Successful stem cell transplantation (SCT) across HLA barriers can be performed with cord blood, megadoses of stem cells, or with nonmyeloablative conditioning strategies. Because the HLA-mismatched transplants are often T-cell depleted, leukemia relapse rates are high. Treatment of relapsed leukemia after HLA- mismatched SCT is difficult. A novel potential strategy to treat relapsed leukemia after HLA-mismatched SCT is the use of patients' mismatched HLA molecules as antigen-presenting molecules to generate hematopoietic system-specific cytotoxic T cells (CTLs) from the stem cell donor. Adoptive transfer of these hematopoletic system-specific CTLs that are re-stricted by nonself HLA molecules may eliminate leukemia without affecting the patient's nonhematopoietic cells or donor hematopoietic cells. We investigated the feasibility of this strategy using the hematopoietic system-specific minor histocompatibility antigen HA-1, which is known to induce HLA-A2-restricted CTLs. HLA-A2- peripheral blood mononuclear cells were stimulated with HLA-A2+ T2 cells pulsed with synthetic HA-1 peptide or with dendritic cells transduced with the HA-1 cDNA. Tetrameric HLA-A2/HA-1 peptide complexes were used to monitor and enrich HA-1-specific CTLs. In the alloreactive cultures, HA-1-specific CTLs were enriched up to 7% by 3 rounds of antigen-specific stimulations and up to 87% by fluorescence-activated cell sorting of tetramer-positive T cells. The HA-1-specific CTLs showed specific lysis of the relevant target cells, including leukemic cells. Because the polyclonal CTL cultures also contained natural killer cells and allo-HLA-A2-specific CTLs, CTL clones were generated that showed the expected HA-1 specificity only. Thus, HA-1-specific CTLs restricted by nonself HLA-A2 molecules can be generated in an HLA-A2-mismatched setting.