Donor natural killer cells trigger production of ß-2-microglobulin to enhance post-bone marrow transplant immunity.

Allogeneic hematopoietic transplantation is a powerful treatment for hematologic malignancies. Posttransplant immune incompetence exposes patients to disease relapse and infections. We previously demonstrated that donor alloreactive natural killer (NK) cells ablate recipient hematopoietic targets, including leukemia. Here, in murine models, we show that infusion of donor alloreactive NK cells triggers recipient dendritic cells (DCs) to synthesize ß-2-microglobulin (B2M) that elicits the release of c-KIT ligand and interleukin-7 that greatly accelerate posttransplant immune reconstitution. An identical chain of events was reproduced by infusing supernatants of alloreactive NK/DC cocultures. Similarly, human alloreactive NK cells triggered human DCs to synthesize B2M that induced interleukin-7 production by thymic epithelial cells and thereby supported thymocyte cellularity in vitro. Chromatography fractionation of murine and human alloreactive NK/DC coculture supernatants identified a protein with molecular weight and isoelectric point of B2M, and mass spectrometry identified amino acid sequences specific of B2M. Anti-B2M antibody depletion of NK/DC coculture supernatants abrogated their immune-rebuilding effect. B2M knock-out mice were unable to undergo accelerated immune reconstitution, but infusion of (wild-type) NK/DC coculture supernatants restored their ability to undergo accelerated immune reconstitution. Similarly, silencing the B2M gene in human DCs, before coculture with alloreactive NK cells, prevented the increase in thymocyte cellularity in vitro. Finally, human recombinant B2M increased thymocyte cellularity in a thymic epithelial cells/thymocyte culture system. Our studies uncover a novel therapeutic principle for treating posttransplant immune incompetence and suggest that, upon its translation to the clinic, patients may benefit from adoptive transfer of large numbers of cytokine-activated, ex vivo-expanded donor alloreactive NK cells.

Blinatumomab Redirects Donor Lymphocytes against CD19+ Acute Lymphoblastic Leukemia without Relevant Bystander Alloreactivity after Haploidentical Hematopoietic Stem Cell Transplantation.

Blinatumomab alone or with donor leukocyte infusions (DLI) has been used after allogeneic hematopoietic stem cell transplantation (HSCT) as a salvage therapy in relapsing patients with CD19+ hematological malignancies. It was effective in a fraction of them, with low incidence of Graft-versus-Host Disease (GvHD). Immunosuppressive drugs used as GvHD prophylaxis hinder T cell function and reduce the efficacy of the treatment. Because T cell-depleted haploidentical HSCT with donor regulatory and conventional T cells (Treg/Tcon haploidentical HSCT) does not require post-transplant immunosuppression, it is an ideal platform for the concomitant use of blinatumomab and DLI. However, the risk of GvHD is high because the donor is haploidentical. We treated two patients with CD19+ acute lymphoblastic leukemia (ALL) who had relapsed after Treg/Tcon haploidentical HSCT with blinatumomab and DLI. Despite the mismatch for one HLA haplotype, they did not develop GvHD and achieved complete remission with negative minimal residual disease. Consistently, we found that blinatumomab did not enhance T cell alloreactivity in vitro. Eventually, the two patients relapsed again because of their high disease risk. This study suggests that treatment with blinatumomab and DLI can be feasible to treat relapse after haploidentical transplantation, and its pre-emptive use should be considered to improve efficacy.

Rifaximin use favoured micafungin-resistant Candida spp. infections in recipients of allogeneic hematopoietic cell transplantation.

Damage to gut mucosa following conditioning regimens may favour bacterial infections that can trigger graft versus host disease (GvHD) in patients undergoing allogeneic hematopoietic stem cell transplantation (HSCT). Rifaximin, an oral and non-absorbable antibiotic, has been recently proposed as effective prophylaxis to reduce bacterial infections in the gut and consequently acute GvHD in this setting. The present study evaluated safety and outcomes of HSCT patients that were treated with rifaximin prophylaxis at Perugia University Hospital. Rifaximin prophylaxis was introduced as standard of care in HSCT patients in May 2018. We retrieved data from 118 consecutive transplants, and we compared the outcomes of rifaximin-treated patients with historical controls that did not receive antibiotic prophylaxis. While incidences of neutropenic fever, documented bacterial infections, and aGvHD were similar in the two groups, we found an increased frequency of invasive candidiasis and clinically relevant Candida spp. infections in rifaximin-treated patients (5 patients vs 1 patient, 25% [± 0.99%] vs 1% [± 0.01%], p < .0001). Three rifaximin-treated patients experienced life-threating candidemia (2 C. krusei, 1 C. orthopsilosis). Rifaximin was the only factor that increased the risk of Candida spp. infections. Rifaximin could have contributed to microbiome disruption which favoured an outbreak of life-threatening Candida infections. This important complication forced us to halt its use. Larger, prospective studies are needed to assess the impact of rifaximin prophylaxis on incidence of bacterial infections, aGvHD, and survival of HSCT patients.

