Indoleamine 2,3-dioxygenase production by human dendritic cells results in the inhibition of T cell proliferation

Non-invasive monitoring of kidney allograft rejection through IDO metabolism evaluation

The immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO) is activated by interferon-gamma (IFN-gamma) and via tryptophan depletion, suppresses adaptive T cell-mediated immunity in inflammation, host immune defense, and maternal tolerance. Its role in solid organ transplantation is still unclear. Therefore, we investigated the usefulness of IDO-mediated tryptophan catabolism in the evaluation of kidney allograft rejection. Blood, urine, and tissue samples were collected from 34 renal transplant patients without rejection and from nine patients with biopsy-confirmed episodes of acute rejection (n=12). Concentrations of kynurenine and tryptophan in serum and urine were analyzed by high-pressure liquid chromatography. Kynurenine to tryptophan ratio (kyn/trp) was calculated to estimate IDO activity. Immunostaining for IDO was performed on renal biopsies. Neopterin was assessed using radioimmunoassay. Kyn/trp and neopterin were detectable at low levels in serum of healthy volunteers and were increased in non-rejecting allograft recipients. Serum levels of kyn/trp were higher in recipients with rejection compared to non-rejectors as early as by day 1 post-surgery. Rejection episodes occurring within 13+/-5.9 days after transplantation were accompanied by elevated kyn/trp in serum (114+/-44.5 micromol/mmol, P=0.001) and urine (126+/-65.9 micromol/mmol, P=0.02) compared to levels during stable graft function. Kyn/trp correlated significantly with neopterin suggesting an IFN-gamma-induced increase in IDO activity. Immunostaining showed upregulation of IDO in rejection biopsies, localized in tubular-epithelial cells. Non-rejected grafts displayed no IDO expression. Acute rejection is associated with simultaneously increased serum and urinary kyn/trp in patients after kidney transplantation. Thus, IDO activity might offer a novel non-invasive means of immunomonitoring of renal allografts.

Cinnabarinic acid generated from 3-hydroxyanthranilic acid strongly induces apoptosis in thymocytes through the generation of reactive oxygen species and the induction of caspase

3-Hydroxyanthranilic acid (3HAA) is one of the tryptophan metabolites along the kynurenine pathway and induces apoptosis in T cells. We investigated the mechanism of 3HAA-induced apoptosis in mouse thymocytes. The optimal concentration of 3HAA for apoptosis induction was 300-500 microM. The induction of apoptosis by a suboptimal concentration (100 microM) of 3HAA was enhanced by superoxide dismutase (SOD) as well as MnCl2 and further promoted in the presence of catalase. The 3HAA-mediated generation of intracellular reactive oxygen species (ROS) was enhanced by SOD or MnCl2 and inhibited by catalase. Corresponding to apoptosis induction, the generation of cinnabarinic acid (CA) through the oxidation of 3HAA was enhanced by SOD or MnCl2 in the presence of catalase. The synthesized CA possessed more than 10 times higher apoptosis-inducing activity than 3HAA. The intracellular ROS generation was induced by CA within 15 min and decreased to the control levels within 4 h, whereas the 3HAA-induced ROS generation increased gradually up to 4 h. Corresponding to ROS generation, the mitochondrial membrane potential was downregulated within 15 min and retained by the CA treatment. Apoptosis induction by 3HAA or CA was dependent on caspases, and caspase-3 was much more strongly activated by CA than 3HAA. In conclusion, the CA generated from 3HAA possesses a strong apoptosis-inducing activity in thymocytes through ROS generation, the loss of mitochondrial membrane potential, and caspase activation.

The tryptophan catabolite l-kynurenine inhibits the surface expression of NKp46- and NKG2D-activating receptors and regulates NK-cell function

Tryptophan (Trp) catabolism mediated by indoleamine 2,3-dioxygenase (IDO) plays a central role in the regulation of T-cell–mediated immune responses. In this study, we also demonstrate that natural killer (NK)–cell function can be influenced by IDO. Indeed, l-kynurenine, a Trp-derived catabolite resulting from IDO activity, was found to prevent the cytokine-mediated up-regulation of the expression and function of specific triggering receptors responsible for the induction of NK-cell–mediated killing. The effect of l-kynurenine appears to be restricted to NKp46 and NKG2D, while it does not affect other surface receptors such as NKp30 or CD16. As a consequence, l-kynurenine–treated NK cells display impaired ability to kill target cells recognized via NKp46 and NKG2D. Instead, they maintain the ability to kill targets, such as dendritic cells (DCs), that are mainly recognized via the NKp30 receptor. The effect of l-kynurenine, which is effective at both the transcriptional and the protein level, can be reverted, since NK cells were found to recover their functional competence after washing.

Improving antitumor immune responses by circumventing immunoregulatory cells and mechanisms

Although numerous immunotherapeutic strategies have been studied in patients with cancer, consistent induction of clinical responses remains a formidable challenge. Cancer vaccines are often successful at generating elevated numbers of tumor-specific T lymphocytes in peripheral blood, however, despite this, tumors usually continue to grow unabated. Recent evidence suggests that endogenous regulatory cells, known to play a major role in the induction of immune tolerance to self and prevention of autoimmunity, as well as suppressive myeloid cells invoked in the tumor-bearing state, may be largely responsible for preventing effective antitumor immune responses. This review will focus on the major regulatory cell subtypes, including CD4(+)CD25(+) T-regulatory cells, type 1 regulatory T cells, natural killer T cells, and immature myeloid cells. Studies in humans and in animal models have shown a role for all of these cells in tumor progression, although the mechanisms by which they act to suppress immunity remain largely undefined. Elucidation of the dominant molecular mechanisms mediating immune suppression in vivo will allow more precise targeting of the relevant regulatory cell populations, as well as the development of novel strategies and clinical reagents that will directly block molecules that induce the suppression of antitumor immunity.

Stromal-cell regulation of dendritic-cell differentiation and function

Dendritic cells (DCs) are the ubiquitous sentinels of the immune system, instructing and shaping the adaptive immune response. As such, DCs are often targeted directly by pathogens as a means of immune evasion. Although DCs in different anatomical locations originate from common bone-marrow-derived progenitors and, hence, share several characteristics, microenvironmental factors have an important influence on DC biology under both steady-state and inflammatory conditions. A growing body of literature suggests that these instructive processes are mediated by tissue stromal cells, empowering these cells with a decisive role in local immune regulation. Here, we review recent progress in this area, focussing on the role of stromal cells in supporting the generation of regulatory DCs, and propose that tissue stromal cells provide an alternate avenue whereby pathogens can influence DC function.

