IDO expression in brain tumors increases the recruitment of regulatory T cells and negatively impacts survival

The role of IDO in brain tumor immunotherapy

Malignant glioma comprises the majority of primary brain tumors. Coincidently, most of those malignancies express an inducible tryptophan catabolic enzyme, indoleamine 2,3 dioxygenase 1 (IDO1). While IDO1 is not normally expressed at appreciable levels in the adult central nervous system, it's rapidly induced and/or upregulated upon inflammatory stimulus. The primary function of IDO1 is associated with conversion of the essential amino acid, tryptophan, into downstream catabolites known as kynurenines. The depletion of tryptophan and/or accumulation of kynurenine has been shown to induce T cell deactivation, apoptosis and/or the induction of immunosuppressive programming via the expression of FoxP3. This understanding has informed immunotherapeutic design for the strategic development of targeted molecular therapeutics that inhibit IDO1 activity. Here, we review the current knowledge of IDO1 in brain tumors, pre-clinical studies targeting this enzymatic pathway, alternative tryptophan catabolic mediators that compensate for IDO1 loss and/or inhibition, as well as proposed clinical strategies and questions that are critical to address for increasing future immunotherapeutic effectiveness in patients with incurable brain cancer.

Involvement of the kynurenine pathway in human glioma pathophysiology

The kynurenine pathway (KP) is the principal route of L-tryptophan (TRP) catabolism leading to the production of kynurenine (KYN), the neuroprotectants, kynurenic acid (KYNA) and picolinic acid (PIC), the excitotoxin, quinolinic acid (QUIN) and the essential pyridine nucleotide, nicotinamide adenine dinucleotide (NAD(+)). The enzymes indoleamine 2,3-dioxygenase-1 (IDO-1), indoleamine 2,3-dioxygenase-2 (IDO-2) and tryptophan 2,3-dioxygenase (TDO-2) initiate the first step of the KP. IDO-1 and TDO-2 induction in tumors are crucial mechanisms implicated to play pivotal roles in suppressing anti-tumor immunity. Here, we report the first comprehensive characterisation of the KP in 1) cultured human glioma cells and 2) plasma from patients with glioblastoma (GBM). Our data revealed that interferon-gamma (IFN-γ) stimulation significantly potentiated the expression of the KP enzymes, IDO-1 IDO-2, kynureninase (KYNU), kynurenine hydroxylase (KMO) and significantly down-regulated 2-amino-3-carboxymuconate semialdehyde decarboxylase (ACMSD) and kynurenine aminotransferase-I (KAT-I) expression in cultured human glioma cells. This significantly increased KP activity but significantly lowered the KYNA/KYN neuroprotective ratio in human cultured glioma cells. KP activation (KYN/TRP) was significantly higher, whereas the concentrations of the neuroreactive KP metabolites TRP, KYNA, QUIN and PIC and the KYNA/KYN ratio were significantly lower in GBM patient plasma (n = 18) compared to controls. These results provide further evidence for the involvement of the KP in glioma pathophysiology and highlight a potential role of KP products as novel and highly attractive therapeutic targets to evaluate for the treatment of brain tumors, aimed at restoring anti-tumor immunity and reducing the capacity for malignant cells to produce NAD(+), which is necessary for energy production and DNA repair.

Host immune response to infection and cancer: unexpected commonalities

Both microbes and tumors activate innate resistance, tissue repair, and adaptive immunity. Unlike acute infection, tumor growth is initially unapparent; however, inflammation and immunity affect all phases of tumor growth from initiation to progression and dissemination. Here, we discuss the shared features involved in the immune response to infection and cancer including modulation by commensal microbiota, reactive hematopoiesis, chronic immune responses and regulatory mechanisms to prevent collateral tissue damage. This comparative analysis of immunity to infection and cancer furthers our understanding of the basic mechanisms underlying innate resistance and adaptive immunity and their translational application to the design of new therapeutic approaches.

A road less traveled paved by IDO silencing: Harnessing the antitumor activity of neutrophils

Orchestrating a cytotoxic polymorphonuclear neutrophil (PMN) response strictly focused within the tumor tissue remains a formidable challenge for the successful therapeutic use of these cells. A Salmonella vector carrying an shRNA against indoleamine 2,3-dioxygenase has been shown to recruit PMNs and enhance their activation specifically in the tumor bed, resulting in significant anticancer effects.

