The evaluation of thrombomodulin activity in porcine to human xenotransplantation

Expression of human thrombomodulin by GalTKO.hCD46 pigs modulates coagulation cascade activation by endothelial cells and during ex vivo lung perfusion with human blood

Thrombomodulin is important for the production of activated protein C (APC), a molecule with significant regulatory roles in coagulation and inflammation. To address known molecular incompatibilities between pig thrombomodulin and human thrombin that affect the conversion of protein C into APC, GalTKO.hCD46 pigs have been genetically modified to express human thrombomodulin (hTBM). The aim of this study was to evaluate the impact of transgenic hTBM expression on the coagulation dysregulation that is observed in association with lung xenograft injury in an established lung perfusion model, with and without additional blockade of nonphysiologic interactions between pig vWF and human GPIb axis. Expression of hTBM was variable between pigs at the transcriptional and protein level. hTBM increased the activation of human protein C and inhibited thrombosis in an in vitro flow perfusion assay, confirming that the expressed protein was functional. Decreased platelet activation was observed during ex vivo perfusion of GalTKO.hCD46 lungs expressing hTBM and, in conjunction with transgenic hTBM, blockade of the platelet GPIb receptor further inhibited platelets and increased survival time. Altogether, our data indicate that expression of transgenic hTBM partially addresses coagulation pathway dysregulation associated with pig lung xenograft injury and, in combination with vWF-GP1b-directed strategies, is a promising approach to improve the outcomes of lung xenotransplantation.

Future of Lung Transplantation: Xenotransplantation and Bioengineering Lungs

Xenotransplantation promises to alleviate the issue of donor organ shortages and to decrease waiting times for transplantation. Recent advances in genetic engineering have allowed for the creation of pigs with up to 16 genetic modifications. Several combinations of genetic modifications have been associated with extended graft survival and life-supporting function in experimental heart and kidney xenotransplants. Lung xenotransplantation carries specific challenges related to the large surface area of the lung vascular bed, its innate immune system's intrinsic hyperreactivity to perceived 'danger', and its anatomic vulnerability to airway flooding after even localized loss of alveolocapillary barrier function. This article discusses the current status of lung xenotransplantation, and challenges related to immunology, physiology, anatomy, and infection. Tissue engineering as a feasible alternative to develop a viable lung replacement solution is discussed.

Accommodation in allogeneic and xenogeneic organ transplantation: Prevalence, impact, and implications for monitoring and for therapeutics

Accommodation refers to acquired resistance of organs or tissues to immune or inflammatory reactions that might otherwise cause severe injury or rejection. As first observed in ABO-incompatible kidney transplants and heterotopic cardiac xenografts, accommodation was identified when organ transplants continued to function despite the presence of anti-graft antibodies and/or other reactants in the blood of recipients. Recent evidence suggests many and perhaps most organ transplants have accommodation, as most recipients mount B cell responses specific for the graft. Wide interest in the impact of graft-specific antibodies on the outcomes of transplants prompts questions about which mechanisms confer protection against such antibodies, how accommodation might be detected and whether and how rejection could be superimposed on accommodation. Xenotransplantation offers a unique opportunity to address these questions because immune responses to xenografts are easily detected and the pathogenic impact of immune responses is so severe. Xenotransplantation also provides a compelling need to apply these and other insights to decrease the intensity and toxicity of immunosuppression that otherwise could limit clinical application.

"You cannot stay in the laboratory forever"*: Taking pig kidney xenotransplantation from the laboratory to the clinic

Progress in life-supporting kidney transplantation in the genetically-engineered pig-to-nonhuman primate model has been encouraging, with pig kidneys sometimes supporting life for > 1 year. What steps need to be taken by (i) the laboratory team, and (ii) the clinical team to prepare for the first clinical trial? The major topics include (i) what currently-available genetic modifications are optimal to reduce the possibility of graft rejection, (ii) what immunosuppressive therapeutic regimen is optimal, and (iii) what steps need to be taken to minimize the risk of transfer of an infectious microorganism with the graft. We suggest that patients who are unlikely to live long enough to receive a kidney from a deceased human donor would benefit from the opportunity of a period of dialysis-free support by a pig kidney, and the experience gained would enable xenotransplantation to progress much more rapidly than if we remain in the laboratory.

Xenotransplantation: Progress Along Paths Uncertain from Models to Application

For more than a century, transplantation of tissues and organs from animals into man, xenotransplantation, has been viewed as a potential way to treat disease. Ironically, interest in xenotransplantation was fueled especially by successful application of allotransplantation, that is, transplantation of human tissue and organs, as a treatment for a variety of diseases, especially organ failure because scarcity of human tissues limited allotransplantation to a fraction of those who could benefit. In principle, use of animals such as pigs as a source of transplants would allow transplantation to exert a vastly greater impact than allotransplantation on medicine and public health. However, biological barriers to xenotransplantation, including immunity of the recipient, incompatibility of biological systems, and transmission of novel infectious agents, are believed to exceed the barriers to allotransplantation and presently to hinder clinical applications. One way potentially to address the barriers to xenotransplantation is by genetic engineering animal sources. The last 2 decades have brought progressive advances in approaches that can be applied to genetic modification of large animals. Application of these approaches to genetic engineering of pigs has contributed to dramatic improvement in the outcome of experimental xenografts in nonhuman primates and have encouraged the development of a new type of xenograft, a reverse xenograft, in which human stem cells are introduced into pigs under conditions that support differentiation and expansion into functional tissues and potentially organs. These advances make it appropriate to consider the potential limitation of genetic engineering and of current models for advancing the clinical applications of xenotransplantation and reverse xenotransplantation.

