Aggregation of human platelets induced by porcine endothelial cells is dependent upon both activation of complement and thrombin generation

Effect of Factor H on Complement Alternative Pathway Activation in Human Serum Remains on Porcine Cells Lacking N-Glycolylneuraminic Acid

Background

Triple knockout (TKO) donor pigs lacking alpha-1,3-galactose (Gal), N-glycolylneuraminic acid (Neu5Gc), and Sd(a) expressions were developed to improve the clinical success of xenotransplantation. Neu5Gc, a sialic acid expressed on cell surfaces, recruits factor H to protect cells from attack by the complement system. Lack of Neu5Gc expression may cause unwanted complement activation, abrogating the potential benefit of gene-modified donor pigs. To investigate whether TKO porcine cells display increased susceptibility to complement activation in human serum, pathway-specific complement activation, apoptosis, and human platelet aggregation by porcine cells were compared between alpha-1,3-galactosyltransferase gene-knockout (GTKO) and TKO porcine cells.

Methods

Primary porcine peripheral blood mononuclear cells (pPBMCs) and endothelial cells (pECs) from GTKO and TKO pigs were used. Cells were incubated in human serum diluted in gelatin veronal buffer (GVB⁺⁺) or Mg⁺⁺-EGTA GVB, and C3 deposition and apoptotic changes in these cells were measured by flow cytometry. C3 deposition levels were also measured after incubating these cells in 10% human serum supplemented with human factor H. Platelet aggregation in human platelet-rich plasma containing GTKO or TKO pECs was analyzed.

Results

The C3 deposition level in GTKO pPBMCs or pECs in GVB⁺⁺ was significantly higher than that of TKO pPBMCs or pECs, respectively, but C3 deposition levels in Mg⁺⁺-EGTA-GVB were comparable between them. The addition of factor H into the porcine cell suspension in 10% serum in Mg⁺⁺ -EGTA-GVB inhibited C3 deposition in a dose-dependent manner, and the extent of inhibition by factor H was similar between GTKO and TKO porcine cells. The percentage of late apoptotic cells in porcine cell suspension in GVB⁺⁺ increased with the addition of human serum, of which the net increase was significantly less in TKO pPBMCs than in GTKO pPBMCs. Finally, the lag time of platelet aggregation in recalcified human plasma was significantly prolonged in the presence of TKO pECs compared to that in the presence of GTKO pECs.

Conclusion

TKO genetic modification protects porcine cells from serum-induced complement activation and apoptotic changes, and delays recalcification-induced human platelet aggregation. It does not hamper factor H recruitment on cell surfaces, allowing the suppression of alternative complement pathway activation.

Differences in platelet aggregometers to study platelet function and coagulation dysregulation in xenotransplantation

Xenotransplantation (ie, cross-species transplantation) using genetically engineered pig organs could be a limitless source to solve the shortage of organs and tissues worldwide. However, despite prolonged survival in preclinical pig-to-nonhuman primate xenotransplantation trials, interspecies coagulation dysregulation remains to be overcome in order to achieve continuous long-term success. Different platelet aggregometry methods have been previously used to study the coagulation dysregulation with wild-type and genetically engineered pig cells, including the impact of possible treatment options. Among these methods, while thromboelastography and rotational thromboelastometry measure the change in viscoelasticity, optical aggregometry measures the change in opacity. Recently, impedance aggregometry has been used to measure changes in platelet aggregation in electrical conductance, providing more information to our understanding of coagulation dysregulation in xenotransplantation compared to previous methods. The present study reviews the merits and differences of the above-mentioned platelet aggregometers in xenotransplantation research.

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.

Regulation of human platelet aggregation by genetically modified pig endothelial cells and thrombin inhibition

BACKGROUND

Coagulation disorders remain barriers to successful pig-to-primate organ xenotransplantation. In vitro, we investigated the impact of pig genetic modifications on human platelet aggregation in response to pig aortic endothelial cells (pAEC).

METHODS

In comparison to human (h)AEC and wild-type (WT) pAEC, the expression of human complement- (CD46, CD55) or coagulation (thrombomodulin [TBM], endothelial protein C receptor [EPCR]) -regulatory proteins on pAEC from WT or α1,3-galactosyltransferase gene-knockout (GTKO) pigs was studied by flow cytometry. Using platelet-aggregometry, human whole blood platelet aggregation was evaluated after co-incubation with various AEC. Further, the inhibitory effect on aggregation of heparin, low molecular weight heparin, and hirudin was assessed.

