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

Xenotransplantate vom Schwein – ist das Ende des Organmangels in Sicht?

Unter „Xenotransplantation“ wird die Übertragung von funktionsfähigen Zellen, Geweben oder Organen zwischen verschiedenen Spezies verstanden, insbesondere von Schweinen auf den Menschen. In den meisten Industrieländern klafft eine große Lücke zwischen der Anzahl geeigneter Spenderorgane und der Anzahl benötigter Transplantate. Weltweit können nur etwa 10% des Organbedarfs durch Spenden gedeckt werden. Eine erfolgreiche Xenotransplantation könnte diesen Mangel mildern oder sogar weitgehend vermeiden. Das Schwein wird aus verschiedenen Erwägungen heraus als am besten geeignete Spenderspezies angesehen. Bei einer Übertragung porziner Organe auf Primaten treten verschiedene immunologisch bedingte Abstoßungsreaktionen auf, die das übertragene Organ innerhalb kurzer Zeit zerstören können, wie die HAR (hyperakute Abstoßung), die AVR (akute vaskuläre Abstoßung) und die spätere zelluläre Abstoßung. Diese Abstoßungsreaktionen müssen durch genetische Modifikationen im Schwein und eine geeignete immunsuppressive Behandlung des Empfängers kontrolliert werden. Dazu müssen Tiere mit mehrfachen genetischen Veränderungen produziert und im Hinblick auf ihre Eignung für eine erfolgreiche Xenotransplantation geprüft werden. Inzwischen können die HAR und auch die AVR durch Knockouts von antigenen Oberflächenepitopen (z. B. αGal [Galaktose-α1,3-Galaktose]) und transgene Expression humaner Gene mit antiinflammatorischer, antiapoptotischer oder antikoagulativer Wirkung zuverlässig kontrolliert werden. Nach orthotopen Transplantationen in nicht humane Primaten konnten inzwischen mit Schweineherzen Überlebensraten von bis zu 264 Tagen und mit porzinen Nieren von 435 Tagen erzielt werden. Eine Übertragung pathogener Erreger auf den Empfänger kann bei Einhaltung einschlägiger Hygienemaßnahmen ausgeschlossen werden. PERV (porzine endogene Retroviren) können durch RNA-(Ribonukleinsäure-)Interferenz oder Gen-Knockout ausgeschaltet werden. Sie stellen damit kein Übertragungsrisiko für den Empfänger mehr dar. Anfang 2022 wurde in Baltimore (USA) ein Schweineherz mit 10 genetischen Modifikationen auf einen Patienten mit schwerem Herzleiden übertragen, mit dem der Empfänger 2 Monate offenbar ohne größere Probleme lebte. Es wird erwartet, dass Xenotransplantate vom Schwein in absehbarer Zeit zur klinischen Anwendungsreife kommen werden. Dazu werden klinische Versuche zur systematischen Erfassung aller Auswirkungen solcher Transplantate auf den Patienten sowie geeignete rechtliche und finanzielle Rahmenbedingungen benötigt.

Advance of genetically modified pigs in xeno-transplantation

As the standard of living improves, chronic diseases and end-stage organ failure have been a regular occurrence in human beings. Organ transplantation has become one of the hopes in the fight against chronic diseases and end-stage organ failure. However, organs available for transplantation are far from sufficient to meet the demand, leading to a major organ shortage crisis. To solve this problem, researchers have turned to pigs as their target since pigs have many advantages as xenograft donors. Pigs are considered the ideal organ donor for human xenotransplantation, but direct transplantation of porcine organs to humans faces many obstacles, such as hyperacute rejection, acute humoral xenograft rejection, coagulation dysregulation, inflammatory response, coagulation dysregulation, and endogenous porcine retroviral infection. Many transgenic strategies have been developed to overcome these obstacles. This review provides an overview of current advances in genetically modified pigs for xenotransplantation. Future genetic engineering-based delivery of safe and effective organs and tissues for xenotransplantation remains our goal.

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.

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.

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.

Kidneys From α1,3-Galactosyltransferase Knockout/Human Heme Oxygenase-1/Human A20 Transgenic Pigs Are Protected From Rejection During Ex Vivo Perfusion With Human Blood

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Multiple modifications of the porcine genome are required to prevent rejection after pig-to-primate xenotransplantation. Here, we produced pigs with a knockout of the α1,3-galactosyltransferase gene (GGTA1-KO) combined with transgenic expression of the human anti-apoptotic/anti-inflammatory molecules heme oxygenase-1 and A20, and investigated their xenoprotective properties.

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.

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.

Xeno-kidney transplantation: from idea to reality

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

Transgenic expression of human heme oxygenase-1 in pigs confers resistance against xenograft rejection during ex vivo perfusion of porcine kidneys

BACKGROUND

The major immunological hurdle to successful porcine-to-human xenotransplantation is the acute vascular rejection (AVR), characterized by endothelial cell (EC) activation and perturbation of coagulation. Heme oxygenase-1 (HO-1) and its derivatives have anti-apoptotic, anti-inflammatory effects and protect against reactive oxygen species, rendering HO-1 a promising molecule to control AVR. Here, we report the production and characterization of pigs transgenic for human heme oxygenase-1 (hHO-1) and demonstrate significant protection in porcine kidneys against xenograft rejection in ex vivo perfusion with human blood and transgenic porcine aortic endothelial cells (PAEC) in a TNF-α-mediated apoptosis assay.

METHODS

Transgenic and non-transgenic PAEC were tested in a TNF-α-mediated apoptosis assay. Expression of adhesion molecules (ICAM-1, VCAM-1, and E-selectin) was measured by real-time PCR. hHO-1 transgenic porcine kidneys were perfused with pooled and diluted human AB blood in an ex vivo perfusion circuit. MHC class-II up-regulation after induction with IFN-γ was compared between wild-type and hHO-1 transgenic PAEC.

RESULTS

Cloned hHO-1 transgenic pigs expressed hHO-1 in heart, kidney, liver, and in cultured ECs and fibroblasts. hHO-1 transgenic PAEC were protected against TNF-α-mediated apoptosis. Real-time PCR revealed reduced expression of adhesion molecules like ICAM-1, VCAM-1, and E-selectin. These effects could be abrogated by the incubation of transgenic PAECs with the specific HO-1 inhibitor zinc protoporphorine IX (Zn(II)PPIX, 20 μm). IFN-γ induced up-regulation of MHC class-II molecules was significantly reduced in PAECs from hHO-1 transgenic pigs. hHO-1 transgenic porcine kidneys could successfully be perfused with diluted human AB-pooled blood for a maximum of 240 min (with and without C1 inh), while in wild-type kidneys, blood flow ceased after ∼60 min. Elevated levels of d-Dimer and TAT were detected, but no significant consumption of fibrinogen and antithrombin was determined. Microthrombi could not be detected histologically.

CONCLUSIONS

These results are encouraging and warrant further studies on the biological function of heme oxygenase-I expression in hHO-1 transgenic pigs in the context of xenotransplantation.

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.

Controlling coagulation dysregulation in xenotransplantation

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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