group (n = 4) of genetically unmodified pigs and an experimental group (n = 5) of pigs transgenic for CD55, CD59, and H-transferase as donors. All the donating piglets obtained through hysterectomy were maintained in specific pathogen-free conditions. The selection of transgenic pig donors followed demonstration of transgene expression using monoclonal antibodies (antiCD55, antiCD59) and immunohistological studies on liver biopsies.

RESULTS

All animals in the control group developed hyperacute rejection with survival rates less than 16 hours without function of transplanted livers. In the experimental group none of the animals suffered hyperacute rejection. Survival in this group was between 13 and 24 hours. The livers were functional, producing bile and maintaining above 35% prothrombin activity. Only in one case was there primary dysfunction of the xenograft.

CONCLUSION

Polytransgenic livers for complement regulatory proteins prevent hyperacute rejection when xenotransplanted into a baboon.

Foundation Review: Transgenic animals and their impact on the drug discovery industry

The ability to direct genetic changes at the molecular level has resulted in a revolution in biology. Nowhere has this been more apparent than in the production of transgenic animals. Transgenic technology lies at the junction of several enabling techniques in such diverse fields as embryology, cell biology and molecular genetics. A host of techniques have been used to effect change in gene expression and develop new pharmaceutical and nutraceutical compounds cost-effectively. Scientific advances gained by transgenic capabilities enable further understanding of basic biological pathways and yield insights into how changes in fundamental processes can perturb programmed development or culminate in disease pathogenesis.

Warfarin or low-molecular-weight heparin therapy does not prolong pig-to-primate cardiac xenograft function

Microvascular thrombosis is a prominent feature in cardiac delayed xenograft rejection (DXR). We investigated the impact of warfarin or low-molecular-weight heparin (LMWH) anti-coagulation on xenograft function using a heterotopic pig-to-primate model. Donor hearts were from CD46 transgenic pigs and baboon immunosuppression included tacrolimus, sirolimus, anti-CD20 and TPC, an alpha-galactosyl-polyethylene glycol conjugate. Three groups of animals were studied. Group 1 (n = 9) was treated with warfarin, Group 2 (n = 13) with LMWH and Group 3, received no anti-coagulant drugs. The median duration of xenograft function was 20 days (range 3-62 days), 18 days (range 5-109 days) and 15 days (range 4-53 days) in Groups 1 to 3 respectively. Anti-coagulation achieved the targeted international normalized prothrombin ratio (INR) and anti-factor Xa levels consistent with effective in vivo therapy yet, no significant impact on median xenograft function was observed. At rejection, a similar histology of thrombosis and ischemia was apparent in each group and the levels of fibrin deposition and platelet thrombi in rejected tissue was the same. Anti-coagulation with warfarin or LMWH did not have a significant impact on the onset of DXR and microvascular thrombosis. However, a role for specific anti-coagulant strategies to achieve long-term xenograft function cannot be excluded.

Complete inhibition of acute humoral rejection using regulated expression of membrane-tethered anticoagulants on xenograft endothelium

Xenotransplantation promises an unlimited supply of organs for clinical transplantation. However, an aggressive humoral immune response continues to limit the survival of pig organs after transplantation into primates. Because intravascular thrombosis and systemic coagulopathy are prominent features of acute humoral xenograft rejection, we hypothesized that expression of anticoagulants on xenogeneic vascular endothelium might inhibit the process. Hearts from novel transgenic mice, expressing membrane-tethered fusion proteins based on human tissue factor pathway inhibitor and hirudin, respectively, were transplanted into rats. In contrast to control non-transgenic mouse hearts, which were all rejected within 3 days, 100% of the organs from both strains of transgenic mice were completely resistant to humoral rejection and survived for more than 100 days when T-cell-mediated rejection was inhibited by administration of ciclosporin A. These results demonstrate the critical role of coagulation in the pathophysiology of acute humoral rejection and the potential for inhibiting rejection by targeting the expression of anticoagulants to graft endothelial cells. This genetic strategy could be applied in a clinically relevant species such as the pig.

Effective antiplatelet therapy does not prolong transgenic pig to baboon cardiac xenograft survival

BACKGROUND

Microvascular thrombosis is a prominent characteristic of delayed xenograft rejection, therefore the effects of antiplatelet therapy with aspirin and clopidogrel on long-term cardiac xenograft function was investigated in a heterotopic pig-to-baboon cardiac transplant model.

METHODS

Donor hearts from human CD46 transgenic pigs were transplanted heterotopically to baboons. The recipients received immunosuppression that included tacrolimus, sirolimus, corticosteroids, anti-CD20 monoclonal antibody and TPC, an alpha-galactosyl-polyethylene glycol conjugate. In group 1 (n = 9) in addition to immunosuppression, the recipients received combination therapy consisting of aspirin (80 mg/day) and clopidogrel (75 mg/day) beginning 2 days after transplant and continuing until cessation of graft function. Antiaggregatory efficacy was evaluated by platelet aggregation assay. In group 2 (n = 9) antiplatelet drugs were not given.