RESULTS

Heparin, low molecular weight heparin and hirudin almost completely prevented platelet aggregation induced by WT pAEC. The level of expression of human CD46, CD55, TBM and EPCR on pAEC was comparable to that on hAEC. Platelet aggregation induced by all genetically modified pAEC was significantly less (P < 0.05) than that by WT pAEC (which was 54%). GTKO/CD46/TBM pAEC induced the least platelet aggregation (27%)-a reduction of almost 50%-but this remained significantly greater (P < 0.01) than aggregation induced by hAEC (4%). There was significant positive correlation between reduction of aggregation and TBM or EPCR expression on pAEC (r = 0.89 and r = 0.86, respectively; P < 0.05). Platelet aggregation induced by GTKO/CD46/TBM pAEC in the presence of hirudin (1 IU/ml) was comparable to platelet aggregation induced by hAEC.

CONCLUSIONS

Genetic modification of pAEC is associated with significant reduction of human platelet aggregation in vitro. With concomitant thrombin inhibition, platelet aggregation was comparable to that stimulated by hAEC.

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.

Feasibility of xeno-transplantation

Within a relatively short time span, a significant number of barriers to xeno-transplantation have been identified and potential solutions generated; however, the survival rates for pig-to-primate heart transplantation remain modest at best, with the longest functioning heterotopic heart transplant surviving only 99 days and the longest functioning orthotopic heart transplant surviving only 39 days. A great deal of improvement in immunological strategies will be needed to make xeno-transplantation a clinical reality. The most exciting prospect in the near term is the use of organs from homozygous alphaGal knockout pigs. The diversity of the biological pathways involved in the total spectrum of xenograft rejection, however, makes it highly likely that the clinical feasibility of xeno-transplantation will depend on a multipronged approach that incorporates the advantages of genetically eliminating the alphaGal epitope on hyperacute and acute xenograft rejection and the advantages of tolerance induction on cellular and chronic xenograft rejection.

Aggregation of human platelets in plasma by porcine blood cells in vitro is probably mediated by thrombin generation

The infusion of pig progenitor cells into baboons is associated with a thrombotic microangiopathy probably related to the interaction of these cells with the baboon endothelial cells and platelets. We have shown previously that pig peripheral blood mononuclear cells (p-PBMC), are able to activate the human coagulation cascade as they are able to generate thrombin when added to defibrinated plasma. In this work, we have tested the interaction of p-PBMC with human platelets to assess the capacity of p-PBMC to cause platelet aggregation and the possible role of complement activation in this aggregation. Human platelet aggregation assays, using collagen (1 or 2 microg/ml), were performed with platelets in platelet-rich plasma (PRP) or platelets washed by filtration. PRP or washed platelets were also incubated with p-PBMC or human PBMC (h-PBMC) at several concentrations and aggregation was measured. The effect of Dansylarginine N-(3-ethyl-1,5-pentanediyl)amide (DAPA), an inhibitor of thrombin, was studied on platelet aggregation caused by the pig cells. Complement activation was measured by deposition of fragment c derived from C3 splitting (C3c) on pig cells incubated with citrated platelet poor plasma (PPP). When human PRP was incubated with p-PBMC, aggregation was a consistent event quantitatively similar to that induced by collagen. No aggregation of washed platelets was observed when these were incubated with p-PBMC or h-PBMC. Aggregation of human platelets in PRP, induced by p-PBMC, was inhibited when DAPA (100 microm) was added to the incubation mixture (23%), indicating that the thrombin inhibitor blocked the capacity of p-PBMC to aggregate human platelets. No deposition of C3c fragments on p-PBMC was detected when the porcine cells were incubated for up to 20 min with citrated PPP. The fact is that p-PBMC induces human platelet aggregation in plasma being thrombin generation a likely explanation for this observation. Our data suggest that, in the system assayed, complement activation is not a cause of platelet aggregation. These findings are relevant for the clarification of the reported thrombotic microangiopathy complicating the intravenous infusion of pig cells in primates in attempts to induce pig tolerance in baboons.

Genetic engineering for xenotransplantation

Xenotransplantation is being pursued vigorously to solve the shortage of allogeneic donor organs. Experimental studies of the major xenoantigen (Gal) and of complement regulation enable model xenografts to survive hyperacute rejection. When the Gal antigen is removed or reduced and complement activation is controlled, the major barriers to xenograft survival include unregulated coagulation within the graft and cellular reactions involving macrophages, neutrophils, natural killer (NK) cells, and T lymphocytes. Unlike allografts, where specific immune responses are the sole barrier to graft survival, molecular differences between xenograft and recipient that affect normal receptor-ligand interactions (largely active at the cell surface and which may not be immunogenic), are also involved in xenograft failure. Transgenic strategies provide the best options to control antigen expression, complement activation, and coagulation. Although the Gal antigen can be eliminated by gene knockout in mice, that outcome has only become a possibility in pigs due to the recent cloning of pigs after nuclear transfer. Instead, the use of transgenic glycosyl transferase enzymes and glycosidases, which generate alternative terminal carbohydrates on glycolipids and glycoproteins, has reduced antigen in experimental models. As a result, novel strategies are being tested to seek the most effective solution. Transgenic pigs expressing human complement-regulating proteins (DAF/CD55, MCP/CD46, or CD59) have revealed that disordered regulation of the coagulation system requires attention. There will undoubtedly be other molecular incompatibilities that need addressing. Xenotransplantation, however, offers hope as a therapeutic solution and provides much information about homeostatic mechanisms.

