Endothelial cell activation and thromboregulation during xenograft rejection

Relevance of circulating markers of endothelial activation for cardiovascular risk assessment in rheumatoid arthritis: a narrative review

Rheumatoid arthritis (RA) is associated with excessive cardiovascular mortality secondary to premature atherosclerosis, in which endothelial activation (EA) plays a central role. EA is characterized by loss of vascular integrity, expression of leucocyte adhesion molecules, transition from antithrombotic to prothrombotic phenotype, cytokines production, shedding of membrane microparticles and recruitment of endothelial progenitor cells. As EA is an early event in atherogenesis, circulating markers of EA are putative markers of vascular pathology and cardiovascular (CV) risk. After a presentation of biology of EA, the present review analyzed the available data regarding changes in EA markers in RA in link with the vascular pathology and CV events, discussed their relevance as biomarkers of CV risk and proposed future directions.

Stem Cell Therapy, Artificial Heart or Xenotransplantation: What will be New “Regenerative” Strategies in Heart Failure during the Next Decade?

The main limitation of allotransplantation and in particular heart transplantation is the insufficient supply of donor organs. As alternative strategies to heart transplantation, stem cells opened the way of regenerative medicine in early 2000. While new biotechnologies tried to minimize side effects due to hemocompatibility in artificial hearts, progress in xenotransplantation allowed in 2022 to realize the first pig-to-human heart transplant on a compassionate use basis. This xenotransplantation has been successful thanks to genetically modified pigs using the CRISPR-Cas9 technology. Indeed, gene editing allowed modifications of immune responses and thrombotic potential to modulate graft and systemic reaction. Academic research and preclinical studies of xenogeneic tissues already used in clinic such as bioprosthesis valve and of new xenotransplantation options will be necessary to evaluate immune-thrombosis and organ/vascular damages more deeply to make this hope of xenotransplantation a clinical reality. Stem cells, artificial heart and xenotransplantation are all in line to overcome the lack of donor hearts. Combination of stem cell approaches and/or xenogeneic tissue and/or artificial organs are probably part of the research objectives to make these projects real in the short term.

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

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

Porcine genome engineering for xenotransplantation

The extreme shortage of human donor organs for treatment of patients with end-stage organ failures is well known. Xenotransplantation, which might provide unlimited organ supply, is a most promising strategy to solve this problem. Domestic pigs are regarded as ideal organ-source animals owing to similarity in anatomy, physiology and organ size to humans as well as high reproductive capacity and low maintenance cost. However, several barriers, which include immune rejection, inflammation and coagulative dysfunctions, as well as the cross-species transmission risk of porcine endogenous retrovirus, blocked the pig-to-human xenotransplantation. With the rapid development of genome engineering technologies and the potent immunosuppressive medications in recent years, these barriers could be eliminated through genetic modification of pig genome together with the administration of effective immunosuppressants. A number of candidate genes involved in the regulation of immune response, inflammation and coagulation have been explored to optimize porcine xenograft survival in non-human primate recipients. PERV inactivation in pigs has also been accomplished to firmly address the safety issue in pig-to-human xenotransplantation. Many encouraging preclinical milestones have been achieved with some organs surviving for years. Therefore, the clinical trials of some promising organs, such as islet, kidney and heart, are aimed to be launched in the near future.

Effect of prolonged treatment with phosphodiesterase-5-inhibitors on endothelial dysfunction in vascular diseases and vascular risk conditions: A systematic review analysis and meta-analysis of randomized double-blind placebo-controlled trials

OBJECTIVE

To challenge the argument that continuous use of phosphodiesterase-5-selective inhibitors may reduce endothelial cell dysfunction in patients with vascular diseases or vascular risk conditions.

DESIGN

This study included systematic reviews and meta-analysis of randomized double-blind placebo-controlled trials dealing with the prolonged use of phosphodiesterase-5-selective inhibitors. The risk of bias and quality of trials were assessed by the Cochrane algorithm. Fixed or random effect models, standardised mean differences and heterogeneity were estimated in the study.

DATA SOURCES

Systematic search for randomized double-blind placebo-controlled trials was done in PubMed, Scopus, CINAHL, Science direct and the Cochrane Library.

ELIGIBILITY CRITERIA FOR SELECTING STUDIES

Randomized double-blind placebo-controlled trials reporting measures of endothelial cell dysfunction and/or endothelial cell activation were included.

RESULTS

On the whole, 469 subjects were allocated to the phosphodiesterase-5-selective inhibitor group, while 463 were assigned to the placebo group in 13 randomized double-blind placebo-controlled trials. Flow-mediated dilation of the brachial artery was found to improve after the administration of phosphodiesterase-5-selective inhibitors (P < 0.0001). The results were questioned by the elevated and uncorrectable heterogeneity (I²  = 92%) and the asymmetry of the funnel plot suggested a publication bias. Phosphodiesterase-5-selective inhibitors have no effect on endothelial cell dysfunction, as assessed in the resistance vessels by digital arterial tonometry. The blood level of endothelin-1 was observed to be decreased in phosphodiesterase-5-selective inhibitors arm (P = 0.03), although the effect disappeared once the publication bias and heterogeneity were corrected. The effect of phosphodiesterase-5-selective inhibitors on biomarkers of endothelial cell activation was found to be inconsistent.

CONCLUSIONS

The results on the benefits of a prolonged use of phosphodiesterase-5-selective inhibitors, with the objective of lowering endothelial cell dysfunction in patients with vascular diseases or vascular risk conditions are not convincing. This is because of the overall low quality of evidence, giving an unclear scientific support to this treatment. Systematic review registration: PROSPERO registration: CRD42017055399.

Up-regulation of fibrinogen-like protein 2 in porcine endothelial cells by xenogeneic CD40 signal

Acute humoral xenograft rejection (AHXR), characterized by thrombin generation and endothelial cell activation, should be overcome for the success of xenotransplantation. Fibrinogen-like protein 2 (fgl2) expressed on endothelial cells can convert prothrombin to thrombin directly, which indicates that the induced fgl2 expression in activated endothelial cells can contribute to thrombosis. In xenotransplant condition, the interaction between human CD40L and porcine endothelial CD40 can activate endothelial cells. In this study, we investigated the effect of endothelial cell activation through the interaction between human CD40L and porcine CD40 on fgl2 expression and its function as a direct prothrombinase. We found that CD40 stimulation up-regulated fgl2 expression as well as its enzymatic activity in porcine endothelial cells. Moreover, functional studies using knock-down system showed that the major factor converting human prothrombin to thrombin is fgl2 protein expressed on porcine endothelial cells. Overall, this study demonstrates that fgl2 expression can be induced by xenogeneic CD40 signal on endothelial cells and contribute to thrombin generation.

