Leukemia (2014) 28, 1784–1792 & 2014 Macmillan Publishers Limited All rights reserved 0887-6924/14 www.nature.com/leu

REVIEW

The targeting of immunosuppressive mechanisms in hematological malignancies MH Andersen 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. Leukemia (2014) 28, 1784–1792; doi:10.1038/leu.2014.108

INTRODUCTION Leukemia and lymphoma remain difficult conditions to treat. Modulating the immune system may improve the survival of leukemia patients, as the immune system is evidently capable of recognizing and attacking leukemic cells.1 The best evidence for the potential effect of immunotherapy against leukemia lies in the often sustained remissions associated with hematopoietic stem cell transplantation (HSCT) in hematological malignancies.2 Although initially believed to be a method of bone marrow rescue after high-dose chemotherapy, it is now clear that HSCT promotes a graft-versus-leukemia response, which can be further enhanced with donor lymphocyte infusion regardless of leukemic relapse after the transplantation. HSCT is currently the best therapeutic option for most hematological malignancies, although it remains a dangerous procedure with many possible complications. In addition, HSCT is unfeasible for many leukemia patients because of stringent major histocompatibility complex requirements. Thus, cancer immunologists have sought additional approaches to stimulate leukemia-specific cytotoxic T lymphocytes (CTLs) that recognize human leukocyte antigenrestricted antigens on the surface of leukemia cells in order to engage the adoptive immune system (for example, vaccination3,4 or the adoptive transfer of in vitro expanded T cells5). Chimeric T-cell antigen receptors have recently been explored for adoptive T-cell therapy. In this approach, polyclonal T cells can be redirected toward cancer cells that express defined antigens by

the transfer of genes encoding those target antigen-specific receptors. A successful example has been the use of genetically engineered T cells redirected toward B-cell lineage antigen CD19, which have demonstrated impressive clinical results in patients with chronic lymphocytic leukemia (CLL).6 Unfortunately, immune tolerance mechanisms that otherwise protect healthy tissues from autoimmune reactions are major obstacles for the development of effective anticancer immunity. In fact, some of the mechanisms that prevent autoimmunity are hijacked by cancers to attain immune escape. This evasion of immune destruction is based on several mechanisms, including the degradation of metabolically important amino acids; the expansion of regulatory immune cells, such as regulatory T cells (Tregs), myeloid-derived suppressor cells (MDSCs) and macrophages; the production of immunosuppressive cytokines; and the expression of negative co-stimulatory ligands such as programmed death ligand 1 (PD-L1) on cancer cells (Figure 1). In addition, poor co-stimulation by tumor cells induces T-cell anergy in leukemia-specific T cells. A detailed understanding of the factors responsible for this evasion of immune destruction in hematopoietic malignancies, which becomes more pronounced with disease progression, is indispensable for the development of novel immunotherapeutic treatment modalities in leukemia and lymphoma.7 The current review focuses on three well-defined immunosuppressive mechanisms: (i) co-inhibitory T-cell pathways such as PD-L1/ programmed death 1 (PD-1); (ii) regulatory immune cells; and

Department of Hematology, Center for Cancer Immune Therapy (CCIT), Copenhagen University Hospital, Herlev, Denmark. Correspondence: Professor MH Andersen, Department of Hematology, Center for Cancer Immune Therapy (CCIT), Copenhagen University Hospital, Herlev DK-2730, Denmark. E-mail: [email protected] Received 30 January 2014; revised 20 February 2014; accepted 13 March 2014; accepted article preview online 18 March 2014; advance online publication, 1 April 2014

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Figure 1. Leukemic cells can suppress immunity by contact-dependent and contact-independent mechanisms. Leukemia-specific CTL has the capacity to recognize and kill leukemia cells (red arrow). However, leukemic cells use several different immunosuppressive mechanisms to evade otherwise effective T-cell responses (black bar-headed lines). This immunosuppression is not only generated by leukemic cells themselves but in addition by the expansion and attraction of regulatory immune cells, that is, Tregs, MDSCs and tumor-associated DC. Expression of negative co-stimulatory ligands, increased levels of immunosuppressive cytokines and increased IDO expression that leads to increased depletion of the essential amino acid tryptophan are some of the important mechanisms by which leukemia cells evade from immune surveillance.

(iii) metabolic enzymes such as indoeamine-2,3-dioxygenase (IDO). Although highly relevant in many leukemic settings, these mechanisms have mainly been studied in solid tumors, where they are the targets for several different immunotherapeutic interventions. In contrast to solid tumors, leukemia is a systemic disease from its onset. Subsequently, it appears likely that the mechanisms that regulate immune activation versus tolerance in leukemia patients differ somewhat from those in patients with solid cancers. However, as discussed in this review, the abovementioned pathways may indeed influence the pathogenesis of hematological malignancies; consequently, their targeting (alone or in combination) may be highly favorable in therapeutic interventions against hematological malignancies. THE TARGETING OF INHIBITORY T-CELL PATHWAYS Although leukemic cells express antigens that are recognizable by leukemia-specific CTL, antigen presentation alone is not enough to initiate an effective immune response. The T-cell receptor costimulatory pathways are centrally involved in maintaining homeostasis of the immune system by regulating T-cell activation. The rearrangement of cell components to form a distinctive immunological synapse emerges when immune cells polarize in response to the recognition of an antigen presented in the context of an human leukocyte antigen molecule. The CD28 family of receptors are key elements of the immunological synapse that include several members including CD28, CTLA-4 (CTL antigen 4), & 2014 Macmillan Publishers Limited

ICOS and PD-1, that, upon interaction with their corresponding ligands, are able to generate potent co-stimulatory or inhibitory signals in T cells. The negative receptors limit T-cell-mediated damage to self-tissues by inhibiting T-cell responses. Importantly, tumor cells can engage these pathways within T cells by expressing ligands for the respective inhibitory receptor on their surface. The cell surface molecules CD28 and CTLA-4 provide positive (CD28) and negative (CTLA-4) signals in the early stages of an immune response.8 CD28 binds its ligands B7-1 (CD80) and B7-2 (CD86) on the surface of the antigen-presenting cell (APC), thereby facilitating and maintaining a T-cell response at least partly through increased cytokine expression. Quite the opposite, CTLA-4 basically arrests T-cell activation by triggering an inhibitory signal within the T cell.8 Thus, CTLA-4 is a key inhibitory receptor that critically affects peripheral T-cell tolerance and T-cell function.8 Two models of CTLA4 regulation of T-cell expansion have been proposed: the threshold model and the attenuation model. The threshold model proposes that CTLA-4 sets a threshold of activation above the background ‘noise’ of T-cell receptor signals. The attenuation model suggests that CTLA-4 limits the capacity of a T cell to divide after initiation of activation.9 In fact, both models may be at work, depending on the strength of the T-cell receptor signal. It has been described that blocking of CTLA-4 by means of monoclonal antibodies can shift the immune system balance toward T-cell activation, resulting in rejection of tumors by the host. Hence, the anti-CTLA-4 blocking antibody ipilimumab Leukemia (2014) 1784 – 1792