Durable therapeutic efficacy utilizing combinatorial blockade against IDO, CTLA-4, and PD-L1 in mice with brain tumors

PURPOSE

Glioblastoma (GBM) is the most common form of malignant glioma in adults. Although protected by both the blood-brain and blood-tumor barriers, GBMs are actively infiltrated by T cells. Previous work has shown that IDO, CTLA-4, and PD-L1 are dominant molecular participants in the suppression of GBM immunity. This includes IDO-mediated regulatory T-cell (Treg; CD4(+)CD25(+)FoxP3(+)) accumulation, the interaction of T-cell-expressed, CTLA-4, with dendritic cell-expressed, CD80, as well as the interaction of tumor- and/or macrophage-expressed, PD-L1, with T-cell-expressed, PD-1. The individual inhibition of each pathway has been shown to increase survival in the context of experimental GBM. However, the impact of simultaneously targeting all three pathways in brain tumors has been left unanswered.

EXPERIMENTAL DESIGN AND RESULTS

In this report, we demonstrate that, when dually challenged, IDO-deficient tumors provide a selectively competitive survival advantage against IDO-competent tumors. Next, we provide novel observations regarding tryptophan catabolic enzyme expression, before showing that the therapeutic inhibition of IDO, CTLA-4, and PD-L1 in a mouse model of well-established glioma maximally decreases tumor-infiltrating Tregs, coincident with a significant increase in T-cell-mediated long-term survival. In fact, 100% of mice bearing intracranial tumors were long-term survivors following triple combination therapy. The expression and/or frequency of T cell expressed CD44, CTLA-4, PD-1, and IFN-γ depended on timing after immunotherapeutic administration.

CONCLUSIONS

Collectively, these data provide strong preclinical evidence that combinatorially targeting immunosuppression in malignant glioma is a strategy that has high potential value for future clinical trials in patients with GBM.

Clinical significance of tryptophan metabolism in the nontumoral hemisphere in patients with malignant glioma

α-(11)C-methyl-L-tryptophan (AMT) PET allows evaluation of brain serotonin synthesis and can also track upregulation of the immunosuppressive kynurenine pathway in tumor tissue. Increased AMT uptake is a hallmark of World Health Organization grade III-IV gliomas. Our recent study also suggested decreased frontal cortical AMT uptake in glioma patients contralateral to the tumor. The clinical significance of extratumoral tryptophan metabolism has not been established. In the present study, we investigated clinical correlates of tryptophan metabolic abnormalities in the nontumoral hemisphere of glioma patients.

METHODS

Standardized AMT uptake values (SUVs) and the uptake rate constant of AMT (K [mL/g/min], a measure proportional to serotonin synthesis in nontumoral gray matter) were quantified in the frontal and temporal cortex and thalamus in the nontumoral hemisphere in 77 AMT PET scans of 66 patients (41 men, 25 women; mean age ± SD, 55 ± 15 y) with grade III-IV gliomas. These AMT values were determined before treatment in 35 and after treatment in 42 patients and were correlated with clinical variables and survival.

RESULTS

AMT uptake in the thalamus showed a moderate age-related increase before treatment (SUV, r = 0.39, P = 0.02) but decrease after treatment (K, r = -0.33, P = 0.057). Women had higher thalamic SUVs before treatment (P = 0.037) and higher thalamic (P = 0.013) and frontal cortical K values (P = 0.023) after treatment. In the posttreatment glioma group, high thalamic SUVs and high thalamocortical SUV ratios were associated with short survival in Cox regression analysis. The thalamocortical ratio remained strongly prognostic (P < 0.01) when clinical predictors, including age, glioma grade, and time since radiotherapy, were entered in the regression model. Long interval between radiotherapy and posttreatment AMT PET as well as high radiation dose affecting the thalamus were associated with lower contralateral thalamic or cortical AMT uptake values.

CONCLUSION

These observations provide evidence for altered tryptophan uptake in contralateral cortical and thalamic brain regions in glioma patients after initial therapy, suggesting treatment effects on the serotonergic system. Low thalamic tryptophan uptake appears to be a strong, independent predictor of long survival in patients with previous glioma treatment.

Metabolic control of tumour progression and antitumour immunity

PURPOSE OF REVIEW

Loss of cell growth control is not sufficient to explain why tumours form as the immune system recognizes many malignant cells and keeps them in check. The local inflammatory microenvironment is a pivotal factor in tumour formation, as tumour-associated inflammation actively suppresses antitumour immunity. The purpose of this review is to evaluate emerging evidence that amino acid catabolism is a key feature of tumour-associated inflammation that supports tumour progression and immune resistance to therapy.

