Prolonged function of macrophage, von Willebrand factor-deficient porcine pulmonary xenografts

Lung Xenotransplantation

Although efforts have been made to expand the pool of donor lung allografts for human lung transplantation, a shortage remains. Lung xenotransplantation has been proposed as an alternative approach, but lung xenotransplantation in humans has not yet been reported. In addition, significant biological and ethical barriers will have to be addressed before clinical trials can be undertaken. However, significant progress has been made toward addressing biological incompatibilities that present a barrier, and recent advances in genetic engineering tools promise to accelerate further progress.

Humanized von Willebrand factor reduces platelet sequestration in ex vivo and in vivo xenotransplant models

The transplantation of organs across species offers the potential to solve the shortage of human organs. While activation of human platelets by human von Willebrand factor (vWF) requires vWF activation by shear stress, contact between human platelets and porcine vWF (pvWF) leads to spontaneous platelet adhesion and activation. This non-physiologic interaction may contribute to the thrombocytopenia and coagulation pathway dysregulation often associated with xenotransplantation of pig organs in nonhuman primates. Pigs genetically modified to decrease antibody and complement-dependent rejection (GTKO.hCD46) were engineered to express humanized pvWF (h*pvWF) by replacing a pvWF gene region that encodes the glycoprotein Ib-binding site with human cDNA orthologs. This modification corrected for non-physiologic human platelet aggregation on exposure to pig plasma, while preserving in vitro platelet activation by collagen. Organs from pigs with h*pvWF demonstrated reduced platelet sequestration during lung (p ≤ .01) and liver (p ≤ .038 within 4 h) perfusion ex vivo with human blood and after pig-to-baboon lung transplantation (p ≤ .007). Residual platelet sequestration and activation were not prevented by the blockade of canonical platelet adhesion pathways. The h*pvWF modification prevents physiologically inappropriate activation of human or baboon platelets by porcine vWF, addressing one cause of the thrombocytopenia and platelet activation observed with xenotransplantation.

The pig as a medical model for acquired respiratory diseases and dysfunctions: An immunological perspective

By definition no model is perfect, and this also holds for biology and health sciences. In medicine, murine models are, and will be indispensable for long, thanks to their reasonable cost and huge choice of transgenic strains and molecular tools. On the other side, non-human primates remain the best animal models although their use is limited because of financial and obvious ethical reasons. In the field of respiratory diseases, specific clinical models such as sheep and cotton rat for bronchiolitis, or ferret and Syrian hamster for influenza and Covid-19, have been successfully developed, however, in these species, the toolbox for biological analysis remains scarce. In this view the porcine medical model is appearing as the third, intermediate, choice, between murine and primate. Herein we would like to present the pros and cons of pig as a model for acquired respiratory conditions, through an immunological point of view. Indeed, important progresses have been made in pig immunology during the last decade that allowed the precise description of immune molecules and cell phenotypes and functions. These progresses might allow the use of pig as clinical model of human respiratory diseases but also as a species of interest to perform basic research explorations.

CRISPR/Cas Technology in Pig-to-Human Xenotransplantation Research

CRISPR/Cas (clustered regularly interspaced short palindromic repeats linked to Cas nuclease) technology has revolutionized many aspects of genetic engineering research. Thanks to it, it became possible to study the functions and mechanisms of biology with greater precision, as well as to obtain genetically modified organisms, both prokaryotic and eukaryotic. The changes introduced by the CRISPR/Cas system are based on the repair paths of the single or double strand DNA breaks that cause insertions, deletions, or precise integrations of donor DNA. These changes are crucial for many fields of science, one of which is the use of animals (pigs) as a reservoir of tissues and organs for xenotransplantation into humans. Non-genetically modified animals cannot be used to save human life and health due to acute immunological reactions resulting from the phylogenetic distance of these two species. This review is intended to collect and summarize the advantages as well as achievements of the CRISPR/Cas system in pig-to-human xenotransplantation research. In addition, it demonstrates barriers and limitations that require careful evaluation before attempting to experiment with this technology.

Application of α2 -adrenergic agonists combined with anesthetics and their implication in pulmonary intravascular macrophages-insulted pulmonary edema and hypoxemia in ruminants

Alpha₂ -adrenergic agonists have been implicated in the development of pulmonary edema (PE) and sustained hypoxemia that lead to life-threatening pulmonary distress in ruminants, especially with sensitive and compromised animals. Recently, there is limited understanding of exact mechanism underlying pulmonary alterations associated with α₂ -adrenergic agonist administration. Ruminants have a rich population of pulmonary intravascular macrophages (PIMs) in the pulmonary circulation, which may be involved in the development of pulmonary alveolo-capillary barrier damage. Hence, the central thesis of this review is overviewing the literatures regarding the systemic use of α₂ -adrenergic agonists in domestic ruminants, focusing on their pulmonary side effects, especially on the influence of PIMs on the lung. At this moment, further studies are needed to provide a clear emphasis and better understanding of the potential role of PIMs in the lung pathophysiology associated with α₂ -adrenergic agonists. These preliminary studies would be potentially to develop future medications and intervention targets that may be helpful to alleviate or prevent the critical striking pulmonary effects, and thereby improving the safety of α₂ -agonist application in ruminants.

Evaluation of the CRISPR/Cas9 Genetic Constructs in Efficient Disruption of Porcine Genes for Xenotransplantation Purposes Along with an Assessment of the Off-Target Mutation Formation

The increasing life expectancy of humans has led to an increase in the number of patients with chronic diseases and organ failure. However, the imbalance between the supply and the demand for human organs is a serious problem in modern transplantology. One of many solutions to overcome this problem is the use of xenotransplantation. The domestic pig (Sus scrofa domestica) is currently considered as the most suitable for human organ procurement. However, there are discrepancies between pigs and humans that lead to the creation of immunological barriers preventing the direct xenograft. The introduction of appropriate modifications to the pig genome to prevent xenograft rejection is crucial in xenotransplantation studies. In this study, porcine GGTA1, CMAH, β4GalNT2, vWF, ASGR1 genes were selected to introduce genetic modifications. The evaluation of three selected gRNAs within each gene was obtained, which enabled the selection of the best site for efficient introduction of changes. Modifications were examined after nucleofection of porcine primary kidney fibroblasts with CRISPR/Cas9 system genetic constructs, followed by the tracking of indels by decomposition (TIDE) analysis. In addition, off-target analysis was carried out for selected best gRNAs using the TIDE tool, which is new in the research conducted so far and shows the utility of this tool in these studies.