Immune privilege and HIV-1 persistence in the CNS

Human immunodeficiency virus-1 (HIV-1) neuroinvasion occurs early (during period of initial viremia), leading to infection of a limited amount of susceptible cells with low CD4 expression. Protective cellular and humoral immunity eliminate and suppress viral replication relatively quickly due to peripheral immune responses and the low level of initial central nervous system (CNS) infection. Upregulation of the brain protective mechanisms against lymphocyte entry and survival (related to immune privilege) helps reduce viral load in the brain. The local immune compartment dictates local viral evolution as well as selection of cytotoxic lymphocytes and immunoglobulin G specificity. Such status can be sustained until peripheral immune anti-viral responses fail. Activation of microglia and astrocytes, due to local or peripheral triggers, increases chemokine production, enhances traffic of infected cells into the CNS, upregulates viral replication in resident brain macrophages, and significantly augments the spread of viral species. The combination of these factors leads to the development of HIV-1 encephalitis-associated neurocognitive decline and patient death. Understanding the immune-privileged state created by virus, the brain microenvironment, and the ability to enhance anti-viral immunity offer new therapeutic strategies for treatment of HIV-1 CNS infection.

Inhibitors of human indoleamine 2,3-dioxygenase identified with a target-based screen in yeast

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan degradation enzyme that is emerging as an important drug target. IDO is expressed by many human tumors to help them escape immune detection, and it has been implicated in depression and in the formation of senile nuclear cataracts. There is a need for potent and selective IDO inhibitors for use in research and as lead compounds for drug development. We show that expression of human IDO in a Saccharomyces cerevisiae tryptophan auxotroph restricts yeast growth in the presence of low tryptophan concentrations and that inhibition of IDO activity can restore growth. We use this assay to screen for IDO inhibitors in collections of pure chemicals and crude natural extracts. We identify NSC 401366 (imidodicarbonimidic diamide, N-methyl-N'-9-phenanthrenyl-, monohydrochloride) as a potent nonindolic IDO inhibitor (Ki=1.5 +/- 0.2 microM) that is competitive with respect to tryptophan. We also use this assay to identify the active compound caulerpin from a crude algal extract. The yeast growth restoration assay is simple and inexpensive. It combines desirable attributes of cell- and target-based screens and is an attractive tool for chemical biology and drug screening.

1-Methyl-tryptophan can interfere with TLR signaling in dendritic cells independently of IDO activity

The compound 1-methyl-tryptophan (1-MT) is a competitive inhibitor of IDO that can break tolerance and induce fetus, graft, and tumor rejection. Because of its broad effect on immune-related mechanisms, the direct action of 1-MT on human monocyte-derived dendritic cells (DC) was analyzed. It is shown here that the effect of 1-MT on DC is dependent on the maturation pathway. Although 1-MT had no effect on DC stimulated by the TLR3 ligand poly(I:C), it strongly enhanced the Th1 profile of DC stimulated with TLR2/1 or TLR2/6 ligands. Drastic changes in the function of DC stimulated by the TLR4 ligand LPS were induced by 1-MT. These cells could still activate allogeneic and syngeneic T cells but stimulation yielded T cells secreting IL-5 and IL-13 rather than IFN-gamma. This action of 1-MT correlated with an increased phosphorylation of p38 and ERK MAPKs and sustained activation of the transcription factor c-Fos. Inhibiting p38 and ERK phosphorylation with synthetic inhibitors blocked the effect of 1-MT on LPS-stimulated DC. Thus, 1-MT can modulate DC function depending on the maturation signal and independently of its action on IDO. This is consistent with previous observations and will help further understanding the mechanisms of DC polarization.

Cyclooxygenase-2 inhibitor enhances the efficacy of a breast cancer vaccine: role of IDO

We report that administration of celecoxib, a specific cyclooxygenase-2 (COX-2) inhibitor, in combination with a dendritic cell-based cancer vaccine significantly augments vaccine efficacy in reducing primary tumor burden, preventing metastasis, and increasing survival. This combination treatment was tested in MMTV-PyV MT mice that develop spontaneous mammary gland tumors with metastasis to the lungs and bone marrow. Improved vaccine potency was associated with an increase in tumor-specific CTLs. Enhanced CTL activity was attributed to a significant decrease in levels of tumor-associated IDO, a negative regulator of T cell activity. We present data suggesting that inhibiting COX-2 activity in vivo regulates IDO expression within the tumor microenvironment; this is further corroborated in the MDA-MB-231 human breast cancer cell line. Thus, a novel mechanism of COX-2-induced immunosuppression via regulation of IDO has emerged that may have implications in designing future cancer vaccines.

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.

Integrated molecular signature of disease: analysis of influenza virus-infected macaques through functional genomics and proteomics

Recent outbreaks of avian influenza in humans have stressed the need for an improved nonhuman primate model of influenza pathogenesis. In order to further develop a macaque model, we expanded our previous in vivo genomics experiments with influenza virus-infected macaques by focusing on the innate immune response at day 2 postinoculation and on gene expression in affected lung tissue with viral genetic material present. Finally, we sought to identify signature genes for early infection in whole blood. For these purposes, we infected six pigtailed macaques (Macaca nemestrina) with reconstructed influenza A/Texas/36/91 virus and three control animals with a sham inoculate. We sacrificed one control and two experimental animals at days 2, 4, and 7 postinfection. Lung tissue was harvested for pathology, gene expression profiling, and proteomics. Blood was collected for genomics every other day from each animal until the experimental endpoint. Gross and microscopic pathology, immunohistochemistry, viral gene expression by arrays, and/or quantitative real-time reverse transcription-PCR confirmed successful yet mild infections in all experimental animals. Genomic experiments were performed using macaque-specific oligonucleotide arrays, and high-throughput proteomics revealed the host response to infection at the mRNA and protein levels. Our data showed dramatic differences in gene expression within regions in influenza virus-induced lesions based on the presence or absence of viral mRNA. We also identified genes tightly coregulated in peripheral white blood cells and in lung tissue at day 2 postinoculation. This latter finding opens the possibility of using gene expression arrays on whole blood to detect infection after exposure but prior to onset of symptoms or shedding.

