Clinical impact of natural killer cell reconstitution after allogeneic hematopoietic transplantation

Impact of Natural Killer Cell-Associated Factors on Acute Leukemia Outcomes after Haploidentical Hematopoietic Stem Cell Transplantation with αβ T Cell Depletion in a Pediatric Cohort

The technique of αβ T cell depletion (αβTCD) is a well-established method of hematopoietic stem cell transplantation (HSCT) for children with acute leukemia owing to the low rates of graft-versus-host disease and nonrelapse mortality (NRM). The graft-versus-leukemia effect is generally ascribed to natural killer (NK) cells conserved within the graft. It is not known whether NK-related factors affect the outcome of αβTCD HSCT, however. The aim of this retrospective study was to explore the impact of NK alloreactivity (based on donor-recipient killer immunoglobulin-like receptor [KIR] mismatch), graft NK cell dose, and blood NK cell recovery on day +30 post-HSCT on the incidences of leukemia relapse and NRM. The pediatric acute leukemia cohort comprised 295 patients who underwent their first HSCT from a haploidentical donor in complete remission. During post hoc analysis, the total cohort was divided into subcohorts by diagnosis (acute lymphoblastic leukemia [ALL]/acute myeloid leukemia [AML]), NK alloreactivity prediction (KIR match/KIR mismatch), graft NK cell dose (less than versus greater than the median value), and blood NK cell recovery on day +30 post-HSCT (less than versus greater than the median value). We also investigated the influence of serotherapy (antithymocyte globulin [ATG] group) versus abatacept + tocilizumab combination [aba+toci] group) on relapse risk in the context of KIR mismatch. The risks of relapse and NRM were calculated by the cumulative risk method, and groups were compared using the Gray test. Multivariate analysis revealed no apparent impact of predicted NK alloreactivity or any other studied NK cell-related factors for the entire cohort. For patients with AML, a significantly higher relapse risk associated with high NK cell graft content on the background of no predicted KIR mismatch (P = .002) was shown. Multivariate analysis confirmed this finding (P = .018); on the other hand, for the KIR-mismatched patients, there was a trend toward a lower risk of relapse associated with high NK cell dose. The use of ATG was associated with a trend toward reduced relapse risk (P = .074) in the AML patients. There was no significant impact of NK-related factors in the ALL patients. Overall, the evaluated NK-related factors did not show a clear and straightforward correlation with the key outcomes of HSCT in our cohort of children with acute leukemia. In practice, the data support prioritization of KIR-mismatched donors for patients with AML. Importantly, a potential interaction of KIR ligand mismatch and NK cell content in the graft was identified. Indirect evidence suggests that additional cellular constituents of the graft could influence the function of NK cells after HSCT and affect their role as graft-versus-leukemia effectors.

Clinical Characteristics and Outcome Analysis for HLA Loss Patients Following Partially Mismatched Related Donor Transplantation Using HLA Chimerism for Loss of Heterozygosity Analysis by Next-Generation Sequencing

Genomic loss of mismatched human leukocyte antigen (HLA loss) is one of the most vital immune escape mechanisms of leukemic cells after allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, the methods currently used for HLA loss analysis have some shortcomings. Limited literature has been published, especially in lymphoid malignancies. This study aims to evaluate the incidences, risk factors of HLA loss, and clinical outcomes of HLA loss patients. In all, 160 patients undergoing partially mismatched related donor (MMRD) transplantation from 18 centers in China were selected for HLA loss analysis with the next-generation sequencing (NGS)-based method, which was validated by HLA-KMR. Variables of the prognostic risk factors for HLA loss or HLA loss–related relapse were identified with the logistic regression or the Fine and Gray regression model. An HLA loss detection system, HLA-CLN [HLA chimerism for loss of heterozygosity (LOH) analysis by NGS], was successfully developed. Forty (25.0%) patients with HLA loss were reported, including 27 with myeloid and 13 with lymphoid malignancies. Surprisingly, 6 of those 40 patients did not relapse. The 2-year cumulative incidences of HLA loss (22.7% vs 22.0%, P = 0.731) and HLA loss–related relapse (18.4% vs 20.0%, P = 0.616) were similar between patients with myeloid and lymphoid malignancies. The number of HLA mismatches (5/10 vs <5/10) was significantly associated with HLA loss in the whole cohort [odds ratio (OR): 3.15, P = 0.021] and patients with myeloid malignancies (OR: 3.94, P = 0.021). A higher refined-disease risk index (OR: 6.91, P = 0.033) and donor–recipient ABO incompatibility (OR: 4.58, P = 0.057) contributed to HLA loss in lymphoid malignancies. To sum up, HLA-CLN could overcome the limitations of HLA-KMR and achieve a better HLA coverage for more patients. The clinical characteristics and outcomes were similar in patients with HLA loss between myeloid and lymphoid malignancies. In addition, the results suggested that a patient with HLA loss might not always relapse.

