Inhibition of T cell proliferation by macrophage tryptophan catabolism

Functional expression of indoleamine 2,3-dioxygenase by murine CD8 alpha(+) dendritic cells

Immunoregulatory antigen-presenting cells (APC) play an important role in maintaining T cell homeostasis and self-tolerance. In particular, recent evidence demonstrates a role for inhibition of T cell proliferation by macrophage tryptophan catabolism involving the activity of the enzyme indoleamine 2,3-dioxygenase (IDO). Dendritic cells (DC) have also been shown to exert immunoregulatory effects mediated by tryptophan catabolism and to cause T cell apoptosis. In the present study, we have comparatively analyzed the expression of IDO activity by murine macrophages and splenic DC. By means of PCR, Western blotting and measurements of enzyme functional activity, we obtained evidence that, different from macrophages, DC constitutively express IDO. Following activation by IFN-gamma, the latter cells, in particular the CD8 alpha(+) subset, exhibit high functional activity and, unlike macrophages, mediate apoptosis of T(h) cells in vitro. Therefore, in the mouse, CD8 alpha(+) DC may be unique APC capable of fully expressing the IDO mechanism functionally.

T-helper-1 and T-helper-2 responses in psychiatric disorders

The expanding field of psychoneuroimmunology has markedly increased knowledge about the interference of the central nervous system and the immune system. Immunological abnormalities in psychiatric patients have been repeatedly described in the last century. Modern concepts of immunology and the growing knowledge of psychoneuroimmunology may help in understanding the distinct immunological mechanisms in psychiatric disorders. One of these concepts regarding the adaptive immune system is the discrimination between Th1-like cell-mediated and Th2-like antibody-related immune responses. This article systematically describes alterations of Th1- or Th2-specific parameters in the major psychiatric disorders schizophrenia, major depression, and Alzheimer's disease. There are several hints of associations of these two distinct arms of immune response with subgroups of schizophrenia and major depression. The immunological research in Alzheimer's disease has already led to a preclinical model of immunotherapy. Categorization of immune parameters may also help to identify a possible immune-related pathophysiology in psychotic and affective disorders, resulting in specific treatment strategies.

Interleukin-10 contributes development of macrophage suppressor activities by macrophage colony-stimulating factor, but not by granulocyte-macrophage colony-stimulating factor

Macrophages are known to possess suppressor activities in immune responses. To determine the effects of GM-CSF and M-CSF on the expression of macrophage suppressor activities, monocyte-derived macrophages cultured with GM-CSF (GM-Mphis) were compared with those cultured with M-CSF (M-Mphis) for antigen-specific proliferation and interferon-gamma (IFN-gamma) production by lymphocytes. Both GM-Mphis and M-Mphis equally suppressed lymphocyte proliferation, but only M-Mphis suppressed IFN-gamma production in response to purified protein derivative (PPD). M-Mphis, but not GM-Mphis, released IL-10 not only in the course of macrophage differentiation but also in response to PPD after maturation to macrophages. From the results that (i) exogenous IL-10 suppressed IFN-gamma production, but not proliferation of lymphocytes, and that (ii) neutralizing antibody to IL-10 reversed suppressor activities of M-Mphis on IFN-gamma production, but not lymphocyte proliferation, it appeared that IL-10 was the major factor responsible for suppression of IFN-gamma production. Thus, these results suggest that only M-CSF augments IL-10-dependent suppressor activity of macrophages on IFN-gamma production and that both GM-CSF and M-CSF induce IL-10-independent macrophage suppressor activity on lymphocyte proliferation.

Tumor-induced immune dysfunctions caused by myeloid suppressor cells

In the late 1970s, several findings suggested that accessory cells distinct from lymphocytes might suppress immune reactivity in tumor-bearing hosts. Studies in animal models and patients later confirmed that cells driven to act as dominant immune suppressors by growing cancers could subvert the immune system. These cells have also been termed natural suppressors, a functional definition connoting their ability to hamper various T- and B-lymphocyte responses without prior activation and independently from antigen and MHC restriction. These properties were attributed to distinct cell populations. The phenotypic discrepancies, together with the lack of antigen specificity, have generated serious restraints to research on tumor-induced suppression. Recent evidence indicates that suppressor cells are closely related to immature myeloid precursors and can be found in several situations that can exert adverse effects on the immunotherapy of cancer. The present review is an attempt to address the nature and properties of immature myeloid suppressors and their relationship to dendritic cells and macrophages, with the aim of clarifying the complex network of tumor-induced, negative regulators of the immune system.

