Thrombotic microangiopathy: the next big hurdle for xenotransplantation

Intra-Abdominal Heterotopic Cardiac Xenotransplantation: Pearls and Pitfalls

Heterotopic cardiac xenotransplantation in the intra-abdominal position has been studied extensively in a pig-to-baboon model to define the optimal donor genetics and immunosuppressive regimen to prevent xenograft rejection. Extensive investigation using this model is a necessary stepping stone toward the development of a life-supporting animal model, with the ultimate goal of demonstrating suitability for clinical cardiac xenotransplantation trials. Aspects of surgical technique, pre- and post-operative care, graft monitoring, and minimization of infectious risk have all required refinement and optimization of heterotopic cardiac xenotransplantation over time. This review details non-immunologic obstacles relevant to this model described by our group and in the literature, as well as strategies that have been developed to address these specific challenges.

Altered expression of eNOS, prostacyclin synthase, prostaglandin G/H synthase, and thromboxane synthase in porcine aortic endothelial cells after exposure to human serum-relevance to xenotransplantation

Under normal conditions, the activity of platelets is stringently and precisely balanced between activation and quiescent state. This guarantees rapid hemostasis and avoids uncontrolled thrombosis. However, excessive platelet activation and resulting thrombotic microangiopathy are frequently observed in pig-to-primate xenotransplantation models. Endothelium-derived inhibitory mechanisms play an important role in regulation of platelet activation. These mainly include nitric oxide (NO), prostacyclin PGI₂ , and adenosine, which are synthesized by endothelial NO synthases (eNOS), prostacyclin synthase, and CD39/CD73, respectively. We investigated whether endothelium-derived regulatory mechanisms are affected in porcine aortic endothelial cells (PAECs) after exposure to human serum. In the present study, exposure of PAECs or porcine iliac arteries to human serum suppressed gene expression of eNOS and prostacyclin synthase, while induced gene expression of prostaglandin G/H synthase and thromboxane synthase. Simultaneously, exposure to human serum reduced NO and PGI₂ production in PAEC culture supernatants. Thus, human serum altered the balance of endothelium-derived inhibitory mechanisms in PAECs, which may indicate a regulatory mechanism of excessive platelet activation in pig-to-primate xenotransplantation.

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.

Genetically engineered pigs and target-specific immunomodulation provide significant graft survival and hope for clinical cardiac xenotransplantation

OBJECTIVES

Cardiac transplantation and available mechanical alternatives are the only possible solutions for end-stage cardiac disease. Unfortunately, because of the limited supply of human organs, xenotransplantation may be the ideal method to overcome this shortage. We have recently seen significant prolongation of heterotopic cardiac xenograft survival from 3 to 12 months and beyond.

METHODS

Hearts from genetically engineered piglets that were alpha 1-3 galactosidase transferase knockout and expressed the human complement regulatory gene, CD46 (groups A-C), and the human thrombomodulin gene (group D) were heterotropically transplanted in baboons treated with antithymocyte globulin, cobra venom factor, anti-CD20 antibody, and costimulation blockade (anti-CD154 antibody [clone 5C8]) in group A, anti-CD40 antibody (clone 3A8; 20 mg/kg) in group B, clone 2C10R4 (25 mg/kg) in group C, or clone 2C10R4 (50 mg/kg) in group D, along with conventional nonspecific immunosuppressive agents.

RESULTS

Group A grafts (n = 8) survived for an average of 70 days, with the longest survival of 236 days. Some animals in this group (n = 3) developed microvascular thrombosis due to platelet activation and consumption, which resulted in spontaneous hemorrhage. The median survival time was 21 days in group B (n = 3), 80 days in group C (n = 6), and more than 200 days in group D (n = 5). Three grafts in group D are still contracting well, with the longest ongoing graft survival surpassing the 1-year mark.

CONCLUSIONS

Genetically engineered pig hearts (GTKOhTg.hCD46.hTBM) with modified targeted immunosuppression (anti-CD40 monoclonal antibody) achieved long-term cardiac xenograft survival. This potentially paves the way for clinical xenotransplantation if similar survival can be reproduced in an orthotopic transplantation model.

Pathologic characteristics of transplanted kidney xenografts

For xenotransplantation to become a clinical reality, we need to better understand the mechanisms of graft rejection or acceptance. We examined pathologic changes in α1,3-galactosyltransferase gene-knockout pig kidneys transplanted into baboons that were treated with a protocol designed to induce immunotolerance through thymic transplantation (n=4) or were treated with long-term immunosuppressants (n=3). Hyperacute rejection did not occur in α1,3-galactosyltransferase gene-knockout kidney xenografts. By 34 days, acute humoral rejection led to xenograft loss in all three xenografts in the long-term immunosuppression group. The failing grafts exhibited thrombotic microangiopathic glomerulopathy with multiple platelet-fibrin microthrombi, focal interstitial hemorrhage, and acute cellular xenograft rejection. Damaged glomeruli showed IgM, IgG, C4d, and C5b-9 deposition. They also demonstrated endothelial cell death, diffuse endothelial procoagulant activation with high expression of tissue factor and vWF, and low expression of the ectonucleotidase CD39. In contrast, in the immunotolerance group, two of four grafts had normal graft function and no pathologic findings of acute or chronic rejection at 56 and 83 days. One of the remaining kidneys had mild but transient graft dysfunction with reversible, mild microangiopathic glomerulopathy, probably associated with preformed antibodies. The other kidney in the immunotolerance group developed unstable graft function at 81 days and developed chronic xenograft glomerulopathy. In summary, the success of pig-to-primate xenotransplantation may necessitate immune tolerance to inhibit acute humoral and cellular xenograft rejection.

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.