Hyperacute Xenograft Rejection Research Articles

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

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. 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). 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. 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.

Functional activity of the membrane-associated complement inhibitor CD59 in a pig-to-human in vitro model for hyperacute xenograft rejection.

Hyperacute rejection triggered by activation of the recipient's complement system represents the major barrier to successful xenotransplantation. Transfer of human membrane-associated complement regulators to donor organs has been suggested as one strategy to interfere with complement-mediated hyperacute xenograft rejection. Pigs are discussed as potential organ donors. We therefore investigated a putative protective function of the membrane-bound complement inhibitor CD59 in a pig-to-human in vitro model of hyperacute xenograft rejection. Aortic porcine endothelial cells were transfected with human CD59 cDNA. Expression of human CD59 was demonstrated by cytofluorimetric and RNA analysis. Removal of CD59 from the cell surface by phosphatidylinositol-specific phospholipase C (PI-PLC) demonstrated its production as a glycosyl phosphatidylinositol (GPI)-anchored protein. Functional activity of the transfected CD59 was tested by a lactate dehydrogenase (LDH) release assay for complement-mediated lysis. Porcine endothelial cells expressing human CD59 were significantly protected from lysis by human serum complement compared with CD59- cells. The protective effect was abolished by preincubating the cells with anti-CD59 antibodies or PI-PLC. We calculated by Scatchard analysis that the established CD59+ cell line expressed a CD59 level comparable to that of human endothelial cells. Our results recommend the production of pigs transgenic for CD59.

REACTION OF COMPLEMENT WITH ENDOTHEUAL CELLS IN A MODEL OF XEN0TRANSPLANTATION

We review our studies on the role of complement (C) as mediator of xenograft hyperacute rejection using an in vitro model consisting of porcine endothelial cells as target and human serum as source of natural antibodies and C. Cytotoxicity of endothelial cells required IgM antibodies to porcine endothelial cells, and the classical pathway and membrane attack complex of C. These findings correlated with in vivo results of porcine organs transplanted into rhesus monkeys, which showed a) co-deposition of IgM, C3, C4 and C9, along blood vessels of rejecting organs, with trace deposits of factors B or P, and b) minimal deposition of IgM and C components in transplants with prolonged survival that were performed in rhesus monkeys depleted of natural antibodies but with normal C levels. Human serum causes activation of porcine endothelial cells manifested by release of heparan sulfate proteoglycan. Heparan sulfate release was induced by C5a alone. A new approach to avert xenograft hyperacute rejection was tested. To inhibit cytotoxicity of porcine endothelial cells by human C, the membrane-associated C inhibitor decay-accelerating factor (DAF) of human origin was incorporated into endothelial cells. Human DAF was able to efficiently inhibit C-mediated killing of porcine endothelial cells, suggesting that the use of DAF and other C inhibitors could be used to interfere with C-mediated xenograft hyperacute rejection.

Current cellular innate immune hurdles in pig-to-primate xenotransplantation

The generation of pigs lacking Galalpha1,3Gal (alphaGal) or overexpressing human regulators of complement has largely overcome the barrier of hyperacute xenograft rejection. Nevertheless, delayed xenograft rejection remains a major hurdle, including humoral responses against nonalphaGal epitopes, and cellular innate immune responses. This review addresses the early interactions between the porcine endothelium and human natural killer cells, neutrophils, and monocytes/macrophages. Whether human leukocyte recruitment to, and lysis of, porcine endothelial cells includes direct recognition of alphaGal remains a matter of debate. Although the human natural killer receptor natural killerRP1A may directly recognize alphaGal, several studies did not reveal significant differences regarding adhesion, transmigration, and cytotoxicity using porcine endothelial cells expressing or lacking alphaGal. The strong human monocyte response against pig cells partly relies on the inability of porcine ligands to ligate inhibitory human receptors, such as SIRPalpha. Strategies to overcome cellular innate immune responses include transgenic expression in porcine cells of human ligands for inhibitory receptors, together with species-specific blocking of porcine ligands mediating human leukocyte interactions. Cellular human innate immune responses are increasingly recognized as barriers to successful pig-to-human xenotransplantation, and only a detailed knowledge of the molecular mechanisms involved will allow us to overcome this barrier.

