HLA-Haploidentical Stem Cell Transplantation for Hematologic Malignancies Academic research paper on "Clinical medicine"

ASBMI

American Society for Blood and Marrow Transplantation

HLA-Haploidentical Stem Cell Transplantation for Hematologic Malignancies

Ephraim J. Fuchs, Xiao-jun Huang, Jeffrey S. Miller

Partially HLA-mismatched related, or HLA-haploidentical, donor stem cell transplantation (SCT) is a feasible therapeutic option for advanced hematologic malignancies patients who lack an HLA-matched related or unrelated donor. Advances in conditioning regimens, graft manipulation, and pharmacologic prophylaxis of graft-versus-host disease (GVHD) have reduced the risk of fatal graft failure and severe GVHD, two of the most serious complications of traversing the HLA barrier. Clinical observations reveal a potential role for natural killer (NK) cell alloreactivity in reducing the risk of relapse of acute myeloid leukemia after HLA-haploidentical SCT. NK cell infusions attempt to harness the graft-versus-leukemia effect without producing GVHD. The availability of multiple potential HLA-haploidentical related donors for most patients opens the possibility of optimizing transplantation outcome through intelligent donor selection. Biol Blood Marrow Transplant 16: S57-S63 (2010) © 2010 American Society for Blood and Marrow Transplantation

KEY WORDS: Stem cell transplantation, Adoptive immunotherapy, Natural killer cells, Graft-versus-host disease, Human leukocyte antigens, Transplantation conditioning

INTRODUCTION

Donor availability is one of the major obstacles to the success of allogeneic hematopoietic stem cell transplantation (HSCT) for the treatment of hematologic malignancies or nonmalignant hematologic disorders. Because of historically superior outcomes of human leukocyte antigen (HLA)-matched compared to partially HLA-mismatched HSCT [1,2], an HLA-matched sibling or unrelated donor (URD) is the preferred source of stem cells for transplantation. However, an HLA-matched donor can be identified for only 50% to 60% of patients referred for HSCT, lower still for patients in ethnic minorities. The ability to cross the HLA boundary safely would increase the

From the 1Division of Hematologic Malignancies, Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, Baltimore, Maryland; Institute of Hematology, People's Hospital, Peking University, Beijing, People's Republic of China; and ^Division of Hematology, Oncology and Transplantation, University of Minnesota Cancer Center, Minneapolis, Minnesota. Financial disclosure: See Acknowledgments on page 62. Correspondence and reprint requests: Ephraim J. Fuchs, M.D., M.B.A., Division of Hematologic Malignancies, Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins, 488 Cancer Research Building, 1650 Orleans Street, Baltimore, MD 21231 (e-mail: fuchsep@hmi.edu). Received September 15, 2009; accepted October 29, 2009 © 2010 American Society for Blood and Marrow Transplantation 1083-8791/10/161S-00011$36.00/0 doi:10.1016/j.bbmt.2009.10.032

availability of a stem cell donor to nearly 100% of patients referred for allogeneic HSCT.

There are two potential sources of grafts for patients lacking HLA-matched donors: (1) unrelated umbilical cord blood (UCB), and (2) partially HLA-mismatched, or HLA-haploidentical, related donors. Results of UCB transplantation in children are encouraging [3], and transplantation of 2 UCB units generates cell doses that are sufficient for engraft-ment in adults [4,5]. The initial studies of HLA-hap-loidentical HSCT employed lethal conditioning, infusion of T cell-replete marrow grafts, and graft-versus-host disease (GVHD) prophylaxis with methotrexate (MTX), with or without cyclosporine (CsA) [6]. These transplants were complicated by excessive bidirectional alloreactivity resulting in high rates of graft failure [7], severe GVHD, and nonrelapse mortality (NRM) [8]. Consequently, event-free survival (EFS) was poor, especially when donors and recipients were mismatched for 2 or more HLA antigens [1,8]. Results of HLA-haploidentical HSCT have improved significantly over the past decade owing to the development of highly immunosuppressive yet non-myeloablative (NMA) conditioning, novel graft manipulation, and improved prophylaxis of GVHD. Further, HLA-haploidentical HSCT harnesses the potential of natural killer (NK) cell alloreactivity to kill tumor cells and reduce the risk of posttransplantation relapse. These recent developments are the subject of this review.