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1786 (Yervoy, Bristol–Myers Squibb) was recently approved by the Food and Drug Administration for the treatment of melanoma after demonstrating efficacy in clinical phase III studies (Figure 2). Data supporting a significant negative regulatory role for CTLA-4 in leukemia is still limited, although CTLA-4 polymorphisms have been described to influence the incidence of relapse of acute myeloid leukemia (AML) after complete remission. Therefore, CTLA-4 might participate in the control of minimal residual disease by the immune system.10 Furthermore, CTLA-4 blockade by monoclonal antibodies has been shown to enhance T-cell responses against AML.11 Ipilimumab was examined in a study of 29 patients with different recurrent or progressive hematological malignancies after allogeneic HSCT. Ipilimumab did not appear to induce graft-versus-host disease (GVHD) or graft rejection in the patients. Complete remission was observed in two patients with Hodgkin lymphoma, and partial remission was observed in a patient with refractory mantle cell lymphoma.12 Clinical trials are currently evaluating the safety and effect of ipilimumab in AML patients in relapse (NCT01757639) and after allogeneic HSCT (NCT01822509 and NCT00060372). Regarding the latter, Fevery et al.13 recently showed in a model system that blockading CTLA-4 induces a host-derived antileukemic effect without GVHD. PD-1 is another important regulatory surface molecule that delivers inhibitory signals to maintain T-cell functional silence against their cognate antigens. Its ligands, known as PD-L1 (B7-H1) and PD-L2 (B7-H2), are expressed on APCs, placental cells and nonhematopoietic cells found in an inflammatory microenvironment. In general, interactions between PD-1 on T cells and the ligand PD-L1 (B7-H1) control the induction and maintenance of peripheral T-cell tolerance during normal immune responses.14 The interaction

between PD-1 and PD-L1 has been reported to negatively regulate the proliferation and cytokine production of T cells. PD-L1 expression has been reported on many different tumor cells; accordingly, cancers use it to evade the host immune system.15–23 PD-L1 expression has been correlated with poor prognosis in cancer.24 PD-L1 expression on tumor cells has been further correlated with increased tumor aggressiveness and increased risk of death for a number of solid cancers, including ovarian cancer and pancreatic cancer.25,26 In addition, surface expression of PD-L1 on cancer cells has been described in several hematological cancers.18–23 In this regard, PD-L1 is expressed on both malignant cells and infiltrating immune cells in subsets of aggressive B-cell lymphomas.27 The upregulation of PD-L1 expression on myeloma cells reportedly induces T-cell apoptosis and anergy of tumor-specific T cells, and enhances aggressive myeloma cell characteristics.22 In addition, Christiansson et al.28 have reported increased PD-1 expression on T cells among peripheral blood mononuclear cells from patients with leukemia in comparison with healthy donors. Thus, upregulation of both PD-1 and PD-L1 might contribute to immunosuppression in leukemia patients, leading to T-cell immunodeficiency as well as lower proliferation and activation. The PD-1/PD-L1 pathway has also been implicated in tumor escape by the induction of CTL exhaustion during chronic human T-cell leukemia virus-1 infection in adult T-cell leukemia/lymphoma (ATLL).29 The importance of PD-1 and PD-L1 has recently been highlighted because the blockade of either PD-1 or PD-L1 results in outstanding clinical responses (Figure 2). A multicenter phase 1 trial reported that the antibody-mediated blockade of PD-L1 induces durable tumor regression and prolonged stabilization in

Figure 2. Immune checkpoint blockade as an immunotherapeutic modality. T-cell receptor binding to an human leukocyte antigen/peptide complex is a central event in CTL-mediated elimination of leukemia cells as it induces an activation signal within the CTL. Immune checkpoints refer to a number of inhibitory pathways that are crucial for maintaining self-tolerance and modulating the duration and amplitude of immune responses in peripheral tissues in order to minimize collateral tissue damage. It is well described that tumors use different immune checkpoint pathways as a major mechanism of immune resistance against leukemia-specific CTL. CTLA-4 is upregulated after antigen-specific activation of a naive or memory T cell in lymphatic tissue, leading to decreased effector function (early activation phase). PD-1 is mainly expressed on antigenexperienced memory T cells in peripheral tissues (effector phase). Leukemia cells use this regulatory mechanism to evade a leukemia-directed CTL response by upregulating PD-L1, which is the ligand for PD-1, on its surface. Blocking monoclonal antibodies (orange squares) directed at the inhibitory immune receptors CTLA-4, PD-1 and PD-L1 have all emerged as promising novel treatment approaches for cancer patients. Leukemia (2014) 1784 – 1792

& 2014 Macmillan Publishers Limited

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1787 patients with different solid cancers.30 Another study showed similar objective clinical responses in different cancer patients treated with anti-PD-1 blocking antibodies.31 A phase I clinical trial that focused on patients with different hematopoietic malignancies (AML, CLL, non-Hodgkin lymphoma, Hodgkin lymphoma or multiple myeloma) also recently showed that blockade of PD-1 might have an impact on hematological malignancies.32 The treatment was well tolerated and clinical benefit was observed in approximately one-third of patients. PD-1 blockade is currently being investigated in AML patients in combination with a cancer vaccine (NCT01096602). We recently reported that the immune system itself appears to have established a counteractive mechanism such as PD-L1-specific CTLs.33,34 PD-L1-reactive CTLs are readily isolated from the peripheral blood of cancer patients and to a lesser extent from the blood of healthy donors. PD-L1-specific CTLs recognized and killed cutaneous T-cell lymphoma (CTCL) cells. Furthermore, PD-L1-specific CTLs were able to kill non-malignant PD-L1expressing dendritic cells (DCs) in a PD-L1-dependent manner. Our observations illustrate that the immune system is able to react toward immunosuppressive mechanisms of cancerous cells. Inducing or boosting CTL responses against PD-L1 might thus be an intriguing strategy to treat different hematopoietic malignancies that express PD-L1 to evade the host immune system. Several widespread viruses may be involved in the pathogenesis of certain hematological malignancies (for example, cytomegalovirus (CMV) in CLL35 and Epstein-Barr virus in different malignant lymphomas).36 Because the PD-L1/PD-1 pathway is important to the regulation of both viral and anticancer CTL responses, we considered the possibility of using PD-L1-specific CTLs to influence antiviral (for example, EpsteinBarr virus-specific or CMV-specific) immunity. Indeed, the addition of PD-L1-specific CTLs 1 week after virus stimulation resulted in an immense increase in the number of virus-specific CD8 þ T cells in culture; in three independent experiments, the number of virus-specific CD8 þ T cells increased by 0.5- to 3-fold after only 2 weeks of culture.37 Hence, PD-L1-specific CTLs may effectively boost the effector phase of the immune response by removing PD-L1-expressing regulatory immune cells that inhibit PD-1positive effector T cells. If this finding were translated to the clinic, vaccination against PD-L1 might be a way to strengthen antiviral responses (for example, in ATLL or CLL patients). Reactivation of exhausted T cells by PD-L1 vaccination might thus be an effective strategy to treat virus-related malignancies. In fact, it might be useful to harness PD-L1-specific CTL in antileukemia immunotherapy in general. It should be noted that the function and effect of PD-L1-specific CTLs might vary depending on the microenvironment and the state of the immune response. Therefore, the major role of the PD-1 pathway is believed to be the regulation of effector T-cell responses in order to control tissue damage, rather than at the initial T-cell activation stage.38 Accordingly, the presence of PD-L1specific CTLs during the activation phase of an immune response may not increase this response. In fact, the addition of PD-L1specific CTLs simultaneously with virus antigen stimulation slightly decreased the numbers of viral-specific T cells,37 possibly because of the expression of PD-L1 on APCs or resting T cells. Finally, it should be mentioned that other co-inhibitory receptors, such as TIM-3 and LAG-3, are emerging as potential cancer targets.38 Combinatorial targeting of negative regulatory receptors and their ligands is an interesting strategy to further improve antitumor immunity. TARGETING OF REGULATORY IMMUNE CELLS Tregs are an important component in the complex network of cells responsible for self-tolerance and immune homeostasis. & 2014 Macmillan Publishers Limited