RECENT FINDINGS

Enhanced amino acid catabolism in inflammatory tumour microenvironments correlates with carcinogen resistance and immune regulation mediated by tumour-associated immune cells that protect tumours from natural and vaccine-induced immunity. Interfering with metabolic pathways exploited by tumours is a promising antitumour strategy, especially when combined with other therapies. Moreover, molecular sensors that evolved to detect pathogens may enhance evasion of immune surveillance to permit tumour progression.

SUMMARY

Innate immune sensing that induces amino acid catabolism in tumour microenvironments may be pivotal in initiating and sustaining local inflammation that promotes immune resistance and attenuates antitumour immunity. Targeting molecular sensors that mediate these metabolic changes may be an effective strategy to enhance antitumour immunity that prevents tumour progression, as well as improving the efficacy of cancer therapy.

The indoleamine 2,3-dioxygenase pathway controls complement-dependent enhancement of chemo-radiation therapy against murine glioblastoma

Background

Indoleamine 2,3-dioxygenase (IDO) is an enzyme with immune-suppressive properties that is commonly exploited by tumors to evade immune destruction. Anti-tumor T cell responses can be initiated in solid tumors, but are immediately suppressed by compensatory upregulation of immunological checkpoints, including IDO. In addition to these known effects on the adaptive immune system, we previously showed widespread, T cell-dependent complement deposition during allogeneic fetal rejection upon maternal treatment with IDO-blockade. We hypothesized that IDO protects glioblastoma from the full effects of chemo-radiation therapy by preventing vascular activation and complement-dependent tumor destruction.

Methods

To test this hypothesis, we utilized a syngeneic orthotopic glioblastoma model in which GL261 glioblastoma tumor cells were stereotactically implanted into the right frontal lobes of syngeneic mice. These mice were treated with IDO-blocking drugs in combination with chemotherapy and radiation therapy.

Results

Pharmacologic inhibition of IDO synergized with chemo-radiation therapy to prolong survival in mice bearing intracranial glioblastoma tumors. We now show that pharmacologic or genetic inhibition of IDO allowed chemo-radiation to trigger widespread complement deposition at sites of tumor growth. Chemotherapy treatment alone resulted in collections of perivascular leukocytes within tumors, but no complement deposition. Adding IDO-blockade led to upregulation of VCAM-1 on vascular endothelium within the tumor microenvironment, and further adding radiation in the presence of IDO-blockade led to widespread deposition of complement. Mice genetically deficient in complement component C3 lost all of the synergistic effects of IDO-blockade on chemo-radiation-induced survival.

Conclusions

Together these findings identify a novel mechanistic link between IDO and complement, and implicate complement as a major downstream effector mechanism for the beneficial effect of IDO-blockade after chemo-radiation therapy. We speculate that this represents a fundamental pathway by which the tumor regulates intratumoral vascular activation and protects itself from immune-mediated tumor destruction.

Indoleamine 2,3-dioxygenase promotes peritoneal metastasis of ovarian cancer by inducing an immunosuppressive environment

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-catabolizing enzyme that has immunoregulatory functions. Our prior study showed that tumoral IDO overexpression is involved in disease progression and impaired patient survival in human ovarian cancer, although its mechanism remains unclear. The purpose of the present study is to clarify the role of IDO during the process of peritoneal dissemination of ovarian cancer. Indoleamine 2,3-dioxygenase cDNA was transfected into the murine ovarian carcinoma cell line OV2944-HM-1, establishing stable clones of IDO-overexpressing cells (HM-1-IDO). Then HM-1-IDO or control vector-transfected cells (HM-1-mock) were i.p. transplanted into syngeneic immunocompetent mice. The HM-1-IDO-transplanted mice showed significantly shortened survival compared with HM-1-mock-transplanted (control) mice. On days 11 and 14 following transplantation, the tumor weight of peritoneal dissemination and ascites volume were significantly increased in HM-1-IDO-transplanted mice compared with those of control mice. This tumor-progressive effect was coincident with significantly reduced numbers of CD8⁺ T cells and natural killer cells within tumors as well as increased levels of transforming growth factor-β and interleukin-10 in ascites. Finally, treatment with the IDO inhibitor 1-methyl-tryptophan significantly suppressed tumor dissemination and ascites with reduced transforming growth factor-β secretion. These findings showed that tumor-derived IDO promotes the peritoneal dissemination of ovarian cancer through suppression of tumor-infiltrating effector T cell and natural killer cell recruitment and reciprocal enhancement of immunosuppressive cytokines in ascites, creating an immunotolerogenic environment within the peritoneal cavity. Therefore, IDO may be a promising molecular target for the therapeutic strategy of ovarian cancer.

