Expression of indoleamine 2,3-dioxygenase by plasmacytoid dendritic cells in tumor-draining lymph nodes

A SREBF2-dependent gene program drives an immunotolerant dendritic cell population during cancer progression

Dendritic cells (cDCs) are essential mediators of anti-tumor immunity. Cancers have developed mechanisms to render DCs dysfunctional within the tumor microenvironment. Utilizing CD63 as a unique surface marker, we demonstrate that mature regulatory DCs (mregDCs) suppress DC antigen cross-presentation while driving T _H 2 and regulatory T cell differentiation within tumor-draining lymph node tissues. Transcriptional and metabolic studies show that mregDC functionality is dependent upon the mevalonate biosynthetic pathway and the master transcription factor, SREBP2. Melanoma-derived lactate activates DC SREBP2 in the tumor microenvironment (TME) and drives mregDC development from conventional DCs. DC-specific genetic silencing and pharmacologic inhibition of SREBP2 promotes anti-tumor CD8 ⁺ T cell activation and suppresses melanoma progression. CD63 ⁺ mregDCs reside within the sentinel lymph nodes of melanoma patients. Collectively, this work describes a tumor-driven SREBP2-dependent program that promotes CD63 ⁺ mregDC development and function while serving as a promising therapeutic target for overcoming immune tolerance in the TME.

One Sentence Summary

The metabolic transcription factor, SREBF2, regulates the development and tolerogenic function of the mregDC population within the tumor microenvironment.

Reshaping the tumor microenvironment with oncolytic viruses, positive regulation of the immune synapse, and blockade of the immunosuppressive oncometabolic circuitry

BACKGROUND

Oncolytic viruses are considered part of immunotherapy and have shown promise in preclinical experiments and clinical trials. Results from these studies have suggested that tumor microenvironment remodeling is required to achieve an effective response in solid tumors. Here, we assess the extent to which targeting specific mechanisms underlying the immunosuppressive tumor microenvironment optimizes viroimmunotherapy.

METHODS

We used RNA-seq analyses to analyze the transcriptome, and validated the results using Q-PCR, flow cytometry, and immunofluorescence. Viral activity was analyzed by replication assays and viral titration. Kyn and Trp metabolite levels were quantified using liquid chromatography-mass spectrometry. Aryl hydrocarbon receptor (AhR) activation was analyzed by examination of promoter activity. Therapeutic efficacy was assessed by tumor histopathology and survival in syngeneic murine models of gliomas, including Indoleamine 2,3-dioxygenase (IDO)-/- mice. Flow cytometry was used for immunophenotyping and quantification of cell populations. Immune activation was examined in co-cultures of immune and cancer cells. T-cell depletion was used to identify the role played by specific cell populations. Rechallenge experiments were performed to identify the development of anti-tumor memory.

RESULTS

Bulk RNA-seq analyses showed the activation of the immunosuppressive IDO-kynurenine-AhR circuitry in response to Delta-24-RGDOX infection of tumors. To overcome the effect of this pivotal pathway, we combined Delta-24-RGDOX with clinically relevant IDO inhibitors. The combination therapy increased the frequency of CD8+ T cells and decreased the rate of myeloid-derived suppressor cell and immunosupressive Treg tumor populations in animal models of solid tumors. Functional studies demonstrated that IDO-blockade-dependent activation of immune cells against tumor antigens could be reversed by the oncometabolite kynurenine. The concurrent targeting of the effectors and suppressors of the tumor immune landscape significantly prolonged the survival in animal models of orthotopic gliomas.

CONCLUSIONS

Our data identified for the first time the in vivo role of IDO-dependent immunosuppressive pathways in the resistance of solid tumors to oncolytic adenoviruses. Specifically, the IDO-Kyn-AhR activity was responsible for the resurface of local immunosuppression and resistance to therapy, which was ablated through IDO inhibition. Our data indicate that combined molecular and immune therapy may improve outcomes in human gliomas and other cancers treated with virotherapy.

Immune landscape in vulvar cancer-draining lymph nodes indicates distinct immune escape mechanisms in support of metastatic spread and growth

Background

Therapeutic immune intervention is highly dependent on the T-cell priming and boosting capacity of tumor-draining lymph nodes (TDLN). In vulvar cancer, in-depth studies on the immune status of (pre)metastatic TDLN is lacking.

Methods

We have phenotyped and enumerated various T-cell and myeloid subsets in tumor-free (LN−, n=27) and metastatic TDLN (LN+, n=11) using flow cytometry. Additionally, we studied chemokine and cytokine release profiles and assessed expression of indoleamine 2,3-dioxygenase (IDO) in relation to plasmacytoid dendritic cell (pDC) or myeloid subsets.

Results

Metastatic involvement of TDLN was accompanied by an inflamed microenvironment with immune suppressive features, marked by hampered activation of migratory DC, increased cytokine/chemokine release, and closely correlated elevations of pDC and LN-resident conventional DC (LNR-cDC) activation state and frequencies, as well as of terminal CD8⁺ effector-memory T-cell (TemRA) differentiation, regulatory T-cell (Treg) rates, T-cell activation, and expression of cytotoxic T-lymphocyte protein-4 (CTLA-4) and programmed cell death protein-1 (PD-1) immune checkpoints. In addition, high indoleamine 2,3-dioxygenase (IDO) expression and increased frequencies of monocytic myeloid-derived suppressor cells (mMDSC) were observed. Correlation analyses with primary and metastatic tumor burden suggested respective roles for Tregs and suppression of inducible T cell costimulator (ICOS)⁺ T helper cells in early metastatic niche formation and for CD14⁺ LNR-cDC and terminal T-cell differentiation in later stages of metastatic growth.

Conclusions

Metastatic spread in vulvar TDLN is marked by an inflamed microenvironment with activated effector T cells, which are likely kept in check by an interplay of suppressive feedback mechanisms. Our data support (neoadjuvant) TDLN-targeted therapeutic interventions based on CTLA-4 and PD-1 blockade, to reinvigorate memory T cells and curb early metastatic spread and growth.

Neuroinflammation in Autoimmune Disease and Primary Brain Tumors: The Quest for Striking the Right Balance

According to classical dogma, the central nervous system (CNS) is defined as an immune privileged space. The basis of this theory was rooted in an incomplete understanding of the CNS microenvironment, however, recent advances such as the identification of resident dendritic cells (DC) in the brain and the presence of CNS lymphatics have deepened our understanding of the neuro-immune axis and revolutionized the field of neuroimmunology. It is now understood that many pathological conditions induce an immune response in the CNS, and that in many ways, the CNS is an immunologically distinct organ. Hyperactivity of neuro-immune axis can lead to primary neuroinflammatory diseases such as multiple sclerosis and antibody-mediated encephalitis, whereas immunosuppressive mechanisms promote the development and survival of primary brain tumors. On the therapeutic front, attempts are being made to target CNS pathologies using various forms of immunotherapy. One of the most actively investigated areas of CNS immunotherapy is for the treatment of glioblastoma (GBM), the most common primary brain tumor in adults. In this review, we provide an up to date overview of the neuro-immune axis in steady state and discuss the mechanisms underlying neuroinflammation in autoimmune neuroinflammatory disease as well as in the development and progression of brain tumors. In addition, we detail the current understanding of the interactions that characterize the primary brain tumor microenvironment and the implications of the neuro-immune axis on the development of successful therapeutic strategies for the treatment of CNS malignancies.