CTLA-4 overexpression inhibits T cell responses through a CD28-B7-dependent mechanism

CTLA-4 has been shown to be an important negative regulator of T cell activation. To better understand its inhibitory action, we constructed CTLA-4 transgenic mice that display constitutive cell surface expression of CTLA-4 on CD4 and CD8 T cells. In both in vivo and in vitro T cell responses, CTLA-4 overexpression inhibits T cell activation. This inhibition is dependent on B7 and CD28, suggesting that overexpressed CTLA-4 inhibits responses by competing with CD28 for B7 binding or by interfering with CD28 signaling. In addition, expression of the transgene decreases the number of CD25+Foxp3+ T cells in these mice, but does not affect their suppressive ability. Our data confirm the activity of CTLA-4 as a negative regulator of T cell activation and that its action may be by multiple mechanisms.

Shaping immune responses through the activation of dendritic cells' P2 receptors

Dendritic cells (DCs) activate and shape the adaptive immune response by capturing antigens, migrating to peripheral lymphoid organs where naïve T cells reside, expressing high levels of MHC and costimulatory molecules and secreting cytokines and chemokines. DCs are endowed with a high degree of functional plasticity and their functions are tightly regulated. Besides initiating adaptive immune responses, DCs play a key role in maintaining peripheral tolerance toward self-antigens. On the basis of the information gathered from the tissue where they reside, DCs adjust their functional activity to ensure that protective immunity is favoured while unwanted or exaggerated immune responses are prevented. A wide variety of signals from neighbouring cells affecting DC functional activity have been described. Here we will discuss the complex role of extracellular nucleotides in the regulation of DC function and the role of P2 receptors as possible tools to manipulate immune responses.

Inhibition of decay-accelerating factor (CD55) attenuates prostate cancer growth and survival in vivo

Decay-accelerating factor (CD55) is a member of membrane-bound complement-regulatory proteins. CD55 expression correlates with poor survival in patients with colorectal cancer and has been implicated in the survival and tumorigenesis of blood-borne malignancies. Histologic analysis of clinical specimens from patients with advanced prostate cancer revealed an increase in CD55 expression in prostate tumor epithelial cells. CD55 was shown to be functionally active and to inhibit complement-mediated lysis in PC-3 and DU145 cells. The percentage of lysis was correlative with the CD55 expression profile observed in these prostate cancer cell lines. These data suggest that CD55 is an important regulator of prostate cancer cell survival. As a result, we have hypothesized that CD55 expression on prostate cancer cells promotes cell survival and contributes to the metastatic potential of prostate cancer cells. To determine the role of CD55 in prostate cancer tumorigenesis and metastasis, we generated PC-3(Luc) prostate cancer cells with CD55 siRNA-targeted disruption. We found that PC-3(Luc)/CD55 siRNA constructs in SCID mice resulted in a significant attenuation of overall tumor burden. Further investigation into the mechanisms of CD55-mediated tumor cell/microenvironment interaction is necessary to understand the role of CD55 in tumor cell survival and metastatic lesion formation.

Interleukin-10-secreting type 1 regulatory T cells in rodents and humans

Interleukin-10 (IL-10)-secreting T regulatory type 1 (Tr1) cells are defined by their specific cytokine production profile, which includes the secretion of high levels of IL-10 and transforming growth factor-beta(TGF-beta), and by their ability to suppress antigen-specific effector T-cell responses via a cytokine-dependent mechanism. In contrast to the naturally occurring CD4+ CD25+ T regulatory cells (Tregs) that emerge directly from the thymus, Tr1 cells are induced by antigen stimulation via an IL-10-dependent process in vitro and in vivo. Specialized IL-10-producing dendritic cells, such as those in an immature state or those modulated by tolerogenic stimuli, play a key role in this process. We propose to use the term Tr1 cells for all IL-10-producing T-cell populations that are induced by IL-10 and have regulatory activity. The full biological characterization of Tr1 cells has been hampered by the difficulty in generating these cells in vitro and by the lack of specific marker molecules. However, it is clear that Tr1 cells play a key role in regulating adaptive immune responses both in mice and in humans. Further work to delineate the specific molecular signature of Tr1 cells, to determine their relationship with CD4+ CD25+ Tregs, and to elucidate their respective role in maintaining peripheral tolerance is crucial to advance our knowledge on this Treg subset. Furthermore, results from clinical protocols using Tr1 cells to modulate immune responses in vivo in autoimmunity, transplantation, and chronic inflammatory diseases will undoubtedly prove the biological relevance of these cells in immunotolerance.

BeWo Trophoblasts are Unable to Control Replication of Toxoplasma gondii, Even in the Presence of Exogenous IFN-γ

The ability of RH strain of Toxoplasma gondii to invade and grow into BeWo cells was investigated in the present study using IFN-gamma, l-tryptophan, or alpha-methyl-tryptophan treatments. HeLa cells were used in the same conditions for comparison purposes. It was demonstrated that BeWo cells are more permissive to T. gondii infection, making them more susceptible to this pathogen when compared to HeLa cells. Infection rates of BeWo cells do not show any significant alteration in different protocols using IFN-gamma. In addition, BeWo treated with l-tryptophan was unable to significantly increase parasite growth. In contrast, HeLa cells treated with IFN-gamma or IFN-gamma plus l-tryptophan are able to impair or increase, respectively, parasite replication, providing evidence that this indoleamine-2,3-dioxygenase-dependent phenomenon is operant in these cells, whereas it is inactive in BeWo. Therefore, our data support the hypothesis that the immunological mechanisms controlling infection at the maternal-fetal interface are different from those occurring in the periphery. At the same time that operating regulatory mechanisms work inside and outside the cells located at that microenvironment to prevent maternal rejection of the concept, these events might facilitate the progression of infection caused by intracellular pathogens, as T. gondii.

Immune modulation by mesenchymal stem cells

Mesenchymal stem cells (MSCs) have been shown to suppress activation of T cells both in vivo and in vitro. In vivo, this may be a way for the body to maintain homeostasis and inhibit immune activation in distinct compartments, such as the bone marrow and the interface between mother and fetus. MSCs modulate the immune function of the major cell populations involved in alloantigen recognition and elimination, including antigen presenting cells, T cells, and natural killer cells. The molecular mechanism that mediates the immunosuppressive effect of MSCs is not completely understood.