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

Disordered regulation of coagulation and platelet activation in xenotransplantation

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

Acute humoral xenograft rejection: destruction of the microvascular capillary endothelium in pig-to-nonhuman primate renal grafts

The major cause of xenograft loss beyond hyperacute rejection is a form of injury, traditionally termed delayed xenograft rejection (DXR), whose pathogenesis is unknown. Here we analyze the immunologic and morphologic features of DXR that develops in pig kidney xenografts transplanted into nonhuman primates. Kidneys from miniature swine were transplanted into cynomolgus monkeys (n = 14) or baboons (n = 11) that received regimens aimed to induce mixed chimerism and tolerance. No kidney was rejected hyperacutely. Morphologic and immunohistochemical studies were performed on serial biopsies, and an effort was made to quantify the pathologic features seen. The early phase of DXR (Days 0-12) was characterized by focal deposition of IgM, IgG, C3, and scanty neutrophil and macrophage infiltrates. The first abnormality recognized was glomerular and peritubular capillary endothelial cell death as defined by in situ DNA nick-end labeling (TUNEL). Damaged endothelial cells underwent apoptosis and, later, frank necrosis. The progressive phase developed around Day 6 and was characterized by progressive deposition of IgM, IgG, C3, and prominent infiltration of cytotoxic T cells and macrophages, with a small number of NK cells. Thrombotic microangiopathy developed in the glomeruli and peritubular capillaries with TUNEL+ endothelial cells, platelet aggregation, and destruction of the capillary network. Only rare damaged arterial endothelial cells and tubular epithelial cells were observed, with rare endothelialitis and tubulitis. In the advanced phase of DXR, interstitial hemorrhage and infarction occurred. During the development of DXR, the number of TUNEL+ cells increased, and this correlated with progressive deposition of antibody. The degree of platelet aggregation correlated with the number of TUNEL+ damaged endothelial cells. We conclude that peritubular and glomerular capillary endothelia are the primary targets of renal DXR rather than tubular epithelial cells or arterial endothelium and that the earliest detectable change is endothelial cell death. DXR was characterized by progressive destruction of the microvasculature (glomeruli and peritubular capillaries) and formation of fibrin-platelet thrombi. Both cytotoxic cells and antibodies potentially mediate the endothelial damage in DXR; however, in this model, DXR is largely humorally mediated and is better termed "acute humoral xenograft rejection."

Factors in xenograft rejection

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

A model of xenograft hyperacute rejection attenuates endothelial nitric oxide production: a mechanism for graft vasospasm?

BACKGROUND

The deposition of complement components within grafts, complement consumption, and prolongation of graft function by complement inactivation imply a pivotal role for complement in xenograft hyperacute rejection. The current investigations examined the endothelial production of vasoactive substances in pulmonary arteries during simulated hyperacute rejection.

METHODS AND RESULTS

Canine pulmonary arteries were suspended in organ chambers and exposed to either autologous canine serum for 90 minutes or heterologous porcine serum for 30, 60, or 90 minutes. Following serum exposure, the vessels were allowed a one-hour equilibration in buffered crystalloid solution. Dose-response curves were obtained with acetylcholine, sodium nitroprusside, and calcium ionophore A23187 following contraction with phenylephrine (10(-6) M) in the presence of indomethacin (10(-5) M). Receptor-dependent, endothelial-dependent relaxations to acetylcholine (10(-9)-10(-4) M) were impaired with 30-, 60-, or 90-minute porcine serum exposure when compared to vessels exposed to autologous canine serum (n = 10, 7, 9, respectively; p < .05; 2-way ANOVA). Receptor-independent, endothelial-dependent relaxations to calcium inophore (10(-9)-10(-6) M) were significantly impaired at 60- and 90-minute porcine exposures only (n = 7, 8; p < .05). Endothelial-independent relaxations to sodium nitroprusside (10(-9)-10(-4) M) were not impaired with either canine or porcine serum exposure. Oxyhemoglobin (10(-6) M) abolished acetylcholine-mediated relaxations, indicating that nitric oxide was the predominant mediator.

CONCLUSIONS

Simulated hyperacute xenograft rejection impairs endothelium-dependent relaxation of canine pulmonary arteries. Both basal and stimulated production of nitric oxide is impaired by heterologous serum exposure and, subsequently, complement activation. Reduced production of nitric oxide may explain, in part, the vasospasm and thrombosis of xenografts during hyperacute rejection.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

Systemic coagulation disturbances are associated with delayed xenograft rejection.

