Human indoleamine 2,3-dioxygenase inhibits Toxoplasma gondii growth in fibroblast cells

The Role of the Placental Enzyme Indoleamine 2,3-Dioxygenase in Normal and Abnormal Human Pregnancy

The biologically significant phenomenon that the fetus can survive immune attacks from the mother has been demonstrated in mammals. The survival mechanism depends on the fetus and placenta actively defending themselves against attacks by maternal T cells, achieved through the localized depletion of the amino acid L-tryptophan by an enzyme called indoleamine 2,3-dioxygenase. These findings were entirely unexpected and pose important questions regarding diseases related to human pregnancy and their prevention during human pregnancy. Specifically, the role of this mechanism, as discovered in mice, in humans remains unknown, as does the extent to which impaired activation of this process contributes to major clinical diseases in humans. We have, thus, elucidated several key aspects of this enzyme expressed in the human placenta both in normal and abnormal human pregnancy. The questions addressed in this brief review are as follows: (1) localization and characteristics of human placental indoleamine 2,3-dioxygenas; (2) overall tryptophan catabolism in human pregnancy and a comparison of indoleamine 2,3-dioxygenase expression levels between normal and pre-eclamptic pregnancy; (3) controlling trophoblast invasion by indoleamine 2,3-dioxygenase and its relation to the pathogenesis of placenta accrete spectrum.

A Toxoplasma gondii lipoxygenase-like enzyme is necessary for virulence and changes localization associated with the host immune response

IMPORTANCE

Lipoxygenases (LOXs) are enzymes that catalyze the deoxygenation of polyunsaturated fatty acids such as linoleic and arachidonic acid. These modifications create signaling molecules that are best characterized for modulating the immune response. Deletion of the first lipoxygenase-like enzyme characterized for Toxoplasma gondii (TgLOXL1) generated a less virulent strain, and infected mice showed a decreased immune response. This virulence defect was dependent on the mouse cytokine interferon gamma IFNγ. TgLOXL1 changes location from inside the parasite in tissue culture conditions to vesicular structures within the host immune cells during mouse infection. These results suggest that TgLOXL1 plays a role in the modification of the host immune response in mice.

Indoleamine 2,3-Dioxygenase (IDO) Activity: A Perspective Biomarker for Laboratory Determination in Tumor Immunotherapy

Indoleamine 2,3-dioxygenase 1 (IDO1) is a heme enzyme involved in catalyzing the conversion of tryptophan (Trp) into kynurenine (Kyn) at the first rate-limiting step in the kynurenine pathway of L-tryptophan metabolism. It has been found to be involved in several biological functions such as aging, immune microorganism, neurodegenerative and infectious diseases, and cancer. IDO1 plays an important role in immune tolerance by depleting tryptophan in the tumor microenvironment and inhibiting the proliferation of effector T cells, which makes it an important emerging biomarker for cancer immunotherapy. Therefore, the research and development of IDO1 inhibitors are of great importance for tumor therapy. Of interest, IDO activity assays are of great value in the screening and evaluation of inhibitors. Herein, we mainly review the biological functions of IDO1, immune regulation, key signaling molecules in the response pathway, and the development of IDO1 inhibitors in clinical trials. Furthermore, this review provides a comprehensive overview and, in particular, a discussion of currently available IDO activity assays for use in the evaluation of IDO inhibitors in human blood. We believe that the IDO activity is a promising biomarker for the immune escape and laboratory evaluation of tumor immunotherapy.

Toxoplasma gondii infection in patients with brain tumors in Southern Iran: a case-control study

Considerable evidence points to a dominant role of inflammation in tumor pathology. The biological response of the immune system can be triggered by Toxoplasma gondii as a common brain-tropic parasite. The aim of this study was to investigate an association between Toxoplasma infection and brain tumors. This case-control study was performed on sera of brain tumor patients (n = 124) and age- and sex-matched control subjects (n = 124) in Southern Iran. Data related to tumor site and type were collected during sample collection. Anti-Toxoplasma IgG was assessed by enzyme-linked immunosorbent assay (ELISA). Seroprevalence anti-Toxoplasma IgG was significantly higher in brain tumor patients 30.6% (38/124) compared with 12.1% (15/124) of the healthy controls (OR 3.211; 95% CI 1.658 to 6.219; p = 0.001). The highest seroprevalence was detected in patients with ependymoma (100%), followed by glioblastoma (83%), pituitary adenoma (47.3%), astrocytoma (27.2%), schwannoma (23%), and meningioma (22.6%). The parasite infection was correlated to brain tumor's location i.e., the patients with frontal lobe and sella region tumors had higher seropositivity compared with others (P < 0.05). The higher prevalence of Toxoplasma infection among patients with brain tumor compared with the control group indicates a probable association between the infection and brain tumors.

CRISPR Screens Identify Toxoplasma Genes That Determine Parasite Fitness in Interferon Gamma-Stimulated Human Cells

Toxoplasma virulence depends on its ability to evade or survive the toxoplasmacidal mechanisms induced by interferon gamma (IFNγ). While many Toxoplasma genes involved in the evasion of the murine IFNγ response have been identified, genes required to survive the human IFNγ response are largely unknown. In this study, we used a genome-wide loss-of-function screen to identify Toxoplasma genes important for parasite fitness in IFNγ-stimulated primary human fibroblasts. We generated gene knockouts for the top six hits from the screen and confirmed their importance for parasite growth in IFNγ-stimulated human fibroblasts. Of these six genes, three have homology to GRA32, localize to dense granules, and coimmunoprecipitate with each other and GRA32, suggesting they might form a complex. Deletion of individual members of this complex leads to early parasite egress in IFNγ-stimulated cells. Thus, prevention of early egress is an important Toxoplasma fitness determinant in IFNγ-stimulated human cells. IMPORTANCE Toxoplasma infection causes serious complications in immunocompromised individuals and in the developing fetus. During infection, certain immune cells release a protein called interferon gamma that activates cells to destroy the parasite or inhibit its growth. While most Toxoplasma parasites are cleared by this immune response, some can survive by blocking or evading the IFNγ-induced restrictive environment. Many Toxoplasma genes that determine parasite survival in IFNγ-activated murine cells are known but parasite genes conferring fitness in IFNγ-activated human cells are largely unknown. Using a Toxoplasma adapted genome-wide loss-of-function screen, we identified many Toxoplasma genes that determine parasite fitness in IFNγ-activated human cells. The gene products of four top hits play a role in preventing early parasite egress in IFNγ-stimulated human cells. Understanding how IFNγ-stimulated human cells inhibit Toxoplasma growth and how Toxoplasma counteracts this, could lead to the development of novel therapeutics.

