The vascular and coagulation issues in 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.

Glycocalyx dynamics and the inflammatory response of genetically modified porcine endothelial cells

Xenotransplantation is a promising approach to reduce organ shortage, while genetic modification of donor pigs has significantly decreased the immunogenic burden of xenotransplants, organ rejection is still a hurdle. Genetically modified pig organs are used in xenotransplantation research, and the first clinical pig-to-human heart transplantation was performed in 2022. However, the impact of genetic modification has not been investigated on a cellular level yet. Endothelial cells (EC) and their sugar-rich surface known as the glycocalyx are the first barrier encountering the recipient's immune system, making them a target for rejection. We have previously shown that wild type venous but not arterial EC were protected against heparan sulfate (HS) shedding after activation with human serum or human tumor necrosis factor alpha (TNF𝛼). Using a 2D microfluidic system we investigated the glycocalyx dynamics of genetically modified porcine arterial and venous EC (Gal𝛼1,3 Gal knock-out, transgenic for human CD46 and thrombomodulin, GTKO/hCD46/hTM) after activation with human serum or human TNF𝛼. Interestingly, we observed that GTKO/hCD46/hTM arterial cells, additionally to venous cells, do not shed HS. Unscathed HS on GTKO/hCD46/hTM EC correlated with reduced complement deposition, suggesting that protection against complement activation contributes to maintaining an intact glycocalyx layer on arterial EC. This protection was lost on GTKO/hCD46/hTM cells after simultaneous perfusion with human serum and human TNF𝛼. HS shedding on arterial cells and increased complement deposition on both arterial and venous cells was observed. These findings suggest that GTKO/hCD46/hTM EC revert to a proinflammatory phenotype in an inflammatory xenotransplantation setting, potentially favoring transplant rejection.

Expression of human thrombomodulin on porcine endothelial cells can reduce platelet aggregation but did not reduce activation of complement or endothelium - an experimental study

Clinical xenotransplantation will only be feasible when present limitations can be controlled sufficiently. Activation of endothelium and complement as well as coagulopathy and thrombotic microangiopathy (TMA) is important barriers. Transgenic expression of hTBM on porcine endothelial cells is a reasonable approach to reduce activation of haemostasis. Endothelial cells from wild-type pigs as well from pigs expressing hTBM alone or in combination with hCD46 and knockout of the alpha-1,3,-galactosyltransferase (GTKO) were perfused with platelet-rich plasma in a microfluidic flow chamber. Platelet aggregation and activation, coagulation, complement and endothelial cell activation were assessed. Perfusion of wild-type porcine aortic endothelial cells (PAEC) resulted in distinct platelet aggregation. Expression of hTBM in either mono-transgenic or triple-transgenic (GTKO/hCD46/hTBM) PAEC showed significantly reduced or absent platelet aggregation. Flow cytometric analysis of platelets showed an increased CD62P expression in wild-type PAEC and significantly reduced expression in mono- or triple-transgenic PAEC. Activation of coagulation measured by TAT occured in WT PAEC and was clearly reduced in hTBM and GTKO/hCD46/hTBM PAEC. Activation of complement and endothelial cells was only reduced in GTKO/hCD46/hTBM but not in PAEC expressing hTBM alone. Expression of hTBM was able to prevent activation of coagulation and platelet aggregation in mono- and triple-transgenic PAEC, while activation of complement and endothelial cells was not reduced in mono-transgenic PAEC.

A potential role of TLR2 in xenograft rejection of porcine iliac endothelial cells: An in vitro study

BACKGROUND

Porcine vascular endothelial cells are a major participant in xenograft rejection. The Toll-like receptor 2 (TLR2) pathway plays an important role in both innate and adaptive immunity. The specific role of TLR2 in the response to a xenograft has not been reported. Whether the TLR2 pathway in pig vascular endothelial cells is involved in acute rejection needs to be investigated, and the mechanism is explored.

