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.
Statement of Benefit to California:
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.
Year 1Our goal is to identify clinically translatable strategies to induce tolerance to stem cell transplants. Host rejection of stem cells can occur by several mechanisms, all of which need to be circumvented. Towards this goal, in this past year we have laid the foundation for the use of natural killer (NK) cells in our animal models of bone marrow and embryonic stem cell transplantation to achieve stem cell transplant tolerance. We studied the immunogenicity of human and mouse embryonic stem cells with a focus on NK cells to tell us what hurdles stem cell based therapies have to overcome when encountering the patient’s immune system in the bloodstream. We found that a patient’s NK cells, which are part of the innate immune system, reside mainly in the bloodstream and kill cells which lack a molecule called major histocompatibility complex class I (MHCI), play a significant role in embryonic stem cell immunogenicity. This finding must be taken into consideration when applying regenerative therapies but may also be exploited beneficially to remove unwanted leftover pluripotent stem cells from stem cell-derived therapies. Furthermore, we studied the effectiveness of human NK cells to veto T and NK cell activity in vitro yielding findings that we now can apply to in vivo models of human stem cell transplantation rejection. Lastly, we have made progress in the use of specific subsets of mouse donor NK cells to improve their effectiveness in suppressing host T cells so that we can now translate those results into our mouse bone marrow transplantation model.
Year 2Our goal is to identify strategies to improve the engraftment of stem cell transplants in patients. During the previous funding years, we have demonstrated that a small subset of white blood cells called natural killer (NK) cells are involved in the rejection of embryonic stem cells in animal models. However, we have now shown that not all NK cells have the same potential to reject embryonic stem (ES) cells. NK cells which express receptors that can bind a molecule called MHC are termed “licensed” because they have a greater ability to kill virally-infected and tumor cells. However, we have now demonstrated for the first time that these cells are also the predominant subset of NK cells which are involved in the rejection of ES cells. This may lead to clinical strategies to improve ES engraftment since we have identified a key player in the rejection of these cells. Furthermore, we have studied how NK cells can act as “veto” cells: cells which can attack the T and NK cells in the patient which are responsible for rejecting transplants. Combined, our studies suggest that NK cells from both the donor and patient can be utilized and carefully observed in stem cell transplants to improve the likelyhood of engraftment and clinical responses.
Year 3The aim of this proposal was to find ways to improve the outcomes of treatments involving stem cell transplants. One of the difficulties with these treatments is that the donor cells must be “matched” to the patient to prevent the patient’s own immune cells from rejecting the transplant. Otherwise, the patient would need immune suppressants to prevent this rejection. Our proposal aims to specifically target the white blood cells that are responsible for rejecting donor cells with an immunotherapy. Over the course of the funding period, we have demonstrated that T cells and NK cells, both of which are subsets of white blood cells, are responsible for rejecting stem cells. However, immunotherapy with NK cells derived from the donor can kill the “alloreactive” immune cells—those cells which are responsible for rejecting the stem cells. Specifically over the past year, we have demonstrated that host NK cells can infiltrate stem cell grafts and mediate the rejection of these grafts. We have also shown that donor NK cells, when activated, can overpower host NK and T cell responses to prevent the rejection of donor tissue.