IMPROVED HEPATIC MICROCIRCULATION BY HUMAN SOLUBLE URINARY THROMBOMODULIN IN THE XENO-PERFUSED PORCINE LIVER

A review of pig liver xenotransplantation: Current problems and recent progress

Pig liver xenotransplantation appears to be more perplexing when compared to heart or kidney xenotransplantation, even though great progress has been achieved. The relevant molecular mechanisms involved in xenogeneic rejection, including coagulopathy, and particularly thrombocytopenia, are complex, and need to be systematically investigated. The deletion of expression of Gal antigens in the liver graft highlights the injurious impact of nonGal antigens, which continue to induce humoral rejection. Innate immunity, particularly mediated by macrophages and natural killer cells, interplays with inflammation and coagulation disorders. Kupffer cells and liver sinusoidal endothelial cells (LSECs) together mediate leukocyte, erythrocyte, and platelet sequestration and phagocytosis, which can be exacerbated by increased cytokine production, cell desialylation, and interspecies incompatibilities. The coagulation cascade is activated by release of tissue factor which can be dependent or independent of the xenoreactive immune response. Depletion of endothelial anticoagulants and anti-platelet capacity amplify coagulation activation, and interspecies incompatibilities of coagulation-regulatory proteins facilitate dysregulation. LSECs involved in platelet phagocytosis and transcytosis, coupled with hepatocyte-mediated degradation, are responsible for thrombocytopenia. Adaptive immunity could also be problematic in long-term liver graft survival. Currently, relevant evidence and study results of various genetic modifications to the pig donor need to be fully determined, with the aim of identifying the ideal transgene combination for pig liver xenotransplantation. We believe that clinical trials of pig liver xenotransplantation should initially be considered as a bridge to allotransplantation.

Regulatory sequences of the porcine THBD gene facilitate endothelial-specific expression of bioactive human thrombomodulin in single- and multitransgenic pigs

BACKGROUND

Among other mismatches between human and pig, incompatibilities in the blood coagulation systems hamper the xenotransplantation of vascularized organs. The provision of the porcine endothelium with human thrombomodulin (hTM) is hypothesized to overcome the impaired activation of protein C by a heterodimer consisting of human thrombin and porcine TM.

METHODS

We evaluated regulatory regions of the THBD gene, optimized vectors for transgene expression, and generated hTM expressing pigs by somatic cell nuclear transfer. Genetically modified pigs were characterized at the molecular, cellular, histological, and physiological levels.

RESULTS

A 7.6-kb fragment containing the entire upstream region of the porcine THBD gene was found to drive a high expression in a porcine endothelial cell line and was therefore used to control hTM expression in transgenic pigs. The abundance of hTM was restricted to the endothelium, according to the predicted pattern, and the transgene expression of hTM was stably inherited to the offspring. When endothelial cells from pigs carrying the hTM transgene--either alone or in combination with an aGalTKO and a transgene encoding the human CD46-were tested in a coagulation assay with human whole blood, the clotting time was increased three- to four-fold (P<0.001) compared to wild-type and aGalTKO/CD46 transgenic endothelial cells. This, for the first time, demonstrated the anticoagulant properties of hTM on porcine endothelial cells in a human whole blood assay.

CONCLUSIONS

The biological efficacy of hTM suggests that the (multi-)transgenic donor pigs described here have the potential to overcome coagulation incompatibilities in pig-to-primate xenotransplantation.

The coagulation barrier in xenotransplantation: incompatibilities and strategies to overcome them

PURPOSE OF REVIEW

Dysregulated coagulation is now recognized as a major contributor to graft loss in xenotransplantation. This review summarizes recent data on putative mechanisms of pathogenic coagulation in xenotransplantation and discusses progress on strategies to overcome them.

