Increased level of myeloid-derived suppressor cells, programmed death receptor ligand 1/programmed death receptor 1, and soluble CD25 in Sokal high risk chronic myeloid leukemia

Paradoxical counteraction by imatinib against cell death in myeloid progenitor 32D cells expressing p210BCR-ABL

Chronic myeloid leukemia (CML) is believed to be caused by the tyrosine kinase p210BCR-ABL, which exhibits growth-promoting and anti-apoptotic activities. However, mechanisms that allow cell differentiation in CML still remain elusive. Here we established tetracycline (Tet)-regulatable p210BCR-ABL-expressing murine 32D myeloid progenitor (32D/TetOff-p210) cells to explore p210BCR-ABL-induced cell death and differentiation. Tet-regulatable overexpression of p210BCR-ABL induced cell death due to the activation of both caspase-1 and caspase-3, coincident with the differentiation from myeloid progenitors into CD11b⁺Ly6C⁺Ly6G⁺ cells with segmented nuclei, exemplified as granulocytic myeloid-derived suppressor cells (G-MDSC), and the ability to secrete IL-1β, TNF-α, and S100A8/A9 into the culture supernatant. Treatment with imatinib almost completely abrogated all these phenotypes. Moreover, overexpression of a sensor of activated caspase-1 based on fluorescence resonance energy transfer (FRET) probe enabled us to detect activation of caspase-1 in a human CML cell line, K562. Furthermore, increased numbers of splenic G-MDSC associated with enhancement of S100A8/A9 production were observed in transgenic mice expressing p210BCR-ABL compared with that in wild-type mice. We also propose the novel mode of cell death in this 32D/TetOff-p210 system termed as myeloptosis.

Blood-based analyses of cancer: Circulating myeloid-derived suppressor cells - is a new era coming?

Progress in cancer treatment made by the beginning of the 21st century has shifted the paradigm from one-size-fits-all to tailor-made treatment. The popular vision, to study solid tumors through the relatively noninvasive sampling of blood, is one of the most thrilling and rapidly advancing fields in global cancer diagnostics. From this perspective, immune-cell analysis in cancer could play a pivotal role in oncology practice. This approach is driven both by rapid technological developments, including the analysis of circulating myeloid-derived suppressor cells (cMDSCs), and by the increasing application of (immune) therapies, the success or failure of which may depend on effective and timely measurements of relevant biomarkers. Although the implementation of these powerful noninvasive diagnostic capabilities in guiding precision cancer treatment is poised to change the ways in which we select and monitor cancer therapy, challenges remain. Here, we discuss the challenges associated with the analysis and clinical aspects of cMDSCs and assess whether the problems in implementing tumor-evolution monitoring as a global tool in personalized oncology can be overcome.

Targeting the Immune Microenvironment in Acute Myeloid Leukemia: A Focus on T Cell Immunity

Immunotherapies, such as chimeric antigen receptor T cells, bispecific antibodies, and immune checkpoint inhibitors, have emerged as promising modalities in multiple hematologic malignancies. Despite the excitement surrounding immunotherapy, it is currently not possible to predict which patients will respond. Within solid tumors, the status of the immune microenvironment provides valuable insight regarding potential responses to immune therapies. Much less is known about the immune microenvironment within hematologic malignancies but the characteristics of this environment are likely to serve a similar predictive role. Acute myeloid leukemia (AML) is the most common hematologic malignancy in adults, and only 25% of patients are alive 5 years following their diagnosis. There is evidence that manipulation of the immune microenvironment by leukemia cells may play a role in promoting therapy resistance and disease relapse. In addition, it has long been documented that through modulation of the immune system following allogeneic bone marrow transplant, AML can be cured, even in patients with the highest risk disease. These concepts, along with the poor prognosis associated with this disease, have encouraged many groups to start exploring the utility of novel immune therapies in AML. While the implementation of these therapies into clinical trials for AML has been supported by preclinical rationale, many questions still exist surrounding their efficacy, tolerability, and the overall optimal approach. In this review, we discuss what is known about the immune microenvironment within AML with a specific focus on T cells and checkpoints, along with their implications for immune therapies.

Chronic Myeloid Leukemia: Immunobiology and Novel Immunotherapeutic Approaches

Imatinib has revolutionized the treatment and prognosis of chronic myeloid leukemia (CML) with survival rates now approaching those of the age-matched healthy population. To be able to discontinue tyrosine kinase inhibitor (TKI) treatment, it is necessary to develop complementary therapies to target minimal residual disease. Recent findings by a number of investigators in both CML mouse models and CML patients offer evidence that many factors in the leukemic microenvironment can collectively contribute to immune escape, including expansion of myeloid-derived suppressor cells, programmed death-1/programmed death-1 ligand interactions resulting in T-cell impairment, expression of soluble suppressive factors such as soluble CD25, and down-regulation of MHC molecules by CML cells. Other investigators have studied the role of cytokines on the resistance to TKIs by leukemic stem cells (LSCs) and have highlighted the implication of the JAK/STAT pathway as well as the interleukin 1 (IL-1) signaling pathway. Distinct immunologic strategies have been considered to harness the immune system or trigger LSC death to allow more CML patients to discontinue TKI treatment (so-called functional cure). Successful immunotherapy and TKI combination and the optimal timing of immunotherapy determination represent major challenges for the future.

