Flt-3 internal tandem duplication hampers differentiation of AML blasts towards leukemic dendritic cells

Dendritic Cells of Leukemic Origin: Specialized Antigen-Presenting Cells as Potential Treatment Tools for Patients with Myeloid Leukemia

The prognosis of elderly patients with acute myeloid leukemia (AML) and high-grade myelodysplastic syndrome (MDS) is limited due to the lack of therapy options and high relapse rates. Dendritic cell (DC)-based immunotherapy seems to be a promising treatment tool. DC are potent antigen-presenting cells and play a pivotal role on the interface of the innate and the adaptive immune system. Myeloid leukemia blasts can be converted to DC of leukemic origin (DC_leu), expressing costimulatory molecules along with the whole leukemic antigen repertoire of individual patients. These generated DC_leu are potent stimulators of various immune reactive cells and increase antileukemic immunity ex vivo. Here we review the generating process of DC/DC_leu from leukemic peripheral blood mononuclear cells as well as directly from leukemic whole blood with "minimized" Kits to simulate physiological conditions ex vivo. The purpose of adoptive cell transfer of DC/DC_leu as a vaccination strategy is discussed. A new potential therapy option with Kits for patients with myeloid leukemia, which would render an adoptive DC/DC_leu transfer unnecessary, is presented. In summary, DC/DC_leu-based therapies seem to be promising treatment tools for patients with AML or MDS but ongoing research including trials in animals and humans have to be performed.

Procedures for the expansion of CD14(+) precursors from acute myeloid leukemic cells to facilitate dendritic cell-based immunotherapy

AIMS

Vaccination with acute myeloid leukemia (AML)-derived dendritic cells (DCs) is a promising immunotherapeutic approach to prevent relapse of AML. However, in clinical practice AML-derived DC culture is unfeasible in 40% of cases. Here, we demonstrate that AML cells can be expanded in vitro prior to differentiation with cocktails of cytokines with known myeloid growth-promoting effects.

RESULTS

Nine out of 13 initially CD14(-) samples gain de novo CD14 (>10%) expression (69% increment; p = 0.01) after in vitro expansion. These expanded CD14(+) leukemic cells displayed a high probability (six out of six initially CD14(-) samples tested) to differentiate into DCs upon culture with GM-CSF, TNF-α and IL-4.

CONCLUSION

Induction of CD14 on initially CD14(-) AML cells potentially increases the number of patients eligible for DC-based immunotherapy.

Clinical trials of dendritic cell-based cancer vaccines in hematologic malignancies

The potential for the immune system to target hematological malignancies is demonstrated in the allogeneic transplant setting, where durable responses can be achieved. However, allogeneic transplantation is associated with significant morbidity and mortality related to graft versus host disease. Cancer immunotherapy has the capacity to direct a specific cytotoxic immune response against cancer cells, particularly residual cancer cells, in order to reduce the likelihood of disease relapse in a more targeted and tolerated manner. Ex vivo dendritic cells can be primed in various ways to present tumor associated antigen to the immune system, in the context of co-stimulatory molecules, eliciting a tumor specific cytotoxic response in patients. Several approaches to prime dendritic cells and overcome the immunosuppressive microenvironment have been evaluated in pre-clinical and early clinical trials with promising results. In this review, we summarize the clinical data evaluating dendritic cell based vaccines for the treatment of hematological malignancies.

Dendritic cell-based immunotherapy for myeloid leukemias

Acute and chronic myeloid leukemia (AML, CML) are hematologic malignancies arising from oncogene-transformed hematopoietic stem/progenitor cells known as leukemia stem cells (LSCs). LSCs are selectively resistant to various forms of therapy including irradiation or cytotoxic drugs. The introduction of tyrosine kinase inhibitors has dramatically improved disease outcome in patients with CML. For AML, however, prognosis is still quite dismal. Standard treatments have been established more than 20 years ago with only limited advances ever since. Durable remission is achieved in less than 30% of patients. Minimal residual disease (MRD), reflected by the persistence of LSCs below the detection limit by conventional methods, causes a high rate of disease relapses. Therefore, the ultimate goal in the treatment of myeloid leukemia must be the eradication of LSCs. Active immunotherapy, aiming at the generation of leukemia-specific cytotoxic T cells (CTLs), may represent a powerful approach to target LSCs in the MRD situation. To fully activate CTLs, leukemia antigens have to be successfully captured, processed, and presented by mature dendritic cells (DCs). Myeloid progenitors are a prominent source of DCs under homeostatic conditions, and it is now well established that LSCs and leukemic blasts can give rise to "malignant" DCs. These leukemia-derived DCs can express leukemia antigens and may either induce anti-leukemic T cell responses or favor tolerance to the leukemia, depending on co-stimulatory or -inhibitory molecules and cytokines. This review will concentrate on the role of DCs in myeloid leukemia immunotherapy with a special focus on their generation, application, and function and how they could be improved in order to generate highly effective and specific anti-leukemic CTL responses. In addition, we discuss how DC-based immunotherapy may be successfully integrated into current treatment strategies to promote remission and potentially cure myeloid leukemias.