Phase II trial of the IDO pathway inhibitor indoximod plus pembrolizumab for the treatment of patients with advanced melanoma

Background

The indoleamine 2,3-dioxygenase (IDO) pathway is a key counter-regulatory mechanism that, in cancer, is exploited by tumors to evade antitumor immunity. Indoximod is a small-molecule IDO pathway inhibitor that reverses the immunosuppressive effects of low tryptophan (Trp) and high kynurenine (Kyn) that result from IDO activity. In this study, indoximod was used in combination with a checkpoint inhibitor (CPI) pembrolizumab for the treatment for advanced melanoma.

Methods

Patients with advanced melanoma were enrolled in a single-arm phase II clinical trial evaluating the addition of indoximod to standard of care CPI approved for melanoma. Investigators administered their choice of CPI including pembrolizumab (P), nivolumab (N), or ipilimumab (I). Indoximod was administered continuously (1200 mg orally two times per day), with concurrent CPI dosed per US Food and Drug Administration (FDA)-approved label.

Results

Between July 2014 and July 2017, 131 patients were enrolled. (P) was used more frequently (n=114, 87%) per investigator’s choice. The efficacy evaluable population consisted of 89 patients from the phase II cohort with non-ocular melanoma who received indoximod combined with (P).

The objective response rate (ORR) for the evaluable population was 51% with confirmed complete response of 20% and disease control rate of 70%. Median progression-free survival was 12.4 months (95% CI 6.4 to 24.9). The ORR for Programmed Death-Ligand 1 (PD-L1)-positive patients was 70% compared with 46% for PD-L1-negative patients. The combination was well tolerated, and side effects were similar to what was expected from single agent (P).

Conclusion

In this study, the combination of indoximod and (P) was well tolerated and showed antitumor efficacy that is worth further evaluation in selected patients with advanced melanoma.

Altered ratio of dendritic cell subsets in skin-draining lymph nodes promotes Th2-driven contact hypersensitivity

Plasmacytoid dendritic cells (pDCs) specialize in the production of type I IFN (IFN-I). pDCs can be depleted in vivo by injecting diphtheria toxin (DT) in a mouse in which pDCs express a diphtheria toxin receptor (DTR) transgene driven by the human CLEC4C promoter. This promoter is enriched for binding sites for TCF4, a transcription factor that promotes pDC differentiation and expression of pDC markers, including CLEC4C. Here, we found that injection of DT in CLEC4C-DTR⁺ mice markedly augmented Th2-dependent skin inflammation in a model of contact hypersensitivity (CHS) induced by the hapten fluorescein isothiocyanate. Unexpectedly, this biased Th2 response was independent of reduced IFN-I accompanying pDC depletion. In fact, DT treatment altered the representation of conventional dendritic cells (cDCs) in the skin-draining lymph nodes during the sensitization phase of CHS; there were fewer Th1-priming CD326⁺ CD103⁺ cDC1 and more Th2-priming CD11b⁺ cDC2. Single-cell RNA-sequencing of CLEC4C-DTR⁺ cDCs revealed that CD326⁺ DCs, like pDCs, expressed DTR and were depleted together with pDCs by DT treatment. Since CD326⁺ DCs did not express Tcf4, DTR expression might be driven by yet-undefined transcription factors activating the CLEC4C promoter. These results demonstrate that altered DC representation in the skin-draining lymph nodes during sensitization to allergens can cause Th2-driven CHS.

Proteostasis in dendritic cells is controlled by the PERK signaling axis independently of ATF4

Differentiated dendritic cells display an unusual activation of the integrated stress response, which is necessary for normal type-I Interferon production and cell migration.

In stressed cells, phosphorylation of eukaryotic initiation factor 2α (eIF2α) controls transcriptome-wide changes in mRNA translation and gene expression known as the integrated stress response. We show here that DCs are characterized by high eIF2α phosphorylation, mostly caused by the activation of the ER kinase PERK (EIF2AK3). Despite high p-eIF2α levels, DCs display active protein synthesis and no signs of a chronic integrated stress response. This biochemical specificity prevents translation arrest and expression of the transcription factor ATF4 during ER-stress induction by the subtilase cytotoxin (SubAB). PERK inactivation, increases globally protein synthesis levels and regulates IFN-β expression, while impairing LPS-stimulated DC migration. Although the loss of PERK activity does not impact DC development, the cross talk existing between actin cytoskeleton dynamics; PERK and eIF2α phosphorylation is likely important to adapt DC homeostasis to the variations imposed by the immune contexts.

Plasmacytoid dendritic cells in granulomatous variant of mycosis fungoides

INTRODUCTION

Granulomatous mycosis fungoides (MF) is a rare variant in which granulomas are associated with other typical signs of MF. Its prognosis is worse than that of classical MF. Plasmacytoid dendritic cells (PDCs) are a subset of interferon-producing dendritic cells that link the innate and the adaptative immune responses. They have also been related to tolerance to certain tumors such as melanoma.

MATERIALS AND METHODS

In this article, we examined for the presence of CD123+ PDC in six cases of granulomatous MF from our archives.

RESULTS

We found clusters of 10 or more positive cells in three of six cases of granulomatous MF (two women and a man, in their sixth and seventh decade). Although in two of these three cases the granulomatous response was extensive, in the other, it only represented 10% of the infiltrate of the biopsy. In all three cases, the granulomas were epithelioid, sarcoidal type.

CONCLUSIONS

CD123+ PDC can be identified in granulomatous MF. The pathogenic and prognostic role of this finding requires further clarification.

Regulatory T Cells: From Discovery to Autoimmunity

Multiple sclerosis (MS) is a genetically mediated autoimmune disease of the central nervous system. Allelic variants lead to lower thresholds of T-cell activation resulting in activation of autoreactive T cells. Environmental factors, including, among others, diet, vitamin D, and smoking, in combination with genetic predispositions, play a substantial role in disease development and activation of autoreactive T cells. FoxP3⁺ regulatory T cells (Tregs) have emerged as central in the control of autoreactive T cells. A consistent finding in patients with MS is defects in Treg cell function with reduced suppression of effector T cells and production of proinflammatory cytokines. Emerging data suggests that functional Tregs become effector-like T cells with loss of function associated with T-bet expression and interferon γ (IFN-γ) secretion.

The Regulatory Status Adopted by Lymph Node Dendritic Cells and T Cells During Healthy Aging Is Maintained During Cancer and May Contribute to Reduced Responses to Immunotherapy

Aging is associated with an increased incidence of cancer. One contributing factor could be modulation of immune cells responsible for anti-tumor responses, such as dendritic cells (DCs) and T cells. These immunological changes may also impact the efficacy of cancer immunotherapies in the elderly. The effects of healthy aging on DCs and T cells, and their impact on anti-mesothelioma immune responses, had not been reported. This study examined DCs and T cells in young (2–5 months; equivalent to 16–26 human years) and elderly (20–24 months; equivalent to 60–70 human years) healthy and mesothelioma-bearing C57BL/6J mice. During healthy aging, elderly lymph nodes adopted a regulatory profile, characterized by: (i) increased plasmacytoid DCs, (ii) increased expression of the adenosine-producing enzyme CD73 on CD11c⁺ cells, and (iii) increased expression of multiple regulatory markers (including CD73, the adenosine A2B receptor, CTLA-4, PD-1, ICOS, LAG-3, and IL-10) on CD8⁺ and CD4⁺ T cells, compared to lymph nodes from young mice. Although mesotheliomas grew faster in elderly mice, the increased regulatory status observed in healthy elderly lymph node DCs and T cells was not further exacerbated. However, elderly tumor-bearing mice demonstrated reduced MHC-I, MHC-II and CD80 on CD11c⁺ cells, and decreased IFN-γ by CD8⁺ and CD4⁺ T cells within tumors, compared to young counterparts, implying loss of function. An agonist CD40 antibody based immunotherapy was less efficient at promoting tumor regression in elderly mice, which may be due to: (i) failure of elderly CD8⁺ T cells to up-regulate perforin, and (ii) increased expression of multiple regulatory markers on CD11c⁺ cells and T cells in elderly tumor-draining lymph nodes (including CD73, PD-1, ICOS, LAG-3, and TGF-β). Our findings suggest that checkpoint blockade may improve responses to immunotherapy in elderly hosts with mesothelioma, and warrants further investigation.

