Antiparasitic and antiproliferative effects of indoleamine 2,3-dioxygenase enzyme expression in human fibroblasts

Cytotoxic CD4+ T Cells Are Induced during Infection with Chlamydia trachomatis

Chlamydia trachomatis is the most common cause of bacterial sexually transmitted infection in both men and women. Immunity to C. trachomatis involves many cell types, but CD4+ T cells play a key role in protecting the host during natural infection. Specifically, IFN-γ production by CD4+ T cells is the main effector responsible for bacterial clearance, yet the exact mechanism by which IFN-γ confers protection is poorly defined. In our efforts to define the specific mechanisms for bacterial clearance, we now show that IFN-γ upregulates expression of MHC class II (MHCII) on nonhematopoietic cells during C. trachomatis infection in vivo. We also find that MHCII expression on epithelial cells of the upper genital tract contributes to the efficient clearance of bacteria mediated by pathogen-specific CD4+ Th1 cells. As we further cataloged the protective mechanisms of C. trachomatis-specific CD4+ T cells, we found that the T cells also express granzyme B (GzmB) when coincubated with infected cells. In addition, during C. trachomatis infection of mice, primed activated-naive CD4+ Th1 cells displayed elevated granzyme transcripts (GzmA, GzmB, GzmM, GzmK, GzmC) compared with memory CD4+ T cells in vivo. Finally, using intracellular cytokine staining and a GzmB-/- mouse strain, we show that C. trachomatis-specific CD4+ Th1 cells express GzmB upon Ag stimulation, and that this correlates with Chlamydia clearance in vivo. Together these results have led us to conclude that Chlamydia-specific CD4+ Th1 cells develop cytotoxic capacity through engagement with nonhematopoietic MHCII, and this correlates to C. trachomatis clearance.

Deficiency in astrocyte CCL2 production reduces neuroimmune control of Toxoplasma gondii infection

Toxoplasma gondii is an obligate intracellular parasite that infects one-third of the world's human population and establishes infection in the brain. Cerebral immune cell infiltration is critical for controlling the parasite, but little is known about the molecular cues guiding immune cells to the brain during infection. Activated astrocytes produce CCL2, a chemokine that mediates inflammatory monocyte recruitment to tissues by binding to the CCR2 receptor. We detected elevated CCL2 production in the brains of C57BL/6J mice by 15 days after T. gondii infection. Utilizing confocal microscopy and intracellular flow cytometry, we identified microglia and brain-infiltrating myeloid cells as the main producers of CCL2 during acute infection, and CCL2 was specifically produced in regions of parasite infection in the brain. In contrast, astrocytes became the dominant CCL2 producer during chronic T. gondii infection. To determine the role of astrocyte-derived CCL2 in mobilizing immune cells to the brain and controlling T. gondii infection, we generated GFAP-Cre x CCL2fl/fl mice, in which astrocytes are deficient in CCL2 production. We observed significantly decreased immune cell recruitment and increased parasite burden in the brain during chronic, but not acute, infection of mice deficient in astrocyte CCL2 production, without an effect on peripheral immune responses. To investigate potential mechanisms explaining the reduced control of T. gondii infection, we analyzed key antimicrobial and immune players in host defense against T. gondii and detected a reduction in iNOS+ myeloid cells, and T. gondii-specific CD4+ T cells in the knockout mice. These data uncover a critical role for astrocyte-derived CCL2 in immune cell recruitment and parasite control in the brain during chronic, but not acute, T. gondii infection.

Monocytes as primary defenders against Toxoplasma gondii infection

Monocytes are recruited from the bone marrow to sites of infection where they release cytokines and chemokines, function in antimicrobial immunity, and differentiate into macrophages and dendritic cells to control infection. Although many studies have focused on monocyte-derived macrophages and dendritic cells, recent work has examined the unique roles of monocytes during infection to promote immune defense. We focus on the effector functions of monocytes during infection with the parasite Toxoplasma gondii, and discuss the signals that mobilize monocytes to sites of infection, their production of inflammatory cytokines and antimicrobial mediators, their ability to shape the adaptive immune response, and their immunoregulatory functions. Insights from other infections, including Plasmodium and Listeria are also included for comparison and context.

The Complexity of Interferon Signaling in Host Defense against Protozoan Parasite Infection

Protozoan parasites, such as Plasmodium, Leishmania, Toxoplasma, Cryptosporidium, and Trypanosoma, are causative agents of health-threatening diseases in both humans and animals, leading to significant health risks and socioeconomic losses globally. The development of effective therapeutic and prevention strategies for protozoan-caused diseases requires a full understanding of the pathogenesis and protective events occurring in infected hosts. Interferons (IFNs) are a family of cytokines with diverse biological effects in host antimicrobial defense and disease pathogenesis, including protozoan parasite infection. Type II IFN (IFN-γ) has been widely recognized as the essential defense cytokine in intracellular protozoan parasite infection, whereas recent studies also revealed the production and distinct function of type I and III IFNs in host defense against these parasites. Decoding the complex network of the IFN family in host-parasite interaction is critical for exploring potential new therapeutic strategies against intracellular protozoan parasite infection. Here, we review the complex effects of IFNs on the host defense against intracellular protozoan parasites and the crosstalk between distinct types of IFN signaling during infections.

Overview of Apoptosis, Autophagy, and Inflammatory Processes in Toxoplasma gondii Infected Cells

Toxoplasma gondii (T. gondii) is an obligate intracellular parasite. During the parasitic invasion, T. gondii creates a parasitophorous vacuole, which enables the modulation of cell functions, allowing its replication and host infection. It has effective strategies to escape the immune response and reach privileged immune sites and remain inactive in a controlled environment in tissue cysts. This current review presents the factors that affect host cells and the parasite, as well as changes in the immune system during host cell infection. The secretory organelles of T. gondii (dense granules, micronemes, and rhoptries) are responsible for these processes. They are involved with proteins secreted by micronemes and rhoptries (MIC, AMA, and RONs) that mediate the recognition and entry into host cells. Effector proteins (ROP and GRA) that modify the STAT signal or GTPases in immune cells determine their toxicity. Interference byhost autonomous cells during parasitic infection, gene expression, and production of microbicidal molecules such as reactive oxygen species (ROS) and nitric oxide (NO), result in the regulation of cell death. The high level of complexity in host cell mechanisms prevents cell death in its various pathways. Many of these abilities play an important role in escaping host immune responses, particularly by manipulating the expression of genes involved in apoptosis, necrosis, autophagy, and inflammation. Here we present recent works that define the mechanisms by which T. gondii interacts with these processes in infected host cells.