Clinical-Grade Expanded Regulatory T Cells Are Enriched with Highly Suppressive Cells Producing IL-10, Granzyme B, and IL-35.

In the setting of T cell-depleted, full-haplotype mismatched transplantation, adoptive immunotherapy with regulatory T cells (Tregs) and conventional T cells (Tcons) can prevent graft-versus-host disease (GVHD) and improve post-transplantation immunologic reconstitution and is associated with a powerful graft-versus-leukemia effect. To improve the purity and the quantity of the infused Tregs, good manufacturing practices (GMP)-compatible expansion protocols are needed. Here we expanded Tregs using an automated, clinical-grade protocol. Cells were extensively characterized in vitro, and their efficiency was tested in vivo in a mouse model. Tregs were selected by CliniMacs (CD4+CD25+, 94.5 ± 6.3%; FoxP3+, 63.7 ± 11.5%; CD127+, 20 ± 3%; suppressive activity, 60 ± 7%), and an aliquot of 100 × 106 was expanded for 14 days using the CliniMACS Prodigy System, obtaining 684 ± 279 × 106 cells (CD4+CD25+, 99.6 ± 0.2%; FoxP3+, 82 ± 8%; CD127+, 1.1 ± 0.8%; suppressive activity, 75 ± 12%). CD39 and CTLA4 expression levels increased from 22.4 ± 12% to 58.1 ± 13.3% (P < .05) and from 20.4 ± 6.7% to 85.4 ± 9.8% (P < .01), respectively. TIM3 levels increased from .4 ± .05% to 29 ± 16% (P < .05). Memory Tregs were the prevalent population, whereas naive Tregs almost disappeared at the end of the culture. mRNA analysis displayed significant increases in CD39, IL-10, granzyme B, and IL-35 levels at the end of culture period (P < .05). Conversely, IFNγ expression decreased significantly by day +14. Expanded Tregs were sorted according to TIM3, CD39, and CD62L expression levels (purity >95%). When sorted populations were analyzed, TIM3+ cells showed significant increases in IL-10 and granzyme B (P < .01) .When expanded Tregs were infused in an NSG murine model, mice that received Tcons only died of GVHD, whereas mice that received both Tcons and Tregs survived without GVHD. GMP grade expanded cells that display phenotypic and functional Treg characteristics can be obtained using a fully automated system. Treg suppression is mediated by multiple overlapping mechanisms (eg, CTLA-4, CD39, IL-10, IL-35, TGF-ß, granzyme B). TIM3+ cells emerge as a potentially highly suppressive population. © 2020 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc.

TNF-α priming enhances CD4+FoxP3+ regulatory T-cell suppressive function in murine GVHD prevention and treatment.

CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) have been shown to effectively prevent graft-versus-host disease (GVHD) when adoptively transferred in murine models of hematopoietic cell transplantation and in phase 1/2 clinical trials. Critical limitations to Treg clinical application are the paucity of cells and limited knowledge of the mechanisms of in vivo function. We hypothesized that inflammatory conditions in GVHD modify Treg characteristics and activity. We found that peripheral blood of recipient animals during acute GVHD (aGVHD) induces Treg activation and enhances their function. The serum contains high levels of tumor necrosis factor-α (TNF-α) that selectively activates Tregs without impacting CD4(+)FoxP3(-) T cells. TNF-α priming induces Treg in vivo proliferation, whereas it limits the ability of CD4 and CD8 conventional T cells (Tcons) to proliferate and induce GVHD. TNF-α-primed Tregs prolong animal survival as compared with unprimed Tregs when used at an unfavorable Treg:Tcon ratio, demonstrating enhanced in vivo efficacy of TNF-α-primed Tregs. Because TNF-α is produced by several immune cells during inflammation, our work elucidates aspects of the physiologic mechanisms of Treg function. Furthermore, TNF-α priming of Tregs provides a new tool to optimize Treg cellular therapies for GVHD prevention and treatment.

