T cells that accompany allogeneic hematopoietic grafts for treating leukemia enhance engraftment and mediate the graft-versus-leukemia effect. Unfortunately, alloreactive T cells also cause graft-versus-host disease (GVHD). T cell depletion prevents GVHD but increases the risk of graft rejection and leukemic relapse. In human transplants, we show that donor-versus-recipient natural killer (NK)-cell alloreactivity could eliminate leukemia relapse and graft rejection and protect patients against GVHD. In mice, the pretransplant infusion of alloreactive NK cells obviated the need for high-intensity conditioning and reduced GVHD. NK cell alloreactivity may thus provide a powerful tool for enhancing the efficacy and safety of allogeneic hematopoietic transplantation.

The main limitations of transplantation of bone marrow from donors who are matched with the recipient for only one HLA haplotype GVHD and graft failure - can be overcome. Since most patients have a relative with one haplotype mismatch, advances in this method will increase the availability of hematopoietic-cell transplantation as curative therapy for acute leukemia.

IntroductionA viable option for high-risk, acute leukemia patients without matched donors is hematopoietic stem cell transplantation (HSCT) from human leukocyte antigen (HLA)-haploidentical 3-locimismatched family members who were readily available for almost all patients. 1,2 Until the 1990s, full-haplotype-mismatched, T cell-replete transplantations were unsuccessful because donoralloreactive T cells triggered a high incidence of severe graft-versushost disease (GVHD) despite posttransplantation immune suppression. 3,4 The breakthrough came with the use of a megadoses of extensively ex vivo T cell-depleted peripheral blood hematopoietic progenitor cells and a highly myeloablative conditioning regimen containing antithymocyte globulin (ATG), which exerts additional T-cell depletion in vivo. This approach ensures a high rate of primary engraftment in the absence of GVHD, 5 with more than 40% long-term event-free survival and excellent quality of life. 1,2 However, extensive ex vivo and in vivo T-cell depletion delays the recovery of immune responses against pathogens, leading to a high incidence of life-threatening infections. 1,2 Strategies to hasten posttransplantation immune reconstitution without triggering GVHD have included infusion of donor T cells after engineering with a suicide gene, 6 photodynamic purging, 7 and the use of an anti-CD25 monoclonal antibody (mAb) 8 to remove alloreactive cells. An alternative strategy might be based on donor CD4 ϩ CD25 ϩ T-regulatory cells (Tregs). In murine models of HSCT across major histocompatibility complex barriers, CD4 ϩ CD25 ϩ Tregs suppressed lethal GVHD 9 and favored posttransplantation immune reconstitution when coinfused with conventional T cells (Tcons). 10 The main obstacle to clinical application of human Tregs is their paucity in the peripheral blood. Although ex vivo-expanded polyclonal 11 or recipient-specific Tregs 12 were proposed to circumvent this potential barrier, we opted for closed, automated immunoselection 13 of naturally occurring Tregs. In the present study, for the first time in humans, we show that the early infusion of freshly isolated donor Tregs followed by Tcons at the time of full-haplotypemismatched HSCT prevented GVHD while favoring Tconmediated posttransplantation immune reconstitution. MethodsStudy design, conditioning regimen, stem cell mobilization, and supportive careIn 2008, the Umbria Regional Hospital Ethics Committee (CEAS Umbria) approved the protocol entitled "Adoptive Immunotherapy with Natural Regulatory T cells (Treg) and Effector T Cells in Allogeneic Hematopoietic Stem Cell Transplantation from 2-3 Loci Mismatched HLA-Haploidentical Family Donors for Patients with High Risk Haematologic Malignancies" (Protocol No 01/08). Written informed consent was obtained for all patients and donors in accordance with the Declaration of Helsinki. Inclusion criteria were: acute myeloid leukemia (AML) or acute lymphoid leukemia (ALL) in remission at high risk of relapse; acute leukemia with primary induction failure, in chemoresist...

Our transplantation procedure provides reliable, reproducible CD34+ cell purification, high engraftment rates, and prevention of GvHD. The mismatched-related transplant emerges as a viable, alternative source of stem cells for acute leukemia patients without matched donors and/or those who urgently need transplantation.

Natural killer (NK) cells are primed to kill by several activating receptors. NK cell killing of autologous cells is prevented because NK cells coexpress inhibitory receptors (killer cell immunoglobulin-like receptors [KIR]) that recognize groups of (self) major histocompatibility complex class I alleles. Because KIRs are clonally distributed, the NK cell population in any individual are constituted of a repertoire with a variety of class I specificities. NK cells in the repertoire mediate alloreactions when the allogeneic targets do not express the class I alleles that block them. After haploidentical hematopoietic transplantation, NK cell-mediated donor-versus-recipient alloresponses reduce the risk of relapse in acute myeloid leukemia patients while improving engraftment and protecting against graft-versus-host disease. High-resolution molecular HLA typing of recipient and donor, positive identification of donor KIR genes, and, in some cases, functional assessment of donor NK clones identify haploidentical donors who are able to mount donor-versus-recipient NK alloreactions.