Xenotransplantation: Current Status in Preclinical Research

The increasing life expectancy of humans has led to a growing numbers of patients with chronic diseases and end-stage organ failure. Transplantation is an effective approach for the treatment of end-stage organ failure; however, the imbalance between organ supply and the demand for human organs is a bottleneck for clinical transplantation. Therefore, xenotransplantation might be a promising alternative approach to bridge the gap between the supply and demand of organs, tissues, and cells; however, immunological barriers are limiting factors in clinical xenotransplantation. Thanks to advances in gene-editing tools and immunosuppressive therapy as well as the prolonged xenograft survival time in pig-to-non-human primate models, clinical xenotransplantation has become more viable. In this review, we focus on the evolution and current status of xenotransplantation research, including our current understanding of the immunological mechanisms involved in xenograft rejection, genetically modified pigs used for xenotransplantation, and progress that has been made in developing pig-to-pig-to-non-human primate models. Three main types of rejection can occur after xenotransplantation, which we discuss in detail: (1) hyperacute xenograft rejection, (2) acute humoral xenograft rejection, and (3) acute cellular rejection. Furthermore, in studies on immunological rejection, genetically modified pigs have been generated to bridge cross-species molecular incompatibilities; in the last decade, most advances made in the field of xenotransplantation have resulted from the production of genetically engineered pigs; accordingly, we summarize the genetically modified pigs that are currently available for xenotransplantation. Next, we summarize the longest survival time of solid organs in preclinical models in recent years, including heart, liver, kidney, and lung xenotransplantation. Overall, we conclude that recent achievements and the accumulation of experience in xenotransplantation mean that the first-in-human clinical trial could be possible in the near future. Furthermore, we hope that xenotransplantation and various approaches will be able to collectively solve the problem of human organ shortage.

Xenogeneic Lung Transplantation Models

Study of lung xenografts has proven useful to understand the remaining barriers to successful transplantation of other organ xenografts. In this chapter, the history and current status of lung xenotransplantation will be briefly reviewed, and two different experimental models, the ex vivo porcine-to-human lung perfusion and the in vivo xenogeneic lung transplantation, will be presented. We will focus on the technical details of these lung xenograft models in sufficient detail, list the needed materials, and mention analysis techniques to allow others to adopt them with minimal learning curve.

GalT-KO pig lungs are highly susceptible to acute vascular rejection in baboons, which may be mitigated by transgenic expression of hCD47 on porcine blood vessels

BACKGROUND

Despite recent progress in survival times of xenografts in non-human primates, there are no reports of survival beyond 5 days of histologically well-aerated porcine lung grafts in baboons. Here, we report our initial results of pig-to-baboon xeno-lung transplantation (XLTx).

METHODS

Eleven baboons received genetically modified porcine left lungs from either GalT-KO alone (n = 3), GalT-KO/humanCD47(hCD47)/hCD55 (n = 3), GalT-KO/hD47/hCD46 (n = 4), or GalT-KO/hCD39/hCD46/hCD55/TBM/EPCR (n = 1) swine. The first 2 XLTx procedures were performed under a non-survival protocol that allowed a 72-hour follow-up of the recipients with general anesthesia, while the remaining 9 underwent a survival protocol with the intention of weaning from ventilation.

RESULTS

Lung graft survivals in the 2 non-survival animals were 48 and >72 hours, while survivals in the other 9 were 25 and 28 hours, at 5, 5, 6, 7, >7, 9, and 10 days. One baboon with graft survival >7 days, whose entire lung graft remained well aerated, was euthanized on POD 7 due to malfunction of femoral catheters. hCD47 expression of donor lungs was detected in both alveoli and vessels only in the 3 grafts surviving >7, 9, and 10 days. All other grafts lacked hCD47 expression in endothelial cells and were completely rejected with diffuse hemorrhagic changes and antibody/complement deposition detected in association with early graft loss.

CONCLUSIONS

To our knowledge, this is the first evidence of histologically viable porcine lung grafts beyond 7 days in baboons. Our results indicate that GalT-KO pig lungs are highly susceptible to acute humoral rejection and that this may be mitigated by transgenic expression of hCD47.

In vitro characterization of neonatal, juvenile, and adult porcine islet oxygen demand, β-cell function, and transcriptomes

BACKGROUND

There is currently a shortage of human donor pancreata which limits the broad application of islet transplantation as a treatment for type 1 diabetes. Porcine islets have demonstrated potential as an alternative source, but a study evaluating islets from different donor ages under unified protocols has yet to be conducted.

METHODS

Neonatal porcine islets (NPI; 1-3 days), juvenile porcine islets (JPI; 18-21 days), and adult porcine islets (API; 2+ years) were compared in vitro, including assessments of oxygen consumption rate, membrane integrity determined by FDA/PI staining, β-cell proliferation, dynamic glucose-stimulated insulin secretion, and RNA sequencing.

RESULTS

Oxygen consumption rate normalized to DNA was not significantly different between ages. Membrane integrity was age dependent, and API had the highest percentage of intact cells. API also had the highest glucose-stimulated insulin secretion response during a dynamic insulin secretion assay and had 50-fold higher total insulin content compared to NPI and JPI. NPI and JPI had similar glucose responsiveness, β-cell percentage, and β-cell proliferation rate. Transcriptome analysis was consistent with physiological assessments. API transcriptomes were enriched for cellular metabolic and insulin secretory pathways, while NPI exhibited higher expression of genes associated with proliferation.