DR3 signaling modulates the function of Foxp3+ regulatory T cells and the severity of acute graft-versus-host disease.

CD4+Foxp3+ regulatory T cells (Treg) are a subpopulation of T cells, which regulate the immune system and enhance immune tolerance after transplantation. Donor-derived Treg prevent the development of lethal acute graft-versus-host disease (GVHD) in murine models of allogeneic hematopoietic stem cell transplantation. We recently demonstrated that a single treatment of the agonistic antibody to DR3 (death receptor 3, αDR3) to donor mice resulted in the expansion of donor-derived Treg and prevented acute GVHD, although the precise role of DR3 signaling in GVHD has not been elucidated. In this study, we comprehensively analyzed the immunophenotype of Treg after DR3 signal activation, demonstrating that DR3-activated Treg (DR3-Treg) had an activated/mature phenotype. Furthermore, the CD25+Foxp3+ subpopulation in DR3-Treg showed stronger suppressive effects in vivo. Prophylactic treatment of αDR3 to recipient mice expanded recipient-derived Treg and reduced the severity of GVHD, whereas DR3 activation in mice with ongoing GVHD further promoted donor T-cell activation/proliferation. These data suggest that the function of DR3 signaling was highly dependent on the activation status of the T cells. In conclusion, our data demonstrated that DR3 signaling affects the function of Treg and T-cell activation after alloantigen exposure in a time-dependent manner. These observations provide important information for future clinical testing using human DR3 signal modulation and highlight the critical effect of the state of T-cell activation on clinical outcomes after activation of DR3.

Clinical-Grade-Expanded Regulatory T Cells Prevent Graft-versus-Host Disease While Allowing a Powerful T Cell-Dependent Graft-versus-Leukemia Effect in Murine Models.

We developed a good manufacturing practices-compatible expansion protocol to improve number and purity of regulatory T cells (Tregs) available for clinical trials. Six clinical-grade separation procedures were performed, followed by expansion with high-dose interleukin (IL)-2, anti-CD3/anti-CD28 TCR stimulation, and rapamycin for 19 days achieving a median of 8.5-fold (range, 6.25 to 13.7) expansion. FOXP3 expression was stably maintained over the culture period, while the percentage of CD127 was significantly reduced. The in vitro suppression assay showed a strong Mixed Lymphocytes Reaction inhibition. In vitro amplification did not induce any karyotypic modification. To evaluate the graft-versus-host disease (GVHD)/graft-versus-leukemia (GVL) bifunctional axis, expanded Tregs and conventional T cells (Tcons) were tested in NOD/SCID/IL2Rgnull mice injected with primary acute myeloid leukemia (AML) cells, AML cell line, acute lymphoid leukemia Philadelphia cell line, or Burkitt-like lymphoma cell line. All mice that received leukemia cells together with expanded Tregs and Tcons were rescued from leukemia and survived without GVHD, showing that Treg expansion procedure did not compromise GVHD control and the strong Tcon-mediated GVL activity. This report might represent the basis for treating high-risk leukemia and/or relapsed/refractory leukemia patients with high-dose Treg/Tcons.

Third-party CD4+ invariant natural killer T cells protect from murine GVHD lethality.

Graft-versus-host disease (GVHD) is driven by extensive activation and proliferation of alloreactive donor T cells causing significant morbidity and mortality following allogeneic hematopoietic cell transplantation (HCT). Invariant natural killer T (iNKT) cells are a potent immunoregulatory T-cell subset in both humans and mice. Here, we explored the role of adoptively transferred third-party CD4(+) iNKT cells for protection from lethal GVHD in a murine model of allogeneic HCT across major histocompatibility barriers. We found that low numbers of CD4(+) iNKT cells from third-party mice resulted in a significant survival benefit with retained graft-versus-tumor effects. In vivo expansion of alloreactive T cells was diminished while displaying a T helper cell 2-biased phenotype. Notably, CD4(+) iNKT cells from third-party mice were as protective as CD4(+) iNKT cells from donor mice although third-party CD4(+) iNKT cells were rejected early after allogeneic HCT. Adoptive transfer of third-party CD4(+) iNKT cells resulted in a robust expansion of donor CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) that were required for protection from lethal GVHD. However, in vivo depletion of myeloid-derived suppressor cells abrogated both Treg expansion and protection from lethal GVHD. Despite the fact that iNKT cells are a rare cell population, the almost unlimited third-party availability and feasibility of in vitro expansion provide the basis for clinical translation.