Tumor conditions induce bone marrow expansion of granulocytic, but not monocytic, immunosuppressive leukocytes with increased CXCR2 expression in mice

Myeloid-derived suppressor cells (MDSCs) promote tumor growth through, in part, inhibiting T-cell immunity. However, mechanisms underlying MDSC expansion and guidance of MDSCs toward the tumor microenvironment remain unclear. Employing Percoll density gradients, we separate bone marrow (BM) leukocytes from tumor-bearing mice into four density-increasing bands with myeloid leukocytes enriched in bands III and IV. Band III comprises monocytes and low-density granulocytes, both confirmed to be M-MDSCs and G-MDSCs, respectively, by displaying potent inhibition of T-cell proliferation. However, monocytes act as M-MDSCs not only under tumor conditions but also the healthy condition. In contrast, band IV contains non-inhibitory, mature granulocytes. Only band III G-MDSCs display significant expansion in mice bearing B16 melanoma, Lewis lung carcinoma, or MC38 colon carcinoma. The expanded G-MDSCs also show increased CXCR2 expression, which guides egress out of BM, and produce arginase-1 and ROS upon encountering antigen-activated T cells. Adoptive transfer assays demonstrate that both G-MDSCs and mature granulocytes infiltrate tumors, but only the former displays sustention and accumulation. Intratumoral administrations of granulocytes further demonstrate that G-MDSCs promote tumor growth, whereas mature granulocytes exert minimal effects, or execute powerful anti-tumor effects providing the presence of PMN activation mechanisms in the tumor microenvironment.

Monocytic myeloid-derived suppressor cells as prognostic factor in chronic myeloid leukaemia patients treated with dasatinib

Myeloid suppressor cells are a heterogeneous group of myeloid cells that are increased in patients with chronic myeloid leukaemia (CML) inducing T cell tolerance. In this study, we found that therapy with tyrosine kinase inhibitors (TKI) decreased the percentage of granulocytic MDSC, but only patients treated with dasatinib showed a significant reduction in the monocytic subset (M‐MDSC). Moreover, a positive correlation was observed between number of persistent M‐MDSC and the value of major molecular response in dasatinib‐treated patients. Serum and exosomes from patients with CML induced conversion of monocytes from healthy volunteers into immunosuppressive M‐MDSC, suggesting a bidirectional crosstalk between CML cells and MDSC. Overall, we identified M‐MDSC as prognostic factors in patients treated with dasatinib. It might be of interest to understand whether MDSC may be a candidate predictive markers of relapse risk following TKI discontinuation, suggesting their potential significance as practice of precision medicine.

Overexpression of Arginase 1 is linked to DNMT3A and TET2 mutations in lower-grade myelodysplastic syndromes and chronic myelomonocytic leukemia

Immune dysregulation is a common feature of myelodysplastic syndromes (MDS) and chronic myelomonocytic leukemia (CMML), particularly in early stages. However, the genetic basis remains poorly understood. We recently reported that macrophages from mice deficient in tet methylcytosine dioxygenase 2 (Tet2), a model of MDS/CMML, are hyperinflammatory and have increased expression of arginase 1 (Arg1). In macrophages and myeloid derived suppressor cells (MDSCs) expression of Arg1 contributes to T-cell suppression and immune evasion by L-arginine depletion, in the setting of chronic inflammation and cancer. Since human MDS and CMML are driven by TET2 mutations and associated with chronic inflammation, we hypothesized that arginase enzymatic activity and ARG1 expression would be increased in human MDS/CMML bone marrow. Elevated arginase activity was observed in bone marrow mononuclear cells of MDS and CMML patients with lower-grade features. Immunohistochemical studies confirmed that myelomonocytic cells overexpress ARG1. Additionally, mutations in the epigenetic regulators TET2 and DNMT3A corresponded to high ARG1 expression and activity. These findings suggest ARG1 is a biomarker of immune dysregulation in early MDS and CMML. Recent murine findings have implicated Tet2 and Dnmt3a in regulation of innate immunity. Our study suggests similar changes may be driven by human TET2 and DNMT3A mutations.

The hOCT1 and ABCB1 polymorphisms do not influence the pharmacodynamics of nilotinib in chronic myeloid leukemia

First-line nilotinib in chronic myeloid leukemia is more effective than imatinib to achieve early and deep molecular responses, despite poor tolerability or failure observed in one-third of patients. The toxicity and efficacy of tyrosine kinase inhibitors might depend on the activity of transmembrane transporters. However, the impact of transporters genes polymorphisms in nilotinib setting is still debated. We investigated the possible correlation between single nucleotide polymorphisms of hOCT1 (rs683369 [c.480C>G]) and ABCB1 (rs1128503 [c.1236C>T], rs2032582 [c.2677G>T/A], rs1045642 [c.3435C>T]) and nilotinib efficacy and toxicity in a cohort of 78 patients affected by chronic myeloid leukemia in the context of current clinical practice. The early molecular response was achieved by 81% of patients while 64% of them attained deep molecular response (median time, 26 months). The 36-month event-free survival was 86%, whereas 58% of patients experienced toxicities. Interestingly, hOCT1 and ABCB1 polymorphisms alone or in combination did not influence event-free survival or the adverse events rate. Therefore, in contrast to data obtained in patients treated with imatinib, hOCT1 and ABCB1 polymorphisms do not impact on nilotinib efficacy or toxicity. This could be relevant in the choice of the first-line therapy: patients with polymorphisms that negatively condition imatinib efficacy might thus receive nilotinib as first-line therapy.

