Early Translational I
A primary bottleneck that must be overcome for all stem cell based therapies (SCBT) is rejection by the patient’s immune system. The immune system is educated to recognize what is “self” versus “foreign”, leading to “immunological tolerance.” Breakdown of immunological tolerance can lead to uncontrolled immune responses, autoimmune disease, and rejection of transplanted cells and organs by the patient’s immune system. Breakdown of immunological tolerance could also lead to rejection of SCBT. We will adapt the body’s normal mechanisms for inducing self-tolerance to facilitate induction of long-term tolerance to foreign stem cells. We will utilize regeneration of the thymus as an approach to induce immunological tolerance to ESC-derived grafts. The thymus is the location of self versus non-self education for T cells, key players in the immune response. In mouse and humans, the thymus loses its function due to changes in sex hormones that occur during puberty. Thymic epithelial cells (TECs), which provide structural support and cell signaling to promote T cell development, are particularly sensitive to these changes. Consistent with this, the number of TECs is reduced in the aged thymus. Interestingly, this can be increased by sex steroid ablation (SSA) therapy, suggesting that changes occur in TECs that mimic a younger thymus and endow them with an increased ability to support T cell development. Sex hormone production can be blocked using FDA-approved drugs (Lupron) or surgically. Both approaches result in enhanced engraftment of T cell progenitor cells and regeneration of thymus function. As older patients are likely to be recipients of stem cell therapies, we will use aged mice as our model system. We will test whether SSA and thymus regeneration can enhance T cell development from ESC-derived cells in antigen-matched and mismatched models. It is critical to test protocols in models of diseases that can benefit from SCBT and in human systems. Using a mouse model of autoimmune multiple sclerosis, we will test if SSA plus ESC-derived T cell progenitors can induce immunological tolerance to ESC-derived neural stem cells and repair damaged tissue. We will also treat humanized mouse models with SSA and transplant human T cell progenitors derived from existing human ESC lines. We will test if immune cell development and if tolerance is achieved by transplanting hESC-derived tissues and using non-invasive biological imaging to monitor their growth. Results from pilot clinical trials indicate that SSA increases the success of hematopoietic cell transplants for cancer treatment. We aim to extend this therapeutic approach to ESC-derived transplants and provide a mechanistic understanding of the effects of SSA on regeneration of the thymus and induction of immunological tolerance. If successful, SSA could also be applied to other types of SCBT. Taken together, these data will aid the advancement of SCBT to the clinical setting.
Statement of Benefit to California:
Results from the proposed research project will benefit the State of California and its citizens at several levels. Direct Impacts: This research project aims to target an obvious barrier to ESC-therapy for any disease: the avoidance of immune rejection in the patient. If immunological tolerance to ESC-derived grafts is not achieved, the therapeutic ESC is destroyed and disease persists. The immune system is formed from hematopoietic (blood) stem cells, and our research goals are to establish protocols which promote engraftment of ES-derived blood stem cells in a recipient to induce immunological tolerance to the ESC-graft. If successful, this will provide preclinical data to demonstrate that immunological tolerance to ESC-based therapies can be achieved. As immunological tolerance is important for all potential ESC-therapies, this work can have broad applications to a wide diversity of stem-cell based therapies and diseases. The work will also have indirect impacts outside of the research, such as notoriety to CIRM as the funding agency for this groundbreaking research, and be the springboard for improvements in health care, increase in tax revenues, and improvements in education for California residents. Health Care: We will test our protocols in well-studied models of transplantation and human multiple sclerosis. If successful, this research could provide stem cell therapies for these patients. As this research is funded by CIRM, it is highly likely that Californians would be the primary recipients of therapies designed using my research. Furthermore, my research plan is designed to have broad applicability, so ESC-therapies for other illnesses such as cancer, Alzheimer’s, Parkinson’s, diabetes, and cardiovascular disease can next be evaluated. Biotechnology: My research already relies on a number of products and tools manufactured and sold in the state of California. If successful, research will require a scaled-up version of protocols designed in my studies. This could attract new biotechnology companies in the state, boosting the tax revenue in the state. This in turn, will provide new jobs for California state residents. Education: Establishment of successful ESC-therapeutics in California will encourage institutions of higher education to promote science education to fill the jobs created by stem cell research. This will retain California students in the state that are interested in biomedical research and medical careers. Furthermore, it could attract out-of-state students seeking degrees that will allow them access to careers in stem cell research. It is envisioned that this will trickle down to the K-12 levels and provide funding to promote science education at all levels.
This application proposes to utilize thymic regeneration to overcome a major bottleneck of immune reactions to allogeneic embryonic stem cell (ESC)-derived grafts. The scientific rationale proposed by the applicants is that tolerance to foreign grafts has not been achieved clinically to date, due to age–related thymic atrophy. Thymic atrophy and subsequent loss of function is attributed to exposure to sex hormones that occurs during puberty. The applicant proposes to utilize sex steroid ablation (SSA) to induce immune tolerance to ESC-derived grafts. The applicant hypothesizes that SSA-induced thymic regeneration enhances acceptance of ESC-derived hematopoietic progenitor (HP) cells to enable the formation of mixed hematopoietic chimerism and tolerance induction to a secondary graft derived from the same ESC. The applicant proposes to investigate whether SSA plus ESC-derived HP cells can induce immunological tolerance to ESC-derived neural stem cells in a murine model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE). Ultimately, the applicant proposes to test the regimen with human ESC-derived cells in a humanized murine model. Reviewers concurred that the application addresses an important bottleneck in development of cell-based therapies, allowing transplanted ESC-derived cells to engraft and avoid immune rejection. Some aspects of the proposal were called intriguing; however reviewers had difficulty with the poorly written proposal. Reviewers expressed major concerns with the research plan presented in the application. The research Aims were judged to be loosely related, and some poorly designed. In Aim 1, reviewers felt that the proof-of-concept studies could be more simply done with CD34+ progenitor/stem cells from fresh marrow, in order to establish suitable conditions for transplantation with ESC-derived HP cells in Aim 2. Reviewers commented that Aim 3, to study the effects of SSA on adult murine thymic epithelial cells, did not fit well with the overall proposal. Finally, in Aim 4, reviewers questioned whether the study of the survival of hESC-derived HPs in a profoundly immunodeficient murine model will allow true assessment of any tolerance induction, supported by evidence that other SCID mouse with human hematopoietic stem cell transplant models have not shown much ability to reject grafts. Finally, the preliminary data included in the application did not increase the confidence that that SSA will overcome immunological barriers to allogeneic ESC-derived grafts. The principal applicant is highly trained and has experience with the creation of embryonic stem cells and derived progenitors; however, one reviewer expressed reservations at the principal investigator’s (PI’s) lack of recent publications. The PI’s home campus promotes an inter-disciplinary approach and provides the necessary environment to execute successful research. The proposal is greatly aided by the addition of collaborators who have published extensively on thymic regeneration using SSA and related topics. The budget was noted to be large but would be required if all of the proposed studies are carried out. In summary, reviewers judged the proposal to address an important bottleneck in the development of cell-based therapies; however the review panel was not convinced that the proposal as written was cohesive or feasible.