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.

Pig-to-baboon heterotopic heart transplantation--exploratory preliminary experience with pigs transgenic for human thrombomodulin and comparison of three costimulation blockade-based regimens

BACKGROUND

Three costimulation blockade-based regimens have been explored after transplantation of hearts from pigs of varying genetic backgrounds to determine whether CTLA4-Ig (abatacept) or anti-CD40mAb+CTLA4-Ig (belatacept) can successfully replace anti-CD154mAb.

METHODS

All pigs were on an α1,3-galactosyltransferase gene-knockout/CD46 transgenic (GTKO.CD46) background. Hearts transplanted into Group A baboons (n=4) expressed additional CD55, and those into Group B (n=3) expressed human thrombomodulin (TBM). Immunosuppression included anti-thymocyte globulin with anti-CD154mAb (Regimen 1: n=2) or abatacept (Regimen 2: n=2) or anti-CD40mAb+belatacept (Regimen 3: n=2). Regimens 1 and 2 included induction anti-CD20mAb and continuous heparin. One further baboon in Group B (B16311) received a modified Regimen 1. Baboons were followed by clinical/laboratory monitoring of immune/coagulation parameters. At biopsy, graft failure, or euthanasia, the graft was examined by microscopy.

RESULTS

Group A baboons survived 15 to 33 days, whereas Group B survived 52, 99, and 130 days, respectively. Thrombocytopenia and reduction in fibrinogen occurred within 21 days in Group A, suggesting thrombotic microangiopathy (TM), confirmed by histopathology. In Group B, with follow-up for >4 m, areas of myofiber degeneration and scarring were seen in two hearts at necropsy. A T-cell response was documented only in baboons receiving Regimen 2.

CONCLUSIONS

The combination of anti-CD40mAb+belatacept proved effective in preventing a T-cell response. The expression of TBM prevented thrombocytopenia and may possibly delay the development of TM and/or consumptive coagulopathy.

Lung xenotransplantation: recent progress and current status

Xenotransplantation has undergone important progress in controlling initial hyperacute rejection in many preclinical models, with some cell, tissue, and organ xenografts advancing toward clinical trials. However, acute injury, driven primarily by innate immune and inflammatory responses, continues to limit results in lung xenograft models. The purpose of this article is to review the current status of lung xenotransplantation--including the seemingly unique challenges posed by this organ-and summarize proven and emerging means of overcoming acute lung xenograft injury.

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.

Histopathologic insights into the mechanism of anti-non-Gal antibody-mediated pig cardiac xenograft rejection

The histopathology of cardiac xenograft rejection has evolved over the last 20 yr with the development of new modalities for limiting antibody-mediated injury, advancing regimens for immune suppression, and an ever-widening variety of new donor genetics. These new technologies have helped us progress from what was once an overwhelming anti-Gal-mediated hyperacute rejection to a more protracted anti-Gal-mediated vascular rejection to what is now a more complex manifestation of non-Gal humoral rejection and coagulation dysregulation. This review summarizes the changing histopathology of Gal- and non-Gal-mediated cardiac xenograft rejection and discusses the contributions of immune-mediated injury, species-specific immune-independent factors, transplant and therapeutic procedures, and donor genetics to the overall mechanism(s) of cardiac xenograft rejection.

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.

Clinical lung xenotransplantation--what donor genetic modifications may be necessary?

Barriers to successful lung xenotransplantation appear to be even greater than for other organs. This difficulty may be related to several macro anatomic factors, such as the uniquely fragile lung parenchyma and associated blood supply that results in heightened vulnerability of graft function to segmental or lobar airway flooding caused by loss of vascular integrity (also applicable to allotransplants). There are also micro-anatomic considerations, such as the presence of large numbers of resident inflammatory cells, such as pulmonary intravascular macrophages and natural killer (NK) T cells, and the high levels of von Willebrand factor (vWF) associated with the microvasculature. We have considered what developments would be necessary to allow successful clinical lung xenotransplantation. We suggest this will only be achieved by multiple genetic modifications of the organ-source pig, in particular to render the vasculature resistant to thrombosis. The major problems that require to be overcome are multiple and include (i) the innate immune response (antibody, complement, donor pulmonary and recipient macrophages, monocytes, neutrophils, and NK cells), (ii) the adaptive immune response (T and B cells), (iii) coagulation dysregulation, and (iv) an inflammatory response (e.g., TNF-α, IL-6, HMGB1, C-reactive protein). We propose that the genetic manipulation required to provide normal thromboregulation alone may include the introduction of genes for human thrombomodulin/endothelial protein C-receptor, and/or tissue factor pathway inhibitor, and/or CD39/CD73; the problem of pig vWF may also need to be addressed. It would appear that exploration of every available therapeutic path will be required if lung xenotransplantation is to be successful. To initiate a clinical trial of lung xenotransplantation, even as a bridge to allotransplantation (with a realistic possibility of survival long enough for a human lung allograft to be obtained), significant advances and much experimental work will be required. Nevertheless, with the steadily increasing developments in techniques of genetic engineering of pigs, we are optimistic that the goal of successful clinical lung xenotransplantation can be achieved within the foreseeable future. The optimistic view would be that if experimental pig lung xenotransplantation could be successfully managed, it is likely that clinical application of this and all other forms of xenotransplantation would become more feasible.