CONCLUSIONS

The oxygen demand, membrane integrity, β-cell function and proliferation, and transcriptomes of islets from API, JPI, and NPI provide a comprehensive physiological comparison for future studies. These assessments will inform the optimal application of each age of porcine islet to expand the availability of islet transplantation.

Genetically Modified Pigs as Organ Donors for Xenotransplantation

The growing shortage of available organs is a major problem in transplantology. Thus, new and alternative sources of organs need to be found. One promising solution could be xenotransplantation, i.e., the use of animal cells, tissues and organs. The domestic pig is the optimum donor for such transplants. However, xenogeneic transplantation from pigs to humans involves high immune incompatibility and a complex rejection process. The rapid development of genetic engineering techniques enables genome modifications in pigs that reduce the cross-species immune barrier.

Lung xenotransplantation

PURPOSE OF REVIEW

This review describes the most recent progress in xeno lung transplantation (XLTx) to date. It describes the potential mechanisms of early xeno lung graft loss, as well as the latest therapeutic strategies to overcome them.

RECENT FINDINGS

Using ex-vivo perfusion models of porcine lungs with human blood, the use of genetically modified pig lungs along with novel pharmaceutical approaches has recently been studied. Strategies that have demonstrated improved lung survival include the knockout of known xenoantigens (GalTKO and N-glycolylneuraminic acid-KO), genes that regulate complement activation (hCD46 and hCD55), as well as the inflammation/coagulation cascade (human leukocyte antigen-E, human thrombomodulin, human endothelial protein C receptor, hCD47, hCD39, hCD73 and heme oxygenase-1). Furthermore, pharmacologic interventions including the depletion of pulmonary intravascular macrophages or von Willebrand factor, inhibition of thromboxane synthase and blockade of histamine receptors have also demonstrated protective effects on xeno lung grafts. Using in-vivo pig to nonhuman primate lung transplant models, these approaches have been shown to extend pulmonary xenograft survival to 5 days.

SUMMARY

The development of new multitransgenic GalTKO pigs has demonstrated prolongation of porcine xenograft survival; however, advancement in XLTx has remained frustratingly limited. Further intensive and innovative strategies including genetic manipulation of donors, as well as inflammation/coagulation dysregulation, are required to make XLTx a clinical possibility.

The Sda and Cad glycan antigens and their glycosyltransferase, β1,4GalNAcT-II, in xenotransplantation

Antibody-mediated rejection is a barrier to the clinical application of xenotransplantation, and xenoantigens play an important role in this process. Early research suggested that N-acetyl-D-galactosamine (GalNAc) could serve as a potential xenoantigen. GalNAc is the immunodominant glycan of the Sda antigen. Recently, knockout of β1,4-N-acetylgalactosaminyltransferase 2 (β1,4GalNAcT-II) from the pig results in a decrease in binding of human serum antibodies to pig cells. It is believed that this is the result of the elimination of the GalNAc on the Sda antigen, which is catalyzed by the enzyme, β1,4GalNAcT-II. However, research into human blood group antigens suggests that only a small percentage (1%-2%) of people express anti-Sda antibodies directed to Sda antigen, and yet a majority appear to have antibodies directed to the products of pig B4GALNT2. Questions can therefore be asked as to (i) whether the comprehensive structure of the Sda antigen in humans, that is, the underlying sugar structure, is identical to the Sda antigen in pigs, (ii) whether the human anti-Sda antibody binds ubiquitously to pig cells, but not to human cells, and (iii) what role the Sda⁺⁺ (also called Cad) antigen is playing in this discrepancy. We review what is known about these antigens and discuss the discrepancies that have been noted above.

Xenotransplantation: back to the future?

The field of xenotransplantation has fluctuated between great optimism and doubts over the last 50 years. The initial clinical attempts were extremely ambitious but faced technical and ethical issues that prompted the research community to go back to preclinical studies. Important players left the field due to perceived xenozoonotic risks and the lack of progress in pig-to-nonhuman-primate transplant models. Initial apparently unsurmountable issues appear now to be possible to overcome due to progress of genetic engineering, allowing the generation of multiple-xenoantigen knockout pigs that express human transgenes and the genomewide inactivation of porcine endogenous retroviruses. These important steps forward were made possible by new genome editing technologies, such as CRISPR/Cas9, allowing researchers to precisely remove or insert genes anywhere in the genome. An additional emerging perspective is the possibility of growing humanized organs in pigs using blastocyst complementation. This article summarizes the current advances in xenotransplantation research in nonhuman primates, and it describes the newly developed genome editing technology tools and interspecific organ generation.

Effects of carbon monoxide on early dysfunction and microangiopathy following GalT-KO porcine pulmonary xenotransplantation in cynomolgus monkeys

BACKGROUND

Despite progress in the current genetic manipulation of donor pigs, most non-human primates were lost within a day of receiving porcine lung transplants. We previously reported that carbon monoxide (CO) treatment improved pulmonary function in an allogeneic lung transplant (LTx) model using miniature swine. In this study, we evaluated whether the perioperative treatment with low-dose inhalation of CO has beneficial effects on porcine lung xenografts in cynomolgus monkeys (cynos).

METHODS

Eight cynos received orthotopic left LTx using either α-1,3-galactosyltransferase knockout (GalT-KO; n = 2) or GalT-KO with human decay accelerating factor (hDAF) (GalT-KO/hDAF; n = 6) swine donors. These eight animals were divided into three groups. In Group 1 (n = 2), neither donor nor recipients received CO therapy. In Group 2 (n = 4), donors were treated with inhaled CO for 180-minute. In Group 3 (n = 2), both donors and recipients were treated with CO (donor: 180-minute; recipient: 360-minute). Concentration of inhaled CO was adjusted based on measured levels of carboxyhemoglobin in the blood (15%-20%).

