Microcoagulation processes after xenotransplantation:

Four-dimensional characterization of thrombosis in a live-cell, shear-flow assay: development and application to xenotransplantation

Background

Porcine xenografts are a promising source of scarce transplantable organs, but stimulate intense thrombosis of human blood despite targeted genetic and pharmacologic interventions. Current experimental models do not enable study of the blood/endothelial interface to investigate adhesive interactions and thrombosis at the cellular level under physiologic conditions. The purpose of this study was to develop and validate a live-cell, shear-flow based thrombosis assay relevant to general thrombosis research, and demonstrate its potential in xenotransplantation applications.

Methodology/Principal Findings

Confluent wild-type (WT, n = 48) and Gal transferase knock-out (GalTKO, which resist hyperacute rejection; n = 11) porcine endothelia were cultured in microfluidic channels. To mimic microcirculatory flow, channels were perfused at 5 dynes/cm² and 37°C with human blood stained to fluorescently label platelets. Serial fluorescent imaging visualized percent surface area coverage (SA, for adhesion of labeled cells) and total fluorescence (a metric of clot volume). Aggregation was calculated by the fluorescence/SA ratio (FR). WT endothelia stimulated diffuse platelet adhesion (SA 65 ± 2%) and aggregation (FR 120 ± 1 a.u.), indicating high-grade thrombosis consistent with the rapid platelet activation and consumption seen in whole-organ lung xenotransplantation models. Experiments with antibody blockade of platelet aggregation, and perfusion of syngeneic and allo-incompatible endothelium was used to verify the biologic specificity and validity of the assay. Finally, with GalTKO endothelia thrombus volume decreased by 60%, due primarily to a 58% reduction in adhesion (P < 0.0001 each); importantly, aggregation was only marginally affected (11% reduction, P < 0.0001).

Conclusions/Significance

This novel, high-throughput assay enabled dynamic modeling of whole-blood thrombosis on intact endothelium under physiologic conditions, and allowed mechanistic characterization of endothelial and platelet interactions. Applied to xenogeneic thrombosis, it enables future studies regarding the effect of modifying the porcine genotype on sheer-stress-dependent events that characterize xenograft injury. This in-vitro platform is likely to prove broadly useful to study thrombosis and endothelial interactions under dynamic physiologic conditions.

Lung xenotransplantation: recent progress and current status

Xenotransplantation has undergone important progress in controlling initial hyperacute rejection in many preclinical models, with some cell, tissue, and organ xenografts advancing toward clinical trials. However, acute injury, driven primarily by innate immune and inflammatory responses, continues to limit results in lung xenograft models. The purpose of this article is to review the current status of lung xenotransplantation--including the seemingly unique challenges posed by this organ-and summarize proven and emerging means of overcoming acute lung xenograft injury.

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.

Antibody-mediated xenograft injury: mechanisms and protective strategies

The use of porcine organs for clinical transplantation is a promising potential solution to the shortage of human organs. Preformed anti-pig antibody is the primary cause of hyperacute rejection, while elicited antibody can contribute to subsequent "delayed" xenograft rejection. This article will review recent progress to overcome antibody mediated xenograft rejection, through modification of the host immunity and use of genetically engineered pig organs.

The spectrum of thrombotic thrombocytopenic purpura: a clinicopathologic demonstration of tacrolimus-induced thrombotic thrombocytopenic purpura in a lung transplant patient

Immunosuppressive drugs used post-transplantation are among the most common causes of thrombotic thrombocytopenic purpura (TTP). Diagnosis is often confounded not only by its myriad presentations, but also because these manifestations may be explained by the comorbidities or complications of transplantation. A 61-year-old female who had a single lung transplant for severe chronic obstructive pulmonary disease maintained on corticosteroids, tacrolimus and mycophenolate mofetil, was admitted for fever, headache with confusion and lethargy. She was mildly anemic and thrombocytopenic. Peripheral smear showed rare fragmented red cells. Muddy brown casts were present on urinalysis. She was diagnosed with TTP. Tacrolimus was discontinued and the mental status of the patient, anemia and thrombocytopenia improved significantly.