Indoleamine 2,3-Dioxygenase 1: A Promising Therapeutic Target in Malignant Tumor

Tumorigenesis is a complex multifactorial and multistep process in which tumors can utilize a diverse repertoire of immunosuppressive mechanisms to evade host immune attacks. The degradation of tryptophan into immunosuppressive kynurenine is considered an important immunosuppressive mechanism in the tumor microenvironment. There are three enzymes, namely, tryptophan 2,3-dioxygenase (TDO), indoleamine 2,3-dioxygenase 1 (IDO1), and indoleamine 2,3-dioxygenase 2 (IDO2), involved in the metabolism of tryptophan. IDO1 has a wider distribution and higher activity in catalyzing tryptophan than the other two; therefore, it has been studied most extensively. IDO1 is a cytosolic monomeric, heme-containing enzyme, which is now considered an authentic immune regulator and represents one of the promising drug targets for tumor immunotherapy. Collectively, this review highlights the regulation of IDO1 gene expression and the ambivalent mechanisms of IDO1 on the antitumoral immune response. Further, new therapeutic targets via the regulation of IDO1 are discussed. A comprehensive analysis of the expression and biological function of IDO1 can help us to understand the therapeutic strategies of the inhibitors targeting IDO1 in malignant tumors.

Amino Acid Trp: The Far Out Impacts of Host and Commensal Tryptophan Metabolism

Tryptophan (Trp) is an essential amino acid primarily derived from the diet for use by the host for protein synthesis. The intestinal tract is lined with cells, both host and microbial, that uptake and metabolize Trp to also generate important signaling molecules. Serotonin (5-HT), kynurenine and its downstream metabolites, and to a lesser extent other neurotransmitters are generated by the host to signal onto host receptors and elicit physiological effects. 5-HT production by neurons in the CNS regulates sleep, mood, and appetite; 5-HT production in the intestinal tract by enterochromaffin cells regulates gastric motility and inflammation in the periphery. Kynurenine can signal onto the aryl hydrocarbon receptor (AHR) to elicit pleiotropic responses from several cell types including epithelial and immune cells, or can be further metabolized into bioactive molecules to influence neurodegenerative disease. There is a remarkable amount of cross-talk with the microbiome with regard to tryptophan metabolites as well. The gut microbiome can regulate the production of host tryptophan metabolites and can use dietary or recycled trp to generate bioactive metabolites themselves. Trp derivatives like indole are able to signal onto xenobiotic receptors, including AHR, to elicit tolerogenic effects. Here, we review studies that demonstrate that tryptophan represents a key intra-kingdom signaling molecule.

Intestinal microbiota regulates tryptophan metabolism following oral infection with Toxoplasma gondii

INTRODUCTION

The intestinal microbiota plays an important role in modulating host immune responses. Oral Toxoplasma gondii infection can promote intestinal inflammation in certain mice strains. The IDO-AhR axis may control tryptophan (Trp) metabolism constituting an important immune regulatory mechanism in inflammatory settings.

AIMS

In the present study, we investigated the role of the intestinal microbiota on Trp metabolism during oral infection with T gondii.

METHODS AND RESULTS

Mice were treated with antibiotics for four weeks and then infected with T gondii by gavage. Histopathology and immune responses were evaluated 8 days after infection. We found that depletion of intestinal microbiota by antibiotics contributed to resistance against T gondii infection and led to reduced expression of AhR on dendritic and Treg cells. Mice depleted of Gram-negative bacteria presented higher levels of systemic Trp, downregulation of AhR expression and increased resistance to infection whereas depletion of Gram-positive bacteria did not affect susceptibility or expression of AhR on immune cells.

CONCLUSION

Our findings indicate that the intestinal microbiota can control Trp availability and provide a link between the AhR pathway and host-microbiota interaction in acute infection with T gondii.

Toxoplasma gondii Parasitophorous Vacuole Membrane-Associated Dense Granule Proteins Regulate Maturation of the Cyst Wall

After differentiation is triggered, the tachyzoite-stage Toxoplasma gondii parasitophorous vacuole membrane (PVM) has been hypothesized to transition into the cyst membrane that surrounds the cyst wall and encloses bradyzoites. Here, we tracked the localization of two PVM dense granule (GRA) proteins (GRA5 and GRA7) after in vitro differentiation of the tachyzoite stage parasitophorous vacuole into the mature cyst. GRA5 and GRA7 were visible at the cyst periphery at 6 h and at all later times after differentiation, suggesting that the PVM remained intact as it transitioned into the cyst membrane. By day 3 postdifferentiation, GRA5 and GRA7 were visible in a continuous pattern at the cyst periphery. In mature 7- and 10-day-old cysts permeabilized with a saponin pulse, GRA5 and GRA7 were localized to the cyst membrane and the cyst wall regions. Cysts at different stages of cyst development exhibited differential susceptibility to saponin permeabilization, and, correspondingly, saponin selectively removed GRA5 from the cyst membrane and cyst wall region in 10-day-old cysts. GRA5 and GRA7 were localized at the cyst membrane and cyst wall region at all times after differentiation of the parasitophorous vacuole, which supports a previous model proposing that the PVM develops into the cyst membrane. In addition, evaluation of Δgra3, Δgra5, Δgra7, Δgra8, and Δgra14 mutants revealed that PVM-localized GRAs were crucial to support the normal rate of accumulation of cyst wall proteins at the cyst periphery.IMPORTANCEToxoplasma gondii establishes chronic infection in humans by forming thick-walled cysts that persist in the brain. Once host immunity wanes, cysts reactivate to cause severe, and often lethal, toxoplasmic encephalitis. There is no available therapy to eliminate cysts or to prevent their reactivation. Furthermore, how the cyst membrane and cyst wall structures develop is poorly understood. Here, we visualized and tracked the localization of Toxoplasma parasitophorous vacuole membrane (PVM) dense granules (GRA) proteins during cyst development in vitro. PVM-localized GRA5 and GRA7 were found at the cyst membrane and cyst wall region throughout cyst development, suggesting that the PVM remains intact and develops into the cyst membrane. In addition, our results show that genetic deletion of PVM GRAs reduced the rate of accumulation of cyst wall cargo at the cyst periphery and suggest that PVM-localized GRAs mediate the development and maturation of the cyst wall and cyst membrane.

Innate, adaptive, and cell-autonomous immunity against Toxoplasma gondii infection

Hosts have been fighting pathogens throughout the evolution of all infectious diseases. Toxoplasma gondii is one of the most common infectious agents in humans but causes only opportunistic infection in healthy individuals. Similar to antimicrobial immunity against other organisms, the immune response against T. gondii activates innate immunity and in turn induces acquired immune responses. After activation of acquired immunity, host immune cells robustly produce the proinflammatory cytokine interferon-γ (IFN-γ), which activates a set of IFN-γ-inducible proteins, including GTPases. IFN-inducible GTPases are essential for cell-autonomous immunity and are specialized for effective clearance and growth inhibition of T. gondii by accumulating in parasitophorous vacuole membranes. Recent studies suggest that the cell-autonomous immune response plays a protective role in host defense against not only T. gondii but also various intracellular bacteria. Moreover, the negative regulatory mechanisms of such strong immune responses are also important for host survival after infection. In this review, we will discuss in detail recent advances in the understanding of host defenses against T. gondii and the roles played by cell-autonomous immune responses.