KIR Receptors as Key Regulators of NK Cells Activity in Health and Disease

Natural killer (NK) cells are part of the cellular immune response. They target mainly cancer and virally infected cells. To a high extent cytotoxic activity of NK cells is regulated inter alia by signals from killer immunoglobulin-like receptors (KIR). The major histocompatibility complex (MHC) class I molecules are important ligands for KIR receptors. Binding of ligands (such as MHC I) to the KIR receptors has the important role in solid organ or hematopoietic cell transplantation. Of note, the understanding of the relationship between KIR and MHC receptors may contribute to the improvement of transplant results. Donor-recipient matching, which also includes the KIR typing, may improve monitoring, individualize the treatment and allow for predicting possible effects after transplantation, such as the graft-versus-leukemia effect (GvL) or viral re-infection. There are also less evident implications of KIR/MHC matching, such as with pregnancy and cancer. In this review, we present the most relevant literature reports on the importance of the KIR/MHC relationship on NK cell activity and hematopoietic stem cell transplantation (HSCT)/solid organ transplantation (SOT) effects, the risk of allograft rejection, protection against post-transplant cytomegalovirus (CMV) infection, pregnancy complications, cancer and adoptive therapy with NK cells.

NK cells sense tumors, course of disease and treatments: Consequences for NK-based therapies

The recent findings on NK activation indicate that these cells are important antitumor effectors. NK cells participate in the graft-vs.-leukemia effect to control the relapse in leukemic patients transplanted with allogeneic hematopoietic stem cells. In various tumors, correlation between NK cell infiltrates and prognosis were reported. However, tumor-infiltrating NK cells are yet poorly characterized. We here summarize our results and the recent studies of the literature on tumor-infiltrating NK cells, and discuss the impact of these novel insights into NK cell responses against tumors for the design of NK cell-based therapies.

High-Risk Leukemia: Past, Present, and Future Role of NK Cells

Natural killer (NK) cells are a population of cytotoxic innate lymphocytes that evolved prior to their adaptive counterparts and constitute one of the first lines of defense against infected/mutated cells. Several studies have shown that in patients with acute leukemia given haploidentical hematopoietic stem cell transplantation, donor-derived NK cells play a key role in the eradication of cancer cells. The antileukemic effect is mostly related to the presence of “alloreactive” NK cells, that is, mature KIR+ NK cells that express inhibitory KIR mismatched with HLA class I (KIR-L) of the patient. A genotypic analysis detecting KIR B haplotype and the relative B content is an additional donor selection criterion. These data provided the rationale for implementing phase I/II clinical trials of adoptive infusion of either selected or ex vivo-activated NK cells, often from an HLA-mismatched donor. In this review, we provide a historical perspective on the role played by NK cells in patients with acute leukemia, focusing also on the various approaches to adoptive NK cell therapy and the unresolved issues therein. In addition, we outline new methods to enhance NK activity, including anti-KIR monoclonal antibody, bi-/trispecific antibodies linking NK cells to cytokines and/or target antigens, and CAR-engineered NK cells.