The role of the kynurenine pathway of tryptophan metabolism in cardiovascular disease

Coronary heart disease and stroke, the deadliest forms of cardiovascular disease (CVD), are mainly caused by atherosclerosis, a chronic inflammatory disease of the artery wall driven by maladaptive immune responses in the vessel wall. Various risk factors for CVD influence this pathogenic process, including diabetes mellitus, hypertension, dyslipidaemia, and obesity. Indoleamine 2,3-dioxygenase (IDO), an enzyme catalyzing the rate-limiting step in the kynurenine pathway of tryptophan degradation, is strongly induced by inflammation in several tissues, including the artery wall. An increasing body of evidence indicates that IDO promotes immune tolerance, decreases inflammation, and functions as a homeostatic mechanism against excessive immune reactions.

This review provides an overview of the emerging field of the kynurenine pathway of tryptophan degradation in CVD, emphasizing the role of IDO-mediated tryptophan metabolism and its metabolites in the modulation of ‘classical’ cardiovascular risk factors, such as hypertension, obesity, lipid metabolism, diabetes mellitus, and in the development of atherosclerotic CVD.

The rationale of indoleamine 2,3-dioxygenase inhibition for cancer therapy

Indoleamine 2,3-dioxygenase (IDO, also referred to as IDO1) has been demonstrated to be a normal endogenous mechanism of acquired peripheral immune tolerance in vivo. In the field of oncology, IDO expression and/or activity has been observed in several cancer types and has usually been associated with negative prognostic factors and worse outcome measures. This manuscript reviews current available data on the role of IDO in cancer and the current results obtained with IDO inhibition, both in animal models and in phase 1 and 2 clinical trials in humans. Preliminary results with IDO inhibitors, usually combined with other anti-cancer drugs, seem encouraging. Further studies are needed to clarify the conditions in which IDO inhibitors can be of value as an anti-cancer strategy. In addition, further research should address whether the expression of IDO in tissue or blood can be a marker to select patients who can benefit most from IDO inhibition.

Plasmacytoid Dendritic Cells Respond Directly to Apoptotic Cells by Secreting Immune Regulatory IL-10 or IFN-α

Plasmacytoid dendritic cells (pDCs) play a pivotal role in driving the autoimmune disease systemic lupus erythematosus, via the secretion of IFN-α in response to nuclear self-antigens complexed with autoantibodies. Apoptotic cells, generated at sites of inflammation or secondary lymphoid organs, are exposed to activated pDCs and also express the same nuclear antigens on their cell surface. Here, we show that in the absence of autoantibodies, activated pDCs directly respond to apoptotic cell-expressed chromatin complexes by secreting IL-10 and IL-6, which also induces T cells to secrete IL-10. Conversely, when activated by the viral mimetic CpG-A, apoptotic cells enhance their secretion of IFN-α. This study demonstrates that activated pDCs respond directly to apoptotic cells and may maintain tolerance via IL-10, or promote inflammation through secretion of IFN-α, depending on the inflammatory context.

Tumor-induced MDSC act via remote control to inhibit L-selectin-dependent adaptive immunity in lymph nodes

Myeloid-derived suppressor cells (MDSC) contribute to an immunosuppressive network that drives cancer escape by disabling T cell adaptive immunity. The prevailing view is that MDSC-mediated immunosuppression is restricted to tissues where MDSC co-mingle with T cells. Here we show that splenic or, unexpectedly, blood-borne MDSC execute far-reaching immune suppression by reducing expression of the L-selectin lymph node (LN) homing receptor on naïve T and B cells. MDSC-induced L-selectin loss occurs through a contact-dependent, post-transcriptional mechanism that is independent of the major L-selectin sheddase, ADAM17, but results in significant elevation of circulating L-selectin in tumor-bearing mice. Even moderate deficits in L-selectin expression disrupt T cell trafficking to distant LN. Furthermore, T cells preconditioned by MDSC have diminished responses to subsequent antigen exposure, which in conjunction with reduced trafficking, severely restricts antigen-driven expansion in widely-dispersed LN. These results establish novel mechanisms for MDSC-mediated immunosuppression that have unanticipated implications for systemic cancer immunity.

DOI:http://dx.doi.org/10.7554/eLife.17375.001

IDO1 is an Integral Mediator of Inflammatory Neovascularization

The immune tolerogenic effects of IDO1 (indoleamine 2,3-dioxygenase 1) have been well documented and genetic studies in mice have clearly established the significance of IDO1 in tumor promotion. Dichotomously, the primary inducer of IDO1, the inflammatory cytokine IFNγ (interferon-γ), is a key mediator of immune-based tumor suppression. One means by which IFNγ can exert an anti-cancer effect is by decreasing tumor neovascularization. We speculated that IDO1 might contribute to cancer promotion by countering this anti-neovascular effect of IFNγ, possibly through IDO1-potentiated elevation of the pro-tumorigenic inflammatory cytokine IL6 (interleukin-6). In this study, we investigated how genetic loss of IDO1 affects neovascularization in mouse models of oxygen-induced retinopathy and lung metastasis. Neovascularization in both models was significantly reduced in mice lacking IDO1, was similarly reduced with loss of IL6, and was restored in both cases by concomitant loss of IFNγ. Likewise, the lack of IDO1 or IL6 resulted in reduced metastatic tumor burden and increased survival, which the concomitant loss of IFNγ abrogated. This insight into IDO1's involvement in pro-tumorigenic inflammatory neovascularization may have important ramifications for IDO1 inhibitor development, not only in cancer where clinical trials are currently ongoing, but in other disease indications associated with neovascularization as well.

The Kynurenine Pathway Is a Double-Edged Sword in Immune-Privileged Sites and in Cancer: Implications for Immunotherapy

The term “immune privilege” was originally coined to describe the suppression of inflammatory responses within organs protected by anatomic barriers, ie, the eyes, brain, placenta, and testes. However, cellular and metabolic processes, which orchestrate immune responses, also control inflammation within these sites. Our current understanding of tolerogenic mechanisms has extended the definition of immune privilege to include hair follicles, the colon, and cancer. By catabolizing tryptophan, cells expressing the enzyme indoleamine-2,3-dioxygenase produce kynurenine metabolites, which orchestrate local and systemic responses to control inflammation, thus maintaining immune privilege. This review highlights the double-edged role played by the kynurenine pathway (KP), which establishes and maintains immune-privileged sites while contributing to cancer immune escape. The identification of the underlying molecular drivers of the KP in immune-privileged sites and in cancer is essential for the development of novel therapies to treat autoimmunity and cancer and to improve transplantation outcomes.