HLA-Haploidentical HSCTafter NMA Conditioning

Graft failure is a major complication of HLA-haploidentical HSCT [6,9] and is usually a fatal event after myeloablative (MA) conditioning. Truly NMA conditioning offers the safeguard of reconstitution of autologous hematopoiesis in the event of graft failure. Most NMA conditioning regimens incorporate the highly immunosuppressive drug fludabarine (Flu). Studies from Tuebingen, Germany, and from Duke University in the United States have combined Flu-based conditioning with in vivo T cell depletion (TCD) using OKT3 [10] or CAMPATH [11], respectively, to enable the engraftment of HLA-haploidenti-cal stem cells. These regimens were associated with acceptable nonhematologic toxicities and sustained en-graftment of donor cells in patients up to the age of 66 years. Overall survival (OS) at 1 year after transplantation ranged from 31% to 37% [11,12], establishing the feasibility of HLA-haploidentical HSCT after NMA conditioning.

The groups at Johns Hopkins in Baltimore and the Fred Hutchinson Cancer Research Center in Seattle have been pioneering the use of high-dose, posttransplantation cyclophosphamide (Cy) to achieve the selective depletion of alloreactive cells after NMA conditioning and HLA-haploidentical HSCT. In an early report, 68 patients with poor-risk hematologic malignancies were conditioned with Flu, Cy, and 2 Gy total body irradiation (TBI) prior to receiving T cell-replete bone marrow (BM) from HLA-haploidentical, first-degree relatives (Figure 1) [13]. Donors and recipients were mismatched at a median of 4 HLA alleles. GVHD prophylaxis comprised Cy 50 mg/kg i.v. on day 3 (n = 28) or on days 3 and 4 (n = 40) after transplantation, followed by tacrolimus and mycophe-nolate mofetil (MMF), each beginning on day 5. Graft failure occurred in 9 patients (13%) but was fatal in only 1. Grades II-IVand III-IV acute GVHD (aGVHD) occurred in 34% and 6% of patients, respectively, and chronic GVHD (cGVHD) developed in 15% of patients. The cumulative incidences of relapse and NRM at 1 year after transplantation were 15% and 51%, respectively, and OS and EFS at 2 years after transplantation were 36% and 26%. Only 6 patients died of infection (n = 4) or GVHD (n = 2). In this early report, patients with lymphoid diseases had a superior EFS compared to patients receiving HSCT for mye-logenous diseases (P = .02).

A subsequent report retrospectively compared the outcomes of Hodgkin lymphoma (HL) patients treated with NMA conditioning and grafts from HLA-matched related (n = 38), URD (n = 24), or HLA-haploidentical related (n = 28) donors [14]. Recipients of HLA-haploidentical grafts were conditioned as in Figure 1. Patients had received a median

Cyclophosphamide (Cy) 14.6 mg/kg/day

BMT I I

Bone Marrow Infusion

200 cGy

1 Tacrolimus

10 20 30 40' 90 180

Day -6 -5 -4 -3 -2 -l 0 u

I f t t t I:

Fl ucfarabine 30 mg/m2/day Cy 50 m g/kg/day: * day 3 (n=28) or days 3,4 (n=40)

Figure 1. Treatment schema for nonmyeloablative conditioning and HLA-haploidentical bone marrow transplantation. From ref. 11.

of 5 prior regimens, including autologous HSCT in 92%. With a median follow-up of 25 months, 2-year OS, EFS, and incidences of relapsed/progressive disease were 53%, 23%, and 56% (HLA-matched related), 58%, 29%, and 63% (URD), and 58%, 51%, and 40% (HLA-haploidentical related), respectively. NRM was significantly lower for HLA-haploidentical related recipients compared to HLA-matched related recipients (P = .02). There were also significantly decreased risks of relapse for HLA-haploidentical related recipients compared to HLA-matched related (P = .01) and URD (P = .03) recipients. In a recent report from the Center for International Blood and Marrow Transplant Research (CIBMTR), HL patients receiving reduced intensity, unrelated donor HSCT had a 2-year OS and EFS of 37% and 20%, respectively [15]. HLA-haploidentical HSCT may therefore be uniquely effective for patients with relapsed or refractory HL.