Tregs function to downregulate immune responses in a variety of inflammatory conditions and secure peripheral T-cell tolerance.39 The most widely used and reliable Treg marker is the transcription factor FoxP3. The best-characterized subset of Tregs is a subpopulation of CD4 þ T cells that feature intracellular FoxP3 expression, high amounts of aCD25 (interleukin (IL)-2 receptor a-chain) on the cell surface, and low CD127 expression.40,41 However, many different subsets of Tregs have been characterized, and several other molecules have been used as Treg markers, including the glucocorticoid-induced tumor necrosis factor receptor family-related gene (GITR) and CTLA-4.42 Tregs suppress effector T-cell activity in many different ways, including the production of immunosuppressive cytokines such as IL-10, Transforming growth factor-b and IL-35; the expression of inhibitory molecules, such as CTLA-4, on the cell surface; the induction of anti-inflammatory biochemical signal pathways in APCs and effector T cells; the consumption of pro-inflammatory cytokines, especially IL-2; and the direct or indirect killing of effector T cells and/or APCs.43–45 Deficiency and/or altered function of Tregs have been described in several autoimmune disorders. In solid tumors, malignant cells engage Tregs and thereby inhibit antitumor immunity in the tumor microenvironment, limiting the efficiency of immune surveillance and anticancer immunotherapy. In fact, a correlation between tumorinfiltrating Tregs and poor prognosis has been described.46 Fewer investigations have focused on Tregs in hematological malignancies. However, Tregs have been recognized as a contributing factor to the evasion of immune surveillance that may be recruited and exploited by leukemic cells. Studies have shown that the frequencies of marrow and blood Tregs are higher in patients with AML than in controls.47 Furthermore, Tregs from AML patients are more suppressive than those from healthy controls. Similarly, enhanced frequencies of Tregs have been reported in B-cell CLL.48 Furthermore, FoxP3 has been described as a selective marker for a subset of ATLL. It should be noted that despite the well-described ability of Tregs to suppress antitumor immunity, some studies have shown that high numbers of tumor-infiltrating FoxP3-postive improved overall survival in, for example, follicular lymphoma.49 Even though this might be due to an antitumor effector phenotype of the FoxP3-positive T cells, it underlines that there might be potentially severe adverse effects of manipulating Tregs. Several therapeutic strategies for cancer involve the depletion or modulation of Tregs, even though agents that specifically target Tregs are currently unavailable. Certain chemotherapeutic agents have appeared as clinically feasible agents that can suppress Tregs and allow more effective induction of antitumor immune responses (Figure 3).50 Thus, the administration of metronomic cyclophosphamide in advanced cancer patients has been shown to induce the reduction of circulating Tregs.51 Similarly, thalidomide both induces apoptosis of CLL cells and reduces the number of Tregs in patients.52,53 Fludarabine reportedly reduces the frequency and suppressive function of Tregs in CLL patients.51 In different animal models, anti-CD25 depleting antibodies may improve antitumor immunity by the targeting of Tregs (Figure 3). Hence, the depletion of Tregs before tumor cell inoculation reportedly leads to efficient rejection of tumor cells.54 Clinical trials that use a CD25-directed denileukin diftitox (Ontak) to eliminate Tregs have been performed in patients with renal cell carcinoma or melanoma.55 However, although the initial results from the denileukin diftitox studies were promising, the full potential of targeting Tregs was likely limited by unwanted effects on tumorreactive, CD25-positive effector T cells.56 Interestingly, in a murine model, denileukin diftitox reduced Tregs and improved the efficacy of adoptive cytotoxic T-cell immunotherapy in murine AML.57 This study demonstrated that Tregs present at AML disease sites suppress adoptively transferred CTLs. Depleting of Tregs Leukemia (2014) 1784 – 1792

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Figure 3. Examples of therapeutic interventions being investigated that target immune inhibitory pathways. Modulating the immune system may improve survival in patients suffering from hematological malignancies. Unfortunately, immunosuppressive mechanisms that normally protect healthy tissues from autoimmune reactions are major obstacles for the development of effective anticancer immunity. The targeting of such immunosuppressive mechanisms by small molecule inhibitors of IDO by the depletion of Tregs by CD25-targeting agents (for example, anti-CD25 antibodies or denileukin) as well as by different chemotherapeutic drugs (for example, cyclophosphamide, thalidomide, fludarabine and 5-fluorouracil) are all being investigated clinically (orange squares).