Tryptophan, Kynurenine and Kynurenic Acid Concentrations in Milk and Serum of Dairy Cows with Prototheca Mastitis

The aim of this work was to investigate serum and milk levels of tryptophan (TRP), kynurenine (KYN), and kynurenic acid (KYNA), as well as the activity of indoleamine 2,3-dioxygenase (IDO) in cows with mastitis due to Prototheca algae. The study was prompted by previous research showing a link between the KYN pathway of TRP metabolism and bovine mastitis of bacterial etiology. The study was carried out over a 2-year period (2018-2019) and included quarter milk and serum samples collected from six dairy herds in Poland. The samples were obtained from healthy cows and cows with Prototheca mastitis of either clinical and subclinical manifestation, as determined upon direct measurement of the somatic cell count or indirectly by performing a California Mastitis Test on suspected quarters. Both TRP and KYN concentrations were significantly lower in milk of mastitic cows compared to healthy animals (0.8 vs. 8.72 µM, p = 0.001; 0.07 vs. 0.32 µM, p = 0.001, respectively). The difference in TRP and KYN concentrations in the sera of the two animal groups was much less pronounced (25.55 vs. 27.57 µM, 3.03 vs. 3.56 nM, respectively). The concentration of KYNA was almost at the same level in milk (1.73 vs. 1.70 nM) and in serum (80.47 vs. 75.48 nM) of both mastitic and healthy cows. The data showed that the level of TRP and its metabolites in serum was conspicuously higher compared to milk in all cows under the study. The activity of IDO was significantly higher in milk of cows with Prototheca mastitis compared to healthy animals (71.4 vs. 40.86, p < 0.05), while in serum it was pretty much the same (135.94 vs. 124.98, p > 0.05). The IDO activity differed significantly between serum and milk both for mastitic (135.94 vs. 71.4, p < 0.05) and healthy cows (124.98 vs. 40.86, p < 0.001). In conclusion, low values of TRP and KYN concentrations or elevated IDO activity in milk samples might be used as markers of mastitis due to infectious causes, including Prototheca spp.

IFN-γ mediates Paneth cell death via suppression of mTOR

Paneth cells constitutively produce antimicrobial peptides and growth factors that allow for intestinal homeostasis, host protection, and intestinal stem cell replication. Paneth cells rely heavily on the glycolytic metabolic program, which is in part controlled by the kinase complex Mechanistic target of rapamycin (mTORC1). Yet, little is known about mTOR importance in Paneth cell integrity under steady-state and inflammatory conditions. Our results demonstrate that IFN-γ, a crucial mediator of the intestinal inflammation, acts directly on murine Paneth cells to alter their mitochondrial integrity and membrane potential, resulting in an TORC1-dependent cell death mechanism distinct from canonical cell death pathways including apoptosis, necroptosis, and pyroptosis. These results were established with the purified cytokine and a physiologically relevant common Th1-inducing human parasite Toxoplasma gondii. Given the crucial role for IFN-γ, which is a cytokine frequently associated with the development of inflammatory bowel disease and compromised Paneth cell functions, the identified mechanisms underlying mTORC1-dependent Paneth cell death downstream of IFN-γ may provide promising novel approaches for treating intestinal inflammation.

Modulation of Crustacean Innate Immune Response by Amino Acids and Their Metabolites: Inferences From Other Species

Aquaculture production of crustaceans (mainly shrimp and crabs) has expanded globally, but disease outbreaks and pathogenic infections have hampered production in the last two decades. As invertebrates, crustaceans lack an adaptive immune system and mainly defend and protect themselves using their innate immune system. The immune system derives energy and metabolites from nutrients, with amino acids constituting one such source. A growing number of studies have shown that amino acids and their metabolites are involved in the activation, synthesis, proliferation, and differentiation of immune cells, as well as in the activation of immune related signaling pathways, reduction of inflammatory response and regulation of oxidative stress. Key enzymes in amino acid metabolism have also been implicated in the regulation of the immune system. Here, we reviewed the role played by amino acids and their metabolites in immune-modulation in crustaceans. Information is inferred from mammals and fish where none exists for crustaceans. Research themes are identified and the relevant research gaps highlighted for further studies.

The Uniqueness of Tryptophan in Biology: Properties, Metabolism, Interactions and Localization in Proteins

Tryptophan (Trp) holds a unique place in biology for a multitude of reasons. It is the largest of all twenty amino acids in the translational toolbox. Its side chain is indole, which is aromatic with a binuclear ring structure, whereas those of Phe, Tyr, and His are single-ring aromatics. In part due to these elaborate structural features, the biosynthetic pathway of Trp is the most complex and the most energy-consuming among all amino acids. Essential in the animal diet, Trp is also the least abundant amino acid in the cell, and one of the rarest in the proteome. In most eukaryotes, Trp is the only amino acid besides Met, which is coded for by a single codon, namely UGG. Due to the large and hydrophobic π-electron surface area, its aromatic side chain interacts with multiple other side chains in the protein, befitting its strategic locations in the protein structure. Finally, several Trp derivatives, namely tryptophylquinone, oxitriptan, serotonin, melatonin, and tryptophol, have specialized functions. Overall, Trp is a scarce and precious amino acid in the cell, such that nature uses it parsimoniously, for multiple but selective functions. Here, the various aspects of the uniqueness of Trp are presented in molecular terms.

Influence of the Host and Parasite Strain on the Immune Response During Toxoplasma Infection

Toxoplasma gondii is an exceptionally successful parasite that infects a very broad host range, including humans, across the globe. The outcome of infection differs remarkably between hosts, ranging from acute death to sterile infection. These differential disease patterns are strongly influenced by both host- and parasite-specific genetic factors. In this review, we discuss how the clinical outcome of toxoplasmosis varies between hosts and the role of different immune genes and parasite virulence factors, with a special emphasis on Toxoplasma-induced ileitis and encephalitis.

Paradoxical Pro-inflammatory Responses by Human Macrophages to an Amoebae Host-Adapted Legionella Effector

Legionella pneumophila has co-evolved with amoebae, their natural hosts. Upon transmission to humans, the bacteria proliferate within alveolar macrophages causing pneumonia. Here, we show L. pneumophila injects the effector LamA, an amylase, into the cytosol of human macrophage (hMDMs) and amoebae to rapidly degrade glycogen to generate cytosolic hyper-glucose. In response, hMDMs shift their metabolism to aerobic glycolysis, which directly triggers an M1-like pro-inflammatory differentiation and nutritional innate immunity through enhanced tryptophan degradation. This leads to a modest restriction of bacterial proliferation in hMDMs. In contrast, LamA-mediated glycogenolysis in amoebae deprives the natural host from the main building blocks for synthesis of the cellulose-rich cyst wall, leading to subversion of amoeba encystation. This is non-permissive for bacterial proliferation. Therefore, LamA of L. pneumophila is an amoebae host-adapted effector that subverts encystation of the amoebae natural host, and the paradoxical hMDMs' pro-inflammatory response is likely an evolutionary accident.