P- and E-selectin receptor antagonism prevents human leukocyte adhesion to activated porcine endothelial monolayers and attenuates porcine endothelial damage

BACKGROUND

Alongside the need to develop more effective and less toxic immunosuppression, the shortage of human organs available for organ transplantation is one of the major hurdles facing the field. Research into xenotransplantation, as an alternative source of organs, has unveiled formidable challenges. Porcine lungs perfused with human blood rapidly sequester the majority of circulating neutrophils and platelets, which leads to inflammation and organ failure within hours, and is not significantly attenuated by genetic modifications to the pig targeted to diminish antibody binding and complement and coagulation cascade activation.

METHODS

Here, we model the interaction of freshly isolated human leukocytes with xenotransplanted vasculature under physiologic flow conditions using microfluidic channels coated with porcine endothelial cells. Both isolated human neutrophils and whole human blood were perfused over transgenic pig aortic endothelial cells that had been activated with rhTNF-α or rhIL-4 using the BioFlux system. Novel compounds GMI-1271 and rPSGL1.Fc were tested as E- and P- selectin antagonists, respectively. Cellular adhesion and rolling events were tracked using FIJI (imageJ).

RESULTS

Porcine endothelium activated with either rhTNF-α or rhIL-4 expressed high amounts of selectins, to which isolated human neutrophils readily rolled and tethered. Both E-and P-selectin antagonism significantly reduced the number of neutrophils rolling and rolling distance in a dose-dependent manner, with near total inhibition at higher doses (P < .001). Similarly, with whole human blood, selectin blocking compounds exhibited dose-dependent inhibition of prevalent leukocyte adhesion and severe endothelial injury (Untreated: 394 ± 97 PMNs/hpf, 57 ± 6% loss EC; GMI1271+rPSGL1.Fc: 23 ± 9 PMNs/hpf, 8 ± 6% loss EC P < .01).

CONCLUSIONS

Selectin blockade may be useful as part of an integrated strategy to prevent neutrophil-mediated organ xenograft injury, especially during the early time points following reperfusion.

Biomaterials trigger endothelial cell activation when co-incubated with human whole blood

Endothelial cell activation resulting from biomaterial contact or biomaterial-induced blood activation may in turn also affect hemostasis and inflammatory processes in the blood. Current in vitro hemocompatibility assays typically ignore these modulating effects of the endothelium. This study describes a co-incubation system of human whole blood, biomaterial and endothelial cells (ECs) that was developed to overcome this limitation. First, human endothelial cells were characterized in terms of their expression of coagulation- and inflammation-relevant markers in response to various activators. Subsequently, their capacity to regulate hemostasis as well as complement and granulocyte activation was monitored in a hemocompatibility assay. After blood contact, quiescent ECs exhibited anticoagulant and anti-inflammatory properties. When they were co-incubated with surfaces exhibiting pro-coagulant or pro-inflammatory characteristics, the ECs down-regulated coagulation but not complement or leukocyte activation. Analysis of intracellular levels of the endothelial activation markers E-selectin and tissue factor showed that co-incubation with model surfaces and blood significantly increased the activation state of ECs. Finally, the coagulation- and inflammation-modulating properties of the ECs were tested after blood/biomaterial exposure. Pre-activation of ECs by biomaterials in the blood induced a pro-coagulant and pro-inflammatory state of the ECs, wherein the pro-coagulant response was higher for biomaterial/blood pre-activated ECs than for TNF-α-pre-activated cells. This work provides evidence that biomaterials, even without directly contacting the endothelium, affect the endothelial activation state with and have consequences for plasmatic and cellular reactions in the blood.

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.

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 Coronary Microcirculation in Hamster-to-Rat Cardiac Xenografts

BACKGROUND

The aim of this study was to establish a new experimental model to directly analyse the coronary microcirculation in cardiac xenografts.

METHODS

Intravital fluorescence microscopy (IVM) of the subepicardial microcirculation in heterotopically transplanted hamster-to-rat cardiac xenografts was performed at 30 and 90 min of reperfusion. We quantitatively assessed the microcirculatory perfusion characteristics as well as the interactions of leukocytes and platelets with the endothelium of postcapillary coronary venules in non-sensitised as well as sensitised recipients.

RESULTS

In this first experimental IVM study of cardiac xenografts, we successfully visualised the subepicardial microcirculation, i.e. feeding arterioles, nutritive capillaries and draining postcapillary venules, during reperfusion. Leukocyte-endothelial and platelet-endothelial cell interactions could be quantified. In the non-sensitised group, the myocardial microcirculation remained stable during the observation period of 90 min, whereas in the sensitised group, xenografts were rejected immediately.

CONCLUSIONS

We established a model for the assessment of the microcirculatory dysfunction and inflammation during ischaemia/reperfusion injury in hamster-to-rat cardiac xenografts.

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.

Suppression of NF-kappaB p65 expression attenuates delayed xenograft rejection

BACKGROUND

Delayed xenograft rejection (DXR) involves type II vascular endothelial cell (VEC) activation including upregulation of pro-inflammatory genes, which contributes to infiltration into the graft and a complex process of cytokine production. Approaches to prevent DXR have shown limited success. In this study, we modified heart donors using siRNA in an attempt to attenuate DXR and to improve xenograft survival in the mouse-to-rat heterotopic heart transplant model.

METHODS

siRNA technology was used to inhibit NF-kappaB p65 gene expression in vivo in mice. After the donor was transfected with siRNA, the effects of NF-kappaB siRNA on DXR and expression of NF-kappaB and pro-inflammatory genes were evaluated in a concordant mouse-to-rat cardiac xenograft model.

RESULTS

Treatment of NF-kappaB siRNA prolonged median heart graft survival time in the recipient rats from 1.7 days in a PBS control group to 5.4 days in the NF-kappaB siRNA-treated group (P < 0.05). Compared with normal mouse hearts, the NF-kappaB p65 mRNA relative levels following siRNA injection in the donors decreased significantly (approximately 70% reduction) in grafts harvested 12 h after transplantation. The mRNA levels of VCAM-1, ICAM-1, and interleukin-1 displayed a similar reduction. Histological evaluation using light and electron microscopy showed that damage of endothelial cells after NF-kappaB siRNA treament occured at a later time.

CONCLUSION

Transfection of NF-kappaB p65 siRNA in donor animals can delay the emergence of DXR. This treatment may be used as part of strategies to minimize the complex and multi-faceted rejection responses in vascularized xenografts.