Small molecule drugs with immunomodulatory effects in cancer

Immunotherapy shows promise for positively changing the landscape of the management of many advanced solid tumors, including gastrointestinal (GI) malignancies. Many of these developments have been focused on vaccine-based, monoclonal antibody therapies and more recently, checkpoint inhibitors, although many small molecule inhibitors can function as immunomodulators. Small molecule compounds have several advantages over conventional immunotherapeutic agents including: ease of production and the potential for oral administration. There is a potential niche for small molecule immunomodulators to enhance the efficacy of existing immunotherapeutic and cytotoxic agents. This article focuses on two categories of small molecule compounds with immunomodulatory effects: IDO and MEK inhibitors. Indoleamine -2, 3- dioxygenase (IDO) is known for its effects in tumor immunity. IDO inhibitors are generally well-tolerated and have the potential to enhance anti-tumor responses when combined with checkpoint inhibitors. MEK inhibitors affect signal transduction of the RAS-RAF-MEK pathway and numerous MEK inhibitors are currently being investigated in solid tumors. Small molecule immunomodulators are currently being investigated for their potential role in augmenting the effects of conventional immunotherapeutic agents although further research is required to identify those patients most likely to respond to combination therapy.

Trial Watch-Small molecules targeting the immunological tumor microenvironment for cancer therapy

Progressing malignancies establish robust immunosuppressive networks that operate both systemically and locally. In particular, as tumors escape immunosurveillance, they recruit increasing amounts of myeloid and lymphoid cells that exert pronounced immunosuppressive effects. These cells not only prevent the natural recognition of growing neoplasms by the immune system, but also inhibit anticancer immune responses elicited by chemo-, radio- and immuno therapeutic interventions. Throughout the past decade, multiple strategies have been devised to counteract the accumulation or activation of tumor-infiltrating immunosuppressive cells for therapeutic purposes. Here, we review recent preclinical and clinical advances on the use of small molecules that target the immunological tumor microenvironment for cancer therapy. These agents include inhibitors of indoleamine 2,3-dioxigenase 1 (IDO1), prostaglandin E2, and specific cytokine receptors, as well as modulators of intratumoral purinergic signaling and arginine metabolism.

Virus Infections Incite Pain Hypersensitivity by Inducing Indoleamine 2,3 Dioxygenase

Increased pain sensitivity is a comorbidity associated with many clinical diseases, though the underlying causes are poorly understood. Recently, chronic pain hypersensitivity in rodents treated to induce chronic inflammation in peripheral tissues was linked to enhanced tryptophan catabolism in brain mediated by indoleamine 2,3 dioxygenase (IDO). Here we show that acute influenza A virus (IAV) and chronic murine leukemia retrovirus (MuLV) infections, which stimulate robust IDO expression in lungs and lymphoid tissues, induced acute or chronic pain hypersensitivity, respectively. In contrast, virus-induced pain hypersensitivity did not manifest in mice lacking intact IDO1 genes. Spleen IDO activity increased markedly as MuLV infections progressed, while IDO1 expression was not elevated significantly in brain or spinal cord (CNS) tissues. Moreover, kynurenine (Kyn), a tryptophan catabolite made by cells expressing IDO, incited pain hypersensitivity in uninfected IDO1-deficient mice and Kyn potentiated pain hypersensitivity due to MuLV infection. MuLV infection stimulated selective IDO expression by a discreet population of spleen cells expressing both B cell (CD19) and dendritic cell (CD11c) markers (CD19+ DCs). CD19+ DCs were more susceptible to MuLV infection than B cells or conventional (CD19neg) DCs, proliferated faster than B cells from early stages of MuLV infection and exhibited mature antigen presenting cell (APC) phenotypes, unlike conventional (CD19neg) DCs. Moreover, interactions with CD4 T cells were necessary to sustain functional IDO expression by CD19+ DCs in vitro and in vivo. Splenocytes from MuLV-infected IDO1-sufficient mice induced pain hypersensitivity in uninfected IDO1-deficient recipient mice, while selective in vivo depletion of DCs alleviated pain hypersensitivity in MuLV-infected IDO1-sufficient mice and led to rapid reduction in splenomegaly, a hallmark of MuLV immune pathogenesis. These findings reveal critical roles for CD19+ DCs expressing IDO in host responses to MuLV infection that enhance pain hypersensitivity and cause immune pathology. Collectively, our findings support the hypothesis elevated IDO activity in non-CNS due to virus infections causes pain hypersensitivity mediated by Kyn. Previously unappreciated links between host immune responses to virus infections and pain sensitivity suggest that IDO inhibitors may alleviate heightened pain sensitivity during infections.

IDO2 Modulates T Cell-Dependent Autoimmune Responses through a B Cell-Intrinsic Mechanism

Mechanistic insight into how adaptive immune responses are modified along the self-nonself continuum may offer more effective opportunities to treat autoimmune disease, cancer, and other sterile inflammatory disorders. Recent genetic studies in the KRN mouse model of rheumatoid arthritis demonstrate that the immunomodulatory molecule IDO2 modifies responses to self-antigens; however, the mechanisms involved are obscure. In this study, we show that IDO2 exerts a critical function in B cells to support the generation of autoimmunity. In experiments with IDO2-deficient mice, adoptive transplant experiments demonstrated that IDO2 expression in B cells was both necessary and sufficient to support robust arthritis development. IDO2 function in B cells was contingent on a cognate, Ag-specific interaction to exert its immunomodulatory effects on arthritis development. We confirmed a similar requirement in an established model of contact hypersensitivity, in which IDO2-expressing B cells are required for a robust inflammatory response. Mechanistic investigations showed that IDO2-deficient B cells lacked the ability to upregulate the costimulatory marker CD40, suggesting IDO2 acts at the T-B cell interface to modulate the potency of T cell help needed to promote autoantibody production. Overall, our findings revealed that IDO2 expression by B cells modulates autoimmune responses by supporting the cross talk between autoreactive T and B cells.

Targeting the indoleamine 2,3-dioxygenase pathway in cancer

Tumor cells escape the immune surveillance system of the host through a process called immune tolerance. Immunotherapy targets molecules that serve as checks and balances in the regulation of immune response. Indoleamine-2,3-dioxygenase (IDO) is an intracellular enzyme, which through the process of tryptophan depletion exerts an immunosuppressive effect, facilitating immune escape of tumors. This review summarizes our current knowledge on IDO expression in malignancies, the IDO inhibitors that are currently available and those under clinical development.

Deletion of LRP5 and LRP6 in dendritic cells enhances antitumor immunity

The tumor microenvironment (TME) contains high levels of the Wnt family of ligands, and aberrant Wnt-signaling occurs in many tumors. Past studies have been directed toward how the Wnt signaling cascade regulates cancer development, progression and metastasis. However, its effects on host antitumor immunity remain unknown. In this report, we show that Wnts in the TME condition dendritic cells (DCs) to a regulatory state and suppress host antitumor immunity. DC-specific deletion of Wnt co-receptors low-density lipoprotein receptor-related protein 5 and 6 (LRP5/6) in mice markedly delayed tumor growth and enhanced host antitumor immunity. Mechanistically, loss of LRP5/6-mediated signaling in DCs resulted in enhanced effector T cell differentiation and decreased regulatory T cell differentiation. This was due to increased production of pro-inflammatory cytokines and decreased production of IL-10, TGF-β1 and retinoic acid (RA). Likewise, pharmacological inhibition of the Wnts' interaction with its cognate co-receptors LRP5/6 and Frizzled (Fzd) receptors had similar effects on tumor growth and effector T cell responses. Moreover, blocking Wnt-signaling in DCs resulted in enhanced capture of tumor-associated antigens and efficient cross-priming of CD8⁺ T cells. Hence, blocking the Wnt pathway represents a potential therapeutic to overcome tumor-mediated immune suppression and augment antitumor immunity.