Recombinant pig TFPI efficiently regulates human tissue factor pathways

Rejected pig-to-primate organ xenografts almost invariably exhibit significant microvascular thrombosis, believed to be due in part to several molecular incompatibilities affecting the regulation of coagulation. In this study, we tested one such proposed incompatibility: whether there is, at least in part, a functional incompatibility in pig tissue factor pathway inhibitor (TFPI) that impedes binding of human factor Xa and regulation of human tissue factor-initiated coagulation. TFPIalpha cDNA was cloned from pig aortic endothelial cells and found to encode a 279-residue mature protein with 79% overall identity to human TFPIalpha, increasing to 88 to 90% in the functional Kunitz-1 and Kunitz-2 domains. Transfected primate cells expressing equivalent levels of GPI-linked pig or human TFPIalpha were assayed for binding of human factor Xa and inhibition of the human factor VIIa/tissue factor complex. The activity of the expressed pig anticoagulant was equivalent to that of the human protein in both measures of TFPI function in these systems. These data indicate that there are no apparent incompatibilities between recombinant pig TFPI and the human tissue factor pathway. Other factors must account for the thromboregulatory failure of pig endothelium and aberrant tissue factor activity in xenograft rejection.

Expression of tissue factor and initiation of clotting by human platelets and monocytes after incubation with porcine endothelial cells

OBJECTIVES

Intravascular thrombosis remains a major barrier to successful pig-to-primate xenotransplantation. However, the precise factors initiating thrombosis are unknown. In this study, we investigated the contribution of recipient platelets and monocytes.

METHODS

Primary pig aortic endothelial cells (PAECs) were incubated with combinations of fresh or heat-inactivated human plasma, platelets, or monocytes, after which they were separated and analyzed individually by flow cytometry for tissue factor (TF) expression and for their ability to clot recalcified normal or factor-VII-deficient plasma.

RESULTS

Procoagulant porcine TF was induced in PAECs only by fresh human plasma, and not by heat-inactivated plasma, platelets, or monocytes. In contrast, procoagulant human TF was induced on platelets and monocytes after incubation with PAEC, irrespective of whether the plasma was present or not. In addition, human platelets caused the shedding of procoagulant TF-expressing aggregates from PAEC.

CONCLUSIONS

This work defines a cell-based in vitro assay system to address complex interactions among PAECs, human platelets, and monocytes. The induction of procoagulant TF on PAECs by fresh human plasma was most likely dependent on xenoreactive natural antibody and complement present in fresh human plasma. In contrast, the shedding of procoagulant platelet-PAEC aggregates, induced by human platelets, and the induction of procoagulant TF on human platelets and monocytes by PAEC, occurred independently of these factors. These results suggest that different mechanisms may contribute to the initiation of thrombosis after xenotransplantation, some of which may not be influenced by the further manipulation of the immune response against pig xenografts.

Distribution of the alphaGal- and the non-alphaGal T-antigens in the pig kidney: potential targets for rejection in pig-to-man xenotransplantation

Carbohydrate antigens, present on pig vascular endothelial cells, seem to be the prime agents responsible for graft rejection, and although genetically modified animals that express less amounts of carbohydrate antigen are available, it is still useful to decide the localization of the reactive xenoantigens in organs contemplated for xenotransplantation. Here we compare the distribution in pig kidney of antigens important in xenograft destruction, namely the Galalpha1-3Gal (alphaGal) glycans, with the localization of the T-antigen (Galbeta1-3GalNAc). The alpha-galactose-specific lectin Griffonia simplicifolia isolectin 1B4 was used to detect the Galalpha1-3Gal glycans, whereas Arachis hypogaea (PNA) lectin and a monoclonal antibody (3C9) detected T-antigen. In addition, two vascular markers (anti-caveolin-1 and anti-von Willebrand factor) served to identify vascular structures of the kidney. Both conventional fluorescence and confocal microscopy were used to distinguish lectin and immunohistochemical staining. On the basis of fluorescence signals, the results indicate that the carbohydrate antigens are heterogeneously distributed in the pig kidney. alphaGal epitopes were sparse in the capillary loops forming the glomeruli and in the capillaries surrounding the convoluted tubules, but showed stronger staining in capillaries surrounding the limbs of Henle. In addition, the brush border and basement membranes of the convoluted tubules strongly reacted with the GS1-B4-lectin. Finally, the Galalpha1-3Gal glycans were also present on epithelial cells of the large collecting tubules. Regarding the T-antigen, PNA and 3C9 reacted with different glomerular cells, whereas both reacted strongly with the endothelial cells lining the large kidney vessels. Human serum incubation of pig kidney sections, in which the alphaGal epitopes were blocked by unconjugated GS1-B4, showed staining of the same vascular structures as were obtained after incubation with the T-antigen-detecting agents. The study thus proves a complex spatial distribution of carbohydrate antigens relevant for xenotransplantation of pig kidney.

