This proposal focuses on the role of the immune system in transplantation of derivatives of human pluripotent stem cells (hPSCs). A critical roadblock to successful cell replacement therapies, no matter what the disease or injury, is the fact that the immune system’s main function is to prevent the introduction of foreign substances into our bodies. Unfortunately, this means that transplantation of organs or cells will inevitably lead to rejection unless the immune system is repressed or “tricked” into accepting the transplants as non-foreign. Long term immune suppression is harmful, because of the toxicity of the drugs used and because the repressed immune system is unable to protect against infections and monitor for cancerous cells. Our strategy is to “trick” the immune system into recognizing the cells used for replacement therapy as “self” rather than as foreign. We have had early success with these methods, and now propose to test them in a realistic situation that may lead directly to human applications. Each human being has a particular complement of proteins that are exposed on the surfaces of cells and serve to distinguish “self” from “non-self”. During our development, our immune system is trained to recognize our own molecules (called HLA or MHC) and not reject cells carrying them. However, as we age, our bodies lose this ability to “teach” the immune system what to reject and what to protect. Our scientific strategy is to use specific hPSCs (with specific HLA types) to regenerate the body’s ability to instruct the immune system. We will do this by regenerating the thymus, an organ that is active in childhood but atrophies in adulthood. The thymus is integral to determining what is self, removing the immune system cells that would attack one’s own cells. For this pre-clinical study, we will regenerate the thymus in experimental animals by transplanting thymus cells derived from specific hPSC lines. If successful, this will cause the immune system to recognize the new cells as "self” and not reject cells from this particular cell line when they are later used to repair degenerated tissues. This method, called “inducing tolerance”, will be tested using three diverse PSC lines, derived from Caucasians and Africans, which have very different HLA types. The research team includes three well-established researchers: 1. An expert in deriving and characterizing hPSC lines, who has already made the cell lines to be used; 2. A leader in the field of immune tolerance, who has developed methods for transplantation of thymus cells; and 3. An expert in a widespread human degenerative disease, multiple sclerosis (MS), who will transplant hPSC-derived neural stem cells to a mouse model of MS that has been tolerized to accept these cells. This method will be applicable to any disease that can benefit from cell therapy, and this team has all of the expertise and knowledge necessary to develop a successful strategy.
One in seven Americans lives in California, and these people make up the single largest health care market in the United States. The diseases and injuries that affect Californians affect the rest of the US and the world. Many of these diseases involve degeneration of healthy cells and tissues, including neuronal tissue in multiple sclerosis, Parkinson disease, Alzheimer disease, stroke, and ALS, connective tissue in arthritis, pancreatic islet cells in diabetes, heart muscle in cardiovascular disease, the immune system in HIV/AIDs, and other cell types in a host of other diseases, genetic disorders and injuries. There is a great deal of optimism about the use of stem cell-derived cells for replacement therapy for these disorders, and a great deal of progress has been made toward developing the appropriate cell types from human pluripotent and multipotent (adult) stem cells. However, a huge roadblock to the success of these potential therapies remains: how can transplanted cells be successfully engrafted without rejection? The immune system has evolved to protect individuals from infection and the result is that it specifically eliminates “non-self” organ, tissues, cells or molecules. A long history of research on the immune system predicts that all cells that are recognized as being non-self will ultimately be rejected by the host. The goal of this project is to develop a general method that can be used for any cell replacement therapy, to allow the transplanted cells to survive and function, while retaining the immune system’s ability to guard against infection and destroy cancerous cells. Our strategy is based on the idea of immune “tolerance”, which is a procedure that is designed to allow transplanted cells to be recognized as "self” and not be rejected. If our research efforts are successful, this method will enable the whole range of cell replacement therapies that will provide cures to previously incurable degenerative disease and injury.
This proposal will examine ways to improve engraftment and tolerance to derivatives of human pluripotent stem cells (hPSCs), with the goal of enabling therapies for treatment of human disease. The project brings together three research groups with expertise in the areas of human hPSC biology, transplant immunology, and autoimmune disease. For Aim 1, the Principal Investigator (PI) proposes to generate clinically relevant derivatives from three hPSCs lines with diverse HLA haplotypes. In collaboration with the Partner PI, the PI plans to develop a method to differentiate hPSC into thymic epithelial progenitor cells for transplantation, towards the goal of generating a chimeric thymic environment. The partner PI will perform Aim 2, which is to test the immunogenicity of the hPSC derivatives and the effects of chimeric thymus-based tolerance induction on their engraftment. Aim 3 will be performed by a co-investigator, who will test transplants of hPSC derived neural stem cells in an animal model of autoimmune disease.