Function of indoleamine 2,3-dioxygenase in corneal allograft rejection and prolongation of allograft survival by over-expression

Indoleamine 2,3-dioxygenase (IDO) suppresses T cell responses by its action in catabolising tryptophan. It is important in maintenance of immune privilege in the placenta. We investigated the activity of IDO in the cornea, following corneal transplantation and the effect of IDO over-expression in donor corneal endothelium on the survival of corneal allografts. IDO expression was analysed and functional activity was quantified in normal murine cornea and in corneas following transplantation as allografts. Low levels of IDO, at both mRNA and protein levels, was detected in the normal cornea, up-regulated by IFN-gamma and TNF. Expression of IDO in cornea was significantly increased following corneal transplantation. However, inhibition of IDO activity in vivo had no effect on graft survival. Following IDO cDNA transfer, murine corneal endothelial cells expressed functional IDO, which was effective at inhibiting allogeneic T cell proliferation. Over-expression of IDO in donor corneal allografts resulted in prolonged graft survival. While, on one hand, our data indicate that IDO may augment corneal immune privilege, up-regulated IDO activity following cytokine stimulation may serve to inhibit inflammatory cellular responses. While increasing IDO mRNA expression was found in allogeneic corneas at rejection, over-expression in donor cornea was found to significantly extend survival of allografts.

Therapeutic vaccines for melanoma: current status

Considerable clinical research is focused on improving systemic treatments for melanoma. Unfortunately, the disease is generally resistant to standard chemotherapy, and surgical excision remains the best treatment option whenever possible. However, complete spontaneous regression of melanoma has been observed in some patients, a phenomenon thought to be mediated by the immune system. This has stimulated attempts to manipulate the immune system for therapeutic purposes. Vaccination is a form of active specific immunotherapy, such that the response against the tumor is actively generated by the patient's immune system, and is directed against a particular cellular target or specific membrane antigen. Numerous approaches to vaccination for melanoma have been investigated, and have become more complex as our understanding of anti-tumor immunity has increased. Vaccines have been shown to induce measurable immunologic responses that may be correlated with improved clinical outcomes in patients with melanoma. Large phase III clinical trials using peptide, ganglioside, and whole-cell tumor antigens are ongoing. Although anti-tumor vaccination has shown promising results in patients with melanoma, to date no vaccine has been approved for routine therapy of melanoma. Recently, a phase III trial evaluating the Canvaxin whole-cell vaccine in stage IV melanoma was halted because of a low likelihood of significant benefit. However, a larger phase III trial for patients with stage III disease was continued and results are awaited with interest.

Differential effects of the tryptophan metabolite 3-hydroxyanthranilic acid on the proliferation of human CD8+ T cells induced by TCR triggering or homeostatic cytokines

Production of indoleamine 2,3-dioxygenase (IDO) by tumor cells, leading to tryptophan depletion and production of immunosuppressive metabolites, may facilitate immune tolerance of cancer. IDO gene is also expressed in dendritic cells (DC) upon maturation induced by lipopolysaccarides or IFN. We investigated IDO gene expression in melanoma cell lines and clinical specimens as compared to mature DC (mDC). Furthermore, we explored effects of L-kynurenine (L-kyn) and 3-hydroxyanthranilic acid (3-HAA) on survival and antigen-dependent and independent proliferation of CD8(+) cells. We observed that IDO gene expression in cultured tumor cells and freshly excised samples is orders of magnitude lower than in mDC, providing highly efficient antigen presentation to CD8(+) T cells. Non toxic concentrations of L-kyn or 3-HAA did not significantly inhibit antigen-specific CTL responses. However, 3-HAA, but not L-kyn markedly inhibited antigen-independent proliferation of CD8(+) T cells induced by common receptor gamma-chain cytokines IL-2, -7 and -15. Our data suggest that CD8(+) T cell activation induced by antigenic stimulation, a function exquisitely fulfilled by mDC, is unaffected by tryptophan metabolites. Instead, in the absence of effective T cell receptor triggering, 3-HAA profoundly affects homeostatic proliferation of CD8(+) T cells.

CD25 and indoleamine 2,3-dioxygenase are up-regulated by prostaglandin E2 and expressed by tumor-associated dendritic cells in vivo: additional mechanisms of T-cell inhibition

Immune tolerance is a central mechanism counteracting tumor-specific immunity and preventing effective anticancer immunotherapy. Induction of tolerance requires a specific environment in which tolerogenic dendritic cells (DCs) play an essential role deviating the immune response away from effective immunity. It was recently shown that maturation of DCs in the presence of PGE2 results in upregulation of indoleamine 2,3-dioxygenase (IDO) providing a potential mechanism for the development of DC-mediated Tcell tolerance. Here, we extend these findings, demonstrating a concomitant induction of IDO and secretion of soluble CD25 after DC maturation in the presence of PGE2. While maturation of DCs induced IDO expression on transcriptional level, only integration of PGE2 signaling led to up-regulation of functional IDO protein as well as significant expression of cell-surface and soluble CD25 protein. As a consequence, T-cell proliferation and cytokine production were significantly inhibited, which was mediated mainly by IDO-induced tryptophan depletion. Of importance, we demonstrate that different carcinoma entities associated with elevated levels of PGE2 coexpress CD25 and IDO in peritumoral dendritic cells, suggesting that PGE2 might influence IDO expression in human DCs in the tumor environment. We therefore suggest PGE2 to be a mediator of early events during induction of immune tolerance in cancer.

Indoleamine 2,3-Dioxygenase Participates in the Interferon-gamma-Induced Cell Death Process in Cultured Bovine Luteal Cells1

Interferon-gamma (IFNG) induces apoptotic cell death in bovine luteal cells, but the pathway(s) involved in this process are not well defined. Evidence supporting the involvement of an IFNG-inducible enzymatic pathway that degrades tryptophan in IFNG-induced death of bovine luteal cells is presented in this study. The IFNG-inducible enzyme indoleamine 2,3-dioxygenase (INDO) catalyzes the first step in a metabolic pathway that degrades tryptophan. In the first experiment, RT-PCR revealed the presence of INDO mRNA in luteal cells treated with IFNG, but not in untreated cells. To determine whether INDO participates in IFNG-induced death of bovine luteal cells, an experiment was performed to test the effect of 1-methyl-D-tryptophan (1-MT), an inhibitor of INDO, on IFNG-induced DNA fragmentation in luteal cells. Single-cell gel electrophoresis and microscopic image analysis revealed that 1-MT inhibited DNA fragmentation induced by IFNG. To determine whether supplementation of cell cultures with additional tryptophan could also protect luteal cells from IFNG-induced DNA fragmentation, luteal cells were cultured in the presence of IFNG, and L-tryptophan was added to cultures to achieve final concentrations that were 5-, 10-, or 25-fold higher than the concentration of L-tryptophan found in nonsupplemented culture medium. Supplementation of IFNG-treated luteal cell cultures with elevated concentrations of tryptophan also prevented IFNG-induced DNA fragmentation. We conclude that INDO participates in IFNG-induced death of bovine luteal cells, through a mechanism that involves degradation of tryptophan, thereby reducing tryptophan concentrations to a point insufficient to meet luteal cells needs.