before CTL transfer resulted in therapeutic efficacy in experiments where antileukemic CTL infusion alone was ineffective. CTCL is the most frequent primary lymphoma in the skin and is believed to involve malignant proliferation of Tregs. In Sezary syndrome, a leukemic variant of CTCL, malignant T cells often express CD25 and FoxP3 and may function as Tregs. Using denileukin diftitox to target malignant T cells has provided promising results in a fraction of CTCL patients, although only B30% of patients with advanced disease experience a sustained response to the drug.58 FoxP3-specific CTL might provide an unconventional means of targeting FoxP3 þ Tregs (Figure 4). Gilboa and colleagues59 showed that vaccination of mice against FoxP3 resulted in a FoxP3-specific CTL response that led to the elimination of FoxP3 þ Tregs and enhanced antitumor immunity. A similar FoxP3-specific CTL response exaggerated atherosclerotic lesion formation in an atherosclerosis model by significantly decreasing the number of FoxP3 þ Tregs.60 We recently reported that natural CD8 reactivity toward FoxP3 also exists in humans.61 FoxP3-specific CTLs not only recognized Tregs, but also killed malignant T cells that expressed high levels of FoxP3, suggesting that vaccination with FoxP3 might be useful for lymphoma patients with FoxP3 þ malignant T cells. MDSC is another regulatory immune cell type that is reportedly involved in immunosuppression in cancer patients. MDSCs are a heterogeneous population of immature cells derived from monocytes or granulocytes. Many different phenotypes have been described in humans, including CD11b þ CD14-CD33 þ cells. MDSCs suppress effector T cell and natural killer cell activation, proliferation, and cytotoxicity, and also induce the differentiation and expansion of Tregs. Although much effort has been put into establishing an important role for MDSCs in solid cancers, our knowledge regarding their role in suppressing antileukemic T cells remains limited. However, MDSC expansion has been reported in animal AML models.62 Furthermore, MDSC numbers are increased in CML patients.28 Hence, therapeutic targeting of MDSCs might Leukemia (2014) 1784 – 1792

very well benefit patients with hematological malignancies. In this regard, 5-fluorouracil has been reported to selectively kill tumor-associated MDSCs in the EL4 lymphoma model, resulting in enhanced T-cell-dependent antitumor immunity.63 This finding is another example of the effects that chemotherapeutic agents might have on immunosuppression (Figure 3). TARGETING OF THE METABOLIC ENZYMES An altered tumor metabolism results in the depletion of essential nutrients and may lead to the accumulation of immunosuppressive metabolites. An important enzyme in this regard is IDO. IDO degrades the essential amino acid tryptophan. IDO expression is critical in limiting potentially exaggerated inflammatory reactions in response to danger signals64 and in assisting the function of Tregs. Effector T cells starved of tryptophan are unable to proliferate.65 The immunosuppressive effect of IDO is mediated through local tryptophan depletion, as well as direct immunosuppressive tryptophan metabolites.66 Regulation of tryptophan metabolism by IDO in DCs is a highly flexible modulator of immunity. When IDO þ DCs are injected in vivo, they create suppression and anergy in antigen-specific T cells in the lymph node draining the injection site. Another effect of IDO is mediated through the enhancement of local Treg-mediated immunosuppression. Constitutive IDO expression in DCs imparts T cells with regulatory properties.67 The expression of IDO is extensively upregulated in patients with cancer. Most studies have been performed in patients with solid tumors, and elevated IDO expression has been described in DCs in tumor-draining lymph nodes.67 Such IDO-expressing DCs isolated from tumor-draining lymph nodes mediated profound immunosuppression and T-cell anergy in vivo. It has been described that it is the stimulation of DCs in tumor-draining lymph nodes by Tregs that induces the IDO expression. The induction of IDO converts the tumor-draining lymph node from an immunizing to a tolerizing milieu. Furthermore, IDO is commonly & 2014 Macmillan Publishers Limited

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Figure 4. The targeting of immunosuppressive cells by vaccination. Antigen-specific CTL recognizing normal self proteins expressed in regulatory immune cells such as PD-L1, IDO and FoxP3 are able to eliminate regulatory immune cells as well as leukemia cells (red arrows). Hence, the boosting of specific CTL by vaccination may directly modulate immune regulation and potentially altering tolerance to leukemic antigens. Because immunosuppressive cells might antagonize the desired effects of cancer vaccines, the addition of antigens such as PD-L1, IDO and FoxP3 would consequently be easily implementable and highly synergistic. The induction of specific CTL represents a novel immunotherapeutic approach, in which the specific depletion of target cells is not limited to targeting proteins that are expressed on the cell surface.

expressed within solid tumors, where it inhibits the effector phase of immune responses.68 However, IDO expression and activity are also increased in many hematological malignancies. Hence, the expression of IDO has been reported in blasts of AML patients,69 where it directly convert CD25  cells into CD25 þ Tregs.70 Furthermore, IDO expression in blasts from AML patients has been correlated with significantly shortened overall and relapse-free survival.71 IDO is also highly expressed in both ATLL72 and CLL.73 Because of its immunosuppressive effects, IDO has become a very appealing target for the design of new anticancer drugs. Several IDO inhibitors are also being investigated74 (Figure 3). In particular, the compound 1-methyl-tryptophan has been widely studied as an inhibitor of IDO activity.75 An IDO inhibitor is also currently being examined in a clinical phase II trial in patients with myelodysplastic syndrome, a disease that can develop into AML (NCT01822691). Despite this progress, some challenges remain regarding IDO-targeting therapy; so far, IDO inhibitors exert only minor effects on the enzymatic and cellular activities of IDO.76 Thus, the current IDO inhibitors might not be optimal candidates for clinical application, and additional therapeutic strategies targeting IDO might be more beneficial. We have recently described spontaneous CD8 þ and CD4 þ T-cell reactivity against IDO in the tumor microenvironment and the peripheral blood of cancer patients.77 IDO-reactive CD8 þ T cells were specific CTLs that were able to recognize and kill IDO-expressing cells. Importantly, AML blasts enriched directly from patients were killed by IDO-specific T cells ex vivo. Therefore, IDO might also be an appealing anticancer vaccine target (Figure 4). This possibility was first examined recently in a clinical phase I trial that demonstrated long-lasting disease stabilization, as well as a partial response to liver metastasis in metastatic lung cancer patients vaccinated with an IDO-derived peptide in the absence of toxicity (NCT01219348).78 IDO expression is induced in monocytes when infected with CMV. This might confer an advantage for the virus by enabling CMV-infected monocytes to escape T-cell responses. Intriguingly, & 2014 Macmillan Publishers Limited

IDO-specific T cells were found to boost immunity against CMV antigens by eradicating IDO-expressing cells and changing the regulatory microenvironment.77 Furthermore, the presence of IDO-specific T-cell responses in the periphery has been described to correlate with the presence of CMV-specific T-cell responses.49 Importantly, as described above, CMV is not only an infectious agent but also may be a central antigen for the B-cell clone in CLL.35 Depletion of antiviral T cells is a major side effect of many different agents used to treat CLL. Therefore, boosting IDO-specific T-cell responses might be a way to strengthen antivirus-specific responses in CLL patients. In fact, IDO vaccination may in general be synergistic with additional immunotherapy. It should also be noted that IDO was recently described as a crucial factor in the MDSC-mediated suppression of antitumor immune responses.68,79 Therefore, the therapeutic targeting of IDO might suppress MDSCs, which (as discussed above) could be highly valuable in hematological malignancies. This possibility is emphasized by the finding that IDO-positive MDSCs expand in patients after allergenic HSCT.80 IDO inhibits T-cell responses by depleting tryptophan and producing kynurenine. Tumors may also suppress immunity through other enzymatic mechanisms. Another way that tumor cells suppress T cells is by manipulating the metabolism of L-arginine through the enzymes nitric oxide synthase (NOS) and arginase. Many tumors exhibit the increased expression of arginase and inducible NOS (iNOS), which leads to the depletion of arginine from the tumor microenvironment.81 Several studies have emphasized the importance of this altered tumor arginine metabolism for the suppression of tumor-specific T-cell responses. It was recently shown that AML blasts have an arginase-dependent ability to inhibit T-cell proliferation and hematopoietic stem cells. Furthermore, arginase and iNOS inhibitors reduce the suppressive activity of AML.82 iNOS leads to increased production of nitric oxide, thereby promoting angiogenesis, metastasis, and immunosuppression in tumors. In addition to malignant cells, macrophages, granulocytes, and MDSCs can also suppress immunity by expressing arginase and NOS. Leukemia (2014) 1784 – 1792