Effects of upregulated indoleamine 2, 3-dioxygenase 1 by interferon γ gene transfer on interferon γ-mediated antitumor activity

Interferon γ (IFN-γ), an anticancer agent, is a strong inducer of indoleamine 2,3-dioxygenase 1 (IDO1), which is a tryptophan-metabolizing enzyme involved in the induction of tumor immune tolerance. In this study, we investigated the IDO1 expression in organs after IFN-γ gene transfer to mice. IFN-γ gene transfer greatly increased the mRNA expression of IDO1 in many tissues with the highest in the liver. This upregulation was associated with reduced L-tryptophan levels and increased L-kynurenine levels in serum, indicating that IFN-γ gene transfer increased the IDO activity. Then, Lewis lung carcinoma (LLC) tumor-bearing wild-type and IDO1-knockout (IDO1 KO) mice were used to investigate the effects of IDO1 on the antitumor activity of IFN-γ. IFN-γ gene transfer significantly retarded the tumor growth in both strains without any significant difference in tumor size between the two groups. By contrast, the IDO1 activity was increased only in the wild-type mice by IFN-γ gene transfer, suggesting that cells other than LLC cells, such as tumor stromal cells, are the major contributors of IDO1 expression in LLC tumor. Taken together, these results imply that IFN-γ gene transfer mediated IDO1 upregulation in cells other than LLC cells has hardly any effect on the antitumor activity of IFN-γ.

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.

Increased tryptophan uptake on PET has strong independent prognostic value in patients with a previously treated high-grade glioma

BACKGROUND

Previously, we demonstrated the high accuracy of alpha-[(11)C]methyl-L-tryptophan (AMT) PET for differentiating recurrent gliomas from radiation injury. The present study evaluated the prognostic value of increased AMT uptake in patients with previously treated high-grade glioma.

METHODS

AMT-PET was performed in 39 patients with suspected recurrence of World Health Organization grades III-IV glioma following surgical resection, radiation, and chemotherapy. Mean and maximum standardized uptake values (SUVs) and unidirectional AMT uptake (K) were measured in brain regions suspicious for tumor and compared with the contralateral cortex (ie, background). Optimal cutoff thresholds for 1-year survival prediction were determined for each AMT parameter and used for calculating the prognostic value of high (above threshold) versus low (below threshold) values for post-PET overall survival (OS).

RESULTS

In univariate analyses, 1-year survival was strongly associated with 3 AMT parameters (SUVmax, SUVmean, and tumor-to-background K-ratio; odds ratios: 21.3-25.6; P ≤ .001) and with recent change in MRI contrast enhancement (odds ratio: 14.7; P = .02). Median OS was 876 days in the low- versus 177 days in the high-AMT groups (log-rank P < .001). In multivariate analyses, all 3 AMT parameters remained strong predictors of survival: high AMT values were associated with unfavorable 1-year survival (binary regression P ≤ .003) and shorter overall survival in the whole group (Cox regression hazard ratios: 5.3-10.0) and in patients with recent enhancement change on MRI as well (hazard ratios: 7.0-9.3; P ≤ .001).

CONCLUSION

Increased AMT uptake on PET is highly prognostic for 1-year and overall survival, independent of MRI contrast enhancement and other prognostic factors in patients with a previously treated high-grade glioma.

Dendritic cell vaccination for glioblastoma multiforme: review with focus on predictive factors for treatment response

Glioblastoma multiforme (GBM) is the most common and most aggressive type of primary brain cancer. Since median overall survival with multimodal standard therapy is only 15 months, there is a clear need for additional effective and long-lasting treatments. Dendritic cell (DC) vaccination is an experimental immunotherapy being tested in several Phase I and Phase II clinical trials. In these trials, safety and feasibility have been proven, and promising clinical results have been reported. On the other hand, it is becoming clear that not every GBM patient will benefit from this highly personalized treatment. Defining the subgroup of patients likely to respond to DC vaccination will position this option correctly amongst other new GBM treatment modalities, and pave the way to incorporation in standard therapy. This review provides an overview of GBM treatment options and focuses on the currently known prognostic and predictive factors for response to DC vaccination. In this way, it will provide the clinician with the theoretical background to refer patients who might benefit from this treatment.