We have recently analyzed, retrospectively, the effect of HLA mismatching on the outcome of 185 he-matologic malignancies patients treated with NMA, HLA-haploidentical SCT and posttransplantation Cy [16]. The cumulative incidences of grade II-IV aGVHD and cGVHD were 31% and 15%, respectively. The cumulative incidences of NRM and relapse or progression at 1 year were 15% and 50%, respectively. Actuarial EFS at 1 year was 35%. Increasing degrees of HLA mismatch at either class I or class II loci had no significant effect on the cumulative incidence of aGVHD or cGVHD or NRM. In contrast, the presence of an HLA-DRB1 antigen mismatch in the GVH direction was associated with a significantly lower cumulative incidence of relapse (Figure 2A; P = .04) and improved EFS (Figure 2C; P = .009), whereas HLA-DQB1 antigen mismatch status had no effect. Additionally, the presence of 2 or more class I allele mismatches (composite of HLA-A, -B, and -Cw) in either direction was associated with a significantly lower cumulative incidence of relapse (Figure 2B; P = .045 for GVH direction, P = .01 for HVG direction) and improved EFS (Figure 2D; P = .07 for GVH direction, P = .001 for HVG direction). Although the analysis was limited by its retrospective nature and the small numbers of pairs with 2 or fewer HLA antigen mismatches (n = 26), the results

Figure 2. Effect of HLA-DRBI antigen mismatch (a,c) or HLA Class I allele mismatches (b,d) in the GVH direction on relapse, NRM (a,b), and EFS (c,d) after nonmyeloablative, HLA-haploidentical bone marrow transplantation with high-dose, posttransplantation cyclophosphamide.

raise the possibility that increasing HLA disparity is associated with improved outcomes after NMA, HLA-haploidentical HSCT with high-dose, posttransplantation Cy.

Previous studies of MA, HLA-haploidentical HSCT have shown that increasing HLA disparity was associated with a reduced incidence of relapse, but an inferior EFS because of increased GVHD and NRM [8,17]. In contrast, our study of NMA, HLA-haploidentical HSCT with posttransplantation Cy showed that increasing HLA disparity was associated with a reduced risk of relapse with no effect on NRM, resulting in an improved EFS. What accounts for the difference? Although conceding the pitfalls of retrospective analyses, we raise the possibility that posttransplantation Cy differentially affects the populations of cells mediating GVHD versus those producing graft-versus-leukemia (GVL) effects. Further clinical and laboratory studies will be required to understand the effects of high-dose, posttransplantation Cy on host tolerance and antitumor immunity.

HLA-Haploidentical HSCT after MA Conditioning

As mentioned previously, the initial trials of HLA-haploidentical BM transplantation (BMT) for leukemia used lethal conditioning and T cell-replete grafts and were complicated by high rates of severe GVHD and NRM, and 5-year EFS among recipients of grafts mismatched for 2 to 3 HLA loci was approximately 10% [8]. Although T reduced the risk of GVHD after HLA-haploidentical HSCT, it increased the risk of graft failure and did not improve leukemia-free

survival (LFS) [18]. Investigators in Perugia, Italy, achieved low rates of graft failure and GVHD by conditioning patients intensively and transplanting them with rigorously T cell-depleted grafts containing "megadoses" of CD341 HSC [19]. EFS (6 standard deviation) rate was 48% 6 8% and 46% 6 10%, respectively, for 42 acute myelogenous leukemia

Haploidentical HCT Ara-C 4 g/m2

Bu 1mg/kg,q6h

Cy1.8g/m2 MeCCNU 250 mg/m2

Conditioning

ATG 2.5mg/kg(r)

-10 -9 -8 -7 -6

-5 -4 -3 -2 -1

01 BMSC

02 PBSC

Figure 3. The myeloablative conditioning regimen for HLA-haploi-dentical HSCT at Peking University. Ara-C: cytosine arabinoside; Bu: busulfan; Cy: cyclophosphamide; MeCCNU: simustine; ATG: antithymo-cyte globulin.

(AML) and 24 acute lymphoblastic leukemia (ALL) patients receiving transplantation in remission [20]. These studies also established a potential role for donor NK cells in mediating GVL effects after TCD, HLA-haploidentical BMT for AML, but not ALL [21].

Peking University researchers developed a novel approach to HLA-mismatched/haploidentical transplantation without in vitro TCD. This approach, shown in Figure 3, was first reported by Huang et al. [22] in a study of 58 patients undergoing HLA-mismatched/haploidentical HSCT. Since this initial report, 831 additional patients have received HLA-haploidentical SCTs at the Peking University Institute of Hematology.

