The 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.