Antibody, T-cell and dendritic cell immunotherapy for malignant brain tumors

Modest improvement in brain tumor patient survival has been achieved through advances in surgical, adjuvant radiation and chemotherapeutic strategies. However, these traditional approaches have been unsuccessful in permanently controlling these aggressive tumors, with recurrence being quite common. Hence, there is a need for novel therapeutic approaches that specifically target the molecularly diverse brain tumor cell population. The ability of the immune system to recognize altered tumor cells while avoiding surrounding normal cells offers an enormous advantage over the nonspecific nature of the conventional treatment schemes. Therefore, immunotherapy represents a promising approach that may supplement the standard therapies in eliminating the residual brain tumor cells. This review summarizes different immunotherapeutic approaches currently being tested for malignant brain tumor treatment.

The role of regulatory T-cells in glioma immunology

Despite recent advances in treatment, the prognosis for glioblastoma multiforme (GBM) remains poor. The lack of response to treatment in GBM patients may be attributed to the immunosuppressed microenvironment that is characteristic of invasive glioma. Regulatory T-cells (Tregs) are immunosuppressive T-cells that normally prevent autoimmunity when the human immune response is evoked; however, there have been strong correlations between glioma-induced immunosuppression and Tregs. In fact, induction of Treg activity has been correlated with glioma development in both murine models and patients. While the exact mechanisms by which regulatory T-cells function require further elucidation, various cytokines such as interleukin-10 (IL-10) and transforming growth factor-β (TFG-β) have been implicated in these processes and are currently under investigation. In addition, hypoxia is characteristic of tumor development and is also correlated with downstream induction of Tregs. Due to the poor prognosis associated with immunosuppression in glioma patients, Tregs remain a promising area for immunotherapeutic research.

The role of inflammation in brain cancer

Malignant brain tumors are among the most lethal of human tumors, with limited treatment options currently available. A complex array of recurrent genetic and epigenetic changes has been observed in gliomas that collectively result in derangements of common cell signaling pathways controlling cell survival, proliferation, and invasion. One important determinant of gene expression is DNA methylation status, and emerging studies have revealed the importance of a recently identified demethylation pathway involving 5-hydroxymethylcytosine (5hmC). Diminished levels of the modified base 5hmC is a uniform finding in glioma cell lines and patient samples, suggesting a common defect in epigenetic reprogramming. Within the tumor microenvironment, infiltrating immune cells increase oxidative DNA damage, likely promoting both genetic and epigenetic changes that occur during glioma evolution. In this environment, glioma cells are selected that utilize multiple metabolic changes, including changes in the metabolism of the amino acids glutamate, tryptophan, and arginine. Whereas altered metabolism can promote the destruction of normal tissues, glioma cells exploit these changes to promote tumor cell survival and to suppress adaptive immune responses. Further understanding of these metabolic changes could reveal new strategies that would selectively disadvantage tumor cells and redirect host antitumor responses toward eradication of these lethal tumors.

Long-lasting disease stabilization in the absence of toxicity in metastatic lung cancer patients vaccinated with an epitope derived from indoleamine 2,3 dioxygenase

PURPOSE

To investigate targeting of indoleamine 2,3 dioxygenase (IDO) enzyme using a synthetic peptide vaccine administered to patients with metastatic non-small cell lung cancer (NSCLC).

EXPERIMENTAL DESIGN

In a clinical phase I study, we treated 15 HLA-A2-positive patients with stage III-IV NSCLC in disease stabilization after standard chemotherapy. Patients were treated with imiquimod ointment and subcutaneous vaccinations (100 μg IDO5 peptide, sequence ALLEIASCL, formulated in 900 μL Montanide). Primary endpoint was toxicity. Clinical benefit and immunity were assessed as secondary endpoints.

RESULTS

No severe toxicity was observed. One patient developed a partial response (PR) after one year of vaccine treatment, whereas long-lasting stable disease (SD) ≥ 8.5 months was demonstrated in another six patients. The median overall survival (OS) was 25.9 months. Patients demonstrated significant improved OS (P = 0.03) when compared with the group of patients excluded because of HLA-A2 negativity. IDO-specific CD8(+) T-cell immunity was demonstrated by IFN-γ Elispot and Tetramer staining. Fluorescence-activated cell sorting analyses demonstrated a significant reduction of the Treg population (P = 0.03) after the sixth vaccine (2.5 months) compared with pretreatment levels. Furthermore, expression of IDO was detected in nine of ten tumor biopsies by immunohistochemistry. High-performance liquid chromatography analyses of kynurenine/tryptophan (Kyn/Trp) ratio in sera were performed. In long-term analyses of two clinical responding patients, the ratio of Kyn/Trp remained stable.

CONCLUSIONS

The vaccine was well tolerated with no severe toxicity occurring. A median OS of 25.9 months was demonstrated and long-lasting PR+SD was seen in 47% of the patients.

