Both mature KIR+ and immature KIR- NK cells control pediatric acute B-cell precursor leukemia in NOD.Cg-Prkdcscid IL2rgtmWjl/Sz mice

Epigenetic targeting to enhance acute myeloid leukemia-directed immunotherapy

AML is a malignant disease of hematopoietic progenitor cells with unsatisfactory treatment outcome, especially in patients that are ineligible for intensive chemotherapy. Immunotherapy, comprising checkpoint inhibition, T-cell engaging antibody constructs, and cellular therapies, has dramatically improved the outcome of patients with solid tumors and lymphatic neoplasms. In AML, these approaches have been far less successful. Discussed reasons are the relatively low mutational burden of AML blasts and the difficulty in defining AML-specific antigens not expressed on hematopoietic progenitor cells. On the other hand, epigenetic dysregulation is an essential driver of leukemogenesis, and non-selective hypomethylating agents (HMAs) are the current backbone of non-intensive treatment. The first clinical trials that evaluated whether HMAs may improve immune checkpoint inhibitors' efficacy showed modest efficacy except for the anti-CD47 antibody that was substantially more efficient against AML when combined with azacitidine. Combining bispecific antibodies or cellular treatments with HMAs is subject to ongoing clinical investigation, and efficacy data are awaited shortly. More selective second-generation inhibitors targeting specific chromatin regulators have demonstrated promising preclinical activity against AML and are currently evaluated in clinical trials. These drugs that commonly cause leukemia cell differentiation potentially sensitize AML to immune-based treatments by co-regulating immune checkpoints, providing a pro-inflammatory environment, and inducing (neo)-antigen expression. Combining selective targeted epigenetic drugs with (cellular) immunotherapy is, therefore, a promising approach to avoid unintended effects and augment efficacy. Future studies will provide detailed information on how these compounds influence specific immune functions that may enable translation into clinical assessment.

Epigenetic remodeling of the immune landscape in cancer: therapeutic hurdles and opportunities

The tumor immune microenvironment represents a sophisticated ecosystem where various immune cell subtypes communicate with cancer cells and stromal cells. The dynamic cellular composition and functional characteristics of the immune landscape along the trajectory of cancer development greatly impact the therapeutic efficacy and clinical outcome in patients receiving systemic antitumor therapy. Mounting evidence has suggested that epigenetic mechanisms are the underpinning of many aspects of antitumor immunity and facilitate immune state transitions during differentiation, activation, inhibition, or dysfunction. Thus, targeting epigenetic modifiers to remodel the immune microenvironment holds great potential as an integral part of anticancer regimens. In this review, we summarize the epigenetic profiles and key epigenetic modifiers in individual immune cell types that define the functional coordinates of tumor permissive and non-permissive immune landscapes. We discuss the immunomodulatory roles of current and prospective epigenetic therapeutic agents, which may open new opportunities in enhancing cancer immunotherapy or overcoming existing therapeutic challenges in the management of cancer.

KIR-favorable TCR-αβ/CD19-depleted haploidentical HCT in children with ALL/AML/MDS: primary analysis of the PTCTC ONC1401 trial

We performed a prospective multicenter study of T-cell receptor αβ (TCR-αβ)/CD19-depleted haploidentical hematopoietic cell transplantation (HCT) in children with acute leukemia and myelodysplastic syndrome (MDS), to determine 1-year disease-free survival (DFS) and compare 2-year outcomes with recipients of other donor cell sources. Fifty-one patients aged 0.7 to 21 years were enrolled; donors were killer immunoglobulin-like receptor (KIR) favorable based on ligand mismatch and/or high B content. The 1-year DFS was 78%. Superior 2-year DFS and overall survival (OS) were noted in patients <10 years of age, those treated with reduced toxicity conditioning (RTC) rather than myeloablative conditioning, and children with minimal residual disease <0.01% before HCT. Multivariate analysis comparing the KIR-favorable haploidentical cohort with controls showed similar DFS and OS compared with other donor cell sources. Multivariate analysis also showed a marked decrease in the risk of grades 2 to 4 and 3 to 4 acute graft versus host disease (aGVHD), chronic GVHD, and transplant-related mortality vs other donor cell sources. Ethnic and racial minorities accounted for 53% of enrolled patients, and data from a large cohort of recipients/donors screened for KIR showed that >80% of recipients had a KIR-favorable donor by our definition, demonstrating that this approach is broadly applicable to groups often unable to find donors. This prospective, multicenter study showed improved outcomes using TCR-αβ/CD19-depleted haploidentical donors using RTC for children with acute leukemia and MDS. Randomized trials comparing this approach with matched unrelated donors are warranted. This trial was registered at https://clinicaltrials.gov as #NCT02646839.

