Myelodysplastic syndrome (MDS) is a heterogeneous group of progressive, irreversible, hematopoietic stem cell disorders characterized by progressive cytopenia and for which there are no effective therapies. Experimental and clinical evidence indicates that lymphocytes from patients with MDS exert an inhibitory effect on autologous hematopoietic colony growth, and that this contributes to cytopenia. Immunosuppressive treatments that decrease the number of lymphocytes or suppress their function such as corticosteroids, cyclosporine, and antithymocyte globulin (ATG) have been shown to reverse that cytopenia, and in some cases to reduce the number of blasts in the marrow. How these lymphocytes recognize their target antigens and inhibit hematopoietic precursors is unknown. Identification of relevant hematopoietic target antigens, however, might lead to useful therapies for MDS, and would provide insight into other bone marrow failure states such as aplastic anemia where T lymphocytes are also thought to play a key role in the development of pancytopenia. As a strategy to search for those target antigens, we hypothesize that in myelodysplastic syndrome, lymphocyte inhibition of hematopoietic progenitors is mediated by clonal or oligoclonal activated T lymphocytes through MHC-restricted antigen recognition. The long-term goal of this project is to investigate whether clonal T cells associated with inhibition of marrow progenitors can be isolated from MDS patients and then used to further identify relevant target antigens. These clonal T cells could then be more specifically targeted in the treatment of MDS patients and identification of T cell target cells/antigens could help determine the proportional contribution of lymphocytes to the development of cytopenia in MDS. We have shown that patients with MDS who respond to ATG treatment have activated CD8+ lymphocytes that inhibit colony forming unit-granulocyte macrophage (CFU-GM) in a MHC class I-restricted manner. Dominant clonal and oligoclonal lymphocyte populations that are present in peripheral blood and bone marrow in some MDS patients are later replaced by a normal polyclonal distribution, which coincides with reestablishment of effective hematopoiesis after ATG treatment. The proposed studies will isolate and characterize clonal T cells from MDS patients, determine how these T cell clones suppress hematopoiesis, whether T cell-mediated inhibition of hematopoiesis is directed against dysplastic or normal progenitors, and whether additional T-cell-directed immunosuppressive agents added to ATG treatment can enhance recovery from cytopenia in a randomized clinical trial.