Increased human tumor necrosis factor-α levels induce procoagulant change in porcine endothelial cells in vitro

BACKGROUND

Intravascular thrombosis and systemic coagulation abnormalities are major hurdles to successful xenotransplantation and are signs of acute humoral rejection. Increased expression of tissue factor (TF) is associated with the development of microvascular thrombosis in xenografts. To develop an effective strategy to prevent accelerated coagulation in xenografts, we investigated the mechanism by which porcine endothelial cells (PECs) become procoagulant after contact with human blood.

METHODS

The changes in TF mRNA levels and activity in PECs after incubation with 20% human serum or human bioactive molecules, including C5a, tumor necrosis factor-α (TNFα) and interleukin (IL)-1α, were evaluated using real-time PCR and the factor Xa chromogenic assay, respectively. The procoagulant changes in PECs by these agonists were evaluated by measuring the coagulation time of human citrated plasma suspended with PECs pretreated with each agonist. TF expression and coagulation times were also assessed in PECs transfected with short interfering RNA (siRNA) designed to knock down porcine TF. We also examined the production of proinflammatory cytokines in human whole-blood or plasma after contact with PECs, which were screened using the cytometric bead array system. TNFα levels were measured using ELISA in whole-blood after contact with PECs, with or without the addition of xenoreactive antibodies or C1 esterase inhibitor.

RESULTS

Porcine TF mRNA and activity in PECs were up-regulated in response to human TNFα and IL-1α but were not affected by C5a or 20% human serum. Up-regulation of TF expression by human TNFα or IL-1α shortened PEC-induced coagulation time, while siRNA-mediated knockdown of TF expression prolonged coagulation time. The incubation of PECs with human whole-blood led to a significant increase in human TNFα levels in the blood, which was promoted by the addition of xenoreactive antibodies and prevented by C1 esterase inhibitor.

CONCLUSIONS

Human TNFα level increases in human blood after contact with PECs, which is attributed to xenoreactive antibody binding and subsequent complement activation. Human TNFα induces procoagulant changes in PECs with increased TF expression. This study suggests that human TNFα may be one of the mediators linking complement activation with procoagulant changes in the xenoendothelium.

Immunobiology of liver xenotransplantation

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

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

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

Clinical 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.

The Porcine Aortic Tissue Culture System in vitro for Stem Cell Research

BACKGROUND AND OBJECTIVES

Due to the shortage of human donors for transplantation, the use of animal organs for xenotransplantation has come into great interest. Xeno-derived vessels and cardiac valves would be possible alternatives for the patient suffering from cardiovascular diseases. Therefore, we established in vitro culture system of a porcine vessel that could be helpful for the research of xenograft and stem cell research.

METHODS AND RESULTS

We primarily isolated porcine thoracic aorta, cultured square-shaped pieces up to 17 days and analyzed its morphology and characters. The endothelial cells were primarily isolated from cultured porcine aortic pieces and their morphology, function and character were analyzed in order to confirm them as endothelial cells at day 3, 4, 8, 10 and 17. Even at day 17, the morphology exhibited the intact endothelial layer as well as specifically expressed CD31 and von Willebrand factor. The morphology of primarily isolated cells from cultured tissues was identical as an endothelial cell. By flow cytometry analysis, more than 80% of the isolated cells expressed CD31 and up to 80% took up acetyl low density lipoprotein (ac-LDL) until day 10 of tissue culture period even though it decreased to about 50% at day 17 that means they not only showed typical endothelial cell characters but also functioned properly.

CONCLUSIONS

We successfully established and optimized a porcine vascular tissue in vitro culture system that could be a valuable model for in vitro study of xenotransplantation and stem cell research.

The immense potential of xenotransplantation in surgery

There is a limited availability of deceased human organs and cells for the purposes of clinical transplantation. Genetically-engineered pigs may provide an alternative source. Although several immune barriers need to be overcome, considerable progress has been made in experimental models in recent years, largely through the increasing availability of pigs with new genetic modifications. Pig heterotopic heart graft survival in nonhuman primates has extended for 8 months, with orthotopic grafts supporting life for almost 2 months. Life-supporting kidney transplants have functioned for almost 3 months. The current barriers are related to coagulation dysfunction between pig and primate that results in thrombotic microangiopathy and/or a consumptive coagulopathy, which may in part be related to molecular incompatibilities in the coagulation systems of pigs and primates. Current efforts are concentrated on genetically-modifying the organ- or islet-source pigs by the introduction of 'anticoagulant' or 'anti-thrombotic' genes to provide protection from the recipient coagulation cascade and platelet activation. Progress with pig islet xenotransplantation has been particularly encouraging with complete control of glycemia in diabetic monkeys extending in one case for >12 months. Other areas where experimental data suggest the possibility of early clinical trials are corneal xenotransplantation and pig neuronal cell xenotransplantation, for example, in patients with Parkinson's disease. With the speed of advances in genetic engineering increasing steadily, it is almost certain that the remaining problems will be overcome within the foreseeable future, and clinical allotransplantation will eventually become of historical interest only.

Histopathology of xenografts in pig to non-human primate discordant xenotransplantation

Xenotransplantation could provide a solution to the critical shortage of organs for transplantation in humans. Swine have been proposed as a suitable donor species. Swine organs, however, when transplanted to primates, are rapidly rejected by hyperacute rejection (HAR) and acute humoral xenograft rejection (AHXR). Both HAR and AHXR are triggered by xenoreactive natural antibodies directed against a specific epitope (galactose alpha1-3 galactose: Gal) on porcine vascular endothelium. In attempt to prevent HAR and AHXR, alpha1,3-galactosyltransferase gene knockout (GalT-KO) pigs have been produced. GalT-KO pig organs do not express the Gal epitope (antigen), and it therefore can eliminate the anti-Gal antibody--Gal antigen immunoreaction in xenotransplantation. We reported our initial study of kidney transplantation from GalT-KO miniature swine to baboons with either immunosuppression protocol or with a tolerance inducing protocol. Here, we discussed the pathology of xenografts in GalT-KO pig to non-human primate kidney transplantation.