Heme oxygenase-1 inhibits rat and human breast cancer cell proliferation: mutual cross inhibition with indoleamine 2,3-dioxygenase

Heme oxygenase-1 (HO-1) is the rate limiting enzyme of heme catabolism whereas indoleamine 2,3 dioxygenase (IDO) catabolizes tryptophan through the kynurenine pathway. We analyzed the expression and biological effects of these enzymes in rat and human breast cancer cell lines. We show that rat (NMU and 13762) but not human cells (MCF-7 and T47D) express HO-1. When overexpressed, we found this enzyme to have anti-proliferative and proapoptotic effects by antioxidant mechanisms in these four cell lines. We show that IDO is expressed by rat and human breast cancer cells. IDO inhibition with 1-MT and siRNA leads to diminished proliferation in rat cells. In contrast, HO-1 negative human cell lines increase proliferation upon IDO inhibition. Since we also demonstrate that IDO inhibits the anti-proliferative HO-1, we propose that IDO has opposite effects on proliferation depending on the coexpression or not of HO-1. We also describe that HO-1 inhibits IDO at the post-translational level through heme starvation. In vivo, we show that rat normal breast expresses HO-1 and IDO. In contrast, N-nitrosomethylurea-induced breast adenocarcinomas only express IDO. In conclusion, we show that HO-1/IDO cross-regulation modulates apoptosis and proliferation in rat and human breast cancer cells.

Indoleamine 2,3-dioxygenase, tumor-induced tolerance and counter-regulation

Tumors create an abnormal state of tolerance toward themselves and their antigens. One mechanism that might contribute to this tolerance is the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO). IDO-expressing antigen-presenting cells are found in tumor-draining lymph nodes, where they can create a tolerogenic microenvironment. IDO can also be expressed within the tumor itself, by tumor cells or host stromal cells, where it can inhibit the effector phase the immune response. Finally, emerging evidence suggests that IDO might also constitute a significant counter-regulatory mechanism, induced by clinically relevant pro-inflammatory signals, such as IFN-gamma, IFN-alpha, CpG oligodeoxynucleotides, and 4-1BB ligation. Strategies to inhibit the IDO pathway may thus assist in breaking tolerance to tumors, and might enhance the efficacy of other immunotherapy strategies by removing unwanted counter-regulation.

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.

IDO expression on decidual and peripheral blood dendritic cells and monocytes/macrophages after treatment with CTLA-4 or interferon-gamma increase in normal pregnancy but decrease in spontaneous abortion

Recent data demonstrated that CD4+CD25+ regulatory T (Treg) cells and an enzyme called indoleamine 2,3-dioxygenase (IDO) mediate maternal tolerance to the fetus. Interestingly, Treg cells express the CTLA-4 molecule on their surface, and B7 (CD80/86) ligation by CTLA-4 enhanced IDO activity of dendritic cells (DCs) and monocytes by the induction of interferon gamma (IFN-gamma) production. In this study, we studied the IDO expression on peripheral blood monocytes and decidual monocytes or DCs after treatment with CTLA-4/Fc fusion protein or IFN-gamma using flow cytometry. IDO expressions on both peripheral blood DC and decidual DC and monocytes were up-regulated during normal pregnancy. On the other hand, both IDO expression on DC and monocytes after IFN-gamma treatment or CTLA-4 treatment were decreased in spontaneous abortion cases. The expression of CD86 on peripheral blood and decidual monocytes and DC in spontaneous abortion cases was lower compared with those in normal pregnancy subjects. Also, IFN-gamma production by decidual and peripheral blood mononuclear cells after CTLA-4/Fc treatment in spontaneous abortion cases was significantly lower than those in normal pregnancy subjects. These data suggest that CTLA-4 on Treg cells up-regulates IDO expression on decidual and peripheral blood DC and monocytes by the induction of IFN-gamma production.

Indoleamine 2,3 dioxygenase and human leucocyte antigen-G inhibit the T-cell alloproliferative response through two independent pathways

Both human leucocyte antigen (HLA)-G and indoleamine 2,3 dioxygenase (IDO) are key molecules involved in immune tolerance. HLA-G is a non-classical HLA class I molecule that can be expressed in both membrane-bound (HLA-G1) and soluble (HLA-G5) forms, both of which exhibit tolerogenic properties via interaction with inhibitory receptors present on natural killer (NK) cells, T cells and antigen-presenting cells (APC). IDO is an enzyme that acts by depleting the surrounding microenvironment of the essential amino acid, tryptophan, thereby inhibiting T-cell proliferation. Our present study was aimed at analysing the potential link that may exist between IDO and HLA-G. Our results showed that during allogeneic reactions, soluble HLA-G expression was not regulated by the addition of IDO substrate (i.e. tryptophan), metabolite (i.e. kynurenine) or inhibitor (i.e. 1-methyl-tryptophan), that IDO activity was not altered by HLA-G5 treatment, and that HLA-G5-mediated inhibition of the T-cell alloproliferative response was neither affected by the presence of tryptophan and kynurenine nor reversed after IDO activity blockage, demonstrating that HLA-G5 can exert its function in the absence of functional IDO. Similarly, inhibition of the T-cell alloresponse, induced by HLA-G1-expressing antigen-presenting cells, was not altered by IDO metabolites or inhibitor. Taken together, these findings show that the function and expression of IDO and HLA-G5 are not mutually influenced, but rather inhibit the T-cell alloproliferative response through two independent pathways. IDO and HLA-G are thus complementary for inducing and maintaining immune tolerance in physiological (pregnancy) and pathological (tumour and allograft) situations.