Immune checkpoint blockade in human cancer therapy: lung cancer and hematologic malignancies

Tumor immune evasion is one of the hallmarks of cancer, and expression of the B7 family of immune checkpoints (PD-L1, PD-L2, B7-H3, B7x and HHLA2) is one mechanism of immune evasion by tumors to suppress T-cell function. Antibodies blocking these interactions of B7-1/B7-2/CTLA-4 and PD-L1/PD-L2/PD-1 have had remarkable clinical success in several cancers and are less toxic than traditional chemotherapy. Even though only a small proportion of patients respond to checkpoint blockade, the duration of such responders due to immunological memory is remarkable and is longer than would be expected with any other agent in refractory disease. In this article, we review the therapeutic trials of blocking these pathways in human lung cancer and hematological malignancies.

MUC1 inhibition leads to decrease in PD-L1 levels via upregulation of miRNAs

The PD-L1/PD-1 pathway is a critical component of the immunosuppressive tumor microenvironment in acute myeloid leukemia (AML), but little is known about its regulation. We investigated the role of the MUC1 oncoprotein in modulating PD-L1 expression in AML. Silencing of MUC1 in AML cell lines suppressed PD-L1 expression without a decrease in PD-L1 mRNA levels, suggesting a post-transcriptional mechanism of regulation. We identified the microRNAs miR-200c and miR-34a as key regulators of PD-L1 expression in AML. Silencing of MUC1 in AML cells led to a marked increase in miR-200c and miR-34a levels, without changes in precursor microRNA, suggesting that MUC1 might regulate microRNA-processing. MUC1 signaling decreased the expression of the microRNA-processing protein DICER, via the suppression of c-Jun activity. NanoString (Seattle, WA, USA) array of MUC1-silenced AML cells demonstrated an increase in the majority of probed microRNAs. In an immunocompetent murine AML model, targeting of MUC1 led to a significant increase in leukemia-specific T cells. In concert, targeting MUC1 signaling in human AML cells resulted in enhanced sensitivity to T-cell-mediated lysis. These findings suggest MUC1 is a critical regulator of PD-L1 expression via its effects on microRNA levels and represents a potential therapeutic target to enhance anti-tumor immunity.

Immune checkpoint blockade for hematologic malignancies: a review

Immune checkpoint blockade has revolutionized the treatment of cancer, with impressive responses seen in a broad variety of tumor types. Blockade of immune checkpoints and immune signaling antibodies has shown promise in multiple types of hematologic malignancies (HMs), with dramatic single agent responses for pembrolizumab and nivolumab in Hodgkin lymphoma (HL). In this review, we outline the current state of immune checkpoint blockade drug development in HMs, and discuss mechanisms of activity and resistance, and highlight potential targets in the immune tumor microenvironment (TME). Blockade of T-cell checkpoint molecules PD-1/PD-L1 and CTLA-4 are the most clinically mature of the immune checkpoint strategies. Novel and upcoming strategies for immune checkpoint blockade drug development in HMs using innovative combinations to modulate immunologic targets shows significant promise as a way to expand the number of patients with blood cancers who could benefit from immunotherapy.

Myeloid-derived suppressor cells as effectors of immune suppression in cancer

The tumor microenvironment consists of an immunosuppressive niche created by the complex interactions between cancer cells and surrounding stromal cells. A critical component of this environment are myeloid-derived suppressor cells (MDSCs), a heterogeneous group of immature myeloid cells arrested at different stages of differentiation and expanded in response to a variety of tumor factors. MDSCs exert diverse effects in modulating the interactions between immune effector cells and the malignant cells. An increased presence of MDSCs is associated with tumor progression, poorer outcomes, and decreased effectiveness of immunotherapeutic strategies. In this article, we will review our current understanding of the mechanisms that underlie MDSC expansion and their immune-suppressive function. Finally, we review the preclinical studies and clinical trials that have attempted to target MDSCs, in order to improve responses to cancer therapies.

Mesenchymal Stem Cells (MSC) Regulate Activation of Granulocyte-Like Myeloid Derived Suppressor Cells (G-MDSC) in Chronic Myeloid Leukemia Patients

It is well known that mesenchymal stem cells (MSC) have a role in promotion of tumor growth, survival and drug-resistance in chronic myeloid leukemia (CML). Recent reports indicated that a subpopulation of myeloid cells, defined as granulocyte-like myeloid-derived suppressor cells (G-MDSC) is increased in these patients. So far, the role of MSC in MDSC expansion and activation into the BM microenvironment remains unexplored. To address this question, here we use a specific experimental model in vitro, co-culturing MSC with peripheral blood mononucleated cells (PBMC) from normal individuals, in order to generate MSC-educated G-MDSC. Although MSC of healthy donors (HD) and CML patients were able to generate the same amount of MDSC, only CML-MSC-educated G-MDSC exhibited suppressive ability on autologous T lymphocytes. In addition, compared with HD-MSC, CML-MSC over-expressed some immunomodulatory factors including TGFβ, IL6 and IL10, that could be involved in MDSC activation. CML-MSC-educated G-MDSC expressed higher levels of ARG1, TNFα, IL1β, COX2 and IL6 than G-MDSC isolated from co-culture with HD-MSC. Our data provide evidence that CML-MSC may play a critical role in tumor microenvironment by orchestrating G-MDSC activation and regulating T lymphocytes-mediated leukemia surveillance, thus contributing to CML immune escape.

