Transplantation Tolerance: One Step Closer?

September 29, 2006

Frank JMF Dor, MD, surgeon-in-training in Rotterdam, The Netherlands, will defend his PhD thesis, entitled "Investigations relating to the induction of immunological tolerance through spleen transplantation in miniature swine" on October 11th 2006. Dor demonstrates in his research that the induction of immunological tolerance is feasible through spleen transplantation.

Significant progress has been achieved in the field of transplantation and immunosuppressive therapy in the last 50 years. However, rejection of transplanted organs still hampers long-term survival. The ultimate goal in clinical transplantation is to achieve donor-specific immunological tolerance towards the transplanted organ. If tolerance can be realised, it will be feasible to offer transplant patients an organ that functions long-term, without the complications of immunosuppressive therapy, including cardiovascular disease, opportunistic infections, and malignancies.

Spleen transplantation has been described as a succesful method to induce tolerance in rodent models. However, this approach has never been extended to large animal studies or large-scale clinical application.

Dor investigated spleen transplantation in miniature swine at the Transplantation Biology Research Center of the Massachusetts General Hospital / Harvard Medical School in Boston, USA, under supervision of Professor David KC Cooper and Professor David H Sachs. Spleen transplant recipients received a short course of immunosuppressive therapy, after which the transplanted spleen was accepted. The recipient became unresponsive towards the donor in vitro, allowing a kidney from the same donor to be accepted without immunosuppression. One of the possible mechanisms leading to tolerance though spleen transplantation, is the fact that hematopoietic stem cells from the donor spleen engraft in the recipients lymphoid organs, including the bone marrow, thereby teaching the immune system to consider the donor as "self".

Because of these hopeful results, Dor and colleagues conclude that spleen transplantation might be a clinically-applicable strategy to induce transplantation tolerance in humans. However, further confirmatory studies in primate models would be needed to allow the introduction of clinical spleen transplantation.

Scientific abstract:

The ultimate goal in transplantation is to achieve donor-specific immunological tolerance. The induction of tolerance would result in the long-term survival of a transplanted organ without the need for continuous immunosuppressive therapy, thus avoiding its many attendant risks and complications. In this dissertation, spleen transplantation will be investigated as a method to induce immunological tolerance.

In the literature, there is evidence for a tolerogenic effect of spleen transplantation in rodents. However, the relevance of this phenomenon remains to be demonstrated in large animal models and in humans. To investigate whether the approach of spleen transplantation would be clinically applicable, we developed a large animal model for spleen transplantation. The Massachusetts General Hospital miniature swine model was chosen, based on the fact that the Major Histocompatibility Complex (MHC) is well-defined, enabling clinically-relevant donor-recipient combinations. Also, the last 30 years, a large amount of experience in transplantation immunology has been gained in this model.

In miniature swine, a transplanted spleen (a highly immunogeneic organ), is rejected when no immunosuppressive therapy is given. However, it will be accepted when a short course of cyclosporine A is administered, and, depending on MHC mismatches between donor and recipient, additional mild whole body irradiation (100 cGy) and thymic irradiation (700 cGy). After successful spleen transplantation, mixed hematopoietic chimerism is induced in the blood and lymphoid organs of the recipients. Donor cells migrate from the spleen to lymph nodes, thymus, and bone marrow of the recipients (engraftment), in the absence of graft-versus-host-disease. Meanwhile, the donor spleen will be replaced slowly by recipient-type cells.

Creating a state of mixed chimerism is known to be a highly-effective means to induce immunological tolerance. When the spleen is successfully transplanted, recipients demonstrate donor-specific T cell tolerance in vitro. After discontinuation of immunosuppression, T cells remain unresponsive to donor antigen (CML, MLR), and regulatory T cells can be detected in the circulation of the recipients. These regulatory T cells can suppress naïve T cell responses in a donor-specific manner. Recipients of spleen transplants remain immunocompetent. Therefore, we have investigated whether in vivo donor-specific tolerance occurs. In pigs tolerized by spleen transplantation, donor-specific kidneys can survive long-term without immunosuppression, while in control experiments, they will be rejected within 2 weeks.

The fact that donor cells engraft in recipient bone marrow, suggests that hematopoietic stem cells are present in the donor spleen. Therefore, we investigated the presence of hematopoietic stem cells in adult naïve spleen, using flow cytometry and a variety of functional stem cell assays. We have been able to demonstrate that the spleen of pigs, baboons, and humans, harbors very primitive hematopoietic cells, in frequencies comparable to those in the bone marrow, which has not been described thus far.

In the view of our results, we conclude that spleen transplantation indeed has the capacity to induce donor-specific immunological tolerance. Also, the spleen is a relatively rich source of hematopoietic progenitor cells, possibly stem cells.

More studies are underway to investigate whether these results can have clinical implementations.

Title dissertation:

INVESTIGATIONS RELATING TO THE INDUCTION OF IMMUNOLOGICAL TOLERANCE THROUGH SPLEEN TRANSPLANTATION IN MINIATURE SWINE

PhD defense:

Wednesday October 11, 2006 at 11.45, Medical Faculty of the Erasmus Medical Center Rotterdam, The Netherlands.

Promotors:

Prof. Dr Jan N.M. IJzermans (Erasmus MC Rotterdam, The Netherlands) and Prof. Dr David K.C. Cooper (Thomas E. Starzl Transplantation Institute, UPMC, Pittsburgh, USA)

Copyright © 2006 MediLexicon International Ltd.

This article posted October 11, 2006.