Biomarkers of endothelial cell activation serve as potential surrogate markers for drug-induced vascular injury

Drug-induced vascular injury (DIVI) is a nonclinical finding that often confounds the toxicological evaluation of investigational drugs, but there is an absence of qualified biomarkers that can be used to detect and monitor its appearance in animals and patients during drug development and clinical use. It is well known that endothelial cell (EC) activation plays a key role in the expression and evolution of DIVI, and the various immunological and inflammatory factors involved in its expression may serve as potential biomarker candidates. Activated ECs change their morphology and gene expression, generating endothelial adhesion molecules, pro-coagulant molecules, cytokines, chemokines, vasodilators, nitric oxide, and acute-phase reactants. This review provides a brief historical background of EC activation and the search for biomarkers of early EC activation for monitoring DIVI. At present, no biomarkers of EC activation have been qualified to predict DIVI in the nonclinical or clinical context, and a robust pathologic foundation for their use is still lacking. We propose three categories of EC activation biomarkers: recommended surrogate markers, potentially useful markers, and emerging candidate markers. This review alerts pharmaceutical companies, research institutions, and regulatory agencies to the continuing need for reliable biomarkers of EC activation in drug development.

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

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

Overcoming the barriers to xenotransplantation: prospects for the future

Cross-species transplantation (xenotransplantation) has immense potential to solve the critical need for organs, tissues and cells for clinical transplantation. The increasing availability of genetically engineered pigs is enabling progress to be made in pig-to-nonhuman primate experimental models. Potent pharmacologic immunosuppressive regimens have largely prevented T-cell rejection and a T-cell-dependent elicited antibody response. However, coagulation dysfunction between the pig and primate is proving to be a major problem, and this can result in life-threatening consumptive coagulopathy. This complication is unlikely to be overcome until pigs expressing a human 'antithrombotic' or 'anticoagulant' gene, such as thrombomodulin, tissue factor pathway inhibitor or CD39, become available. Progress in islet xenotransplantation has been more encouraging, and diabetes has been controlled in nonhuman primates for periods in excess of 6 months, although this has usually been achieved using immunosuppressive protocols that might not be clinically applicable. Further advances are required to overcome the remaining barriers.

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.

Renal and cardiac endothelial heterogeneity impact acute vascular rejection in pig-to-baboon xenotransplantation

Xenograft outcomes are dictated by xenoantigen expression, for example, Gal alpha1, 3Gal (Gal), but might also depend on differing vascular responses. We investigated whether differential vascular gene expression in kidney and cardiac xenografts correlate with development of thrombotic microangiopathy (TM) and consumptive coagulation (CC). Immunosuppressed baboons underwent miniswine or hDAF pig kidney (n = 6) or heart (n = 7), or Gal-transferase gene-knockout (GalT-KO) (thymo)kidney transplantation (n = 14). Porcine cDNA miniarrays determined donor proinflammatory, apoptosis-related and vascular coagulant/fibrinolytic gene expression at defined time points; validated by mRNA, protein levels and immunopathology. hDAF-transgenic and GalT-KO xenografts, (particularly thymokidneys) exhibited prolonged survival. CC was seen with Gal-expressing porcine kidneys (3 of 6), only 1 of 7 baboons postcardiac xenotransplantation and was infrequent following GalT-KO grafts (1 of 14). Protective-type genes (heme oxygenase-I, superoxide dismutases and CD39) together with von Willebrand factor and P-selectin were upregulated in all renal grafts. Transcriptional responses in Gal-expressing xenografts were comparable to those seen in the infrequent GalT-KO rejection. In cardiac xenografts, fibrin deposition was associated with increased plasminogen activator inhibitor-1 expression establishing that gene expression profiles in renal and cardiac xenografts differ in a quantitative manner. These findings suggest that therapeutic targets may differ for renal and cardiac xenotransplants.

Coagulation dysregulation as a barrier to xenotransplantation in the primate

PURPOSE OF REVIEW

The ability to generate pigs expressing a human complement regulatory protein (hCRP) and/or pigs in which the alpha1,3-galactosyltransferase gene has been knocked out (GT-KO) has largely overcome the barrier of hyperacute rejection of a pig organ transplanted into a primate. However, acute humoral xenograft rejection (AHXR), presenting as microvascular thrombosis and/or consumptive coagulopathy, remains a major hurdle to successful xenotransplantation. This review summarizes recent studies of the coagulation problems associated with xenotransplantation, and discusses potential strategies to overcome them.

RECENT PROGRESS

Organ transplantation into nonhuman primates from GT-KO pigs that express a hCRP are not susceptible to hyperacute rejection. Nevertheless, most recipients of GT-KO and/or hCRP transgenic pig organs develop a consumptive coagulopathy, even when the graft remains functioning. This is associated with platelet aggregation, thrombocytopenia, anemia, and a tendency to bleed. Whilst this may reflect an ongoing immune response against the graft, (as exposure to anti-nonGal antibodies in vitro induces procoagulant changes in porcine ECs, even in the absence of complement), histological examination of the graft often shows only minimal features of immune injury, unlike grafts undergoing typical AHXR. Importantly, recent in vitro studies have indicated that the coincubation of porcine endothelial cells (ECs) with human platelets activates the platelets to express tissue factor, independent of a humoral immune response. These observations suggest that the use of organs from GT-KO pigs that express a hCRP may not be sufficient to prevent the development of a coagulation disorder following xenotransplantation, even if complete immunological tolerance can be achieved.

SUMMARY

Both thrombotic microangiopathy and systemic consumptive coagulopathy are increasingly recognized as barriers to successful xenotransplantation. The breeding of transgenic pigs with one or more human anticoagulant genes, such as CD39 or tissue factor pathway inhibitor, is anticipated to inhibit the procoagulant changes that take place on the graft ECs, and thus may prevent or reduce platelet activation that arises as a result of immune-mediated injury. The identification of the molecular mechanisms that develop between porcine ECs and human platelets may allow pharmacological approaches to be determined that inhibit the development of thrombotic microangiopathy and consumptive coagulopathy. Hopefully, further genetic modification of the organ-source pigs, combined with systemic drug therapy to the recipient, will prolong graft survival further.

Pre-treatment of porcine pulmonary xenograft with desmopressin: a novel strategy to attenuate platelet activation and systemic intravascular coagulation in an ex-vivo model of swine-to-human pulmonary xenotransplantation

BACKGROUND

Von Willebrand factor (vWF) has been proposed as a major contributor to the development of coagulopathy in pulmonary xenotransplantation. Pretreatment of donor swine with 1-deamino-8-d-arginine vasopressin (DDAVP), an analog of vasopressin, can reduce the content of vWF in pulmonary xenografts. Here, we investigate the effects of DDAVP pre-treatment in an ex-vivo perfusion model of pulmonary xenotransplantation.