How important is NK alloreactivity and KIR in allogeneic transplantation?

Relapse of acute myelogenous leukemia (AML) after allogeneic hematopoietic cell transplantation (allo HCT) is a major cause of death in transplant recipients. Efforts to control relapse by promoting donor T-cell alloreactivity, such as withdrawal of immune suppression or donor lymphocyte infusions, are limited by the propensity to induce graft versus host disease (GVHD) and by inadequate efficacy. Therefore, options for AML patients who have relapsed AML after allo HCT are few and outcomes are poor. Similar to T-cells, natural killer (NK) cells have potent anti-leukemia effector capacity, and yet unlike T-cells, NK cells do not mediate GVHD. Furthermore, their function does not require matching of human leukocyte antigens (HLA) between donor and recipient. Maximizing donor NK alloreactivity thus holds the exciting possibility to induce the graft versus leukemia (GVL) effect without engendering GVHD. Among the array of activating and inhibitory NK cell surface receptors, the killer Ig-like receptors (KIR) play a central role in modulating NK effector function. Here we will review how KIR mediates donor alloreactivity, discuss the role of KIR gene and allele typing to optimize allo HCT donor selection, and discuss how KIR may aid adoptive NK and other cell therapies.

Immunotherapy in acute myeloid leukemia

Despite the remarkable progress made in some leukemias such as CML and CLL, cytotoxic treatment for AML remains essentially unchanged over the last 4 decades. Several lines of evidence, including the graft versus leukemia effect associated with allogeneic hematopoietic stem cell transplantation (HSCT), suggest that immunotherapy is an active modality in AML. Given the lack of progress for chemotherapy in this disease, many novel immunologic treatment approaches have been explored. The goals of non-transplant-based immune approaches have largely consisted of the stimulation or restoration of endogenous immune responses or the targeting of specific tumor antigens by immune cells. These strategies have been associated with less toxicity than allogeneic HSCT but typically have inferior efficacy. Allogeneic HSCT exploits major and minor histocompatibility differences between the donor and recipient in order to recognize and eradicate malignancy. With the recognition that the immune system itself provides a basis for treating AML, immunotherapy continues to be an attractive modality to exploit in the treatment of this disease.

Ascorbic acid promotes proliferation of natural killer cell populations in culture systems applicable for natural killer cell therapy

BACKGROUND AIMS

Natural killer (NK) cell-based immunotherapy is a promising treatment for a variety of malignancies. However, generating sufficient cell numbers for therapy remains a challenge. To achieve this, optimization of protocols is required.

METHODS

Mature NK cells were expanded from peripheral blood mononuclear cells PBMCs in the presence of anti-CD3 monoclonal antibody and interleukin-2. Additionally, NK-cell progenitors were generated from CD34(+) hematopoietic stem cells or different T/NK-cell progenitor populations. Generated NK cells were extensively phenotyped, and functionality was determined by means of cytotoxicity assay.

RESULTS

Addition of ascorbic acid (AA) resulted in more proliferation of NK cells without influencing NK-cell functionality. In more detail, PBMC-derived NK cells expanded 2362-fold (median, range: 90-31,351) in the presence of AA and were capable of killing tumor cells under normoxia and hypoxia. Moreover, hematopoietic stem cell-derived progenitors appeared to mature faster in the presence of AA, which was also observed in the NK-cell differentiation from early T/NK-cell progenitors.

CONCLUSIONS

Mature NK cells proliferate faster in the presence of phospho-L-AA, resulting in higher cell numbers with accurate functional capacity, which is required for adoptive immunotherapy.

