Given their practically, unlimited availability and capabilities to produce every organ in the body, embryonic stem cells are well poised as a cellular source for tissue regeneration therapy. However, it has been shown that ES cells are rejected by the recipient's immune system and that any future use of ESC is likely to require the use of systemic medications to overcome the rejection of these grafts. These treatments are usually quite toxic and result in significant morbidity and mortality. We have developed a novel and practical model of bone marrow transplantation to induce tolerance to graft from different genetic backgrounds of the recipient. Unlike the usual bone marrow transplantation protocols, this is a non-toxic and therefore well tolerated regimen that requires no long term treatment with anti-rejection drugs. In this model, blood cells developed from hematopoietic stem cells will be used as a means to induce a tolerance to future grafts made from the same embryonic stem cells for regenerative purposes. While the focus of this grant will be on induction of tolerance to grafts made from embryonic stem cells, the application of this approach can quickly spread for therapies for a wide variety of other disorders. Examples of such an applications are the treatment of blood cancers, as a part of immunotherapy for solid tumors, to prevent rejection of adult grafts such as kidney or heart transplant, the treatment of autoimmune disorders such as severe cases of arthritis and treatment of a wide variety of hereditary disorders such as thalassemia and sickle cell diseases.
Statement of Benefit to California:
While the potential clinical application of embryonic stem (ES) cells and their derivatives are expanding, there has been very little focus on the immunological aspects of their use. There is a very high chance that ES derived tissues are easily rejected by the host without aggressive immunosuppressive medications. On the other hand, the side effects of immunosuppressive drugs are, in most cases, overwhelming for the patients and result in significant morbidity and even mortality for these patients. The widespread clinical application of any ES cell related therapeutic approach will rely on overcoming the problems associated with graft rejection. The use of induced pluripotent stem cells (iPS cells) will be limited at this point due to issues with gene therapy. This is specifically a bigger problem in the case of hereditary disorders and malignancies when permanent cloning of the cells is needed. Furthermore, it is not clear if autologous iPS cells from the older patients, the target of most of the regenerative medicine protocols, has the same pluripotency and safety profile of already established ES cells. We believe that by inducing low levels of stable mixed hematopoietic chimerism from ES derived hematopoietic stem cells using a non-toxic transplant model, we will achieve long term tolerance to the graft and therefore prevent its rejection. More importantly, the clinical application of this approach is not limited to the induction of tolerance to the ES derived organs. Induction of stable mixed chimerism can be used in a wide spectrum of clinical applications including but not limited to treatment of hematological malignancies, induction of tolerance to the allogeneic immune reactive cells as a part of immunotherapy for solid tumors, induction of tolerance for solid organ transplants, treatment of autoimmune disorders and treatment of a wide variety of hereditary disorders such as sickle cell anemia and thalassemias. Each of these clinical scenarios currently create significant clinical problems for the patients as well as a significant financial burden on the California Budget.
This proposal is focused on developing hematopoietic stem cell (HSC) transplant protocols designed to create a state of stable mixed chimerism, in which there is representation of both donor and host HSCs in the recipient. The goal is to create an immune system that is tolerant to subsequent tissue transplantation from the same donor while retaining anti-third party immune responses. The applicant proposes to achieve this goal in rodents by combining and fine-tuning two protocols that are currently used in human patients. Three specific aims are proposed: (1) to optimize the transplant protocol in a mouse model by evaluating the effects of different pre and post-transplant conditioning regimens and different ratios of autologous and allogeneic cells; (2) to investigate the immune system recovery and the mechanism of tolerance induction after the transplant; and (3) to study in a mouse model, co-transplantation of autologous bone marrow cells with major histocompatibility complex (MHC)-mismatched, embryonic stem cells (ESC)-derived HSCs, to establish hematopoietic mixed chimerism and tolerance to ESC-derived secondary grafts. Reviewers did not find this proposal to be particularly innovative. They noted that the proposed approaches for generating mixed chimerism are not novel but have been established by other investigators and have been extensively evaluated previously. The only novel aspect is their combination. They noted that mixed chimerism has become relatively easy to achieve in small animal models, but to date has had very limited success when translated to a clinical setting. The reviewers did acknowledge that the proposal could have a major impact if successful, as the safe induction of mixed chimerism under reduced conditioning could pave the road for transplantation of human ESC-derived tissues. In addition, the generation of engraftment- and reconstitution-capable HSCs from mESCs would be a significant advance for the field. However, reviewers cautioned that while stable mixed chimerism has been achieved in mouse models, similar protocols have not been effective in human subjects, which could limit the potential impact of the proposal. The reviewers also raised a number of concerns about the rationale, design and feasibility of the research plan. They found the proposal’s guiding hypothesis, that roadblocks to establishing mixed chimerism can be eliminated by combining and fine-tuning two established protocols, to be overly optimistic. Reviewers commented that translating findings from rodent models to human patients is extremely challenging. In addition, mixed chimerism may not be desirable for patients with malignancy, as tolerance will presumably lead to the loss of graft-versus-leukemia activity. Reviewers raised a number of issues with the mouse model and experimental design. For example, they questioned the use of T cell replete allogeneic bone marrow in Aim 1 when the eventual goal of the proposal is to use mESC-derived HSCs which are devoid of T cells. They indicated that this discrepancy is critical because of the major role played by T cells in determining host immunity and engraftment, and that a more relevant model should use purified allogeneic hematopoietic stem cells rather than bone marrow. They also questioned whether, as argued in the preliminary data, the addition of autologous bone marrow (BM) to an allogeneic BM transplant truly reduces GVHD, because the absolute numbers of allogeneic cells used for the comparison were not equivalent. Finally, reviewers were not convinced of the feasibility of a key step required for Aim 3, that is, the generation of engraftment-capable HSCs from mESCs. They noted that this is a major obstacle in the field and the brief description of proposed experiments, together with the limited preliminary data do not engender confidence that this goal can be realized. Reviewers questioned whether the qualifications of the Principal Investigator (PI) and assembled research team are sufficient to enable the success of the project. They noted that the PI has not been prolific in the areas of chimerism or using HSCs for regenerative treatments. Reviewers did appreciate the stem cell expertise contributed by the Co-Investigator and consultants but felt that collectively, the team would be better able to address the ESC differentiation aspects than the tolerance aspects and that this is reflected by major flaws in the design of the mouse experiments. Overall, reviewers did not find this proposal to be innovative and they raised several serious concerns about both the research plan and the team that caused them to doubt its feasibility.