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1790 CONCLUSIONS AND PERSPECTIVES The immune system is clearly capable of recognizing and killing malignant cells in patients with hematological malignancies. However, the presence of several different immunosuppressive mechanisms results in vast immune dysregulation, which causes the eventual escape from natural immune control (Figure 1). Regulatory feedback mechanisms, such as the upregulation of PD-L1 or IDO, are essential to limit the intensity and extent of immune responses, which might otherwise cause dangerous harm to the host. However, this immune evasion is harmful in the context of cancer immunotherapy. Thus, the targeting of one or more immunosuppressive pathways may be highly useful in combination with antileukemic immunotherapy in which immunosuppressive mechanisms antagonize the required effects. By definition, every successful immune therapy aims to initiate immunological activation. It is also appealing to speculate that the targeting of immunosuppressive pathways could be used to stimulate a graft-versus-leukemia effect after allogeneic HSCT. However, owing to the non-antigen-specific T-cell stimulatory capabilities in this setting, this approach might also amplify GVHD. In the current review, several different potential therapeutic strategies with which to target immunosuppression in hematological cancers have been discussed. First of all, blocking inhibitory pathways (such as the PD-1/PD-L1 co-stimulatory pathway) is an attractive strategy (Figure 2). Recent clinical trials have suggested that PD-1/PD-L1 inhibition has a significant therapeutic impact on a variety of solid tumors. PD-L1 is expressed at particularly high levels in AML, multiple myeloma, and different B-cell lymphomas; consequently, monoclonal antibodies that block the PD-1/PD-L1 pathway might be beneficial in these settings. The use of chemotherapy in combination with immunotherapy has been controversial. However, accumulated data point to the possible advantages of combining these two treatments. There is convincing preclinical and accumulating clinical evidence in support of the notion that successful antineoplastic therapies reinstate immune surveillance (Figure 3). To this end, chemotherapy can have various ancillary effects on the immune system. Some chemotherapeutic drugs lead to an immunogenic death of cancer cells. Furthermore, the state of lymphopenia after high-dose chemotherapy seems to provide a phase of enhanced responsiveness to immunotherapy.83 Hence, certain chemotherapeutic drugs may assist in breaking immune tolerance by preferentially depleting immunoregulatory cell subsets.51 For example, cyclophosphamide not only induces immunological cell death of malignant cells, but also stimulates the differentiation of Th17 cells and weakens the immunosuppressive effects of Tregs. Nonetheless, chemotherapy alone is insufficient to break tolerance to tumors, probably because the original tolerogenic mechanisms rapidly restore tolerance after each cycle of chemotherapy. In principle, the blocking of IDO could therefore have a synergistic effect with chemotherapy by preventing the reacquisition of tolerance. This hypothesis is supported by data showing that the IDO blocker 1-methyl-tryptophan works synergistically with different chemotherapy drugs in mouse models.84 As discussed in this review, several IDO-blocking drugs are in clinical development, and may be included in treatments involving cancer vaccines or chemotherapy (Figure 3). We have suggested the use of specific CTLs as yet another approach to target immunosuppression (Figure 4). In this review, naturally occurring, specific T cells that recognize several different antigens expressed in both immunosuppressive cells and malignant cells have been described. Taken together, these data suggest that natural T-cell responses against epitopes derived from proteins expressed in regulatory immune cells are quite common, and that such T cells might be a useful tool for the treatment of hematopoietic malignancies. Thus, the boosting of specific T cells may directly modulate immune regulation and alter Leukemia (2014) 1784 – 1792

tolerance to tumor antigens. Because immunosuppressive cells might antagonize the desired effects of cancer vaccines, the addition of such antigens would consequently be easily implementable and highly synergistic. Furthermore, the loss of expression of antigens such as IDO, PD-L1, or FoxP3 during vaccination therapy as a means of immune escape might save target cells from immune-mediated destruction by vaccineinduced CTLs. However, this should lead to the removal of local immunosuppression thereby enabling circulating effector cells to function or to become activated. The induction of specific T cells represents a new and attractive immunotherapeutic approach, in which the specific depletion of target cells is not limited to targeting proteins that are expressed on the cell surface. An additional principal difference between therapeutically induced T cells and surface blockade by antibodies is that the former reduces not only the target protein-mediated immunosuppression but also other immunosuppressive effects mediated by the target cells. CONFLICT OF INTEREST The authors declare no conflict of interest.

ACKNOWLEDGEMENTS I would like to thank Charlotte Hald Andersen for her assistance with the preparation of the figures. This work was supported by the Danish Cancer Society, the Danish Council for Independent Research and Herlev Hospital.

DISCLAIMER It should be noted, however, that MHA is an author and co-author of two filed patent applications based on the use of PD-L1 or IDO for vaccination. The rights of the patent applications have been transferred to Copenhagen University Hospital, Herlev according to Danish Law of Public Inventions at Public Research Institutions.