Favorable impact of natural killer cell reconstitution on chronic graft-versus-host disease and cytomegalovirus reactivation after allogeneic hematopoietic stem cell transplantation

Natural killer cells are the first lymphocyte subset to reconstitute, and play a major role in early immunity after allogeneic hematopoietic stem cell transplantation. Cells expressing the activating receptor NKG2C seem crucial in the resolution of cytomegalovirus episodes, even in the absence of T cells. We prospectively investigated natural killer-cell reconstitution in a cohort of 439 adult recipients who underwent non-T-cell-depleted allogeneic hematopoietic stem cell transplantation between 2005 and 2012. Freshly collected blood samples were analyzed 3, 6, 12 and 24 months after transplantation. Data were studied with respect to conditioning regimen, source of stem cells, underlying disease, occurrence of graft-versus-host disease, and profiles of cytomegalovirus reactivation. In multivariate analysis we found that the absolute numbers of CD56(bright) natural killer cells at month 3 were significantly higher after myeloablative conditioning than after reduced intensity conditioning. Acute graft-versus-host disease impaired reconstitution of total and CD56(dim) natural killer cells at month 3. In contrast, high natural killer cell count at month 3 was associated with a lower incidence of chronic graft-versus-host disease, independently of a previous episode of acute graft-versus-host disease and stem cell source. NKG2C(+)CD56(dim) and total natural killer cell counts at month 3 were lower in patients with reactivation of cytomegalovirus between month 0 and month 3, but expanded greatly afterwards. These cells were also less numerous in patients who experienced later cytomegalovirus reactivation between month 3 and month 6. Our results advocate a direct role of NKG2C-expressing natural killer cells in the early control of cytomegalovirus reactivation after allogeneic hematopoietic stem cell transplantation.

A two-step approach to allogeneic haploidentical hematopoietic stem cell transplantation

Strategies that exploit natural killer (NK) cell alloreactivity or attenuate rather than deplete T cells have resulted in improved outcomes after haploidentical hematopoietic stem cell transplantation (HSCT). However, no approach has consistently produced the triad of optimal immune reconstitution, avoidance of significant graft-versus-host disease (GVHD), and durable control of malignancy. We developed a two-step approach to haploidentical HSCT in which the lymphoid and myeloid portions of the graft are given in two separate steps in order to control and optimize T-cell dosing. The initial results from these trials have included robust immune reconstitution, low rates of toxicity and significant GVHD, and durable disease control in good-risk patients, as well as insights regarding a threshold for T-cell dosing above which graft-versus-tumor (GVT) effects might be expected. Patients who were not in remission at the time of HSCT had higher rates of relapse requiring efforts to further strengthen GVT effects. Second-generation trials are underway to further exploit changes in the dosing and timing of administration of T cells and to optimize donor selection in an effort to decrease relapse rates in high-risk patients.

The role of natural killer cells in hematopoietic stem cell transplantation

Natural killer (NK) cells are important elements of innate immunity, and a large body of evidence supports the significant role of NK in immune surveillance against infections and tumors. Regulation of cytotoxic activity is mediated through activating and inhibitory receptors expressed on the cell surface. NK cells are key players of allogeneic hematopoietic stem cell transplantation (allo-SCT), and previous studies showed the beneficial effect of NK alloreactivity in prevention of relapse, especially in the setting of haploidentical SCT. Biology of human NK cells is an area of active research. Exploitation of the molecular mechanisms regulating NK maturation, tolerance to self, and NK-mediated cytotoxicity will help in the development of innovative NK cell immunotherapy methods.

Blood dendritic cells suppress NK cell function and increase the risk of leukemia relapse after hematopoietic cell transplantation

NK cells play an important role in hematopoietic stem cell transplantation (HCT) and in cross talk with dendritic cells (DCs) to induce primary T cell response against infection. Therefore, we hypothesized that blood DCs should augment NK cell function and reduce the risk of leukemia relapse after HCT. To test this hypothesis, we conducted laboratory and clinical studies in parallel. We found that although, phenotypically, NK cells could induce DC maturation and DCs could in turn increase activating marker expression on NK cells, paradoxically, both BDCA1(+) myeloid DCs and BDCA4(+) plasmacytoid DCs suppressed the function of NK cells. Patients who received an HLA-haploidentical graft containing a larger number of BDCA1(+) DCs or BDCA4(+) DCs had a higher risk of leukemia relapse and poorer survival. Further experiments indicated that the potent inhibition on NK cell cytokine production and cytotoxicity was mediated in part through the secretion of IL-10 by BDCA1(+) DCs and IL-6 by BDCA4(+) DCs. These results have significant implications for future HCT strategies.