Controlling coagulation dysregulation in xenotransplantation

PURPOSE OF REVIEW

Deletion of the α1,3-galactosyltransferase (GalT) gene in pigs has removed a major xenoantigen but has not eliminated the problem of dysregulated coagulation and vascular injury. Rejecting GalT knockout organ xenografts almost invariably show evidence of thrombosis and platelet sequestration, and primate recipients frequently develop consumptive coagulopathy. This review examines recent findings that illuminate potential mechanisms of this current barrier to successful xenotransplantation.

RECENT FINDINGS

The coagulation response to xenotransplantation differs depending on the type of organ and quite likely the distinct vasculatures. Renal xenografts appear more likely to initiate consumptive coagulopathy than cardiac xenografts, possibly reflecting differential transcriptional responses. Liver xenografts induce rapid and profound thrombocytopenia resulting in recipient death within days due to bleeding; ex-vivo data suggest that liver endothelial cells and hepatocytes are responsible for platelet consumption by a coagulation-independent process.It has been proposed that expression of recipient tissue factor on platelets and monocytes is an important trigger of consumptive coagulopathy. Finally, pigs transgenic for human anticoagulants and antithrombotics are slowly but surely coming on line, but have not yet been rigorously tested to date.

SUMMARY

Successful control of coagulation dysregulation in xenotransplantation may require different combinatorial pharmacological and genetic strategies for different organs.

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.

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.

Effects of pharmacological intervention on coagulopathy and organ function in xenoperfused kidneys

BACKGROUND

Following pig to primate kidney transplantation, xenogenic activation of the coagulation (XAC) system of the recipient eventually leading to organ dysfunction and disseminated intravascular coagulation (DIC) can be observed.

METHODS

Using an ex-vivo perfusion circuit based on low-dose heparin-mediated anticoagulation and exogenous complement inhibition by C1- Inhibitor (C1-Inh), we have analysed XAC following contact of human blood with porcine endothelium. Porcine kidneys (n = 23) were recovered following in situ cold perfusion with histidine-tryptophan-ketoglutarate (HTK) solution and were connected to a perfusion circuit utilizing freshly drawn pooled human AB blood.

RESULTS

Kidney survival during organ perfusion with human blood, CI-Inh, heparin but without any further pharmacological intervention was 126 +/- 78 min. XAC was observed with significantly elevated levels of D-dimer and thrombin antithrombin complexes (TAT). Pharmacological intervention with nitroprusside and prostacycline resulted in increased organ survival (220 +/- 28 min and 180 +/- 85 min respectively) but failed to inhibit XAC. In contrast, addition of activated protein C (APC) significantly reduced the increase in D-dimer and TAT and prolonged organ survival to 240 min (+/-0). On histology, no remarkable signs of XAC were observed.

CONCLUSIONS

We conclude that exogenous APC is able to reduce XAC in this ex vivo perfusion model.

Pig thrombomodulin binds human thrombin but is a poor cofactor for activation of human protein C and TAFI

Incompatibility between pig thrombomodulin (TM) and primate thrombin is thought to be an important factor in the development of microvascular thrombosis in rejecting pig-to-primate xenografts. To examine this interaction at the molecular level, we cloned pig TM and measured its ability to bind human thrombin and act as a cofactor for the activation of human protein C and TAFI. The 579-residue pig TM protein showed approximately 69% sequence identity to human TM. Within the EGF domains necessary for binding of thrombin (EGF56), protein C (EGF4) and TAFI (EGF3), all of the amino acids previously identified as critical for the function of human TM, with the exception of Glu-408 in EGF5, were conserved in pig TM. Comparison of transfected cells expressing pig or human TM demonstrated that both proteins bound human thrombin and inhibited its procoagulant activity. However, pig TM was a poor cofactor for the activation of human protein C and TAFI, with domain swapping showing that EGF5 was the most important determinant of compatibility. Thus, while pig TM may be capable of binding thrombin generated in the vicinity of xenograft endothelium, its failure to promote the activation of human protein C remains a significant problem.

Alpha1,3-galactosyltransferase gene-knockout pigs for xenotransplantation: where do we go from here?

The ability to genetically engineer pigs that no longer express the Galalpha1,3Gal (Gal) oligosaccharide has been a significant step toward the clinical applicability of xenotransplantation. Using a chronic immunosuppressive regimen based on costimulatory blockade, hearts from these pigs have survived from 2 to 6 months in baboons. Graft failure was predominantly from the development of a thrombotic microangiopathy. Potential contributing factors include the presence of preformed anti-nonGal antibodies or the development of low levels of elicited antibodies to nonGal antigens, natural killer (NK) cell or macrophage activity, and inherent coagulation dysregulation between pigs and primates. The breeding of pigs transgenic for an "anticoagulant" gene, such as human tissue factor pathway inhibitor, hirudin, or CD39, or lacking the gene for the prothrombinase, fibrinogen-like protein-2, is anticipated to inhibit the change in the endothelium to a procoagulant state that takes place in the pig organ after transplantation. The identification of the targets for anti-nonGal antibodies and/or human macrophages might allow further genetic modification of the pig, and xenogeneic NK cell recognition and activation may be inhibited by the transgenic expression of human leukocyte antigen molecules and/or by blocking the function of activating NK receptors. The ultimate goal of induction of T-cell tolerance may be possible only if these hurdles in the coagulation system and innate immunity can be overcome.

