One of the great promises of stem cell research is that it will one day be possible to prepare replacement cells or organs from stem cells such as embryonic stem cells, which can be transplanted to patients to substitute for diseased or defective patient tissues or organs. Unfortunately, the immune system reacts against, and rejects, transplanted tissues that are not perfectly matched with the recipient. A promising approach around this problem is a two step procedure, in which a patient is first transplanted with blood stem cells of a specific type, and later with replacement tissues or cells derived from the same embryonic stem cells as the blood stem cells. If the blood cell transplant is successful, the patient’s blood cells will forever after be composed of a mixture of their own blood cells and the donor blood cells (“chimerism”). It is known that blood cell chimerism induces the recipient to be accept diverse types of grafts of the same source as the blood cells. Thus, the blood cell graft prepares the recipient to accept other types of grafts derived from the same stem cells. Unfortunately, blood stem cell grafts are themselves subject to a specific type of immune rejection, mediated by natural killer (NK) cells. Hence, successful application of the two step procedure requires the development of methods to prevent NK cells from rejecting blood cell grafts.
We have developed evidence that NK cells can be induced to become tolerant of mismatched blood cell grafts. We propose studies to develop a general procedure to induce tolerance of a recipient’s NK cells to mismatched blood cell grafts. Using an experimental model, we will test whether the procedure facilitates transplantation of blood cells derived from embryonic stem cells, the generation of blood cell chimerism, and the subsequent transplantation of other tissues in a two step procedure.
The proposed research is designed to provide novel methods to facilitate therapeutic transplantation of stem cell derived cells and tissues to patient’s suffering from numerous disorders and diseases. Such approaches will ultimately benefit millions of Californians suffering from diabetes, heart disease, neurodegenerative diseases, etc. Breakthroughs in stem cell research in California will also generate new industries, reducing joblessness and bolstering the California economy.
The goal of this proposal is to develop cell-based methods to promote natural killer (NK) cell tolerance of major histocompatibility complex (MHC) Class I mismatched hematopoietic stem cell (HSC) grafts, and then to harness this process to induce mixed hematopoietic chimerism and donor specific graft acceptance. Investigators will utilize a series of chimeric HSC transplant studies in order to: 1) identify the cell type responsible and 2) determine the tolerizing protocol for inducing NK cell-mediated tolerance to (MHC)-deficient hematopoietic cell grafts. The next series of studies will test whether rejection of allogeneic cells (MHC mis-matched) can be prevented using the same identified cells and tolerizing protocol developed above. Finally, investigators will validate the concept by testing the capacity of allogeneic embryonic stem cell (ESC)-derived cells to induce tolerance to ESC-derived allogeneic HSC, and whether the resultant hematopoietic chimerism enables donor specific, allogeneic skin transplant acceptance.
Overall, reviewers were extremely enthusiastic about the proposal’s innovative approach, directed at solving a major problem in HSC transplantation. NK cells are radioresistant and therefore may present a bottleneck to successful HSC engraftment and the development of hematopoetic chimerism, even in conditioned hosts. Resolution of NK rejection, while retaining the remainder of the host’s NK function, could have a major impact upon both therapeutic cell and solid organ transplantation.
Reviewers unanimously praised the proposal as novel and well crafted, and appreciated that it addresses fundamental NK cell related rejection issues. Preliminary data that even mature NK cells can be tolerized to MHC Class I-deficient HSC transplants provides convincing support for the proposal’s rationale. Reviewers viewed the group’s familiarity with and access to the required technologies and tools to perform the work a strength of the proposal. Reviewers judged the experimental design to be excellent. In particular, reviewers were impressed by Aim 1’s elegant, well-detailed and straightforward series of chimeric transplantation and cell depletion experiments to identify tolerizing cell types. Reviewers agreed the data generated should lead to meaningful results and clinically evaluable strategies. The panel did note, however, that experiments in Aim 4 to derive the tolerizing cell type from ESC and to test it’s ultimate ability to promote hematapoeitic chimerism and allogeneic graft acceptance are technically demanding. They requested further development of alternative plans for this Aim. Finally, a discussion regarding the relationship between transplanted cell numbers and NK mediated rejection would strengthen the proposal. Reviewers felt these concerns could be addressed and did not affect the strong enthusiasm for the proposed project.
Reviewers uniformly praised that the established principal investigator (PI) who is well funded and has an exceptional publication record. They referred to the PI as a world leader in NK and T cell biology. Although the reviewers noted the proposed team is entirely contained in the PI’s laboratory, rather than consisting of a broad based collaborative effort, they were confident this talented group can successfully execute the experimental plan.
In summary, this proposal provides a novel approach to establish and harness hematopoeitic chimerism to achieve donor specific tolerance. The reviewers were extremely enthusiastic about the proposal’s exceptional PI, potential impact and high likelihood of success.