A novel form of immune signaling revealed by transmission of the inflammatory mediator serotonin between dendritic cells and T cells

Adaptive immunity is triggered at the immune synapse, where peptide-major histocompatibility complexes and costimulatory molecules expressed by dendritic cells (DCs) are physically presented to T cells. Here we describe transmission of the inflammatory monoamine serotonin (5-hydroxytryptamine [5-HT]) between these cells. DCs take up 5-HT from the microenvironment and from activated T cells (that synthesize 5-HT) and this uptake is inhibited by the antidepressant, fluoxetine. Expression of 5-HT transporters (SERTs) is regulated by DC maturation, exposure to microbial stimuli, and physical interactions with T cells. Significantly, 5-HT sequestered by DCs is stored within LAMP-1+ vesicles and subsequently released via Ca2+-dependent exocytosis, which was confirmed by amperometric recordings. In turn, extracellular 5-HT can reduce T-cell levels of cAMP, a modulator of T-cell activation. Thus, through the uptake of 5-HT at sites of inflammation, and from activated T cells, DCs may shuttle 5-HT to naive T cells and thereby modulate T-cell proliferation and differentiation. These data constitute the first direct measurement of triggered exocytosis by DCs and reveal a new and rapid type of signaling that may be optimized by the intimate synaptic environment between DCs and T cells. Moreover, these results highlight an important role for 5-HT signaling in immune function and the potential consequences of commonly used drugs that target 5-HT uptake and release.

Production and purification of human indoleamine 2,3‐dioxygenase (HuIDO) protein in a baculovirus expression system and production and characterization of egg yolk antibody against the purified HuIDO

The human indoleamine 2,3-dioxygenase (HuIDO) baculoviral construct, for expression of HuIDO protein with a hexa-histidine and FLAG (DYKDDDDK) tag, was produced using the BacPAK Baculovirus Expression System. HuIDO baculovirus was used to infect Sf21 insect cells to produce functionally active protein in large amounts. Conditions for protein purification by metal affinity chromatography were determined and optimized. Addition of haemin ensured optimal activity of the purified heme-containing oxygenase. The soluble purified protein was used to immunize a chicken to produce large quantities of polyclonal IgY against HuIDO. The anti-HuIDO IgY antibody specifically detected HuIDO produced by a range of cell types including transfectants and native HuIDO expression induced in IFN-gamma-stimulated cells. The antibody detected HuIDO in cell lysates by western blotting and in the cytoplasm of cells by microscopy. The antibody was unable to block the function of the enzyme, indicating that this antibody binds outside the active site of HuIDO.

CD40 ligation prevents onset of tolerogenic properties in human dendritic cells treated with CTLA-4-Ig

The synthetic immunomodulator cytotoxic T lymphocyte antigen 4-Ig (CTLA-4-Ig) initiates effects in human monocyte-derived dendritic cells (DC) that rely on immunosuppressive tryptophan catabolism. However, it is unable to induce suppressive properties in DC matured by CD40 engagement. Thus, CD40-driven events may physiologically set human DC free from restraint by regulatory cells expressing surface CTLA-4.

Monitoring tryptophan metabolism in chronic immune activation

The essential amino acid tryptophan is a constituent of proteins and is also a substrate for two important biosynthetic pathways: the generation of neurotransmitter 5-hydroxytryptamine (serotonin) by tryptophan 5-hydroxylase, and the formation of kynurenine derivatives and nicotinamide adenine dinucleotides. The latter pathway is initiated by the enzymes tryptophan pyrrolase (tryptophan 2,3-dioxygenase, TDO) and indoleamine 2,3-dioxygenase (IDO). TDO is located in liver cells, whereas IDO is expressed in a variety of cells including monocyte-derived macrophages and dendritic cells and is preferentially induced by Th1-type cytokine interferon-gamma. Tryptophan depletion via IDO is part of the cytostatic and antiproliferative activity mediated by interferon-gamma in cells. In vivo tryptophan concentration can be measured by HPLC by monitoring its natural fluorescence (285 nm excitation and 365 nm emission wavelength). IDO activity is characterized best by the kynurenine to tryptophan ratio which correlates with concentrations of immune activation markers such as neopterin. Low serum/plasma tryptophan concentration is observed in infectious, autoimmune, and malignant diseases and disorders that involve cellular (Th1-type) immune activation as well as during pregnancy due to accelerated tryptophan conversion. Thus, in states of persistent immune activation, low tryptophan concentration may contribute to immunodeficiency. Decreased serum tryptophan can also effect serotonin biosynthesis and thus contribute to impaired quality of life and depressive mood. As such, monitoring tryptophan metabolism in chronic immunopathology provides a better understanding of the association between immune activation and IDO and its role in the development of immunodeficiency, anemia and mood disorders.

Studying the immunosuppressive role of indoleamine 2,3-dioxygenase: tryptophan metabolites suppress rat allogeneic T-cell responses in vitro and in vivo

Pregnancy is a natural model of successful tolerance induction against allogeneic tissues. Recent studies pointed to a role of indoleamine 2,3-dioxygenase (IDO), a tryptophan-degrading enzyme expressed in the placenta, in mediation of T-cell suppression. We want to apply to organ transplantation what nature has developed for suppression of fetal rejection during pregnancy. Here we analyze whether IDO-induced tryptophan metabolites are able to suppress the allogeneic T-cell response and allograft rejection in rats. Rat lymphocytes were stimulated with allogeneic dendritic cells in vitro in the presence of increasing amounts of tryptophan metabolites (kynurenine, 3-hydroxykynurenine, anthranilic acid, 3-hydroxyanthranilic acid and quinolinic acid) and T-cell proliferation was determined. The findings showed that kynurenine, 3-hydroxykynurenine and 3-hydroxyanthranilic acid strongly suppress the T-cell response, whereas anthranilic and quinolinic acid are non-effective. Vital staining of cells with subsequent fluorescence-activated cell sorter analyses demonstrated that suppression is mediated by T-cell death. Thereafter, the action of metabolites was analyzed in a skin allograft model (BN-->LEW). Lewis recipients received daily s.c. injections of tryptophan metabolite mixture (kynurenine + 3-hydroxyanthranilic acid), cyclosporin A (positive control), or no treatment (negative control). The metabolites induced a significant prolongation (P = 0.0018) of graft survival. We conclude that IDO-induced tryptophan metabolites suppress the T-cell response and prolong allograft survival in rats.

