Refractory or relapsed acute myeloid leukemia (AML) represents a frequent complication after allogeneic hematopoietic stem-cell transplantation (HSCT). We show herein that primary in vitro stimulation of CD45RA-selected CD4 T cells of stem-cell donors with 10/10 HLA-matched AML blasts results in expansion of cytolytic T-lymphocytes (CTL) that almost all recognize HLA-DPB1 mismatch alleles, which clinically occur in up to 80% of donor-patient pairs. Primary AML blasts were found to strongly express HLA-DPB1, whereas fibroblasts and keratinocytes used as surrogate target cells for graft-versus-host disease did express HLA-DPB1 only upon IFN-γ pre-treatment. Since patients' AML blasts are rarely available in clinical routine, we developed a protocol based on stimulation of donor-derived CD45RA-selected CD4 T cells with autologous dendritic cells electroporated with RNA encoding patients' HLA-DPB1 mismatch alleles. Short-term stimulated T cell-lines specifically lysed HLA-DPB1 mismatch-expressing AML blasts, but not fibroblasts and keratinocytes without IFN-γ pre-treatment. Notably, these CD4 CTL efficiently eliminated AML blasts upon adoptive transfer into leukemia-engrafted NSG mice. In conclusion, we show strong immunogenicity of HLA-DPB1 mismatch alleles in CD45RA-selected CD4 T cells of stem-cell donors and introduce a novel strategy to reliably generate HLA-DPB1-specific CD4 CTL that might be powerful cellular therapeutics in relapsed or refractory AML after HSCT.