Allelic variation of the porcine α-1,3-galactosyltransferase 1 (GGTA1) gene

The alpha-1,3-galactosyltransferase 1 enzyme (GGTA1) produces the alpha-Gal epitopes, responsible for pig-to-human hyperacute xenograft rejection. Recently, efforts have been directed at inactivating the porcine GGTA1 gene in order to reduce hyperacute rejection. As very little is known about the genetic variability of this key gene among pig breeds, we investigated the variation in its nucleotide sequence, by amplification of the entire coding region with the use of polymerase chain reaction followed by DNA sequencing. Eight commercial pig populations were analysed and 17 single nucleotide polymorphisms (SNPs) were detected: 11 in intronic regions and 6 in the 3' untranslated region (UTR). No SNPs change the encoded protein; however, 8 of these SNPs may alter the transcriptional regulation and pre-mRNA splicing of GGTA1.

Transgenic mice designed to express human α-1,2-fucosyltransferase in combination of human DAF and CD59 to avoid xenograft rejection

The expression of human alpha-1,2-fucosyltransferase (HT) or complement regulatory proteins has been proved as an strategy to overcome hypercute rejection in discordant xenogeneic organ transplantation. In this study, we examined whether peripheral blood mononuclear cells (PBMCs) from polytransgenic mice expressing the human HT, and complement regulatory proteins (DAF and CD59), can provide more effective protection against xenograft rejection. Transgenic mice were produced by co-injection of gene constructs for human HT, DAF and/or CD59. Flow Cytometry (FCM) was used to screen the positive transgenic mice. PBMCs from transgenic mice were incubated with 15% human serum to evaluate natural antibody binding, complement activation and expression of adhesion molecules. Three transgenes were strongly expressed in PBMCs of transgenic mice, and HT expression significantly reduced expression of the major xenoepitope galactose-alpha-1,3-galactose (alpha-Gal). Functional studies with PBMCs showed that co-expression of HT and DAF or CD59 markedly increased their resistance to human serum-mediated cytolysis when compared with single transgenic PBMCs. Moreover, the combined expression of triple transgenes in PBMCs led to the greatest protection against human serum-mediated cytolysis, avoided hyperacute rejection and reduced expression of adhesion molecules. Strong co-expression of triple transgenes was completely protected from xenograft hyperacute rejection and partially inhibited acute vascular rejection. The studies suggest that engineering mice to express triple molecules represents an critical step toward prolonging xenograft survival and might be more suitable for xenotransplantation.

Anti‐Gal antibody‐mediated skin graft rejection requires a threshold level of Gal expression

Despite overcoming xenograft hyperacute rejection (HAR), Gal (galactose-alpha1,3-galactose) expression may not be completely eliminated from the alpha1,3-galactosyltransferase gene knockout (Gal KO) pig because of alternative galactosyltransferases. Whether low levels of "residual" Gal are still susceptible to either complement fixing or non-complement fixing antibody beyond the HAR barrier remains unknown. Furthermore, it would be impossible to analyze the immune response specific to low-level Gal in a xenograft setting given the multitude of xenoantigens that could induce a recipient response. To investigate this question, we therefore used a skin graft model in BALB/c mice where the sole difference between donor and recipient was the expression of Gal, where rejection is caused by passively administered anti-Gal monoclonal antibody and where HAR does not occur. Gal expression over time was examined by immunohistochemistry in wildtype-to-Gal KO skin grafts. Graft rejection in response to passively administered anti-Gal monoclonal antibody at early and late time points was studied to determine changes in susceptibility to antibody. To independently test the effect of reduced Gal expression on antibody-mediated rejection, we used two separate lines of alpha1,2-fucosyltransferase transgenic mice as skin donors in the model. These mice have known reduced but different levels of Gal as determined by flow cytometry on peripheral blood leukocytes. Gal expression on skin grafts diminished with time with a corresponding reduction in susceptibility to antibody-mediated rejection. Skin grafts at day 30 (n = 7) and 150 (n = 11) had a rejection rate of 100% and 45% respectively in response to non-complement fixing anti-Gal antibody administered to the recipient. Similar results were demonstrated with a complement fixing anti-Gal antibody. When alpha1,2-fucosyltransferase transgenic mice skin was used in the model, the line with lowest level of Gal expression was resistant to antibody-induced rejection with a rate 0% (n = 9) vs. 60% (n = 5) in the alternative line with relatively more Gal expressed but still much less than normal mice. Resistance to anti-Gal antibody-mediated damage in the model was observed in skin grafts 100 to 150 days post-grafting but not earlier and was associated with a reduction in Gal expression. It is possible that below a threshold level of Gal expression, the grafts were not susceptible to anti-Gal antibody.