DC generation from peripheral blood mononuclear cells in patients with chronic myeloid leukemia: Influence of interferons on DC yield and functional properties

In Chronic Myeloid Leukemia (CML), standard treatment consists of modern tyrosine-kinase inhibitors (TKI). Nevertheless, there is evidence that immune responses against leukemia-associated antigens (LAA) may play an important role in disease control. Dendritic cell (DC)- based immunotherapy is able to induce T cell responses against LAA and might therefore pose an interesting therapeutic option in CML, especially in the setting of minimal residual disease (MRD). GMP production of DC for clinical vaccination remains a time- and cost- intensive procedure and standardized DC generation is warranted. We asked whether maturation-induction with IFN-γ and IFN-α has an influence on functional properties of DC derived from peripheral blood mononuclear cells (PBMC) in CML patients. Monocyte-derived DC from healthy donors and from patients with CML were analyzed after maturation-induction with our TNF-α-containing standard cytokine cocktail with or without addition of IFN-α and/or IFN-γ. Our results confirm that the addition of IFN-γ leads to enhanced IL-12 secretion in healthy donors. In contrast, in CML patients, IFN-γ was not able to increase IL-12 secretion, possibly due to a higher degree of cell adherence and lower cell yield during the cell culture. Our data suggest, that- in contrast to healthy donors-, additional interferons are not beneficial for maturation induction during large-scale DC production in patients with CML.

Programming the immune checkpoint to treat hematologic malignancies

INTRODUCTION

Hematologic malignancies manipulate the immune suppressive pathways involving CTLA-4, PD-1, and others to promote immune tolerance of cancer. New monoclonal antibodies targeting immune checkpoints are showing meaningful responses in the treatment of relapsed and refractory Hodgkin lymphoma, diffuse large B-cell lymphoma, follicular lymphoma, and chronic lymphocytic leukemia. The basis for success of anti-PD-1 therapy appears to be expression of PD-L1 on tumor cells and cells of the tumor microenvironment (TME). While adverse events associated with immune checkpoint inhibitors are capable of generating auto-immune phenomena, in general these therapies are well tolerated.

AREAS COVERED

In this review, the authors discuss the development of immune checkpoint inhibitors and activators which hold promise as useful therapies in malignancies of hematologic origin, since many exploit endogenous pathways to induce tolerance. By programming the immune response to attack hematologic malignancies, unique regimens can be developed to optimally treat patients with curative potential.

EXPERT OPINION

The utilization of immune checkpoint targeting agents to boost the innate and acquired immune systems to eradicate human malignancies represents a unique opportunity to develop novel therapies with increased clinical efficacy. Side effects of these therapies come with the price of auto-immune phenomena that require appropriate management.

Myeloid-derived suppressor cells: The green light for myeloma immune escape

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous, immature myeloid cell population with the ability to suppress innate and adaptive immune responses that promote tumor growth. MDSCs are increased in patients with multiple myeloma (MM) and have bidirectional interaction with tumors within the MM microenvironment. MM-MDSCs promote MM tumor growth and induce immune suppression; conversely, MM cells induce MDSC development and survival. Although the role of MDSCs in infections, inflammatory diseases and solid tumors has been extensively characterized, their tumor-promoting and immune-suppressive role in MM and the MM microenvironment is only beginning to emerge. The presence and activation of MDSCs in MM patients has been well documented; however, the direct actions and functional consequences of MDSCs on cancer cells is poorly defined. Immunosuppressive MDSCs play an important role in tumor progression primarily because of their capability to promote immune-escape, angiogenesis, drug resistance and metastasis. However, their role in the bone marrow (BM), the primary MM site, is poorly understood. MM remains an incurable malignancy, and it is likely that the BM microenvironment protects MM against chemotherapy agents and the host immune system. A growing body of evidence suggests that host immune cells with a suppressive phenotype contribute to a myeloma immunosuppressive network. Among the known suppressor cells, MDSCs and T regulatory cells (Tregs) have been found to be significantly increased in myeloma patients and their levels correlate with disease stage and clinical outcome. Furthermore, it has been shown that MDSC can mediate suppression of myeloma-specific T-cell responses through the induction of T-cell anergy and Treg development in the MM microenvironment. Here, we review clinical correlations and the preclinical proof-of-principle data on the role of MDSCs in myeloma immunotolerance and highlight the mechanistically relevant MDSC-targeted compounds and their potential utility in a new approach for anti-myeloma therapy.

Immune checkpoint pathways: perspectives on myeloid malignancies

Immunologic tolerance to cancer has recently been shown to have major implications for the ability of tumors to survive despite a variety of therapeutic approaches. A critical mechanism underlying this microenvironment dysfunction relates to the ability of tumor cells to block immune check points through expression of specific proteins that interfere with immune cell effector function. Recent advances based on this model have led translational work showing therapeutic efficacy in a variety of solid and lymphoid tumors. Myeloid malignancies, in particular myelodysplastic syndromes (MDS), have significant immune dysregulation of variable degree based on their clinical stages which makes feasible extending such therapeutic approaches to this group of diseases. This review will discuss recent advances in the field of immune checkpoint biology including recent clinical trials with checkpoint inhibitors in patients with a variety of clinical conditions, with focus on such potential therapy in patients with myeloid malignancies.

Novel immunotherapeutic approaches for the treatment of acute leukemia (myeloid and lymphoblastic)

There have been major advances in our understanding of the multiple interactions between malignant cells and the innate and adaptive immune system. While the attention of immunologists has hitherto focused on solid tumors, the specific immunobiology of acute leukemias is now becoming defined. These discoveries have pointed the way to immune interventions building on the established graft-versus-leukemia (GVL) effect from hematopoietic stem-cell transplant (HSCT) and extending immunotherapy beyond HSCT to individuals with acute leukemia with a diversity of immune manipulations early in the course of the leukemia. At present, clinical results are in their infancy. In the coming years larger studies will better define the place of immunotherapy in the management of acute leukemias and lead to treatment approaches that combine conventional chemotherapy, immunotherapy and HSCT to achieve durable cures.