RESULTS

Two recipients survived for 3 days; 75 hours (no-CO) and 80 hours (CO in both the donor and the recipient), respectively. Histology showed less inflammatory cell infiltrates, intravascular thrombi, and hemorrhage in the 80-hour survivor with the CO treatment than the 75-hours non-CO treatment. Anti-non-Gal cytotoxicity levels did not affect the early loss of the grafts. Although CO treatment did not prolong overall xeno lung graft survival, the recipient/donor CO treatment helped to maintain platelet counts and inhibit TNF-α and IL-6 secretion at 2 hours after revascularization of grafts. In addition, lung xenografts that were received recipient/donor CO therapy demonstrated fewer macrophage and neutrophil infiltrates. Infiltrating macrophages as well as alveolar epithelial cells in the CO-treated graft expressed heme oxygenase-1.

CONCLUSION

Although further investigation is required, CO treatment may provide a beneficial strategy for pulmonary xenografts.

Pig Liver Xenotransplantation: A Review of Progress Toward the Clinic

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

Clinical xenotransplantation, a closer reality: Literature review

Xenotransplantation could provide an unlimited supply of organs and solve the current shortage of organs for transplantation. To become a reality in clinical practice, the immunological and physiological barriers and the risk of xenozoonosis that they possess should be resolved. From the immunological point of view, in the last 30 years a significant progress in the production of transgenic pigs has prevented the hyperacute rejection. About xenozoonosis, attention has been focused on the risk of transmission of porcine endogenous retroviruses; however, today, it is considered that the risk is very low and the inevitable transmission should not prevent the clinical xenotransplantation. Regarding the physiological barriers, encouraging results have been obtained and it's expected that the barriers that still need to be corrected can be solved in the future through genetic modifications.

Beneficial effect of a nitric oxide donor in an ex vivo model of pig-to-human pulmonary xenotransplantation

BACKGROUND

Nitric oxide (NO) can reduce platelet adhesion and vascular resistance. Tempol can scavenge the reactive oxygen species (ROS) that induce tissue injury. As xenograft rejection attenuates endogenous NO production and generates ROS, we evaluated the potential effect of an NO donor (SIN-1, 3-morpholinosydnonimine) and tempol on hyperacute xenograft dysfunction using an ex vivo porcine lung perfusion model.

METHODS

For the evaluation of von Willebrand factor (vWF) secretion, human endothelial cells were stimulated with thrombin. Porcine lungs were perfused with either fresh human whole blood (unmodified control group [n = 4]), SIN-1 (n = 4), or SIN and tempol (n = 4).

RESULTS

SIN-1 and tempol significantly inhibited vWF secretion from endothelial cells in vitro. However, they did not suppress xenogeneic complement activation. In an ex vivo pulmonary perfusion model, SIN-1 improved pulmonary xenograft function by reducing pulmonary vascular resistance (PVR), inhibiting complement activation, and inhibiting thrombin generation. Combined treatment with tempol and SIN-1 potentiated PVR reduction, but slightly enhanced complement activation.

CONCLUSIONS

An NO donor is expected to improve pulmonary xenograft function through inhibition of vWF secretion, vasoconstriction, thrombin generation, and indirectly through inhibition of complement activation. The additional effects of tempol on an NO donor were not considered significant in an ex vivo xenograft system.

Comparative biology of decellularized lung matrix: Implications of species mismatch in regenerative medicine

Lung engineering is a promising technology, relying on re-seeding of either human or xenographic decellularized matrices with patient-derived pulmonary cells. Little is known about the species-specificity of decellularization in various models of lung regeneration, or if species dependent cell-matrix interactions exist within these systems. Therefore decellularized scaffolds were produced from rat, pig, primate and human lungs, and assessed by measuring residual DNA, mechanical properties, and key matrix proteins (collagen, elastin, glycosaminoglycans). To study intrinsic matrix biologic cues, human endothelial cells were seeded onto acellular slices and analyzed for markers of cell health and inflammation. Despite similar levels of collagen after decellularization, human and primate lungs were stiffer, contained more elastin, and retained fewer glycosaminoglycans than pig or rat lung scaffolds. Human endothelial cells seeded onto human and primate lung tissue demonstrated less expression of vascular cell adhesion molecule and activation of nuclear factor-κB compared to those seeded onto rodent or porcine tissue. Adhesion of endothelial cells was markedly enhanced on human and primate tissues. Our work suggests that species-dependent biologic cues intrinsic to lung extracellular matrix could have profound effects on attempts at lung regeneration.

Platelet sequestration and activation during GalTKO.hCD46 pig lung perfusion by human blood is primarily mediated by GPIb, GPIIb/IIIa, and von Willebrand Factor

BACKGROUND

Here, we ask whether platelet GPIb and GPIIb/IIIa receptors modulate platelet sequestration and activation during GalTKO.hCD46 pig lung xenograft perfusion.

METHODS

GalTKO.hCD46 transgenic pig lungs were perfused with heparinized fresh human blood. Results from perfusions in which αGPIb Fab (6B4, 10 mg/l blood, n = 6), αGPIIb/IIIa Fab (ReoPro, 3.5 mg/l blood, n = 6), or both drugs (n = 4) were administered to the perfusate were compared to two additional groups in which the donor pig received 1-desamino-8-d-arginine vasopressin (DDAVP), 3 μg/kg (to pre-deplete von Willebrand Factor (pVWF), the main GPIb ligand), with or without αGPIb (n = 6 each).

RESULTS

Platelet sequestration was significantly delayed in αGPIb, αGPIb+DDAVP, and αGPIb+αGPIIb/IIIa groups. Median lung "survival" was significantly longer (>240 vs. 162 min reference, p = 0.016), and platelet activation (as CD62P and βTG) were significantly inhibited, when pigs were pre-treated with DDAVP, with or without αGPIb Fab treatment. Pulmonary vascular resistance rise was not significantly attenuated in any group, and was associated with residual thromboxane and histamine elaboration.