Rapid loss of intraportally transplanted islets: an overview of pathophysiology and preventive strategies

Islets isolated from multiple pancreas donors are often necessary to achieve euglycemia in type 1 diabetic patients treated by islet allotransplantation. This increases the burden on the limited pool of donor organs. After infusion into the portal vein, a substantial percentage of islets are lost in the immediate post-transplant period through an inflammatory response termed the instant blood-mediated inflammatory reaction (IBMIR). IBMIR is equally, if not more of a problem after islet xenotransplantation, e.g., using pig islets in non-human primates. Coagulation, platelet aggregation, complement activation, and neutrophil and monocyte infiltration play roles in this reaction. IBMIR is potentially triggered by islet surface molecules, such as tissue factor and collagen residues that are normally not in direct contact with the blood. Also, stress during the islet isolation process results in the expression and production of several inflammatory mediators by the islets themselves. The potential mechanisms involved in this rapid graft loss and treatment options to reduce this loss are reviewed. Preventive strategies for IBMIR can include systemic treatment of the recipient, pre-conditioning of the isolated islets, or, in the case of xenotransplantation, genetic modification of the organ-source pig. Pre-conditioning of islets in culture by exposure to anti-inflammatory agents or by genetic modification harbors fewer risks of systemic complications in the recipient. The future of clinical islet transplantation will, at least in part, depend on the success of efforts made to reduce rapid graft loss, and thus allow islet transplantation to become a more efficient therapy by the use of single donors.

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.

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.

Cardiac xenotransplantation: recent preclinical progress with 3-month median survival

OBJECTIVES

Transplantation is limited by a lack of human organ donors. Organs derived from animals, most likely the pig, represent a potential solution to this problem. For the heart, 90-day median graft survival of life-supporting pig hearts transplanted to nonhuman primates has been considered a reasonable standard for entry into the clinical arena. Overcoming the immune barrier to successful cardiac xenotransplantation is most appropriately first explored with the non-life-supporting heterotopic model.

METHODS

We performed a series of 7 heterotopic heart transplantations from CD46 transgenic pigs to baboons using a combination of therapeutic agents largely targeted at controlling the synthesis of anti-pig antibodies. Rituximab (anti-CD20) and Thymoglobulin (rabbit antithymocyte globulin [ATG]; SangStat Medical Corp, Fremont, Calif) were used as induction therapy. Baseline immunosuppression consisted of splenectomy, tacrolimus, sirolimus, steroids, and TPC (an anti-Gal antibody therapeutic). Rejection events were not treated.

RESULTS

By using Kaplan-Meier analysis, median graft survival was 96 days (range, 15-137 days; 95% confidence interval, 38-99 days). Only 2 grafts were lost as a result of rejection, as defined by cessation of graft palpation. There was no evidence of a consumptive coagulopathy, infectious complications were treatable, and no posttransplantation lymphoproliferative disorders occurred. No cellular infiltration was observed.

CONCLUSIONS

This study reports the longest median survival to date (96 days) of pig hearts transplanted heterotopically into baboons. Duplication of these results in the orthotopic life-supporting position could bring cardiac xenotransplantation to the threshold of clinical application.

Xenotransplantation: an update on recent progress and future perspectives

Currently, the number of patients awaiting transplantation is continuously increasing, and shortage of available deceased organ donors is the major limitation for organ and cell allotransplantation. Research to develop alternative sources of tissues is ongoing and xenogeneic organs or cells represent an attractive solution. This review focuses on recent progress achieved in this field, including the development of newly genetically modified animal donors and new immunosuppressive approaches. As xenotransplantation is moving closer to clinical application, future perspectives must establish guidelines to ensure that future clinical trials are carried out ethically and safely.

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.

Gene therapy in tissue-engineered blood vessels

Cardiovascular disease is the leading cause of morbidity and mortality in Western society. More than 1 million arterial bypass procedures are performed annually in the United States, where either autologous veins or synthetic grafts are used to replace arteries in the coronary or peripheral circulation. Tissue engineering of blood vessels from autologous cells has the potential to produce biological grafts for use in bypass surgery. Ex vivo development of vascular grafts also provides an ideal target of site-specific gene therapy to optimize the physiology of the developing conduit, and for the possible delivery of other therapeutic genes to a vascular bed of interest. In this article, we demonstrate that by using a novel retroviral gene delivery system, a target gene of interest can be specifically delivered to the endothelial cells of a developing engineered vessel. Further, we demonstrate that this technique results in stable incorporation of the delivered gene into the target endothelial cells for more than 30 days. These data demonstrate the utility of the retroviral gene delivery approach for optimizing the biologic phenotype of engineered vessels. This also provides the framework for testing an array of genes that may improve the function of engineered blood vessels after surgical implantation.

Pathways to acute humoral rejection

Acute humoral rejection, also known as acute vascular rejection, is a devastating condition of organ transplants and a major barrier to clinical application of organ xenotransplantation. Although initiation of acute humoral or vascular rejection is generally linked to the action of antibodies and complement on the graft, other factors such as ischemia, platelets, T cells, natural killer cells, and macrophages have also been implicated. Central to any understanding of the pathogenesis of acute humoral rejection, and to developing means of preventing it, is to know whether these factors injure the graft independently or through one or few pathways. We addressed this question by examining early events in a severe model of vascular rejection in which guinea pig hearts transplanted heterotopically into rats treated with cobra venom factor (CVF) develop disease over 72 hours. The early steps in acute vascular rejection were associated with expression of a set of inflammatory genes, which appeared to be controlled by availability of interleukin (IL)-1. Interruption of IL-1 signaling by IL-1 receptor antagonist (IL-1ra) averted expression of these genes and early tissue changes, including coagulation and influx of inflammatory cells. These findings suggest IL-1 plays an important role in initiation of acute humoral rejection.