L-tryptophan contaminant 'peak E' induces the release of IL-5 and IL-10 by peripheral blood mononuclear cells from patients with functional somatic syndromes

In 1989, the development of eosinophilia myalgia syndrome (EMS) was observed in some patients after the intake of l-tryptophan containing several contaminants, including 1,1'-ethylidenebis[l-tryptophan] ('peak E'). Since l-tryptophan has been taken particularly by individuals suffering from functional somatic syndromes (FSS), such as fibromyalgia syndrome (FMS), we put forward the hypothesis that EMS may have developed preferentially in patients with FSS as an allergic reaction towards the contaminant peak E. We therefore studied the immunological reactivity towards l-tryptophan and peak E in these individuals (n = 12) and compared these data with those obtained in 12 healthy controls and 12 patients with other chronic disorders. Peripheral blood mononuclear cells (PBMC) were cultured for 7 days with pure l-tryptophan and peak E. Supernatant fluids were collected at day 7. The type 2 cytokines IL-4, IL-5 and IL-10, and the type 1 cytokines IL-2 and IFN-gamma, were determined by a double sandwich ELISA. PBMC from seven of the 12 FSS patients, but only three of the 24 controls, produced cytokines after incubation with peak E (P < 0.05). Interestingly, six of the seven FSS patients reacting with peak E produced IL-5 and/or IL-10. In contrast, PBMC from only one patient with other chronic disorders and one healthy control secreted type 2 cytokines in response to peak E. The observed heightened type 2 reactivity towards the more immunogenic contaminant 1,1'-ethylidenebis[l-tryptophan] in FSS patients may therefore be taken as an additional argument for our concept that EMS may have developed as a kind of drug-induced allergic disease.

Why isn't the fetus rejected?

The long-standing question of how the fetal allograft avoids immune rejection during pregnancy has lately been generating renewed interest. Recent insights have emerged from studies in mice on uterine NK cells, NKT cells, complement inhibition and the reproductive effects of 1-methyl-tryptophan.

Restriction of Toxoplasma gondii growth in human brain microvascular endothelial cells by activation of indoleamine 2,3-dioxygenase

One of the first steps in the development of cerebral toxoplasmosis is the penetration of the blood-brain barrier, which is comprised of microvascular endothelial cells. We examined the capacity of human brain microvascular endothelial cells (HBMEC) to interact with Toxoplasma gondii. We found that stimulation of HBMEC with gamma interferon (IFN-gamma) resulted in the induction of toxoplasmostasis. The capacity of HBMEC to restrict Toxoplasma growth after IFN-gamma stimulation was enhanced in the presence of tumor necrosis factor alpha (TNF-alpha). In addition, we found that IFN-gamma induced a strong induction of indoleamine 2,3-dioxygenase (IDO) activity in HBMEC, and this enzyme activity was enhanced by costimulation with TNF-alpha. The addition of excess amounts of tryptophan to the HBMEC cultures resulted in a complete abrogation of the IFN-gamma-TNF-alpha-mediated toxoplasmostasis. We therefore conclude that IDO induction contributed to the antiparasitic effector mechanism inducible in HBMEC by IFN-gamma and TNF-alpha.

Enhancement of innate immunity against Mycobacterium avium infection by immunostimulatory DNA is mediated by indoleamine 2,3-dioxygenase

Bacterial DNA and its synthetic immunostimulatory oligodeoxynucleotide analogs (ISS-ODN) activate innate immunity and promote Th1 and cytotoxic T-lymphocyte immune responses. Based on these activities, we investigated whether ISS-ODN could modify the course of Mycobacterium avium infection. M. avium growth in vitro was significantly inhibited by ISS-ODN treatment of human and mouse macrophages, and M. avium growth in vivo was similarly inhibited in C57BL/6 mice treated with ISS-ODN. This protective effect of ISS-ODN was largely independent of tumor necrosis factor alpha (TNF-alpha), interleukin 12 (IL-12), nitric oxide, NADPH oxidase, alpha/beta interferon (IFN-alpha/beta), and IFN-gamma. In contrast, we found that the induction of indoleamine 2,3-dioxygenase (IDO) was required for the antimycobacterial effect of ISS-ODN. To evaluate the potential for synergism between ISS-ODN and other antimycobacterial agents, treatment with a combination of ISS-ODN and clarithromycin (CLA) was tested in vitro and in vivo. ISS-ODN significantly enhanced the therapeutic effect of CLA in both human and mouse macrophages and in C57BL/6 mice. This study newly identifies IDO as being involved in the antimicrobial activity of ISS-ODN and suggests the usefulness of ISS-ODN when used in combination with conventional chemotherapy for microbial infections.