Researchers are extensively studying immune responses to the single-celled parasite Toxoplasma gondii, which infects around one-third of humans, often harmlessly, but can cause life-threatening toxoplasmosis infections in patients with weakened immune systems. Masahiro Yamamoto and Miwa Sasai at Osaka University in Japan review recent advances in understanding the interactions between the immune system and the parasite. They consider non-specific ‘innate’ immune responses and also the ‘acquired’ responses that target specific parts of the parasite, referred to as antigens. Methods that selectively switch off genes in mice are revealing details presumed to also be relevant for humans. Significant molecules, molecular signaling pathways and immune-regulating processes are being identified. Recent studies suggest cell-autonomous immunity, the ability of host cells to defend themselves against attack, plays a significant role in fighting Toxoplasma gondii infection.

Host immune responses to Toxoplasma gondii

Toxoplasma gondii can infect homoeothermic animals including humans and cause lethal toxoplasmosis in immunocompromised individuals. When hosts are infected with T. gondii, the cells induce immune responses against T. gondii. The pathogen infection is recognized by immune sensors that directly detect T. gondii structural components, leading to production of pro-inflammatory cytokines and chemokines. Antigen-presenting cells such as macrophages and dendritic cells strongly activate T cells and induce development of Th1 cells and antigen-specific killer CD8 T cells. These T cells and Group 1 innate lymphoid cells are main producers of IFN-γ, which robustly stimulates cell-autonomous immunity in cells infected with T. gondii. IFN-γ-inducible effectors such as IFN-inducible GTPases, inducible nitric oxide synthase and indoleamine-2,3-dioxygenase differentially play important roles in suppression of T. gondii growth and its direct killing in anti-T. gondii cell-autonomous immune responses. In this review, we will describe our current knowledge of innate, adaptive and IFN-γ-mediated cell-autonomous immunity against T. gondii infection.

Tryptophan catabolism in epithelial ovarian carcinoma

Ovarian cancers are the most common cause of gynecological death, and the five-year survival rate for women diagnosed with epithelial ovarian carcinoma (EOC) remains extremely low at only 47%. Recent studies have highlighted the importance of the anti-tumor immune response in determining EOC clinical outcomes, and much research is currently being undertaken in an effort to reverse tumor immune evasion. One mechanism known to promote tumor immune evasion in multiple cancer types is tryptophan catabolism. Here we review the potential role of two rate-limiting enzymes that evolved separately to catabolize tryptophan, indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase 2 (TDO2), that may be active in ovarian cancers and result in the production of immune suppressive catabolites. Research to date has focused on IDO inhibitors, currently in clinical trials, but these therapies fail to inhibit TDO2. However, our mining of publically available data from clinical specimens suggest that TDO2 may also need to be targeted in ovarian cancer.

A latent ability to persist: differentiation in Toxoplasma gondii

A critical factor in the transmission and pathogenesis of Toxoplasma gondii is the ability to convert from an acute disease-causing, proliferative stage (tachyzoite), to a chronic, dormant stage (bradyzoite). The conversion of the tachyzoite-containing parasitophorous vacuole membrane into the less permeable bradyzoite cyst wall allows the parasite to persist for years within the host to maximize transmissibility to both primary (felids) and secondary (virtually all other warm-blooded vertebrates) hosts. This review presents our current understanding of the latent stage, including the factors that are important in bradyzoite induction and maintenance. Also discussed are the recent studies that have begun to unravel the mechanisms behind stage switching.

Exposing Toxoplasma gondii hiding inside the vacuole: a role for GBPs, autophagy and host cell death

The intracellular parasite Toxoplasma gondii resides inside a vacuole, which shields it from the host’s intracellular defense mechanisms. The cytokine interferon gamma (IFNγ) upregulates host cell effector pathways that are able to destroy the vacuole, restrict parasite growth and induce host cell death. Interferon-inducible GTPases such as the Guanylate Binding Proteins (GBPs), autophagy proteins and ubiquitin-driven mechanisms play important roles in Toxoplasma control in mice and partly also in humans. The host inflammasome is regulated by GBPs in response to bacterial infection in murine cells and may also respond to Toxoplasma infection. Elucidation of murine Toxoplasma defense mechanisms are guiding studies on human cells, while inevitably leading to the discovery of human-specific pathways that often function in a cell type-dependent manner.

Huntingtons Disease Mice Infected with Toxoplasma gondii Demonstrate Early Kynurenine Pathway Activation, Altered CD8+ T-Cell Responses, and Premature Mortality

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a polyglutamine-repeat expansion in the huntingtin protein. Activation of the kynurenine pathway of tryptophan degradation is implicated in the pathogenesis of HD. Indoleamine-2,3-dioxygenase (IDO) catalyzes the oxidation of tryptophan to kynurenine, the first step in this pathway. The prevalent, neuroinvasive protozoal pathogen Toxoplasma gondii (T. gondii) results in clinically silent life-long infection in immune-competent individuals. T. gondii infection results in activation of IDO which provides some protection against the parasite by depleting tryptophan which the parasite cannot synthesize. The kynurenine pathway may therefore represent a point of synergism between HD and T. gondii infection. We show here that IDO activity is elevated at least four-fold in frontal cortex and striata of non-infected N171-82Q HD mice at 14-weeks corresponding to early–advanced HD. T. gondii infection at 5 weeks resulted in elevation of cortical IDO activity in HD mice. HD-infected mice died significantly earlier than wild-type infected and HD control mice. Prior to death, infected HD mice demonstrated decreased CD8+ T-lymphocyte proliferation in brain and spleen compared to wild-type infected mice. We demonstrate for the first time that HD mice have an altered response to an infectious agent that is characterized by premature mortality, altered immune responses and early activation of IDO. Findings are relevant to understanding how T. gondii infection may interact with pathways mediating neurodegeneration in HD.

Fecal Indole as a Biomarker of Susceptibility to Cryptosporidium Infection

Cryptosporidium causes significant diarrhea worldwide, especially among children and immunocompromised individuals, and no effective drug treatment is currently available for those who need it most. In this report, previous volunteer infectivity studies have been extended to examine the association between fecal indole and indole-producing (IP) gut microbiota on the outcome of a Cryptosporidium infection. Fecal indole concentrations (FICs) of 50 subjects and 19 taxa of common gut microbiota, including six IP taxa (11 subjects) were determined in stool samples collected before and after a challenge with Cryptosporidium oocysts. At the baseline, the mean FIC (± the standard deviation) was 1.66 ± 0.80 mM in those who became infected after a challenge versus 3.20 ± 1.23 mM in those who remained uninfected (P = 0.0001). Only 11.1% of the subjects with a FIC of >2.5 mM became infected after a challenge versus 65.2% of the subjects with a FIC of <2.5 mM. In contrast, the FICs of infected subjects at the baseline or during diarrhea were not correlated with infection intensity or disease severity. The relative abundances (percent) of Escherichia coli, Bacillus spp., and Clostridium spp. were greater ≥2.5-fold in volunteers with a baseline FIC of >2.5 mM, while those of Bacteroides pyogenes, B. fragilis, and Akkermansia muciniphila were greater in those with a baseline FIC of <2.5 mM. These data indicate that some IP bacteria, or perhaps indole alone, can influence the ability of Cryptosporidium to establish an infection. Thus, preexisting indole levels in the gut join the oocyst dose and immune status as important factors that determine the outcome of Cryptosporidium exposure.

