Bone marrow contains "stem" cells that can restore normal blood cell production in patients transplanted with these cells. This procedure now allows patients with certain leukemias and non-malignant blood diseases to be cured. However, many cannot benefit from this type of therapy because the numbers of cells available are insufficient. The procedure would also become much safer and cheaper if it were possible to expand the cells that produce new blood cells very rapidly after the transplant is performed. Over the past few years, tissue culture conditions have been developed that appear useful at first glance, but subsequent studies showed these cultured cells no longer work properly when transplanted. I have confirmed this finding and shown it develops very quickly (within a day). These findings are important clinically as they indicate that additional methods are required to make these cells clinically useful. The goal of my project is to better understand the mechanism that activates the engrafting defect these cultured cells acquire, and identify new agents that can prevent or reverse the problem. My preliminary experiments indicate that certain unidentified proteins can achieve this result. I will now use several strategies to determine what these are and how they work. This will include looking for changes in the molecules known to be involved in the normal engrafting ability of these cells. A second approach is to screen for genes that are key to the defect and small molecules that have the desired corrective effect. Candidates thus identified will finally be tested in a model in which human cells are transplanted into mice that are genetically immunodeficient. The results will provide a significant contribution to our understanding of why cultured cells perform poorly in patients and how this situation can be corrected with clinically useful agents, leading to improved outcomes in patients whose prognosis relies on a transplant.