Inhibition of indoleamine 2,3-dioxygenase (IDO) enhances elimination of virus-infected macrophages in an animal model of HIV-1 encephalitis

Indoleamine 2,3-dioxygenase (IDO) is the rate-limiting enzyme in the kynurenine pathway of tryptophan metabolism. IDO activity is linked with immunosuppression by its ability to inhibit lymphocyte proliferation, and with neurotoxicity through the generation of quinolinic acid and other toxins. IDO is induced in macrophages by HIV-1 infection, and it is up regulated in macrophages in human brain tissue with HIV-1 encephalitis (HIVE). Using a model of HIVE, we investigated whether IDO inhibitor 1-methyl-d-tryptophan (1-MT) could affect the generation of cytotoxic T lymphocytes (CTLs) and clearance of virus-infected macrophages from the brain. Severe combined immunodeficient mice were reconstituted with human peripheral blood lymphocytes, and encephalitis was induced by intracranial injection of autologous HIV-1-infected monocyte-derived macrophages (MDMs). Animals treated with 1-MT demonstrated increased numbers of human CD3+, CD8+, CD8+/interferon-gamma+ T cells, and HIV-1(gag/pol)-specific CTLs in peripheral blood compared with controls. At week 2 after MDM injection in the basal ganglia, mice treated with 1-MT showed a 2-fold increase in CD8+ T lymphocytes in the areas of the brain containing HIV-1-infected MDMs compared with untreated controls. By week 3, 1-MT-treated mice showed 89% reduction in HIV-infected MDMs in brain as compared with controls. Thus, manipulation of immunosuppressive IDO activity in HIVE may enhance the generation of HIV-1-specific CTLs, leading to elimination of HIV-1-infected macrophages in brain.

A two-step induction of indoleamine 2,3 dioxygenase (IDO) activity during dendritic-cell maturation

Prostaglandins, a family of lipidic molecules released during inflammation, display immunomodulatory properties in several models. One use includes exposure of monocyte-derived dendritic cells (DCs) to a cocktail of cytokines that contains prostaglandin E2 (PGE2) for purposes of maturation; such cells are currently being used for cancer immunotherapy trials. Our analysis of the transcription profile of DCs matured in the presence of tumor necrosis factor alpha (TNFalpha) and PGE2 revealed a strong up-regulation of indoleamine 2-3 dioxygenase (IDO), an enzyme involved in tryptophan catabolism and implicated in both maternal and T-cell tolerance. Using quantitative assays to monitor levels of IDO mRNA, protein expression, and enzyme activity, we report that PGE2 induces mRNA expression of IDO; however, a second signal through TNF receptor (TNF-R) or a Toll-like receptor (TLR) is necessary to activate the enzyme. Interestingly, use of TNFalpha, lipopolysaccharide, or Staphylococcus aureus Cowan I strain (SAC) alone does not induce IDO. The effect of PGE2 is mediated by activation of adenylate cyclase via the Gs-protein-coupled receptor E prostanoid-2 (EP2). A better understanding of these regulatory mechanisms and the crosstalk between TNF-R/TLR and EP2 signaling pathways will provide insight into the regulation of T-cell activation by DCs and may help to improve existing immunotherapy protocols.

Dendritic cells in melanoma immunotherapy

No consensus exists regarding optimal systemic treatment for melanoma in the adjuvant or metastatic disease setting. Dendritic cell vaccine therapy, though investigational at present, offers very promising preliminary data that warrant exploration, and patients with existing disease and those seeking adjuvant treatment should evaluate open protocols using this strategy.

Eosinophil granulocytes account for indoleamine 2,3-dioxygenase-mediated immune escape in human non-small cell lung cancer

Indoleamine 2,3-dioxygenase (IDO), a catabolizing enzyme of tryptophan, is supposed to play a role in tumor immune escape. Its expression in solid tumors has not yet been well elucidated: IDO can be expressed by the tumor cells themselves, or by ill-defined infiltrating cells, possibly depending on tumor type. We have investigated IDO expression in 25 cases of non-small cell lung cancer (NSCLC). Using histochemistry and immunohistochemistry, we found that IDO was expressed not by tumor cells, but by normal cells infiltrating the peritumoral stroma. These cells were neither macrophages nor dendritic cells, and were identified as eosinophil granulocytes. The amount of IDO-positive eosinophils varied in different cases, ranging from a few cells to more than 50 per field at x200 magnification. IDO protein in NSCLC was enzymatically active. Therefore, at least in NSCLC cases displaying a large amount of these cells in the inflammatory infiltrate, IDO-positive eosinophils could exert an effective immunosuppressive action. On analyzing the 17 patients with adequate follow-up, a significant relationship was found between the amount of IDO-positive infiltrate and overall survival. This finding suggests that the degree of IDO-positive infiltrate could be a prognostic marker in NSCLC.

Regulation of human auto- and alloreactive T cells by indoleamine 2,3-dioxygenase (IDO)–producing dendritic cells: too much ado about IDO?

Although dendritic cells (DCs) strongly stimulate the immune response, they can also induce unresponsiveness. Recently, a human monocyte-derived DC subpopulation was described that constitutively expresses indoleamine 2,3-dioxygenase (IDO). These DCs were defined as nonadherent CD123+/CC chemokine receptor 6+ (CCR6+) cells that suppress the allogeneic T-cell response. In the present study, we generated nonadherent, mature DCs from human blood monocytes. As expected, in addition to the classic markers, these cells expressed CD123 and CCR6. Reverse transcription–polymerase chain reaction (RT-PCR), however, did not show IDO gene transcription, nor did we detect enzymatic IDO activity. Treating the cells with interferon-γ (IFN-γ) resulted in significant IDO production. Subsequently, we studied the regulatory properties of IDO-producing DCs on autologous and allogeneic T-cell responses. Neither OKT3-stimulated T cells of healthy donors nor myelin basic protein (MBP)–specific T cells of patients with multiple sclerosis (MS) were suppressed by autologous IDO DCs. However, whereas IDOneg DCs supported further stimulation of preactivated MBP-specific T cells of an MS patient, IDOpos DCs had lost this capacity. The allogeneic T-cell response was only marginally suppressed by IDO DCs. Our findings show that nonadherent CD123+/CCR6+ human DCs do not constitutively express IDO, and, even if they express the enzyme after IFN-γ treatment, they possess only limited T-cell regulatory function.

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.