Clinical tolerance in allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) has been a curative therapeutic option for a wide range of immune hematologic malignant and non-malignant disorders including genetic diseases and inborn errors. Once in the host, allogeneic transplanted cells have not only to ensure myeloid repopulation and immunological reconstitution but also to acquire tolerance to host human leukocyte antigens via central or peripheral mechanisms. Peripheral tolerance after allogeneic HSCT depends on several regulatory mechanisms aimed at blocking alloimmune reactivity while preserving immune responses to pathogens and tumor antigens. Patients transplanted with HSCT represent an ideal model system in humans to identify and characterize the key cellular and molecular players underlying these mechanisms. The knowledge gained from these studies has allowed the development of novel therapeutic strategies aimed at inducing long-term peripheral tolerance, which can be applicable not only in allogeneic HSCT but also in autoimmune diseases and solid-organ transplantation. In the present review, we describe Type 1 regulatory T cells, initially discovered and characterized in chimeric patients transplanted with human leukocyte antigen-mismatched HSCT, and how their presence correlates to tolerance induction and maintenance. Furthermore, we summarize different cell therapy approaches with regulatory T cells, designed to facilitate tolerance induction, minimizing pharmaceutical interventions.

Expression profiling of major histocompatibility and natural killer complex genes reveals candidates for controlling risk of graft versus host disease

Background

The major histocompatibility complex (MHC) is the most important genomic region that contributes to the risk of graft versus host disease (GVHD) after haematopoietic stem cell transplantation. Matching of MHC class I and II genes is essential for the success of transplantation. However, the MHC contains additional genes that also contribute to the risk of developing acute GVHD. It is difficult to identify these genes by genetic association studies alone due to linkage disequilibrium in this region. Therefore, we aimed to identify MHC genes and other genes involved in the pathophysiology of GVHD by mRNA expression profiling.

Methodology/Principal Findings

To reduce the complexity of the task, we used genetically well-defined rat inbred strains and a rat skin explant assay, an in-vitro-model of the graft versus host reaction (GVHR), to analyze the expression of MHC, natural killer complex (NKC), and other genes in cutaneous GVHR. We observed a statistically significant and strong up or down regulation of 11 MHC, 6 NKC, and 168 genes encoded in other genomic regions, i.e. 4.9%, 14.0%, and 2.6% of the tested genes respectively. The regulation of 7 selected MHC and 3 NKC genes was confirmed by quantitative real-time PCR and in independent skin explant assays. In addition, similar regulations of most of the selected genes were observed in GVHD-affected skin lesions of transplanted rats and in human skin explant assays.

Conclusions/Significance

We identified rat and human MHC and NKC genes that are regulated during GVHR in skin explant assays and could therefore serve as biomarkers for GVHD. Several of the respective human genes, including HLA-DMB, C2, AIF1, SPR1, UBD, and OLR1, are polymorphic. These candidates may therefore contribute to the genetic risk of GVHD in patients.

Expansion of CD56-negative, CD16-positive, KIR-expressing natural killer cells after T cell-depleted haploidentical hematopoietic stem cell transplantation

The main functions of natural killer (NK) cells are early protection against viruses or tumor cells and production of cytokines that regulate immune functions. The present study assessed the role of different NK subsets in exerting graft-versus-leukemia effects in recipients of human leukocyte antigen (HLA) haploidentical hematopoietic transplants and monitored for the first time CD3-/CD56- lymphocyte expansion. CD3-/CD56- cells expressed NK cell-associated molecules, such as CD16, NKp46, NKp30, CD244 (2B4), CD161, and killer cell immunoglobulin-like receptors. CD3-/CD56- cells further exhibited the classical functional characteristics of NK cells: cytolysis of target cells lacking HLA class I, antibody-dependent cellular cytotoxicity and cytokine production. These results demonstrate that CD56- NK cells are functional, recognize missing self and, like their CD56+ counterparts, may contribute to graft-versus-leukemia reactions.