Tryptophan catabolism prevents maternal T cells from activating lethal anti-fetal immune responses

The murine conceptus is protected from maternal immunity by cells expressing indoleamine dioxygenase (IDO), which catabolizes tryptophan. Induction of lethal maternal anti-fetal immunity requires effective pharmacologic inhibition of IDO enzyme activity and the presence of maternal T cells, but not B cells and also depends on the degree of maternal-fetal tissue incompatibility. Based on these findings, we propose a model to explain the role of IDO in suppressing maternal immunity and the mechanism of fetal allograft rejection, when IDO activity is inhibited during gestation. This model incorporates observations that fetal allograft rejection is T cell dependent, antibody-independent and is accompanied by a novel type of inflammation involving extensive complement deposition at the maternal-fetal interface, when IDO activity is blocked during murine pregnancy.

Tryptophan degradation by human placental indoleamine 2,3-dioxygenase regulates lymphocyte proliferation

1. The physiological importance of human placental indoleamine 2,3-dioxygenase (EC 1.13.11.42), the first and rate-limiting enzyme in tryptophan metabolism, in regulating feto-maternal immunology has been studied. 2. Concentrations were measured in placental villous explant conditioned media of 14 amino acids that are known to be required for lymphocyte proliferation. In the absence of interferon-gamma only tryptophan and threonine were significantly lowered; in the presence of interferon-gamma (known to stimulate indoleamine 2,3-dioxygenase) tryptophan but not threonine depletion was much greater. 3. Peripheral blood mononuclear cell proliferation determined by measuring thymidine incorporation into DNA following culture in the medium previously conditioned by culture of villous explants was markedly reduced when placental indoleamine 2,3-dioxygenase was stimulated with interferon-gamma. Inhibition of placental indoleamine 2,3-dioxygenase by 1-methyl-tryptophan prevented inhibition of thymidine incorporation. Supplementation of the conditioned medium with tryptophan but no other amino acid completely reversed the inhibition of thymidine incorporation. 4. Flow cytometric analysis showed that CD4-positive T lymphocyte division was specifically suppressed by indoleamine 2,3-dioxygenase-mediated tryptophan depletion. This inhibition of T cell proliferation was due to arrest of cell cycle progression. 5. To study the mechanism of tryptophan sensing we examined the ability of 11 L-tryptophan analogues to support lymphocyte proliferation. Only L-tryptophan methyl and ethyl esters were able to stimulate proliferation in tryptophan-free media. Since both of these molecules are readily degraded to tryptophan by intracellular esterases this suggests that the tryptophan sensor is intracellular. 6. Our results show that mechanisms are present in the human placenta which are able to regulate cellular proliferation of the maternal immune system. This mechanism is dependent both on placental indoleamine 2,3-dioxygenase-mediated tryptophan degradation and on tryptophan sensing systems within lymphocytes.

Extinguishing maternal immune responses during pregnancy: implications for immunosuppression

Mammals owe their existence to immunosuppressive processes that prevent fetal rejection in utero. Blocking tryptophan catabolism during murine pregnancy allows maternal T cells to provoke fetal allograft rejection. Cells expressing indoleamine 2,3-dioxygenase (IDO), which catabolizes tryptophan, prevent T cell cycle progression and enhance activation induced T cell death. Here, we discuss the role of cells expressing IDO in regulating maternal T cell immunity during pregnancy and consider whether this mechanism might contribute to immunological discrimination by promoting T cell tolerance in other circumstances.

Induction of indoleamine 2,3-dioxygenase in vivo by IFN-con1

Indoleamine 2,3-dioxygenase (IDO) activity as determined by increases in serum kynurenine was measured in a group of hepatitis C patients treated with consensus interferon (IFN-con1). Kynurenine levels increased significantly within 2 days of initiation of treatment but returned to normal values by week 4 after treatment. Although IDO is normally induced by IFN-gamma, no such IFN was detected by ELISA or biologic assays. Thus, consensus IFN induces low levels of IDO in vivo without an IFN-gamma intermediate.