REFERENCES 1 Heine A, Held SA, Bringmann A, Holderried TA, Brossart P. Immunomodulatory effects of anti-angiogenic drugs. Leukemia 2011; 25: 899–905. 2 Clarkson B, Strife A, Wisniewski D, Lambek CL, Liu C. Chronic myelogenous leukemia as a paradigm of early cancer and possible curative strategies. Leukemia 2003; 17: 1211–1262. 3 Hus I, Schmitt M, Tabarkiewicz J, Radej S, Wojas K, Bojarska-Junak A et al. Vaccination of B-CLL patients with autologous dendritic cells can change the frequency of leukemia antigen-specific CD8 þ T cells as well as CD4 þ CD25 þ FoxP3 þ regulatory T cells toward an antileukemia response. Leukemia 2008; 22: 1007–1017. 4 Giannopoulos K, Dmoszynska A, Kowal M, Rolinski J, Gostick E, Price DA et al. Peptide vaccination elicits leukemia-associated antigen-specific cytotoxic CD8 þ T-cell responses in patients with chronic lymphocytic leukemia. Leukemia 2010; 24: 798–805. 5 Fabricius D, Breckerbohm L, Vollmer A, Queudeville M, Eckhoff SM, Fulda S et al. Acute lymphoblastic leukemia cells treated with CpG oligodeoxynucleotides, IL-4 and CD40 ligand facilitate enhanced anti-leukemic CTL responses. Leukemia 2011; 25: 1111–1121. 6 Brentjens RJ, Riviere I, Park JH, Davila ML, Wang X, Stefanski J et al. Safety and persistence of adoptively transferred autologous CD19-targeted T cells in patients with relapsed or chemotherapy refractory B-cell leukemias. Blood 2011; 118: 4817–4828. 7 Herreros B, Sanchez-Aguilera A, Piris MA. Lymphoma microenvironment: culprit or innocent? Leukemia 2008; 22: 49–58. 8 Fife BT, Bluestone JA. Control of peripheral T-cell tolerance and autoimmunity via the CTLA-4 and PD-1 pathways. Immunol Rev 2008; 224: 166–182. 9 Egen JG, Kuhns MS, Allison JP. CTLA-4: new insights into its biological function and use in tumor immunotherapy. Nat Immunol 2002; 3: 611–618. 10 Perez-Garcia A, Brunet S, Berlanga JJ, Tormo M, Nomdedeu J, Guardia R et al. CTLA-4 genotype and relapse incidence in patients with acute myeloid leukemia in first complete remission after induction chemotherapy. Leukemia 2009; 23: 486–491.

& 2014 Macmillan Publishers Limited

Immunosuppressive mechanisms in leukemia MH Andersen

1791 11 Zhong RK, Loken M, Lane TA, Ball ED. CTLA-4 blockade by a human MAb enhances the capacity of AML-derived DC to induce T-cell responses against AML cells in an autologous culture system. Cytotherapy 2006; 8: 3–12. 12 Bashey A, Medina B, Corringham S, Pasek M, Carrier E, Vrooman L et al. CTLA4 blockade with ipilimumab to treat relapse of malignancy after allogeneic hematopoietic cell transplantation. Blood 2009; 113: 1581–1588. 13 Fevery S, Billiau AD, Sprangers B, Rutgeerts O, Lenaerts C, Goebels J et al. CTLA-4 blockade in murine bone marrow chimeras induces a host-derived antileukemic effect without graft-versus-host disease. Leukemia 2007; 21: 1451–1459. 14 Dong H, Zhu G, Tamada K, Chen L. B7-H1 a third member of the B7 family, co-stimulates T-cell proliferation and interleukin-10 secretion. Nat Med 1999; 5: 1365–1369. 15 Kozako T, Yoshimitsu M, Fujiwara H, Masamoto I, Horai S, White Y et al. PD-1/ PD-L1 expression in human T-cell leukemia virus type 1 carriers and adult T-cell leukemia/lymphoma patients. Leukemia 2009; 23: 375–382. 16 Atanackovic D, Luetkens T, Kro¨ger N. Coinhibitory molecule PD-1 as a potential target for the immunotherapy of multiple myeloma. Leukemia 2014; 28: 993–1000. 17 Yang H, Bueso-Ramos C, DiNardo C, Estecio MR, Davanlou M, Geng Q-R et al. Expression of PD-L1, PD-L2, PD-1 and CTLA4 in myelodysplastic syndromes is enhanced by treatment with hypomethylating agents. Leukemia 2014; 28: 1280–1288. 18 Krejsgaard T, Odum N, Geisler C, Wasik MA, Woetmann A. Regulatory T cells and immunodeficiency in mycosis fungoides and Sezary syndrome. Leukemia 2012; 26: 424–432. 19 Kollgaard T, Petersen SL, Hadrup SR, Masmas TN, Seremet T, Andersen MH et al. Evidence for involvement of clonally expanded CD8 þ T cells in anticancer immune responses in CLL patients following nonmyeloablative conditioning and hematopoietic cell transplantation. Leukemia 2005; 19: 2273–2280. 20 Ame-Thomas P, Le PJ, Yssel H, Caron G, Pangault C, Jean R et al. Characterization of intratumoral follicular helper T cells in follicular lymphoma: role in the survival of malignant B cells. Leukemia 2012; 26: 1053–1063. 21 van deDonk NW, Kamps S, Mutis T, Lokhorst HM. Monoclonal antibody-based therapy as a new treatment strategy in multiple myeloma. Leukemia 2012; 26: 199–213. 22 Tamura H, Ishibashi M, Yamashita T, Tanosaki S, Okuyama N, Kondo A et al. Marrow stromal cells induce B7-H1 expression on myeloma cells, generating aggressive characteristics in multiple myeloma. Leukemia 2013; 27: 464–472. 23 Greaves P, Gribben JG. The role of B7 family molecules in hematologic malignancy. Blood 2013; 121: 734–744. 24 Thompson RH, Gillett MD, Cheville JC, Lohse CM, Dong H, Webster WS et al. Costimulatory B7-H1 in renal cell carcinoma patients: Indicator of tumor aggressiveness and potential therapeutic target. Proc Natl Acad Sci USA 2004; 101: 17174–17179. 25 Hamanishi J, Mandai M, Iwasaki M, Okazaki T, Tanaka Y, Yamaguchi K et al. Programmed cell death 1 ligand 1 and tumor-infiltrating CD8 þ T lymphocytes are prognostic factors of human ovarian cancer. Proc Natl Acad Sci USA 2007; 104: 3360–3365. 26 Nomi T, Sho M, Akahori T, Hamada K, Kubo A, Kanehiro H et al. Clinical significance and therapeutic potential of the programmed death-1 ligand/ programmed death-1 pathway in human pancreatic cancer. Clin Cancer Res 2007; 13: 2151–2157. 27 Chen BJ, Chapuy B, Ouyang J, Sun HH, Roemer MG, Xu ML et al. PD-L1 expression is characteristic of a subset of aggressive B-cell lymphomas and virus-associated malignancies. Clin Cancer Res 2013; 19: 3462–3473. 28 Christiansson L, Soderlund S, Svensson E, Mustjoki S, Bengtsson M, Simonsson B et al. 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. PLoS One 2013; 8: e55818. 29 Hatta Y, Koeffler HP. Role of tumor suppressor genes in the development of adult T cell leukemia/lymphoma (ATLL). Leukemia 2002; 16: 1069–1085. 30 Brahmer JR, Tykodi SS, Chow LQ, Hwu WJ, Topalian SL, Hwu P et al. Safety and activity of anti-PD-L1 antibody in patients with advanced cancer. N Engl J Med 2012; 366: 2455–2465. 31 Topalian SL, Hodi FS, Brahmer JR, Gettinger SN, Smith DC, McDermott DF et al. Safety, activity, and immune correlates of anti-PD-1 antibody in cancer. N Engl J Med 2012; 366: 2443–2453. 32 Berger R, Rotem-Yehudar R, Slama G, Landes S, Kneller A, Leiba M et al. Phase I safety and pharmacokinetic study of CT-011, a humanized antibody interacting with PD-1, in patients with advanced hematologic malignancies. Clin Cancer Res 2008; 14: 3044–3051. 33 Munir S, Andersen GH, Met O, Donia M, Frosig TM, Larsen SK et al. HLA-restricted cytotoxic T cells that are specific for the immune checkpoint ligand PD-L1 occur with high frequency in cancer patients. Cancer Res 2013; 73: 1674–1776.