Characterization of long-term mixed donor-donor chimerism after double cord blood transplantation

Double cord blood transplantation (DCBT) with two matched or partially matched cord blood units has been implemented successfully to circumvent the limitations of graft cell dose associated with single CBT. After DCBT, sustained haematopoiesis is derived almost exclusively from only one of the donated units. None the less, we previously observed two of six evaluable DCBT patients still having mixed donor-donor chimerism at 28 and 45 months post-transplantation, respectively. In the present study we utilize flow cytometry techniques to perform the first thorough analysis of phenotype and functionality of cord blood units in patients with mixed donor-donor chimerism. Our results suggest that the two stable cord blood units are different phenotypically and functionally: one unit shows more naive T cells, lower T cell cytokine production and higher frequencies of natural killer cells, the other shows higher frequencies of well-differentiated and functional lymphocytes. Additionally, in comparison with control patients having a single prevailing cord blood unit, the patients with donor-donor chimerism exhibit less overall T cell cytokine production and a smaller fraction of memory T cells. Furthermore, our results indicate that human leucocyte antigen-C match of donor units may partly explain the development of a donor-donor mixed chimerism.

Monitoring human leukocyte antigen class I molecules by micro-Raman spectroscopy at single-cell level

Human leukocyte antigen (HLA) class I molecules are formed by three immunoglobulin-like domains (alpha1, alpha2, and alpha3) once folded by peptide and beta(2)-microglobulin show the presence of two alpha-helix streams and one beta-sheet limiting the pocket for the antigenic peptide. The loss of HLA class I expression in tumors and virus-infected cells, on one hand, prevents T cell recognition, while on the other hand, it leads to natural killer (NK) cell mediated cytotoxicity. We propose the possibility of using Raman spectroscopy to measure the relative expression of HLA class I molecules at the single-cell level. Raman spectra are recorded for three cell lines (K562, T2, and T3) and monomers (HLA class I folded, unfolded and peptide+beta(2)-microlobulin refolded) using 830 nm laser line. Our data are consistent with the hypothesis that in the Raman spectra, ranging from 1600 to 1800 cm(-1), the intensity variation of cells associated with HLA class I molecules could be measured.

Relapse of leukemia with loss of mismatched HLA resulting from uniparental disomy after haploidentical hematopoietic stem cell transplantation

We investigated human leukocyte antigen (HLA) expression on leukemic cells derived from patients at diagnosis and relapse after hematopoietic stem cell transplantation (HSCT) using flow cytometry with locus-specific antibodies. Two of 3 patients who relapsed after HLA-haploidentical HSCT demonstrated loss of HLA alleles in leukemic cells at relapse; on the other hand, no loss of HLA alleles was seen in 6 patients who relapsed after HLA-identical HSCT. Single-nucleotide polymorphism array analyses of sorted leukemic cells further revealed the copy number-neutral loss of heterozygosity, namely, acquired uniparental disomy on the short arm of chromosome 6, resulting in the total loss of the mismatched HLA haplotype. These results suggest that the escape from immunosurveillance by the loss of mismatched HLA alleles may be a crucial mechanism of relapse after HLA-haploidentical HSCT. Accordingly, the status of mismatched HLA on relapsed leukemic cells should be checked before donor lymphocyte infusion.

The graft-versus-tumor effect in pediatric malignancy

Because severe forms of the graft-versus-host reaction directed against normal tissues (also termed graft-versus-host disease [GVHD]) also contribute to morbidity and mortality following allogeneic hematopoietic stem cell transplantation, major efforts have focused on strategies to separate GVHD from the potentially beneficial immune reactivity against tumor (also called the graft-versus-tumor [GVT] effect). This article focuses on the data supporting the contribution of the GVT effect to cure of malignancy, what is known about the biology of the GVT reaction, and, finally, strategies to manipulate the GVT effect to increase the potency of HSCT.