Hypomethylating agents (HMA) for the treatment of acute myeloid leukemia and myelodysplastic syndromes: mechanisms of resistance and novel HMA-based therapies

Aberrant DNA methylation plays a pivotal role in tumor development and progression. DNA hypomethylating agents (HMA) constitute a class of drugs which are able to reverse DNA methylation, thereby triggering the re-programming of tumor cells. The first-generation HMA azacitidine and decitabine have now been in standard clinical use for some time, offering a valuable alternative to previous treatments in acute myeloid leukemia and myelodysplastic syndromes, so far particularly in older, medically non-fit patients. However, the longer we use these drugs, the more we are confronted with the (almost inevitable) development of resistance. This review provides insights into the mode of action of HMA, mechanisms of resistance to this treatment, and strategies to overcome HMA resistance including next-generation HMA and HMA-based combination therapies.

Epigenetic Regulation of NK Cell-Mediated Antitumor Immunity

Natural killer (NK) cells are critical innate lymphocytes that can directly kill target cells without prior immunization. NK cell activation is controlled by the balance of multiple germline-encoded activating and inhibitory receptors. NK cells are a heterogeneous and plastic population displaying a broad spectrum of functional states (resting, activating, memory, repressed, and exhausted). In this review, we present an overview of the epigenetic regulation of NK cell-mediated antitumor immunity, including DNA methylation, histone modification, transcription factor changes, and microRNA expression. NK cell-based immunotherapy has been recognized as a promising strategy to treat cancer. Since epigenetic alterations are reversible and druggable, these studies will help identify new ways to enhance NK cell-mediated antitumor cytotoxicity by targeting intrinsic epigenetic regulators alone or in combination with other strategies.

Fit-For-All iPSC-Derived Cell Therapies and Their Evaluation in Humanized Mice With NK Cell Immunity

Human induced pluripotent stem cells (iPSCs) can be limitlessly expanded and differentiated into almost all cell types. Moreover, they are amenable to gene manipulation and, because they are established from somatic cells, can be established from essentially any person. Based on these characteristics, iPSCs have been extensively studied as cell sources for tissue grafts, blood transfusions and cancer immunotherapies, and related clinical trials have started. From an immune-matching perspective, autologous iPSCs are perfectly compatible in principle, but also require a prolonged time for reaching the final products, have high cost, and person-to-person variation hindering their common use. Therefore, certified iPSCs with reduced immunogenicity are expected to become off-the-shelf sources, such as those made from human leukocyte antigen (HLA)-homozygous individuals or genetically modified for HLA depletion. Preclinical tests using immunodeficient mice reconstituted with a human immune system (HIS) serve as an important tool to assess the human alloresponse against iPSC-derived cells. Especially, HIS mice reconstituted with not only human T cells but also human natural killer (NK) cells are considered crucial. NK cells attack so-called “missing self” cells that do not express self HLA class I, which include HLA-homozygous cells that express only one allele type and HLA-depleted cells. However, conventional HIS mice lack enough reconstituted human NK cells for these tests. Several measures have been developed to overcome this issue including the administration of cytokines that enhance NK cell expansion, such as IL-2 and IL-15, the administration of vectors that express those cytokines, and genetic manipulation to express the cytokines or to enhance the reconstitution of human myeloid cells that express IL15R-alpha. Using such HIS mice with enhanced human NK cell reconstitution, alloresponses against HLA-homozygous and HLA-depleted cells have been studied. However, most studies used HLA-downregulated tumor cells as the target cells and tested in vitro after purifying human cells from HIS mice. In this review, we give an overview of the current state of iPSCs in cell therapies, strategies to lessen their immunogenic potential, and then expound on the development of HIS mice with reconstituted NK cells, followed by their utilization in evaluating future universal HLA-engineered iPSC-derived cells.