METHODS

We set up and performed the ex-vivo perfusion using porcine pulmonary accessory lobes and fresh human whole blood (n = 12). Half of the donor swine were given 3 mug/kg DDAVP intravenously for 3 days before ex-vivo perfusion (DDAVP group) and half of them were left untreated (control group). The porcine lung was perfused with fresh blood for 1 h and changes in the following parameters were monitored: pulmonary arterial pressure, pulmonary vascular resistance, blood cell counts, fibrinogen, antithrombin, platelet factor 4, D-dimer, C3a, C4d, and xenoreactive IgM. The release of Galalpha1-3Gal xenoantigen (alphaGal) from porcine lung which had been perfused and retained for 30 min with human blood was assessed by enzyme-linked immunosorbent assay using alphaGal-binding lectin.

RESULTS

Both DDAVP and control groups showed typical findings of immediate pulmonary dysfunction: an increase of pulmonary vascular resistance and sequestration of leukocytes and platelets after ex-vivo perfusion. However, in the DDAVP group, the increase of platelet factor 4, C3a, and C4d after perfusion was attenuated compared to that in the control group. The release of alphaGal after blood retention was significantly lower in the DDAVP group than that of the control group.

CONCLUSION

Pre-infusion of DDAVP to the donor swine was beneficial in attenuating platelet activation as well as complement/coagulation activation. These effects of DDAVP are likely to relate to the reduction of porcine vWF content in the xenograft. Therefore, the modulation of vWF secretion in donor lungs could be an additional therapeutic way to reduce systemic coagulopathy in pulmonary xenotransplantation.

Platelet aggregation and thrombosis in xenotransplantation between pigs and humans

Allografts are currently short for clinical potential recipients. Organs and tissues from pigs could be a potential alternative source for clinical transplantation because of their high similarity in anatomical and physiological aspects. Thrombosis could be a consequence of the immunological response or the physiological incompatibilities in cell and molecular levels across species. Platelets play an essential role in haemostasis and the incompatibility of platelets between pigs and humans could be related to rejection and dysfunction of xenografts. Pig blood components, including plasma, leukocytes, red blood cells and platelets, could induce aggregation of human platelets directly, which then resulted in severe thrombosis after xenotransplantation. On the other hand, the existence of potential incompatibilities in coagulation and fibrinolytic system between pigs and humans in the context of xenotransplantation is an important consideration. Here we reviewed platelet incompatibility between pigs and humans related to thrombosis after xenotransplantation, and contribution of immunosuppressive agents to minimizing thrombosis and rejection.

Alpha1,3-galactosyltransferase gene-knockout pigs for xenotransplantation: where do we go from here?

The ability to genetically engineer pigs that no longer express the Galalpha1,3Gal (Gal) oligosaccharide has been a significant step toward the clinical applicability of xenotransplantation. Using a chronic immunosuppressive regimen based on costimulatory blockade, hearts from these pigs have survived from 2 to 6 months in baboons. Graft failure was predominantly from the development of a thrombotic microangiopathy. Potential contributing factors include the presence of preformed anti-nonGal antibodies or the development of low levels of elicited antibodies to nonGal antigens, natural killer (NK) cell or macrophage activity, and inherent coagulation dysregulation between pigs and primates. The breeding of pigs transgenic for an "anticoagulant" gene, such as human tissue factor pathway inhibitor, hirudin, or CD39, or lacking the gene for the prothrombinase, fibrinogen-like protein-2, is anticipated to inhibit the change in the endothelium to a procoagulant state that takes place in the pig organ after transplantation. The identification of the targets for anti-nonGal antibodies and/or human macrophages might allow further genetic modification of the pig, and xenogeneic NK cell recognition and activation may be inhibited by the transgenic expression of human leukocyte antigen molecules and/or by blocking the function of activating NK receptors. The ultimate goal of induction of T-cell tolerance may be possible only if these hurdles in the coagulation system and innate immunity can be overcome.

Pathology of renal xenograft rejection in pig to non-human primate transplantation

Xenotransplantation has the potential to alleviate the critical shortage of organs for transplantation in humans. Miniature swine are a promising donor species for xenotransplantation. However, when swine organs are transplanted into primates, hyperacute rejection (HAR), acute humoral xenograft rejection (AHXR), acute cellular xenograft rejection (ACXR), and chronic xenograft rejection prevent successful engraftment. Developing a suitable regimen for preventing xenograft rejection requires the ability to accurately diagnosis the severity and type of rejection in the graft. For this purpose, histopathology remains the most definitive and reliable tool. We discuss here the characteristic features of xenograft rejection in a preclinical pig-to-non-human primate transplantation model. In miniature swine to baboon xenotransplantation, marked interstitial hemorrhage develops in HAR, and renal microvascular injury develops with multiple platelet-fibrin microthrombi in both HAR and AHXR. T-cell-mediated cellular immunity plays an important role in ACXR. Chronic humoral and cellular rejection may induce chronic xenograft rejection, and will be a major cause of graft loss in discordant xenotransplantation.

Immunological analogy between allograft rejection, recurrent abortion and pre-eclampsia - the same basic mechanism?

There are still controversies concerning the role of immunological mechanisms engaged both in recurrent abortions (RA) and pre-eclampsia (PE). According to some opinions, recurrent miscarriage is comparable to organ-specific autoimmune disease. Analysis of immune reactions shows that graft rejection shares many similar mechanisms with RA and PE. This fact allows us to conclude that rejection of transplanted alloantigenic organs and pregnancy loss have probably the same evolutionary origin. Subsets and functions of immunocompetent cells (T CD4, suppressor gammadeltaT, cytotoxic T CD8, Treg, Tr1, uterine NK cells), over-activation of innate immunity (activation of NK cytotoxic cells, macrophages, neutrophils and complement), changes of Th1/Th2 cytokine balance (IL-2, IL-12, IL-15, IL-18, IFNgamma, TNFalpha vs. IL-4, IL-10, TGFbeta), importance of HLA-G molecule, CD200/CD200R interaction, over-expression of adhesion molecules, fgl2 prothrombinase activation and stimulation of IDO and HO expression, all suggest that RA and PE are syndromes of fetal allograft rejection, and not organ-specific autoimmune diseases. According to that supposition, an analogy might exist between acute graft rejection and recurrent abortion, and between chronic graft rejection and pre-eclampsia.