Heme oxygenase-1 protects regulatory T cells from hypoxia-induced cellular stress in an experimental mouse brain tumor model

Two characteristic features of malignant gliomas (MG) are the presence of hypoxia and accumulation of regulatory T cells (Tregs). Heme-oxygenase-1 (HO1) is a cytoprotective enzyme expressed in high level by Tregs in glioma. In this study, we show that higher HO1 expression in Tregs is associated with increased survival under hypoxic conditions and that HO1 inhibitor, tin protoporphyrin (SnPP), abrogates the survival benefits. Moreover, SnPP preferentially eliminates Tregs and treatment with SnPP of tumor bearing mice significantly increases survival (23 to 31days (p<0.05)). Thus HO1 inhibition provides another alternative way of therapeutically targeting Tregs in MG.

Immune-checkpoint blockade and active immunotherapy for glioma

Cancer immunotherapy has made tremendous progress, including promising results in patients with malignant gliomas. Nonetheless, the immunological microenvironment of the brain and tumors arising therein is still believed to be suboptimal for sufficient antitumor immune responses for a variety of reasons, including the operation of “immune-checkpoint” mechanisms. While these mechanisms prevent autoimmunity in physiological conditions, malignant tumors, including brain tumors, actively employ these mechanisms to evade from immunological attacks. Development of agents designed to unblock these checkpoint steps is currently one of the most active areas of cancer research. In this review, we summarize recent progresses in the field of brain tumor immunology with particular foci in the area of immune-checkpoint mechanisms and development of active immunotherapy strategies. In the last decade, a number of specific monoclonal antibodies designed to block immune-checkpoint mechanisms have been developed and show efficacy in other cancers, such as melanoma. On the other hand, active immunotherapy approaches, such as vaccines, have shown encouraging outcomes. We believe that development of effective immunotherapy approaches should ultimately integrate those checkpoint-blockade agents to enhance the efficacy of therapeutic approaches. With these agents available, it is going to be quite an exciting time in the field. The eventual success of immunotherapies for brain tumors will be dependent upon not only an in-depth understanding of immunology behind the brain and brain tumors, but also collaboration and teamwork for the development of novel trials that address multiple layers of immunological challenges in gliomas.

Home sweet home: the tumor microenvironment as a haven for regulatory T cells

CD4⁺Foxp3⁺ regulatory T cells (T_regs) have a fundamental role in maintaining immune balance by preventing autoreactivity and immune-mediated pathology. However this role of T_regs extends to suppression of anti-tumor immune responses and remains a major obstacle in the development of anti-cancer vaccines and immunotherapies. This feature of T_reg activity is exacerbated by the discovery that T_reg frequencies are not only elevated in the blood of cancer patients, but are also significantly enriched within tumors in comparison to other sites. These observations have sparked off the quest to understand the processes through which T_regs become elevated in cancer-bearing hosts and to identify the specific mechanisms leading to their accumulation within the tumor microenvironment. This manuscript reviews the evidence for specific mechanisms of intra-tumoral T_reg enrichment and will discuss how this information may be utilized for the purpose of manipulating the balance of tumor-infiltrating T cells in favor of anti-tumor effector cells.

Differentiation of glioblastomas from metastatic brain tumors by tryptophan uptake and kinetic analysis: a positron emission tomographic study with magnetic resonance imaging comparison

Differentiating high-grade gliomas from solitary brain metastases is often difficult by conventional magnetic resonance imaging (MRI); molecular imaging may facilitate such discrimination. We tested the accuracy of α[11C]methyl-l-tryptophan (AMT)-positron emission tomography (PET) to differentiate newly diagnosed glioblastomas from brain metastases. AMT-PET was performed in 36 adults with suspected brain malignancy. Tumoral AMT accumulation was measured by standardized uptake values (SUVs). Tracer kinetic analysis was also performed to separate tumoral net tryptophan transport (by AMT volume of distribution [VD]) from unidirectional uptake rates using dynamic PET and blood input function. Differentiating the accuracy of these PET variables was evaluated and compared to conventional MRI. For glioblastoma/metastasis differentiation, tumoral AMT SUV showed the highest accuracy (74%) and the tumor/cortex VD ratio had the highest positive predictive value (82%). The combined accuracy of MRI (size of contrast-enhancing lesion) and AMT-PET reached up to 93%. For ring-enhancing lesions, tumor/cortex SUV ratios were higher in glioblastomas than in metastatic tumors and could differentiate these two tumor types with > 90% accuracy. These results demonstrate that evaluation of tryptophan accumulation by PET can enhance pretreatment differentiation of glioblastomas and metastatic brain tumors. This approach may be particularly useful in patients with a newly diagnosed solitary ring-enhancing mass.