A personal account of the earliest attempts to define the coagulopathy associated with hyperacute xenograft rejection

A personal account of the earliest attempts to define the coagulopathy associated with hyperacute xenograft rejection

Xenograft rejection and the innate immune system

Purpose of review Major advances have been made in the understanding of the immune mechanisms underlying hyperacute and acute vascular xenograft rejection. Key roles of T and B lymphocytes have been revealed and significant progress has been made in understanding how they contribute to xenograft rejection and how they can be tolerized. In addition to the well established contribution of naturally occurring anti-Galα1,3Galβ1,4GlcNAc antibodies, the importance of active participation in xenograft rejection by other components of the innate immune system is increasingly being acknowledged. Recent findings Naturally occurring anti-Galα1,3Galβ1,4GlcNAc antibodies and complement play a central role in hyperacute xenograft rejection and are also involved in acute vascular xenograft rejection. Reports indicate that natural killer cells can contribute to xenograft rejection, and several mechanisms of action have been proposed. Furthermore, evidence indicates that macrophages and neutrophils are able to directly recognize xenogeneic target cells and to directly mediate xenograft damage. Although a role of Toll-like receptor signalling in allograft rejection is emerging, data on its role in xenograft rejection are limited. Summary Several components of the innate immune system can actively participate in xenograft rejection and thus represent a barrier to xenograft acceptance that may be particularly difficult to overcome.

Spectrum of the Early Xenograft Response: From Hyperacute Rejection to Delayed Xenograft Injury

Hyperacute xenograft rejection is a well-defined barrier to clinical pig-to-human xenotransplantation, and intense research in this area has identified potential solutions. In contrast, the next phase of xenograft injury, which can occur days to weeks later, has introduced a new series of immunological and nonimmunological barriers with complex mechanisms. This review addresses mechanisms of the immediate and delayed xenograft response with a focus on the relevant components. The key individual elements include carbohydrate antigens and natural antibodies to these epitopes, the role of the complement and coagulation systems, and the inflammatory cellular xenograft response that is predominantly mediated by the innate immune system. The vascular elements are central targets in this process, and the role of the endothelial cell is discussed. Important recent developments in xenotransplantation include the production of genetically modified pigs (deficient in alphaGal transferase and pigs transgenic for complement regulators) and a progressive understanding of xenograft-induced thrombotic microangiopathy, which threatens the long-term survival of transplanted pig organs and tissue. However, a clear standardized classification of the immunopathological mechanisms involved is essential. Further studies into the delayed xenograft response, using primates, are required before the routine use of pig organs for clinical transplantation.

Reactivity of Human Natural Antibodies to Endothelial Cells From Gal??(1,3)Gal-Deficient Pigs

Xenoreactive human natural antibodies (NAb) are predominantly directed against galactose-alpha(1,3)galactose (Gal). Binding of immunoglobulin (Ig) G and IgM NAb activates porcine endothelial cells (pEC) and triggers complement lysis responsible for hyperacute xenograft rejection. In vitro, IgG NAb induce human natural killer (NK) cell-mediated lysis of pEC by antibody-dependent cell-mediated cytotoxicity (ADCC). The present study examined the levels of anti-porcine NAb in a large number of individuals and addressed the functional role of non-Gal anti-porcine NAb. Sera from 120 healthy human blood donors were analyzed for the presence of anti-porcine NAb by flow cytometry using porcine red blood cells (pRBC), lymphoblastoid cells (pLCL), and pEC derived from control or Gal-deficient pigs. Xenogeneic complement lysis was measured by flow cytometry using human serum and rabbit complement. ADCC was analyzed by chromium-release assays using human serum and freshly isolated NK cells. Human IgM binding to pRBC was found in 93% and IgG binding in 86% of all samples. Non-Gal NAb comprised 13% of total IgM and 36% of total IgG binding to pEC. NAb/complement-induced lysis and ADCC of Gal-deficient compared to Gal-positive pEC were 21% and 29%, respectively. The majority of anti-Gal and non-Gal IgG NAb were of the IgG2 subclass. The generation of Gal-deficient pigs has overcome hyperacute anti-Gal-mediated xenograft rejection in nonhuman primates. Non-Gal anti-porcine NAb represent a potentially relevant immunological hurdle in a subgroup of individuals by inducing endothelial damage in xenografts.

