Immune Responses in humanized Mice with Xenogeneic Chimerism

Organ transplantation is the only curative procedure with the potential to restore normal life quality in patients with end-stage organ-failure. The number of available donor organs, however, limits its applicability. Xenotransplantation poses a solution for this problem, since it allows the generation of organs "on demand". Despite recent advances, the use of the procedure is limited due to strong rejection-reactions mediated by antibodies, T-cells and NK-cells. Problematically, these rejection-reactions are too strong to be controlled by conventional immunosuppression. Immune tolerance is therefore likely needed for the success of clinical xenotransplantation. Studies in rodents have shown that mixed chimerism can promote tolerance of T-cells, B-cells and NK-cells. The host laboratory has adapted the study into a human/porcine setting and generated a humanized mouse model demonstrating that mixed xenogeneic chimerism can induce tolerance of human T-cells. By employing the humanized mouse model we will investigate the impact of porcine mixed xenochimerism on T- cell functionality, on the induction of T-cell tolerance and on a possible carryover of porcine pathogens to human cells within humanized mice. The proposed experiments will deliver valuable information about xenochimerism and its potential and feasibility to induce tolerance. Since a potential carryover of porcine pathogens to the human recipient cannot be excluded, the presented study will also have profound impact on the conception of future studies. Overall, we anticipate that the project will contribute to move the xenogeneic organ transplantation towards a clinical applicability.

Humanized mice are mice are animals which harbor a human immune system. The human immune system can grow and develop in those mice which makes it possible to monitor, investigate and even manipulate this immune system At Columbia University medical center I wanted to use the humanized mouse to investigate if it is possible to transplant pig organs into an organism with a human immune system. Furthermore, we wanted to see if the human immune cells can protect the pig cells from pathogens that would otherwise threaten to destroy the pig organs. For obvious reasons, this procedure cannot be done directly with humans because it would cause life-threatening immune reactions. However, the same procedure is possible when humanized mice are used as proxy organism. During the course of my research stay at Columbia University Medical Center, I generated numerous humanized mice and furthermore transplanted pig cells into those mice. The human immune system, was accepting the pig cells without rejecting them. However, and this was startling us, the human immune system could not recognize pig cells and human control cells at all. The reason for this was a failure in the humanized mouse system to mount a successful vaccination response. In order to find out, why vaccination did not work, we started to set up several experiments. These experiments addressed several facets of the humanized mouse, namely the mouse thymus and the mouse lymph nodes. By our experiments, we found out that the mouse thymus was causing problems and allowed the development of human cells in it. We therefore concluded that its presence is disturbing and decided that it is best to remove it. Indeed, we could confirm that the removal of the mouse thymus solved to problem. We furthermore found out, that the mouse lymph nodes were underdeveloped and not normal. Since lymph nodes are important for successful vaccination responses, we concluded that humanized mice have problems to build functioning lymph nodes and we decided to develop methods to repair the mouse lymph nodes. These methods involved the injection of human immune cells into newborn mice, because we assumed that the presence of immune cells at birth is necessary to develop good lymph nodes. We found out that newborn mice which received human cells indeed developed bigger lymph nodes and better lymph node structure. However, many of those mice started to show severe signs of autoimmunity. Concluding in brief, the humanized mouse model, in theory, is an attractive tool which would allow the investigation and manipulation of an in vivo immune system. Practically, however, the model has to be optimized so that certain flaws, like the inability to induce a vaccination response and the spontaneous occurrence of autoimmune reactions, can be overcome.