Prolonged function of extracorporeal hDAF transgenic pig livers perfused with human blood

BACKGROUND

The development of genetically modified pigs has renewed interest in the use of porcine liver perfusion in the treatment of acute liver failure.

METHOD

A previously developed model of extracorporeal perfusion has been used to test the function of porcine livers transgenic for human decay accelerating factor when perfused with fresh, whole, human blood. Three experimental groups were studied: alloperfusions (normal pig livers perfused with pig blood) and xenoperfusions of both unmodified and transgenic pig livers with human blood. All livers were perfused for up to 72 hr.

RESULTS

Alloperfusion resulted in the maintenance of good function and histological structure. Stable hemodynamic, synthetic, and metabolic parameters were demonstrated in both unmodified and transgenic liver xenoperfusions; hyperacute rejection was not seen. In both groups, however, the measured parameters of liver function deteriorated toward the end of the 72 hr perfusion period; deterioration was more marked in the nontransgenic group. Xenoperfusions were characterized by a progressive and marked decrease in hematocrit of the circulating blood. Histologically, patchy necrosis was noted in both groups and more retained erythrocytes were seen in the sinusoids of nontransgenic livers, but no other consistent differences were apparent.

CONCLUSIONS

These studies have demonstrated that porcine liver xenoperfusions can be performed for prolonged periods while maintaining good liver function. The use of organs from animals transgenic for a human complement regulator protein confers improvement in some measures of liver function. This preclinical model provides evidence that extracorporeal liver xenoperfusion may be effective in temporary liver support for patients in acute liver failure.

Histopathology of cardiac xenograft rejection in the pig-to-baboon model

BACKGROUND

The use of pig organs transgenic for human decay accelerating factor (hDAF) has largely overcome the problems of hyperacute rejection. With improved immunosuppressive protocols, life supporting grafts are showing greater survival times bringing the possibility of clinical xenotransplantation closer. Examination of the histopathology of the rejection process provides insight into the underlying mechanism and may suggest ways in which new immunosuppressive strategies should be directed.

METHODS

44 baboons (Papio anubis) underwent heart transplants of which 39 were from transgenic donors. The transplanted organs were examined histologically and stained for evidence of immunoglobulin and complement deposition as well as cellular infiltrates.

RESULTS

In the transgenic animals survival times were 2 to 99 days (mean 23.5) and the heterotopic group and 1 to 39 days (mean 11.7) in the orthotopic group. There were 3 cases of hyperacute rejection between the 2 groups. Rejected organs showed areas of old and recent myocardial infarction associated with vascular thrombosis. There was widespread deposition within vessels of immunoglobulins IgM and IgG together with complement fractions C3 and C5b to 9 in those organs that were rejected. The amount of complement positive in the longer surviving organs was less than those rejecting early. Cellular infiltate was predominantly macrophage with some later appearing T or natural killer cells.

CONCLUSIONS

The histopathological changes support the importance of immunoglobulin and complement in delayed xenograft or acute vascular rejection. With time there is an increase in cellular infiltrate predominantly macrophages and these findings suggest an increasingly important role for the cells and the rejection process. The presence of areas of infarction and underlying vascular thrombosis is in keeping with endothelial activation and the establishment of procoagulant phenotype which may be due to immunoglobulin, complement, secreted cytokines and direct cellular effects.

A new in vitro model for the study of pig-to-human vascular hyperacute rejection

Studies on vascular hyperacute xenograft rejection (HAR) are usually conducted in vitro on cultured endothelial cells (EC) exposed to human serum, in complex whole organ perfusion models using heparinized blood or in vivo models. Here we describe a new model allowing perfusion of pig vessels with human whole blood without anticoagulants. Segments of the porcine iliac artery were connected to circular polyvinyl chloride (PVC) tubing, whose inner surface was conjugated with immobilized heparin. The vessels were perfused with 7 to 8 ml of fresh, non-anticoagulated human blood by rocking of the tubing device for 5, 15 or 60 min in an incubator at 37 degrees C. Human iliac arteries (n = 4) were perfused with fresh human ABO-compatible blood as controls. Perfusion of human vessels resulted in changes in the blood and plasma parameters similar to those in the PVC control loop. Overall, perfusion of the porcine vessels generated high levels of C3a, sC5b-9 and thrombin-anti-thrombin (TAT). Platelet consumption was near total (97.2 +/- 1.2%; "high" responders) in six of 13 vessels perfused and only moderate (55.8 +/- 9.9%; "low" responders) in the remaining seven vessels. The "high" responder vessel group showed a significantly higher platelet reduction, neutrophil loss and monocyte consumption and higher C3a and TAT factor at 60 min compared with the human vessels. The "low" responder porcine vessel group also generated significantly higher TAT levels at 60 min compared with the human vessels, but lower levels compared with the "high" responder porcine vessel group. Immunohistochemical examination of perfused porcine vessels revealed binding of human IgM, IgG, IgA, C1q, C3, fibrin and platelets at 5 min. The binding of these proteins was even stronger at 15 and 60 min, and at 60 min C9 could also be detected. Addition of soluble complement receptor 1 (sCR1) to the blood resulted in a significant reduction in C3a and sC5b-9 (P = 0.046 and P = 0.046, respectively). However, sCR1 did not reduce C1q, C3c or C5 staining, but did abolish C9 binding to the endothelium. In conclusion, in vitro perfusion of porcine vessel segments with non-heparinized, fresh human blood triggered events characterizing HAR. The small quantity of blood and xenogenic tissue that is needed makes this model ideal for investigations of the mechanisms and treatments of rejections associated with xenogeneic pig-to-human xenotransplantation.