Reviewers felt that this proposal has some innovative aspects, particularly the possible generation of thymic tissue from hPSCs in Aim 1, but will also require great effort to achieve. The project will use a systematic approach to define the immunogenic potential of hPSCs, establish thymus-based immune tolerance to hPSC-derived tissues, and demonstrate the effectiveness of a proposed tolerogenic approach in a mouse model of multiple sclerosis. If successful, it will have broad implications for stem cell-based therapies and organ transplantation by providing a novel approach for the induction of tolerance toward transplanted tissues.
Reviewers appreciated the collaborative efforts of the stem cell and thymic manipulation experts towards the accomplishment of the bulk of the first two aims.
However, they also noted a lack of cohesiveness to some project elements and felt some of them lacked a clear rationale. They noted that the third aim, in particular, stood apart from the rest of the proposal, and they faulted the rationale behind its choice of cells for transplantation. The supplied preliminary data suggested that mouse oligodendrocyte precursor cells (OPC) can remyelinate neurons in the disease model, but that preliminary data does not support a rationale for this proposal, which plans to transplant an entirely different cell type, neural stem cells, which most likely act through a distinct immunomodulatory mechanism. Also, the rationale behind performing the proposed thymic rejuvenation studies in Aim 2 was unclear, as the effects of the described factors upon restoring endogenous thymic function are already known. Furthermore, investigators provided no clear indication of the relationship between these studies and those focused on human stem cells.
In general, most of the experiments were thought to be feasible, but reviewers doubted whether the overall project could be achieved in the proposed time frame. For Aim 1, reviewers felt the logistics for quality control and distribution of cells to different labs were straightforward and well thought out. Aim 2 contains some innovative components, but the methodology, end points, and expected outcomes were poorly described. For example, no description was provided regarding the immuno-competence of the specific animal model proposed. This left reviewers unclear as to whether the transplantation study data pertained to translationally relevant allogeneic responses or to xenogeneic responses, which are very different and less clinically applicable. A second part of this aim will seek to differentiate thymic epithelial progenitors from hESCs for construction of a thymic organoid. Reviewers expressed concern that accomplishing this critical goal, upon which testing the central hypothesis of the proposal depends, will require longer than anticipated and may not be achievable within the time frame of the grant. Reviewers noted that the transcription factor to be used to detect differentiation lacks sufficient specificity for thymic epithelial progenitor cells, since it is also expressed in extra-thymic tissues derived from pharyngeal endoderm and could therefore mark a broad range of pharyngeal endoderm precursors. There was also a lack of justification to support the idea of using a pluripotency factor to promote longevity of the thymic epithelial progenitor cells. Because Aims 1 and 3 are dependent on this part of the project, reviewers had serious doubts about the ability of the team to complete those aims within the time frame of the grant.
The research team was judged to be very qualified to conduct the proposed studies, bringing together appropriate and recognized leaders in stem cells, immunology/thymus biology, and animal model of autoimmune disease. All primary investigators have an excellent publication and funding track records. The PI was viewed as highly qualified to lead this project; however, the panel expressed concern that the role of the California PI appeared limited to supplying characterized cells to the rest of the team. Overall, the budget seemed appropriate for the project, but reviewers found that individual roles of several of requested personnel to be inadequately explained or justified.
Overall, reviewers found this proposal to have attractive elements, particularly in bringing together three distinct research groups to address issues of immune tolerance. However, enthusiasm for the project was seriously diminished by a lack of detail regarding immunological function in key xenogeneic transplant models and serious doubts about whether the group could achieve a critical intermediate goal, the generation of thymic epithelial cells from hPSC. Although the collaborating research groups were viewed as well qualified, the project’s disparate components were inadequately integrated and did not comprise a cohesive investigation.
A motion was made to move this application into Tier 1, Recommended for Funding. The maker of the motion argued that the project is unique in its use of ethnically diverse cell lines and contribution of key expertise in the area of thymic development to address tolerance. Other reviewers suggested that the project does not come together well and were unconvinced that the project would have a significant impact on the field. The motion was opposed by a majority of the GWG. Because the motion was supported by more than 35% of members, supporters have exercised their right to have that position reported to the ICOC.
The minority position cites three unique strengths in support of recommending this application for funding. First, the proposal makes use of three ethnically diverse hPSC lines. Second, the project involves outstanding investigators with expertise in thymic development. Third, the project proposes an advanced model for identification and isolation of thymic progenitors.
- Bruce Blazar