METHODS

We used a modified antibody-dependent complement-mediated cytotoxicity (ADCC) assay to conduct in vitro experiments. In porcine iliac artery endothelial cells (PIECs), siRNA was used to knock down the expression of TLR2, CXCL8, and CCL2. The effect of human serum or inactivated human serum on the expression of TLR2 was analyzed by real-time PCR and Western blotting, and transwell assays were used to assess the chemotactic efficiency of PIECs on human monocyte-macrophages (THP-1 cells) and human neutrophils. The downstream signaling pathways activated by human serum were detected by Western blotting, and the regulation of proinflammatory chemokines and cytokines by TLR2 signaling was assessed by real-time PCR and ELISA.

RESULTS

TLR2 was significantly upregulated in PIECs after exposure to human serum, and porcine proinflammatory chemokines, CXCL8 and CCL2, were induced, at least partially, in a TLR2-dependent pattern; the upregulated chemokines participated in the chemotaxis of human neutrophils and THP-1 cells across the species barrier.

CONCLUSIONS

(i) TLR2 is significantly upregulated in PIECs by human serum, (ii) the elevated TLR2 participates in the chemotaxis of inflammatory cells through the secretion of chemokine CCL2 and CXCL8, and (iii) blockade of TLR2 would be beneficial for xenograft survival.

Investigation of the influence of xenoreactive antibodies on activation of complement and coagulation in an ex vivo perfusion animal study using porcine kidneys

During pig-to-primate xenotransplantation or perfusion of porcine organs with human blood, a xenogeneic coagulopathy with consecutive development of thrombotic microangiopathy (TMA) can be observed. The aim of this study was to elucidate the influence of the reduction of xenoreactive natural antibodies on the coagulopathy using an ex vivo perfusion system. Thirteen perfusion experiments using landrace wild-type porcine kidneys were performed in three different experimental groups: autologous, xenogeneic, and immunoadsorption. During and after perfusion, blood and tissue samples were collected to assess markers of coagulation, complement, inflammation, and endothelial activation. Immunoadsorption prior to perfusion did not prolong perfusion time (174 min ±28) compared to xenogeneic (182 min ±22) experiments, whereas autologous perfusion was possible for maximum of 240 min in all experiments. Activation of coagulation was similar comparing perfusions after immunoadsorption (D-Dimer 24 186 μg/l ±5813; TAT 566 μg/l ±34) to xenogeneic (D-Dimer 22 175 μg/l ±7826, TAT 600 μg/l ±0) experiments. But antibody-mediated complement activation was reduced in the immunoadsorption group. TNF-alpha and markers of endothelial cell activation were lower in the immunoadsorption group compared to the xenogeneic experiments. In this ex vivo perfusion model, we observed that marked removal of xenogeneic antibodies can reduce complement activation via the classical pathway as well as endothelial cell activation and inflammation. Immunoadsorption cannot prevent the activation of the terminal complement cascade and coagulation.

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.

Effect of TNF-alpha blockade on coagulopathy and endothelial cell activation in xenoperfused porcine kidneys

BACKGROUND

Following pig-to-primate kidney transplantation, endothelial cell activation and xenogenic activation of the recipient's coagulation eventually leading to organ dysfunction and microthrombosis can be observed. In this study, we examined the effect of a TNF-receptor fusion protein (TNF-RFP) on endothelial cell activation and coagulopathy utilizing an appropriate ex vivo perfusion system.

METHODS

Using an ex vivo perfusion circuit based on C1-Inhibitor (C1-Inh) and low-dose heparin administration, we have analyzed consumptive coagulopathy following contact of human blood with porcine endothelium. Porcine kidneys were recovered following in situ cold perfusion with Histidine-tryptophan-ketoglutarate (HTK) organ preservation solution and were immediately connected to a perfusion circuit utilizing freshly drawn pooled porcine or human AB blood. The experiments were performed in three individual groups: autologous perfusion (n = 5), xenogenic perfusion without any further pharmacological intervention (n = 10), or with addition of TNF-RFP (n = 5). After perfusion, tissue samples were obtained for real-time PCR and immunohistological analyses. Endothelial cell activation was assessed by measuring the expression levels of E-selectin, ICAM-1, and VCAM-1.