Physiologic and immunologic hurdles to xenotransplantation

The major problem in the field of renal transplantation is currently the shortage of available kidneys. However, the use of animals as a source of kidneys, i.e., xenotransplantation, is increasingly being viewed as a potential solution to this problem. One preeminent hurdle to xenotransplantation is the immune response of the recipient against the graft; other hurdles include the physiologic limitations of the transplant, infection, and ethical considerations. This review summarizes what is currently known regarding the obstacles to xenotransplantation and some potential solutions to those problems.

Coagulation and thrombotic disorders associated with pig organ and hematopoietic cell transplantation in nonhuman primates

BACKGROUND

Efforts to achieve tolerance to transplanted pig organs in nonhuman primates by the induction of a state of mixed hematopoietic chimerism have been associated with disorders of coagulation and thrombosis. Activation of recipient vascular endothelium and platelets by porcine hematopoietic cells and/or activation of donor organ vascular endothelium and/or molecular differences between the species may play roles. Irradiation or drug therapy could possibly potentiate endothelial cell activation and/or injury.

METHODS

We have investigated parameters of coagulation and platelet activation in nonhuman primates after (1) a regimen aimed at inducing mixed hematopoietic chimerism and tolerance (TIR that included total body irradiation, T cell depletion, and splenectomy; (2) pig bone marrow or pig peripheral blood mobilized progenitor cell transplantation (PCTx); and/or (3) pig organ transplantation (POTx). Five experimental groups were studied. Baboons were the recipient subjects in all groups except Group 1. Gp 1 Cynomolgus monkeys (n=6) underwent TIR + allotransplantation of hematopoietic cells and a kidney or heart or TIR + concordant xenotransplantation (using baboons as donors) of cells and a kidney; Gp 2 Baboons (n=4) underwent TIR with or without (+/-) autologous hematopoietic cell infusion; Gp 3 (n=12) PCTx+/-TIR; Gp 4 (n=5) POTx+/-TIR; Gp 5 (n=4) TIR + PCTx + POTx. Platelet counts, with plasma prothrombin time, partial thromboplastin time, fibrinogen levels, fibrin split products and/or D-dimer were measured.

RESULTS

In the absence of a discordant (porcine) cellular or organ transplant (Groups 1 and 2), TIR resulted in transient thrombocytopenia only, in keeping with bone marrow depression from irradiation. PCTx alone (Group 3) was associated with the rapid development of a thrombotic thrombocytopenic (TTP)-like microangiopathic state, that persisted longer when PCTx was combined with TIR. POTx (+/-TIR) (Group 4) was associated with a gradual fall (over several days) in platelet counts and fibrinogen with disseminated intravascular coagulation (DIC); after graft excision, the DIC generally resolved. When TIR, PCTx and POTx were combined (Group 5), an initial TTP-like state was superseded by a consumptive picture of DIC within the first week, necessitating graft removal.

CONCLUSIONS

Both PCTx and POTx lead to profound alterations in hemostasis and coagulation parameters that must be overcome if discordant xenotransplantation of hematopoietic cells and organs is to be fully successful. Disordered thromboregulation could exacerbate vascular damage and potentiate activation of coagulation pathways after exposure to xenogeneic cells or a vascularized xenograft.

Xenotransplantation of the liver

Xenotransplantation of the liver, in its broadest conception, might involve the transplantation of an intact organ or xenogeneic hepatocytes, or the use of an intact xenogeneic liver or cells as an ex vivo "device." The indications for xenotransplantation include not only hepatic failure but also, potentially, the treatment of metabolic diseases. The hurdles to xenotransplantation include immune, physiologic, and infectious complications. New information and progress in experimental systems are bringing xenotransplantation closer to clinical application.

Disordered regulation of coagulation and platelet activation in xenotransplantation

Rejection of xenografts is associated with vascular-based inflammation, thrombocytopenia and the consumption of coagulation factors that may evolve into disseminated intravascular coagulation (DIC). Similarly, bone marrow-derived cellular xenotransplantation procedures are associated with endothelial cell activation and thrombotic microangiopathic injury. These complications generally develop despite the best available measures for depletion of xenoreactive natural antibody, inhibition of complement activation and suppression of T- and B-cell mediated immune responses. The mechanisms underlying the DIC and thrombotic microangiopathy associated with xenotransplantation are unclear. A proposed primary biological dysfunction of xenografts with respect to regulation of clotting could amplify vascular injury, promote immunological responses and independently contribute to graft failure. Disordered thromboregulation could have deleterious effects, comparable to unregulated complement activation, in the pathogenesis of xenograft rejection and may therefore represent a substantive barrier to xenotransplantation.

Cardiac xenotransplantation: clinical experience and future direction

The shortage of human organs has focused research on finding an animal source of replacement organs. The immunological barriers to xenotransplantation are now more clearly defined, allowing retrospective interpretation of past clinical experience in humans. Due to physiological compatibilities as well as ethical and infectious considerations, pigs have now emerged as the most likely source of future xenografts. The introduction of transgenic pigs expressing human complement regulatory proteins and new immunosuppressive regimens have shown early promise in the laboratory, although further advancements are needed to advance to clinical trials. Additional clarification of infectious risks and patient strategies are remaining obstacles to application in the clinical arena.