Treatment with agonistic DR3 antibody results in expansion of donor Tregs and reduced graft-versus-host disease.

The paucity of regulatory T cells (Tregs) limits clinical translation to control aberrant immune reactions including graft-versus-host disease (GVHD). Recent studies showed that the agonistic antibody to DR3 (αDR3) expanded CD4(+)FoxP3(+) Tregs in vivo. We investigated whether treating donor mice with a single dose of αDR3 could alleviate acute GVHD in a MHC-mismatched bone marrow transplantation model. αDR3 induced selective proliferation of functional Tregs. CD4(+) T cells isolated from αDR3-treated mice contained higher numbers of Tregs and were less proliferative to allogeneic stimuli. In vivo GVHD studies confirmed that Tregs from αDR3-treated donors expanded robustly and higher frequencies of Tregs within donor CD4(+) T cells were maintained, resulting in improved survival. Conventional T cells derived from αDR3-treated donors showed reduced activation and proliferation. Serum levels of proinflammatory cytokines (IFNγ, IL-1ß, and TNFα) and infiltration of donor T cells into GVHD target tissues (gastrointestinal tract and liver) were decreased. T cells from αDR3-treated donors retained graft-vs-tumor (GVT) effects. In conclusion, a single dose of αDR3 alleviates acute GVHD while preserving GVT effects by selectively expanding and maintaining donor Tregs. This novel strategy will facilitate the clinical application of Treg-based therapies.

Haploidentical hematopoietic transplantation from KIR ligand-mismatched donors with activating KIRs reduces nonrelapse mortality.

Because activating killer cell immunoglobulinlike receptors (KIRs) are heterogeneously expressed in the population, we investigated the role of donor activating KIRs in haploidentical hematopoietic transplants for acute leukemia. Transplants were grouped according to presence vs absence of KIR-ligand mismatches in the graft-vs-host direction (ie, of donor-vs-recipient natural killer [NK]-cell alloreactivity). In the absence of donor-vs-recipient NK-cell alloreactivity, donor activating KIRs had no effects on outcomes. In the 69 transplant pairs with donor-vs-recipient NK-cell alloreactivity, transplantation from donors with KIR2DS1 and/or KIR3DS1 was associated with reduced risk of nonrelapse mortality, largely infection related (KIR2DS1 present vs absent: hazard ratio [HR], 0.25; P = .01; KIR3DS1 present vs absent: HR, 0.18; P = .006), and better event-free survival (KIR2DS1 present vs absent: HR, 0.31; P = .011; KIR3DS1 present vs absent: HR, 0.30; P = .008). Transplantation from donors with KIR2DS1 and/or KIR3DS1 was also associated with a 50% reduction in infection rate (P = .003). In vitro analyses showed that KIR2DS1 binding to its HLA-C2 ligand upregulated inflammatory cytokine production by alloreactive NK cells in response to infectious challenges. Because ∼40% of donors able to exert donor-vs-recipient NK-cell alloreactivity carry KIR2DS1 and/or KIR3DS1, searching for them may become a feasible, additional criterion in donor selection.

Donor Requirements for Regulatory T Cell Suppression of Murine Graft-versus-Host Disease.

Adoptive transfer of freshly isolated natural occurring CD4(+)CD25(+)Foxp3(+) regulatory T cells (Treg) prevents graft-versus-host disease (GVHD) in several animal models and following hematopoietic cell transplantation (HCT) in clinical trials. Donor-derived Treg have been mainly used, as they share the same MHC with CD4(+) and CD8(+) conventional T cells (Tcon) that are primarily responsible for GVHD. Third party-derived Treg are a promising alternative for cellular therapy, as they can be prepared in advance, screened for pathogens and activity, and banked. We explored MHC disparities between Treg and Tcon in HCT to evaluate the impact of different Treg populations in GVHD prevention and survival. Third-party Treg and donor Treg are equally suppressive in ex vivo assays, whereas both donor and third-party but not host Treg protect from GVHD in allogeneic HCT, with donor Treg being the most effective. In an MHC minor mismatched transplantation model (C57BL/6 → BALB/b), donor and third-party Treg were equally effective in controlling GVHD. Furthermore, using an in vivo Treg depletion mouse model, we found that Treg exert their main suppressive activity in the first 2 d after transplantation. Third-party Treg survive for a shorter period of time after adoptive transfer, but despite the shorter survival, they control Tcon proliferation in the early phases of HCT. These studies provide relevant insights on the mechanisms of Treg-mediated protection from GVHD and support for the use of third-party Treg in clinical trials.