& 2014 Macmillan Publishers Limited

34 Munir S, Andersen GH, Woetmann A, Odum N, Becker JC, Andersen MH. Cutaneous T cell lymphoma cells are targets for immune checkpoint ligand PD-L1-specific, cytotoxic T cells. Leukemia 2013; 27: 2251–2253. 35 Mous R, Savage P, Remmerswaal EB, van Lier RA, Eldering E, van Oers MH. Redirection of CMV-specific CTL towards B-CLL via CD20-targeted HLA/CMV complexes. Leukemia 2006; 20: 1096–1102. 36 Peric Z, Cahu X, Chevallier P, Brissot E, Malard F, Guillaume T et al. Features of Epstein-Barr Virus (EBV) reactivation after reduced intensity conditioning allogeneic hematopoietic stem cell transplantation. Leukemia 2011; 25: 932–938. 37 Ahmad SM, Larsen SK, Svane IM, Andersen MH. Harnessing PD-L1-specific cytotoxic T cells for anti-leukemia immunotherapy to defeat mechanisms of immune escape mediated by the PD-1 pathway. Leukemia 2014; 28: 236–238. 38 Pardoll DM. The blockade of immune checkpoints in cancer immunotherapy. Nat Rev Cancer 2012; 12: 252–264. 39 Wing K, Onishi Y, Prieto-Martin P, Yamaguchi T, Miyara M, Fehervari Z et al. CTLA-4 control over Foxp3 þ regulatory T cell function. Science 2008; 322: 271–275. 40 Liu W, Putnam AL, Xu-Yu Z, Szot GL, Lee MR, Zhu S et al. CD127 expression inversely correlates with FoxP3 and suppressive function of human CD4 þ T reg cells. J Exp Med 2006; 203: 1701–1711. 41 Seddiki N, Santner-Nanan B, Martinson J, Zaunders J, Sasson S, Landay A et al. Expression of interleukin (IL)-2 and IL-7 receptors discriminates between human regulatory and activated T cells. J Exp Med 2006; 203: 1693–1700. 42 Sakaguchi S. Naturally arising CD4 þ regulatory t cells for immunologic self-tolerance and negative control of immune responses. Annu Rev Immunol 2004; 22: 531–562. 43 Roncarolo MG, Gregori S, Battaglia M, Bacchetta R, Fleischhauer K, Levings MK. Interleukin-10-secreting type 1 regulatory T cells in rodents and humans. Immunol Rev 2006; 212: 28–50. 44 Chen W, Jin W, Hardegen N, Lei KJ, Li L, Marinos N et al. Conversion of peripheral CD4 þ CD25- naive T cells to CD4 þ CD25 þ regulatory T cells by TGF-beta induction of transcription factor Foxp3. J Exp Med 2003; 198: 1875–1886. 45 Collison LW, Workman CJ, Kuo TT, Boyd K, Wang Y, Vignali KM et al. The inhibitory cytokine IL-35 contributes to regulatory T-cell function. Nature 2007; 450: 566–569. 46 Chen KJ, Lin SZ, Zhou L, Xie HY, Zhou WH, Taki-Eldin A et al. Selective recruitment of regulatory T cell through CCR6-CCL20 in hepatocellular carcinoma fosters tumor progression and predicts poor prognosis. PLoS One 2011; 6: e24671. 47 Szczepanski MJ, Szajnik M, Czystowska M, Mandapathil M, Strauss L, Welsh A et al. Increased frequency and suppression by regulatory T cells in patients with acute myelogenous leukemia. Clin Cancer Res 2009; 15: 3325–3332. 48 Beyer M, Kochanek M, Darabi K, Popov A, Jensen M, Endl E et al. Reduced frequencies and suppressive function of CD4 þ CD25hi regulatory T cells in patients with chronic lymphocytic leukemia after therapy with fludarabine. Blood 2005; 106: 2018–2025. 49 Munir S, Larsen SK, Iversen TZ, Donia M, Klausen TW, Svane IM et al. Natural CD4( þ ) T-cell responses against indoleamine 2,3-dioxygenase. PLoS One 2012; 7: e34568. 50 Le DT, Jaffee EM. Regulatory T-cell modulation using cyclophosphamide in vaccine approaches: a current perspective. Cancer Res 2012; 72: 3439–3444. 51 Ghiringhelli F, Menard C, Puig PE, Ladoire S, Roux S, Martin F et al. Metronomic cyclophosphamide regimen selectively depletes CD4 þ CD25 þ regulatory T cells and restores T and NK effector functions in end stage cancer patients. Cancer Immunol Immunother 2007; 56: 641–648. 52 Giannopoulos K, Schmitt M, Wlasiuk P, Chen J, Bojarska-Junak A, Kowal M et al. The high frequency of T regulatory cells in patients with B-cell chronic lymphocytic leukemia is diminished through treatment with thalidomide. Leukemia 2008; 22: 222–224. 53 Giannopoulos K, Dmoszynska A, Kowal M, Wasik-Szczepanek E, Bojarska-Junak A, Rolinski J et al. Thalidomide exerts distinct molecular antileukemic effects and combined thalidomide/fludarabine therapy is clinically effective in high-risk chronic lymphocytic leukemia. Leukemia 2009; 23: 1771–1778. 54 Teng MW, Swann JB, von SB, Sharkey J, Zerafa N, McLaughlin N et al. Multiple antitumor mechanisms downstream of prophylactic regulatory T-cell depletion. Cancer Res 2010; 70: 2665–2674. 55 Dannull J, Su Z, Rizzieri D, Yang BK, Coleman D, Yancey D et al. Enhancement of vaccine-mediated antitumor immunity in cancer patients after depletion of regulatory T cells. J Clin Invest 2005; 115: 3623–3633. 56 Barnett BG, Ruter J, Kryczek I, Brumlik MJ, Cheng PJ, Daniel BJ et al. Regulatory T cells: a new frontier in cancer immunotherapy. Adv Exp Med Biol 2008; 622: 255–260. 57 Zhou Q, Bucher C, Munger ME, Highfill SL, Tolar J, Munn DH et al. Depletion of endogenous tumor-associated regulatory T cells improves the efficacy of adoptive cytotoxic T-cell immunotherapy in murine acute myeloid leukemia. Blood 2009; 114: 3793–3802.

