The thymus is an organ that plays a key role in controlling immune responses and immune tolerance. The thymus promotes immune tolerance by deleting and removing self-reactive T cells from the immune system. In addition, the thymus also helps drive the production of important suppressor T cell populations like regulatory T cells that also control immune tolerance. Thus, strategies that expand and improve thymic function could be critical in improving transplantation of tissues derived from embryonic stem cells. The thymus consists of a supporting network of thymic epithelial cells that help bone marrow derived T cell precursors mature and differentiate into fully functional T lymphocytes. Despite their importance, there has been little progress in methods to grow and expand out the supportive thymic epithelial network. This project will explore strategies to grow and expand out functional thymic epithelial cells from human embryonic stem cells using a multi-step culturing technique. These expanded thymic epithelial cells will be characterized and tested for the ability to support T cell development and differentiation. Finally, the expanded thymic epithelial cells will be put into transplantation models in humanized mice to test their ability to improve and enhance the acceptance of transplanted tissues. These studies offer enormous potential for promoting graft-specific immune tolerance in that embryonic stem cells could be differentiated into both a replacement tissue and into functional thymus
The work in this proposal is designed to help improve the effectiveness of stem cell treatments by preventing immunological rejection of transplanted tissue derived from stem cells. Although significant progress and promise has been shown to use stem cells to regenerate damaged organs for the treatment of a wide variety of diseases, an important barrier to bringing this to the clinic is the potential of the immune system to reject or damage this regenerated tissue. Currently, there are efforts underway to use stem cells to treat diseases that have a wide impact on the health of Californians, including diabetes, Parkinson’s disease, Alzheimer’s disease, retinal eye diseases, and musculoskeletal diseases to name just a few.
The work proposed here will help improve treatment for these diseases by improving the ability to put a break on the immune system to reject or destroy cells or tissues that are derived from stem cells for the treatment of these diseases. Here we will improve methods to grow and expand an important organ that controls the ability of the immune system to be “tolerant” of transplanted tissues called the thymus. If methods to grow and expand the thymus from stem cells can be done, this would represent a significant advance in improving stem cell therapies. Thus, the impact of this work could have a broad impact on a large number of the disease treatments that involve stem cells.
The overall goals of this proposal are to generate thymic epithelial cells (TECs) from human pluripotent stem cells and assess their functionality in mouse transplantation studies. The thymus is an organ of the immune system that plays a crucial role in promoting tolerance. The applicant hypothesizes that TECs derived from human embryonic stem cells (hESCs) will have the ability to induce tolerance to other tissue grafts derived from the same hESCs. There are two Specific Aims: (1) to differentiate TECs from hESCs and induced pluripotent stem cells (iPSCs); and (2) to assess the function of differentiated TECs both in vitro and in vivo following transplant into a mouse model with a humanized immune system. Aim 2 includes co-transplantation studies to test the ability of hESC-derived TECs to improve donor acceptance of other transplanted tissues.
Reviewers agreed that this proposal could have a profound impact if successful. The generation of TECs from hESCs or iPSCs could pave the way for clinical strategies to induce graft specific tolerance without immunosuppression. Reviewers also found the proposal to be innovative in its staged approach to TEC differentiation and use of humanized mouse models to assess the functionality of TECs in vivo.
The reviewers were impressed by the research plan. They found its scientific rationale to be logical and compelling and appreciated that the approach to generating TECs is based on the known developmental biology of thymic epithelium. The overall research plan was presented clearly and appears straightforward. Reviewers were concerned that the “brute force” strategy proposed for determining conditions for TEC differentiation may not succeed, but noted that an alternative approach, forced transcription factor expression, is presented. They did suggest that the use of a reporter construct for this transcription factor might be superior, as it would allow for high-throughput screening of differentiation conditions while avoiding potential problems associated with forced gene expression. Use of humanized mouse model for the functional assessment of hESC-derived TEC was judged to be particularly innovative. Reviewers also appreciated the substantial preliminary data presented but noted that the lack of data demonstrating thymic differentiation makes the proposal somewhat risky.
Reviewers described the Principal Investigator and Co-Investigator as exceptionally well-qualified to carry out the proposed research. They praised their outstanding track records as well as their substantial and complementary expertise in the fields of immunology and ESC biology.
Overall, reviewers were extremely enthusiastic about this proposal. They noted that the generation of TECs from hESCs or iPSCs would have a major impact on the field of regenerative medicine. Reviewers acknowledged that the project is high-risk but were confident that the outstanding research team is capable of accomplishing the proposed aims.