Phenotypic and Functional Characterization of NK Cells in αβT-Cell and B-Cell Depleted Haplo-HSCT to Cure Pediatric Patients with Acute Leukemia

NK cells can exert remarkable graft-versus-leukemia (GvL) effect in HLA-haploidentical hematopoietic stem cell transplantation (haplo-HSCT). Here, we dissected the NK-cell repertoire of 80 pediatric acute leukemia patients previously reported to have an excellent clinical outcome after αβT/B-depleted haplo-HSCT. This graft manipulation strategy allows the co-infusion of mature immune cells, mainly NK and γδT cells, and hematopoietic stem cells (HSCs). To promote NK-cell based antileukemia activity, 36/80 patients were transplanted with an NK alloreactive donor, defined according to the KIR/KIR-Ligand mismatch in the graft-versus-host direction. The analysis of the reconstituted NK-cell repertoire in these patients showed relatively high proportions of mature and functional KIR⁺NKG2A^-CD57⁺ NK cells, including the alloreactive NK cell subset, one month after HSCT. Thus, the NK cells adoptively transfused with the graft persist as a mature source of effector cells while new NK cells differentiate from the donor HSCs. Notably, the alloreactive NK cell subset was endowed with the highest anti-leukemia activity and its size in the reconstituted repertoire could be influenced by human cytomegalovirus (HCMV) reactivation. While the phenotypic pattern of donor NK cells did not impact on post-transplant HCMV reactivation, in the recipients, HCMV infection/reactivation fostered a more differentiated NK-cell phenotype. In this cohort, no significant correlation between differentiated NK cells and relapse-free survival was observed.

iPSC-Derived Platelets Depleted of HLA Class I Are Inert to Anti-HLA Class I and Natural Killer Cell Immunity

The ex vivo production of platelets depleted of human leukocyte antigen class I (HLA-I) could serve as a universal measure to overcome platelet transfusion refractoriness caused by HLA-I incompatibility. Here, we developed human induced pluripotent cell-derived HLA-I-deficient platelets (HLA-KO iPLATs) in a clinically applicable imMKCL system by genetic manipulation and assessed their immunogenic properties including natural killer (NK) cells, which reject HLA-I downregulated cells. HLA-KO iPLATs were deficient for all HLA-I but did not elicit a cytotoxic response by NK cells in vitro and showed circulation equal to wild-type iPLATs upon transfusion in our newly established Hu-NK-MSTRG mice reconstituted with human NK cells. Additionally, HLA-KO iPLATs successfully circulated in an alloimmune platelet transfusion refractoriness model of Hu-NK-MISTRG mice. Mechanistically, the lack of NK cell-activating ligands on platelets may be responsible for evading the NK cell response. This study revealed the unique non-immunogenic property of platelets and provides a proof of concept for the clinical application of HLA-KO iPLATs.

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Clinically applicable iPSC-derived HLA class I knockout platelets (HLA-KO iPLATs)

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HLA-KO iPLATs do not elicit NK cell activation in vitro

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HLA-KO iPLATs circulate comparably with wild type in human NK cell-reconstituted mice

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HLA-KO iPLATs circulate competently in alloimmune PTR model mice

The Role of Natural Killer Cells as a Platform for Immunotherapy in Pediatric Cancers

PURPOSE OF REVIEW

We aim to review the most recent findings in the use of NK cells in childhood cancers.

RECENT FINDINGS

Natural killer cells are cytotoxic to tumor cells. In pediatric leukemias, adoptive transfer of NK cells can bridge children not in remission to transplant. Interleukins (IL2, IL15) can enhance NK cell function. NK cell-CAR therapy has advantages of shorter life span that lessens chronic toxicities, lower risk of graft versus host disease when using allogeneic cells, ability of NK cells to recognize tumor cells that have downregulated MHC to escape T cells, and possibly less likelihood of cytokine storm. Cytotoxicity to solid tumors (rhabdomyosarcoma, Ewing's sarcoma, neuroblastoma) is seen with graft versus tumor effect in transplant and in combination with antibodies. Challenges lie in the microenvironment which is suppressive for NK cells. NK cell immunotherapy in childhood cancers is promising and recent works aim to overcome challenges.