Immunostimulatory Gene Therapy Using Oncolytic Viruses as Vehicles

Immunostimulatory gene therapy has been developed during the past twenty years. The aim of immunostimulatory gene therapy is to tilt the suppressive tumor microenvironment to promote anti-tumor immunity. Hence, like a Trojan horse, the gene vehicle can carry warriors and weapons into enemy territory to combat the tumor from within. The most promising immune stimulators are those activating and sustaining Th1 responses, but even if potent effects were seen in preclinical models, many clinical trials failed to show objective responses in cancer patients. However, with new tools to control ongoing immunosuppression in cancer patients, immunostimulatory gene therapy is now emerging as an interesting option. In parallel, oncolytic viruses have been shown to be safe in patients. To prolong immune stimulation and to increase efficacy, these two fields are now merging and oncolytic viruses are armed with immunostimulatory transgenes. These novel agents are racing towards approval as established cancer immunotherapeutics.

Synergistic immunotherapeutic effects of Lycium barbarum polysaccharide and interferon-α2b on the murine Renca renal cell carcinoma cell line in vitro and in vivo

Novel therapeutic strategies to improve clinical efficacy in patients with renal cell carcinoma (RCC) are required. The possibility of combination therapy with Lycium barbarum polysaccharides (LBP) and recombinant interferon (IFN)-α2b remains to be elucidated in RCC. The present study investigated the putative synergistic immunotherapeutic roles of LBP and IFN-α2b against RCC in vitro and in vivo. The mouse RCC cell line, Renca, was used for in vitro experiments. Treatment of the cells with a combination of LBP and IFN-α2b markedly inhibited cell proliferation, retarded cell cycle growth and promoted apoptosis in the Renca cells. Western blot analysis revealed that LBP and IFN-α2b synergistically downregulated the expression levels of cyclin D1, c-Myc and Bcl-2, and upregulated the expression of the antiapoptotic protein, Bax. Myeloid-derived suppressor cells (MDSCs) were markedly upregulated during tumour progression and promoted tumour growth by inhibiting the T-cell-mediated immune response. In vivo, a marked reduction in the MDSC ratio and tumour volume was observed in a group receiving combined treatment with LBP and IFN-α2b in a xenograft tumour model. In conclusion, the present study suggested that the combination of LBP and IFN-α2b is likely to be more effective in treating murine RCC compared with the less pronounced immunotherapeutic effects of administering LBP or IFN-α2b alone.

T cells are functionally not impaired in AML: increased PD-1 expression is only seen at time of relapse and correlates with a shift towards the memory T cell compartment

Background

T cell function is crucial for the success of several novel immunotherapeutic strategies for the treatment of acute myeloid leukemia (AML). However, changes in phenotype and function of T cells have been described in various hematologic malignancies, mimicking T cell exhaustion known from chronic viral infections. Detailed knowledge about phenotype and function of T cells in AML patients at different stages of the disease is indispensable for optimal development and application of immunotherapeutic strategies for this disease.

Methods

We used flow cytometry-based assays to characterize T cell phenotype and function in peripheral blood and bone marrow of AML patients at diagnosis, at relapse after intensive chemotherapy, and at relapse after allogeneic stem cell transplantation (SCT). Surface expression of CD244, PD-1, CD160, and TIM-3 was determined, and proliferation and production of IFN-γ, TNF-α, and IL-2 were measured.

Results

We detected similar expression of inhibitory molecules on T cells from patients at diagnosis and from age-matched healthy controls. At relapse after SCT, however, PD-1 expression was significantly increased compared to diagnosis, both on CD4⁺ and CD8⁺ T cells. This pattern was not associated with age and cytomegalovirus (CMV) status but with a shift towards effector memory cells in relapsed AML patients. Proliferation and cytokine production assays did not reveal functional defects in T cells of AML patients, neither at diagnosis nor at relapse.

Conclusion

We thus conclude that T cell exhaustion does not play a major role in AML. Immunotherapeutic strategies targeting autologous T cells thus have particularly good prospects in the setting of AML.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-015-0189-2) contains supplementary material, which is available to authorized users.

Myeloid derived suppressor cells in chronic myeloid leukemia

The suppression of the immune system creates a permissive environment for development and progression of cancer. One population of immunosuppressive cells that have become the focus of intense study is myeloid derived suppressor cells (MDSCs), immature myeloid cells able to induce immune-escape, angiogenesis, and tumor progression. Two different subpopulations have been identified and studied: granulocytic and monocytic MDSCs, with a different immunophenotype and immunosuppressive properties. Recently, an accumulation of both Gr-MDSCs and Mo-MDSCs cells has been found in the peripheral blood of chronic myeloid leukemia (CML) patients. They are part of the tumor clone showing BCR/ABL expression. Imatinib therapy decreases both MDSCs and arginase 1 levels to normal ones. This review will focus on actual knowledge for human MDSCs and their immunosuppressive activity in CML patients, with a critical attention to comparison of Gr-MDSCs and polymorphonuclear cells (PMNs). We will then suggest the monitoring of MDSCs in patients who have discontinued tyrosine kinase inhibitors (TKIs) therapy to evaluate if their increase could correlate with disease relapse.

