Cell therapy for Parkinson’s disease remains one of the most highly anticipated applications for stem cells in the brain repair field. Clinical trials with fetal tissue have provided proof of principle that new dopamine neurons implanted into the brain of Parkinson’s disease patients can provide sustained improvement in motor function. However, the primary source of tissue to be used dopaminergic neurons (DA) cell replacement has not yet been fully determined for a variety of reasons, including ethical considerations surrounding the acquisition of embryonic or fetal stem cells. Despite the so-called ‘immuno-privileged’ environment offered by the brain, extensive work using fetal tissue in animal models and in studies utilizing human tissue it has been shown that while cells transplanted into the brain can survive long-term without immunosuppressive treatment, a long-lasting inflammatory response is present and ultimately compromises the grafted cells. Furthermore, in clinical trials, patients undergoing different immunosuppressive regimes suggest that the host immune response may well have a decisive impact on graft survival and function. Our understanding of the complex interaction between the immune system and the CNS has significantly improved in the recent past and it has become clear that our immune system is intimately involved in CNS damage/repair and disease progression and presumably etiology. Consequently, we propose to assess how dominant adaptive immune factors mediate graft survival, but also determine how these may affect finer aspects of graft function under circumstances of immune limitation, which may occur with advanced CNS disease, extreme youth, or advanced age. Our ability to manipulate cytotoxic T cells and their function allows us to systematically determine the contribution of dominant effectors of systemic graft rejection in the successful engraftment and functioning of transplanted CNS cells. Secondary to this, we will also probe the possibility that other immune factors, including non-cytotoxic and non-ab T cells, pro- and anti-inflammatory cytokines, and innate immune cells and molecules, may modify graft-modulating ability of cytotoxic T cells. Coupled together, we have the unique ability to examine the integrity and function of transplanted cells in an animal model for Parkinson’s Disease (PD) and subsequently define the role of specific immune cells at various stages of appropriate engraftment and functional recovery.
Our propose study will directly and systematically address the immunologic limitations of allogeneic stem cell grafts and begin to elucidate the specific immune cells that influence appropriate engraftment. Furthermore, our proposal will examine the potential of modified immunotherapy in the promotion of successful transplantation for cell replacement therapy in the CNS.
Statement of Benefit to California:
The propose research will directly examine the potential of stem cells to become dopaminergic neurons, survive and establish appropriate connections in the brain for functional recovery in an animal model for Parkinson's disease. These neurons are the same neurons that are damaged in Parkinson’s disease. Furthermore, we will directly examine the role of T cells or immune cells in graft rejection and acceptance. The results to be obtained from this study will provide critical information regarding the role of host immunity in cell replacement therapy for Parkinson’s disease. Therefore, providing the State of California the platform to directly translate what has been learned from this study to the clinic. California has the unique advantage to examine the potential of embryonic derived stem cells as the citizens of this state have invested valuable resources to study and determine the utility of these cells for treating patients with various diseases.
The overall aim of this project is to examine the immunological consequences of intrastriatal transplantation of both fetal neural and embryonic stem cell (ESC) derived dopaminergic progenitors in a mouse model of Parkinson’s disease (PD). Specifically, the applicant proposes to assess the immune response elicited by ESC grafts relative to transplants of fetal brain-derived tissue, and to test the hypothesis that allograft rejection in the CNS after transplantation of fetal or ESC derived dopaminergic neurons is overwhelmingly mediated by a specific population of cytotoxic T cells (CTL) that can be identified by their cell surface marker expression. Latter aspects of this investigation will employ transplantation studies into wild-type and various immune-deficient mouse recipients. Reconstitution of various T-cell populations in immune deficient mice are also proposed for assessing both the sufficiency of the proposed CTL population in mediating graft rejection and the impact of CTL levels on structural and functional engraftment.
Parkinson’s disease is an important therapeutic target for stem cell transplantation, and immunogenicity of transplanted cells is a major challenge in moving cell therapy forward to widespread clinical use. However, this project tests only mouse cells in mouse animal models; from an immune perspective it focuses exclusively on one specific CTL population. Reviewers therefore concluded that it would have only an incremental influence on the immunobiology of cell therapy. In this regard, the reviewers disagreed with the PI’s assertion that the experiments in this proposal would predict how human patients would respond to grafted cells. In addition, except for the immunodeficient mouse model, reviewers commented that the project lacked novelty. Overall, the probable impact of this project on PD was viewed as limited.
Although the overall scientific rationale for this project is generally sound, reviewers raised serious issues with the project. The rationale for targeting the proposed CTL population for experimental manipulation is poorly articulated. One reviewer commented that the cell surface marker used to identify the CTL population is expressed not only in T cells but also in B cells, intestinal intraepithelial cells and dendritic cells, which may complicate interpretation of the results. In addition, the use of the mouse model was viewed as having serious limitations; the degree to which it may represent the human immune response to embryonic stem cells is questionable, especially since human cells will not be studied. The proposed mouse model of Parkinson's disease has additional limitations, since many aspects of the progressive neurological diminishment typical of the human disease are not evident in the mouse model. Furthermore, this differing disease progression may be characterized by different immune responses.
Reviewers also noted concerns with the research design. The challenges of neuronal grafting, integration and innervation were poorly considered. Outcome measures for dopamine neuron survival did not account for possible migration of ES cells or the generation of growth factor-producing astrocytes. Important experiments, such as the comparison of fetal and ES cell grafts into the ventral mesencephalon to provide a better opportunity for neural control of grafted neurons were not proposed. Additionally, the use of more sophisticated behavioral tests, such as balance beam, with transplanted mice were not included.
The preliminary data was unconvincing and raised serious questions about the project’s feasibility. Procedures for ES cell differentiation into dopaminergic (DA) neurons were glossed over and no data were provided on reproducibility, cell yields, or percentages of DA+ neurons obtained. The complexity of DA neuron differentiation was of further concern, since the research group appears to possess limited experience in stem cell research.
The principal investigator’s track record was viewed as adequate. The PI’s expertise is in the field of CNS tumor immunology with some experience in the therapy of neurodegenerative diseases. The PI is relatively well published but has little previous experience managing large-scale international research projects. The leadership plan and description of workflow were not well articulated. The collaborators have requisite experience, although reviewers would have appreciated more information on the qualifications of the investigators from the collaborating partner’s groups. The budget was heavy on personnel and travel but light on consumables; this raised further concern about the team’s inexperience with stem cell research and neuronal derivation the techniques and its ability to complete the proposed project.
Overall, reviewers doubted that this project would have a significant impact on the field. Furthermore, reviewers raised significant concerns about the project’s feasibility including reservations regarding the research design and the team’s lack of experience with stem cell-derived dopaminergic cells, lack of a defined leadership plan, and an inappropriate budget.