Engraftment

Huang et al. [23] reported 171 patients, including 86 with high-risk disease, receiving grafts from HLA-mismatched/haploidentical family donors. All patients achieved hematopoietic recovery after transplantation. The median time for myeloid engraftment was 12 days (range: 9-26 days) and median time to platelet recovery was 15 days (range: 8-151 days). There was no significant association between the extent of HLA disparity and the time to myeloid or platelet recovery. On multivariate analysis, a low number of CD341 cells (<2.19 x 106/kg) in the graft, and advanced disease stage were independently associated with an increased risk of platelet nonengraftment

[24]. Among children who received HLA-haploidentical grafts, only the dose of infused CD341 cells/kg of recipient weight was significantly associated with an increased risk of platelet engraftment.

Our results suggest that the incidences of grade III-IV aGVHD and extensive cGVHD were acceptable in patients after unmanipulated HLA-mis-matched/haploidentical transplantation, although the T cell dose in grafts was more than 108/kg. At 100 days after transplantation, the cumulative incidence was 55.0% for grade II-IV aGVHD, and 23.1% for grade III-IV aGVHD. The incidence of cGVHD was 44.67%, with 21.3% for limited and 23.3% for extensive, respectively [23]. We further reported 42 children below 14 years of age with hematologic malignancies treated with HLA-haploidentical HSCT

[25]. The cumulative incidence of aGVHD of grade II-IV was 57.2%, and that of grade III-IV was 13.8%. The cumulative incidence of cGVHD was 56.7% for total and 29.5% for extensive. Apparently, the incidence of grade III-IV aGVHD in pediatric patients was lower than that of adult patients. In contrast to previously published data, there was no significant association of HLA disparity with the incidence or severity of aGVHD or cGVHD in this protocol.

These findings may be related to (1) T cell hypo-responsiveness maintained after in vitro mixture of filgrastim (G-CSF)-mobilized peripheral blood (G-PB) and filgrastim-mobilized bone marrow (G-BM) in different proportions [26,27]; (2) the use of antithymo-cyte globulin (ATG) before transplantation, which may induce depletion of infused donor T lymphocytes in vivo and thus lower the incidence of GVHD; (3) a possible effect of the combination of CsAMTX, and MMF as postgrafting immunosuppression; (4) the application of granulocyte-colony stimulating factor (G-CSF) day 5 posttransplant, which may further regulate T cell function; or (5) the immunomodulatory effect of mesenchymal stem cells (MSCs)/mesenchy-mal (stroma) progenitor cells (MPCs) from the G-CSF mobilized BM and peripheral blood stem cell (PBSC) grafts, respectively.

Factors correlating the high incidence of aGVHD are killer cell immunoglobulin-like receptor (KIR) li-gand mismatch and a higher dose of CD56bright NK cells (41.9 x 106/kg) in the allografts, whereas a higher CD56dim/CD56bright NK cell ratio (more than 8.0) in allografts was correlated with a decreased risk of III-IV aGVHD after unmanipulated HLA-mismatched/hap-loidentical transplantation.

Relapse and management

We studied the incidence and management of relapsed malignancy in 250 recipients of HLA-haploidentical transplants at Peking University [28]. The 3-year probabilities of relapse in the standard-risk group were 11.9% for a AML and 24.3% for ALL, and in the high-risk group were 20.2% for AML and 48.5% for ALL. Advanced disease status, a higher CD4/CD8 ratio in G-BM [29], and delayed lymphocyte recovery at day 30 posttransplantation correlated with an increased relapse rate. Conversely, a higher CD56dim/CD56bright NK cell ratio (more than 8.0) was correlated with a decreased rate of relapse after haploidentical transplantation without in vitro TCD.

Modified donor lymphocyte infusions (DLI) was used to treat relapse of patients after unmanipulated HLA-mismatched/haploidentical transplantation [30]. Twenty patients who underwent T cell-depleted, HLA-haploidentical HSCT between April 1, 2002 and May 1, 2005 were included in this study. After DLI, 11 patients received CsA (blood concentration of 150-250 ng/mL for 2-4 weeks) or a low dose of MTX (10 mg once per week for 2-4 weeks) to prevent GVHD, and 9 patients received no GVHD prophylaxis. The incidence of grade III-IV aGVHD was significantly lower in patients with GVHD prophylaxis than those without (9.1% versus 55.6%, P = .013). Fifteen patients achieved complete remission (CR) at a median of 289 (40-1388) days after

DLI. The 1-year and 2-year LFS were 60% and 40%.