The tyrosine kinase inhibitors imatinib and dasatinib reduce myeloid suppressor cells and release effector lymphocyte responses

Immune escape mechanisms promote tumor progression and are hurdles of cancer immunotherapy. Removing immunosuppressive cells before treatment can enhance efficacy. Tyrosine kinase inhibitors (TKI) may be of interest to combine with immunotherapy, as it has been shown that the inhibitor sunitinib reduces myeloid suppressor cells in patients with renal cell carcinoma and dasatinib promotes expansion of natural killer-like lymphocytes in chronic myeloid leukemia (CML). In this study, the capacity of dasatinib and imatinib to reduce myeloid suppressor cells and to induce immunomodulation in vivo was investigated ex vivo. Samples from CML patients treated with imatinib (n = 18) or dasatinib (n = 14) within a Nordic clinical trial (clinicalTrials.gov identifier: NCT00852566) were investigated for the presence of CD11b(+)CD14(-)CD33(+) myeloid cells and inhibitory molecules (arginase I, myeloperoxidase, IL10) as well as the presence of natural killer cells, T cells (naïve/memory), and stimulatory cytokines (IL12, IFNγ, MIG, IP10). Both imatinib and dasatinib decreased the presence of CD11b(+)CD14(-)CD33(+) myeloid cells as well as the inhibitory molecules and the remaining myeloid suppressor cells had an increased CD40 expression. Monocytes also increased CD40 after therapy. Moreover, increased levels of CD40, IL12, natural killer cells, and experienced T cells were noted after TKI initiation. The presence of experienced T cells was correlated to a higher IFNγ and MIG plasma concentration. Taken together, the results demonstrate that both imatinib and dasatinib tilted the immunosuppressive CML tumor milieu towards promoting immune stimulation. Hence, imatinib and dasatinib may be of interest to combine with cancer immunotherapy.

Immunology of chronic myeloid leukemia: current concepts and future goals

Although chronic myeloid leukemia is a rare malignancy, it has developed into a model system for the study of a variety of aspects of cancer biology and immunology. The introduction of tyrosine kinase inhibitors has resulted in a significant prolongation of the survival rates of chronic myeloid leukemia patients but has not resulted in a cure. There is a growing conviction that this aim can be achieved through immunotherapy. For this concept to be successful, a considerable increase in the present understanding of chronic myeloid leukemia immunology is required. The authors attempt to review and evaluate the current findings that demonstrate a number of immunological aberrations in patients prior to the start of any therapy and their normalization after achieving remission. They also discuss the recent clinical trials with experimental therapeutic vaccines and then present their own strategy on how to address the problem.

Complexity and challenges in defining myeloid-derived suppressor cells

Study of myeloid cells endowed with suppressive activity is an active field of research which has particular importance in cancer, in view of the negative regulatory capacity of these cells to the host's immune response. The expansion of these cells, called myeloid-derived suppressor cells (MDSCs), has been documented in many models of tumor-bearing mice and in patients with tumors of various origin, and their presence is associated with disease progression and reduced survival. For this reason, monitoring this type of cell expansion is of clinical importance, and flow cytometry is the technique of choice for their identification. Over the years, it has been demonstrated that MDSCs comprise a group of immature myeloid cells belonging both to monocytic and granulocytic lineages, with several stages of differentiation; their occurrence depends on tumor-derived soluble factors, which guide their expansion and determine their block of differentiation. Because of their heterogeneous composition, accurate phenotyping of these cells requires a multicolor approach, so that the expansion of all MDSC subsets can be appreciated. This review article focuses on identifying MDSCs and discusses problems associated with phenotyping circulating and tumor-associated MDSCs in humans and in mouse models.

Myeloid-derived suppressor cells: their role in the pathophysiology of hematologic malignancies and potential as therapeutic targets

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells at various stages of differentiation/maturation that have a role in cancer induction and progression. They function as vasculogenic and immunosuppressive cells, utilizing multiple mechanisms to block both innate and adaptive anti-tumor immunity. Recently, their mechanism of action and clinical importance have been defined, and the cross-talk between myeloid cells and cancer cells has been shown to contribute to tumor induction, progression, metastasis and tolerance. In this review, we focus on the role of MDSCs in hematologic malignancies and the therapeutic approaches targeting MDSCs that are currently in clinical studies.

Myeloid-derived suppressor cells as therapeutic target in hematological malignancies

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of immature myeloid cells that accumulate during pathological conditions such as cancer and are associated with a poor clinical outcome. MDSC expansion hampers the host anti-tumor immune response by inhibition of T cell proliferation, cytokine secretion, and recruitment of regulatory T cells. In addition, MDSC exert non-immunological functions including the promotion of angiogenesis, tumor invasion, and metastasis. Recent years, MDSC are considered as a potential target in solid tumors and hematological malignancies to enhance the effects of currently used immune modulating agents. This review focuses on the characteristics, distribution, functions, cell–cell interactions, and targeting of MDSC in hematological malignancies including multiple myeloma, lymphoma, and leukemia.

Tumor-induced myeloid dysfunction and its implications for cancer immunotherapy

Immune function relies on an appropriate balance of the lymphoid and myeloid responses. In the case of neoplasia, this balance is readily perturbed by the dramatic expansion of immature or dysfunctional myeloid cells accompanied by a reciprocal decline in the quantity/quality of the lymphoid response. In this review, we seek to: (1) define the nature of the atypical myelopoiesis observed in cancer patients and the impact of this perturbation on clinical outcomes; (2) examine the potential mechanisms underlying these clinical manifestations; and (3) explore potential strategies to restore normal myeloid cell differentiation to improve activation of the host antitumor immune response. We posit that fundamental alterations in myeloid homeostasis triggered by the neoplastic process represent critical checkpoints that govern therapeutic efficacy, as well as offer novel cellular-based biomarkers for tracking changes in disease status or relapse.