Cooperation between Human DAF and CD59 in Protecting Cells from Human Complement-mediated Lysis

The complement (C) regulatory proteins decay accelerating factor (DAF, CD55) and CD59 could protect host cells using different mechanisms from C-mediated damage at two distinct levels within the C pathway. Co-expression of DAF and CD59 would be an effective strategy to help overcome host C-induced xenograft hyperacute rejection. In this study, we made a construct of recombinant expression vector containing DAF and CD59 cDNA and the stable cell lines were obtained by G418 selection. Extraneous genes integration and co-expression were identified by PCR, RT-PCR and Western blot analysis. Human c-mediated cytolysis assays showed that NIH/3T3 cells transfected stably with pcDNA3-CD59, pcDNA3-DAF, and pcDNA3-CD59DAF-DP were protected from Cmediated damage and that synchronously expressed human CD59 and DAF provided the most excellent protection for host cells as compared with either human CD59 or DAF expressed alone. Therefore, the construct represents an effective and efficacy strategy to overcome C-mediated damage in cells and, ultimately, in animals.

Combination of hDAF-transgenic pig hearts and immunoadsorption in heterotopic xenotransplantation of immunosuppressed baboons

Objectives: Hyperacute xenograft rejection (HXR) and acute vascular rejection (AVR) after xenotransplantation is triggered by xenoreactive antibodies (XAb) and an activated complement cascade. In a heterotopic (abdominal) xenotransplantation model we combined immunoadsorption (IA, Ig-Therasorb column) and a quadruple immunosuppressive therapy in recipient baboons with donor pig hearts transgenic for human decay accelerating factor (hDAF).

Endothelial cell protection and complement inhibition in xenotransplantation: a novel in vitro model using whole blood

Studying the interactions between xenoreactive antibodies, complement and coagulation factors with the endothelium in hyperacute and acute vascular rejection usually necessitates the use of in vivo models. Conventional in vitro or ex vivo systems require either serum, plasma or anti-coagulated whole blood, making analysis of coagulation-mediated effects difficult. Here a novel in vitro microcarrier-based system for the study of endothelial cell (EC) activation and damage, using non-anticoagulated whole blood is described. Once established, the model was used to study the effect of the characterized complement- and coagulation inhibitor dextran sulfate (DXS, MW 5000) for its EC protective properties in a xenotransplantation setting. Porcine aortic endothelial cells (PAEC), grown to confluence on microcarrier beads, were incubated with non-anticoagulated whole human blood until coagulation occurred or for a maximum of 90 min. PAEC-beads were either pre- or co-incubated with DXS. Phosphate buffered saline (PBS) experiments served as controls. Fluid phase and surface activation markers for complement and coagulation were analyzed as well as binding of DXS to PAEC-beads. Co- as well as pre-incubation of DXS, followed by washing of the beads, significantly prolonged time to coagulation from 39 +/- 12 min (PBS control) to 74 +/- 23 and 77 +/- 20 min, respectively (P < 0.005 vs. PBS). DXS treatment attenuated surface deposition of C1q, C4b/c, C3b/c and C5b-9 without affecting IgG or IgM deposition. Endothelial integrity, expressed by positivity for von Willebrand Factor, was maintained longer with DXS treatment. Compared with PBS controls, both pre- and co-incubation with DXS significantly prolonged activated partial thromboplastin time (>300 s, P < 0.05) and reduced production of thrombin-antithrombin complexes and fibrinopeptide A. Whilst DXS co-incubation completely blocked classical pathway complement activity (CH50 test) DXS pre-incubation or PBS control experiments showed no inhibition. DXS bound to PAEC-beads as visualized using fluorescein-labeled DXS. This novel in vitro microcarrier model can be used to study EC damage and the complex interactions with whole blood as well as screen ''endothelial protective'' substances in a xenotransplantation setting. DXS provides EC protection in this in vitro setting, attenuating damage of ECs as seen in hyperacute xenograft rejection.