Host cell depletion of tryptophan by IFNγ-induced Indoleamine 2,3-dioxygenase 1 (IDO1) inhibits lysosomal replication of Coxiella burnetii

Most intracellular pathogens that reside in a vacuole prevent transit of their compartment to lysosomal organelles. Effector mechanisms induced by the pro-inflammatory cytokine Interferon-gamma (IFNγ) can promote the delivery of pathogen-occupied vacuoles to lysosomes for proteolytic degradation and are therefore important for host defense against intracellular pathogens. The bacterial pathogen Coxiella burnetii is unique in that, transport to the lysosome is essential for replication. The bacterium modulates membrane traffic to create a specialized autophagolysosomal compartment called the Coxiella-containing vacuole (CCV). Importantly, IFNγ signaling inhibits intracellular replication of C. burnetii, raising the question of which IFNγ-activated mechanisms restrict replication of a lysosome-adapted pathogen. To address this question, siRNA was used to silence a panel of IFNγ-induced genes in HeLa cells to identify genes required for restriction of C. burnetii intracellular replication. This screen demonstrated that Indoleamine 2,3-dioxygenase 1 (IDO1) contributes to IFNγ-mediated restriction of C. burnetii. IDO1 is an enzyme that catabolizes cellular tryptophan to kynurenine metabolites thereby reducing tryptophan availability in cells. Cells deficient in IDO1 function were more permissive for C. burnetii replication when treated with IFNγ, and supplementing IFNγ-treated cells with tryptophan enhanced intracellular replication. Additionally, ectopic expression of IDO1 in host cells was sufficient to restrict replication of C. burnetii in the absence of IFNγ signaling. Using differentiated THP1 macrophage-like cells it was determined that IFNγ-activation resulted in IDO1 production, and that supplementation of IFNγ-activated THP1 cells with tryptophan enhanced C. burnetii replication. Thus, this study identifies IDO1 production as a key cell-autonomous defense mechanism that limits infection by C. burnetii, which suggests that peptides derived from hydrolysis of proteins in the CCV do not provide an adequate supply of tryptophan for bacterial replication.

Tryptophan, kynurenine, kynurenic acid concentrations and indoleamine 2,3-dioxygenase activity in serum and milk of dairy cows with subclinical mastitis caused by coagulase-negative staphylococci

The aim of the study was to investigate serum and milk concentrations of tryptophan (TRP), kynurenine (KYN), kynurenic acid (KYNA), and indoleamine 2,3-dioxygenase (IDO) activity in cows suffering from subclinical mastitis caused by coagulase-negative staphylococci (MSCNS). TRP and kynurenines were determined by high-performance liquid chromatography (HPLC), and IDO activity was calculated as the KYN/TRP ratio. The blood and milk samples were collected from 40 midlactation Holstein-Fresian cows from two herds in the Lublin region in Poland. In the milk samples from 20 cows with subclinical mastitis, coagulase-negative staphylococci were isolated and in the milk obtained from healthy cows growth of microorganisms was not detected. TRP, KYN and KYNA concentrations were significantly lower in milk of cows with MSCNS compared to control animals (4.47 vs. 7.24 µM, 0.14 vs. 0.21 µM, 1.58 vs. 2.18 nM, respectively). There was no statistically significant difference in TRP and KYNA concentrations in serum between the studied animal groups (32.97 vs. 39.29 µM, 31.3 vs. 26.5 nM, respectively). In turn, the level of KYN was lower in the serum (0.81 vs. 1.13 µM) of cows with mastitis compared to healthy ones. No statistically significant differences in IDO activity, both in serum and in milk (25.24 and 27.55, 28.56 and 27.17, respectively) was revealed between the studied groups. These findings may have potential implications for diagnosis of mastitis in cows because reduction of these parameters in milk might be a marker predicting the occurrence of the disease.

Hepatic Metabolomics Investigation in Acute and Chronic Murine Toxoplasmosis

Toxoplasma gondii poses a great threat to human health, with no approved vaccine available for the treatment of T. gondii infection. T. gondii infections are not limited to the brain, and may also affect other organs especially the liver. Identification of host liver molecules or pathways involved in T. gondii replication process may lead to the discovery of novel anti-T. gondii targets. Here, we analyzed the metabolic profile of the liver of mice on 11 and 30 days postinfection (dpi) with type II T. gondii Pru strain. Global metabolomics using liquid chromatography-tandem mass spectrometry (LC-MS/MS) identified 389 significant metabolites from acutely infected mice; and 368 from chronically infected mice, when compared with control mice. Multivariate statistical analysis revealed distinct metabolic signatures from acutely infected, chronically infected and control mice. Infection influenced several metabolic processes, in particular those for lipids and amino acids. Metabolic pathways, such as steroid hormone biosynthesis, primary bile acid biosynthesis, bile secretion, and biosynthesis of unsaturated fatty acids were perturbed during the whole infection process, particularly during the acute stage of infection. The present results provide insight into hepatic metabolic changes that occur in BALB/c mice during acute and chronic T. gondii infection.

Translational Control in the Latency of Apicomplexan Parasites

Apicomplexan parasites Toxoplasma gondii and Plasmodium spp. use latent stages to persist in the host, facilitate transmission, and thwart treatment of infected patients. Therefore, it is important to understand the processes driving parasite differentiation to and from quiescent stages. Here, we discuss how a family of protein kinases that phosphorylate the eukaryotic initiation factor-2 (eIF2) function in translational control and drive differentiation. This translational control culminates in reprogramming of the transcriptome to facilitate parasite transition towards latency. We also discuss how eIF2 phosphorylation contributes to the maintenance of latency and provides a crucial role in the timing of reactivation of latent parasites towards proliferative stages.