CONCLUSIONS

The GPIb-VWF and GPIIb/IIIa axes play important roles in platelet sequestration and coagulation cascade activation during GalTKO.hCD46 lung xenograft injury. GPIb blockade significantly reduces platelet activation and delays platelet sequestration in this xenolung rejection model, an effect amplified by adding αGPIIb/IIIa blockade or depletion of VWF from pig lung.

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.

Progress towards overcoming coagulopathy and hemostatic dysfunction associated with xenotransplantation

Dysregulation of coagulation and disordered hemostasis are frequent complications in the pig-to-nonhuman primate preclinical xenotransplantation model. The most extreme manifestations are the systemic development of a life-threatening consumptive coagulopathy, characterized by thrombocytopenia and bleeding, which is balanced at the opposite extreme by local complications of graft loss due to thrombotic microangiopathy. The contributing mechanisms include inflammation, vascular injury, heightened innate, humoral and cellular immune responses, and molecular incompatibilities affecting the regulation of coagulation. There also appear to be organ-specific factors that have been linked to vascular heterogeneity. As examples, liver xenografts rapidly induce thrombocytopenia by sequestering human/primate platelets; renal xenografts cause a broader coagulopathy, linked in some cases to reactivation of porcine CMV, whereas cardiac xenografts often succumb to microvascular thrombosis without associated systemic coagulopathy but with local perturbations in fibrinolysis. Overcoming coagulation dysfunction will require a combination of genetic and pharmacological strategies. Deletion of the xenoantigen αGal, transgenic expression of human complement regulatory proteins, and refinement of immunosuppression to blunt the antibody response have all had some impact, without providing a complete solution. More recently, the addition of approaches specifically targeted at coagulation have produced promising results. As an example, heterotopic cardiac xenografts from donors expressing human thrombomodulin have survived for more than a year in immunosuppressed baboons, with no evidence of thrombotic microangiopathy or coagulopathy.

The need for xenotransplantation as a source of organs and cells for clinical transplantation

The limited availability of deceased human organs and cells for the purposes of clinical transplantation remains critical worldwide. Despite the increasing utilization of 'high-risk', 'marginal', or 'extended criteria' deceased donors, in the U.S. each day 30 patients either die or are removed from the waiting list because they become too sick to undergo organ transplantation. In certain other countries, where there is cultural resistance to deceased donation, e.g., Japan, the increased utilization of living donors, e.g., of a single kidney or partial liver, only very partially addresses the organ shortage. For transplants of tissues and cells, e.g., pancreatic islet transplantation for patients with diabetes, and corneal transplantation for patients with corneal blindness (whose numbers worldwide are potentially in the millions), allotransplantation will never prove a sufficient source. There is an urgent need for an alternative source of organs and cells. The pig could prove to be a satisfactory source, and clinical xenotransplantation using pig organs or cells, particularly with the advantages provided by genetic engineering to provide resistance to the human immune response, may resolve the organ shortage. The physiologic compatibilities and incompatibilities of the pig and the human are briefly reviewed.

Meta-analysis of the independent and cumulative effects of multiple genetic modifications on pig lung xenograft performance during ex vivo perfusion with human blood

BACKGROUND

Genetically modified pigs are a promising potential source of lung xenografts. Ex vivo xenoperfusion is an effective platform for testing the effect of new modifications, but typical experiments are limited by testing of a single genetic intervention and small sample sizes. The purpose of this study was to analyze the individual and aggregate effects of donor genetic modifications on porcine lung xenograft survival and injury in an extensive pig lung xenoperfusion series.

METHODS

Data from 157 porcine lung xenoperfusion experiments using otherwise unmodified heparinized human blood were aggregated as either continuous or dichotomous variables. Lungs were wild type in 17 perfusions (11% of the study group), while 31 lungs (20% of the study group) had one genetic modification, 40 lungs (39%) had 2, and 47 lungs (30%) had 3 or more modifications. The primary endpoint was functional lung survival to 4 h of perfusion. Secondary analyses evaluated previously identified markers associated with known lung xenograft injury mechanisms. In addition to comparison among all xenografts grouped by survival status, a subgroup analysis was performed of lungs incorporating the GalTKO.hCD46 genotype.

RESULTS

Each increase in the number of genetic modifications was associated with additional prolongation of lung xenograft survival. Lungs that exhibited survival to 4 h generally had reduced platelet activation and thrombin generation. GalTKO and the expression of hCD46, HO-1, hCD55, or hEPCR were associated with improved survival. hTBM, HLA-E, and hCD39 were associated with no significant effect on the primary outcome.

CONCLUSION

This meta-analysis of an extensive lung xenotransplantation series demonstrates that increasing the number of genetic modifications targeting known xenogeneic lung injury mechanisms is associated with incremental improvements in lung survival. While more detailed mechanistic studies are needed to explore the relationship between gene expression and pathway-specific injury and explore why some genes apparently exhibit neutral (hTBM, HLA-E) or inconclusive (CD39) effects, GalTKO, hCD46, HO-1, hCD55, and hEPCR modifications were associated with significant lung xenograft protection. This analysis supports the hypothesis that multiple genetic modifications targeting different known mechanisms of xenograft injury will be required to optimize lung xenograft survival.

Xenotransplantation of Cells, Tissues, Organs and the German Research Foundation Transregio Collaborative Research Centre 127

Human organ transplantation is the therapy of choice for end-stage organ failure. However, the demand for organs far exceeds the donation rate, and many patients die while waiting for a donor. Clinical xenotransplantation using discordant species, particularly pigs, offers a possible solution to this critical shortfall. Xenotransplantation can also increase the availability of cells, such as neurons, and tissues such as cornea, insulin producing pancreatic islets and heart valves. However, the immunological barriers and biochemical disparities between pigs and primates (human) lead to rejection reactions despite the use of common immunosuppressive drugs. These result in graft vessel destruction, haemorrhage, oedema, thrombus formation, and transplant loss. Our consortium is pursuing a broad range of strategies to overcome these obstacles. These include genetic modification of the donor animals to knock out genes responsible for xenoreactive surface epitopes and to express multiple xenoprotective molecules such as the human complement regulators CD46, 55, 59, thrombomodulin and others. We are using (new) drugs including complement inhibitors (e.g. to inhibit C3 binding), anti-CD20, 40, 40L, and also employing physical protection methods such as macro-encapsulation of pancreatic islets. Regarding safety, a major objective is to assure that possible infections are not transmitted to recipients. While the aims are ambitious, recent successes in preclinical studies suggest that xenotransplantation is soon to become a clinical reality.