Hyperacute Lung Rejection in the Pig-to-Human Model 4: Evidence for Complement and Antibody Independent Mechanisms

We assessed whether the combination of complement regulation and depletion of xenoreactive antibodies improves the outcome of pulmonary xenografts compared with either strategy alone. Lungs from pigs heterozygous (hDAF(+/-)) or homozygous (hDAF(+/+)) for the human decay accelerating factor transgene (hDAF) or their nontransgenic litter mates (hDAF(-/-)) were perfused with heparinized whole human blood. In additional groups, xenoreactive natural antibodies (XNA) were depleted by pig lung perfusion (hDAF(-/-)/AbAbs, hDAF(+/-)/AbAbs) before the experiment. This combined approach was augmented by adding soluble complement receptor 1 (sCR1) to the perfusate in one further group (hDAF(+/-)/AbAbs/sCR1). HDAF(-/-) lungs perfused with unmodified human blood were rejected after 32.5 min (interquartile range, IQR 5 to 210). HDAF(+/-) lungs survived for 90 min (IQR 10 to 161, P = 0.54). Both groups showed a rapid rise in pulmonary vascular resistance (PVR), which is a characteristic feature of hyperacute rejection (HAR). This phenomenon was blunted in the hDAF(+/+) group, although survival (48 min, IQR 14 to 111) was not further prolonged. Antibody depletion (AbAbs) led to a significant increase in survival time (hDAF(-/-)/AbAbs: 315 min, IQR 230 to 427; hDAF(+/-)/AbAbs: 375 min, IQR 154 to 575), reduced PVR and less complement production. Addition of sCR1 reduced complement elaboration but did not further improve survival (200 min, IQR 128 to 580) and surprisingly tended to increase PVR. Depletion of xenoreactive antibodies is more effective than membrane-bound complement regulation to blunt hyperacute rejection of pulmonary xenografts, but even the combined approach including soluble-phase complement inhibition is not sufficient to reliably prevent organ failure within hours. It therefore seems likely that other factors independent of antibody and complement contribute to HAR in this model.

Relationship between αGal epitope expression and decrease of tumorigenicity in pancreatic adenocarcinoma model

The alphaGal epitope is a carbohydrate structure, Galalpha1,3Galbeta1,4GlcNAc-R, synthesized on glycoconjugates in many mammals by alpha1,3galactosyltransferase. Humans do not express this epitope and present in serum large amounts of naturally occuring antibodies, which recognize the alphaGal epitopes and participate in the hyperacute rejection of xenograft. Studies indicated that the fundamental mechanism of hyperacute rejection involving the alphaGal epitope expression can be used in cancer therapy. We have previously suggested that the alphaGal epitope expression by human pancreatic tumoral cells could decrease the tumorigenic behavior of these cells. To determine whether the expression of the alphaGal epitope can modify the tumorigenicity of pancreatic cancer cells, we used a Syrian golden hamster pancreatic adenocarcinoma experimental model. The expression of alphaGal epitopes in the Syrian golden hamster pancreatic cancer cell line HaP-T1 was obtained by selecting stable cell clones transfected with murine alpha1,3galactosyltransferase gene. The alphaGal epitope expression resulted in a delay in the tumoral development of HaP-T1 cells in vivo after allograft transplantation of Syrian golden hamsters (2.5-fold, P < 0.05) and of nude mice. This result is associated with an 100% increase in survival time of nude mice bearing tumors expressing the alphaGal epitope. Our results confirm that the cell surface expression of alphaGal epitope decreases the tumorigenic behavior of pancreatic cancer cells. This novel property may be useful for the development of cancer gene immunotherapy strategy.

Activation of porcine endothelium in response to xenogeneic serum causes thrombosis independently of platelet activation