RESULTS

Kidney survival during organ perfusion with human blood, C1-Inh, and heparin, but without any further pharmacological intervention was 126 ± 78 min. Coagulopathy was observed with significantly elevated concentrations of D-dimer and thrombin-antithrombin complex (TAT), resulting in the formation of multiple microthrombi. Endothelial cell activation was pronounced, as shown by increased expression of E-selectin and VCAM-1. In contrast, pharmacological intervention with TNF-RFP prolonged organ survival to 240 ± 0 min (max. perfusion time; no difference to autologous control). Formation of microthrombi was slightly reduced, although not significantly, if compared to the xenogenic control. D-dimer and TAT were elevated at similar levels to the xenogenic control experiments. In contrast, endothelial cell activation, as shown by real-time PCR, was significantly reduced in the TNF-RFP group.

CONCLUSION

We conclude that although coagulopathy was not affected, TNF-RFP is able to suppress inflammation occurring after xenoperfusion in this ex vivo perfusion model.

Initial in vitro studies on tissues and cells from GTKO/CD46/NeuGcKO pigs

BACKGROUND

The impact that the absence of expression of NeuGc in pigs might have on pig organ or cell transplantation in humans has been studied in vitro, but only using red blood cells (pRBCs) and peripheral blood mononuclear cells (pPBMCs) as the target cells for immune assays. We have extended this work in various in vitro models and now report our initial results.

METHODS

The models we have used involve GTKO/hCD46 and GTKO/hCD46/NeuGcKO pig aortas and corneas, and pRBCs, pPBMCs, aortic endothelial cells (pAECs), corneal endothelial cells (pCECs), and isolated pancreatic islets. We have investigated the effect of the absence of NeuGc expression on (i) human IgM and IgG binding, (ii) the T-cell proliferative response, (iii) human platelet aggregation, and (iv) in an in vitro assay of the instant blood-mediated inflammatory reaction (IBMIR) following exposure of pig islets to human blood/serum.

RESULTS

The lack of expression of NeuGc on some pig tissues (aortas, corneas) and cells (RBCs, PBMCs, AECs) significantly reduces the extent of human antibody binding. In contrast, the absence of NeuGc expression on some pig tissues (CECs, isolated islet cells) does not reduce human antibody binding, possibly due to their relatively low NeuGc expression level. The strength of the human T-cell proliferative response may also be marginally reduced, but is already weak to GTKO/hCD46 pAECs and islet cells. We also demonstrate that the absence of NeuGc expression on GTKO/hCD46 pAECs does not reduce human platelet aggregation, and nor does it significantly modify the IBMIR to pig islets.

CONCLUSION

The absence of NeuGc on some solid organs from GTKO/hCD46/NeuGcKO pigs should reduce the human antibody response after clinical transplantation when compared to GTKO/hCD46 pig organs. However, the clinical benefit of using certain tissue (e.g., cornea, islets) from GTKO/hCD46/NeuGcKO pigs is questionable.

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

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

The role of genetically engineered pigs in xenotransplantation research

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

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.

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.

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.

Expression of human CD46 modulates inflammation associated with GalTKO lung xenograft injury

Evaluation of lungs from GalTKO.hCD46 pigs, genetically modified to lack the galactose-α(1,3)-galactose epitope (GalTKO) and to express human CD46, a complement regulatory protein, has not previously been described. Physiologic, hematologic and biochemical parameters during perfusion with heparinized fresh human blood were measured for 33 GalTKO.hCD46, GalTKO (n = 16), and WT pig lungs (n = 16), and 12 pig lungs perfused with autologous pig blood. Median GalTKO.hCD46 lung survival was 171 min compared to 120 for GalTKO (p = 0.27) and 10 for WT lungs (p < 0.001). Complement activation, platelet activation and histamine elaboration were significantly reduced during the first 2 h of perfusion in GalTKO.hCD46 lungs compared to GalTKO (ΔC3a at 120' 812 ± 230 vs. 1412 ± 1047, p = 0.02; ΔCD62P at 120' 9.8 ± 7.2 vs. 25.4 ± 18.2, p < 0.01; Δhistamine at 60' 97 ± 62 vs. 189 ± 194, p = 0.03). We conclude that, in addition to significant down-modulation of complement activation, hCD46 expression in GalTKO lungs diminished platelet and coagulation cascade activation, neutrophil sequestration and histamine release. Because GalTKO.hCD46 lung failure kinetics correlated directly with platelet and neutrophil sequestration, coagulation cascade activation and a rise in histamine levels within the first hour of perfusion, further progress will likely depend upon improved control of these pathways, by rationally targeted additional modifications to pigs and pharmacologic interventions.