Effect of parasitic infection on dopamine biosynthesis in dopaminergic cells

Infection by the neurotropic agent Toxoplasma gondii alters rodent behavior and can result in neuropsychiatric symptoms in humans. Little is understood regarding the effects of infection on host neural processes but alterations to dopaminergic neurotransmission are implicated. We have previously reported elevated levels of dopamine (DA) in infected dopaminergic cells however the involvement of the host enzymes and fate of the produced DA were not defined. In order to clarify the effects of infection on host DA biosynthetic enzymes and DA packaging we examined enzyme levels and activity and DA accumulation and release in T. gondii-infected neurosecretory cells. Although the levels of the host tyrosine hydroxylase (TH) and DOPA decarboxylase and AADC (DDC) did not change significantly in infected cultures, DDC was found within the parasitophorous vacuole (PV), the vacuolar compartment where the parasites reside, as well as in the host cytosol in infected dopaminergic cells. Strikingly, DDC was found within the intracellular parasite cysts in infected brain tissue. This finding could provide some explanation for observations of DA within tissue cysts in infected brain as a parasite-encoded enzyme with TH activity was also localized within tissue cysts. In contrast, cellular DA packaging appeared unchanged in single-cell microamperometry experiments and only a fraction of the increased DA was accessible to high potassium-induced release. This study provides some understanding of how this parasite produces elevated DA within dopaminergic cells without the toxic ramifications of free cytosolic DA. The mechanism for synthesis and packaging of DA by T. gondii-infected dopaminergic cells may have important implications for the effects of chronic T. gondii infection on humans and animals.

Complex immune cell interplay in the gamma interferon response during Toxoplasma gondii infection

Toxoplasma gondii is an obligate intracellular parasite of clinical importance, especially in immunocompromised patients. Investigations into the immune response to the parasite found that T cells are the primary effector cells regulating gamma interferon (IFN-γ)-mediated host resistance. However, recent studies have revealed a critical role for the innate immune system in mediating host defense independently of the T cell responses to the parasite. This body of knowledge is put into perspective by the unifying theme that immunity to the protozoan parasite requires a strong IFN-γ host response. In the following review, we discuss the role of IFN-γ-producing cells and the signals that regulate IFN-γ production during T. gondii infection.

Evolution to a chronic disease niche correlates with increased sensitivity to tryptophan availability for the obligate intracellular bacterium Chlamydia pneumoniae

The chlamydiae are obligate intracellular parasites that have evolved specific interactions with their various hosts and host cell types to ensure their successful survival and consequential pathogenesis. The species Chlamydia pneumoniae is ubiquitous, with serological studies showing that most humans are infected at some stage in their lifetime. While most human infections are asymptomatic, C. pneumoniae can cause more-severe respiratory disease and pneumonia and has been linked to chronic diseases such as asthma, atherosclerosis, and even Alzheimer's disease. The widely dispersed animal-adapted C. pneumoniae strains cause an equally wide range of diseases in their hosts. It is emerging that the ability of C. pneumoniae to survive inside its target cells, including evasion of the host's immune attack mechanisms, is linked to the acquisition of key metabolites. Tryptophan and arginine are key checkpoint compounds in this host-parasite battle. Interestingly, the animal strains of C. pneumoniae have a slightly larger genome, enabling them to cope better with metabolite restrictions. It therefore appears that as the evolutionarily more ancient animal strains have evolved to infect humans, they have selectively become more "susceptible" to the levels of key metabolites, such as tryptophan. While this might initially appear to be a weakness, it allows these human C. pneumoniae strains to exquisitely sense host immune attack and respond by rapidly reverting to a persistent phase. During persistence, they reduce their metabolic levels, halting progression of their developmental cycle, waiting until the hostile external conditions have passed before they reemerge.

Role of mouse and human autophagy proteins in IFN-γ-induced cell-autonomous responses against Toxoplasma gondii

IFN-γ mediates cellular innate immunity against an intracellular parasite, Toxoplasma gondii, by inducing immunity-related GTPases such as p47 IFN-γ-regulated GTPases (IRGs) and p65 guanylate-binding proteins (GBPs), which also participate in antibacterial responses via autophagy. An essential autophagy protein, Atg5, was previously shown to play a critical role in anti-T. gondii cell-autonomous immunity. However, the involvement of other autophagy proteins remains unknown. In this study, we show that essential autophagy proteins differentially participate in anti-T. gondii cellular immunity by recruiting IFN-γ-inducible GTPases. IFN-γ-induced suppression of T. gondii proliferation and recruitment of an IRG Irgb6 and GBPs are profoundly impaired in Atg7- or Atg16L1-deficient cells. In contrast, cells lacking other essential autophagy proteins, Atg9a and Atg14, are capable of mediating the anti-T. gondii response and recruiting Irgb6 and GBPs to the parasites. Although IFN-γ also stimulates anti-T. gondii cellular immunity in humans, whether this response requires GBPs and human autophagy proteins remains to be seen. To analyze the role of human ATG16L1 and GBPs in IFN-γ-mediated anti-T. gondii responses, human cells lacking ATG16L1 or GBPs were generated by the Cas9/CRISPR genome-editing technique. Although both ATG16L1 and GBPs are dispensable for IFN-γ-induced inhibition of T. gondii proliferation in the human cells, human ATG16L1 is also required for the recruitment of GBPs. Taken together, human ATG16L1 and mouse autophagy components Atg7 and Atg16L1, but not Atg9a and Atg14, participate in the IFN-γ-induced recruitment of the immunity-related GTPases to the intracellular pathogen.

Cell death of gamma interferon-stimulated human fibroblasts upon Toxoplasma gondii infection induces early parasite egress and limits parasite replication

The intracellular protozoan parasite Toxoplasma gondii is a major food-borne illness and opportunistic infection for the immunosuppressed. Resistance to Toxoplasma is dependent on gamma interferon (IFN-γ) activation of both hematopoietic and nonhematopoietic cells. Although IFN-γ-induced innate immunity in nonhematopoietic cells has been extensively studied in mice, it remains unclear what resistance mechanisms are relied on in nonhematopoietic human cells. Here, we report an IFN-γ-induced mechanism of resistance to Toxoplasma in primary human foreskin fibroblasts (HFFs) that does not depend on the deprivation of tryptophan or iron. In addition, infection is still controlled in HFFs deficient in the p65 guanylate binding proteins GBP1 or GBP2 and the autophagic protein ATG5. Resistance is coincident with host cell death that is not dependent on the necroptosis mediator RIPK3 or caspases and is correlated with early egress of the parasite before replication. This IFN-γ-induced cell death and early egress limits replication in HFFs and could promote clearance of the parasite by immune cells.