IL-6 inhibits the tolerogenic function of CD8 alpha+ dendritic cells expressing indoleamine 2,3-dioxygenase

The outcome of dendritic cell (DC) presentation of tumor and/or self peptides, including P815AB (a tumor peptide of murine mastocytoma cells) and NRP-A7 (a synthetic peptide mimotope recognized by diabetogenic T cells), may depend on a balance between the activities of immunogenic (CD8alpha(-)) and tolerogenic (CD8alpha(+)) DC. By virtue of their respective actions on CD8(-) and CD8(+) DC, IL-12 and IFN-gamma have functionally opposing effects on peptide presentation by the CD8(-) DC subset, and IFN-gamma-activated CD8(+) DC mediate tolerogenic effects that prevail over the adjuvant activity of IL-12 on CD8(-) DC. We have previously shown that CD40 ligation abrogates the tolerogenic potential of CD8(+) DC, an effect associated with an impaired capacity of the CD40-modulated and IFN-gamma-treated DC to degrade tryptophan and initiate T cell apoptosis in vitro. We report here that IL-6 may both replace (upon administration of the recombinant cytokine) and mediate (as assessed by the use of neutralizing Abs) the effect of CD40 ligation in ablating the tolerogenic activity of CD8(+) DC. The activity of IL-6 includes down-regulation of IFN-gammaR expression in the CD8(+) DC subset and correlates to a reduced ability of these cells to metabolize tryptophan and initiate T cell apoptosis in vitro.

A short course of methylprednisolone immunosuppression inhibits both rejection and spontaneous acceptance of rat liver allografts

BACKGROUND

The effects of immunosuppressive drugs on transplant tolerance have not been extensively studied, although their effect on rejection is well established.

METHODS

We examined the effects of a short course of treatment with the immunosuppressive drug methylprednisolone (MP) on the survival of PVG liver allografts in Dark Agouti (DA) recipients that accepted the livers and in Lewis recipients that rejected the livers. Infiltration of liver allografts was examined by immunohistochemical staining of liver sections, and apoptosis was measured by terminal deoxynucleotide transferase-mediated dUTP nick end labeling.

RESULTS

A 5-day course of MP (days 0 to 4) led to rejection of four of six livers (mean survival time [MST] 99 days) in DA recipients compared with long-term survival (MST >100 days) in untreated animals. Delayed administration of MP (days 3 to 7) exacerbated rejection in DA recipients, and all eight animals rejected the graft (MST 68.5 days). Treatment of Lewis recipients with MP did not significantly prolong survival when administered from days 0 to 4 (MST 13 days), although delay of administration improved the outcome. Treatment from days 3 to 7 resulted in an MST of 21 days, whereas treatment from days 7 to 11 resulted in an MST of 41.5 days. MP treatment from day 3 to day 7 reduced T cells and interleukin 2 receptor-expressing cells but increased the numbers of apoptotic cells infiltrating both DA and Lewis strain allografts.

CONCLUSIONS

These results show that immunosuppression with MP inhibits both spontaneous tolerance and rejection of liver allografts in a rat model and question the efficacy of administering MP to all liver allograft recipients from the time of transplantation.

Extrathymic development of V alpha 11 T cells in placenta during pregnancy and their possible physiological role

The molecular and cellular mechanisms of the feto-maternal immune responses in the placenta in connection with natural abortion remain unclear. In this report we provide evidence that V(alpha11) T cells developed in the placenta may be responsible for the induction of natural abortion. The majority of V(alpha11) TCRs detected during pregnancy showed a consensus motif in the CDR3 region, similar to that of anti-GM3 TCR clones, and were of maternal origin. V(alpha11) TCRs were found in the middle to late stages of gestation due to de novo generation in the placenta, not to migration from the maternal side, as evidenced by the significant increases in the out-of-frame V(alpha11) TCR mRNA and the copy number of circular DNA generated by V(alpha11) gene rearrangements. Furthermore, administration of anti-V(alpha11) Ab to pregnant mice resulted in a significant decrease in the incidence of fetal demise, suggesting that V(alpha11) T cells detected in the placenta develop extrathymically and are involved in natural abortion.