Complement dependent early immunological responses during ex vivo xenoperfusion of hCD46/HLA-E double transgenic pig forelimbs with human blood

BACKGROUND

Besides α1,3-galactosyltransferase gene (GGTA1) knockout, several transgene combinations to prevent pig-to-human xenograft rejection are currently being investigated. In this study, the potential of combined overexpression of human CD46 and HLA-E to prevent complement- and NK-cell-mediated xenograft rejection was tested in an ex vivo pig-to-human xenoperfusion model.

METHODS

α1,3-Galactosyltransferase knockout heterozygous, hCD46/HLA-E double transgenic (transgenic) as well as wild-type pig forelimbs were ex vivo perfused with whole, heparinized human and autologous pig blood, respectively. Blood samples were analyzed for the production of porcine and/or human inflammatory cytokines as well as complement activation products. Biopsy samples were examined for deposition of human and porcine C3b/c, C4b/c, and C6 as well as CD62E (E-selectin) and CD106 (VCAM-1) expression. Apoptosis was measured in the porcine muscle tissue using TUNEL assays. Finally, the formation of thrombin-antithrombin (TAT) complexes was measured in EDTA plasma samples.

RESULTS

No hyperacute rejection was seen in this model. Extremity perfusions lasted for up to 12 h without increase in vascular resistance and were terminated due to continuous small blood losses. Plasma levels of porcine cytokines IL1β, IL-6, IL-8, IL-10, TNF-α, and MCP-1 as well as human complement activation markers C3a (P = 0.0002), C5a (P = 0.004), and soluble C5b-9 (P = 0.03) were lower in blood perfused through transgenic as compared to wild-type limbs. Human C3b/c, C4b/c, and C6 as well as CD62E and CD106 were deposited in tissue of wild-type limbs, but significantly lower levels (P < 0.0001) of C3b/c, C4b/c, and C6 deposition as well as CD62E and CD106 expression were detected in transgenic limbs perfused with human blood. Transgenic porcine tissue was protected from xenoperfusion-induced apoptosis (P < 0.0001). Finally, TAT levels were significantly lower (P < 0.0001) in transgenic limb as compared to wild-type limb xenoperfusions.

CONCLUSION

Transgenic hCD46/HLA-E expression clearly reduced humoral xenoresponses since all, the terminal pathway of complement activation, endothelial cell activation, muscle cell apoptosis, inflammatory cytokine production, as well as coagulation activation, were all downregulated. Overall, this model represents a useful tool to study early immunological responses during pig-to-human vascularized xenotransplantation in the absence of hyperacute rejection.

Regulatory sequences of the porcine THBD gene facilitate endothelial-specific expression of bioactive human thrombomodulin in single- and multitransgenic pigs

BACKGROUND

Among other mismatches between human and pig, incompatibilities in the blood coagulation systems hamper the xenotransplantation of vascularized organs. The provision of the porcine endothelium with human thrombomodulin (hTM) is hypothesized to overcome the impaired activation of protein C by a heterodimer consisting of human thrombin and porcine TM.

METHODS

We evaluated regulatory regions of the THBD gene, optimized vectors for transgene expression, and generated hTM expressing pigs by somatic cell nuclear transfer. Genetically modified pigs were characterized at the molecular, cellular, histological, and physiological levels.

RESULTS

A 7.6-kb fragment containing the entire upstream region of the porcine THBD gene was found to drive a high expression in a porcine endothelial cell line and was therefore used to control hTM expression in transgenic pigs. The abundance of hTM was restricted to the endothelium, according to the predicted pattern, and the transgene expression of hTM was stably inherited to the offspring. When endothelial cells from pigs carrying the hTM transgene--either alone or in combination with an aGalTKO and a transgene encoding the human CD46-were tested in a coagulation assay with human whole blood, the clotting time was increased three- to four-fold (P<0.001) compared to wild-type and aGalTKO/CD46 transgenic endothelial cells. This, for the first time, demonstrated the anticoagulant properties of hTM on porcine endothelial cells in a human whole blood assay.