RESULTS

Functional assays confirmed inhibition of platelet aggregation in group 1 suggesting sufficient systemic effects of the treatment. However, anticoagulant therapy did not result in significant prolongation of xenograft function (group 1: median survival 22 days, range 15 to 30 days; group 2: median survival 15 days, range 4 to 53 days). Histologic analysis at rejection revealed no difference in the level of platelet containing thrombi between the groups.

CONCLUSIONS

Inhibition of platelet aggregation by a combination of aspirin and clopidogrel did not have a significant impact on the length of xenograft survival or on the development of microvascular thrombosis in this pig-to-primate model.

Thromboregulatory manifestations in human CD39 transgenic mice and the implications for thrombotic disease and transplantation

Extracellular nucleotides play an important role in thrombosis and inflammation, triggering a range of effects such as platelet activation and recruitment, endothelial cell activation, and vasoconstriction. CD39, the major vascular nucleoside triphosphate diphosphohydrolase (NTPDase), converts ATP and ADP to AMP, which is further degraded to the antithrombotic and anti-inflammatory mediator adenosine. Deletion of CD39 renders mice exquisitely sensitive to vascular injury, and CD39-null cardiac xenografts show reduced survival. Conversely, upregulation of CD39 by somatic gene transfer or administration of soluble NTPDases has major benefits in models of transplantation and inflammation. In this study we examined the consequences of transgenic expression of human CD39 (hCD39) in mice. Importantly, these mice displayed no overt spontaneous bleeding tendency under normal circumstances. The hCD39 transgenic mice did, however, exhibit impaired platelet aggregation, prolonged bleeding times, and resistance to systemic thromboembolism. Donor hearts transgenic for hCD39 were substantially protected from thrombosis and survived longer in a mouse cardiac transplant model of vascular rejection. These thromboregulatory manifestations in hCD39 transgenic mice suggest important therapeutic potential in clinical vascular disease and in the control of serious thrombotic events that compromise the survival of porcine xenografts in primates.

Endothelial Induction of fgl2 Contributes to Thrombosis during Acute Vascular Xenograft Rejection

Thrombosis is a prominent feature of acute vascular rejection (AVR), the current barrier to survival of pig-to-primate xenografts. Fibrinogen-like protein 2 (fgl2/fibroleukin) is an inducible prothrombinase that plays an important role in the pathogenesis of fibrin deposition during viral hepatitis and cytokine-induced fetal loss. We hypothesized that induction of fgl2 on the vascular endothelium of xenografts contributes to thrombosis associated with AVR. We first examined fgl2 as a source of procoagulant activity in the pig-to-primate combination. The porcine fgl2 (pfgl2) was cloned and its chromosomal locus was identified. Recombinant pfgl2 protein expressed in vitro was detected on the cell surface and generated thrombin from human prothrombin. Studies of pig-to-baboon kidney xenografts undergoing AVR in vivo revealed induction of pfgl2 expression on graft vascular endothelial cells (ECs). Cultured porcine ECs activated by human TNF-alpha in vitro demonstrated induction of pfgl2 expression and enhanced activation of human prothrombin. The availability of gene-targeted fgl2-deficient mice allowed the contribution of fgl2 to the pathogenesis of AVR to be directly examined in vivo. Hearts heterotopically transplanted from fgl2(+/+) and fgl2(+/-) mice into Lewis rats developed AVR with intravascular thrombosis associated with induction of fgl2 in graft vascular ECs. In contrast, xenografts from fgl2(-/-) mice were devoid of thrombosis. These observations collectively suggest that induction of fgl2 on the vascular endothelium plays a role in the pathogenesis of AVR-associated thrombosis. Manipulation of fgl2, in combination with other interventions, may yield novel strategies by which to overcome AVR and extend xenograft survival.

Suppression of natural and elicited antibodies in pig-to-baboon heart transplantation using a human anti-human CD154 mAb-based regimen