Leukemia (2014) 1784 – 1792

Immunosuppressive mechanisms in leukemia MH Andersen

1792 58 Prince HM, Duvic M, Martin A, Sterry W, Assaf C, Sun Y et al. Phase III placebocontrolled trial of denileukin diftitox for patients with cutaneous T-cell lymphoma. J Clin Oncol 2010; 28: 1870–1877. 59 Nair S, Boczkowski D, Fassnacht M, Pisetsky D, Gilboa E. Vaccination against the forkhead family transcription factor Foxp3 enhances tumor immunity. Cancer Res 2007; 67: 371–380. 60 van ET, van Puijvelde GH, Foks AC, Habets KL, Bot I, Gilboa E et al. Vaccination against Foxp3( þ ) regulatory T cells aggravates atherosclerosis. Atherosclerosis 2010; 209: 74–80. 61 Larsen SK, Munir S, Woetmann A, Froesig TM, Odum N, Svane IM et al. Functional characterization of Foxp3-specific spontaneous immune responses. Leukemia 2013; 27: 2332–2340. 62 Zhang L, Chen X, Liu X, Kline DE, Teague RM, Gajewski TF et al. CD40 ligation reverses T cell tolerance in acute myeloid leukemia. J Clin Invest 2013; 123: 1999–2010. 63 Vincent J, Mignot G, Chalmin F, Ladoire S, Bruchard M, Chevriaux A et al. 5-Fluorouracil selectively kills tumor-associated myeloid-derived suppressor cells resulting in enhanced T cell-dependent antitumor immunity. Cancer Res 2010; 70: 3052–3061. 64 Romani L, Bistoni F, Perruccio K, Montagnoli C, Gaziano R, Bozza S et al. Thymosin alpha1 activates dendritic cell tryptophan catabolism and establishes a regulatory environment for balance of inflammation and tolerance. Blood 2006; 108: 2265–2274. 65 Munn DH, Sharma MD, Baban B, Harding HP, Zhang Y, Ron D et al. GCN2 kinase in T cells mediates proliferative arrest and anergy induction in response to indoleamine 2,3-dioxygenase. Immunity 2005; 22: 633–642. 66 Platten M, Ho PP, Youssef S, Fontoura P, Garren H, Hur EM et al. Treatment of autoimmune neuroinflammation with a synthetic tryptophan metabolite. Science 2005; 310: 850–855. 67 Munn DH, Mellor AL. Indoleamine 2,3-dioxygenase and tumor-induced tolerance. J Clin Invest 2007; 117: 1147–1154. 68 Smith C, Chang MY, Parker KH, Beury DW, DuHadaway JB, Flick HE et al. IDO is a nodal pathogenic driver of lung cancer and metastasis development. Cancer Discov 2012; 2: 722–735. 69 Curti A, Aluigi M, Pandolfi S, Ferri E, Isidori A, Salvestrini V et al. Acute myeloid leukemia cells constitutively express the immunoregulatory enzyme indoleamine 2,3-dioxygenase. Leukemia 2007; 21: 353–355. 70 Curti A, Trabanelli S, Salvestrini V, Baccarani M, Lemoli RM. The role of indoleamine 2,3-dioxygenase in the induction of immune tolerance: focus on hematology. Blood 2009; 113: 2394–2401. 71 Chamuleau ME, van de Loosdrecht AA, Hess CJ, Janssen JJ, Zevenbergen A, Delwel R et al. High INDO (indoleamine 2,3-dioxygenase) mRNA level in blasts of acute myeloid leukemic patients predicts poor clinical outcome. Haematologica 2008; 93: 1894–1898.

Leukemia (2014) 1784 – 1792

72 Hoshi M, Ito H, Fujigaki H, Takemura M, Takahashi T, Tomita E et al. Changes in serum tryptophan catabolism as an indicator of disease activity in adult T-cell leukemia/lymphoma. Leuk Lymphoma 2009; 50: 1372–1374. 73 Lindstrom V, Aittoniemi J, Jylhava J, Eklund C, Hurme M, Paavonen T et al. Indoleamine 2,3-dioxygenase activity and expression in patients with chronic lymphocytic leukemia. Clin Lymphoma Myeloma Leuk 2012; 12: 363–365. 74 Liu X, Shin N, Koblish HK, Yang G, Wang Q, Wang K et al. Selective inhibition of IDO1 effectively regulates mediators of antitumor immunity. Blood 2010; 115: 3520–3530. 75 Lob S, Konigsrainer A, Schafer R, Rammensee HG, Opelz G, Terness P. Levo- but not dextro-1-methyl tryptophan abrogates the IDO activity of human dendritic cells. Blood 2008; 111: 2152–2154. 76 Lob S, Konigsrainer A, Rammensee HG, Opelz G, Terness P. Inhibitors of indoleamine-2,3-dioxygenase for cancer therapy: can we see the wood for the trees? Nat Rev Cancer 2009; 9: 445–452. 77 Sorensen RB, Hadrup SR, Svane IM, Hjortso MC. thor Straten P, Andersen MH. Indoleamine 2,3-dioxygenase specific, cytotoxic T cells as immune regulators. Blood 2011; 117: 2200–2210. 78 Iversen TZ, Engell-Noerregaard L, Ellebaek E, Andersen R, Larsen SK, Bjoern J et al. Long-lasting disease stabilization in the absence of toxicity in metastatic lung cancer patients vaccinated with an epitope derived from indoleamine 2,3 dioxygenase. Clin Cancer Res 2014; 20: 221–232. 79 Yu J, Du W, Yan F, Wang Y, Li H, Cao S et al. Myeloid-derived suppressor cells suppress antitumor immune responses through IDO expression and correlate with lymph node metastasis in patients with breast cancer. J Immunol 2013; 190: 3783–3797. 80 Mougiakakos D, Jitschin R, von BL, Poschke I, Gary R, Sundberg B et al. Immunosuppressive CD14 þ HLA-DRlow/neg IDO þ myeloid cells in patients following allogeneic hematopoietic stem cell transplantation. Leukemia 2013; 27: 377–388. 81 Bronte V, Zanovello P. Regulation of immune responses by L-arginine metabolism. Nat Rev Immunol 2005; 5: 641–654. 82 Mussai F, De SC, Abu-Dayyeh I, Booth S, Quek L, McEwen-Smith RM et al. Acute myeloid leukemia creates an arginase-dependent immunosuppressive microenvironment. Blood 2013; 122: 749–758. 83 Morgan RA, Dudley ME, Wunderlich JR, Hughes MS, Yang JC, Sherry RM et al. Cancer regression in patients after transfer of genetically engineered lymphocytes. Science 2006; 314: 126–129. 84 Muller AJ, DuHadaway JB, Donover PS, Sutanto-Ward E, Prendergast GC. Inhibition of indoleamine 2,3-dioxygenase, an immunoregulatory target of the cancer suppression gene Bin1, potentiates cancer chemotherapy. Nat Med 2005; 11: 312–319.

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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 t...
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