New strategies in acute myelogenous leukemia: leukemogenesis and personalized medicine

Recent advances in molecular technology have unraveled the complexity of leukemogenesis and provided the opportunity to design more personalized and pathophysiology-targeted therapeutic strategies. Despite the use of intensive chemotherapy, relapse remains the most common cause for therapeutic failure in acute myelogenous leukemia (AML). The interactions between leukemia stem cells (LSC) and marrow microenvironment appear to be critical in promoting therapeutic resistance through progressive acquisition of genetic and epigenetic changes within leukemia cells and immune evasion, resulting in leukemia cell survival. With advances in genomic-sequencing efforts, epigenetic and phenotypic characterization, personalized therapeutic strategies aimed at critical leukemia survival mechanisms may be feasible in the near future. Here, we review select novel approaches to therapy of AML such as targeting LSC, altering leukemia/marrow microenvironment interactions, inhibiting DNA repair or cell-cycle checkpoints, and augmenting immune-based antileukemia activity.

Immunology and immunotherapy of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a clonal bone marrow stem cell neoplasia known to be responsive to immunotherapy. Despite the success of tyrosine kinase inhibitors (TKIs) targeting the BCR-ABL1 oncokinase, patients are not considered to be cured with the current therapy modalities. However, there have been recent advancements in understanding the immunobiology of the disease (such as tumor specific antigens and immunostimulatory agents), and this may lead to the development of novel, curative treatment strategies. Already there are promising results showing that a small proportion of CML patients are able to discontinue the therapy although they have a minimal amount of residual leukemia cells left. This implies that the immune system is able to restrain the tumor cell expansion. In this review, we aim to give a brief update of the novel aspects of the immune system in CML patients and of the developing strategies for controlling CML by the means of immunotherapy.

An autologous leukemia cell vaccine prevents murine acute leukemia relapse after cytarabine treatment

Acute leukemias with adverse prognostic features carry a high relapse rate without allogeneic stem cell transplantation (allo-SCT). Allo-SCT has a high morbidity and is precluded for many patients because of advanced age or comorbidities. Postremission therapies with reduced toxicities are urgently needed. The murine acute leukemia model C1498 was used to study the efficacy of an intravenously administered vaccine consisting of irradiated leukemia cells loaded with the natural killer T (NKT)-cell agonist α-galactosylceramide (α-GalCer). Prophylactically, the vaccine was highly effective at preventing leukemia development through the downstream activities of activated NKT cells, which were dependent on splenic langerin(+)CD8α(+) dendritic cells and which led to stimulation of antileukemia CD4(+) and CD8(+) T cells. However, hosts with established leukemia received no protective benefit from the vaccine, despite inducing NKT-cell activation. Established leukemia was associated with increases in regulatory T cells and myeloid-derived suppressor cells, and the leukemic cells themselves were highly suppressive in vitro. Although this suppressive environment impaired both effector arms of the immune response, CD4(+) T-cell responses were more severely affected. When cytarabine chemotherapy was administered prior to vaccination, all animals in remission posttherapy were protected against rechallenge with viable leukemia cells.

T-cell immune suppression in patients with hematologic malignancies: clinical implications

SUMMARY 

The reversion of immune suppression and restoration of T-cell function against leukemia remains a significant clinical challenge. However, the advent of improved antileukemia-specific T-cell induction and the generation of gene-modified T cells has extended cellular immunotherapy to hematological malignancies. Numerous immunotherapeutic protocols have been developed aiming to enhance antileukemia T-cell immune function, eliminate leukemic cells and prevent relapse. By contrast, abnormal expression of CTLA-4 and PD1/PD-L1 plays a critical role in effector T-cell responses and increases Treg suppressive activity in patients with tumors; therefore, blocking CTLA-4, PD1 and PD-L1 is a novel approach for immunotherapy.

Myeloid derived suppressor cells (MDSCs) are increased and exert immunosuppressive activity together with polymorphonuclear leukocytes (PMNs) in chronic myeloid leukemia patients

Tumor immune tolerance can derive from the recruitment of suppressor cell population, including myeloid derived suppressor cells (MDSCs), able to inhibit T cells activity. We identified a significantly expanded MDSCs population in chronic myeloid leukemia (CML) patients at diagnosis that decreased to normal levels after imatinib therapy. In addition, expression of arginase 1 (Arg1) that depletes microenvironment of arginine, an essential aminoacid for T cell function, resulted in an increase in patients at diagnosis. Purified CML CD11b+CD33+CD14-HLADR- cells markedly suppressed normal donor T cell proliferation in vitro. Comparing CML Gr-MDSCs to autologous polymorphonuclear leukocytes (PMNs) we observed a higher Arg1 expression and activity in PMNs, together with an inhibitory effect on T cells in vitro. Our data indicate that CML cells create an immuno-tolerant environment associated to MDSCs expansion with immunosuppressive capacity mediated by Arg1. In addition, we demonstrated for the first time also an immunosuppressive activity of CML PMNs, suggesting a strong potential immune escape mechanism created by CML cells, which control the anti-tumor reactive T cells. MDSCs should be monitored in imatinib discontinuation trials to understand their importance in relapsing patients.