Guanylate-binding protein 1 (GBP1) contributes to the immunity of human mesenchymal stromal cells against Toxoplasma gondii

Mesenchymal stromal cells (MSCs) have recently been shown to play important roles in mammalian host defenses against intracellular pathogens, but the molecular mechanism still needs to be clarified. We confirmed that human MSCs (hMSCs) prestimulated with IFN-γ showed a significant and dose-dependent ability to inhibit the growth of two types of Toxoplasma gondii [type I RH strain with green fluorescent proteins (RH/GFP) or type II PLK strain with red fluorescent proteins (PLK/RED)]. However, in contrast to previous reports, the anti-T. gondii activity of hMSCs was not mediated by indoleamine 2,3-dioxygenase (IDO). Genome-wide RNA sequencing (RNA-seq) analysis revealed that IFN-γ increased the expression of the p65 family of human guanylate-binding proteins (hGBPs) in hMSCs, especially hGBP1. To analyze the functional role of hGBPs, stable knockdowns of hGBP1, -2, and -5 in hMSCs were established using a lentiviral transfection system. hGBP1 knockdown in hMSCs resulted in a significant loss of the anti-T. gondii host defense property, compared with hMSCs infected with nontargeted control sequences. hGBP2 and -5 knockdowns had no effect. Moreover, the hGBP1 accumulation on the parasitophorous vacuole (PV) membranes of IFN-γ-stimulated hMSCs might protect against T. gondii infection. Taken together, our results suggest that hGBP1 plays a pivotal role in anti-T. gondii protection of hMSCs and may shed new light on clarifying the mechanism of host defense properties of hMSCs.

5-Hydroxytryptophan, a major product of tryptophan degradation, is essential for optimal replication of human parainfluenza virus

Interferon (IFN) exerts its antiviral effect by inducing a large family of cellular genes, named interferon (IFN)-stimulated genes (ISGs). An intriguing member of this family is indoleamine 2,3-dioxygenase (IDO), which catalyzes the first and rate-limiting step of the main branch of tryptophan (Trp) degradation, the kynurenine pathway. We recently showed that IDO strongly inhibits human parainfluenza virus type 3 (PIV3), a significant respiratory pathogen. Here, we show that 5-hydoxytryptophan (5-HTP), the first product of an alternative branch of Trp degradation and a serotonin precursor, is essential to protect virus growth against IDO in cell culture. We also show that the apparent antiviral effect of IDO on PIV3 is not due to the generation of the kynurenine pathway metabolites, but rather due to the depletion of intracellular Trp by IDO, as a result of which this rare amino acid becomes unavailable for the alternative, proviral 5-HTP pathway.

The potential of targeting indoleamine 2,3-dioxygenase for cancer treatment

INTRODUCTION

Degradation of the essential amino acid tryptophan via indoleamine 2,3-dioxygenase (IDO1) represents an important antiproliferative strategy of the cellular immune response. Tryptophan shortage and accumulation of kynurenine downstream products also affect T-cell responses, providing a negative feedback control of immune activation. IDO1 activity can promote a regulatory phenotype in both T cells and dendritic cells. These phenomena can support tumor immune escape.

AREAS COVERED

IDO1 activity reflects the course of several malignancies, and determination of kynurenine to tryptophan ratio in serum/plasma can be used to assess immune activation. Moreover, the accelerated breakdown of tryptophan has been correlated with the development of cancer-associated disturbances such as anemia, weight loss and depression. Tumoral IDO1 expression was correlated with a poor prognosis in several types of tumors, which makes it to an interesting target for immunotherapy. In addition, according to recent data, a role of trytptophan 2,3-dioxygenase (TDO) in tumorigenesis cannot be excluded.

EXPERT OPINION

Tryptophan metabolism is critical for cell proliferation, inflammation and immunoregulation. Accelerated tryptophan breakdown favors tumor immune escape. Accordingly, targeting IDO1 by immunotherapy may represent a favorable approach; however, blocking crucial immunoregulatory pathways could also introduce the risk of immune system overactivation, finally leading to unresponsiveness.

Evaluation of kynurenine pathway metabolism in Toxoplasma gondii-infected mice: implications for schizophrenia

Toxoplasma gondii, an intracellular protozoan parasite, is a major cause of opportunistic infectious disease affecting the brain and has been linked to an increased incidence of schizophrenia. In murine hosts, infection with T. gondii stimulates tryptophan degradation along the kynurenine pathway (KP), which contains several neuroactive metabolites, including 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN) and kynurenic acid (KYNA). As these endogenous compounds may provide a mechanistic connection between T. gondii and the pathophysiology of schizophrenia, we measured KP metabolites in both the brain and periphery of T. gondii-treated C57BL/6 mice 8 and 28 days post-infection. Infected mice showed early decreases in the levels of tryptophan in the brain and serum, but not in the liver. These reductions were associated with elevated levels of kynurenine, KYNA, 3-HK and QUIN in the brain. In quantitative terms, the most significant increases in these KP metabolites were observed in the brain at 28 days post-infection. Notably, the anti-parasitic drugs pyrimethamine and sulfadiazine, a standard treatment of toxoplasmosis, significantly reduced 3-HK and KYNA levels in the brain of infected mice when applied between 28 and 56 days post-infection. In summary, T. gondii infection, probably by activating microglia and astrocytes, enhances the production of KP metabolites in the brain. However, during the first two months after infection, the KP changes in these mice do not reliably duplicate abnormalities seen in the brain of individuals with schizophrenia.

Direct Effect of Two Naphthalene-Sulfonyl-Indole Compounds on Toxoplasma gondii Tachyzoite

Past studies have stated that the parasitostatic effect of IFN-γ is most likely due to the starvation of Toxoplasma gondii for tryptophan in the host cell. The aim of this study was to evaluate the direct effect of two new Naphthalene-Sulfonyl-Indole compounds as competitive molecules for tryptophan on viability and infectivity of Toxoplasma tachyzoites. Tachyzoites of RH strain were incubated in various concentrations (25–800 μM) of 1-(naphthalene-2-sulfonyl)-2,3-dihydro-1H-indole and 1-[5-(2,3-dihydro-1H-indole-1-sulfonyl)naphthalene-1-sulfonyl]-2,3-dihydro-1H-indole for 1.5 hours. Then, they were stained by PI and analyzed by FACS. To evaluate the infectivity, 2 × 10⁶ tachyzoites exposed to the concentrations mentioned above were intraperitoneally inoculated into five mice from each group. Also, naïve parasites and parasites exposed to DMSO (control) were inoculated in both groups of mice. The LD₅₀ of 1-(naphthalene-2-sulfonyl)-2,3-dihydro-1H-indole was 62 μmol whilst the quantity of 1-[5-(2,3-dihydro-1H-indole-1-sulfonyl)naphthalene-1-sulfonyl]-2,3-dihydro-1H-indole was more than 800 μmol. The infectivity of tachyzoites exposed to both of the compounds preserved and killed mice. No statistical correlation was seen between longevity of mice groups and different doses of the compounds. If we consider a well-organized transporter mechanism for indole compounds in the parasite, thus the designation of an antagonist that has indole groups can assist with the production of new drugs.