Targeting Tregs in Malignant Brain Cancer: Overcoming IDO

One of the hallmark features of glioblastoma multiforme (GBM), the most common adult primary brain tumor with a very dismal prognosis, is the accumulation of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs). Regulatory T cells (Tregs) segregate into two primary categories: thymus-derived natural Tregs (nTregs) that develop from the interaction between immature T cells and thymic epithelial stromal cells, and inducible Tregs (iTregs) that arise from the conversion of CD4⁺FoxP3⁻ T cells into FoxP3 expressing cells. Normally, these Treg subsets complement one another’s actions by maintaining tolerance of self-antigens, thereby suppressing autoimmunity, while also enabling effective immune responses toward non-self-antigens, thus promoting infectious protection. However, Tregs have also been shown to be associated with the promotion of pathological outcomes, including cancer. In the setting of GBM, nTregs appear to be primary players that contribute to immunotherapeutic failure, ultimately leading to tumor progression. Several attempts have been made to therapeutically target these cells with variable levels of success. The blood brain barrier-crossing chemotherapeutics, temozolomide, and cyclophosphamide (CTX), vaccination against the Treg transcriptional regulator, FoxP3, as well as mAbs against Treg-associated cell surface molecules CD25, CTLA-4, and GITR are all different therapeutic approaches under investigation. Contributing to the poor success of past approaches is the expression of indoleamine 2,3-dioxygenase 1 (IDO), a tryptophan catabolizing enzyme overexpressed in GBM, and critically involved in regulating tumor-infiltrating Treg levels. Herein, we review the current literature on Tregs in brain cancer, providing a detailed phenotype, causative mechanisms involved in their pathogenesis, and strategies that have been used to target this population, therapeutically.

Depletion of regulatory T cells in a mouse experimental glioma model through anti-CD25 treatment results in the infiltration of non-immunosuppressive myeloid cells in the brain

The recruitment and activation of regulatory T cells (Tregs) in the micro-environment of malignant brain tumors has detrimental effects on antitumoral immune responses. Hence, local elimination of Tregs within the tumor micro-environment represents a highly valuable tool from both a fundamental and clinical perspective. In the syngeneic experimental GL261 murine glioma model, Tregs were prophylactically eliminated through treatment with PC61, an anti-CD25 mAb. This resulted in specific elimination of CD4+CD25hiFoxp3+ Treg within brain-infiltrating lymphocytes and complete protection against subsequent orthotopic GL261 tumor challenge. Interestingly, PC61-treated mice also showed a pronounced infiltration of CD11b+ myeloid cells in the brain. Phenotypically, these cells could not be considered as Gr-1+ myeloid-derived suppressor cells (MDSC) but were identified as F4/80+ macrophages and granulocytes.

Interferon gamma suppresses collagen-induced arthritis by regulation of Th17 through the induction of indoleamine-2,3-deoxygenase

C57BL/6 mice are known to be resistant to the development of collagen-induced arthritis (CIA). However, they show a severe arthritic phenotype when the Ifng gene is deleted. Although it has been proposed that IFN-γ suppresses inflammation in CIA via suppressing Th17 which is involved in the pathogenesis of CIA, the exact molecular mechanism of the Th17 regulation by IFN-γ is poorly understood. This study was conducted to 1) clarify that arthritogenic condition of IFN-γ knockout (KO) mice is dependent on the disinhibition of Th17 and 2) demonstrate that IFN-γ-induced indoleamine2,3dioxgenase (IDO) is engaged in the regulation of Th17. The results showed that the IFN-γ KO mice displayed increased levels of IL-17 producing T cells and the exacerbation of arthritis. Also, production of IL-17 by the splenocytes of the IFN-γ KO mice was increased when cultured with type II collagen. When Il17 was deleted from the IFN-γ KO mice, only mild arthritis developed without any progression of the arthritis score. The proportion of CD44^highCD62L^low memory-like T cells were elevated in the spleen, draining lymph node and mesenteric lymph node of IFN-γ KO CIA mice. Meanwhile, CD44^lowCD62L^high naïve T cells were increased in IFN-γ and IL-17 double KO CIA mice. When Th17 polarized CD4+ T cells of IFN-γ KO mice were co-cultured with their own antigen presenting cells (APCs), a greater increase in IL-17 production was observed than in co-culture of the cells from wild type mice. In contrast, when APCs from IFN-γ KO mice were pretreated with IFN-γ, there was a significant reduction in IL-17 in the co-culture system. Of note, pretreatment of 1-methyl-DL- tryptophan, a specific inhibitor of IDO, abolished the inhibitory effects of IFN-γ. Given that IFN-γ is a potent inducer of IDO in APCs, these results suggest that IDO is involved in the regulation of IL-17 by IFN-γ.