Endothelial cell protection in xenotransplantation: looking after a key player in rejection

The endothelium, as an organ at the interface between the intra- and extravascular space, actively participates in maintaining an anti-inflammatory and anti-coagulant environment under physiological conditions. Severe humoral as well as cellular rejection responses, which accompany cross-species transplantation of vascularized organs as well as ischemia/reperfusion injury, primarily target the endothelium and disrupt this delicate balance. Activation of pro-inflammatory and pro-coagulant pathways often lead to irreversible injury not only of the endothelial layer but also of the entire graft, with ensuing rejection. This review focuses on strategies targeted at protecting the endothelium from such damaging effects, ranging from genetic manipulation of the donor organ to soluble, as well as membrane-targeted, protective strategies.

Targeted deletion of Fgl-2/fibroleukin in the donor modulates immunologic response and acute vascular rejection in cardiac xenografts

BACKGROUND

Xenografts ultimately fail as a result of acute vascular rejection (AVR), a process characterized by intravascular thrombosis, fibrin deposition, and endothelial cell activation.

METHODS AND RESULTS

We studied whether targeted deletion of Fgl-2, an inducible endothelial cell procoagulant, (Fgl-2-/-) in the donor prevents AVR in a mouse-to-rat cardiac xenotransplantation model. By 3 days after transplant, Fgl-2+/+ grafts developed typical features of AVR associated with increased levels of donor Fgl-2 mRNA. Grafts from Fgl-2-/- mice had reduced fibrin deposition but developed cellular rejection. Treatment with a short course of cobra venom factor and maintenance cyclosporine resulted in long-term acceptance of both Fgl-2+/+ and Fgl-2-/- grafts. On withdrawal of cyclosporine, Fgl-2+/+ grafts developed features of AVR; in contrast, Fgl-2-/- grafts again developed acute cellular rejection. Rejecting Fgl-2+/+ hearts stained positively for IgG, IgM, C3, and C5b-9, whereas rejecting Fgl-2-/- hearts had minimal Ig and complement deposition despite xenoantibodies in the serum. Furthermore, serum containing xenoantibodies failed to stain Fgl-2-/- long-term treated hearts but did stain wild-type heart tissues. Treatment of Fgl-2-/- xenografts with mycophenolate mofetil and tacrolimus, a clinically relevant immune suppression protocol, led to long-term graft acceptance.

CONCLUSIONS

Deletion of Fgl-2 ameliorates AVR by downregulation of xenoantigens and may facilitate successful clinical heart xenotransplantation.

Monocyte-induced endothelial calcium signaling mediates early xenogeneic endothelial activation

Hallmarks of delayed xenograft rejection include monocyte infiltration, endothelial cell activation and disruption of the endothelial barrier. The monocyte is an important initiator of this type of rejection because monocytes accumulate within hours after xenografting and prior monocyte depletion suppresses the development of this type of rejection. However, the mechanisms that mediate monocyte-induced xenograft injury are unclear at present. Here we report that human monocytes activate xenogeneic endothelial cells through calcium signals. Monocyte contact with porcine but not human endothelium leads to an endothelial calcium transient mediated via a G-protein-coupled receptor (GPCR) that results in up-regulation of porcine VCAM-1 and E-selectin. Although human monocyte adhesion was greater to porcine than to human endothelium, especially when studied under laminar flow, blockade of the xeno-specific endothelial calcium signals did not reduce adhesion of human monocytes to porcine endothelium. Human monocyte contact to porcine endothelium also resulted in reorganization of the F-actin cytoskeleton with a concomitant increase in endothelial monolayer permeability. In contrast to the effect on adhesion, these changes appear to be regulated through endothelial calcium signals. Taken together, these data suggest that human monocytes are capable of activating xenogeneic endothelial cells through calcium transients, as well as other distinct pathways.

Xenotransplantation-current status and future perspectives

Research efforts have shed light on the immunological obstacles to long-term survival of pig organs transplanted into primates and allowed the identification of targets for specific immune intervention. Accordingly, the development of genetically engineered animals has overcome the hyperacute rejection barrier, with acute humoral xenograft rejection (AHXR) currently remaining the most important immunological obstacle. At this stage, a better control of the elicited anti-pig humoral immune response and avoidance of coagulation disorders are the two primary research fronts being pursued in order to overcome AHXR. Nonetheless, it is encouraging that porcine xenografts can sustain the life of non-human primates for several months. Proactive research aimed at the development of a safer organ source is also underway. It is anticipated that ongoing research in several fields, including accommodation, tolerance, immune suppression and genetic engineering, will result in further improvements in non-human primate survival. However, until convincing efficacy data and a more favourable risk/benefit ratio can be established in relevant animal models, progression to the clinic should not be viewed as an option.

Combining the hDAF transgene with the GP IIb/IIIa inhibitor tirofiban improves heart performance and reduces myocardial damage following hyperacute rejection in an ex vivo perfusion model

Xenograft rejection is associated with vascular injury resulting at least in part from platelet activation, and rejected xenografts invariably demonstrate intravascular thrombosis. Assuming that complement activation is a major determinant of humoral immune reactions bringing about platelet-endothelial cell interactions, we tested the effects of the specific platelet glycoprotein IIb/IIIa inhibitor tirofiban in combination with the human decay accelerating factor (hDAF) transgene on hyperacute rejection of pig hearts. Four groups were studied in a working heart-perfusion model. Pig hearts transgenic for hDAF and nontransgenic pig hearts were perfused with human blood containing tirofiban or with unmodified human blood. Cardiac output, stroke work index, and creatine phosphokinases were measured for the evaluation of the extent of myocardial damage. Consumption of complement components was determined. Endothelial deposition of fibrin and intravascular thrombosis were evaluated. Tirofiban improved cardiac output and stroke work index of nontransgenic pig hearts and was able to further increase hemodynamic function of hDAF transgenic pig hearts. Low levels of creatine phosphokinases also revealed a cardioprotective effect of tirofiban. However, a further extension of the survival of hDAF transgenic pig hearts could not be achieved, although tirofiban prolonged beating time of nontransgenic pig hearts. Tirofiban was able to reduce the consumption of complement components independently of hDAF. Intravascular evidence of fibrin and thrombosis tended to be particularly reduced by the combination of tirofiban and hDAF. Thus, the application of tirofiban together with hDAF improves the performance of pig hearts by reducing myocardial damage and intravascular thrombosis.