The role of placental indoleamine 2,3-dioxygenase in human pregnancy

Munn et al. made a scientific observation of major biological importance. For the first time they showed that in the mammal the fetus does survive an immune attack mounted by the mother, and that the mechanism responsible for the survival depends on the fetus and placenta 'actively' defending itself from attack by maternal T cells by means of an enzyme indoleamine 2,3-dioxygenase (EC 1.13.11.42) dependent localised depletion of L-tryptophan. These findings raise critical questions for disease and its prevention during human pregnancy. Specifically, the role of this mechanism (discovered in mouse) in the human, and the extent to which defective activation of this process is responsible for major clinical diseases are unknown. Therefore some key facts about this enzyme expressed in the human placenta have been studied in order to test whether Munn et al.'s findings in mouse are met for human pregnancy. This short review attempts to describe our experimental work on human placental indoleamine 2,3-dioxygenase.

Inhibition of increased indoleamine 2,3-dioxygenase activity attenuates Toxoplasma gondii replication in the lung during acute infection

The regulation of local L-tryptophan concentrations by tryptophan-degrading enzyme, indoleamine 2,3-dioxygenase (IDO) induced by various stimuli such as interferon-γ (IFN-γ) is one of the key mechanisms in antimicrobial effect. Recently, IDO is also focused on an immunosuppressive mechanism shared by several different immune cell types. Here, we show that inhibition of increased IDO activity maybe involved in the antiparasitic mechanism during Toxoplasma gondii (T. gondii) infection in vivo. In this study, we investigated the role of IDO by using IDO-gene-deficient (IDO KO) mice and by administering a competitive enzyme inhibitor, 1-methyl-D,L-tryptophan (1MT), to wild-type mice following T. gondii infection. Although depletion of lung l-tryptophan did not occur in IDO KO mice after T. gondii infection, the increased mRNA expression of T. gondii surface antigen gene 2 (SAG2) and the inflammatory cytokines in the lung were drastically reduced in the IDO KO mice following infection. We also found that complete depletion of lung l-tryptophan was observed in wild-type mice after infection, but not in mice treated with 1MT. At the same time, 1MT suppressed the increased mRNA expression of SAG2. Taken together, we observed that the inflammatory damage was significantly decreased by the administration of 1MT in the lung after infection. Inhibition of the IDO activity or the elimination of IDO's substrate may be an effective therapy against microbial diseases.

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.

Anti-infectious activity of tryptophan metabolites in the L-tryptophan-L-kynurenine pathway

To study the anti-infectious effect of a vascular allograft, the antimicrobial activity of tryptophan metabolites mediated by indoleamine 2,3-dioxygenase was determined. The growth of methicillin-resistant Staphylococcus aureus (MRSA) over 10 h in extracts from post-transplantation vascular allograft was significantly slower than that of extracts from non-transplantation vascular allograft regardless of the presence of tryptophan (p<0.05). When the antimicrobial activity of the tryptophan metabolites in the L-tryptophan-L-kynurenine pathway was examined, 3-hydroxy-DL-kynurenine and alpha-picolinic acid had strong antibacterial activity against MRSA, S. epidermidis, Escherichia coli, and multidrug-resistant Pseudomonas aeruginosa, although antimicrobial activities of anthranilic acid, 3-hydroxyanthranilic acid, and quinolinic acid against them were low. The results showed that, of the tested tryptophan metabolites, 3-hydroxy-DL-kynurenine and alpha-picolinic acid contributed to the anti-infectious effects of the allograft by inhibiting of the growth of microorganisms.

Host cell manipulation by the human pathogen Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that can infect virtually any nucleated cell. During invasion Toxoplasma creates the parasitophorous vacuole, a subcellular compartment that acts as an interface between the parasite and host, and serves as a platform for modulation of host cell functions that support parasite replication and infection. Spatial reorganization of host organelles and cytoskeleton around the parasitophorous vacuole are observed following entry, and recent evidence suggests this interior redecorating promotes parasite nutrient acquisition. New findings also reveal that Toxoplasma manipulates host signaling pathways by deploying parasite kinases and a phosphatase, including at least two that infiltrate the host nucleus. Toxoplasma infection additionally controls several cellular pathways to establish an anti-apoptotic environment, and subverts immune cells as a conduit for dissemination. In this review we discuss these recent developments in understanding how Toxoplasma achieves widespread success as a human and animal parasite by manipulating its host.

New host nuclear functions are not required for the modifications of the parasitophorous vacuole of Toxoplasma

The obligate intracellular parasite Toxoplasma develops within a parasitophorous vacuole (PV) uniquely adapted for its survival in mammalian cells. Post-invasion events extensively modify the PV, resulting in interactions with host cell structures. Recent studies emphasized that Toxoplasma is able to co-opt host gene expression, suggesting that host transcriptional activities are required for parasite infection. By using an experimental enucleation model, we investigated the potential need for Toxoplasma to modify its PV by modulating gene expression in the cell wherein it resides. Unexpectedly, cytoplasts can be actively invaded by Toxoplasma and sustain its replication inside a vacuole until egress and transmission to neighbouring cells. Although randomly distributed in the cytoplast, the PV associates with host centrosomes and the Golgi, is surrounded by host microtubules, and recruits host endoplasmic reticulum and mitochondria. Parasites are proficient in diverting exogenous nutrients from the endocytic network of cytoplasts. In enucleated cells invaded by an avirulent strain of T. gondii, the PV can normally transform into cysts. These observations suggest that new host nuclear functions are not proximately required for the post-invasion events underlying the remodelling of the host cell in which the parasites are confined, and therefore for the generation of infectious parasites in vitro.

Suppression of islet allogeneic immune response by indoleamine 2,3 dioxygenase-expressing fibroblasts

Success of transplantation of pancreatic islets which is a promising way for restoring efficient insulin regulation in type 1 diabetes depends on lifelong use of immunosuppressive drugs. To eliminate the use of systemic immunosuppressive drugs for islet transplantation, we examined the potential use of a local immunosuppressive factor, indoleamine 2,3-dioxygenase (IDO). Thus, the aim of this study was to determine whether local expression of IDO in bystander syngeneic fibroblasts could prevent islet allogeneic immune response in vitro. C57BL/6 (B6) mouse fibroblasts were induced to express IDO by either IFN-gamma treatment or transduction with an adenoviral vector and were co-cultured with B6 mouse lymphocytes and BALB/c mouse pancreatic islets in the presence or absence of an IDO inhibitor. Proliferation of lymphocytes were then assessed using [(3)H]-thymidine incorporation assay. IDO-expression by co-cultured syngeneic fibroblasts resulted in a five-fold decrease in lymphocyte proliferation rate upon stimulation of lymphocytes by allogeneic mouse pancreatic islets (21.9% +/- 5.3 and 22.1% +/- 4.9 in the preparations with IFN-gamma treated and genetically modified IDO-expressing fibroblasts, respectively vs. 100% in control groups, P < 0.01). Allogeneic response was restored when IDO inhibitor was added to the culture indicating that suppression was due to IDO. In conclusion, this study shows that local expression of IDO by syngeneic bystander fibroblasts can suppress in vitro proliferation of lymphocytes in response to stimulation with allogeneic pancreatic islets. This local immunosuppressive function of IDO may be employed for development of a novel alternative strategy for preventing allogeneic islet graft rejection.