Expression of human thrombomodulin cofactor activity in porcine endothelial cells

BACKGROUND

Xenograft rejection may predispose to vascular thrombosis because of putative cross-species' functional incompatibilities between natural anticoagulants present on the donor endothelium and host activated coagulation factors. For example, porcine thrombomodulin expressed on porcine aortic endothelial cells (PAEC) does not provide the expected thrombomodulin (TM)-cofactor activity for human protein C in the presence of human thrombin. In addition, TM may be down-regulated after cellular activation. Our aim was to express human TM cofactor activity in PAEC and to study the proinflammatory effect of tumor necrosis factor-alpha (TNF-alpha) on stable expressed human thrombomodulin in vitro.

METHODS AND RESULTS

Retroviral transduction of PAEC with the gene encoding for human thrombomodulin (hTM) resulted in expression of high levels of specific TM cofactor activity on PAEC (0.62 microg/ml activated protein C/10(5) cells). High-level expression of hTM resulted in a 620-fold higher activation of human protein C in the presence of human thrombin when compared with mock-transduced PAEC (0.0001 microg/ml/10(5) cells; P<0.001). Transduced PAEC expressing hTM also bound more human thrombin than control PAEC, as determined by inhibition of thrombin-induced platelet activation (P<0.05). We noted that exposure to TNF-alpha significantly reduced exogenous hTM cofactor activity on transduced PAEC in a time- and dose-dependent fashion; this occurred despite the relatively stable expression of hTM mRNA and hTM antigen in these cells. Treatment of transduced PAEC with selected antioxidants could protect against the loss of hTM cofactor activity directly associated with the oxidative stress induced by TNF-alpha activation responses.

CONCLUSIONS

Our data show that the functional deficiency of the anticoagulant protein C pathway in PAEC may be corrected by viral transduction of these cells. As analysis of the hTM function showed modulation under conditions of cellular activation, we suggest that expression of hTM mutants resistant to oxidation may have greater therapeutic utility in the genetic modification of porcine xenografts.

Xenogeneic endothelial cells activate human prothrombin

BACKGROUND

Delayed xenograft rejection is characterized by platelet activation and fibrin deposition and is thought to occur independently of complement activation. We have therefore investigated the potential for xenogeneic endothelial cells (EC) to regulate the conversion of prothrombin to thrombin, a central component of the final common pathway of coagulation and an important platelet agonist.

METHODS AND RESULTS

Quiescent porcine aortic EC (PAEC) were found to convert high levels of human prothrombin to thrombin (0.234+/-0.019 IU/ml) when compared with human aortic EC (0.017+/-0 IU/ml, 30-min time point, chromogenic assay; P<0.001). PAEC activation by human complement resulted in comparable levels of thrombin generation. Prothrombin conversion by PAEC as determined by generation of F1+2 (1.909+/-0.119 nmol/L) and formation of thrombin-antithrombin III complexes (125.611+/-6.373 microg/L) was significantly greater than the matched human aortic EC values (F1+2: 1.539+/-0.03 nmol/L, P<0.001; thrombin-antithrombin III: 1.833+/-0.104 microg/L, P<0.001). Sequential analysis of prothrombin activation by PAEC indicated generation of the intermediate meizothrombin followed by autolytically accelerated thrombin formation. Subsequent experiments established important cross-species' incompatibilities with respect to porcine thrombomodulin interaction with human thrombin and protein C in that PAEC had a reduced capacity to generate activated human protein C in vitro.

CONCLUSION

These observations indicate a potentially important molecular barrier involving blood coagulation that may impact on the planned clinical application of porcine transgenic organs.

Loss of ATP diphosphohydrolase activity with endothelial cell activation

Quiescent endothelial cells (EC) regulate blood flow and prevent intravascular thrombosis. This latter effect is mediated in a number of ways, including expression by EC of thrombomodulin and heparan sulfate, both of which are lost from the EC surface as part of the activation response to proinflammatory cytokines. Loss of these anticoagulant molecules potentiates the procoagulant properties of the injured vasculature. An additional thromboregulatory factor, ATP diphosphohydrolase (ATPDase; designated as EC 3.6.1.5) is also expressed by quiescent EC, and has the capacity to degrade the extracellular inflammatory mediators ATP and ADP to AMP, thereby inhibiting platelet activation and modulating vascular thrombosis. We describe here that the antithrombotic effects of the ATPDase, like heparan sulfate and thrombomodulin, are lost after EC activation, both in vitro and in vivo. Because platelet activation and aggregation are important components of the hemostatic changes that accompany inflammatory diseases, we suggest that the loss of vascular ATPDase may be crucial for the progression of vascular injury.