The future of transplantation: with particular reference to chimerism and xenotransplantation

Xenotransplantation-reflections on the bioethics

Background

Similar to most countries in the world, China has a severe shortage of human organs, and this is one of the main issues restricting the application of organ transplantation technology. In 2019 alone, only 19,454 (23.90%) of the 81,410 people waiting were able to receive organ transplants. There is an increasing focus from both the medical profession and society on how to fill the gap between supply and demand.

Methods

Xenotransplantation using animal organs is being considered as one option to make up for the shortage of human organs for transplantation. For some years now, the international medical community has been examining the possibility of using animal organs for human transplant. However, the research has faced two important types of challenges: scientific and ethical issues.

Results

In January 2021, the first clinical trial of transgenic pig heart transplantation into a human recipient was completed by the Medical Center of the University of Maryland in the United States. This has stimulated enthusiasm and interest in xenotransplantation.

Conclusions

The trend towards xenotransplantation has highlighted global problems such as the severe shortage of organ transplant donors and the high cost of organ transplantation. China needs to consider how to cope with the scientific, public health, and social ethics challenges of xenotransplantation clinical trials.

A semiquantitative PCR technique for detecting chimerism in hamster-to-rat bone marrow xenotransplantation

Although bone marrow transplantation has been used to induce donor-specific tolerance in many allogeneic models, similar effort in xenogeneic transplantation is met with obstacles like more severe graft versus host disease (GVHD). We are currently engaged in developing a GVHD-free hamster-to-rat xenotransplantation model using splenectomy, total body irradiation, and donor bone marrow transplantation. To test donor cell chimerism, particularly in the solid tissues, we developed a semiquantitative polymerase chain reaction (PCR) method using primers specific for hamster beta-actin and mitochondrial cytochrome C oxidase I and II (MCO I and II) genes and rat sex determination region on the Y chromosome (SRY) gene. Using this method, we estimated the level of hamster cells chimerism in rats subjected to splenectomy, total body irradiation (10 Gy), and hamster bone marrow transplantation (3 x 10(8) cell/recipient) and observed high levels of donor cells in all recipient tissues tested.

Pig peripheral blood mononuclear cells are directly associated with the thrombotic microangiopathy that complicates the induction of chimerism in pig-to-baboon xenotransplantation

The infusion of large numbers of porcine cells into primates in order to induce specific immunologic tolerance by mixed hematopoietic chimerism, results in thrombotic microangiopathy that can be fatal. For this reason, it is important to study in vitro the interaction of primate endothelial cells with pig cells. We show that pig peripheral blood mononuclear cells (p-PBMC) activate human endothelial cells (hECs) through direct contact. Thus, when endothelial cells are cultured in the presence of p-PBMC, overexpression of VCAM-1 and E-selectin adhesion molecules occurs within 3 h of culture and continues for at least 9 h. The co-culture of p-PBMC and hECs also results in an important adhesion of human platelets to both types of cell. Thus, viewed with the microscope, platelets aggregate above the endothelial cells and also around the pig cells. We present data that suggest that the presence of p-PBMC may be more important with regard to the increase of platelet adhesion to the endothelial cells than the activation alone of the cells. Our results also show that p-PBMC, and not the activated endothelia or the culture supernatant of activated hECs, are able to activate the coagulation cascade because they are able to generate thrombin when added to defibrinated human plasma. Overall, these findings suggest that p-PBMC are of primary importance for the development of the thrombotic disorders that occur in primates transplanted with pig progenitor cells.

Human herpesvirus-6 and -7 in transplantation

Infections with the beta-herpesviruses human herpesvirus-6 (HHV-6) and human herpesvirus-7 (HHV-7) are ubiquitous in childhood. The immunosuppression secondary to organ or bone marrow transplantation together with posttransplantation management may favour viral replication and reactivation. HHV-6 and -7 induce immunosuppression by targeting lymphocytes, natural killer cells and monocytes. HHV-6 is commonly detected posttransplantation but variability in definitions of clinical syndromes related to this virus and detection methods have complicated understanding of the clinical relevance of HHV-6 posttransplantation. Clinical symptoms associated with HHV-6 include febrile illness, pneumonitis, hepatitis, encephalitis and bone marrow suppression. However, the majority of HHV-6 infections are asymptomatic. The incidence of HHV-7 infection and its clinical manifestations posttransplantation are even less well characterised. In addition, HHV-6 and HHV-7 are related to CMV disease or acute graft-versus-host disease and, indirectly, to increases in resource utilisation. Based on the potential relevance of these two beta-herpesviruses in transplant recipients, further studies are required to establish their real impact in transplantation. For this, sensitive and specific molecular diagnostic techniques allowing for the rapid detection and quantitation of virus and for the analysis of susceptibility to current antiviral agents are required.

Xenotransplantation

BACKGROUND

The success of clinical transplantation has led to a large discrepancy between donor organ availability and demand; considerable pressure exists to develop an alternative source of organs. The use of animal organs for donation is a possible solution that is not yet clinically applicable.

METHODS AND RESULTS

A literature review was performed based on a Medline search to find articles on xenotransplantation. Keywords included hyperacute, acute vascular, xenograft rejection combined with concordant and discordant. Additional references cited in these articles from journals not included in Medline were obtained from the British Library. Limited information on unpublished, preliminary work has been included from sources known to the authors, based on their research work in the field. One hundred and forty-six references and four personal communications have been included in this review article.

