One of the key issues in stem cell transplant biology is solving the problem of transplant rejection. Despite over three decades of research in human embryonic stem cells, little is known about the factors governing immune system tolerance to grafts derived from these cells. In order for the promise of embryonic stem cell transplantation for treatment of diseases to be realized, focused efforts must be made to overcome this formidable hurdle.
Our proposal will directly address this critically important issue by investigating the importance of matching immune system components known as human leukocyte antigens (HLA). Because mouse and human immune systems are fundamentally different, we will establish cutting-edge mouse models that have human immune systems as suitable hosts within which to conduct our stem cell brain transplant experiments. Such models rely on immunocompromised mice as recipients for human blood-derived stem cells. These mice go on to develop a human immune system, complete with HLAs, and can subsequently be used to engraft embryonic stem cell-derived brain cells that are either HLA matched or mismatched.
Due to our collective expertise in the central nervous system and animal transplantation studies for Parkinson’s disease, our specific focus will be on transplanting embryonic stem cell-derived neural stem cells into brains of both healthy and Parkinson's diseased mice. We will then detect: 1) abundance of brain immune cell infiltrates, 2) production of immune molecules, and 3) numbers of brain-engrafted embryonic stem cells. Establishing this important system would allow for a predictive model of human stem cell transplant rejection based on immune system matching. We would then know how similar HLAs need to be in order to allow for acceptance stem cell grafts.
In this project, we propose to focus on the role of the human immune system in human embryonic stem cell transplant rejection. Specifically, we aim to develop cutting-edge experimental mouse models that possess human immune systems. This will allow us to determine whether immune system match versus mismatch enables embryonic stem cell brain transplant acceptance versus rejection. Further, we will explore this key problem in stem cell transplant biology both in the context of the healthy and diseased brain. Regarding neurological disease, we will focus on neural stem cell transplants for Parkinson's disease, which is one of the most common neurodegenerative diseases, second only to Alzheimer's disease. If successful, our work will pave the way toward embryonic stem cell-based treatment for this devastating neurological disorder for Californians and others.
In order to accomplish these goals, we will utilize two of the most common embryonic stem cell types, known as WiCell H1 and WiCell H9 cells. It should be noted that these particular stem cells will likely not be reauthorized for funding by the federal government due to ethical considerations. This makes our research even more important to the State of California, which would not only benefit from our work but is also in a unique position to offer funding outside of the federal government to continue studies such as these on these two important types of human embryonic stem cells.
This is a proposal to explore stem cell transplant rejection in the Central Nervous System (CNS). The brain has traditionally been considered an immune privileged tissue, but it has become increasingly clear that transplant rejection occurs in both the diseased and healthy CNS. The applicant proposes to define compatibility criteria for transplantation of human embryonic stem cell (hESC)-derived tissue into the CNS using a novel mouse model with a humanized adaptive immune response (huMouse). The hypothesis to be tested is that criteria can be defined for Human Leukocyte Antigen (HLA) haplotype matching in this model that can be used to predict successful engraftment of human embryonic stem cell derived neural lineage cells (hESC-NSCs) in the CNS, thereby eliminating the need for long-term immune suppression. The applicant proposes first to establish and characterize the novel huMouse model in which this hypothesis can be tested (Aim 1). Once established, the applicant will use this huMouse model to define a minimal degree of HLA mismatch that will not induce allogeneic immune responses and will allow successful engraftment of hESC-NSCs into the CNS. The applicant will monitor CNS immune infiltration, the production of immune mediators, and the number of engrafted cells following transplantation of the hESC-NCS into the CNS of healthy animals (Aim 2) or of murine models of Parkinson Disease (PD) (Aim 3). For the PD mouse studies, analysis will also include behavioral assays.
Reviewers agreed that this is a high-risk proposal that is both highly innovative and clinically relevant. There is currently great uncertainty regarding the degree of immunological surveillance in the CNS, and the development of a new huMouse model is an innovative and creative way of exploring the response of the human immune system to stem cells transplanted into the brain. Reviewers praised the use of this huMouse model to assess the interaction between the HLA genotype of grafted cells and the humanized host immune response. Reviewers also commented that matching donor and recipient HLA haplotypes is a clinically viable approach that is currently used as an alternative to long-term immunosuppression. Defining minimal criteria for HLA matching could yield novel and clinically important information that would strongly impact approaches to CNS cellular therapy.
Reviewers commented that this is a well-crafted application that contains logical research aims and well-defined timelines. The scientific rationale for the project is excellent, particularly since the approach of HLA matching is already used clinically in other settings. However, reviewers expressed significant concerns regarding the feasibility of the proposed huMouse model for addressing rejection of transplanted hESC-NSCs. Importantly, they were not convinced that the human cells used to reconstitute the immune system of the mouse would be able to mount allogeneic immune responses strong enough to reject transplanted neural tissue. The mouse would lack HLA-expressing human thymic tissue and mature lymph nodes containing human stromal elements, which could limit normal T cell development. Allogeneic rejection in such models has not been demonstrated. Reviewers suggested that mice transplanted with human fetal liver and thymus, or infusion with human peripheral blood mononuclear cells (PBMCs), would be more appropriate and likely to produce strong allogeneic responses. Preliminary data demonstrating survival of human neural stem cell transplants in the immunodeficient mice and rejection with T cell specific immune response after transplantation in HLA mismatched humanized mice would be critical to address the feasibility of this project. In addition to this feasibility issue, reviewers also noted that in Aim 3, the applicant plans to examine graft rejection for only a 10-week period. Reviewers thought that a longer time period would be a more accurate measure of long-term engraftment potential. Despite these concerns, reviewers commented that rejection in the CNS is not well studied and these HLA mismatch studies are one of the first attempts to understand this critical issue.
The assembled research team is a phenomenal group of investigators with expertise in hESCs and neuronal differentiation and skills in immunology. The principal investigator (PI) is an experienced and productive neuroimmunologist who is highly qualified to direct this research project. The collaboration between the PI and the Co-Investigator (Co-I) is integral to the project, as the Co-I is a prominent neural stem cell biologist who brings great strength to the proposal. The strength of the team assembled for this proposal helped allay some of the reviewers’ concerns regarding the project’s feasibility.
Overall, reviewers determined that the strengths of the proposal (the high quality research team, the novel, innovative approach to examine an understudied issue, and potential high impact) outweighed the weaknesses (the feasibility of using the proposed huMouse model to address allogeneic rejection), and recommended this proposal for funding in spite of its high risk.