NK Cells as Possible Prognostic Factor in Childhood Acute Lymphoblastic Leukemia

Deficiency or impaired function natural killer (NK) cells might result in the development of serious infections and promote the development of malignancies. The aim of our study was to assess the prognostic role of NK cell percentage in bone marrow on the day of acute lymphoblastic leukemia (ALL) diagnosis. 84 children (49 males = 58%; median age 5 yrs) with ALL were enrolled. The NK cell percentage was assessed using flow cytometry with antibodies against the cluster of differentiation (CD): CD3, CD56, and CD16. We evaluated two groups: group I (NK+), patients with NK cells in the bone marrow (n = 74), and group II (NK-), patients without NK cells in the bone marrow (n = 10) (cut-off value of negative <1%). In the patients from group I, the prednisone good response on day 8 and the remission on day 15 of treatment were observed significantly more often (p = .01, p = .03). The children from group I had significantly better survival as compared to those from group II (p = .02) (HR 2.59; 95% CI: 1.38-4.85). The presence of NK cells in the bone marrow at diagnosis can be a prognostic factor in children with ALL. The presented results should be the basis for further research.

CD20-CD19 Bispecific CAR T Cells for the Treatment of B-Cell Malignancies

The treatment of leukemia/lymphoma by chimeric antigen receptor (CAR) redirected T cells with specificity for CD19 induced complete remissions in the majority of patients, with a realistic hope for cure. However, recent follow-up data revealed a substantial risk of relapse through leukemic cells that lack the CAR targeted antigen. In this situation, a bispecific CAR with binding domains for CD19 and CD20 is aimed at recognizing leukemic cells with only one cognate antigen. The anti-CD20-CD19 bispecific CAR induced a full T-cell response upon engagement of CD19 or CD20 on target cells showing a true "OR" gate recognition in redirecting T-cell activation. T cells with the anti-CD20-CD19 CAR efficiently killed patients' chronic lymphocytic leukemia cells in vitro. The bispecific CAR T cells cleared pediatric acute lymphocytic leukemia with a mixed CD19⁺CD20⁺/CD20^- phenotype from the blood and bone marrow of transplanted mice, while anti-CD20 CAR T cells left CD20^- leukemic cells behind without curing the disease. Data indicate the superior anti-leukemic activity in the control of leukemia, implying that the anti-CD20-CD19 bispecific CAR T cells may reduce the risk of relapse through antigen-loss leukemic cells in the long term.

Tumor-priming converts NK cells to memory-like NK cells

Fascinating earlier evidence suggests an intrinsic capacity of human natural killer (NK) cells to acquire adaptive immune features in the context of cytomegalovirus (CMV) infection or pro-inflammatory cytokine stimulation. Since the role of memory NK cells in cancer has so far remained elusive and adoptive NK cell transfer in relapsing pediatric acute B cell precursor leukemia (BCP-ALL) patients awaits improvement, we asked the question whether tumor-priming could promote the generation of memory NK cells with enhanced graft-vs.-leukemia (GvL) reactivity. Here, we provide substantial evidence that priming of naive human NK cells with pediatric acute B cell leukemia or acute myeloid leukemia specimens induces a functional conversion to tumor-induced memory-like (TIML)-NK cells displaying a heightened tumor-specific cytotoxicity and enhanced perforin synthesis. Cell cycles analyses reveal that tumor-priming sustainably alters the balance between NK cell activation and apoptosis in favor of survival. In addition, gene expression patterns differ between TIML- and cytokine-induced memory-like (CIML)-NK cells with the magnitude of regulated genes being distinctly higher in TIML-NK cells. As such, the tumor-induced conversion of NK cells triggers the emergence of a so far unacknowledged NK cell differentiation stage that might promote GvL effects in the context of adoptive cell transfer.