Liver xenotransplantation: changes in lipid and lipoprotein concentration after long-term graft survival

BACKGROUND/AIMS

Today, scientists devote considerable effort to the study of mechanisms of xenograft rejection, but with liver xenotransplantation (XTx) researchers face the added problem of metabolic incompatibility between species. To date, there have been few studies of molecular xenogeneic interactions, perhaps because little progress has been made in solving immunological problems. This study is an initial analysis of lipoprotein metabolism in a hamster-to-rat hepatic xenotransplantation model.

METHODS

There were 6 experimental groups (n=8): (1) male Sprague-Dawley (S.D.) rats (220-280 g); (2) male Golden Syrian hamsters (100-150 g); (3) S.D. rats, "sham" operation with immunosuppression; (4) S.D. rat-to-S.D. rat alloTx; (5) S.D. rat-to-S.D. rat alloTx with immunosuppression; (6) XTx hamster G.S-to-S.D. rat with immunosuppression. Mofetil mycophenolate (25 mg/kg/d) was administered for 14 days and FK506 (0.2 mg/kg/d) for 45 days (groups 3, 5 and 6). After 24 h fasting, animals were sacrificed (day +50 postransplantation) and a complete lipoprotein profile was determined. Serum lipoproteins were subfractioned by ultracentrifugation in density gradient.

RESULTS

There was a large increase in serum lipid levels in xenografted rats compared with control rats and allografted rats. Xenografted rats presented a severely altered lipoprotein profile compared with normal rats. Surprisingly, the characterisation of lipoproteins in xenografted rats displayed the same composition as donor animals. Histological study did not show signs of alteration of the hepatic architecture.

CONCLUSIONS

Since the liver is the main solid organ co-ordinator of metabolic pathways, such as lipid metabolism, hepatic xenotransplantation makes changes in lipid concentrations in the recipient and also changes in lipid compositions of lipoproteins. Hepatic xenotransplantation is not a feasible solution given the organ's metabolic complexity.

Expression of tissue factor mRNA in cardiac xenografts: clues to the pathogenesis of acute vascular rejection

BACKGROUND

Acute vascular rejection destroys vascularized xenografts over a period of hours to days and is now considered the major hurdle to the clinical application of xenotransplantation. The hallmark of acute vascular rejection is diffuse intravascular coagulation; however, the pathogenesis of coagulation is a matter of controversy. One line of evidence points to activated endothelial cells and another to activated inflammatory cells as a source of tissue factor and thus as a primary cause of this lesion. The distinction between the two mechanisms inducing coagulation in the xenograft provides an opportunity for specific intervention.

METHODS

To explore these mechanisms, we studied the expression of tissue factor mRNA by in situ reverse transcriptase-polymerase chain reaction in relation to the histopathologic manifestations of acute vascular rejection in guinea pig hearts transplanted into rats treated by cobra venom factor to avoid the hyperacute rejection.

RESULTS

Three hours after transplantation and before the deposition of fibrin, tissue factor mRNA was expressed in the endothelial cells lining small and medium blood vessels and in smooth muscle cells of guinea pig cardiac xenografts. Sixteen hours after transplantation, while rat tissue factor mRNA was expressed only in occasional infiltrating cells, cardiac xenografts showed prominent deposits of fibrin in small vessels. Maximum expression of tissue factor on rat infiltrating cells was observed 48 hr after transplantation.

CONCLUSIONS

These results suggest that in acute vascular rejection, coagulation is initiated on the donor vascular system, while the procoagulant characteristics of infiltrating cells may reflect a response to tissue injury rather than a cause.

Current status of xenotransplantation

1. The transplantation of organs and tissues from animals into humans (i.e. xenotransplantation) has been a long sought objective to allow xenotransplantation to achieve its full impact in the clinical practice of medicine. 2. The main hurdles to the application of xenotransplantation are the immunological reaction of the recipient against the transplant, the functional limitations of tissues and organs in biogenetically disparate recipients and the possibility of transferring infectious organisms from the graft into the recipient. 3. Advances in a variety of fields have shed new light on these hurdles and have given rise to potential solutions and prospects for the clinical application of xenotransplant and are summarized in the report that follows.

Genetic therapies and xenotransplantation

There is an acute shortage of human organs available for transplantation. Transplanting animal organs or tissues is a potential solution and could also provide a novel means of gene delivery. Xenotransplantation faces many challenges, including the immune response of the recipient against the transplant, the physiological limitations of the transplant and the possibility of introducing infectious organisms into the recipient. A number of medical centres around the world are addressing these issues. Much of this research focuses on the application of genetic therapies because of the unique opportunity this represents for genetic engineering to be used at the source of organ tissue, rather than to the recipient.