DNAM-1/CD155 interactions promote cytokine and NK cell-mediated suppression of poorly immunogenic melanoma metastases

A role for NK cells in therapeutic intervention for hematologic malignancies, such as acute myeloid leukemia and multiple myeloma, and nonhematologic malignancies, such as melanoma, is becoming more apparent. DNAM-1 is an NK cell receptor whose importance in facilitating activation signals received by NK cells in natural and cytokine-driven responses to tumor metastases in vivo is poorly explored. In this study, we used matched tumor lines expressing a variety of relevant ligands, neutralizing monoclonal Abs, and DNAM-1 gene-targeted mice to determine the relative importance of DNAM-1-ligand interactions in controlling tumor metastases. Our results demonstrate that NK cells require DNAM-1 for natural or cytokine (IL-2, IL-12, or IL-21) suppression of tumor metastases or their variants expressing CD70 or CD80. In contrast, DNAM-1 was dispensable when tumor cells were targets of Ab-dependent cellular cytotoxicity or presented ligands for NKG2D. CD155 appeared to be a key ligand recognized by DNAM-1 in NK cell-mediated suppression of metastases, and DNAM-1-mediated suppression coincided with perforin activity. Overall, these data implied a general role for DNAM-1-CD155 interactions in NK cell-mediated killing of tumors, even in the presence of tumor CD70 or CD80 expression, and further defined the optimal efficacy requirements of cytokines that directly activate NK cells.

New approaches to allogeneic hematopoietic stem cell transplantation in pediatric cancers

About 30 years have passed since the first children underwent allogeneic hematopoietic stem cell transplantation (HSCT). Since then, there have been major improvements to identifying and expanding pools of donors, mobilizing and harvesting hematopoietic stem cells, conditioning therapies, transfusion medicine, antimicrobials, immunosuppression, and supportive care. These advances have broadened the application of HSCT to treat malignant and nonmalignant pediatric disorders. Currently, most children and young adults with cancer who undergo allogeneic HSCT are identified as having a malignancy that would be lethal if not for the biologic therapy that HSCT imparts, and remarkably, many of these patients can be cured. However, this cure still comes with costs, including infections, graft-versus-host disease, loss of potential, and psychosocial and financial stresses. New approaches are increasingly available that focus on immune modulation to reduce the burdens of HSCT while improving its therapeutic benefit.

Loss of mismatched HLA in leukemia after stem-cell transplantation

BACKGROUND

Transplantation of hematopoietic stem cells from partially matched family donors is a promising therapy for patients who have a hematologic cancer and are at high risk for relapse. The donor T-cell infusions associated with such transplantation can promote post-transplantation immune reconstitution and control residual disease.

METHODS

We identified 43 patients who underwent haploidentical transplantation and infusion of donor T cells for acute myeloid leukemia or myelodysplastic syndrome and conducted post-transplantation studies that included morphologic examination of bone marrow, assessment of hematopoietic chimerism with the use of short-tandem-repeat amplification, and HLA typing. The genomic rearrangements in mutant variants of leukemia were studied with the use of genomic HLA typing, microsatellite mapping, and single-nucleotide-polymorphism arrays. The post-transplantation immune responses against the original cells and the mutated leukemic cells were analyzed with the use of mixed lymphocyte cultures.

RESULTS

In 5 of 17 patients with leukemia relapse after haploidentical transplantation and infusion of donor T cells, we identified mutant variants of the original leukemic cells. In the mutant leukemic cells, the HLA haplotype that differed from the donor's haplotype had been lost because of acquired uniparental disomy of chromosome 6p. T cells from the donor and the patient after transplantation did not recognize the mutant leukemic cells, whereas the original leukemic cells taken at the time of diagnosis were efficiently recognized and killed.

CONCLUSIONS

After transplantation of haploidentical hematopoietic stem cells and infusion of donor T cells, leukemic cells can escape from the donor's antileukemic T cells through the loss of the mismatched HLA haplotype. This event leads to relapse.