Endothelial cell activation and microvascular thrombosis are hallmarks of hyperacute xenograft rejection. However, the molecular determinants of platelet-endothelial interaction and thrombus formation are poorly understood. This study investigated whether: (i) xenogeneic human serum (HS), as a source of xenoreactive antibodies and complement, activates porcine aortic endothelial cells (PAEC) to promote thrombus formation under high shear stress; (ii) the endothelial adhesive proteins vitronectin receptor and P-selectin are involved in the von Willebrand factor (VWF)-platelet interaction during the thrombotic process under flow; (iii) reactive oxygen species (ROS) are activated by complement and served as intracellular signals for adhesive protein up-regulation. The PAEC were pre-exposed for 90 min in static conditions to medium plus 10, 20, and 50% HS or 20% porcine serum (PS), as control, then cells were perfused at 50 dynes/cm2 in a parallel plate flow chamber with human blood and area occupied by thrombi was measured. The role of complement in HS-induced thrombus formation was assessed by incubating PAEC with 20% HS in the presence of soluble complement receptor type 1 (sCR1) before blood perfusion. The effect of platelet activation was assessed using human blood treated or not with ADP and then flowed over PAEC pre-exposed to 20% HS or 20% PS as control. To identify the endothelial adhesive proteins involved in thrombus formation PAEC treated with 20% HS were then incubated with anti-vitronectin receptor antibody, anti-P-selectin antibody or P-selectin glycoprotein ligand-1 (PSGL-1), the soluble ligand of P-selectin, before the adhesion assay. Confocal microscopy was used to detect changes in endothelial adhesive protein expression. VWF interaction with platelet receptors GPIb and alphaIIbbeta3 was assessed adding aurin tricarboxylic acid (ATA) and anti-alphaIIbbeta3 antibody to blood before perfusion. The ROS involvement in xenogeneic serum-induced thrombus formation was determined studying the intracellular production of hydrogen peroxide (H2O2). The effect of antioxidants and metal chelators on HS-induced thrombus formation was evaluated treating PAEC with pyrrolidine dithiocarbamate (PDTC) or 1,3-dimethyl-2-thiourea (DMTU) before and during incubation with 20% HS followed by blood perfusion. The effect of antioxidants and sCR1 on ROS generation was investigated treating PAEC with PDTC or DMTU before and during incubation with 20% HS. Intracellular ROS generation was measured by fluorescence spectroscopy using the probe dihydrorhodamine 123 (DHR-123). Human serum but not PS caused thrombus formation on PAEC under high shear stress. Blockade of complement activation by sCR1 prevented xenogeneic serum-induced thrombus formation. Activated platelets did not promote thrombus formation on resting endothelium, and did not further increase platelet deposition on xenogeneic serum-treated PAEC. Vitronectin receptor and P-selectin were up-regulated on the endothelial surface by HS. Their functional blockade by specific antibodies prevented platelet deposition and thrombus formation. H2O2 production significantly increased when PAEC were exposed to the xenogeneic condition. Antioxidants and sCR1 completely prevented thrombus formation by reducing excessive ROS production and the expression of vitronectin receptor and P-selectin. Xenogeneic complement induces endothelial cell activation and thrombosis which is independent of platelet activation. Complement deposition elicits a rapid generation of ROS that lead to overexpression of endothelial adhesive molecules instrumental for platelet deposition.

Mean xenograft survival of 14.6 days in a small group of hDAF-transgenic pig hearts transplanted orthotopically into baboons

Introduction In a discordant orthotopic xenotransplantation model (pig-to-baboon) donor pigs expressing human decay accelerating factor (hDAF) as a regulator of complement activity were used to prevent hyperacute xenograft rejection (HXR). We investigated a modified immunosuppressive therapy consisting of ERL080 (Novartis Pharma AG, Base, Switzerland), cyclosporin A (Neoral), steroids, and a cyclophosphamide (CyP) induction protocol with several reduced doses to prevent acute vascular rejection (AVR). Methods Donor hearts were harvested from hDAF-transgenic pigs (18.8 ± 2.6 kg, Imutran Ltd., a Novartis Pharma AG Company). Four adult baboons (25.6 ± 2.7 kg) with high titers of xenoreactive antibodies (XAb) served as recipients. Serological and hemodynamic parameters were measured. Finally, myocardial tissue was sampled for histological and immunohistochemical examinations. Results In the first baboon, an acute graft failure occurred after 1 hour due to preservation injury. The second succumbed after 11.1 day due to an acute renal failure. The third died after 13.1 days of an ileus. The fourth baboon had continuously excellent cardiac function (mean echocardiographic ejection fraction, 69.2%), but succumbed on day 20 due to anemia. Corrected mean xenograft survival (excluding the first baboon because of a technical failure) was 14.6 ± 2.6 days. XAb decreased after day 3 to constantly low levels (<1:64 titer) after CyP induction. White blood cell count decreased from 10.3 ± 0.8 to 0.9 ± 0.3 G/L after day 3. Macroscopically and histologically no typical signs of HXR or severe AVR could be detected. Conclusions These results confirm that hDAF transgen blocks HXR in this life-supporting model. AVR was prevented by using a modified quadruple immunosuppressive drug combination (Neoral, ERL080, steroids, and several small single doses of CyP). An optimum “fine-tuning” of immunosuppression is required to achieve the best risk-benefit ratio.