Human primary CD4 + T cells activated in the presence of IFN-alpha 2b express functional indoleamine 2,3-dioxygenase

Indoleamine 2,3-dioxygenase (IDO) is the rate-limiting enzyme in the catabolism of tryptophan. By creating a local microenvironment in which levels of tryptophan are low, IDO-expressing antigen-presenting cells (APC) could regulate T cell activation. This may be relevant to control both viral and bacterial replication as well as neoplastic cell growth. Interferon-alpha (IFN-alpha) is an antiviral cytokine affecting cellular differentiation. In addition, it reduces proliferation of CD4(+) T cells by several molecular mechanisms. To dissect the molecular steps responsible for the INF-mediated antiproliferative activity, we sought to determine whether activated primary CD4(+) T cells in the presence of IFN-alpha would produce IDO. We demonstrate here that IDO mRNA is not present in resting CD4(+) T cells. Stimulation with anti-CD3 plus interleukin-2 (IL-2) induces expression of IDO mRNA (about 2000 copies/150,000 cells), as determined by semiquantitative RT-PCR. When cells were stimulated in the presence of IFN-alpha, expression of IDO mRNA was significantly increased (more than 12,000 copies/150,000 cells). Functional analysis of IDO activity paralleled the results obtained with RT-PCR, demonstrating increased production of active enzyme in CD4(+) T cells stimulated in the presence of IFN-alpha. Our results indicate that IFN-alpha modulates levels of IDO produced by activated CD4(+) T cells. This would likely affect bystander cells by modifying levels of tryptophan in the local microenvironment.

Regulation of prostaglandin synthesis and cell adhesion by a tryptophan catabolizing enzyme

BACKGROUND

The tryptophan catabolizing enzyme, indoleamine 2,3, dioxygenase (IDO) is one of two mammalian enzymes, which can catabolize the rarest essential amino acid, tryptophan. IDO is inducible by cytokines such as interferon-gamma and plays a role in inflammation and maternal tolerance of fetal allografts, although its exact mode of action is unclear. Therefore, we investigated the circumstances under which IDO is expressed in vitro together with the effects of overexpression of IDO on the growth and morphology of cells.

RESULTS

Overexpression of IDO in the murine macrophage cell line RAW 264.7 and the murine fibrosarcoma cell line MC57, resulted in the growth of macroscopic cell foci, with altered cell adhesion properties. The expression of IDO was also detected during adhesion of wild type, nontransfected cells in tissue culture to standard cell growth substrates. Inhibition of this expression, likewise resulted in alterations in cell adhesion. Overexpression of IDO or inhibition of endogenous IDO expression was accompanied by changes in metalloproteinase expression and also in the expression and activity of the cyclooxygenase enzymes. In the case of RAW cells, IDO effects on cell growth could be reversed by adding back prostaglandins.

CONCLUSIONS

These results suggest that catabolism of the rarest essential amino acid may regulate processes such as cell adhesion and prostaglandin synthesis.

Antioxidants inhibit indoleamine 2,3-dioxygenase in IFN-gamma-activated human macrophages: posttranslational regulation by pyrrolidine dithiocarbamate

Induction of the heme-containing indoleamine 2,3-dioxygenase (IDO) by IFN-gamma is implicated in anti-microbial and pro-inflammatory activities of human macrophages. Antioxidants can modulate the expression of immune and inflammatory genes, and pyrrolidine dithiocarbamate (PDTC) is a frequently used antioxidant to inhibit the transcription factor NF-kappaB. Here we show that IFN-gamma treatment of human monocyte-derived macrophages (hMDMs) increased the proportion of oxidized glutathione. PDTC attenuated this increase and inhibited IDO activity, although it increased IDO protein expression and did not affect IDO mRNA expression and enzyme activity directly. Other antioxidants, 2-ME, ebselen, and t-butyl hydroquinone, inhibited IDO protein expression. Similar to PDTC, the heme biosynthesis inhibitor succinylacetone (SA) and the iron-chelator pyridoxal isonicotinoyl hydrazone inhibited cellular IDO activity without affecting protein expression, whereas addition of hemin or the heme precursor delta-aminolevulinic acid increased IDO activity. Also, incubation of IFN-gamma-activated hMDM with delta-[(14)C]-aminolevulinic acid resulted in the incorporation of label into immunoprecipitated IDO, a process inhibited by PDTC and SA. Furthermore, supplementation of lysates from PDTC- or SA-treated hMDM with hemin fully restored IDO activity to control levels, and hemin also reversed the inhibitory action of SA but not PDTC in intact cells. Together these results establish a requirement for de novo heme synthesis for IDO activity in IFN-gamma-activated hMDM. They show that, similar to other pro-inflammatory proteins, the activity of IDO is modulated by antioxidants though in the case of PDTC this takes place posttranslationally, in part by limiting the availability of heme for the formation of holo-IDO.