Regulation of Toxoplasma gondii multiplication in BeWo trophoblast cells: cross-regulation of nitric oxide production and polyamine biosynthesis

Materno-foetal transmission causes one of the most severe forms of infection with the protozoan parasite Toxoplasma gondii. Several studies have shown T. gondii in placental trophoblast cells, which form the barrier between maternal blood circulation and foetal tissue. Parasite multiplication in trophoblast cells is thus a critical step leading to infection of the foetus. Here, we show that multiplication of T. gondii tachyzoites was slow in BeWo trophoblast cells, compared with MRC-5 fibroblast cells. However, unlike MRC-5 cells, even combined stimulation with interferon-gamma and tumor necrosis factor-alpha did not reduce T. gondii replication in BeWo cells. This was associated with a lack of indoleamine-2,3-dioxygenase induction by these cytokines. Neither low availability of iron salts, nor an immunosuppressive action of cyclooxygenase-2 could be attributed to the low T. gondii multiplication rate in BeWo cells. However, treatment with the nitric oxide synthesis inhibitor N(G)-methyl-l-arginine and addition of ornithine enhanced the proliferation rate of the intracellular pathogen. Despite detection of inducible nitric oxide synthase-II mRNA in BeWo cells, nitric oxide production could not be detected during cell culture. Thus, inhibition of arginase activity by nitric oxide synthesis may be partially responsible for the lower multiplication rate in BeWo cells.

Characterisation of Toxoplasma gondii engineered to express mouse interferon-gamma

Recent studies have shown the feasibility of using Toxoplasma gondii as an expression system for heterologous protein. For better understanding of the mechanism of interferon-gamma (IFN-gamma) dependent immunity to T. gondii, the parasites were stably transfected with IFN-gamma gene, under control of the GRA1 promoter. Immunofluorescence analyses showed that recombinant mouse IFN-gamma localised to discrete punctuate structures consistent with dense granules and secreted into the vacuolar space. The production of IFN-gamma was detectable in both extracellular parasites and the parasite-infected cells. Growth of the recombinant parasites was inhibited in the mouse macrophage cell line (J774A.1 cells), but not in monkey kidney adherent fibroblasts (Vero cells), demonstrating the species-specificity of IFN-gamma. Addition of anti-mouse IFN-gamma antibody resulted in growth recovery of the recombinant parasites, suggesting that IFN-gamma, secreted from the parasitised host cells across the parasitophorous vacuole membrane, acted in a paracrine manner. Reverse transcription (RT)-PCR analysis revealed significant expression of inducible nitric oxide synthase mRNA and high levels of nitric oxide production in recombinant parasite-infected J774A.1 cells. A competitive inhibitor of the L-arginine-dependent effector pathway, N(G)-monomethyl-L-arginine, inhibited the reduction of recombinant parasite growth in J774A.1 cells. Taken together, our data suggest that the T. gondii expression system may provide a new tool for cytokine gene expression and that parasites engineered to express a cytokine gene may be rationally designed for use in studies on immune responses to T. gondii.

Proliferation of peripheral blood mononuclear cells is suppressed by the indoleamine 2,3-dioxygenase expression of interferon-gamma-treated skin cells in a co-culture system

Indoleamine 2,3-dioxygenase (IDO) is an intracellular tryptophan-oxidizing enzyme possessing various immunosuppressive characteristics. In this study, we report the possible use of this enzyme in an allogenic skin substitute to suppress the proliferation of immune cells. Human fetal skin fibroblasts and keratinocytes were treated with the cytokine interferon-gamma to induce expression of IDO mRNA and protein. IDO enzyme activity was evaluated by measurement of kynurenine levels in the interferon-gamma-treated and -untreated cells. Results of Northern analysis showed a dose-dependent response in expression of IDO mRNA to the various concentrations of interferon-gamma used. Northern blot analysis also showed a time-dependent expression of IDO in response to different durations of interferon-gamma treatment. The level of kynurenine measured, as the bioactivity of IDO enzyme, was significantly higher in the interferon-gamma-treated fibroblasts and keratinocytes compared to those of controls (p < 0.001). To illustrate the immunosuppressive effects of IDO on immune cell proliferation, IDO-expressing fibroblasts were cocultured with human peripheral blood mononuclear cells for a period of 5 days. Results of 3H-thymidine incorporation assays showed a significant reduction in proliferation of the mononuclear cells cocultured with IDO-expressing skin cells compared to monocytes cocultured with control (non-IDO-expressing) skin cells (p < 0.001). Furthermore, addition of the IDO-inhibitor (1-methyl-D-tryptophan) significantly reversed the immunosuppressive effects of IDO on monocyte proliferation (p < 0.001). In conclusion, suppression of peripheral blood mononuclear cell proliferation due to interferon-gamma-induced IDO-expression in allogenic human skin cells might shed new light on developing a nonrejectable allogenic skin substitute.

Immune cell proliferation is suppressed by the interferon-gamma-induced indoleamine 2,3-dioxygenase expression of fibroblasts populated in collagen gel (FPCG)

Indoleamine 2,3-dioxygenase (IDO), a tryptophan-catabolizing enzyme, is an intracellular enzyme possessing various immunosuppressive properties. Here, we report the possible use of this enzyme to suppress proliferation of immune cells cocultured with IDO-expressing fibroblasts of an allogenic skin substitute. Fetal skin fibroblasts embedded within bovine collagen were treated with cytokine interferon-gamma (IFN-gamma) to induce expression of IDO mRNA and protein. Expression of IDO mRNA was evaluated by Northern analysis. IDO enzyme activity was evaluated by measurement of kynurenine and tryptophan levels in the IFN-gamma untreated and treated fibroblasts. The results of Northern analysis showed a dose-dependent increase in expression of IDO mRNA in response to various concentrations of IFN-gamma used. The levels of kynurenine and tryptophan measured, as the bioactivity of IDO, were significantly different in the IFN-gamma treated fibroblasts, compared to those of controls (P < 0.001). In a lasting effect experiment, the expression of IDO mRNA was gradually reduced to an undetectable level within 32 h of IFN-gamma removal. The results of Western blot analysis, however, revealed a significantly longer (192 h) lasting effect of IFN-gamma on IDO protein level, relative to that of mRNA expression. To demonstrate immunosuppressive effects of IDO on proliferation of immune cells, IDO-expressing fibroblasts were cocultured with peripheral blood mononuclear cells (PBMC) for a period of 5 days. The results of (3)H-thymidine incorporation showed a significant reduction in proliferation of PBMC when cocultured with IDO-expressing fibroblasts, compared to those cocultured with non-IDO-expressing fibroblasts (P < 0.001). Furthermore, addition of IDO-inhibitor (1-methyl-d-tryptophan) reversed the suppressive effects of IDO on PBMC proliferation in a dose-dependant fashion. To test the viability of immune cells cocultured with IDO-expressing fibroblasts, FACS analysis of the PI stained PBMC was conducted and no significant difference was found between these cells and the controls. In another set of experiments, we showed that migration rate and subsequent proliferation of IDO-expressing fibroblasts are also the same as those of control cells. In conclusion, IDO-expressing allogenic fibroblasts embedded within collagen gel suppress the proliferation of allogenic immune cells, while they still remain viable in this IDO-induced tryptophan-deficient culture environment.

