The research proposed in this project has very high potential to identify new medications to boost the natural ability of stem cells to prevent rejection of transplanted organs. This is a very important goal, because patients that receive a life-saving transplanted organ must take toxic medications that increase their risk for cancer and serious infections. Experimental clinical trials have recently shown that stem cells given to patients at the same time as they receive their transplanted organ can engraft in the patient and prevent rejection of the transplanted organ, without the need to take immunosuppressive medications. The problem though is that the stem cells don't last forever; they are eventually rejected by the patient's own immune system. A promising target to prevent rejection of stem cells in patients is a group of primitive molecules that are receptors on stem cells, as well as many other cells in the body. These primitive receptors are called innate immune receptors and they provide the trigger for activation of a cascade of mechanisms that lead to rejection of the stem cells. If the trigger is not pulled, then the stem cells will not be rejected. Therefore, our proposal focuses on how to block activation of the rejection cascade so that stem cells are able to engraft in the patient and prevent rejection of transplanted organs, without the life-long use of toxic medications. We have extensive experience studying innate immune receptors and transplantation and therefore are poised to make significant advances in our understanding of how stem cells are rejected by signals that depend on innate immune receptors. Furthermore, once we identify which innate immune receptors are relevant, targeted rationale blockade of these receptors can be proposed.
Statement of Benefit to California:
The proposed research will benefit the State of California and its residents by providing important knowledge about new ways to prevent rejection of transplanted organs. Currently, patients with transplanted organs must take life-long toxic medications to prevent rejection of their organs. This proposal will help develop ways to avoid the use of these toxic medications, while allowing life-saving organ transplants to survive in their new host. The use of stem cells in recipients of solid organ transplants is the first new breakthrough in decades for transplantation and therefore it is very important to try to optimize the use of stem cells to allow the survival of transplanted organs without toxic immunosuppressive medications.
This proposal focuses on the role of innate immune receptors in the establishment of hematopoietic stem cell (HSC)-induced mixed chimerism and allograft tolerance. A promising strategy to induce tolerance to allogeneic transplants is to generate mixed chimerism by transplanting bone marrow (BM) HSCs from the donor, thereby inducing tolerance to a transplanted organ from the same donor. This strategy has been successful in animal models but has had only limited success in humans. In this proposal, the applicant hypothesizes that eventual loss of mixed chimerism and rejection of the organ graft in human patients in circumstances of tolerance from combined BM and organ transplants, may be due to innate immune receptor-mediated activation of donor HSCs, leading to differentiation of these cells into antigen presenting cells, with subsequent activation of an immune response against donor cells. The applicant postulates that if innate immune receptors prove to play a significant role, they represent novel therapeutic targets for tolerance-inducing drugs. There are three specific aims: (1) to utilize several mouse models to determine whether innate immune receptors influence the ability of donor HSCs to induce mixed chimerism; (2) to determine the roles of innate immune receptors in T cell activation or tolerance by transplanting HSCs lacking these receptors; and (3) to determine whether HSCs lacking specific innate immune receptors are capable of inducing tolerance to allogeneic skin and/or heart grafts. Reviewers described this proposal as innovative and particularly appreciated the novelty of studying the role of innate immune receptors in the maintenance of mixed chimerism. They agreed that this represent a highly novel and relatively uncharacterized area of investigation in the field of transplantation. They noted the applicant’s creative approach that involves the use of several transgenic mouse strains deficient in different innate immune receptors, applied in models of allogeneic mixed chimerism and organ transplantation. Reviewers agreed that the proposal could have a significant impact, as blockade of innate immune receptors might provide a novel strategy for tolerance induction and could potentially reduce the toxicity of conditioning regimens. Reviewers agreed that the research plan is based on a sound scientific rationale and is composed of logical aims. They described the experimental approaches as sound and logical, and the plans to elucidate T cell reactivity in Aim 2 as elegant and thorough. Pitfalls were clearly identified and discussed and the preliminary data was good. However, they expressed some concern regarding the lack of detail provided for certain aspects of the proposal such as the mixed chimerism model. For example, the applicant states that the conditioning regimen for inducing mixed chimerism will be optimized in pilot experiments. However, since different regimens will provide different inflammatory environments, leading to different levels of innate immune receptor stimulation, different levels of mixed chimerism and possible different mechanisms of tolerance, they felt it was important for the applicant to define the model that will be used, and they viewed the lack of an established mixed chimerism model as a potential weakness. In addition, the applicant does not address the possibility that the engraftment capacity of donor HSCs from the different transgenic mouse strains may be different for reasons unrelated to their immunological profile. For example, if HSCs from one strain are more prone to differentiation, it would reduce engraftment and confound interpretation of the data. There was also a minor concern that the assays described might not be sensitive enough to detect subtle differences. Finally, reviewers were unsure of the feasibility of the some experiments proposed in Aim 3. They noted that meaningful results will depend on the establishment of a model in which mixed chimerism followed by graft rejection occurs predictably to assess inhibition under the proposed conditions. They would have appreciated preliminary data demonstrating that this is achievable. Reviewers described the Principal Investigator (PI) as highly experienced and an expert in T cell biology. They noted that the PI has an impressive track record of high impact publications and extramural funding. The Co-Investigator has a clinical background in organ transplantation and experience with the proposed skin and heart transplant models, providing complementary expertise to the PI. In general, reviewers found the research team to be well qualified to carry out the proposed experiments. Overall, while reviewers expressed concern about the lack of detail provided about certain aspects of the research plan, they appreciated the novelty and creativity of the proposal and its potential to have a significant impact in the field. Reviewers praised the strong research team and the elegant and cutting edge technology they have assembled to dissect the mechanisms proposed, and were ultimately convinced that the team would be successful in carrying out the proposed experiments. PROGRAMMATIC REVIEW A motion was made to move this application into Tier 1, Recommended for Funding. Reviewers noted that studies investigating the innate immune system were not well represented in Tier 1. The motion carried.