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.

Dipterinyl calcium pentahydrate inhibits intracellular mycobacterial growth in human monocytes via the C-C chemokine MIP-1β and nitric oxide

Tuberculosis remains one of the top three leading causes of morbidity and mortality worldwide, complicated by the emergence of drug-resistant Mycobacterium tuberculosis strains and high rates of HIV coinfection. It is important to develop new antimycobacterial drugs and immunomodulatory therapeutics and compounds that enhance antituberculous immunity. Dipterinyl calcium pentahydrate (DCP), a calcium-complexed pterin compound, has previously been shown to inhibit human breast cancer cells and hepatitis B virus (HBV). DCP inhibitory effects were attributed to induction of apoptosis and/or increased production of interleukin 12 (IL-12) and granulocyte-macrophage colony-stimulating factor (GM-CSF). In this study, we tested the ability of DCP to mediate inhibition of intracellular mycobacteria within human monocytes. DCP treatment of infected monocytes resulted in a significant reduction in viability of intracellular but not extracellular Mycobacterium bovis BCG. The antimicrobial activity of DCP was comparable to that of pyrazinamide (PZA), one of the first-line antituberculosis drugs currently used. DCP potentiated monocyte antimycobacterial activity by induction of the cysteine-cysteine (C-C) chemokine macrophage inflammatory protein 1β (MIP-1β) and inducible nitric oxide synthase 2. Addition of human anti-MIP-1β neutralizing antibody or a specific inhibitor of the l-arginase-nitric oxide pathway (N(G)-monomethyl l-arginine [l-NMMA] monoacetate) reversed the inhibitory effects of DCP on intracellular mycobacterial growth. These findings indicate that DCP induced mycobacterial killing via MIP-1β- and nitric oxide-dependent effects. Hence, DCP acts as an immunoregulatory compound enhancing the antimycobacterial activity of human monocytes.

The rhoptry proteins ROP18 and ROP5 mediate Toxoplasma gondii evasion of the murine, but not the human, interferon-gamma response

The obligate intracellular parasite Toxoplasma gondii secretes effector proteins into the host cell that manipulate the immune response allowing it to establish a chronic infection. Crosses between the types I, II and III strains, which are prevalent in North America and Europe, have identified several secreted effectors that determine strain differences in mouse virulence. The polymorphic rhoptry protein kinase ROP18 was recently shown to determine the difference in virulence between type I and III strains by phosphorylating and inactivating the interferon-γ (IFNγ)-induced immunity-related GTPases (IRGs) that promote killing by disrupting the parasitophorous vacuole membrane (PVM) in murine cells. The polymorphic pseudokinase ROP5 determines strain differences in virulence through an unknown mechanism. Here we report that ROP18 can only inhibit accumulation of the IRGs on the PVM of strains that also express virulent ROP5 alleles. In contrast, specific ROP5 alleles can reduce IRG coating even in the absence of ROP18 expression and can directly interact with one or more IRGs. We further show that the allelic combination of ROP18 and ROP5 also determines IRG evasion and virulence of strains belonging to other lineages besides types I, II and III. However, neither ROP18 nor ROP5 markedly affect survival in IFNγ-activated human cells, which lack the multitude of IRGs present in murine cells. These findings suggest that ROP18 and ROP5 have specifically evolved to block the IRGs and are unlikely to have effects in species that do not have the IRG system, such as humans.

Toxoplasma gondii can infect any warm-blooded animal and is transmitted orally by consumption of tissue cysts. To facilitate transmission, the parasite must balance induction and evasion of host immune responses to allow parasite growth and persistence, while avoiding excessive parasite burden, which can kill the host before infectious cysts are formed. Different strains of Toxoplasma have likely evolved specific effector molecules to modulate the immune responses of different hosts. In many mammals, including mice but not humans, the cytokine interferon gamma (IFNγ) induces the immunity-related GTPases (IRGs), which are essential to the murine immune response to Toxoplasma. They function by binding to and disrupting the parasite-containing vacuole. However, some Toxoplasma strains prevent the IRGs from disrupting the parasitophorous vacuole. It was previously shown that the secreted Toxoplasma kinase ROP18 promotes virulence in mice by phosphorylating the IRGs, leading to their inactivation. We report that ROP18 requires another virulence factor, the secreted pseudokinase ROP5, to prevent IRG accumulation, and these two proteins determine the majority of strain differences in IRG evasion, even for divergent strains for which virulence determinants have not been studied. Additionally, we show that ROP18 and ROP5 do not affect Toxoplasma survival in IFNγ-stimulated human cells. Thus, ROP18 and ROP5 are strain- and host-specific determinants of Toxoplasma immune evasion.

Effects of pro-inflammatory cytokines on expression of kynurenine pathway enzymes in human dermal fibroblasts

Background

The kynurenine pathway (KP) is the main route of tryptophan degradation in the human body and generates several neuroactive and immunomodulatory metabolites. Altered levels of KP-metabolites have been observed in neuropsychiatric and neurodegenerative disorders as well as in patients with affective disorders. The purpose of the present study was to investigate if skin derived human fibroblasts are useful for studies of expression of enzymes in the KP.

Methods

Fibroblast cultures were established from cutaneous biopsies taken from the arm of consenting volunteers. Such cultures were subsequently treated with interferon (IFN)-γ 200 U/ml and/or tumor necrosis factor (TNF)-α, 100 U/ml for 48 hours in serum-free medium. Levels of transcripts encoding different enzymes were determined by real-time PCR and levels of kynurenic acid (KYNA) were determined by HPLC.

Results

At base-line all cultures harbored detectable levels of transcripts encoding KP enzymes, albeit with considerable variation across individuals. Following cytokine treatment, considerable changes in many of the transcripts investigated were observed. For example, increases in the abundance of transcripts encoding indoleamine 2,3-dioxygenase, kynureninase or 3-hydroxyanthranilic acid oxygenase and decreases in the levels of transcripts encoding tryptophan 2,3-dioxygenase, kynurenine aminotransferases or quinolinic acid phosphoribosyltransferase were observed following IFN-γ and TNF-α treatment. Finally, the fibroblast cultures released detectable levels of KYNA in the cell culture medium at base-line conditions, which were increased after IFN-γ, but not TNF-α, treatments.

Conclusions

All of the investigated genes encoding KP enzymes were expressed in human fibroblasts. Expression of many of these appeared to be regulated in response to cytokine treatment as previously reported for other cell types. Fibroblast cultures, thus, appear to be useful for studies of disease-related abnormalities in the kynurenine pathway of tryptophan degradation.

