Adoptive immunotherapy with donor lymphocyte infusion (DLI) after allogeneic stem cell transplantation (alloSCT) may not only mediate Graft-versus-Leukemia (GvL) reactivity, but also induce Graft-versus-Host Disease (GvHD). As HLA-class II molecules are predominantly expressed on hematopoietic cells, CD4+ T cells may selectively mediate GvL reactivity without GvHD. Here, we assessed the capacity of human CD4+ T cells to act as sole mediators of GvL reactivity in a NOD/scid mouse model for human acute lymphoblastic leukemia and chronic myeloid leukemia in lymphoid blast crisis. Highly purified CD4+ DLI eradicated the leukemic cells. The anti-tumor immunity was mediated by a polyclonal CD4+ T cell response comprising cytokine-producing T-helper and cytolytic T-effector cells directed against the mismatched HLA-class II molecules of the patients. Furthermore, primary leukemic cells acquired an antigen-presenting cell (APC) phenotype in vivo after DLI, as well as in vitro after co-culture with leukemia-specific CD4+ T cells. In conclusion, our results show that CD4+ T cells can be strong mediators of anti-tumor immunity, and provide evidence that cross-talk between CD4+ T cells and leukemic cells is the basis for induction of leukemic cells with an APC phenotype. These data emphasize the clinical relevance of CD4+ T cell based immunotherapy as treatment modality for hematological malignancies after alloSCT.