Polymorphisms in Chlamydia trachomatis tryptophan synthase genes differentiate between genital and ocular isolates

We previously reported that laboratory reference strains of Chlamydia trachomatis differing in infection organotropism correlated with inactivating mutations in the pathogen's tryptophan synthase (trpBA) genes. Here, we have applied functional genomics to extend this work and find that the paradigm established for reference serovars also applies to clinical isolates - specifically, all ocular trachoma isolates tested have inactivating mutations in the synthase, whereas all genital isolates encode a functional enzyme. Moreover, functional enzyme activity was directly correlated to IFN-gamma resistance through an indole rescue mechanism. Hence, a strong selective pressure exists for genital strains to maintain a functional synthase capable of using indole for tryptophan biosynthesis. The fact that ocular serovars (serovar B) isolated from the genital tract were found to possess a functional synthase provided further persuasive evidence of this association. These results argue that there is an important host-parasite relationship between chlamydial genital strains and the human host that determines organotropism of infection and the pathophysiology of disease. We speculate that this relationship involves the production of indole by components of the vaginal microbial flora, allowing chlamydiae to escape IFN-gamma-mediated eradication and thus establish persistent infection.

The shikimate pathway and its branches in apicomplexan parasites

The shikimate pathway is essential for production of a plethora of aromatic compounds in plants, bacteria, and fungi. Seven enzymes of the shikimate pathway catalyze sequential conversion of erythrose 4-phosphate and phosphoenol pyruvate to chorismate. Chorismate is then used as a substrate for other pathways that culminate in production of folates, ubiquinone, napthoquinones, and the aromatic amino acids tryptophan, phenylalanine, and tyrosine. The shikimate pathway is absent from animals and present in the apicomplexan parasites Toxoplasma gondii, Plasmodium falciparum, and Cryptosporidium parvum. Inhibition of the pathway by glyphosate is effective in controlling growth of these parasites. These findings emphasize the potential benefits of developing additional effective inhibitors of the shikimate pathway. Such inhibitors may function as broad-spectrum antimicrobial agents that are effective against bacterial and fungal pathogens and apicomplexan parasites.

Indoleamine 2,3-dioxygenase expression in transplanted NOD Islets prolongs graft survival after adoptive transfer of diabetogenic splenocytes

Indoleamine 2,3-dioxygenase (IDO) catalyzes the breakdown of the amino acid tryptophan into kyneurenine. It has been shown that IDO production by placental trophoblasts prevents the attack of maternal T-cells activated in response to the paternal HLA alleles expressed by the tissues of the fetus. In this article, we show that adenoviral gene transfer of IDO to pancreatic islets can sufficiently deplete culture media of tryptophan and consequently inhibit the proliferation of T-cells in vitro. Experiments in vivo have also demonstrated that transplantation of IDO-expressing islets from prediabetic NOD mouse donors into NODscid recipient mice is associated with a prolongation in islet graft survival after adoptive transfer of NOD diabetogenic T-cells. This protection is attributed to the depletion of tryptophan at the transplantation site beneath the kidney capsule. These results suggest that local modulation of tryptophan catabolism may be a means of facilitating islet transplantation as a therapy for type 1 diabetes.

L-tryptophan-L-kynurenine pathway metabolism accelerated by Toxoplasma gondii infection is abolished in gamma interferon-gene-deficient mice: cross-regulation between inducible nitric oxide synthase and indoleamine-2,3-dioxygenase

L-Tryptophan degradation by indoleamine 2,3-dioxygenase (IDO) might have an important role in gamma interferon (IFN-gamma)-induced antimicrobial effects. In the present study, the effects of Toxoplasma gondii infection on IDO were investigated by using wild-type and IFN-gamma-gene-deficient (knockout) (IFN-gamma KO) mice. In wild-type C57BL/6J mice, enzyme activities and mRNA levels for IDO in both lungs and brain were markedly increased and lung L-tryptophan concentrations were dramatically decreased following T. gondii infection. In contrast, these metabolic changes did not occur in T. gondii-infected IFN-gamma KO mice or in uninfected IFN-gamma KO mice. The levels of inducible nitric oxide synthase (iNOS) induction in infected IFN-gamma KO mice were high in lungs and low in brain compared to those in infected wild-type mice. The extent of increased mRNA expression of T. gondii surface antigen gene 2 (SAG2) induced in lungs and brain by T. gondii infection was significantly enhanced in IFN-gamma KO mice compared to wild-type mice on day 7 postinfection. Treatment with N-nitro-L-arginine methyl ester, an iNOS inhibitor, increased the levels of SAG2 mRNA in brain but not in lungs and of plasma L-kynurenine after T. gondii infection. This in vivo study provides evidence that L-tryptophan depletion caused by T. gondii is directly mediated by IFN-gamma in the lungs, where iNOS is not induced by IFN-gamma. This study suggests that there is an antitoxoplasma mechanism of cross-regulation between iNOS and IDO and that the expression of the main antiparasite effector mechanisms for iNOS and/or IDO may vary among tissues.

Efficient asymmetric synthesis of biologically important tryptophan analogues via a palladium-mediated heteroannulation reaction

A novel and concise synthesis of optically active tryptophan derivatives was developed via a palladium-catalyzed heteroannulation reaction of substituted o-iodoanilines with an internal alkyne. The required internal alkyne 14a or 25 was prepared in greater than 96% de via alkylation of the Schöllkopf chiral auxiliary 19 employing diphenyl phosphate as the leaving group. The Schöllkopf chiral auxiliary was chosen here for the preparation of L-tryptophans would be available from D-valine while the D-isomers required for natural product total synthesis would originate from the inexpensive L-valine (300-g scale). Applications of the palladium-catalyzed heteroannulation reaction were extended to the first asymmetric synthesis of L-isotryptophan 38 and L-benz[f]tryptophan 39. More importantly, the optically pure 6-methoxy-D-tryptophan 62 was prepared by this protocol on a large scale (>300 g). This should permit entry into many ring-A oxygenated indole alkaloids when coupled with the asymmetric Pictet-Spengler reaction. In addition, an improved total synthesis of tryprostatin A (9a) was accomplished in 43% overall yield employing this palladium-mediated process.