A potential role for indoleamine 2,3-dioxygenase (IDO) in Rhodococcus equi infection

Rhodococcus equi is a facultative intracellular bacterial pathogen of foals and immunocompromised humans that infects and proliferates within host macrophages and dendritic cells (DC). Indoleamine 2,3-dioxygenase (IDO), the initial enzyme in the tryptophan catabolism pathway, is upregulated in R. equi infected equine monocyte-derived DC and alveolar macrophages. Tryptophan requirement of R. equi for extracellular and intracellular growth was assessed. Growth of R. equi in minimal media did not require tryptophan and pharmacologic inhibition of IDO had no effect on intracellular proliferation of R. equi in equine alveolar macrophages. To investigate an immune-regulatory role for INDO in R. equi infection, IDO(-/-) (B6.129-(Indotm1Alm)/J) (n=22) and strain matched control (C57BL/6J) (n=20) mice were infected with R. equi by intraperitoneal injection, for 3 and 6 days. There was no difference in bacterial counts in liver or spleen between the two groups. Histological sections of liver and spleen were assigned inflammation scores and RT-PCR for interferon-gamma (IFNγ), tumor necrosis factor-alpha (TNFα), IL-4, IL-6, IL-10, IL-12, IL-23, forkhead box P3 (FoxP3), and transforming growth factor-beta (TGFβ) was performed on liver and spleen. Liver tissue of IDO(-/-) had higher inflammation scores at 6 days post-infection (PI) (P=0.05) and had decreased expression of TGFβ at 3 days PI (P=0.01), and FOXP3 at 3 days (P=0.02) and 6 days (P=0.03) compared to control mice. Immunostaining for FOXP3 showed lower numbers of FOXP3+ regulatory T cells in liver of IDO(-/-) mice 6 days PI. Prolonged inflammation in the liver tissue of IDO(-/-) mice corresponded with lower expression of FOXP3 and TGFβ in that tissue, and also with lower numbers of FOXP3+ regulatory T cells. We conclude that IDO expression by activated macrophages and DC plays a role in dampening the inflammatory response to R. equi infection in mice.

Identification of immunologically relevant genes in mare and foal dendritic cells responding to infection by Rhodococcus equi

Rhodococcus equi is a facultative intracellular bacterial pathogen of horses; infected foals develop pyogranulomatous pneumonia, however adult horses are largely unaffected. R. equi infects and proliferates within host macrophages and dendritic cells (DCs). DCs initiate the appropriate adaptive immune response, thereby playing a critical role in determining the outcome of infection. Our aim was to identify genes that are differentially expressed in R. equi infected monocyte-derived DCs (mdDCs). Peripheral blood monocytes from mares and foals were used to derive mdDCs by culturing with recombinant equine IL-4 and recombinant human GM-CSF. RNA harvested 24h after infection with R. equi (ATCC 33701+) was used to perform suppression subtractive hybridization (SSH) experiments. Approximately 38 unique sequences were obtained from these experiments. Differential expression of 19 immunologically relevant genes was validated by PCR. These genes are characterized by the following functions: cell adhesion, chemotaxis/migration, immune/inflammatory response, ion transport, signal transduction, T-cell regulation, and vesicular transport. In summary, we identified several novel genes that are differentially expressed in foal and adult mdDCs in response to R. equi infection. These genes provide promising targets for further research into the host response to R. equi, and the susceptibility of foals to this disease.

Indoleamine 2,3-dioxygenase (IDO) is expressed at feto-placental unit throughout mouse gestation: An immunohistochemical study

INTRODUCTION

The cells expressing Indoleamine 2, 3-dioxygenase (IDO) in feto-maternal interface mediate tryptophan catabolism, hence protect allogeneic fetus from lethal rejection by maternal immune responses. In this study, we report immuno-localization of IDO(+) cells in murine reproductive tract and placenta throughout mouse pregnancy by immunohistochemistry.

MATERIALS AND METHODS

Syngeneic pregnant mice were examined for vaginal plug to discover about their state of pregnancy. A total of three pregnant mice were examined at each stage.The examination was further confirmed by the detection of sperm in vaginal smear. On the gestational days of 2(nd), 12(th) and 18(th), the uterus and oviduct were removed and expression of IDO was investigated in the endometrium, placenta and oviduct by immunohistochemistry.

RESULTS

Our results showed that IDO is expressed consistently in feto-maternal interface throughout pregnancy. In endometrium, expression of IDO was predominantly confined to luminal and glandular epithelial cells. Cells at junctional and labyrinth zones of placenta showed strong IDO immunoreactivity as well.

CONCLUSION

Expression of IDO at the protein level in reproductive tract of pregnant mice during entire periods of gestation points to its potential protective role in maintenance of pregnancy. In our knowledge this is the first report of expression of IDO in feto-maternal phase during murine pregnancy.

Induction of iNOS by Chlamydophila pneumoniae requires MyD88-dependent activation of JNK

Innate immune cells produce NO via inducible NO synthase (iNOS) in response to certain infections or upon stimulation with cytokines such as IFN-gamma and TNF. NO plays an important role in host defense against intracellular bacteria including Chlamydophila pneumoniae as a result of its microbicidal activity. In MyD88-deficient mice, which succumb to C. pneumoniae infection, iNOS induction is impaired 6 days postinfection, although pulmonary levels of IFN-gamma and TNF are elevated as in wild-type mice at this time-point. Here, we demonstrate that induction of iNOS in macrophages upon C. pneumoniae infection is controlled by MyD88 via two pathways: NF-kappaB activation and phosphorylation of the MAPK JNK, which leads to the nuclear translocation of c-Jun, one of the two components of the AP-1 complex. In addition, phosphorylation of STAT1 and expression of IFN regulatory factor 1 (IRF-1) were delayed in the absence of MyD88 after C. pneumoniae infection but not after IFN-gamma stimulation. Taken together, our data show that for optimal induction of iNOS during C. pneumoniae infection, the concerted action of the MyD88-dependent transcription factors NF-kappaB and AP-1 and of the MyD88-independent transcription factors phosphorylated STAT1 and IRF-1 is required.

Immunosuppression routed via the kynurenine pathway: a biochemical and pathophysiologic approach

In the past years, it has been shown that kynurenines pathway is a regulator of both the innate and the adaptive immune responses. Particularly, the initial enzyme of this pathway, indoleamine 2,3-dioxygenase (IDO), is implicated in maintaining tolerance during pregnancy, and also can be expressed in tumors to avoid the immune attack. In this chapter, we will describe how the kynurenine pathway affects the immune system with important implications both in physiology and in pathology. The incorrect activation or blockade suppressive properties of the kynurenine pathway are also implicated in a number of other diseases such as AIDS or autoimmune diseases.

