A major issue in the use of stem cells or in organ transplantation in general is the need to overcome graft rejection. Unfortunately, the only means currently available involves the use of systemic immunosuppression which leaves the recipient at risk for opportunistic infections. This proposal will seek to use the donor’s immune cells to prevent rejection. Using a concept in which the donor immune cells (in this case, natural killer cells) are infused first, the recipient’s immune cells will specifically seek to attack and reject it. The donor natural killer cells will be activated and thereby act as a “veto cell” and “attack the attacker” resulting in the eradication of only the host immune cells which would recognize the donor graft. Once eradicated, we will then infuse the donor stem cells which should now engraft without the need for extensive immunosuppression with the goal that the recipient will now become tolerant of the donor cells. This proposal will also examine the impact of recipient age on this process as the vast majority of patients in need of such therapy will be more advanced in age and this can impact both their rejection ability and the ability to suppress such rejection. These patients are also less able to handle the systemic immunosuppression so this can avoid these negative side-effects. Finally, the proposal will evaluate this “veto concept” in both mouse and human models which lends itself to immediate translation into the clinic. Use of these cells is already proceeding for the treatment of cancer. These studies thus will shed insights on using specific immune cell targeting with donor cells to remove the ability of the recipient to reject the graft but preserving immune functions to pathogens.
This proposal will have significant impact and benefit to the State of California and its residents. The complications associated with systemic immunosuppression after transplantation are considerable and costly. In addition, there have been tremendous resources placed on the clinical use of stem cells. However, if there is rejection of these cells by the immune system or unacceptable toxicities due to life-long immunosuppression then the proposed stem cell therapies will not be translated to the clinic. Therefore, a final hurdle will be to safely and specifically abrogate the ability of the recipient to reject the graft. This proposal is eminently translatable to the clinic and would allow the use of the donor’s own cells to specifically promote long-term engraftment. As such, this proposal will have an impact not only on stem cell transplantation but on solid organ transplantation as well.
This is a proposal to induce tolerance of graft tissue or induced pluripotent stem cells (iPSC) from a donor by pre-exposing transplant recipients to natural killer (NK) cells from the same donor. The hypothesis is that activated NK cells, or “veto cells”, can target and eliminate host T-cells that recognize donor tissue. NK cell pre-exposure would therefore blunt the T-cell response to all grafts from the same donor. Aim 1 tests the effects of activated NK cells on the engraftment of allogeneic iPSCs in a murine model under non-myloablative conditioning. Aim 2 uses murine iPSC-derived hepatocytes to determine whether long-term iPSC engraftment can be achieved with minimal immunosuppression in mice, using NK cells that express the same major histocompatibility complex (MHC). Aim 3 will test donor human NK cell effects on sensitized T cell and NK cell responses and on iPSC engraftment in a xenograft model.
Overall, the reviewers felt that this was an innovative proposal that would have a major impact on the field. Although the use of activated NK cells is not novel, reviewers valued the creative approach to testing the veto effect concept in vitro and in vivo using murine models. If successful, these studies would allow the induction of specific graft tolerance without the use of immunosuppressive drugs, which would revolutionize the field of stem cell transplantation. On the other hand, reviewers acknowledged that this proposal is of high risk, as no evidence exists that NK cell-mediated “veto effects” lead to clinically significant tolerance in vivo. It is also unclear whether induced deregulation of the host immune system will be safe for transplant recipients.
The reviewers were positive about the proposal’s rationale, experimental design, and feasibility. Although the veto effect is somewhat controversial and mechanisms of tolerance induced by NK cells are not understood, reviewers were comfortable that the rationale for the experiments is well supported. Published data from the PI’s lab and others have indicated that activated NK cells suppress graft-vs-host disease (GVHD) while amplifying graft-vs-leukemia (GVL) effects after allogeneic bone marrow transplantation. Reviewers were enthusiastic about the carefully designed experiments and the hypothesis-driven approach described in this application. The animal models, cell lines, cell subtypes, and engraftment/rejection events are all described adequately. The PI adequately addressed potential pitfalls and presented logical aims, and the use of mouse and human cells was cited as a strength of the application. One reviewer noted the PI’s appropriate use of differentiated iPSCs as the target for possible immune recognition, instead of the parent undifferentiated line.
In terms of feasibility, the preliminary data supported the PI’s ability to implement the experimental plan, although there were some concerns that the research might not yield anticipated results. One reviewer noted that the applicant presented no data regarding induced tolerance effects of NK cells on the engraftment of iPSCs. Since the mechanisms of NK cell-induced tolerance are not understood, this reviewer cautioned that the veto effect might not be applicable to iPSC transplants. Experiments in renal transplantation have indicated that elimination of T cells is not sufficient to promote allograft acceptance in all cases, even with additional immunosuppression, so it is not apparent that this approach will be clinically successful for iPSC transplantation. Preliminary data or a discussion that considered these clinical observations would have strengthened the application.
The reviewers commented that the PI’s qualifications, the research team, and the research environment are all superb. The PI is highly experienced and very well qualified to perform this study. His/her high level of productivity in NK-biology is complemented by the co-investigator’s expertise with iPSC-derived hepatocytes. Both the PI and the co-investigator allocate a 10% commitment of their time to this project.
In summary, this proposal will determine whether NK cells can be used to eliminate the host immune response and induce donor-specific tolerance to an iPSC graft. The proposal’s strengths include a creative approach, potential high-impact of the findings, and an outstanding research team. The weaknesses include the project’s extensive experimental plan that may not be fully achieved in the award timeframe and lack of convincing evidence that NK veto effects can generate optimal iPSC-derived hepatocyte engraftment or long-term tolerance.
A motion was made to move this application into Tier 1, Recommended for Funding. Reviewers underscored the strength of the research team composition and cited the unique focus on NK veto effects to achieve tolerance as an approach that is not otherwise well represented among Tier 1 proposals. The motion carried.
- Bruce Blazar
- Yair Reisner