Current progress in xenotransplantation and organ bioengineering

Organ transplantation represents a unique method of treatment to cure people with end-stage organ failure. Since the first successful organ transplant in 1954, the field of transplantation has made great strides forward. However, despite the ability to transform and save lives, transplant surgery is still faced with a fundamental problem the number of people requiring organ transplants is simply higher than the number of organs available. To put this in stark perspective, because of this critical organ shortage 18 people every day in the United States alone die on a transplant waiting list (U.S. Department of Health & Human Services, http://organdonor.gov/about/data.html). To address this problem, attempts have been made to increase the organ supply through xenotransplantation and more recently, bioengineering. Here we trace the development of both fields, discuss their current status and highlight limitations going forward. Ultimately, lessons learned in each field may prove widely applicable and lead to the successful development of xenografts, bioengineered constructs, and bioengineered xeno-organs, thereby increasing the supply of organs for transplantation.

Immune modulation in xenotransplantation

The use of animals as donors of tissues and organs for xenotransplantations may help in meeting the increasing demand for organs for human transplantations. Clinical studies indicate that the domestic pig best satisfies the criteria of organ suitability for xenotransplantation. However, the considerable phylogenetic distance between humans and the pig causes tremendous immunological problems after transplantation, thus genetic modifications need to be introduced to the porcine genome, with the aim of reducing xenotransplant immunogenicity. Advances in genetic engineering have facilitated the incorporation of human genes regulating the complement into the porcine genome, knockout of the gene encoding the formation of the Gal antigen (α1,3-galactosyltransferase) or modification of surface proteins in donor cells. The next step is two-fold. Firstly, to inhibit processes of cell-mediated xenograft rejection, involving natural killer cells and macrophages. Secondly, to inhibit rejection caused by the incompatibility of proteins participating in the regulation of the coagulation system, which leads to a disruption of the equilibrium in pro- and anti-coagulant activity. Only a simultaneous incorporation of several gene constructs will make it possible to produce multitransgenic animals whose organs, when transplanted to human recipients, would be resistant to hyperacute and delayed xenograft rejection.

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.

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.

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.

Sustained function of genetically modified porcine lungs in an ex vivo model of pulmonary xenotransplantation

BACKGROUND

Xenotransplantation could provide a solution to the donor shortage that is currently the major barrier to solid-organ transplantation. The ability to breed pigs with multiple genetic modifications provides a unique opportunity to explore the immunologic challenges of pulmonary xenotransplantation.

METHODS

Explanted lungs from wild-type and 3 groups of genetically modified pigs were studied: (i) α1,3-galactosyltransferase gene knockout (GTKO); (ii) GTKO pigs expressing the human complementary regulatory proteins CD55 and CD59 (GTKO/CD55-59); and (iii) GTKO pigs expressing both CD55-59 and CD39 (GTKO/CD55-59/CD39). The physiologic, immunologic and histologic properties of porcine lungs were evaluated on an ex vivo rig after perfusion with human blood.

RESULTS

Lungs from genetically modified pigs demonstrated stable pulmonary vascular resistance and better oxygenation of the perfusate, and survived longer than wild-type lungs. Physiologic function was inversely correlated with the degree of platelet sequestration into the xenograft. Despite superior physiologic profiles, lungs from genetically modified pigs still showed evidence of intravascular thrombosis and coagulopathy after perfusion with human blood.

CONCLUSIONS

The ability to breed pigs with multiple genetic modifications, and to evaluate lung physiology and histology in real-time on an ex vivo rig, represent significant advances toward better understanding the challenges inherent to pulmonary xenotransplantation.

Porcine sialoadhesin: a newly identified xenogeneic innate immune receptor

Extracorporeal porcine liver perfusion is being developed as a bridge to liver allotransplantation for patients with fulminant hepatic failure. This strategy is limited by porcine Kupffer cell destruction of human erythrocytes, mediated by lectin binding of a sialic acid motif in the absence of antibody and complement. Sialoadhesin, a macrophage restricted lectin that binds sialic acid, was originally described as a sheep erythrocyte binding receptor. Given similarities between sialoadhesin and the unidentified macrophage lectin in our model, we hypothesized porcine sialoadhesin contributed to recognition of human erythrocytes. Two additional types of macrophages were identified to bind human erythrocytes-spleen and alveolar. Expression of sialoadhesin was confirmed by immunofluorescence in porcine tissues and by flow cytometry on primary macrophages. A stable transgenic cell line expressing porcine sialoadhesin (pSn CHO) bound human erythrocytes, while a sialoadhesin mutant cell line did not. Porcine macrophage and pSn CHO recognition of human erythrocytes was inhibited approximately 90% by an antiporcine sialoadhesin monoclonal antibody and by human erythrocyte glycoproteins. Furthermore, this binding was substantially reduced by sialidase treatment of erythrocytes. These data support the hypothesis that porcine sialoadhesin is a xenogeneic receptor that mediates porcine macrophage binding of human erythrocytes in a sialic acid-dependent manner.