Stem cells ameliorate EAE via an indoleamine 2,3-dioxygenase (IDO) mechanism

Syngeneic, pluripotent Lin(-)Sca1(+) bone marrow stem cells (SC), transferred to mice with experimental autoimmune encephalomyelitis, a model of multiple sclerosis, enhanced recovery, prevented relapses and promoted myelin repair. SC-treated mice showed elevated interferon-gamma production and induction of indoleamine 2,3-dioxygenase (IDO) in CD11c(+) dendritic cells (DC). IDO induction was specific since in the presence of IDO-producing CD11c(+) DC, PLP stimulated T cell proliferation was inhibited and the IDO-inhibitor, 1-MT, abrogated the SC effect. Relapse prevention during chronic disease correlated with decreased responsiveness to PLP(178-191) and MBP(85-99). Thus, pluripotent SC induce IDO in DC leading to inhibition of antigen reactivity and spreading in EAE.

Immune-mediated control of Chlamydia infection

Infection with the bacterium Chlamydia trachomatis can lead to a variety of diseases, including ectopic pregnancy, infertility and blindness. Exposure of the host to C. trachomatis stimulates multiple innate and adaptive immune effectors that can contribute towards controlling bacterial replication. However, these effectors are often insufficient to resolve the infection and prevent re-infection, and the continued presence of C. trachomatis within the host may induce immune effectors to chronically produce inflammatory cytokines. This may eventually lead to the tissue pathologies associated with the infection. Reducing the incidence and sequelae of infection will ultimately require the development of a C. trachomatis vaccine that can stimulate sterilizing immunity while avoiding immune-mediated pathology.

Dendritic cell based antitumor vaccination: impact of functional indoleamine 2,3-dioxygenase expression

BACKGROUND

Recent reports have demonstrated that the enzyme indoleamine 2,3-dioxygenase (IDO) is upregulated in human dendritic cells (DCs) upon in vitro maturation. IDO is supposed to convey immunosuppressive effects by degrading the essential amino acid tryptophan, thereby downregulating T-cell functions. Hence, we evaluated IDO expression in DC preparations used for therapeutic DC vaccination and its in vivo effects.

PATIENTS, METHODS AND RESULTS

IDO expression was detected by real-time-PCR in a series of human clinical grade DCs (n = 28) prior to vaccination of advanced melanoma patients (n = 11). These analyses revealed an intra- and interpersonal variation in IDO mRNA levels. IDO was strongly upregulated in human DCs on RNA and on protein level upon in vitro maturation by Interleukin-1beta (IL-1beta), tumour necrosis factor-alpha (TNF-alpha), Interleukin-6 (IL-6) and Prostaglandin E2 (PGE2) over a time course of 24 h. The enzymatic activity of induced IDO was demonstrated by measuring tryptophan degradation. Moreover, in biopsies obtained 24 h after application of the DC vaccine a prominent infiltrate of IDO-positive cells was observed by immunohistochemistry. The inflammatory infiltrate of these sites stained positive for the transcription factor Forkhead box P3 (FoxP3), suggesting an IDO-mediated induction of regulatory T-cells. All analysed melanoma patients (n = 11) receiving DC based immunotherapy exhibited rapid disease progression with a short overall survival due to advanced tumour stage.

CONCLUSION

The presented observations suggest a potential clinical relevance of IDO expression in DC-based therapeutic vaccines via the attraction or induction of FoxP3(+) T-cells.

Improving T cell therapy for cancer

Adoptive transfer of antigen-specific T lymphocytes is a powerful therapy for the treatment of opportunistic disease and some virus-associated malignancies such as Epstein-Barr virus-positive post-transplant lymphoproliferative disease. However, this strategy has been less successful in patients with nonviral cancers owing to their many and varied immune evasion mechanisms. These mechanisms include downregulation of target antigens and antigen-presenting machinery, secretion of inhibitory cytokines, and recruitment of regulatory immune cells to the tumor site. With increased understanding of the tumor microenvironment and the behavior and persistence of ex vivo-manipulated, adoptively transferred T cells, two novel approaches for increasing the efficacy of T cell therapy have been proposed. The first involves genetic modification of tumor-specific T cells to improve their biological function, for example by augmenting their ability to recognize tumor cells or their resistance to tumor-mediated immunosuppression. The second requires modifications to the host environment to improve the homeostatic expansion of infused T cells or to eliminate inhibitory T cell subsets. In this review, we discuss current, promising strategies to improve adoptive T cell therapy for the treatment of cancer.

Decreased protein nitration in macrophages that overexpress indoleamine 2, 3-dioxygenase

The activity of indoleamine 2, 3-dioxygenase (IDO; E.C. 1.13.11.42) catalyzes the oxidative cleavage of tryptophan to form kynurenine. IDO activity consumes superoxide anions; therefore, we postulated that over-expression of IDO might mitigate superoxide-anion dependent, oxidative modification of cellular proteins in vitro. We prepared and characterized RAW 264.7 macrophages that were stably transfected with either an IDO expression vector or the control (empty) vector. We detected IDO mRNA, protein, and enzyme activity in the IDO-transfected macrophages, but not in the macrophages transfected with the empty vector. To generate superoxide anions in situ, we treated the IDO-and control-transfected cultures with xanthine or hypoxanthine, and then used ELISA methods to quantitate the relative levels of oxidatively modified proteins in total cell lysates. The levels of protein carbonyls were similar in IDO-transfected and vector-transfected macrophages; however, protein nitration was significantly less in IDO-transfected cells compared to control transfectants. In addition, steady-state levels of superoxide anions were significantly lower in the IDO-transfected cultures compared with control transfectants. Our results are consistent with the concept that, besides degrading tryptophan, IDO activity may protect cells from oxidative damage.

A high-affinity, tryptophan-selective amino acid transport system in human macrophages

Tryptophan catabolism via the enzyme indoleamine 2,3-dioxygenase (IDO) allows human monocyte-derived macrophages (MDM) and other APC to suppress T cell proliferation. IDO helps protect murine fetuses from rejection by the maternal immune system and can promote tolerance and immunosuppression. For tryptophan to be catabolized by IDO, it must first enter the APC via transmembrane transport. It has been shown that MDM in vitro readily deplete tryptophan present in the extracellular medium to nanomolar levels via IDO activity; yet, no currently known amino acid transport system displays high affinity and specificity sufficiently to permit efficient uptake of tryptophan at these low concentrations. Here, we provide biochemical characterization of a novel transport system with nanomolar affinity and high selectivity for tryptophan. Tryptophan transport in MDM was predominantly sodium-independent and occurred via two distinct systems: one consistent with the known system L transporter and a second system with 100-fold higher affinity for tryptophan (Km<300 nM). Competition studies showed that the high-affinity system did not correspond to any known transporter activity and displayed a marked selectivity for tryptophan over other amino acids and tryptophan analogs. This new system was expressed at low levels in fresh monocytes but underwent selective induction during MDM differentiation. In contrast, resting human T cells expressed only the conventional system L. We speculate that the high-affinity, tryptophan-specific transport system allows MDM to take up tryptophan efficiently under conditions of low substrate concentration, such as may occur during interaction between T cells and IDO-expressing APC.