Neopterin in HIV-1 infection

Neopterin is well established as a reliable marker in HIV-1 infection. Neopterin concentrations measured in urine or serum indicate sensitively the course and progression of the disease as well as efficacy of anti-retroviral therapy. The main trigger for neopterin production is Th1-type cytokine interferon-gamma. During acute HIV-1 infection, enhanced formation of neopterin occurs already at a very early time point, before antibody seroconversion takes place. After this stage, neopterin concentrations in serum and urine closely correlate with virus load in the circulation of HIV-1-infected patients. Data provide evidence for an important role of immune activation and Th1-type cytokine interferon-gamma in the pathogenesis of HIV-1 infection. This review subsumes the importance of neopterin as a marker in HIV-1 infection. Further evidence is increasing, that neopterin derivatives might modulate immune response by interfering with the cellular redox balance, activating redox-sensitive transcription factors, or inducing apoptosis in specific cell types. The possible impact of neopterin derivatives and of other biochemical pathways induced by interferon-gamma such as indoleamine 2,3-dioxygenase in chronic diseases like HIV-1 infection is discussed.

Monocyte-mediated T-cell suppression and augmented monocyte tryptophan catabolism after human hematopoietic stem-cell transplantation

T-cell dysfunction after human hematopoietic stem-cell transplantation (HSCT) is generally attributed to intrinsic T-cell defects. Here we show that the characteristic impaired proliferative responses to polyclonal stimulation of post-HSCT peripheral blood mononuclear cells (PB-MCs) were markedly (4-fold) improved by T-cell enrichment. Conversely, addback of post-HSCT monocytes to these enriched T cells dampened their proliferative responses, suggesting that post-HSCT monocytes effectively mediate T-cell suppression. As a mechanism possibly contributing to monocyte-mediated T-cell suppression, we investigated monocyte tryptophan catabolism by indoleamine 2,3-dioxygenase into kynurenine, which has been implicated in regulating T-cell responses. Compared with controls, all post-HSCT monocyte-containing cell cultures (total PBMCs, monocytes, and monocyte/T-cell cocultures), but not monocyte-depleted populations, secreted elevated amounts of kynurenine. Blockade of tryptophan catabolism improved the proliferative responses. The slightly increased kynurenine release and substantial release of neopterin by unstimulated post-HSCT monocytes suggests that they were in a state of continuous activation. Superimposed on this state, stimulation of these cells caused a striking, additional increase (10-fold) in kynurenine release, and they triggered marked apoptosis of autologous post-HSCT T cells. We conclude that the amplified kynurenine release by post-HSCT monocytes, particularly induced upon stimulation, may underlie their suppressor activity, which in turn may contribute to the depressed T-cell immune responses after HSCT.

Inhibition of allogeneic T-cell responses by dendritic cells expressing transduced indoleamine 2,3-dioxygenase

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO) is an enzyme involved in the catabolism of tryptophan and has been shown to prevent rejection of the fetus during pregnancy by inhibiting alloreactive T cells.

METHODS

In this study we investigated dendritic cells (DCs) that are transfected with IDO cDNA in the inhibition of T-cell proliferation after antigen-specific interaction. XS106 DCs, derived from A/J mice (H-2k), were transduced with IDO with a gene-delivery system using a recombinant adenoviral vector.

RESULTS

Western blotting and immune staining revealed IDO expression in XS106 DCs transduced with IDO (XS106-IDO DCs), and its catabolic effect was confirmed by an increase in kynurenine concentration. Fluorescence-activated cell sorting revealed that XS106-IDO DCs were not changeable for Ia, CD80, and CD86 expression. After XS106-IDO DCs were co-cultured with C57BL/6 allogeneic splenic T cells, the proliferation of the T cell was significantly inhibited. The co-cultured T cells with XS106-IDO DCs exhibited cell-cycle arrest. Furthermore, injection of XS160-IDO DCs into the footpads of C57BL/6 (H-2b) mice demonstrated a reduced T-cell response against allo-antigen.

CONCLUSIONS

These results suggest that overexpression of IDO in the DCs effectively inhibited T-cell proliferation, and may expand a new immunomodulatory strategy for the prevention of allo-rejection of organ transplantation.

Gene expression of enzymes for tryptophan degradation pathway is upregulated in the skin lesions of patients with atopic dermatitis or psoriasis

BACKGROUND

Atopic dermatitis (AD) and psoriasis are common inflammatory skin diseases. Although many reports implicate Th2 cytokines in the pathophysiology of AD and Th1 cytokines in psoriasis, the precise etiology of these diseases remains elusive.

OBJECTIVE

We investigated novel AD- or psoriasis-related genes to further understand the pathogenesis of these diseases.

METHODS

We performed a comprehensive analysis of mRNA expression in skin biopsies from AD or psoriasis patients using DNA microarrays. Quantitative PCR was then used to monitor the expression of novel disease-related genes in human keratinocytes or pinnae from NC/Nga mice.

RESULTS

Levels of mRNA for IDO (indoleamine 2,3-dioxygenase) and kynureninase, enzymes constituting the tryptophan degradation pathway, were found to be upregulated in the skin lesions as compared to the uninvolved skin of patients with AD or psoriasis. Expression of these two genes was induced in human epidermal keratinocytes stimulated with IFN-gamma in vitro. Moreover, in NC/Nga mice, the expression of kynureninase mRNA in the ear skin was induced following development of AD-like skin lesions.

CONCLUSION

The tryptophan degradation pathway may play an important role in the pathophysiology of AD and psoriasis.

Drug abuse and neuropathogenesis of HIV infection: role of DC-SIGN and IDO

Dendritic cells are the critical mediators of various immune responses and are the first line of defense against any infection including HIV. They play a major role in harboring HIV and the subsequent infection of T cells and passage of virus through the blood-brain barrier (BBB). The recently discovered DC-specific, CD4-independent HIV attachment receptor, DC-SIGN, and T-cell suppressing factor, indolamine 2,3-dioxygenase (IDO), are known to play a critical role in the immuno-neuropathogenesis of HIV infection. Since brain microvascular cells (BMVEC) express dendritic cell (DC)-specific C type ICAM-3 grabbing nonintegrin (DC-SIGN), it is possible that DC-SIGN may play a critical role in human immunodeficiency virus-type 1 (HIV-1) infection and migration of infected DC across BBB. Matrix metalloproteinases (MMPs) are proteolytic enzymes known to be responsible for maintenance, turnover and integrity of extracellular matrix. Our results show that cocaine upregulates IDO and DC-SIGN expression by DC. Further, cocaine upregulates DC-SIGN and MMPs in BMVEC supporting the hypothesis that cocaine causes membrane permeability facilitating endothelial transmigration of infected DC in to the CNS. Targeting DC-SIGN and IDO with specific monoclonal antibodies, inexpensive synthetic antagonists, antisense oligonucleotides and siRNA may lead to develop novel treatment strategies particularly in high-risk populations such as cocaine users.

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.