A major obstacle to stem cell based therapies is the immune response of the patient to stem cell derived tissue, which can be recognized as foreign and attacked by the patient's immune system. T cells orchestrate immune responses and are "educated" about self versus foreign in an organ called the thymus. It may be possible to educate T cells in a patient to avoid attacking stem cell derived grafts by "re-educating" them in a thymus that contains the same material as the graft. Humanized mouse models have considerable potential as test beds for exploring different therapeutic approaches, including thymic re-education approaches because they allow for human cells to be observed and manipulated experimentally. This proposal aims to refine humanized mouse models to allow for different therapeutic strategies to promote the acceptance of stem cell grafts by a patient's immune system to be modeled and tested. This proposal takes advantage of the most recent innovations in microscopic imaging to probe the interactions between developing T cells and their support cells in living 3D tissues. In addition to probing thymic development, in the future these approaches could be further adapted to reveal the cellular events that occur when stem cell derived grafts are accepted or rejected, and to allow for preclinical testing of drugs. The successful completion of this project will bring us closer to realizing the benefits of stem cell research by providing a viable humanized immune system mouse model for preclinical testing of stem cell-based therapies.
Statement of Benefit to California:
The successful completion of this project will bring us closer to realizing the benefits of stem cell research by providing a viable humanized immune system mouse model for preclinical testing of stem cell-based therapies. This should benefit Californians in three ways: First many Californians are suffering from diseases for which there are currently no adequate treatments and for which stem cell research holds considerable promise. The availability of a preclinical platform for testing stem cell-based therapies based on humanized mice should accelerate the development of these therapies and get them more quickly to patients in need. Second, answers to basic questions related to human immune system will be of interest to educators and researchers in California, and will contribute to California's standing as center for research and education. Finally, there is potential economic benefit, since the biotech and pharmaceutical industry could make use of these methods to advance the development of stem cell based products.
The goal of this application is to assess and adapt human immune system (HIS) models to provide a clinically relevant preclinical model that would be useful for assessing immunological tolerance to human stem cell-derived grafts. To address this, the Principal Investigator (PI) proposes four aims. In Aim 1, the PI proposes to analyze human thymocyte development in different HIS models. Aim 2 studies human thymic development with defined T cell antigen receptors (TCRs). In Aim 3, the PI explores intrathymic migration patterns of human T cells. In Aim 4, the PI uses HIS mice to examine functionality of human stem cell-derived thymic epithelial cells (TECs) and hematopoietic stem cells (HSCs). The reviewers praised the proposal’s innovation, creativity, and impact. They considered the comparison of the human T cell development in the different HIS models using the technology and methods proposed to be highly innovative. Reviewers also cited the creative approach proposed by the PI to address the in vivo migratory potential of human thymocytes. These studies could be of impact by providing validation of clinical relevant humanized models for use in transplantation studies that replicate human immune mechanisms. In addition, these studies could help derive TEC and HSC populations from human embryonic stem cells (hESCs), which have not been reported to date. Though the proposed techniques for the hESC derivations are not novel, the outcomes, if successful, would represent an advance in the field and impact transplant biology. Reviewers all agreed the PI presents a clear rationale for the overall goal of the study to compare and contrast the HIS models. Specifically, a need exists for surrogate models for assessment of the human immune response to stem cell populations, and to investigate human tolerance mechanisms. Reviewers commented that the rationale for Aims 1-3 is generally strong, and they felt the approach for Aims 1-3 is well designed and entirely feasible. In addition, the reviewers valued the emphasis on studying the thymic architecture and the cellular interactions within the thymus. Furthermore, one reviewer thought the thymic analysis was designed and presented and played to the strength of the PI. The reviewers considered a weakness of the proposal to be the lack of analysis of the models outside of the thymus, reflecting neglect of the role of the peripheral immune system. They noted that peripheral mechanisms of tolerance or immune activation will also be as important in the adult as central mechanisms in acceptance or rejection of the graft. Reviewers also believed that Aims 1-3 were logically organized and had appropriate preliminary data as support. All reviewers agreed that Aim 4 was underdeveloped and a weakness of the application. The PI is dependent upon collaborators to perform hTEC derivation, which has not been accomplished thus far. Similarly, hHSC derivation from ESC has not been successful and it is unclear how could be accomplished here given the information provided. Also, insufficient alternative plans were presented for Aim 4; no contingencies were presented if the collaborator cannot provide TECs or if the preliminary experiments that generate TECs fail. Reviewers thought that Aim 4 represents a departure from the primary goals of the application and strays from the PI’s expertise. Despite these concerns, the reviewers remained positive about the PI’s ability to successfully complete Aims 1-3 within three years and produce new and useful information to define the relevance of these models. The reviewers extolled the PI’s qualifications, training, and productivity. The PI’s notable contributions to thymic development and function have made him/her a leader in the field. The reviewers noted there is no expert in the area of ESCs listed to provide expertise in stem cell biology and the derivation of TEC and HSC, although there is a letter of collaboration. Reviewers also believed that the PI’s 10% commitment and the overall effort committed to the project might not be enough especially given the challenges of Aim 4. In summary, the reviewers were enthusiastic about the investigator and favorable to studies examining human T cell development and migration in humanized mouse models. Their enthusiasm was lessened however by an underwhelming Aim 4 and therefore the contribution of the proposed research to achieving tolerance induction for stem cell products. PROGRAMMATIC REVIEW A motion was made to move this application into Tier 1, Recommended for Funding, given that this is one of the few applications to assess human immune system development, specifically thymopoiesis, in the mouse model. The motion passed.