Dendritic Cells in Oncolytic Virus-Based Anti-Cancer Therapy

Dendritic cells (DCs) are specialized antigen-presenting cells that have a notable role in the initiation and regulation of innate and adaptive immune responses. In the context of cancer, appropriately activated DCs can induce anti-tumor immunity by activating innate immune cells and tumor-specific lymphocytes that target cancer cells. However, the tumor microenvironment (TME) imposes different mechanisms that facilitate the impairment of DC functions, such as inefficient antigen presentation or polarization into immunosuppressive DCs. These tumor-associated DCs thus fail to initiate tumor-specific immunity, and indirectly support tumor progression. Hence, there is increasing interest in identifying interventions that can overturn DC impairment within the TME. Many reports thus far have studied oncolytic viruses (OVs), viruses that preferentially target and kill cancer cells, for their capacity to enhance DC-mediated anti-tumor effects. Herein, we describe the general characteristics of DCs, focusing on their role in innate and adaptive immunity in the context of the TME. We also examine how DC-OV interaction affects DC recruitment, OV delivery, and anti-tumor immunity activation. Understanding these roles of DCs in the TME and OV infection is critical in devising strategies to further harness the anti-tumor effects of both DCs and OVs, ultimately enhancing the efficacy of OV-based oncotherapy.

Respiratory syncytial virus induces indoleamine 2,3-dioxygenase activity: a potential novel role in the development of allergic disease

BACKGROUND

Infants that develop severe bronchiolitis due to respiratory syncytial virus (RSV) are at increased risk of developing asthma later in life. We investigated a potential immunological mechanism for the association between RSV and the development of allergic inflammation. The enzyme indoleamine 2,3-dioxygenase (IDO) has been reported to induce selective apoptosis of T helper 1 (Th1) cells and contributed to Th2-biased immune responses.

OBJECTIVE

To determine whether RSV infection in vitro could induce IDO expression and bioactivity in human dendritic cells, leading to a Th2-biased immune response.

METHODS

Human peripheral blood monocytes from healthy adult donors were isolated, differentiated to dendritic cells (moDC), in vitro. We studied RSV infection and mechanisms of IDO activation in moDC with subsequent effect on T-bet expression.

RESULTS

We found that moDC were infected by RSV and that this induced IDO activation. RSV-induced IDO activity was inhibited by palivizumab, UV inactivation, TL4R inhibition, and ribavirin. However, blocking endosomal TLR function with chloroquine did not inhibit IDO activity. Selective inhibitors suggested that RSV-induced IDO activity was dependent on the retinoic acid-inducible gene-I (RIG-I) related pathway via NF-κB and p38 MAPK. Coculture of RSV-infected moDC with activated T cells, in a transwell system, suppressed expression of T-bet (a Th1-associated factor) but not GATA3 (a Th2 regulator). Inhibition of IDO activity with the competitive inhibitor, 1-methyl tryptophan, blocked the effect on T-bet expression.

CONCLUSION AND CLINICAL RELEVANCE

Our data show for the first time that RSV can induce the expression and bioactivity of IDO in human moDC, in a virus replication-dependant fashion. We suggest that RSV activation of IDO could be a potential mechanism for the development of allergic diseases.

Research progress of indoleamine 2,3-dioxygenase inhibitors

Indoleamine 2,3-dioxygenase (IDO, subsequently named IDO1) can degrade the level of essential amino acid tryptophan in mammals, and catalyze the initial and rate-limiting step through the kynurenine pathway. Broad evidence implies that IDO is overexpressed in both tumor cells and antigen-presenting cells, facilitating the escape of malignant tumors from immune surveillance. In the past decades, the inhibition of IDO has been one of the most promising areas in cancer immunotherapy and many potential inhibitors of IDO have been designed, synthesized and evaluated, among which d-1-methyl-tryptophan and INCB24360 have advanced to clinical trial stage. This review aims to give an overview of the rationale for IDO as a therapeutic target as well as the research progress of IDO inhibitors.

Engineering better immunotherapies via RNA interference

The therapeutic potential of dendritic cell (DC) cancer vaccines has gained momentum in recent years. However, clinical data indicate that antitumor immune responses generally fail to translate into measurable tumor regression. This has been ascribed to a variety of tolerance mechanisms, one of which is the expression of immunosuppressive factors by DCs and T cells. With respect to cancer immunotherapies, these factors antagonise the ability to induce robust and sustained immunity required for tumor cell eradication. Gene silencing of immunosuppressive factors in either DCs or adoptive transferred T cells enhanced anti-tumor immune responses and significantly inhibited tumor growth. Therefore, engineered next generation of DC vaccines or adoptive T-cell therapy should include immunomodulatory siRNAs to release the "brakes" imposed by the immune system. Moreover, the combination of gene silencing, antigen targeting to DCs and cytoplasmic cargo delivery will improve clinical benefits.

Effect of the association of 1-methyl-DL-tryptophan with paclitaxel on the expression of indoleamine 2,3-dioxygenase in cultured cancer cells from patients with breast cancer

Breast cancer is the most common type of cancer among women and the survival of patients affected by it is increasing, mainly due to several new approaches in early diagnosis and more effective treatments. The enzyme indoleamine 2,3-dioxygenase (IDO) is expressed in many cells, including tumor cells. IDO acts by inhibiting the proliferation of T lymphocytes, thus compromising their cytotoxic activity. 1-Methyl-DL-tryptophan (1MT) is a competitive inhibitor of IDO, which blocks its immunosuppressive effect. Paclitaxel is an antineoplastic drug largely used in breast cancer therapy. Thus, this study aimed to determine the in vitro effect of the association of 1MT and paclitaxel chemotherapy, as an approach to reduce tumor growth. It is believed that this would allow the restoration of T lymphocyte proliferation capability and its cytotoxic response. The supplemented cultures showed that the most significant differences in the expression of IDO were observed in the group treated with paclitaxel associated with 1-MT continuous supplementation, reducing enzyme expression from 12.06 to 3.56 %. This association was more effective in reducing IDO expression and could collaborate in developing a new therapeutic strategy for breast cancer treatment.

Tumor-Expressed IDO Recruits and Activates MDSCs in a Treg-Dependent Manner

Indoleamine 2,3-dioxygenase (IDO) has been described as a major mechanism of immunosuppression in tumors, though the mechanisms of this are poorly understood. Here, we find that expression of IDO by tumor cells results in aggressive tumor growth and resistance to T-cell-targeting immunotherapies. We demonstrate that IDO orchestrates local and systemic immunosuppressive effects through recruitment and activation of myeloid-derived suppressor cells (MDSCs), through a mechanism dependent on regulatory T cells (Tregs). Supporting these findings, we find that IDO expression in human melanoma tumors is strongly associated with MDSC infiltration. Treatment with a selective IDO inhibitor in vivo reversed tumor-associated immunosuppression by decreasing numbers of tumor-infiltrating MDSCs and Tregs and abolishing their suppressive function. These findings establish an important link between IDO and multiple immunosuppressive mechanisms active in the tumor microenvironment, providing a strong rationale for therapeutic targeting of IDO as one of the central regulators of immune suppression.