Natural and elicited antipig antibodies (Abs) lead to acute humoral xenograft rejection (AHXR). Ten baboons underwent heterotopic heart transplantation (Tx) from human decay-accelerating factor (hDAF) pigs. Depletion of anti-Galalpha1, 3Gal (Gal) Abs was achieved by the infusion of a Gal glycoconjugate from day-1. Immunosuppression included induction of antithymocyte globulin, thymic irradiation, and cobra venom factor, and maintenance with a human antihuman CD154 mAb, mycophenolate mofetil, and methylprednisolone; heparin and prophylactic ganciclovir were also administered. Pig heart survival ranged from 4 to 139 (mean 37, median 27) days, with three functioning for >50 days. Graft failure (n = 8) was from classical AHXR [4], thrombotic microangiopathy [3], or intragraft thrombosis [1], with death (n = 2) from pneumonia [1], or possible drug toxicity (with features of thrombotic microangiopathy) [1]. Anti-Gal Abs (in microg/mL) were depleted by Gal glycoconjugate before graft implantation from means of 41.3 to 6.3 (IgM) and 12.4-4.6 (IgG), respectively, and at graft excision were 6.3 and 1.7 microg/mL, respectively. No elicited Abs developed, and no cellular infiltration was seen. The treatment regimen was effective in maintaining low anti-Gal Ab levels and in delaying or preventing AHXR. The combination of costimulatory blockade and heparin with Tx of a Gal-negative pig organ may prolong graft survival further.

Clinical xenotransplantion--how close are we?

CONTEXT

Xenotransplantation with pig organs offers a medium-term solution to the shortage of organs available for clinical transplantation. The immunological barriers to xenotransplantation have been, and remain, formidable. In the early 1990s, the identification of Galalpha1,3Gal (Gal) as the main target for human xenoreactive (anti-pig) antibodies and the development of pigs transgenic for a human complement regulatory protein, decay-accelerating factor (hDAF), were major advances. The presence of hDAF on the vascular endothelium of pig organs provided some protection against complement-mediated hyperacute rejection. This protection, however, was short-lived, and, until recently, the longest median time for organ survival that had been achieved (with combinations of biological and pharmacological immunosuppressants) in a series of pig-to-primate organ transplants was under a month.

STARTING POINT

Christopher McGregor and colleagues recently reported to the International Society of Heart and Lung Transplantation (J Heart Lung Transplant 2003; 22: S89) that, by combining the use of organs which express hDAF with the administration of a soluble Gal glycoconjugate and other immunosuppressive agents, the survival of pig hearts in baboons can be extended to a median of 76 days. McGregor's work suggests that immunological barriers to xenotransplantation are not insurmountable. WHERE NEXT? The recent generation of pigs that do not express Gal epitopes (alpha1,3-galactosyltransferase gene-knockout pigs) might remove the need both for the expression of hDAF and the administration of a soluble Gal glycoconjugate. The absence of a natural antibody response will allow investigation of the cellular immune response and of any molecular incompatibilities between pig and primate that may be detrimental to graft survival. Furthermore, the absence of a humoral response may open the way for the induction of immunological tolerance (or unresponsiveness in the absence of exogenous immunosuppression) to a transplanted pig organ.

Gal mismatch alone causes skin graft rejection in mice

BACKGROUND

Elimination of galactose-alpha1,3-galactose (Gal), the major xenoantigen between pig and human, may extend pig-to-human xenograft survival beyond the current barrier of acute vascular rejection. However, it has been suggested that Gal is an essential molecule in the pig and that the generation of a Gal-deleted (Gal KO) pig will not be possible. Should this be the case, understanding the Gal-mediated immune response will be crucial in developing strategies to overcome pig xenograft rejection in humans. There are no existing models of xenograft rejection in which the sole difference between donor and recipient is Gal. We describe a model of exclusively Gal-mismatched skin graft rejection.

METHODS

The survival of Gal skin grafts on Gal KO mice with the same genetic background was analyzed. To examine innate anti-Gal immunity, Gal KO recipients that were also deficient in T and B cells (RAG-1 KO) were used. To study the role of cognate immunity, recipients were sensitized with a primary Gal allograft before receiving a second Gal graft that was otherwise isogeneic. To test the role of anti-Gal antibodies in this model, recipients were passively immunized with a non-complement-fixing anti-Gal monoclonal antibody.

RESULTS

Gal KO mice chronically reject Gal skin grafts by 100 days at a rate of 48% (n=25) on a BALB/c background and 25% (n=8) on a C57BL/6 background. The grafts had an infiltrate that consisted predominantly of CD4 T cells and macrophages, whereas recipients deficient in T and B cells were incapable of rejection and survived for more than 120 days (n=5). Sensitization with a primary Gal allograft increased the incidence and the tempo of rejection of a second Gal-only mismatched skin graft with 99% rejection that ranged from 11 to 45 days (n=26). Passive transfer of mouse IgG anti-Gal monoclonal-antibody-induced rejection in Gal KO and RAG-1/Gal double-KO recipients at a rate of 92% (n=13).

CONCLUSIONS

We have established a model to study rejection based solely on a Gal mismatch. Our results indicate that non-complement-fixing anti-Gal antibody can cause rejection in the acute vascular rejection time frame and that T-cell-mediated chronic rejection will be a further barrier to overcome if Gal cannot be deleted from the pig.