Inhibition of indoleamine 2,3-dioxygenase enhances the T-cell response to influenza virus infection

Influenza infection induces an increase in the level of indoleamine 2,3-dioxygenase (IDO) activity in the lung parenchyma. IDO is the first and rate-limiting step in the kynurenine pathway where tryptophan is reduced to kynurenine and other metabolites. The depletion of tryptophan, and production of associated metabolites, attenuates the immune response to infection. The impact of IDO on the primary immune response to influenza virus infection was determined using the IDO inhibitor 1-methyl-D,L-tryptophan (1MT). C57BL/6 mice treated with 1MT and infected with A/HKx31 influenza virus had increased numbers of activated and functional CD4⁺ T-cells, influenza-specific CD8⁺ T-cells and effector memory cells in the lung. Inhibition of IDO increased the Th1 response in CD4⁺ T-cells as well as enhanced the Th17 response. These studies show that inhibition of IDO engenders a more robust T-cell response to influenza virus, and suggests an approach for enhancing the immune response to influenza vaccination by facilitating increased influenza-specific T-cell response.

The metastasis-promoting roles of tumor-associated immune cells

Tumor metastasis is driven not only by the accumulation of intrinsic alterations in malignant cells, but also by the interactions of cancer cells with various stromal cell components of the tumor microenvironment. In particular, inflammation and infiltration of the tumor tissue by host immune cells, such as tumor-associated macrophages, myeloid-derived suppressor cells, and regulatory T cells, have been shown to support tumor growth in addition to invasion and metastasis. Each step of tumor development, from initiation through metastatic spread, is promoted by communication between tumor and immune cells via the secretion of cytokines, growth factors, and proteases that remodel the tumor microenvironment. Invasion and metastasis require neovascularization, breakdown of the basement membrane, and remodeling of the extracellular matrix for tumor cell invasion and extravasation into the blood and lymphatic vessels. The subsequent dissemination of tumor cells to distant organ sites necessitates a treacherous journey through the vasculature, which is fostered by close association with platelets and macrophages. Additionally, the establishment of the pre-metastatic niche and specific metastasis organ tropism is fostered by neutrophils and bone marrow-derived hematopoietic immune progenitor cells and other inflammatory cytokines derived from tumor and immune cells, which alter the local environment of the tissue to promote adhesion of circulating tumor cells. This review focuses on the interactions between tumor cells and immune cells recruited to the tumor microenvironment and examines the factors allowing these cells to promote each stage of metastasis.

In vivo metabolism of tryptophan in meningiomas is mediated by indoleamine 2,3-dioxygenase 1

Expression and activity of indoleamine 2,3-dioxygenase (IDO), the first and rate-limiting step of the kynurenine pathway of tryptophan catabolism, can enable tumor cells to effectively evade the host's immune response. The potential role of this system was investigated in meningiomas. Surgical specimens from 22 patients with meningiomas were used for cellular, immunological and molecular techniques (immunofluorescence, western blotting, RT-PCR and biochemical assay of enzyme activity) to investigate the expression and activity of IDO. In addition, PET imaging was obtained preoperatively in 10 patients using the tracer α-[ ( 11) C]methyl-L-tryptophan (AMT) which interrogates the uptake and metabolism of tryptophan. Strong AMT accumulation was noted in all meningiomas by PET imaging indicating in vivo tryptophan uptake. Freshly-resected meningiomas expressed both LAT1, the tryptophan transporter system and IDO, demonstrating an active kynurenine pathway. Dissociated meningioma cells lost IDO expression. Following exposure to interferon-γ (IFNγ), IDO expression was reinduced and could be blocked by a selective IDO1 inhibitor. IDO activity may represent an element of local self-protection by meningiomas and could be targeted by emerging IDO1 inhibitors.

Regulatory T cells move in when gliomas say "I Do"

Indoleamine 2,3-dioxygenase (IDO) is an enzyme with known immunosuppressive and tolerogenic effects in cancer. Mounting evidence has associated IDO expression with the induction of regulatory T cells (Treg) and malignant progression. IDO inhibition may therefore provide a promising therapeutic approach for glioblastoma, where the need for novel treatment is great.