CD4+ invariant natural killer T cells protect from murine GVHD lethality through expansion of donor CD4+CD25+FoxP3+ regulatory T cells.

Dysregulated donor T cells lead to destruction of host tissues resulting in graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation (HCT). We investigated the impact of highly purified (>95%) donor CD4(+) invariant natural killer T (iNKT) cells on GVHD in a murine model of allogeneic HCT. We found that low doses of adoptively transferred donor CD4(+) iNKT cells protect from GVHD morbidity and mortality through an expansion of donor CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs). These Tregs express high levels of the Ikaros transcription factor Helios and expand from the Treg pool of the donor graft. Furthermore, CD4(+) iNKT cells preserve T-cell-mediated graft-versus-tumor effects. Our studies reveal new aspects of the cellular interplay between iNKT cells and Tregs in the context of tolerance induction after allogeneic HCT and set the stage for clinical translation.

"Designed" grafts for HLA-haploidentical stem cell transplantation.

Today human leukocyte antigen-haploidentical transplantation is a feasible option for patients with high-risk acute leukemia who do not have matched donors. Whether it is T-cell replete or T-cell depleted, it is still, however, associated with issues of transplant-related mortality and posttransplant leukemia relapse. After reports that adoptive immunotherapy with T-regulatory cells controls the alloreactivity of conventional T lymphocytes in animal models, tomorrow's world of haploidentical transplantation will focus on new "designed" grafts. They will contain an appropriate ratio of conventional T lymphocytes and T-regulatory cells, natural killer cells, γ δ T cells, and other accessory cells. Preliminary results of ongoing clinical trials show the approach is feasible. It is associated with better immune reconstitution and a quite powerful graft-versus-leukemia effect with a low incidence of graft-versus-host disease and no need for posttransplant pharmacological prophylaxis. Future strategies will focus on enhancing the clinical benefit of T-regulatory cells by increasing their number and strengthening their function.

HLA-haploidentical transplantation with regulatory and conventional T-cell adoptive immunotherapy prevents acute leukemia relapse.

Posttransplant relapse is still the major cause of treatment failure in high-risk acute leukemia. Attempts to manipulate alloreactive T cells to spare normal cells while killing leukemic cells have been unsuccessful. In HLA-haploidentical transplantation, we reported that donor-derived T regulatory cells (Tregs), coinfused with conventional T cells (Tcons), protected recipients against graft-versus-host disease (GVHD). The present phase 2 study investigated whether Treg-Tcon adoptive immunotherapy prevents posttransplant leukemia relapse. Forty-three adults with high-risk acute leukemia (acute myeloid leukemia 33; acute lymphoblastic leukemia 10) were conditioned with a total body irradiation-based regimen. Grafts included CD34(+) cells (mean 9.7 × 10(6)/kg), Tregs (mean 2.5 × 10(6)/kg), and Tcons (mean 1.1 × 10(6)/kg). No posttransplant immunosuppression was given. Ninety-five percent of patients achieved full-donor type engraftment and 15% developed ≥grade 2 acute GVHD. The probability of disease-free survival was 0.56 at a median follow-up of 46 months. The very low cumulative incidence of relapse (0.05) was significantly better than in historical controls. These results demonstrate the immunosuppressive potential of Tregs can be used to suppress GVHD without loss of the benefits of graft-versus-leukemia (GVL) activity. Humanized murine models provided insights into the mechanisms underlying separation of GVL from GVHD, suggesting the GVL effect is due to largely unopposed Tcon alloantigen recognition in bone marrow.

Regulatory T cells and natural killer T cells for modulation of GVHD following allogeneic hematopoietic cell transplantation.

Alloreactivity of donor lymphocytes leads to graft-versus-host disease (GVHD) contributing to significant morbidity and mortality following allogeneic hematopoietic cell transplantation (HCT). Within the past decade, significant progress has been made in elucidating the mechanisms underlying the immunologic dysregulation characteristic of GVHD. The recent discoveries of different cell subpopulations with immune regulatory function has led to a number of studies aimed at understanding their role in allogeneic HCT and possible application for the prevention and treatment of GVHD and a host of other immune-mediated diseases. Preclinical animal modeling has helped define the potential roles of distinct populations of regulatory cells that have progressed to clinical translation with promising early results.