Immunobiology of liver xenotransplantation

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

Potential factors influencing the development of thrombocytopenia and consumptive coagulopathy after genetically modified pig liver xenotransplantation

Upregulation of tissue factor (TF) expression on activated donor endothelial cells (ECs) triggered by the immune response (IR) has been considered the main initiator of consumptive coagulopathy (CC). In this study, we aimed to identify potential factors in the development of thrombocytopenia and CC after genetically engineered pig liver transplantation in baboons. Baboons received a liver from either an α1,3-galactosyltransferase gene-knockout (GTKO) pig (n = 1) or a GTKO pig transgenic for CD46 (n = 5) with immunosuppressive therapy. TF exposure on recipient platelets and peripheral blood mononuclear cell (PBMCs), activation of donor ECs, platelet and EC microparticles, and the IR were monitored. Profound thrombocytopenia and thrombin formation occurred within minutes of liver reperfusion. Within 2 h, circulating platelets and PBMCs expressed functional TF, with evidence of aggregation in the graft. Porcine ECs were negative for expression of P- and E-selectin, CD106, and TF. The measurable IR was minimal, and the severity and rapidity of thrombocytopenia were not alleviated by prior manipulation of the IR. We suggest that the development of thrombocytopenia/CC may be associated with TF exposure on recipient platelets and PBMCs (but possibly not with activation of donor ECs). Recipient TF appears to initiate thrombocytopenia/CC by a mechanism that may be independent of the IR.

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.

Infection barriers to successful xenotransplantation focusing on porcine endogenous retroviruses

Xenotransplantation may be a solution to overcome the shortage of organs for the treatment of patients with organ failure, but it may be associated with the transmission of porcine microorganisms and the development of xenozoonoses. Whereas most microorganisms may be eliminated by pathogen-free breeding of the donor animals, porcine endogenous retroviruses (PERVs) cannot be eliminated, since these are integrated into the genomes of all pigs. Human-tropic PERV-A and -B are present in all pigs and are able to infect human cells. Infection of ecotropic PERV-C is limited to pig cells. PERVs may adapt to host cells by varying the number of LTR-binding transcription factor binding sites. Like all retroviruses, they may induce tumors and/or immunodeficiencies. To date, all experimental, preclinical, and clinical xenotransplantations using pig cells, tissues, and organs have not shown transmission of PERV. Highly sensitive and specific methods have been developed to analyze the PERV status of donor pigs and to monitor recipients for PERV infection. Strategies have been developed to prevent PERV transmission, including selection of PERV-C-negative, low-producer pigs, generation of an effective vaccine, selection of effective antiretrovirals, and generation of animals transgenic for a PERV-specific short hairpin RNA inhibiting PERV expression by RNA interference.

Clinical xenotransplantation: the next medical revolution?

The shortage of organs and cells from deceased individuals continues to restrict allotransplantation. Pigs could provide an alternative source of tissue and cells but the immunological challenges and other barriers associated with xenotransplantation need to be overcome. Transplantation of organs from genetically modified pigs into non-human primates is now not substantially limited by hyperacute, acute antibody-mediated, or cellular rejection, but other issues have become more prominent, such as development of thrombotic microangiopathy in the graft or systemic consumptive coagulopathy in the recipient. To address these problems, pigs that express one or more human thromboregulatory or anti-inflammatory genes are being developed. The results of preclinical transplantation of pig cells--eg, islets, neuronal cells, hepatocytes, or corneas--are much more encouraging than they are for organ transplantation, with survival times greater than 1 year in all cases. Risk of transfer of an infectious microorganism to the recipient is small.

Pulmonary intravascular macrophages and lung health: what are we missing?

Pulmonary intravascular macrophages (PIMs) are constitutively found in species such as cattle, horse, pig, sheep, goat, cats, and whales and can be induced in species such as rats, which normally lack them. It is believed that human lung lacks PIMs, but there are previous suggestions of their induction in patients suffering from liver dysfunction. Recent data show induction of PIMs in bile-duct ligated rats and humans suffering from hepato-pulmonary syndrome. Because constitutive and induced PIMs are pro-inflammatory in response to endotoxins and bacteria, there is a need to study their biology in inflammatory lung diseases such as sepsis, asthma, chronic obstructive pulmonary diseases, or hepato-pulmonary syndrome. We provide a review of PIM biology to make an argument for increased emphasis and better focus on the study of human PIMs to better understand their potential role in the pathophysiology and mechanisms of pulmonary diseases.

Xenogeneic lung transplantation models

Study of lung xenografts has proven useful to understand the remaining barriers to successful transplantation of other organ xenografts. In this chapter, the history and current status of lung xenotransplantation are briefly reviewed and two different experimental models, the ex vivo porcine-to-human lung perfusion and the in vivo xenogeneic lung transplantation, are presented. We focus on the technical details of these lung xenograft models in sufficient detail, list the needed materials, and mention analysis techniques to allow others to adopt them with minimal learning curve.

A brief history of cross-species organ transplantation

Cross-species transplantation (xenotransplantation) offers the prospect of an unlimited supply of organs and cells for clinical transplantation, thus resolving the critical shortage of human tissues that currently prohibits a majority of patients on the waiting list from receiving transplants. Between the 17th and 20th centuries, blood was transfused from various animal species into patients with a variety of pathological conditions. Skin grafts were carried out in the 19th century from a variety of animals, with frogs being the most popular. In the 1920s, Voronoff advocated the transplantation of slices of chimpanzee testis into aged men whose "zest for life" was deteriorating, believing that the hormones produced by the testis would rejuvenate his patients. Following the pioneering surgical work of Carrel, who developed the technique of blood vessel anastomosis, numerous attempts at nonhuman primate organ transplantation in patients were carried out in the 20th century. In 1963-1964, when human organs were not available and chronic dialysis was not yet in use, Reemtsma transplanted chimpanzee kidneys into 13 patients, one of whom returned to work for almost 9 months before suddenly dying from what was believed to be an electrolyte disturbance. The first heart transplant in a human ever performed was by Hardy in 1964, using a chimpanzee heart, but the patient died within 2 hours. Starzl carried out the first chimpanzee-to-human liver transplantation in 1966; in 1992, he obtained patient survival for 70 days following a baboon liver transplant. With the advent of genetic engineering and cloning technologies, pigs are currently available with a number of different manipulations that protect their tissues from the human immune response, resulting in increasing pig graft survival in nonhuman primate models. Genetically modified pigs offer hope of a limitless supply of organs and cells for those in need of a transplant.