Treatment-related mortality and survival

In a recent report, 250 acute leukemia (AL) patients received allografts from related donors [28]. The NRM at day 100 after transplantation in the standard- and high-risk groups was 6.8% and 5.9% for AML and 6.9% and 25.9% for ALL, respectively.

An improved LFS after unmanipulated HLA-haploidentical blood and marrow transplantation correlated closely with early disease status, higher numbers of CD56bright cells reconstituted day 14 post-transplant, lower CD4/CD8 in G-BM, a short time from diagnosis to transplant (#450 days) for chronic myelogenous leukemia (CML) patients and higher absolute lymphocyte counts (ALC; >300/mL) day 30 posttransplant. In a large cohort of AL patients, the 3-year probabilities of LFS for standard-risk and high-risk patients were 70.7% and 55.9%, respectively, for patients with AML, and 59.7% and 24.8%, respectively, for patients with ALL [28]. With respect to CML patients, the probability of 1-year and 4-year LFS was 76.5% and 74.5% for patients in first chronic phase (CP), 85.7% and 85.7% for CP2/ CR2 patients, 80% and 66.7% for patients in accelerated phase (AP), and 53.8% and 53.8% for patients in blast crisis (BC).

HLA-haploidentical HSCT: current status

The most important development in HLA-haploi-dentical HSCT over the past decade has been the dramatic reduction in treatment-related morbidity and treatment-related mortality (TRM). Highly immuno-suppressive conditioning regimens now permit the transplantation of TCD grafts, resulting in reliable donor cell engraftment without severe GVHD. As a result, the mortality associated with HLA-haploidentical HSCT now approaches that of HLA-matched HSCT [31], making partially mismatched related donor transplantation a viable treatment option for patients lacking an HLA-matched donor. Going forward, there is a need to decrease the risk of posttransplant infections by improving immune reconstitution, to harness both T cell and NK cell alloreactivity for improved antitumor effects without GVHD, and to define the relative roles of HLA-haploidentical related donor versus unrelated umbilical cord blood SCT for various hemato-logic malignancies.

Biology of NK Cell Alloreactivity

Development of NK cell self-tolerance

The mechanism by which NK cells acquire self-tolerance and alloreactivity has been referred to as NK cell education or licensing. This is one of the most widely

debated topics in NK cell biology over the past several years. Several models have been proposed to explain the integration of inhibitory receptor expression with the acquisition of effector function. These concepts differ in their implied mechanisms and whether the process is one of activation or loss of function [32,33]. What is agreed upon between these and other models is that human NK cells lacking inhibitory receptors are hyporesponsive [34,35]. Therefore, rather than being autoreactive, they are self-tolerant. Although the exact mechanism remains unknown, self-tolerance may be the result of coordinated developmental pathways whereby mature NK cell function is synchronized with the acquisition of self-inhibitory receptors.

Therapeutic efficacy ofNK cells is primarily controlled by KIR interactions

The 2 main strategies to harness the therapeutic power of alloreactive NK cells are: (1) HSCT [21] and (2) adoptive transfer ofNK cells [36]. This literature is based on studies from the Perugia group who first proposed the KIR-ligand incompatibility model, which predicts that donor-derived NK cells will be al-loreactive when recipients lack C2, C1, or Bw4 alleles that are present in the donor. Many groups, including our own [37-39], have tested the clinical efficacy of selecting donors for NK cell therapy or transplantation based on their predicted alloreactivity against the host using one of several models. The potential benefits include: (1) decreased GVHD as host dendritic cells are killed by donor NK cells, (2) better antitumor activity via direct cytotoxicity, (3) improved engraft-ment mediated by NK cell release of hematopoietic cytokines, and (4) enhanced immune reconstitution. Additional clinical trials have supported the finding that KIR ligand mismatch is associated with favorable clinical outcomes in myelogenous malignancies [40]. However, other studies looking at outcomes after KIR ligand mismatched, T cell-replete transplants did not find the same effect, perhaps because T cells in the graft interfere with NK cell development and KIR reconstitution after allogeneic donor transplant as we have shown [41]. Taken together, these results suggest that NK cells play a role in allogeneic transplant and cancer therapy; however, the complexities of the KIR system and the presence of other functional receptors on NK cells may explain some of the confusion in interpreting published studies.