CONCLUSIONS

The biological efficacy of hTM suggests that the (multi-)transgenic donor pigs described here have the potential to overcome coagulation incompatibilities in pig-to-primate xenotransplantation.

Pig-to-baboon liver xenoperfusion utilizing GalTKO.hCD46 pigs and glycoprotein Ib blockade

BACKGROUND

Although transplantation of genetically modified porcine livers into baboons has yielded recipient survival for up to 7 days, survival is limited by profound thrombocytopenia, which becomes manifest almost immediately after revascularization, and by subsequent coagulopathy. Porcine von Willebrand's factor (VWF), a glycoprotein that adheres to activated platelets to initiate thrombus formation, has been shown to constitutively activate human platelets via their glycoprotein Ib (GPIb) receptors. Here, we report our pig-to-primate liver xenoperfusion model and evaluate whether targeting the GPIb-VWF axis prevents platelet sequestration.

METHODS

Twelve baboons underwent cross-circulation with the following extracorporeal livers: one allogeneic control with a baboon liver, 4 xenogeneic controls with a GalTKO.hCD46 pig liver, 3 GalTKO.hCD46 pig livers in recipients treated with αGPIb antibody during perfusion, and 4 GalTKO.hCD46 pig livers pre-treated with D-arginine vasopressin (DDAVP) in recipients treated with αGPIb antibody during perfusion.

RESULTS

All perfused livers appeared grossly and macroscopically normal and produced bile. Xenograft liver perfusion experiments treated with αGPIb antibody may show less platelet sequestration during the initial 2 h of perfusion. Portal venous resistance remained constant in all perfusion experiments. Platelet activation studies demonstrated platelet activation in all xenoperfusions, but not in the allogeneic perfusion.

CONCLUSION

These observations suggest that primate platelet sequestration by porcine liver and the associated thrombocytopenia are multifactorial and perhaps partially mediated by a constitutive interaction between porcine VWF and the primate GPIb receptor. Control of platelet sequestration and consumptive coagulopathy in liver xenotransplantation will likely require a multifaceted approach in our clinically relevant perfusion model.

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.

Minimal effect of bortezomib in reducing anti-pig antibodies in human leukocyte antigen-sensitized patients: a pilot study

BACKGROUND

Bortezomib, a proteasome inhibitor used to treat multiple myeloma, has been administered (± plasma exchange ± intravenous immunoglobulin [IVIg]) in attempts to reduce antibodies against human leukocyte antigens (HLA) in sensitized patients undergoing organ transplantation. To our knowledge, bortezomib has not been investigated for its effect on natural anti-pig antibodies. If bortezomib could reduce the production of anti-pig antibodies, this would likely be beneficial to the outcome of pig organ grafts in primates.

METHODS

Nine patients received bortezomib either to reduce anti-HLA antibody levels before organ allotransplantation or to treat antibody-mediated rejection. Patients at the Mayo Clinic (Group 1; n = 4) received bortezomib alone, whereas at the UPMC (Group 2; n = 5), this was combined with plasmaphereses ± IVIg in some cases. Anti-pig IgM and IgG levels against wild-type (WT) and α1,3-galactosyltransferase gene knockout (GTKO) pig aortic endothelial cells (flow cytometry-relative mean fluorescence intensity) and anti-Gal IgM and IgG (ELISA-OD480 nm ) were measured pre- and post-bortezomib therapy.