Imprint of 5-azacytidine on the natural killer cell repertoire during systemic treatment for high-risk myelodysplastic syndrome

5-azacytidine (5-aza) is a hypomethylating agent approved for the treatment of high-risk myelodysplastic syndrome (MDS). It is assumed to act by demethylating tumor suppressor genes and via direct cytotoxic effects on malignant cells. In vitro treatment with hypomethylating agents has profound effects on the expression of killer-cell immunoglobulin-like (KIR) receptors on natural killer (NK) cells, as these receptors are epigenetically regulated via methylation of the promoters. Here we investigated the influence of 5-aza on the NK-cell repertoire during cytokine-induced proliferation in vitro and homeostatic proliferation in vivo in patients with high-risk MDS. In vitro treatment of NK cells from both healthy donors and MDS patients with low doses of 5-aza led to a significant increase in expression of multiple KIRs, but only in cells that had undergone several rounds of cell division. Proliferating 5-aza exposed NK cells exhibited increased IFN-γ production and degranulation towards tumor target cells. MDS patients had lower proportions of educated KIR-expressing NK cells than healthy controls but after systemic treatment with 5-aza, an increased proportion of Ki-67⁺ NK cells expressed multiple KIRs suggesting uptake of 5-aza in cycling cells in vivo. Hence, these results suggest that systemic treatment with 5-aza may shape the NK cell repertoire, in particular during homeostatic proliferation, thereby boosting NK cell-mediated recognition of malignant cells.

TRAIL-mediated killing of acute lymphoblastic leukemia by plasmacytoid dendritic cell-activated natural killer cells

Acute lymphoblastic leukemia (ALL) still frequently recurs after hematopoietic stem cell transplantation (HSCT), underscoring the need to improve the graft-versus-leukemia (GvL) effect. Natural killer (NK) cells reconstitute in the first months following HSCT when leukemia burden is at its lowest, but ALL cells have been shown to be resistant to NK cell-mediated killing. We show here that this resistance is overcome by NK cell stimulation with TLR-9-activated plasmacytoid dendritic cells (pDCs). NK cell priming with activated pDCs resulted in TRAIL and CD69 up-regulation on NK cells and IFN-γ production. NK cell activation was dependent on IFN-α produced by pDCs, but was not reproduced by IFN-α alone. ALL killing was further enhanced by inhibition of KIR engagement. We showed that ALL lysis was mainly mediated by TRAIL engagement, while the release of cytolytic granules was involved when ALL expressed NK cell activating receptor ligands. Finally, adoptive transfers of activated-pDCs in ALL-bearing humanized mice delayed the leukemia onset and cure 30% of mice. Our data therefore demonstrate that TLR-9 activated pDCs are a powerful tool to overcome ALL resistance to NK cell-mediated killing and to reinforce the GvL effect of HSCT. These results open new therapeutic avenues to prevent relapse in children with ALL.

An Fc-optimized CD133 antibody for induction of NK cell reactivity against myeloid leukemia

Antibody-dependent cellular cytotoxicity (ADCC) of natural killer (NK) cells largely contributes to the success of monoclonal antibody (mAb) treatment in cancer. As no antibodies are clinically available for immunotherapy of myeloid leukemias (MLs), we aimed to develop an Fc-optimized CD133 mAb for induction of NK ADCC against MLs. When comparing different available CD133 mAbs, no difference was observed with regard to binding to primary chronic myeloid leukemia cells. However, clone 293C3 recognized acute myeloid leukemia (AML) cells in a substantially higher percentage of patient cases and was thus chosen to generate chimeric mAbs with either wild-type Fc part (293C3-WT) or a variant containing amino-acid exchanges (S239D/I332E) to enhance affinity to CD16 on NK cells (293C3-SDIE). In vitro, treatment with 293C3-SDIE significantly enhanced activation, degranulation and lysis of primary CD133-positive AML cells by allogeneic and autologous NK cells as compared with its wild-type counterpart. In line with the observed lower expression levels of CD133 on healthy cells compared with malignant hematopoietic cells, 293C3-SDIE caused no relevant toxicity towards committed hematopoietic progenitor cells. In a NOD.Cg-PrkdcscidIL2rgtmWjl/Sz xenotransplantation model, 293C3-SDIE facilitated elimination of patient AML cells by human NK cells. Thus, 293C3-SDIE constitutes an attractive immunotherapeutic compound, in particular for elimination of minimal residual disease in the context of allogeneic stem cell transplantation in AML.

