Coagulation/thrombotic disorders associated with organ and cell xenotransplantation

Progress in pig-to-non-human primate transplantation models (1998-2013): a comprehensive review of the literature

BACKGROUND

The pig-to-non-human primate model is the standard choice for in vivo studies of organ and cell xenotransplantation. In 1998, Lambrigts and his colleagues surveyed the entire world literature and reported all experimental studies in this model. With the increasing number of genetically engineered pigs that have become available during the past few years, this model is being utilized ever more frequently.

METHODS

We have now reviewed the literature again and have compiled the data we have been able to find for the period January 1, 1998 to December 31, 2013, a period of 16 yr.

RESULTS

The data are presented for transplants of the heart (heterotopic and orthotopic), kidney, liver, lung, islets, neuronal cells, hepatocytes, corneas, artery patches, and skin. Heart, kidney, and, particularly, islet xenograft survival have increased significantly since 1998.

DISCUSSION

The reasons for this are briefly discussed. A comment on the limitations of the model has been made, particularly with regard to those that will affect progression of xenotransplantation toward the clinic.

Unchanged global fibrinolytic capacity during the course of hematopoietic stem cell transplantation

Hemostatic changes due to vascular endothelial damage are seen during the course of hematopoietic stem cell transplantation (HSCT). The fibrinolytic response to ongoing hemostatic activation in HSCT remains to be elucidated. Global fibrinolytic capacity (GFC) is a novel method, which reflects the amount of generated D-dimer when fibrinolysis of a freeze-dried fibrin clot is stopped by introducing aprotinin. GFC is sensitive to all the factors involved in the process of fibrinolysis. The aim of this study was to serially assess GFC at certain critical time points (days -1, +7, +14, +21 prior to and following stem cell infusion) during the course of HSCT. The study group comprised 16 patients with hematological malignancies (11 women, five men; median age 32+/-9 years) in whom HSCT had been performed. Thirty healthy adults (21 women, nine men; median age 31+/-7 years) served as controls. In this study, global fibrinolytic response, as reflected by GFC, was unchanged despite ongoing hemostatic activation, as indicated by D-dimer, moreover GFC remained stable, despite the development of thrombocytopenia associated with HSCT prior to platelet engraftment. Our results indicate that a global fibrinolytic response was impaired as a compensatory response to endothelial activation and to other hemostatic changes seen in HSCT. Further studies in larger HSCT populations are warranted to better understand the implications of these findings.

Xenograft transplantation

Interest in xenotransplantation has increased because conventional organ transplantation has been limited by a shortage of human organs. Although xenotransplantation could alleviate the existing and anticipated need for tissues and organs, the application is hindered by various biologic obstacles. This article reviews the basis for the demand for xenotransplantation, the obstacles to clinical application, and potential approaches to overcoming those obstacles.

Mechanisms of thrombotic microangiopathy following xenogeneic hematopoietic progenitor cell transplantation

INTRODUCTION

Attempts to induce tolerance though mixed hematopoietic chimerism in the discordant pig-to-baboon xenotransplantation model are sometimes complicated by a potentially fatal thrombotic microangiopathy in the recipient baboons. This state develops immediately after the infusion of porcine mobilized peripheral blood leukocytes, containing progenitor cells (PBPC). In our study, we examined the interaction of infused porcine PBPC with recipient platelets in vivo in baboons and investigated the underlying mechanisms using an in vitro model.

METHODS

Two naïve baboons and six baboons preconditioned with irradiation and immunosuppression that received porcine PBPC were evaluated in vivo. The interaction of porcine and baboon PBPC with baboon platelets was investigated by an in vitro platelet aggregation assay. Fresh and cryopreserved PBPC were evaluated as well as PBPC obtained from growth-factor mobilized and unmobilized pigs. Furthermore, cellular subsets of PBPC were assessed for potential to induce platelet aggregation. Immunohistochemical staining was performed on platelet-leukocyte aggregates and potential inhibition of aggregation with anti-P-selectin and anti-CD154 mAbs, or eptifibatide (a GPIIb/IIIa receptor antagonist), was tested.

RESULTS

All baboons that received porcine PBPC rapidly developed marked thrombocytopenia (<20,000/microl), elevated serum lactate dehydrogenase (>1,500U/liter), schistocytosis, and platelet aggregates on blood smear. Three baboons died (two untreated and one preconditioned), and substantive platelet aggregates containing porcine leukocytes were observed in the microvasculature of lungs and kidneys. In vitro, porcine, but not baboon, PBPC induced aggregation of baboon platelets in a dose-dependent manner. Immunohistological examination of these aggregates confirmed the incorporation of porcine leukocytes. Cryopreserved PBPC caused less aggregation than fresh PBPC, and growth-factor-mobilized PBPC induced less aggregation than unmobilized PBPC. Aggregation was fully abrogated by the addition of eptifibatide, and modulated by anti-P-selectin and anti-CD154 monoclonal antibodies that recognize adhesion receptors on activated platelets. Purified fractions (granulocytes, CD2+, and CD- cells) of porcine PBPC did not initiate aggregation, whereas addition of exogenous porcine PBPC membranes (erythrocytes, dead cells, and/or platelets) to the purified fractions exacerbated the aggregation response.

CONCLUSIONS

These data indicate that porcine PBPC mediate aggregation of baboon platelets. This process likely contributes to the thrombotic microangiopathy observed after PBPC transplantation in the pig-to-baboon model. Eptifibatide can fully abrogate platelet aggregation induced by porcine PBPC in vitro. Purification of the progenitor cells from porcine PBPC and/or treatment of baboons with eptifibatide may be beneficial.