Combination of hDAF-transgenic pig hearts and immunoadsorption in heterotopic xenotransplantation of immunosuppressed baboons

Introduction Hyperacute xenograft rejection (HXR) and acute vascular rejection (AVR) after xenotransplantation are triggered by xenoreactive antibodies (XAb) and an activated complement cascade. In a heterotopic (abdominal) xenotransplantation model we combined immunoadsorption (IA, Ig-Therasorb column) and a quadruple immunosuppressive drug therapy in recipient baboons with donor pig hearts transgenic for human decay accelerating factor (hDAF). Methods According to XAb titers between 6 and 14 cycles of IA were performed preoperatively in 4 recipient baboons (18.6 ± 2.5 kg). Hearts of hDAF-transgenic donor pigs (6.1 ± 1.1 kg, Imutran Ltd., a Novartis Pharma AG Company, Basel, Switzerland) were heterotopically transplanted using the abdominal technique in baboons. Immunosuppression consisted of cyclophosphamide (CyP) induction therapy, ERL080 (Novartis Pharma AG), cyclosporin A (CyA, Neoral), and steroids. Blood levels of mycophenolate, CyA, immunoglobulins (Ig), anti-pig-antibodies, complement factors, and cardiac enzymes were determined. Abdominal electrocardiography (ECG), echocardiography, and palpation were used for monitoring of the pig hearts. Myocardial tissue specimens were examined using immunohistochemistry, light microscope (LM), and electron microscope (EM). Results Ten cycles of IA alone removed 78% of XAb and accordingly IgM, IgG, IgA, complement C3, and C4. None of the xenografts was hyperacutely rejected, but xenograft failure occurred after 5.0 ± 1.3 days (range, 2.4–8.0 days) because of an AVR associated with a rapid XAb increase within 24 hours. White blood cell count (10.3 ± 2.2 G/L) showed a maximum of 13.1 ± 2.1 (day 1) and constant levels (1.4 ± 0.3–2.1 ± 1.3 G/L) between day 3 and 6. Histology (LM/EM) showed massive hemorrhage, necrosis, and vascular thrombi as signs of AVR. Conclusion Although HXR was prevented by using IA and hDAF-transgenic donor hearts, AVR was not avoided due to insufficient immunosuppressive regimen used and a missed postoperative IA treatment as a result of an inefficient control of XAb production.

MEAN XENOGRAFT SURVIVAL OF 14,6 DAYS IN A SMALL GROUP OF HDAF-TRANSGENIC PIG HEARTS TRANSPLANTED ORTHOTOPICALLY INTO BABOONS

O55 Aims: In a discordant orthotopic xenotransplantation (oXHTx) model (pig-to-baboon) donor pigs expressing human decay accelerating factor (hDAF) as a regulator of complement activity were used to prevent hyperacute xenograft rejection (HXR). We investigated a modified immunosuppressive (IS) regimen consisting of ERL (Novartis Pharma AG, Basel), Neoral, steroids and a cyclophosphamide (CyP) induction protocol with several reduced doses to prevent acute vascular rejection (AVR). Methods: Donor hearts were harvested from hDAF-transgenic pigs (18.8+/−2.6 kg, Imutran Ltd., a Novartis Pharma AG Company). Four adult baboons (25.6+/−2.7 kg) with high titres of xenoreactive antibodies (XNAb) served as recipients. Serological and hemodynamic parameters were measured. Finally myocardial tissue was sampled for histological and immunohistochemical examinations. Results: First baboon had an acute graft failure after 1 h due to preservation injury. The second died after 11.1 d because of acute renal. The third died after 13.1 days from an ileus. The fourth baboon had continuously excellent cardiac function (mean echocardiographical ejection fraction: 69.2%), but died on day 20 due to anemia. Corrected mean xenograft survival (excluding first baboon because of technical failure) was 14.6+/−2.6 days. XNAb decreased on day 3 to constantly low levels (<1:64 titer) after CyP induction. White blood cell count decreased from 10.3+/−0.8 to 0.9+/−0.3 G/l after day 3. Macroscopically and histologically no typical signs of HXR or severe AVR could be detected. Conclusions: These results confirm that hDAF transgen blocks HXR in this life supporting model. AVR was prevented by using a modified quadruple IS drug combination (Neoral, ERL, steroids and several single small doses of CyP). An optimum “fine-tuning” of IS is required to achieve the best risk-benefit ratio.