T regulatory cells (Tregs) play a key role in modulating T cell responses. Clinical trials showed that Tregs modulate graft-versus-host disease (GvHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, their ability to mediate anti-leukemic activity (graft-versus-leukemia [GvL]) is largely unknown. Enforced interleukin-10 (IL-10) expression converts human CD4 + T cells into T regulatory type 1 (Tr1)-like (CD4 IL-10 ) cells that suppress effector T cells in vitro and xenoGvHD in humanized mouse models. In the present study, we show that CD4 IL-10 cells mediate anti-leukemic effects in vitro and in vivo in a human leukocyte antigen (HLA) class I-dependent but antigen-independent manner. The cytotoxicity mediated by CD4 IL-10 cells is granzyme B (GzB) dependent, is specific for CD13 + target cells, and requires CD54 and CD112 expression on primary leukemic target blasts. CD4 IL-10 cells adoptively transferred in humanized mouse models directly mediate anti-tumor and anti-leukemic effects. In addition, when co-transferred with peripheral blood mononuclear cells (PBMCs), CD4 IL-10 cells contribute to the GvL activity but suppress xenoGvHD mediated by the PBMCs. These findings provide for the first time a strong rationale for CD4 IL-10 cell immunotherapy to prevent GvHD and promote GvL in allo-HSCT for myeloid malignancies. Tr1 cells are generated by overexpressing IL-10 in CD4 + T cells (CD4 IL-10 ). In this issue of Molecular Therapy, Locafaro et al. (2017) show that CD4 IL-10 cells kill myeloid leukemia in an HLA class I-dependent mechanism, mediate anti-tumor and anti-leukemic effects, and contribute to GvL while preventing GvHD in humanized mice. © 2017.