CD4 and leukemia: In humans, it has been shown that CD4+ T cells can mediate graft-versus-leukemia (GvL) reactivity after allogeneic stem cell transplantation (aSCT) without induction of graft-versus-host-disease (GvHD).2–4 However, it has also been demonstrated that concomitant viral infection leading to upregulation of human leukocyte antigen (HLA) class II on non-hematopoietic cells induce severe GvHD after CD4+ donor lymphocyte infusion (DLI).5