TMVA, a novel GPIb-binding protein, significantly prevents platelet microthrombi formation and prolongs discordant cardiac xenograft survival

In xenotransplantation, donor endothelium is the first target of immunological attack. Activation of the endothelial cell by preformed natural antibodies leads to platelet binding via the interaction of the glycoprotein (GP) Ib and von Willebrand factor (vWF). TMVA is a novel GPIb-binding protein purified from the venom of Trimeresurus mucrosquamatus. In this study, the inhibitory effect of TMVA on platelet aggregation in rats and the effect on discordant guinea pig-to-rat cardiac xenograft survival were investigated. Three doses (8, 20 or 40 microg/kg) of TMVA were infused intravenously to 30 rats respectively. Platelet aggregation rate was assayed 0.5, 12, and 24 h after TMVA administration. Wister rats underwent guinea pig cardiac cervical heterotopic transplantation using single dosing of TMVA (20 microg/kg, i.v., 0.5 h before reperfusion). Additionally, levels of TXB(2) and 6-keto-PGF(1alpha) within rejected graft tissues were determined by radioimmunoassay. Treatment with TMVA at a dose of 20 or 40 microg/kg resulted in complete inhibition of platelet aggregation 0.5 h after TMVA administration. Rats receiving guinea pig cardiac xenografts after TMVA therapy had significantly prolonged xenograft survival. Histologic and immunopathologic analysis of cardiac xenografts in TMVA treatment group showed no intragraft platelet microthrombi formation and fibrin deposition. Additionally, the ratio of 6-keto-PGF(1alpha) to TXB(2) in TMVA treatment group was significantly higher than those in control group. We conclude that the use of this novel GPIb-binding protein was very effective in preventing platelet microthrombi formation and fibrin deposition in a guinea pig-to-rat model and resulted in prolongation of xenograft survival. The increased ratio of PGI(2)/TXA(2) in TMVA treatment group may protect xenografts from the endothelial cell activation and contribute to the prolongation of xenograft survival.

Thromboregulatory manifestations in human CD39 transgenic mice and the implications for thrombotic disease and transplantation

Extracellular nucleotides play an important role in thrombosis and inflammation, triggering a range of effects such as platelet activation and recruitment, endothelial cell activation, and vasoconstriction. CD39, the major vascular nucleoside triphosphate diphosphohydrolase (NTPDase), converts ATP and ADP to AMP, which is further degraded to the antithrombotic and anti-inflammatory mediator adenosine. Deletion of CD39 renders mice exquisitely sensitive to vascular injury, and CD39-null cardiac xenografts show reduced survival. Conversely, upregulation of CD39 by somatic gene transfer or administration of soluble NTPDases has major benefits in models of transplantation and inflammation. In this study we examined the consequences of transgenic expression of human CD39 (hCD39) in mice. Importantly, these mice displayed no overt spontaneous bleeding tendency under normal circumstances. The hCD39 transgenic mice did, however, exhibit impaired platelet aggregation, prolonged bleeding times, and resistance to systemic thromboembolism. Donor hearts transgenic for hCD39 were substantially protected from thrombosis and survived longer in a mouse cardiac transplant model of vascular rejection. These thromboregulatory manifestations in hCD39 transgenic mice suggest important therapeutic potential in clinical vascular disease and in the control of serious thrombotic events that compromise the survival of porcine xenografts in primates.

Endothelial Induction of fgl2 Contributes to Thrombosis during Acute Vascular Xenograft Rejection

Thrombosis is a prominent feature of acute vascular rejection (AVR), the current barrier to survival of pig-to-primate xenografts. Fibrinogen-like protein 2 (fgl2/fibroleukin) is an inducible prothrombinase that plays an important role in the pathogenesis of fibrin deposition during viral hepatitis and cytokine-induced fetal loss. We hypothesized that induction of fgl2 on the vascular endothelium of xenografts contributes to thrombosis associated with AVR. We first examined fgl2 as a source of procoagulant activity in the pig-to-primate combination. The porcine fgl2 (pfgl2) was cloned and its chromosomal locus was identified. Recombinant pfgl2 protein expressed in vitro was detected on the cell surface and generated thrombin from human prothrombin. Studies of pig-to-baboon kidney xenografts undergoing AVR in vivo revealed induction of pfgl2 expression on graft vascular endothelial cells (ECs). Cultured porcine ECs activated by human TNF-alpha in vitro demonstrated induction of pfgl2 expression and enhanced activation of human prothrombin. The availability of gene-targeted fgl2-deficient mice allowed the contribution of fgl2 to the pathogenesis of AVR to be directly examined in vivo. Hearts heterotopically transplanted from fgl2(+/+) and fgl2(+/-) mice into Lewis rats developed AVR with intravascular thrombosis associated with induction of fgl2 in graft vascular ECs. In contrast, xenografts from fgl2(-/-) mice were devoid of thrombosis. These observations collectively suggest that induction of fgl2 on the vascular endothelium plays a role in the pathogenesis of AVR-associated thrombosis. Manipulation of fgl2, in combination with other interventions, may yield novel strategies by which to overcome AVR and extend xenograft survival.

Heme oxygenases in pregnancy II: HO-2 is downregulated in human pathologic pregnancies

PROBLEM

We previously reported a diminished expression of the heme-degrading enzymes heme oxygenases (HO)-1 and HO-2 in decidua and placenta from mice undergoing Th1-mediated abortion, strongly indicating the protective effect of HO in murine pregnancy maintenance. Here we investigated whether the expression of HO-1 and HO-2 is also reduced at the feto-maternal interface of pathologic human pregnancies.

METHOD OF STUDY

Immunohistochemistry was used to detect HOs expression in placental and decidual first-trimester tissue from patients with: spontaneous abortion (n = 14), choriocarcinoma (n = 14), hydatidiform mole (H-mole) (n = 12), compared with normally progressing pregnancies (n = 15). Further, we investigated early third-trimester decidual and placental tissue from patients with pre-eclampsia (n = 13) compared with fetal growth retardation (n = 14) as age-matched controls.

RESULTS

In first trimester tissue, we observed a significant reduction of HO-2 expression in invasive trophoblast cells, endothelial cells, and syncytiotrophoblasts in samples from patients with spontaneous abortion compared with normal pregnancy. H-mole samples showed a diminished expression of HO-2 in invasive trophoblast cells and endothelial cells in comparison with NP, whereas choriocarcinoma samples showed no significant differences compared with the control. In third trimester tissue, HO-2 was also reduced in syncytiotrophoblasts and invasive trophoblast cells from pre-eclampsia compared with samples from fetal growth retardation. HO-1 expression was diminished in all pathologies investigated; however, the differences did not reach levels of significance.

CONCLUSIONS

Our data indicate that HOs play a crucial role in pregnancy and low expression of HO-2, as observed in pathologic pregnancies, may lead to enhanced levels of free heme at the feto-maternal interface, with subsequent upregulation of adhesion molecules, allowing enhanced inflammatory cells migration to the feto-maternal interface.