Parallel Expression of Enzyme Inhibitors of CD8T Cell Activity in Tumor Microenvironments and Secretory Endometrium

The divergent requirement for tolerance to support conception and protective response against sexually transmitted infections defines the unique immunological dynamics in the female reproductive tract (FRT). In part, these requirements are achieved by the cyclic modulation of cytolytic CD8T cell function in the FRT that underlie the respective immunosuppressive and immunocompetent milieus during the secretory and proliferative phases of the menstrual cycle. The CD8T cell function can be dampened by exposure to indoleamine 2,3-dioxygenase and/or arginase enzymes. Indeed, these 2 enzymes are known as primary inducers of immune suppression in tumor microenvironments. This review discusses the intriguing parallel expression of these 2 enzymes in tumor microenvironments and in the secretory endometrium. We surmise that investigating the underlying natural mechanisms that suppress and restore the immunocompetence of CD8T cells in the FRT each month may provide valuable insights into ways to artificially recapitulate these mechanisms and inhibit immune suppression in tumor microenvironments.

Novel immunotherapies for hematologic malignancies

The immune system is designed to discriminate between self and tumor tissue. Through genetic recombination, there is fundamentally no limit to the number of tumor antigens that immune cells can recognize. Yet, tumors use a variety of immunosuppressive mechanisms to evade immunity. Insight into how the immune system interacts with tumors is expanding rapidly and has accelerated the translation of immunotherapies into medical breakthroughs. Herein, we appraise novel strategies that exploit the patient's immune system to kill cancer. We review various forms of immune-based therapies, which have shown significant promise in patients with hematologic malignancies, including (i) conventional monoclonal therapies like rituximab; (ii) engineered monoclonal antibodies called bispecific T-cell engagers; (iii) monoclonal antibodies and pharmaceutical drugs that block inhibitory T-cell pathways (i.e. PD-1, CTLA-4, and IDO); and (iv) adoptive cell transfer therapy with T cells engineered to express chimeric antigen receptors or T-cell receptors. We also assess the idea of using these therapies in combination and conclude by suggesting multi-prong approaches to improve treatment outcomes and curative responses in patients.

Effects of indoleamine 2,3-dioxygenase inhibitor in non-Hodgkin lymphoma model mice

Indoleamine 2,3-dioxygenase (IDO) catalyzes the rate-limiting step in the metabolism of tryptophan along the kynurenine pathway. In tumors, increased IDO activity inhibits proliferation and induces apoptosis of T cells and natural killer cells. We investigated the therapeutic potential of IDO inhibitor 1-methyl-D-tryptophan (D-1MT) with cyclophosphamide (CY) in a mouse model of lymphoma. To examine the effect of D-1MT, mice were killed on day 28. Serum concentrations of L-kynurenine and L-tryptophan were measured by high-performance liquid chromatography. Regulatory T cells (Tregs) were counted by flow cytometry, and mRNA expressions of IDO1, Foxp3, IFN-γ, and COX-2 were examined by quantitative real-time reverse transcription-polymerase chain reaction. D-1MT+CY combination treatment significantly inhibited tumor growth as compared to either treatment alone. There were no significant differences in the serum L-kynurenine/L-tryptophan ratio or the IDO1 expression level in the tumors among the treatment groups. The expression levels of IFN-γ and COX-2 mRNA in tumor-draining lymph nodes (TDLNs) were found to be significantly up-regulated in the CY and D-1MT+CY groups. The number of Tregs in TDLNs in the D-1MT+CY group was significantly lower than that in CY groups on day 17. These results suggest that D-1MT in combination with CY is an effective treatment for lymphoma in a mouse model.

Potential roles of self-reactive T cells in autoimmunity: lessons from cancer immunology

The immune system is a complex arrangement of cells and molecules that preserve the integrity of the organism by eliminating all elements judged to be dangerous. Several regulatory mechanisms function to terminate immune responses to antigens, return the immune system to a basal state after the antigen has been cleared, and maintain unresponsiveness, or tolerance, to self-antigens. In recent years, reports have described T cell responses to several proteins involved in regulating the immune system, particularly under malignant conditions. The present review highlights specific T cells that recognize proteins involved in three, well-defined immunosuppressive mechanisms: (1) inhibitory T cell pathways (i.e., PD-L1), (2) regulatory T cells (i.e., Foxp3(+)), and (3) metabolic enzymes, like indoleamine-2,3-dioxygenase. Cytotoxic T cells can eliminate regulatory cells, thereby suppressing and/or delaying local immune suppression; conversely, regulatory CD4(+) and non-cytotoxic CD8(+) T cells enhance target-mediated immune suppression. The apparent lack of tolerance against endogenous proteins expressed by regulatory cells is intriguing, because it suggests that self-reactive T cells play a general role of fine-tuning the immune system. Thus, T cell responses may be generally used to maintain the homeostasis of the immune system. Further exploration is warranted to investigate the potential role of auto-reactive T cells under different physiological and/or pathological conditions.

B Cell-Intrinsic IDO1 Regulates Humoral Immunity to T Cell-Independent Antigens

Humoral responses to nonproteinaceous Ags (i.e., T cell independent [TI]) are a key component of the early response to bacterial and viral infection and a critical driver of systemic autoimmunity. However, mechanisms that regulate TI humoral immunity are poorly defined. In this study, we report that B cell-intrinsic induction of the tryptophan-catabolizing enzyme IDO1 is a key mechanism limiting TI Ab responses. When Ido1(-/-) mice were immunized with TI Ags, there was a significant increase in Ab titers and formation of extrafollicular Ab-secreting cells compared with controls. This effect was specific to TI Ags, as Ido1 disruption did not affect Ig production after immunization with protein Ags. The effect of IDO1 abrogation was confined to the B cell compartment, as adoptive transfer of Ido1(-/-) B cells to B cell-deficient mice was sufficient to replicate increased TI responses observed in Ido1(-/-) mice. Moreover, in vitro activation with TLR ligands or BCR crosslinking rapidly induced Ido1 expression and activity in purified B cells, and Ido1(-/-) B cells displayed enhanced proliferation and cell survival associated with increased Ig and cytokine production compared with wild-type B cells. Thus, our results demonstrate a novel, B cell-intrinsic, role for IDO1 as a regulator of humoral immunity that has implications for both vaccine design and prevention of autoimmunity.

Role of indoleamine 2,3-dioxygenase in health and disease

IDO1 (indoleamine 2,3-dioxygenase 1) is a member of a unique class of mammalian haem dioxygenases that catalyse the oxidative catabolism of the least-abundant essential amino acid, L-Trp (L-tryptophan), along the kynurenine pathway. Significant increases in knowledge have been recently gained with respect to understanding the fundamental biochemistry of IDO1 including its catalytic reaction mechanism, the scope of enzyme reactions it catalyses, the biochemical mechanisms controlling IDO1 expression and enzyme activity, and the discovery of enzyme inhibitors. Major advances in understanding the roles of IDO1 in physiology and disease have also been realised. IDO1 is recognised as a prominent immune regulatory enzyme capable of modulating immune cell activation status and phenotype via several molecular mechanisms including enzyme-dependent deprivation of L-Trp and its conversion into the aryl hydrocarbon receptor ligand kynurenine and other bioactive kynurenine pathway metabolites, or non-enzymatic cell signalling actions involving tyrosine phosphorylation of IDO1. Through these different modes of biochemical signalling, IDO1 regulates certain physiological functions (e.g. pregnancy) and modulates the pathogenesis and severity of diverse conditions including chronic inflammation, infectious disease, allergic and autoimmune disorders, transplantation, neuropathology and cancer. In the present review, we detail the current understanding of IDO1’s catalytic actions and the biochemical mechanisms regulating IDO1 expression and activity. We also discuss the biological functions of IDO1 with a focus on the enzyme's immune-modulatory function, its medical implications in diverse pathological settings and its utility as a therapeutic target.