Exploitation of natural killer cells for the treatment of acute leukemia

Natural killer (NK) cells play an important role in surveillance and elimination of malignant cells. Their spontaneous cytotoxicity was first demonstrated in vitro against leukemia cell lines, and NK cells might play a crucial role in the therapy of leukemia. NK cell activity is controlled by an array of germ line-encoded activating and inhibitory receptors, as well as modulating coreceptors. This biologic feature can be exploited in allogeneic cell therapy, and the recognition of "missing-self" on target cells is crucial for promoting NK cell-mediated graft-versus-leukemia effects. In this regard, NK cells that express an inhibitory killer immunoglobulin-like receptor (iKIR) for which the respective major histocompatibility complex class I ligand is absent on leukemic target cells can exert alloreactivity in vitro and in vivo. Several models regarding potential donor-patient constellations have been described that have demonstrated the clinical benefit of such alloreactivity of the donor-derived NK cell system in patients with adult acute myeloid leukemia and pediatric B-cell precursor acute lymphoblastic leukemia after allogeneic stem cell transplantation. Moreover, adoptive transfer of mature allogeneic NK cells in the nontransplant or transplant setting has been shown to be safe and feasible, whereas its effectivity needs further evaluation. NK cell therapy can be further improved by optimal donor selection based on phenotypic and genotypic properties, by adoptive transfer of NK cells with ex vivo or in vivo cytokine stimulation, by the use of antibodies to induce antibody-dependent cellular cytotoxicity or to block iKIRs, or by transduction of chimeric antigen receptors.

Naive Donor NK Cell Repertoires Associated with Less Leukemia Relapse after Allogeneic Hematopoietic Stem Cell Transplantation

Acute and latent human CMV cause profound changes in the NK cell repertoire, with expansion and differentiation of educated NK cells expressing self-specific inhibitory killer cell Ig-like receptors. In this study, we addressed whether such CMV-induced imprints on the donor NK cell repertoire influenced the outcome of allogeneic stem cell transplantation. Hierarchical clustering of high-resolution immunophenotyping data covering key NK cell parameters, including frequencies of CD56(bright), NKG2A(+), NKG2C(+), and CD57(+) NK cell subsets, as well as the size of the educated NK cell subset, was linked to clinical outcomes. Clusters defining naive (NKG2A(+)CD57(-)NKG2C(-)) NK cell repertoires in the donor were associated with decreased risk for relapse in recipients with acute myeloid leukemia and myelodysplastic syndrome (hazard ratio [HR], 0.09; 95% confidence interval [CI]: 0.03-0.27; p < 0.001). Furthermore, recipients with naive repertoires at 9-12 mo after hematopoietic stem cell transplantation had increased disease-free survival (HR, 7.2; 95% CI: 1.6-33; p = 0.01) and increased overall survival (HR, 9.3; 95% CI: 1.1-77, p = 0.04). Conversely, patients with a relative increase in differentiated NK cells at 9-12 mo displayed a higher rate of late relapses (HR, 8.41; 95% CI: 6.7-11; p = 0.02), reduced disease-free survival (HR, 0.12; 95% CI: 0.12-0.74; p = 0.02), and reduced overall survival (HR, 0.07; 95% CI: 0.01-0.69; p = 0.02). Thus, our data suggest that naive donor NK cell repertoires are associated with protection against leukemia relapse after allogeneic HSCT.

Central nervous system acute lymphoblastic leukemia: role of natural killer cells

Central nervous system acute lymphoblastic leukemia (CNS-ALL) is a major clinical problem. Prophylactic therapy is neurotoxic, and a third of the relapses involve the CNS. Increased expression of interleukin 15 (IL-15) in leukemic blasts is associated with increased risk for CNS-ALL. Using in vivo models for CNS leukemia caused by mouse T-ALL and human xenografts of ALL cells, we demonstrate that expression of IL-15 in leukemic cells is associated with the activation of natural killer (NK) cells. This activation limits the outgrowth of leukemic cells in the periphery, but less in the CNS because NK cells are excluded from the CNS. Depletion of NK cells in NOD/SCID mice enabled combined systemic and CNS leukemia of human pre-B-ALL. The killing of human leukemia lymphoblasts by NK cells depended on the expression of the NKG2D receptor. Analysis of bone marrow (BM) diagnostic samples derived from children with subsequent CNS-ALL revealed a significantly high expression of the NKG2D and NKp44 receptors. We suggest that the CNS may be an immunologic sanctuary protected from NK-cell activity. CNS prophylactic therapy may thus be needed with emerging NK cell-based therapies against hematopoietic malignancies.