The overall objective of our proposed project is to develop improved approaches for allogeneic bone marrow transplantation (BMT) in chronic myeloid leukemia (CML) patients, primarily by using the megadose CD34 hematopoietic stem cell transplants supplemented with donor non- alloreactive T-cells. The hypothesis to be tested in this project is that toxicity of current conditioning regimens can be reduced by this approach, as it us anticipated that in patients conditioned by a sub-lethal protocol, infusion of hematopoietic progenitors and non-alloreactive T- cells will result in the induction of antigen-specific tolerance, and thereby diminish rejection and enhance long-term engraftment of the donor cells. Within this context, a second major objective of our project is to study how and what types of cells within the megadose CD34 stem cell transplants "paralyze" specifically the significant anti-donor residual immunity which survive the sub-lethal conditioning of the host. TO achieve our overall goal, we will conduct studies in vitro and in mouse models in close coordination with the clinical program (Project #2), to optimize the most suitable sub-lethal conditioning protocol, as well as the appropriate dose of T cell depleted stem cell enriched inoculum, to be used for transplantation. In anticipation of potential difficulty in achieving he minimal stem cell dose that might be required, we shall explore in the mouse model the synergistic role of non-alloreactive T cells, as well as search new means to generate such cells. We hope that if the overall objective of the proposed project will be achieved, it will allow to join forces with another study of the Program Project aiming at developing donor type anti-leukemic CTL clones, as such cells could be adoptively transferred to the patients following the establishment of tolerance towards donor type cells.