Down-regulation of signal transducer and activator of transcription 3 improves human acute myeloid leukemia-derived dendritic cell function

Signal transducer and activator of transcription (STAT) 3 inhibits dendritic cell (DC) differentiation and is constitutively activated in blasts of approximately half of AML patients. We investigated the correlation between STAT3 activity, DC maturation and the ability to stimulate T-cells in primary acute myeloid leukemia (AML)-derived DCs. STAT3 knock-down by shRNAmir increased the ability of AML-DCs to stimulate T-cells. Treatment of AML-DC with arsenic trioxide, but not AG490, JSI-124 or NSC-74859, led to a more mature phenotype and enhanced T-cell stimulation, while having minimal effect on normal DC. We conclude that AML-DCs have improved immunogenicity after reducing STAT3.

Elevated frequencies of leukemic myeloid and plasmacytoid dendritic cells in acute myeloid leukemia with the FLT3 internal tandem duplication

Some 30% of acute myeloid leukemia (AML) patients display an internal tandem duplication (ITD) mutation in the FMS-like tyrosine kinase 3 (FLT3) gene. FLT3-ITDs are known to drive hematopoietic stem cells towards FLT3 ligand independent growth, but the effects on dendritic cell (DC) differentiation during leukemogenesis are not clear. We compared the frequency of cells with immunophenotype of myeloid DC (mDC: Lin⁻, HLA-DR⁺, CD11c⁺, CD86⁺) and plasmacytoid DC (pDC: Lin⁻, HLA-DR⁺, CD123⁺, CD86⁺) in diagnostic samples of 47 FLT3-ITD⁻ and 40 FLT3-ITD⁺ AML patients. The majority of ITD⁺ AML samples showed high frequencies of mDCs or pDCs, with significantly decreased HLA-DR expression compared with DCs detectable in ITD⁻ AML samples. Interestingly, mDCs and pDCs sorted out from ITD⁺ AML samples contained the ITD insert revealing their leukemic origin and, upon ex vivo culture with cytokines, they acquired DC morphology. Notably, mDC/pDCs were detectable concurrently with single lineage mDCs and pDCs in all ITD⁺ AML (n = 11) and ITD⁻ AML (n = 12) samples analyzed for mixed lineage DCs (Lin⁻, HLA-DR⁺, CD11c⁺, CD123⁺). ITD⁺ AML mDCs/pDCs could be only partially activated with CD40L and CpG for production of IFN-α, TNF-α, and IL-1α, which may affect the anti-leukemia immune surveillance in the course of disease progression.

Dendritic cells (DCs) can be successfully generated from leukemic blasts in individual patients with AML or MDS: an evaluation of different methods

Myeloid-leukemic cells (AML, MDS, CML) can be differentiated to leukemia-derived dendritic cell [DC (DCleu)] potentially presenting the whole leukemic antigen repertoire without knowledge of distinct leukemia antigens and are regarded as promising candidates for a vaccination strategy. We studied the capability of 6 serum-free DC culture methods, chosen according to different mechanisms, to induce DC differentiation in 137 cases of AML and 52 cases of MDS. DC-stimulating substances were cytokines ("standard-medium", "MCM-Mimic", "cytokine-method"), bacterial lysates ("Picibanil"), double-stranded RNA ["Poly (I:C)"] or a cytokine bypass method ("Ca-ionophore"). The quality/quantity of DC generated was estimated by flow cytometry studying (co) expressions of "DC"antigens, costimulatory, maturation, and blast-antigens. Comparing these methods on average 15% to 32% DC, depending on methods used, could be obtained from blast-containing mononuclear cells (MNC) in AML/MDS cases with a DC viability of more than 60%. In all, 39% to 64% of these DC were mature; 31% to 52% of leukemic blasts could be converted to DCleu and DCleu-proportions in the suspension were 2% to 70% (13%). Average results of all culture methods tested were comparable, however not every given case of AML could be differentiated to DC with 1 selected method. However performing a pre-analysis with 3 DC-generating methods (MCM-Mimic, Picibanil, Ca-ionophore) we could generate DC in any given case. Functional analyses provided proof, that DC primed T cells to antileukemia-directed cytotoxic cells, although an anti-leukemic reaction was not achieved in every case. In summary our data show that a successful, quantitative DC/DCleu generation is possible with the best of 3 previously tested methods in any given case. Reasons for different functional behaviors of DC-primed T cells must be evaluated to design a practicable DC-based vaccination strategy.