Frankenswine, or bringing home the bacon: How close are we to clinical trials in xenotransplantation?

Xenotransplantation-specifically from pig into human-could resolve the critical shortage of organs, tissues and cells for clinical transplantation. Genetic engineering techniques in pigs are relatively well-developed and to date have largely been aimed at producing pigs that either (1) express high levels of one or more human complement-regulatory protein(s), such as decay-accelerating factor or membrane cofactor protein, or (2) have deletion of the gene responsible for the expression of the oligosaccharide, Galalpha1,3Gal (Gal), the major target for human anti-pig antibodies, or (3) have both manipulations. Currently the transplantation of pig organs in adequately-immunosuppressed baboons results in graft function for periods of 2-6 months (auxiliary hearts) and 2-3 months (life-supporting kidneys). Pig islets have maintained normoglycemia in diabetic monkeys for >6 months. The remaining immunologic barriers to successful xenotransplantation are discussed, and brief reviews made of (1) the potential risk of the transmission of an infectious microorganism from pig to patient and possibly to the public at large, (2) the potential physiologic incompatibilities between a pig organ and its human counterpart, (3) the major ethical considerations of clinical xenotransplantation, and (4) the possible alternatives that compete with xenotransplantation in the field of organ or cell replacement, such as mechanical devices, tissue engineering, stem cell biology and organogenesis. Finally, the proximity of clinical trials is discussed. Islet xenotransplantation is already at the stage where clinical trials are actively being considered, but the transplantation of pig organs will probably require further genetic modifications to be made to the organ-source pigs to protect their tissues from the coagulation/anticoagulation dysfunction that plays a significant role in pig graft failure after transplantation in primates.

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.

The interface between coagulation and immunity

Coagulation proteases are involved in generating fibrin after vascular injury (hemostasis) but they also have multiple other effects, many of which are mediated independently of fibrin generation, via interactions with specific cell membrane-expressed "protease activated receptors". In inflammation, this family of proteins has a complex influence, the facets of which are still incompletely understood, though a common feature in different models appears to be amplification of innate signals that are initially generated by pathogenic elements or, in the context of transplantation, ischemia or anti-graft antibodies, for instance. There is increasing evidence that these proteases may also have specific effects on cells involved in adaptive immunity and on cells that mediate chronic inflammation and fibrosis. Understanding whether these effects are relevant in the responses generated against transplanted organs is important, as it could lead ultimately to the development of novel ways to promote long-term graft survival.

Coagulation and the xenograft endothelium

Acute humoral rejection remains the major barrier to long-term pig-to-primate xenograft survival, and microvascular thrombosis is a critical element of the rejection process. It appears that persistent endothelial cell activation and injury, by even low levels of anti-graft antibodies, eventually overwhelm the cellular anticoagulant defences and promote the development of thrombotic microangiopathy. Porcine endothelium may be particularly vulnerable because of cross-species molecular incompatibilities affecting the function of thrombomodulin and possibly TFPI. Recent data from small animal models suggest that transgenic overexpression of anti-thrombotic molecules on xenograft endothelium is capable of inhibiting intravascular thrombosis and preventing acute humoral rejection. In conjunction with existing genetic modifications (e.g. Gal KO, hDAF), this is a promising strategy to move xenotransplantation to the clinic.