Targeted gene delivery to sinusoidal endothelial cells: DNA nanoassociate bearing hyaluronan-glycocalyx

Liver sinusoidal endothelial cells (SECs) possess unique receptors that recognize and internalize hyaluronic acid (HA). To develop a system for targeting foreign DNA to SECs, comb-type polycations having HA side chains were prepared by coupling HA to poly(L-lysine) (PLL). The HA-grafted-PLL copolymer (PLL-g-HA) thus formed was mixed with DNA in 154 mM NaCl to form soluble nanoassociates bearing hydrated hyaluronate shells. Agarose gel retardation assays revealed selective interaction of the PLL backbone with DNA despite the presence of polyanionic HA side chains. To determine whether the PLL-g-HA/DNA complexes were recognized by SEC HA receptors in vivo, we injected Wistar rats i.v. via the tail vein with PLL-g-HA complexed to a beta-galactosidase expression plasmid (pSV beta-Gal) labeled with 32P. One hour postinjection, >90% of the injected radioactivity remained in the liver. Administration of the PLL-g-HA complexed to an FITC-labeled DNA revealed that the carrier-DNA complex was distributed exclusively in SECs. A large number of SECs expressing beta-galactosidase was detected along the sinusoidal lining after transfection with PLL-g-HA/pSV beta-Gal. Moreover, PLL-g-HA effectively stabilized DNA triplex formation. In conclusion, the new PLL-g-HA/DNA carrier system permits targeted transfer of exogenous genes selectively to the SECs.

Xenotransplantation

The continued and growing success of lung allotransplantation has intensified the worldwide shortage of donor organs. Yet, xenotransplantation remains a daunting challenge. Additional molecular incompatibilities and unforeseen complications will continue to be discovered. Progress has been made, notably on the generation of alpha-Gal double knockout pigs. Progressive increases in organ survival times have been seen for most organs after significant investments of time and money. The lung continues to be an organ with the lowest supply of cadaveric donors and the least potential for expanded living donation or mechanical alternatives. As such, the impetus for xenotransplantation is strong. The lung appears to be exquisitely sensitive to xenograft rejection and resistant to strategies that have been moderately successful in other organs. A complex program involving genetically modified donor organs, recipient preparation for antibody removal or tolerance promotion, and multitargeted drug therapy will likely be required for successful clinical application.

Potential implications of ABO blood group for vascular rejection in pig to human kidney xenotransplantation

BACKGROUND

A substantial hurdle for successful xenotransplantation is to negate the effect of xenoreactive natural antibodies [mainly Galalpha1-3Galbeta1-4GlcNAc (alpha-Gal) specific] that cause hyperacute xenograft rejection. Galalpha1-3Gal molecules (alpha-Gal) have close structural homology with human ABO blood groups and therefore an individual's blood group might influence the formation of alpha-Gal specific antibodies. Genetic heterogeneity controlling alpha-Gal specific antibody formation could have important implications for future pig to human xenotransplantation clinical trials. We have investigated the relationship between ABO blood group and immunoglobulin M (IgM) and immunoglobulin G (IgG) alpha-Gal specific antibody titres in sera obtained from renal dialysis patients and healthy blood donors.

METHODS

Serially diluted sera (n = 166) obtained from renal dialysis patients awaiting kidney transplantation (n = 116) and healthy blood donors (n = 50) were tested for IgM and IgG alpha-Gal antibodies using an enzyme-linked immunosorbent assay (ELISA) specific for alpha-Gal. The study cohort comprised 62, 48, 36 and 20 sera obtained from blood group O, A, B and AB individuals, respectively. Reciprocal alpha-Gal specific antibody titres were calculated from ELISA titration curves and stratified by individual blood group.

RESULTS

No significant heterogeneity was found in IgM alpha-Gal specific antibody titres across ABO blood groups. In contrast, marked heterogeneity was observed in IgG alpha-Gal specific antibody titres when stratified by blood group. IgG alpha-Gal specific antibody titres were higher in sera obtained from blood group O renal dialysis patients [median titre 40, interquartile range (IQR) 14 to 72], compared with blood group A (median titre 18, IQR 7 to 54, P = 0.05), blood group B (median titre 6, IQR 0 to 15, P < 0.001) and blood group AB patients (median titre 3.5, IQR 0 to 16, P = 0.002). A similar correlation was found for IgG alpha-Gal specific antibody titres in sera obtained from healthy blood donors with median titres of 20 (IQR 12 to 34), 37 (10 to 91), 9 (0 to 20), and 5.5 (0 to 12) in blood groups O, A, B and AB individuals, respectively. There was a strong interrelationship between alpha-Gal specific antibody class and blood group, with both IgM and IgG alpha-Gal specific antibodies found in 84% of the blood group O sera, 73% of blood group A sera, 50% of blood group B sera and 40% of blood group AB sera (P < 0.001). In a subgroup of 39 renal dialysis patients, IgM and IgG alpha-Gal specific antibody titres were measured in two serum samples obtained at different time-points (median time interval 581 days, range 42 to 4414), and showed a high degree of stability (correlation coefficient 0.88 and 0.90 for IgM and IgG, respectively).

CONCLUSION

IgG alpha-Gal specific antibody titres are significantly higher in the sera of blood group O and A renal dialysis patients and healthy individuals compared with blood groups B and AB. These data indicate that future clinical trials of pig to human xenotransplantation may be more problematic for non-blood group B patients who are likely to have high levels of IgG alpha-Gal specific antibodies that are associated with acute vascular rejection.

Islet transplantation, stem cells, and transfusion medicine

Despite the widespread use of exogenous insulin, morbidity and mortality caused by type 1 diabetes mellitus (DM) continue to place a significant burden on society, both in terms of human suffering and cost. The transplantation of vascularized pancreas, usually performed concurrently with renal transplantation, can cure type 1 DM, as shown by results in more than 15000 such transplants over about 30 years. Transplantation of isolated pancreatic islets, instead of the whole organ, however, offers an attractive alternative that minimizes surgery and its complications. Although islet transplantation initially met with only modest success (only about 9% insulin independence at 1 year posttransplant), recent changes in patient selection criteria, number and treatment of islets transplanted, and better immunosuppressive regimens dramatically improved the results; spawning widespread enthusiasm for islet transplantation. Despite this promise, organ/islet availability remains an important limitation to this technology. A solution to the problem of limited materials for transplantation may be in the use of stem/progenitor cells. This article reviews the background of the current enthusiasm for pancreatic islet cell transplantation, highlights future research trends in the field, and suggests that the new islet-related cellular therapies belong within the domain of transfusion medicine.