Which anti-platelet therapies might be beneficial in xenotransplantation?

Xenotransplantation could provide an unlimited and elective supply of grafts, once mechanisms of graft loss and vascular injury are better understood. The development of α-1,3-galactosyltransferase gene-knockout (GalT-KO) swine with the removal of a dominant xeno-antigen has been an important advance; however, delayed xenograft and acute vascular reaction in GalT-KO animals persist. These occur, at least in part, because of humoral reactions that result in vascular injury. Intrinsic molecular incompatibilities in the regulation of blood clotting and extracellular nucleotide homeostasis between discordant species may also predispose to thrombophilia within the vasculature of xenografts. Although limited benefits have been achieved with currently available pharmacological anti-thrombotics and anti-coagulants, the highly complex mechanisms of platelet activation and thrombosis in xenograft rejection also require potent immunosuppressive interventions. We will focus on recent thromboregulatory approaches while elucidating appropriate anti-platelet mechanisms. We will discuss potential benefits of additional anti-thrombotic interventions that are possible in transgenic swine and review recent developments in pharmacological anti-platelet therapy.

Coagulopathy in α-galactosyl transferase knockout pulmonary xenotransplants

BACKGROUND

After substantial progress on many fronts, one of the remaining barriers still opposing the clinical application of xenotransplantation is a disseminated intravascular coagulopathy (DIC) that is observed in the pre-clinical model of porcine-to-primate transplantation. The onset of DIC is particularly rapid in recipients of pulmonary xenografts, usually occurring within the first days or even hours of reperfusion.

METHODS

In this study, we describe the results of two porcine-to-baboon transplants utilizing porcine lungs depleted of macrophages, deficient in the α-1,3-galactosyltransferase gene, and with the expression of human decay-accelerating factor, a complement regulatory protein.

RESULTS

In both cases, evidence of DIC was observed within 48 h of reperfusion, with thrombocytopenia and increases in levels of thrombin-antithrombin complex evident in both cases. Depletion of fibrinogen was observed in one graft, whereas elevation of D-dimer levels was observed in the other. One graft, which showed focal lymphocytic infiltrates pre-operatively, failed within 3 h.

CONCLUSIONS

The results indicate that further efforts to address the coagulopathy associated with pulmonary xenotransplantation are needed. Further, evidence suggests that resident porcine immune cells can play an important role in the coagulopathy associated with xenotransplantation.

Recipient tissue factor expression is associated with consumptive coagulopathy in pig-to-primate kidney xenotransplantation

Consumptive coagulopathy (CC) remains a challenge in pig-to-primate organ xenotransplantation (Tx). This study investigated the role of tissue factor (TF) expression on circulating platelets and peripheral blood mononuclear cells (PBMCs). Baboons (n = 9) received a kidney graft from pigs that were either wild-type (n = 2), alpha1,3-galactosyltransferase gene-knockout (GT-KO; n = 1) or GT-KO and transgenic for the complement-regulatory protein, CD46 (GT-KO/CD46, n = 6). In the baboon where the graft developed hyperacute rejection (n = 1), the platelets and PBMCs expressed TF within 4 h of Tx. In the remaining baboons, TF was detected on platelets on post-Tx day 1. Subsequently, platelet-leukocyte aggregation developed with formation of thrombin. In the six baboons with CC, TF was not detected on baboon PBMCs until CC was beginning to develop. Graft histopathology showed fibrin deposition and platelet aggregation (n = 6), but with only minor or no features indicating a humoral immune response (n = 3), and no macrophage, B or T cell infiltration (n = 6). Activation of platelets to express TF was associated with the initiation of CC, whereas TF expression on PBMCs was concomitant with the onset of CC, often in the relative absence of features of acute humoral xenograft rejection. Prevention of recipient platelet activation may be crucial for successful pig-to-primate kidney Tx.

Aurintricarboxylic acid inhibits endothelial activation, complement activation, and von Willebrand factor secretion in vitro and attenuates hyperacute rejection in an ex vivo model of pig-to-human pulmonary xenotransplantation

BACKGROUND

In the xenotransplantation of vascularized organs, such as the lung, a large area of endothelial cell layer is a big hurdle to be overcome. We investigated the potential protective effect of aurintricarboxylic acid (ATA), a known inhibitor of platelet adhesion, on endothelial damage induced by xenogeneic serum. We also assessed its role in hyperacute xenograft rejection using a porcine ex vivo lung perfusion model.

METHODS

Porcine endothelial cells were incubated with human serum and other inflammatory stimuli. For the evaluation of von Willebrand factor (vWF) secretion and tissue factor (TF) expression, we used human endothelial cells. E-selectin expression, complement activation, TF expression and platelet activation were investigated by flow cytometry. In an ex vivo porcine lung perfusion model, the porcine lungs were perfused with fresh human whole blood: unmodified blood (n = 5), ATA-treated blood (n = 5), and ATA and lepirudin-treated blood (n = 5).

RESULTS

Aurintricarboxylic acid significantly inhibited TNF-alpha- or lipopolysaccharide-induced endothelial E-selectin expression in a dose-dependent manner. ATA also prevented human serum induced-E-selectin expression and human monocytic cell adhesion to porcine endothelial cells. Moreover, ATA abolished thrombin-induced vWF secretion as well as complement activation. However, ATA induced endothelial TF expression and platelet activation in vitro. In ex-vivo experiments, ATA treatment improved pulmonary function and attenuated sequestration of leukocytes. Although ATA did not influence thrombin generation, we were able to minimize its activity by adding lepirudin to the blood with ATA.

CONCLUSIONS

Our study demonstrated in vitro protective effect of ATA on the inhibition of endothelial activation and vWF secretion and confirmed detrimental effect of ATA on induction of endothelial TF and platelet activation. The combination of ATA and lepirudin may act beneficially by preventing coagulation perturbation while maintaining improved xenograft survival.