RESULTS

Mean anti-pig IgM levels were 11.2 (WT) and 1.9 (GTKO) pre-bortezomib treatment and 9.4 (WT: P = 0.02) and 1.7 (GTKO: P = 0.33) post-bortezomib treatment, respectively. Mean anti-pig IgG levels were 4.3 (WT) and 1.5 (GTKO) pre-bortezomib treatment and 3.6 (WT: P = 0.21) and 1.4 (GTKO: P = 0.20) post-bortezomib treatment, respectively. Mean anti-Gal IgM and IgG levels were 0.7 and 1.1, respectively, pre-treatment, and 0.6 (P = 0.03) and 1.1 (NS), respectively, post-treatment. When the data were analyzed in Groups 1 and 2 separately, there were no significant differences between the pre- and post-bortezomib levels of anti-pig, anti-non-Gal, or anti-Gal IgM or IgG.

CONCLUSIONS

From this limited study, we conclude that bortezomib might reduce anti-Gal IgM levels in primates, but, in this respect alone, is unlikely to have any significant effect on the outcome of GTKO pig organ transplantation.

Human CD55 expression blocks hyperacute rejection and restricts complement activation in Gal knockout cardiac xenografts

BACKGROUND

Transgenic expression of human complement regulatory proteins reduces the frequency of hyperacute rejection (HAR) in Gal-positive cardiac xenotransplantation. In this study, we examined the impact of human CD55 (hCD55) expression on a Gal knockout (GTKO) background using pig-to-primate heterotopic cardiac xenotransplantation.

METHODS

Cardiac xenotransplantation was performed with GTKO (group 1; n=6) and GTKO.hCD55 (group 2; n=5) donor pigs using similar immunosuppression. Cardiac biopsies were obtained 30 min after organ reperfusion. Rejection was characterized by histology and immunohistology. Intragraft gene expression, serum non-Gal antibody, and antibody recovered from rejected hearts were analyzed.

RESULTS

HAR of a GTKO heart was observed. Remaining grafts developed delayed xenograft rejection. Median survival was 21 and 28 days for groups 1 and 2, respectively. Vascular antibody deposition was uniformly detected 30 min after organ reperfusion and at explant. A higher frequency of vascular C5b deposition was seen in GTKO organs at explant. Serum non-Gal antibody, antibody recovered from the graft, and intragraft gene expression were similar between the groups.

CONCLUSION

HAR of GTKO hearts without hCD55 may occur. Expression of hCD55 seemed to restrict local complement activation but did not improve graft survival. Chronic vascular antibody deposition with evidence of protracted endothelial cell activation was seen. These observations suggest that non-Gal antibody-induced chronic endothelial cell activation coupled to possible hemostatic incompatibilities may be the primary stimulus for delayed xenograft rejection of GTKO hearts. To avoid possible HAR, future clinical studies should use donors expressing human complement regulatory proteins in the GTKO background.

Cardiac xenotransplantation: progress and challenges

PURPOSE OF REVIEW

Cardiac xenotransplantation (CXTx) remains a promising approach to alleviate the chronic shortage of donor hearts. This review summarizes recent results of heterotopic and orthotopic CXTx, highlights the role of non-Gal antibody in xenograft rejection, and discusses challenges to clinical orthotopic CXTx.

RECENT FINDINGS

Pigs mutated in the α 1,3 galactosyltransferase gene (GTKO pigs) are devoid of the galactose α1,3 galactose (αGal) carbohydrate antigen. This situation effectively eliminates any role for anti-Gal antibody in GTKO cardiac xenograft rejection. Survival of heterotopic GTKO cardiac xenografts in nonhuman primates continues to increase. GTKO graft rejection commonly involves vascular antibody deposition and variable complement deposition. Non-Gal antibody responses to porcine antigens associated with inflammation, complement, and hemostatic regulation and to new carbohydrate antigens have been identified. Their contribution to rejection remains under investigation. Orthotopic CXTx is limited by early perioperative cardiac xenograft dysfunction (PCXD). However, hearts affected by PCXD recover full cardiac function and orthotopic survival up to 2 months without rejection has been reported.

SUMMARY

CXTx remains a promising technology for treating end-stage cardiac failure. Genetic modification of the donor and refinement of immunosuppressive regimens have extended heterotopic cardiac xenograft survival from minutes to in excess of 8 months.

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.