CONCLUSION

A greater understanding of the pathogenesis of xenograft rejection is developing rapidly. Strategies to abrogate hyperacute rejection have proved successful, but control of antibody-driven acute vascular rejection has not yet been achieved. The safety and viability of xenotransplantation as a therapeutic modality are still unproven.

Immunological importance of chimerism in transplantation: new conditioning protocol in BMT and the development of chimeric state

Chimerism is an exceptional immunogenetic state, characterized by the survival and collaboration of cell populations originated from two different individuals. The prerequisits to induce chimerism are immuno-suppression, myeloablation, or severe immunodeficiency of the recipients on the one side and donor originated immuno-hematopoietic cells in the graft on the other. The pathologic or special immunogenetic conditions to establish chimerism are combined with bone marrow transplantation, transfusion, and various kinds of solid organ grafting. Different types of chimerism are known including complete, mixed and mosaic, or split chimerism. There are various methods used to detect the type of chimera state, depending on the immunogenetic differences between the donor and recipient. The induction of complete or mixed chimerism is first determinated by the effect of myeloablative therapy. The chimera state seems to be one of the leading factors to influence the course of the post-transplant period, the frequency and severity of GVHD, and the rate of relapse. However, the most important contribution of the chimeric state is in development of graft versus leukemia effect. A new conditioning protocol (DBM/Ara-C/Cy) for allogeneic BMT in CML patients and its consequence on chimera state and GVL effect is demonstrated.

Xenotransplantation

Xenotransplantation is a potentially promising but exceedingly complex issue. It is critical that this subject be discussed within and outside the transplant community. The need for an expanded organ supply is urgent, but the scientific and ethical positions are complex. This article reviews the current status of xenotransplantation, including the potential benefits and risks; discusses multiple ethical issues; and makes recommendations for the transplant coordinator.

Lung transplantation for cystic fibrosis

Lung transplantation currently stands as the only therapeutic option that carries the potential to restore patients with advanced cystic fibrosis to a more normal state of health. Nonetheless, the procedure carries significant risk and median survival following transplantation is only 5 years. This article discusses the currently achievable outcomes and the common short-comings of transplantation. Strategies to optimize outcomes through appropriate patient selection, use of living donors, and novel research initiatives are discussed.

Organ transplantation in the twenty-first century

Major advances in the understanding of the immunologic process responsible for organ or cellular transplant rejection, a dramatic improvement in available immunosuppressive drugs, development of more sophisticated surgical techniques, and important progress in posttransplant intensive care over the last 30 years have led to a remarkable improvement in success following organ transplantation. Whereas excellent short-term survival of most transplanted organs is readily achieved, graft loss because of chronic rejection and the worsening problem of organ donor shortage remain major concerns in the field of transplantation. Recent advances in immunosuppressive drugs, induction of immunologic tolerance, and gene therapy strategies may help to prolong organ allograft survival in the future. Revised criteria for organ donation and xenotransplantation may one day solve the problem of organ supply. Today, as we approach the next millennium, we are optimistic that the elusive goal of immunologic tolerance will be achieved and perhaps applied to animal tissue. Such will certainly be the challenge for the next century.

Evidence for engraftment of human bone marrow cells in non-lethally irradiated baboons

BACKGROUND

Prior to organ harvesting, an attempt was made to modulate the donor's immune responses against prospective xenogeneic recipients by infusion of "recipient-type" bone marrow.

METHODS

For this purpose, baboons conditioned with total lymphoid irradiation were given 6 x 10(8) unmodified human bone marrow cells/kg body weight with no subsequent treatment.

RESULTS

Animals survived until they were euthanized at 18 months. Using primers specific for human chorionic gonadotrophin gene, the presence of human DNA was confirmed by polymerase chain reaction in the blood of one animal for up to 18 months after cell transplantation; in the other animal, xenogeneic chimerism became undetectable in the blood at 6 months after bone marrow infusion. However, tissue samples obtained from both animals at the time they were euthanized had evidence of donor (human) DNA. Additionally, the presence of donor DNA in individually harvested colonies of erythroid and myeloid lineages suggested that infused human bone marrow cells had engrafted across the xenogeneic barrier in both baboons.

CONCLUSIONS

Bone marrow transplantation from human to baboon leads to establishment of chimerism and modulation of donor-specific immune reactivity, which suggests that this strategy could be reproducibly employed to create "surrogate" tolerogenesis in prospective donors for subsequent organ transplantation across xenogeneic barriers.

Bone marrow chimerism and transplantation tolerance

Engraftment of allogeneic or xenogeneic pluripotent hematopoietic stem cells into nonmyeloablated but immunodepleted (preconditioned) recipients can produce a state of immunological tolerance to donor and host. Host and donor hematopoietic cells entering the thymus ensure deletion of both donor- and host-reactive thymocytes. Additional mechanisms are involved in tolerance induced in recipients that are not immunodepleted. Grafting of donor thymic tissue to thymectomized recipients is an alternative approach for inducing central T cell tolerance without the requirement for engraftment of donor hematopoietic stem cells. During the past year, advances have been made in understanding both the requirements for preconditioning and the mechanisms of tolerance induction in the above transplantation models.

Chimerism after organ transplantation

Recent evidence suggests that passenger leukocytes migrate after organ transplantation and produce persistent chimerism, which is essential for sustained survival of the allograft. Here, we describe how this hematolymphopoietic chimerism provides an important framework for interpretation of post-transplant phenomena and for initiation of therapeutically oriented transplantation research.