Dendritic cell vaccines in acute leukaemia

There is a need for novel treatment for acute leukaemia as relapse rates remain unacceptably high. Immunotherapy aims to stimulate the patient's immune responses to recognize and destroy leukaemia cells whilst activating immune memory. The qualities of the most potent professional antigen-presenting cell, the dendritic cell (DC), can be used to stimulate leukaemia-specific cytotoxic T cells. DCs can be loaded with leukaemia antigens, or leukaemia blasts can be modified to express DC-like properties for use in vaccine therapy. This chapter will review the rationale for DC vaccine therapy, the preclinical and clinical trials to date, the barriers to successful DC vaccine therapies and the role of immune adjuncts to improve outcomes.

Human dendritic cell line models for DC differentiation and clinical DC vaccination studies

Dendritic cells (DC) are increasingly applied in the immunotherapy of cancer. As the development of a standardized DC vaccine product is often hampered by the limited availability of DC precursors and inter- and intra-donor variability, and the preparation of individual vaccines is labor-intensive, it would be preferable to use DC from a readily available and unlimited source, such as cell lines can provide. It has been described that leukemia-derived cell lines are able to differentiate into functional DC, creating possibilities for the development of highly reproducible DC vaccines and providing in vitro model systems for in-depth studies about DC physiology. This review discusses the different human DC cell line differentiation models described so far. Based on the available data, characteristics that determine the ability of leukemia cells to differentiate along the different precursor stages into functional DC will be formulated. In addition, evidence will be provided that the human CD34+ acute myeloid leukemia cell line MUTZ-3 provides DC that exhibit the functional properties that are crucial for the in vivo generation of CTL-mediated immunity and thus, currently, represents the most valuable, sustainable model system for myeloid DC differentiation and clinical DC vaccination studies.

Dendritic cells and myeloid leukaemias: plasticity and commitment in cell differentiation

Dendritic cells (DCs) are key antigen-presenting cells (APCs), which link innate and adaptive immunity, ultimately activating antigen-specific T cells. This review examines the relationship between the acute and chronic myeloid leukaemias and cells with DC properties. DCs are non-dividing terminally differentiated cells, and ex vivo leukaemic cells or cell lines show little similarity to DCs. However, many leukaemias differentiate further in response to defined stimuli, and retain a degree of lineage plasticity. Therefore, several studies have explored the response of leukaemic cells to the in vitro regimens used to differentiate ex vivo primary DCs. Recent data suggest that the most 'dendritic-like' cells can be derived from more undifferentiated myeloid leukaemias, such as the myelomonocytic Mutz-3 cell line. These findings have important implications for understanding the developmental origins of DCs, for harnessing the APC properties of this class of tumour to stimulate the therapeutic anti-tumour immunity, and for developing useful models for the study of human DC physiology and pathology. There is a strong rationale for further exploration of this class of tumour and its relationship to the normal DC.

Feasibility of clinical dendritic cell vaccination in acute myeloid leukemia

Dendritic cells (DC) are increasingly being utilized for anti-cancer therapy. Acute myeloid leukemia (AML) blasts are able to differentiate towards leukemia-derived DC enabling efficient presentation of known and unknown leukemic antigens. Advances in culture techniques and AML-DC characterization justify clinical application. However, clinical trials using AML-DC are hampered by patient inclusion criteria which allow selective entering of patients in second complete remission. Clinical relevant responses to DC-based immunotherapy are likely to only occur in non-end-stage patients. Application in early stage disease is mandatory to permit ultimate proof of clinical benefit of AML-DC vaccination strategy.