Modifiers of complement activation for prevention of antibody-mediated injury to allografts

PURPOSE OF REVIEW

Improvements in prevention and management of cellular rejection of solid organ transplants, coupled with increasing numbers of sensitized patients, have focused attention on antibody-mediated rejection (AbMR). Complement is a critical component of AbMR, in addition to interfacing between innate and adaptive immunity and the coagulation cascade. This article reviews complement biology and strategies to overcome complement in AbMR, cognisant that antibody can act independently of complement.

RECENT FINDINGS

The past decade has witnessed an improvement in the prevention and treatment of AbMR as a result of solid-phase assays to determine antibody specificity, definition of histopathological criteria, and use of plasmapheresis and/or intravenous immunoglobulin (IVIG). Nonetheless, AbMR continues to impact adversely on short- and long-term graft survival. Use of B and/or T-lymphocyte-depleting therapies has not shown measurable benefit, and the need remains for therapies that deplete antibody, or provide better protection from complement-mediated damage. Disordered complement activity in human diseases such as paroxysmal nocturnal haemoglobinuria, has provided additional impetus to pursuing therapeutic complement inhibition. Preliminary data from C5 inhibition with eculizumab in the treatment and prevention of AbMR have shown promise. Trials with recombinant human inhibitors of C1 (effective in angioedema) to prevent or treat AbMR are beginning.

SUMMARY

Despite current limitations, 'protection' of the transplant through plasmapheresis and/or IVIG enables many allografts to survive in sensitized recipients. Elucidating the pathways mediating graft acceptance, by constitutive antibody deletion, or 'accommodation' (wherein donor organ remains uninjured despite antibody binding), or other local protective mechanism(s), is an equally important challenge in the quest to overcome AbMR.

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

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

Therapeutic issues in the treatment of vascularized xenotransplants using gal-knockout donors in nonhuman primates

PURPOSE OF REVIEW

Solid organ xenotransplantation could be the future of transplantation, but improved outcomes are required in experimental models before clinical trials are justified. This review summarizes recent advances in solid organ xenotransplantation using organs from α1,3-galactosyltransferase gene-knockout (GTKO) pigs (with or without other genetic modifications) and novel therapeutic approaches.

RECENT FINDINGS

Work on the development of genetically engineered pigs has been considerable during the past few years, with many research institutes reporting the outcomes of research. Multiple gene modifications on a GTKO background have been reported, and the results of transplantation using organs from these pigs have been published. Progress, however, has been variable, and several obstacles, for example, coagulation dysregulation, have been identified. Heterotopic pig heart xenotransplantation has been associated with graft survival up to 8 months, but kidney graft survival has not improved significantly.

SUMMARY

The availability of GTKO pigs with additional genetic modifications aimed toward expression of multiple complement-regulatory proteins and/or human thromboregulatory genes, combined with novel immunosuppressive regimens, for example, the inclusion of B cell-depleting agents, should improve pig organ survival in the near future.

Effects of long-term administration of recombinant human protein C in xenografted primates

BACKGROUND

The role potential of recombinant human activated protein C (rhaPC), a recently developed molecule with anticoagulant and antiinflammatory properties, in prolonging survival in immunosuppressed primate recipients of porcine renal xenografts has been evaluated.

METHODS

rhaPC was administered daily for 5 days (24 μg/kg/hr; group A; n = 3) or throughout the postoperative period (8-24 μg/kg/hr; group B; n = 2; or 24-48 μg/kg/hr; group C; n = 4). Animals in group D (n = 2) received rhaPC daily (24 μg/kg/hr) combined with recombinant human antithrombin (84 U/kg every 8 hr). Two animals served as control (group E).

RESULTS

The results indicate that rhaPC is protective against fibrin deposition early after transplantation but does not prevent fibrin deposition and the occurrence of acute humoral xenograft rejection (AHXR) later on. Animals in the study survived between 8 and 55 days. At the dose used, rhaPC is able to prevent fibrin deposition in the graft in the first 2 weeks after xenotransplantation, except when it is administered in conjunction with antithrombin. However, rhaPC did not prevent the eventual occurrence of AHXR in primate recipients of porcine xenografts.

CONCLUSIONS

In this pig to primate model, rhaPC confers a short advantage in the prevention of early perioperative xenograft damage but does not represent an effective strategy for preventing AHXR.

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.