Local immunosuppression induced by high viral load of human papillomavirus: characterization of cellular phenotypes producing interleukin-10 in cervical neoplastic lesions

A specific immune response to human papillomavirus (HPV) in the cervical microenvironment plays a key role in eradicating infection and eliminating mutated cells. However, high-risk HPVs modulate immune cells to create an immunosuppressive microenvironment, and induce these immune cells to produce interleukin 10 (IL-10). This production of IL-10, in conjunction with HPV infection, contributes to the appearance of cervical neoplastic lesions. We sought to characterize the IL-10-producing cellular phenotype, and investigate the influence of host and HPV factors upon the induction of an immunosuppressive microenvironment. Immunohistochemical analysis demonstrated an increase in IL-10 production by keratinocytes, macrophages and Langerhans cells in high-grade cervical lesions and cervical cancer. This increase was more pronounced in patients older than 30 years, and was also correlated with high viral load, and infection with a single HPV type, particularly high-risk HPVs. Our results indicate the existence of a highly immunosuppressive microenvironment composed of different IL-10-producing cellular phenotypes in cervical cancer samples, and samples classified as high-grade cervical lesions (cervical intraepithelial neoplasia stages II and III). The immunosuppressive microenvironment that developed for these different cellular phenotypes favours viral persistence and neoplastic progression.

High serum concentration of L-kynurenine predicts unfavorable outcomes in patients with acute myeloid leukemia

The immunomodulatory effects of indoleamine 2,3-dioxygenase (IDO) are ascribed to its ability to catalyze the breakdown of the L-tryptophan along the L-kynurenine pathway. Because blasts from patients with acute myeloid leukemia (AML) express IDO, the goal of this study was to investigate the role of L-kynurenine as a prognostic marker for AML. We enrolled 48 AML patients. L-kynurenine concentrations were measured by high-performance liquid chromatography. The median serum L-kynurenine level was 1.67 μM. There was no significant difference in the complete remission rate between patients with L-kynurenine < 2.4 (77%) and ≥ 2.4 μM (75%). However, 3-year overall survival (OS) rates were significantly better in patients with low L-kynurenine levels (76%) than in those with high L-kynurenine levels (11%) (p < 0.0001). Furthermore, in intermediate-risk cytogenetics patients, only L-kynurenine was significantly associated with OS (p < 0.005). Multivariate analyses revealed that L-kynurenine and high leukocyte count were independent prognostic factors.

Cancer and the Immune System: Basic Concepts and Targets for Intervention

A number of consensuses regarding cancer immunology have recently emerged from both preclinical immunotherapy models and analysis of cancer patients. First and foremost, the natural state of endogenous tumor reactive T cells is characterized by general hyporesponsiveness or anergy. This is likely due to a number of mechanisms that tumors use to induce tolerance as they develop. While many of the newer generation vaccines can effectively transfer antigen to and activate dendritic cells, T-cell tolerance remains a major barrier that is difficult to overcome by vaccination alone. Preclinical models demonstrate that for poorly immunogenic tumors, once tolerance has been established, therapeutic vaccines alone are ineffective at curing animals with a significant established tumor burden. However, combination strategies of vaccination together with inhibitors of immunologic checkpoints and agonists for co-stimulatory pathways are proving capable of overcoming tolerance and generating significant anti-tumor responses even in cases of established metastatic cancer.

Dendritic Cell-Based Vaccines that Utilize Myeloid Rather than Plasmacytoid Cells Offer a Superior Survival Advantage in Malignant Glioma

Dendritic cells (DCs) are professional APCs that are traditionally divided into two distinct subsets, myeloid DC (mDCs) and plasmacytoid DC (pDCs). pDCs are known for their ability to secrete large amounts of IFN-α. Apart from IFN-α production, pDCs can also process Ag and induce T cell immunity or tolerance. In several solid tumors, pDCs have been shown to play a critical role in promoting tumor immunosuppression. We investigated the role of pDCs in the process of glioma progression in the syngeneic murine model of glioma. We show that glioma-infiltrating pDCs are the major APC in glioma and are deficient in IFN-α secretion (p < 0.05). pDC depletion leads to increased survival of the mice bearing intracranial tumor by decreasing the number of regulatory T cells (Tregs) and by decreasing the suppressive capabilities of Tregs. We subsequently compared the ability of mDCs and pDCs to generate effective antiglioma immunity in a GL261-OVA mouse model of glioma. Our data suggest that mature pDCs and mDCs isolated from naive mice can be effectively activated and loaded with SIINFEKL Ag in vitro. Upon intradermal injection in the hindleg, a fraction of both types of DCs migrate to the brain and lymph nodes. Compared to mice vaccinated with pDC or control mice, mice vaccinated with mDCs generate a robust Th1 type immune response, characterized by high frequency of CD4(+)T-bet(+) T cells and CD8(+)SIINFEKEL(+) T cells. This robust antitumor T cell response results in tumor eradication and long-term survival in 60% of the animals (p < 0.001).

Inhibition of indoleamine 2,3-dioxygenase activity enhances the anti-tumour effects of a Toll-like receptor 7 agonist in an established cancer model

Toll-like receptor (TLR) agonists have been shown to have anti-tumour activity in basic research and clinical studies. However, TLR agonist monotherapy does not sufficiently eliminate tumours. Activation of the innate immune response by TLR agonists is effective at driving adaptive immunity via interleukin-12 (IL-12) or IL-1, but is counteracted by the simultaneous induction of immunosuppressive cytokines and other molecules, including IL-10, transforming growth factor-β, and indoleamine 2,3-dioxygenase (IDO). In the present study, we evaluated the anti-cancer effect of the TLR7 agonist, imiquimod (IMQ), in the absence of IDO activity. The administration of IMQ in IDO knockout (KO) mice inoculated with tumour cells significantly suppressed tumour progression compared with that in wild-type (WT) mice, and improved the survival rate. Moreover, injection with IMQ enhanced the tumour antigen-specific T helper type 1 response in IDO-KO mice with tumours. Combination therapy with IMQ and an IDO inhibitor also significantly inhibited tumour growth. Our results indicated that the enhancement of IDO expression with TLR agonists in cancer treatment might impair host anti-tumour immunity while the inhibition of IDO could enhance the therapeutic efficacy of TLR agonists via the increase of T helper type 1 immune response.

The plasmacytoid dendritic cell as the Swiss army knife of the immune system: molecular regulation of its multifaceted functions

Plasmacytoid dendritic cells (pDC) have been regarded as the "professional type I IFN-producing cells" of the immune system following viral recognition that relies on the expression of TLR7 and TLR9. Furthermore, pDC link the innate and adaptive immune systems via cytokine production and Ag presentation. More recently, their ability to induce tolerance and cytotoxicity has been added to their "immune skills." Such a broad range of actions, resembling the diverse functional features of a Swiss army knife, requires strong and prompt molecular regulation to prevent detrimental effects, including autoimmune pathogenesis or tumor escape. Over the last decades, we and other investigators have started to unravel some aspects of the signaling pathways that regulate the various functions of human pDC. In this article, we review aspects of the molecular regulatory mechanisms to control pDC function in light of their multifaceted roles during immunity, autoimmunity, and cancer.