RECENT FINDINGS

Evidence continues to grow that the primary cause of failure of pig cardiac and renal xenografts is probably antibody-mediated injury to the endothelium, leading to development of microvascular thrombosis. Several factors that may exacerbate the problem will remain, even in the absence of a humoral response. These include molecular incompatibilities that affect the control of coagulation - in particular the failure of pig thrombomodulin to activate the primate protein C pathway - and platelet reactivity. Expression of anticoagulant and antiplatelet molecules within the graft is a potential solution that has been successfully tested in rodent models and will soon be applied to the pig-to-primate model. This strategy, in parallel with physical methods such as encasing islets in a protective layer, also holds promise for reducing the thrombogenicity of pig islet xenografts.

SUMMARY

Thrombosis is a barrier to long-term survival and function of porcine xenografts, which may eventually be overcome by various combinations of genetic and physical manipulation.

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

BACKGROUND

Following pig to primate kidney transplantation, xenogenic activation of the coagulation (XAC) system of the recipient eventually leading to organ dysfunction and disseminated intravascular coagulation (DIC) can be observed.

METHODS

Using an ex-vivo perfusion circuit based on low-dose heparin-mediated anticoagulation and exogenous complement inhibition by C1- Inhibitor (C1-Inh), we have analysed XAC following contact of human blood with porcine endothelium. Porcine kidneys (n = 23) were recovered following in situ cold perfusion with histidine-tryptophan-ketoglutarate (HTK) solution and were connected to a perfusion circuit utilizing freshly drawn pooled human AB blood.

RESULTS

Kidney survival during organ perfusion with human blood, CI-Inh, heparin but without any further pharmacological intervention was 126 +/- 78 min. XAC was observed with significantly elevated levels of D-dimer and thrombin antithrombin complexes (TAT). Pharmacological intervention with nitroprusside and prostacycline resulted in increased organ survival (220 +/- 28 min and 180 +/- 85 min respectively) but failed to inhibit XAC. In contrast, addition of activated protein C (APC) significantly reduced the increase in D-dimer and TAT and prolonged organ survival to 240 min (+/-0). On histology, no remarkable signs of XAC were observed.

CONCLUSIONS

We conclude that exogenous APC is able to reduce XAC in this ex vivo perfusion model.

Pig thrombomodulin binds human thrombin but is a poor cofactor for activation of human protein C and TAFI

Incompatibility between pig thrombomodulin (TM) and primate thrombin is thought to be an important factor in the development of microvascular thrombosis in rejecting pig-to-primate xenografts. To examine this interaction at the molecular level, we cloned pig TM and measured its ability to bind human thrombin and act as a cofactor for the activation of human protein C and TAFI. The 579-residue pig TM protein showed approximately 69% sequence identity to human TM. Within the EGF domains necessary for binding of thrombin (EGF56), protein C (EGF4) and TAFI (EGF3), all of the amino acids previously identified as critical for the function of human TM, with the exception of Glu-408 in EGF5, were conserved in pig TM. Comparison of transfected cells expressing pig or human TM demonstrated that both proteins bound human thrombin and inhibited its procoagulant activity. However, pig TM was a poor cofactor for the activation of human protein C and TAFI, with domain swapping showing that EGF5 was the most important determinant of compatibility. Thus, while pig TM may be capable of binding thrombin generated in the vicinity of xenograft endothelium, its failure to promote the activation of human protein C remains a significant problem.

The perspectives for porcine-to-human xenografts

The shortage of donated human organs for transplantation continues to be a life threatening problem for patients suffering from complete organ failure. Although this gap is increasing due to the demographic changes in aging Western populations, it is generally accepted that international trading in human organ is not an ethical solution. Alternatives to the use of human organs for transplantation must be developed and these alternatives include stem cell therapy, artificial organs and organs from other species, i.e. xenografts. For practical reasons but most importantly because of its physiological similarity with humans, the pig is generally accepted as the species of choice for xenotransplantation. Nevertheless, before porcine organs can be used in human xenotransplantation, it is necessary to make a series of precise genetic modifications to the porcine genome, including the addition of genes for factors which suppress the rejection of transplanted porcine tissues and the inactivation or removal of undesirable genes which can only be accomplished at this time by targeted recombination and somatic nuclear transfer. This review will give an insight into the advances in transgenic manipulation and cloning in pigs--in the context of porcine-to-human xenotransplantation.

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.