A new hope in immunotherapy for malignant gliomas: adoptive T cell transfer therapy

Immunotherapy emerged as a promising therapeutic approach to highly incurable malignant gliomas due to tumor-specific cytotoxicity, minimal side effect, and a durable antitumor effect by memory T cells. But, antitumor activities of endogenously activated T cells induced by immunotherapy such as vaccination are not sufficient to control tumors because tumor-specific antigens may be self-antigens and tumors have immune evasion mechanisms to avoid immune surveillance system of host. Although recent clinical results from vaccine strategy for malignant gliomas are encouraging, these trials have some limitations, particularly their failure to expand tumor antigen-specific T cells reproducibly and effectively. An alternative strategy to overcome these limitations is adoptive T cell transfer therapy, in which tumor-specific T cells are expanded ex vivo rapidly and then transferred to patients. Moreover, enhanced biologic functions of T cells generated by genetic engineering and modified immunosuppressive microenvironment of host by homeostatic T cell expansion and/or elimination of immunosuppressive cells and molecules can induce more potent antitumor T cell responses and make this strategy hold promise in promoting a patient response for malignant glioma treatment. Here we will review the past and current progresses and discuss a new hope in adoptive T cell therapy for malignant gliomas.

Myeloid derived suppressor cells are numerically, functionally and phenotypically different in patients with multiple myeloma

Myeloid derived suppressor cells (MDSCs) are a heterogeneous population of cells that have been implicated as inhibitors of lymphopoiesis in patients with malignancies. They have a consensus phenotype of CD33+/CD11b+/HLA-DRlo/- and can be further divided into CD15 + granulocytic (G-MDSC) and CD14 + monocytic (M-MDSC) subsets. We characterized MDSCs in patients with multiple myeloma (MM) and found a significant increase in G-MDSCs in the blood of patients with progressive MM. Flow-sorted MDSCs from patients with MM induced the generation of regulatory T cells (Treg). MDSCs from both patients with MM and aged-matched controls demonstrated a dose-dependent inhibition of lymphocyte proliferation in carboxyfluorescein succinimidyl ester (CFSE)-tracking experiments. Granulocyte colony stimulating factor (G-CSF) administered to induce stem cell mobilization caused an increase in the number of MDSCs in the peripheral blood of patients with MM and a concentration of these immune-suppressive cells in peripheral blood stem cell collections. MDSCs are likely to cause immune dysfunction in patients with MM.

The targeting of immunosuppressive mechanisms in hematological malignancies

The adaptive immune system has the capacity to recognize and kill leukemic cells. However, immune tolerance mechanisms that normally protect healthy tissues from autoimmune effects prevent the development of effective antitumor immunity. Tumors use several different immunosuppressive mechanisms to evade otherwise effective T-cell responses. A growing number of immune evasion mechanisms have been characterized mainly in solid tumors. In hematological malignancies, less is known about how different immune escape mechanisms influence tumor immune evasion and the extent of their impact on ongoing immune responses. The present review highlights the potential role of three well-defined immunosuppressive mechanisms in hematological malignancies: (i) inhibitory T-cell pathways (especially programmed death ligand 1/programmed death 1 (PD-L1/PD-1)), (ii) regulatory immune cells, and (iii) metabolic enzymes such as indoeamine-2,3-dioxygenase (IDO). The possible therapeutic targeting of these pathways is also discussed. Exciting new strategies that might affect future antileukemia immunotherapy include monoclonal antibodies that block inhibitory T-cell pathways (PD-1/PD-L1) and the prevention of tryptophan depletion by IDO inhibitors. Furthermore, the clinical effect of several chemotherapeutic drugs may arise from the targeting of immunosuppressive cells. Evidence for a new feedback mechanism to suppress the function of regulatory immune cells was recently provided by the identification and characterization of spontaneous cytotoxic T lymphocyte (CTL) responses against regulatory immune cells. Such specific CTLs may be immensely useful in anticancer immunotherapy (for example, by anticancer vaccination). The targeting of one or more immunosuppressive pathways may be especially interesting in combination with antileukemic immunotherapy in cases in which immunosuppressive mechanisms antagonize the desired effects of the therapy.

The role of PD-1 and PD-L1 in T-cell immune suppression in patients with hematological malignancies

T-cell activation and dysfunction relies on direct and modulated receptors. Based on their functional outcome, co-signaling molecules can be divided as co-stimulators and co-inhibitors, which positively and negatively control the priming, growth, differentiation and functional maturation of a T-cell response. We are beginning to understand the power of co-inhibitors in the context of lymphocyte homeostasis and the pathogenesis of leukemia, which involves several newly described co-inhibitory pathways, including the programmed death-1 (PD-1) and PD-1 ligand (PD-L1) pathway. The aim of this review is to summarize the PD-1 and PD-L1 biological functions and their alterative expression in hematological malignancies. The role of PD-1 and PD-L1 in T-cell immune suppression and the potential for immunotherapy via blocking PD-1 and PD-L1 in hematological malignancies are also reviewed.

Targeting the acute myeloid leukemic stem cell compartment by enhancing tumor cell-based vaccines

Harvesting the potential of the immune system in order to eradicate (residual) acute myeloid leukemia (AML) cells is the long pursued goal of immunotherapy in AML. Strategies using apoptotic tumor cell vaccines have been explored for many years, without significant clinical improvements. In recent years insight has been gained into the mechanisms activating and interfering with tumor-directed immunity. With the arrival of novel immune-modulating agents allowing for the interference with regulatory molecules and interaction with immune-propelling mechanisms, new doors are opening for increasing vaccination efficacy. Combined with advances in the design of apoptotic tumor-based vaccines, we are on the verge of creating an effective AML vaccine strategy, offering a much needed novel therapeutic option for this devastating disease.