Nitration and inactivation of IDO by peroxynitrite

IDO induction can deplete L-tryptophan in target cells, an effect partially responsible for the antimicrobial activities and antiallogeneic T cell responses of IFN-gamma in human macrophages, dendritic cells, and bone marrow cells. L-tryptophan depletion and NO production are both known to have an antimicrobial effect in macrophages, and the interaction of these two mechanisms is unclear. In this study we found that IDO activity was inhibited by the peroxynitrite generator, 3-(4-morpholinyl)sydnonimine, in PMA-differentiated cytokine-induced THP-1 (acute monocytic leukemia) cells and IFN-gamma-stimulated PBMCs, whereas IDO protein expression was unaffected compared with that in untreated cells. Nitrotyrosine was detected in immunoprecipitated (IP)-IDO from PMA-differentiated cytokine-induced THP-1 cells treated with 3-(4-morpholinyl)sydnonimine, but not from untreated cells. Treatment of IP-IDO and recombinant IDO (rIDO) with peroxynitrite significantly decreased enzyme activity. Nitrotyrosine was detected in both peroxynitrite-treated IP-IDO and rIDO, but not in either untreated IP-IDO or rIDO. Peptide analysis by liquid chromatography/electrospray ionization and tandem mass spectrometry demonstrated that Tyr15, Tyr345, and Tyr353 in rIDO were nitrated by peroxynitrite. The levels of Tyr nitration and the inhibitory effect of peroxynitrite on IDO activity were significantly reduced in the Tyr15-to-Phe mutant. These results indicate that IDO is nitrated and inactivated by peroxynitrite and that nitration of Tyr15 in IDO protein is the most important factor in the inactivation of IDO.

The immunoregulatory role of IDO-producing human dendritic cells revisited

Following the finding that indoleamine 2,3-dioxygenase (IDO), an enzyme expressed in the placenta, prevents rejection of allogeneic fetuses in mice, many studies have focused on the role of IDO in the regulation of the immune response. Most arguments for an immunoregulatory role of IDO in vivo are based on observations in mice. Here, we critically examine the arguments for and against a function of IDO-expressing human dendritic cells (DCs) and conclude that proof for an immunoregulatory role in vivo is still lacking.

Indoleamine 2,3-dioxygenase mediates cell type-specific anti-measles virus activity of gamma interferon

Gamma interferon (IFN-gamma) has been shown to be increased in sera from patients with acute measles and after vaccination, to exhibit protective functions in brains of patients with subacute sclerosing panencephalitis, and to mediate a noncytolytic clearance of measles virus (MV) from rodent brains. In order to reveal a possible intracellular antiviral activity in the absence of antigen presentation and cytotoxic T cells, we investigated IFN-gamma-induced effects on MV replication in various tissue culture cells. While attenuated MV strains are more sensitive to IFN-alpha/beta than are wild-type strains, IFN-gamma inhibits the replication of all MV strains in epithelial, endothelial, and astroglial cells, but not in lymphoid and neuronal cell lines. The antiviral activity induced by IFN-gamma correlates with the induction of indoleamine 2,3-dioxygenase (IDO), an enzyme of the tryptophan degradation pathway known to mediate antiviral as well as antibacterial and antiparasitic effects. The IFN-gamma-induced antiviral activity can be overcome by the addition of excess amounts of l-tryptophan, which indicates a specific role of IDO in the anti-MV activity. Our data suggest that the IFN-gamma-induced enzyme IDO plays an important antiviral role in MV infections of epithelial, endothelial, and astroglial cells.

Indoleamine 2, 3-dioxygenase (IDO) is essential for dendritic cell activation and chemotactic responsiveness to chemokines

Indoleamine 2, 3-dioxygenase (IDO) is a rate-limiting enzyme for the tryptophan catabolism. In human and murine cells, IDO inhibits antigen-specific T cell proliferation in vitro and suppresses T cell responses to fetal alloantigens during murine pregnancy. In mice, IDO expression is an inducible feature of specific subsets of dendritic cells (DCs), and is important for T cell regulatory properties. However, the effect of IDO and tryptophan deprivation on DC functions remains unknown. We report here that when tryptophan utilization was prevented by a pharmacological inhibitor of IDO, 1-methyl tryptophan (1MT), DC activation induced by pathogenic stimulus lipopolysaccharide (LPS) or inflammatory cytokine TNF-alpha was inhibited both phenotypically and functionally. Such an effect was less remarkable when DC was stimulated by a physiological stimulus, CD40 ligand. Tryptophan deprivation during DC activation also regulated the expression of CCR5 and CXCR4, as well as DC responsiveness to chemokines. These results suggest that tryptophan usage in the microenvironment is essential for DC maturation, and may also play a role in the regulation of DC migratory behaviors.

Both inducible nitric oxide synthase and NADPH oxidase contribute to the control of virulent phase I Coxiella burnetii infections

Host control of Coxiella burnetii infections is believed to be mediated primarily by activated monocytes/macrophages. The activation of macrophages by cytokines leads to the production of reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) that have potent antimicrobial activities. The contributions of ROI and RNI to the inhibition of C. burnetii replication were examined in vitro by the use of murine macrophage-like cell lines and primary mouse macrophages. A gamma interferon (IFN-gamma) treatment of infected cell lines and primary macrophages resulted in an increased production of nitric oxide (NO) and hydrogen peroxide (H2O2) and a significant inhibition of C. burnetii replication. The inhibition of replication was reversed in the murine cell line J774.16 upon the addition of either the inducible nitric oxide synthase (iNOS) inhibitor NG-monomethyl-L-arginine (NGMMLA) or the H2O2 scavenger catalase. IFN-gamma-treated primary macrophages from iNOS-/- and p47phox-/- mice significantly inhibited replication but were less efficient at controlling infection than IFN-gamma-treated wild-type macrophages. To investigate the contributions of ROI and RNI to resistance to infection, we performed in vivo studies, using C57BL/6 wild-type mice and knockout mice lacking iNOS or p47phox. Both iNOS-/- and p47phox-/- mice were attenuated in the ability to control C. burnetii infection compared to wild-type mice. Together, these results strongly support a role for both RNI and ROI in the host control of C. burnetii infection.

IDO expression by dendritic cells: tolerance and tryptophan catabolism

Indoleamine 2,3-dioxygenase (IDO) is an enzyme that degrades the essential amino acid tryptophan. The concept that cells expressing IDO can suppress T-cell responses and promote tolerance is a relatively new paradigm in immunology. Considerable evidence now supports this hypothesis, including studies of mammalian pregnancy, tumour resistance, chronic infections and autoimmune diseases. In this review, we summarize key recent developments and propose a unifying model for the role of IDO in tolerance induction.