Mast cells suppress murine GVHD in a mechanism independent of CD4+CD25+ regulatory T cells.

To investigate the role of mast cells in hematopoietic cell transplantation, we assessed graft-versus-host disease (GVHD) in C57BL/6-Kit(W-sh/W-sh) recipients, which virtually lack mast cells, compared with C57BL/6 WT recipients. GVHD was severely exacerbated in C57BL/6-Kit(W-sh/W-sh) mice (median survival time = 13 vs 60 days in wild-type [WT] mice; P < .0001). The increased mortality risk in C57BL/6-Kit(W-sh/W-sh) hosts correlated with increased T-cell numbers in lymph nodes, liver, and gastrointestinal tract sites, as indicated by bioluminescence imaging (P < .001). We did not detect any deficit in the number or function of CD4(+)CD25(+) regulatory T cells (Tregs) in C57BL/6-Kit(W-sh/W-sh) mice. Furthermore, Tregs were equally effective at reducing GVHD in C57BL/6-Kit(W-sh/W-sh) recipients compared with WT recipients containing mast cells. Furthermore, we found that survival of C57BL/6-Kit(W-sh/W-sh) mice during GVHD was significantly improved if the mice were engrafted with bone marrow-derived cultured mast cells from WT C57BL/6 mice but not from interleukin (IL)-10-deficient C57BL/6 mice. These data indicate that the presence of mast cells can significantly reduce GVHD independently of Tregs, by decreasing conventional T-cell proliferation in a mechanism involving IL-10. These experiments support the conclusion that mast cells can mediate a novel immunoregulatory role during hematopoietic cell transplantation.

TLR3 essentially promotes protective class I-restricted memory CD8⁺ T-cell responses to Aspergillus fumigatus in hematopoietic transplanted patients.

Aspergillus fumigatus is a model fungal pathogen and a common cause of severe infections and diseases. CD8⁺ T cells are present in the human and murine T-cell repertoire to the fungus. However, CD8⁺ T-cell function in infection and the molecular mechanisms that control their priming and differentiation into effector and memory cells in vivo remain elusive. In the present study, we report that both CD4⁺ and CD8⁺ T cells mediate protective memory responses to the fungus contingent on the nature of the fungal vaccine. Mechanistically, class I MHC-restricted, CD8⁺ memory T cells were activated through TLR3 sensing of fungal RNA by cross-presenting dendritic cells. Genetic deficiency of TLR3 was associated with susceptibility to aspergillosis and concomitant failure to activate memory-protective CD8⁺ T cells both in mice and in patients receiving stem-cell transplantations. Therefore, TLR3 essentially promotes antifungal memory CD8⁺ T-cell responses and its deficiency is a novel susceptibility factor for aspergillosis in high-risk patients.

How adoptive transfer of components of the donor immune system boosts GvL and prevents GvHD in HLA-haploidentical hematopoietic transplantation for acute leukemia.

Why a new Perspective in allogeneic hematopoietic transplantation? A summary. Nowadays, for high-risk acute leukemia patients without an HLA-matched donor (sibling or volunteer), hematopoietic transplants that use HLA-haploidentical grafts combined with enhanced post transplant immune suppression (i.e., high-dose cyclophosphamide) are widely used. They are associated with low TRM rates. However, they are also associated with significant chronic GvHD while they only partially abrogate leukemia relapse rates. One may speculate that post-transplant immune suppression, required for GvHD prophylaxis, weakens the anti-leukemic potential of the graft. Historically, haploidentical transplants became feasible for the first time through transplantation of T cell-depleted peripheral blood hematopoietic progenitor cells. Lack of post-transplant immune suppression allowed the emergence of donor-versus-recipient NK-cell alloreactions that eradicated AML. In an attempt to improve these results we recently combined an age-adapted, irradiation-based conditioning regimen with transplant of T-cell-depleted grafts and infusion of regulatory and conventional T cells, without any post transplant immune suppression. With the obvious limitations of a single center experience, this protocol resulted in extremely low relapse and chronic GvHD rates and, consequently, in a remarkable 75% chronic GvHD/relapse-free survival in over 50 AML patients up to the age of 65 many of whom at high risk of relapse.