The role of dendritic cells and monocytes in the maintenance and loss of respiratory tolerance

Promoting tolerance to inhaled antigens is an active area of study with the potential to benefit the millions of Americans currently suffering from respiratory allergies and asthma. Interestingly, not all individuals with atopy are symptomatic, arguing that sensitization alone does not lead to an allergic clinical phenotype. Respiratory dendritic cells (rDCs), classically associated with inducing inflammatory responses, can actively promote tolerance. Tolerance can be broken when inflammatory stimuli, including viral infections and other environmental exposures, inhibit rDC-mediated tolerance by allowing innocuous antigen to be presented to initiate type-2 immunity. Importantly, rDCs are composed of multiple subsets, each with a unique response to an inhaled antigen that can lead to either tolerance or inflammation. In this review, we will discuss how rDC subsets actively maintain tolerance or, alternatively, break tolerance in response to environmental cues.

Immunosuppressive factor blockade in dendritic cells via siRNAs results in objective clinical responses

Over the past decade, immunotherapy has emerged as a promising new form of cancer treatment with the potential to eradicate tumor metastasis. However, its curative potential is in general limited by the existence of negative feedback mechanisms that control dendritic cells (DCs) and T-cell activation. For clinically effective immunity, there is a need of inhibiting the expression of these immune suppressors. This could enhance the activation of DCs, T cells, and natural killer cells, and might be beneficial for cancer immunotherapy. Among the immune inhibitory molecules expressed by DCs is indoleamine 2,3-dioxygenase (IDO), an enzyme that conveys immunosuppressive effects by degrading tryptophan, an essential amino acid required for T-cell proliferation and survival. Depletion of tryptophan by IDO-positive DCs induces T-cell apoptosis and the conversion of naïve CD4+ T cells into regulatory T cells that further suppress antitumor immunity. Herein, we describe a protocol for in vitro synthesis of small interfering RNA against IDO and other immunosuppressive factors such as interleukin-10 and programmed cell death-1 ligands in order to reverse immune suppression mediated by DCs. Vaccination with IDO-silenced DC vaccines enhanced immune responses and antitumor immunity in cancer patients.

Plasmacytoid dendritic cells, a role in neoplastic prevention and progression

BACKGROUND

Plasmacytoid dendritic cells (pDCs) are multifunctional bone-marrow-derived immune cells that are key players in bridging the innate and adaptive immune systems. Activation of pDCs through toll-like receptor agonists has proven to be an effective treatment for some neoplastic disorders.

MATERIALS AND METHODS

In this mini-review, we will explore the fascinating contribution of pDCs to neoplastic pathology and discuss their potential utilization in cancer immunotherapy.

RESULTS

Current research suggests that pDCs have cytotoxic potential and can effectively induce apoptosis of tumour-derived cells lines. They are also reported to display tolerogenic function with the ability to suppress T-cell proliferation, analogous to regulatory T cells. In this capacity, they are critical in the suppression of autoimmunity but can be exploited by tumour cells to circumvent the expansion of tumour-specific T cells, thereby allowing tumours to persist.

CONCLUSION

Several forms of skin cancer are successfully treated with the topical drug Imiquimod, which activates pDCs through toll-like receptor 7 engagement. Additionally, pDC-based anticancer vaccines have shown encouraging results for the treatment of melanoma in early trials. Future studies regarding the contributions of pDCs to malignancy will likely afford many opportunities for immunotherapy strategies.

Trial watch: IDO inhibitors in cancer therapy

Indoleamine 2,3-dioxigenase 1 (IDO1) is the main enzyme that catalyzes the first, rate-limiting step of the so-called "kynurenine pathway", i.e., the metabolic cascade that converts the essential amino acid L-tryptophan (Trp) into L-kynurenine (Kyn). IDO1, which is expressed constitutively by some tissues and in an inducible manner by specific subsets of antigen-presenting cells, has been shown to play a role in the establishment and maintenance of peripheral tolerance. At least in part, this reflects the capacity of IDO1 to restrict the microenvironmental availability of Trp and to favor the accumulation of Kyn and some of its derivatives. Also, several neoplastic lesions express IDO1, providing them with a means to evade anticancer immunosurveillance. This consideration has driven the development of several IDO1 inhibitors, some of which (including 1-methyltryptophan) have nowadays entered clinical evaluation. In animal tumor models, the inhibition of IDO1 by chemical or genetic interventions is indeed associated with the (re)activation of therapeutically relevant anticancer immune responses. This said, several immunotherapeutic regimens exert robust clinical activity in spite of their ability to promote the expression of IDO1. Moreover, 1-methyltryptophan has recently been shown to exert IDO1-independent immunostimulatory effects. Here, we summarize the preclinical and clinical studies testing the antineoplastic activity of IDO1-targeting interventions.

Microenvironment of tumor-draining lymph nodes: opportunities for liposome-based targeted therapy

The World Health Organization (WHO) recently reported that the total number of global cancer cases in 2013 reached 14 million, a 10% rise since 2008, while the total number of cancer deaths reached 8.2 million, a 5.2% increase since 2008. Metastasis is the major cause of death from cancer, accounting for 90% of all cancer related deaths. Tumor-draining lymph nodes (TDLN), the sentinel nodes, are the first organs of metastasis in several types of cancers. The extent of metastasis in the TDLN is often used in disease staging and prognosis evaluation in cancer patients. Here, we describe the microenvironment of the TDLN and review the recent literature on liposome-based therapies directed to immune cells within the TDLN with the intent to target cancer cells.

Indoleamine 2,3-dioxygenase activity and clinical outcome following induction chemotherapy and concurrent chemoradiation in Stage III non-small cell lung cancer

Indoleamine 2,3-dioxygenase (IDO) has recently been proposed to account for tumor-induced immunosuppression by influencing the conversion of tryptophan (Trp) into kynurenine (Kyn). The objective of our study was to correlate IDO activity with disease outcome in non-small cell lung cancer (NSCLC) patients treated with multimodal combination therapy. In a single-arm Phase II trial involving induction gemcitabine and carboplatin followed by concurrent paclitaxel, carboplatin and 74 Gy thoracic radiation in stage III NSCLC patients, plasma was drawn at baseline, post-induction, and post-concurrent therapy. The mean plasma Kyn/Trp ratio was used as a surrogate indicator of IDO activity. The 33 participants were distributed as follows: 15 females, 18 males; median age = 62; median overall survival (OS) = 22.4 (95% CI 19.3-25.1) months; median progression-free survival (PFS) = 11.5 (95% CI 6.7-16.3) months. The mean Kyn/Trp ratio at baseline (4.5 ± 2.8) was higher than that of healthy controls (2.9 ± 1.9, p = 0.03) and increased after induction therapy (5.2 ± 3.2, p = 0.08) and chemoradiation (5.8 ± 3.9, p = 0.01). The post-treatment Kyn/Trp ratio and radiologic responses were not significantly associated at any time point. No significant correlation was found between baseline Kyn/Trp ratios and OS (HR = 1.1, 95% CI 0.45-2.5) or PFS (HR = 0.74, 95% CI 0.30-1.82). A post-induction chemotherapy increase in IDO activity portended worse OS (HR = 0.43, 95% CI 0.19-0.95, p = 0.037) and PFS (HR = 0.47, 95% CI 0.22-1.0, p = 0.055). This observed increase in IDO transcription may be a means for tumors to evade immunosurveillance.