Adoptive transfer of allogeneic NK Cells in combination with a nonmyeloablative haploidentical transplantation

We have shown that adoptive transfer of haploi-dentical NK cells can induce remissions in 27% of patients with refractory or relapsed AML [6]. The remissions induced by adoptive NK cell transfer were not durable. We hypothesized that this may be

in part related to the lack of in vivo expansion of NK cells on all patients. Because lymphocyte homeostasis is determined by factors resulting from lymphodeple-tion, we increased our preparative regimen and added a CD34+ stem cell infusion to create an NMA haploi-dentical transplantation protocol. Radiation (200 cGy twice a day on day —13) was added to a preparative regimen used in nontransplant patients that included Flu 25 mg/m2 x 5 (day —18 through day —14) and Cy 60 mg/kg x 2 (days —16 and —15). The NK cell product was activated with 1000 U/mL IL-2 and infused on day —12 followed by 6 doses subcutaneous IL-2 (10 million units) given every other day to promote in vivo NK cell expansion. The mean NK cell dose was 1.85 x 107 cells/kg. A CD34-selected peripheral blood graft from the same donor was given with Thymoglo-bulin 3 mg/kg days 0, +1 and +2 as the only additional immunosuppression. In the 13 patients a significantly higher rate of NK cell expansion (75% [9/12 evaluable]; mean 607 6 184 NK cells/mL) was achieved compared to the adoptive NK cell transfer regimen, which did not include radiation. This adoptive NK cell plus allograft protocol led to 66% of relapsed or refractory AML patients (8/12 evaluable) clearing leukemia by day — 1. Patients who did not clear leukemia (N = 4) did not engraft. All others (N = 6) engrafted promptly at a median 17 days [range: 11-31]). None developed GVHD, but infectious complications were common, not unexpected in a high-risk cohort where subjects typically had prolonged neutropenia prior to transplantation. In summary, in patients with refractory AML, addition of haploidentical NK cells to an NMA haploidentical transplantation yields NK cell expansion in a majority of patients, achievement of CR, and quick engraftment without GVHD. This is a promising platform upon which to add other strategies aimed at improving disease-free survival (DFS) in patients with refractory AML. Additional strategies to sensitize NK cells to leukemia, to target leukemic stem cells, to improve in vivo expansion, to interrupt inhibitory receptor interactions with class I major histo-compatibility complex (MHC) and to pick donors are among future strategies to improve this therapy.

KIR genotyping: implications for donor selection

The importance of KIR in determining clinical outcome after HCT remains controversial. We geno-typed donors and recipients from 209 HLA-matched and 239 mismatched T-replete URD transplantations for AML [42]. Three-year OS was significantly higher after transplantation from a KIR B/x donor (31% [95% confidence interval [CI]: 26-36] versus 20% [95% CI: 13-27]; P = .007). Multivariate analysis demonstrated a 30% improvement in the relative risk of relapse-free survival (RFS) with B/x donors compared with A/A donors (relative risk [RR]: 0.70 [95% CI: 0.55-0.88];

P = .002). This demonstrates that unrelated donors with KIR B haplotypes confer significant survival benefit to patients undergoing T-replete HCT for AML. KIR genotyping should be added to donor selection criteria in addition to HLA typing, to identify donors with B KIR haplotypes. Future investigators are aimed at subsetting the KIR B haplotype for a more refined donor selection strategy.

CONCLUSION

NK cells have been of therapeutic interest for decades as they kill tumor targets in vitro and in animal models. Strategies to activate autologous NK cells dominated the early literature but were found to limited efficacy. This was explained by the discovery of inhibitory receptors on NK cells that recognize "self" MHC molecules. Current strategies using allogeneic NK cells are based on the premise that they will result in a higher frequency of donor cells that will be reactive against the recipient. The promising finding in AML strongly support a role for the therapeutic use of NK cells and offers the opportunity to further manipulate these cells to exploit their full potential when combined with allogeneic transplantation.

ACKNOWLEDGMENTS

Financial disclosure: The authors have nothing to disclose.

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