Modulating an immune response in opposite directions represents the holy grail in allogeneic hematopoietic stem cell transplantation (allo-HSCT) to avoid insufficient reactivity of donor T cells and hematologic malignancy relapse while controlling the potential development of graft-versus-host disease (GVHD), in which donor T cells attack the recipient's tissues. IL-2/anti-IL-2 complexes (IL-2Cx) represent a therapeutic option to selectively accentuate or dampen the immune response. In dedicated experimental models of allo-HSCT, including also human cells injected in immunodeficient NSG mice, we evaluated side-by-side the therapeutic effect of two IL-2Cx designed either to boost regulatory T cells (Treg) or alternatively to activate effector T cells (Teff), on GVHD occurrence and tumor relapse. We also evaluated the effect of the complexes on the phenotype and function of immune cells in vivo. Unexpectedly, both pro-Treg and pro-Teff IL-2Cx prevented GVHD development. They both induced Treg expansion and reduced CD8+ T-cell numbers, compared to untreated mice. However, only mice treated with the pro-Treg IL-2Cx, showed a dramatic reduction of exhausted CD8+ T cells, consistent with a potent anti-tumor effect. When evaluated on human cells, pro-Treg IL-2Cx also preferentially induced Treg expansion in vitro and in vivo, while allowing the development of a potent anti-tumor effect in NSG mice. Our results demonstrate the clinical relevance of using a pro-Treg, but not a pro-Teff IL2Cx to modulate alloreactivity after HSCT, while promoting a graft-versus-leukemia effect.