Factors in xenograft rejection

Important mechanisms underlying immediate xenograft loss by hyperacute rejection (HAR), in the pig-to-primate combination, have been recently delineated. There are now several proposed therapies that deal with the problem of complement activation and xenoreactive natural antibody (XNA) binding to the vasculature that have been shown to prevent HAR. However, vascularized xenografts are still lost, typically within days, by delayed xenograft rejection (DXR), alternatively known as acute vascular rejection (AVR). This process is characterized by endothelial cell (EC) perturbation, localization of XNA within the graft vasculature, host NK cell and monocyte activation with platelet sequestration and vascular thrombosis. Alternative immunosuppressive strategies, additive anti-complement therapies with the control of any resulting EC activation processes and induction of protective responses have been proposed to ameliorate this pathological process. In addition, several potentially important molecular incompatibilities between activated human coagulation factors and the natural anticoagulants expressed on porcine EC have been noted. Such incompatibilities may be analogous to cross-species alterations in the function of complement regulatory proteins important in HAR. Disordered thromboregulation is potentially relevant to the progression of inflammatory events in DXR and the disseminated intravascular coagulation seen in primate recipients of porcine renal xenografts. We have recently demonstrated the inability of porcine tissue factor pathway inhibitor (TFPI) to adequately neutralize human factor Xa (FXa), the aberrant activation of both human prothrombin and FXa by porcine EC and the failure of the porcine natural anticoagulant, thrombomodulin to bind human thrombin and hence activate human protein C. The enhanced potential of porcine von Willebrand factor to associate with human platelet GPIb has been demonstrated to be dependent upon the isolated A1 domain of von Willebrand factor. In addition, the loss of TFPI and vascular ATPDase/CD39 activity following EC activation responses would potentiate any procoagulant changes within the xenograft. These developments could exacerbate vascular damage from whatever cause and enhance the activation of platelets and coagulation pathways within xenografts resulting in graft infarction and loss. Analysis of these and the other putative factors underlying DXR should lead to the development and testing of genetic approaches that, in conjunction with selected pharmacological means, may further prolong xenograft survival to a clinically relevant extent.

IgY antiporcine endothelial cell antibodies effectively block human antiporcine xenoantibody binding

Avian IgY antibodies are structurally different from mammalian IgGs and do not fix mammalian complement components or bind human Fc receptors. As these antibody-mediated interactions are believed to play significant roles in both hyperacute rejection (HAR) and acute vascular xenograft rejection (AVXR), IgY antibodies to xenoantigen target epitopes may inhibit these rejection processes. In this report, we show that chicken IgY antibodies to alpha-Gal antigen epitopes and to other porcine aortic endothelial cell (PAEC) antigens block human xenoreactive natural antibody binding to both porcine and rat cardiac tissues and porcine kidney tissues. Chicken IgY antibodies blocked complement-mediated lysis of PAECs by human serum, and inhibited antibody-dependent cell-mediated lysis of PAECs by heat-inactivated human serum plus peripheral blood leukocytes. Binding of IgY to porcine endothelial cells did not affect cell morphology nor expression of E-selectin. These results suggest that avian IgYs could be of potential use in inhibiting pig-to-human xenograft rejection.

Physiologic incompatibilities in discordant xenotransplantation

At present, several incompatibilities between pig and human, not only from the immunologic point of view but also regarding physiological and molecular systems, have been identified. It is anticipated that this is only the tip of the iceberg regarding this topic. However, there are also many systems that are compatible. Strategies have been outlined, and many tools, such as gene modification, are available to solve the problems. Therefore, the statement by Keith Reemtsma in the foreward to the latest issue of the monography Xenotransplantation "The important question is not whether xenotransplantation will succeed, but rather how and under what circumstances xenografts will provide predictable enough results to warrent clinical application" will hopefully be a reality in the future.

Disseminated intravascular coagulation in association with the delayed rejection of pig-to-baboon renal xenografts

BACKGROUND

Intravascular fibrin deposition and platelet sequestration occur with porcine xenograft rejection by baboons. Disseminated intravascular coagulopathy may arise either as a direct consequence of the failure to fully deplete xenoreactive natural antibodies and block complement, or because of putative cross-species molecular incompatibilities in this discordant species combination.

METHODS

Three baboons were conditioned with retrovirally transduced autologous bone marrow to induce tolerance to swine antigens. Xenoreactive natural antibodies and complement were depleted by plasmapheresis and the use of Gal alpha1-3Gal column adsorptions; baboons were then splenectomized and underwent renal xenografting from inbred, miniature pigs. Soluble complement receptor type-1 with protocol immunosuppression (mycophenolate mofetil, 15-deoxyspergualin, steroids, and cyclosporine) was administered.

RESULTS

A bleeding diathesis was clinically evident from days 5 to 12 after transplantation in two baboons. Low levels of circulating C3a, C3d, and iC3b were measured despite the absence of functional circulating complement components. Profound thrombocytopenia with abnormalities in keeping with disseminated intravascular coagulopathy were observed. Prolongation of prothrombin and partial thromboplastin times was accompanied by evidence for tissue factor-mediated coagulation pathways, high levels of thrombin generation (prothrombin fragment F(1+2) production and thrombin-antithrombin complex formation), fibrinogen depletion, and production of high levels of the fibrin degradation product D-dimer. Importantly, these disturbances resolved rapidly after the excision of the rejected xenografts in two surviving animals. Histopathological examination of the rejected xenografts confirmed vascular injury, fibrin deposition, platelet deposition, and localized complement activation.

CONCLUSIONS

Systemic coagulation disturbances are associated with delayed xenograft rejection.

The promise of xenografting

Advances in understanding the rejection of foreign tissues has renewed enthusiasm about the possibility of overcoming the present organ shortage by transplanting porcine kidneys into humans. Currently, three known forms of organ rejection stand as obstacles to xenotransplantation as a clinical reality--hyperacute rejection, acute vascular rejection, and cellular rejection. Progress in the knowledge of xenoreactive antibodies and of complement, along with new transgenic technologies, have enabled researchers to overcome hyperacute rejection in xenografts. The advances have brought into focus such issues as the ability of the porcine kidney to replace the physiologic functions of the human kidney and the risk associated with the potential transmission of infectious agents from animals to humans. Despite the remaining hurdles to clinical application of xenotransplantation, the rapid pace of research and emerging technologies would seem to make xenotransplantation a renal replacement therapy of great promise.