Induction of tumor necrosis factor-alpha and inducible nitric oxide synthase fails to prevent toxoplasmic encephalitis in the absence of interferon-gamma in genetically resistant BALB/c mice

Following infection with Toxoplasma gondii, certain strains of mice, such as BALB/c, are genetically resistant to development of toxoplasmic encephalitis (TE) and establish a latent chronic infection as do humans. Thus, these animals appear to be a suitable model to analyze the mechanism of resistance to TE. Since the mechanism for their genetic resistance is unknown, we examined the role of interferon-gamma (IFN-gamma) tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) in the resistance using BALB/c-background IFN-gamma-deficient (IFN-gamma(-/-)) mice. IFN-gamma(-/-) and control mice were infected with the ME49 strain of T. gondii and treated with sulfadiazine to establish chronic infection. After discontinuing sulfadiazine, the IFN-gamma(-/-) mice all died, whereas the control mice all survived. Histological studies revealed remarkable inflammatory changes associated with large numbers of tachyzoites in brains of the IFN-gamma(-/-) mice but not in the control mice after discontinuation of sulfadiazine. Large amounts of mRNA for tachyzoite-specific SAG1 were detected in brains of only the IFN-gamma(-/-) mice. IFN-gamma mRNA was detected in brains of only the control mice, whereas mRNA for TNF-alpha and iNOS were detected in brains of both strains of mice. The amounts of the mRNA for TNF-alpha and iNOS did not differ between these mice. Treatment of IFN-gamma(-/-) mice with recombinant IFN-gamma prevented development of TE. These results demonstrate that IFN-gamma is crucial for genetic resistance of BALB/c mice against TE and that TNF-alpha and iNOS are insufficient to prevent TE in the absence of IFN-gamma.

Human placental indoleamine 2,3-dioxygenase: cellular localization and characterization of an enzyme preventing fetal rejection

In order to test the hypothesis (Munn, Zhou, Attwood, Bondarev, Conway, Marshall, Brown, Mellor, Science 281 (1998) 1191-1193) that localized placental tryptophan catabolism prevents immune rejection of the mammalian fetus, the cellular localization and characteristics of human placental indoleamine 2,3-dioxygenase (EC 1.13.11.42) were studied. The localization of indoleamine 2, 3-dioxygenase activity was determined quantitatively using cell fractionation by differential and discontinuous sucrose gradient centrifugation. Enzyme activity was looked for in isolated brush border microvillous plasma membranes of placental syncytiotrophoblast. We found that this membrane preparation (which showed a 32.4-fold purification from the starting homogenate with reference to the activity of a membrane marker enzyme, alkaline phosphatase (EC 3.1.3.1)) was strongly negatively enriched with indoleamine 2,3-dioxygenase (which showed a one twenty-fifth decrease in its specific activity). Placental indoleamine 2, 3-dioxygenase is thus not expressed in the maternal facing brush border membrane of syncytiotrophoblast. 1-Methyl-DL-tryptophan which was used by Munn et al. as a key experimental tool for inhibiting indoleamine 2,3-dioxygenase in the murine model showed a competitive inhibition of human placental indoleamine 2,3-dioxygenase with L-tryptophan. The hypothesis, based on experiments performed in mouse, may therefore be applicable to avoidance of immune rejection of the fetus in human pregnancy.

Safe haven: the cell biology of nonfusogenic pathogen vacuoles

Our understanding of both membrane traffic in mammalian cells and the cell biology of infection with intracellular pathogens has increased dramatically in recent years. In this review, we discuss the cell biology of the host-microbe interaction for four intracellular pathogens: Chlamydia spp., Legionella pneumophila, Mycobacterium spp., and the protozoan parasite Toxoplasma gondii. All of these organisms reside in vacuoles inside cells that have restricted fusion with host organelles of the endocytic cascade. Despite this restricted fusion, the vacuoles surrounding each pathogen display novel interactions with other host cell organelles. In addition to the effect of infection on host membrane traffic, we focus on these novel interactions and relate them where possible to nutrient acquisition by the intracellular organisms.

Mechanisms of interferon-induced inhibition of Toxoplasma gondii replication in human retinal pigment epithelial cells

Inflammation associated with retinochoroiditis is a major complication of ocular toxoplasmosis in infants and immunocompetent individuals. Moreover, Toxoplasma gondii-induced retinal disease causes serious complications in patients with AIDS and transplant patients. The retinal pigment epithelial (RPE) cell is an important regulatory cell within the retina and is one of the cells infected with T. gondii in in vivo. We have developed a human RPE (HRPE) cell in vitro model system to evaluate T. gondii replication and the regulation of this replication by cytokines. T. gondii replication was quantitated by counting the foci of infection (plaque formation) and the numbers of tachyzoites released into the supernatant fluids. Pretreatment of cultures with recombinant human tumor necrosis factor alpha, alpha interferon (IFN-alpha), IFN-beta, or IFN-gamma for 24 h prior to inoculation inhibited T. gondii replication in a dose-dependent manner. Of these cytokines, IFN-gamma was the most potent, and T. gondii replication was completely inhibited at a concentration of 100 U/ml. The anti-toxoplasmotic activity of IFN-gamma was significantly blocked by monoclonal antibody to IFN-gamma. Treatment of the cultures with IFN-gamma from day 1 or 2 postinoculation with T. gondii also offered protection against the parasite. The anti-toxoplasmotic activity of tumor necrosis factor alpha or IFN-alpha, -beta, or -gamma in these cultures was found to be independent of the nitric oxide (NO) pathway, since NO production was not found in HRPE cells treated with these cytokines. However, addition of tryptophan to IFN-gamma-treated cells significantly reversed the inhibitory effects of IFN-gamma, suggesting that IFN-gamma acts by depleting cellular tryptophan. This effect was further confirmed by reverse transcription-PCR and Northern (RNA) blot analysis, which indicated induction of indoleamine 2,3-dioxygenase (IDO), an enzyme that converts tryptophan to kynurenine. These results indicated that interferons inhibited T. gondii replication in HRPE by NO-independent but IDO-dependent mechanisms. This in vitro model of T. gondii replication in HRPE may be useful in evaluating the effects of cytokines and drugs on T. gondii replication within the retina.