Acute activation of peripheral lymphocytes during treatment of diabetic ketoacidosis

Activated peripheral T-lymphocytes are increased in both pre-insulin-dependent diabetes mellitus (IDDM) patients and in recently diagnosed IDDM patients, as well as in various forms of acute stress. We studied the in vivo T-lymphocyte activation in six patients in severe diabetic ketoacidosis (DKA) prior to treatment, after 24 h of treatment and > or =5 days after admission. Five of the six patients showed an increased percentage of activated T-lymphocytes based on the expression of HLA-DR at 24 h of treatment when compared to the admission percentage of activation (P<.05). There was no correlation to the admission serum glucose, osmolality, or electrolytes. Serum pH showed a trend toward an inverse correlation, but was not statistically significant. We speculate that T-lymphocyte activation plays a role in the progression of the acute complications of subclinical brain edema and interstitial pulmonary edema of DKA. This process could also be another factor in the progression of the chronic complications of IDDM in addition to the well-established effects of hyperglycemia and hypertension.

Expression of indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase in early concepti

Indoleamine 2,3-dioxygenase (IDO)-initiated tryptophan degradation in the placenta has been implicated in the prevention of the allogeneic fetus rejection [Munn, Zhou, Attwood, Bondarev, Conway, Marshall, Brown, and Mellor (1998) Science 281, 1191-1193]. To determine how IDO is associated with the development of the fetus and placenta, the time course of IDO expression (tryptophan-degrading activity, IDO protein and IDO mRNA) in the embryonic and extra-embryonic tissues as well as maternal tissues of mice was examined. A high tryptophan-degrading activity was detected in early concepti on days 6.5 and 7.5, whereas IDO protein and its mRNA were not expressed during early gestation, but appeared 2-3 days later, lasted for about 3 days and declined rapidly thereafter. The expression of IDO basically coincided with the formation of the placenta. On the contrary, the early tryptophan-degrading activity was due to gene expression of tryptophan 2,3-dioxygenase (TDO), as shown by Northern and Western analysis. These findings indicate that IDO is transiently expressed in the placenta but that the expression does not last until birth, and that the IDO expression is preceded by expression of another tryptophan-degrading enzyme, TDO, in the maternal and/or embryonic tissues in early concepti.

Interleukin 1 receptor antagonist polymorphism in women with idiopathic recurrent miscarriage

OBJECTIVE

Proinflammatory cytokines have been described as etiologic factors in idiopathic recurrent miscarriage. We investigated the relation between idiopathic recurrent miscarriage and polymorphisms in the gene encoding for the interleukin 1 receptor antagonist, an indigenous modulator of proinflammatory immune response.

DESIGN

Prospective case control study.

SETTING

Academic research institution.

PATIENT(S)

One hundred five women with a history of three or more consecutive pregnancy losses before 20 weeks of gestation and 91 healthy, postmenopausal controls with at least two live births and no history of pregnancy loss.

INTERVENTION(S)

Peripheral venous puncture.

MAIN OUTCOME MEASURE(S)

Polymerase chain reaction was performed to identify the different alleles of the gene encoding for interleukin 1 receptor antagonist.

RESULT(S)

Allele frequencies among women with idiopathic recurrent miscarriage and controls were 0.34 and 0.11, respectively, for the polymorphic allele 2 (P=.002; odds ratio: 7.4, confidence interval: 2.9--10.8) and.05 and.05, respectively, for the polymorphic allele 3 (P=.6; odds ratio: 1.3, confidence interval: 0.8--2.3). Allele 2 was present in homozygous form in 9% of women with idiopathic recurrent miscarriage. In contrast, 1% of the control women were homozygous for this allele (P<.001; odds ratio: 13.5, confidence interval: 7.5--21.8).

CONCLUSION(S)

These data support a role for allele 2 of the gene encoding for interleukin 1 receptor antagonist as genetic determinant of idiopathic recurrent miscarriage.

The role of L-tryptophan transport in L-tryptophan degradation by indoleamine 2,3-dioxygenase in human placental explants

The physiological importance of L-tryptophan transport for placental indoleamine 2,3-dioxygenase-mediated degradation of L-tryptophan has been studied using human placental chorionic villous explants. L-Tryptophan influx into villous explants is supported exclusively by transport system L and is substantially inhibited by the L-system-specific substrate 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH) and also by 1-methyl-tryptophan which is also an inhibitor of indoleamine 2,3-dioxygenase. L-Tryptophan influx is enhanced 2.3-fold following in vitro culture of the villous explant. Interferon-gamma, which increases villous explant indoleamine 2,3-dioxygenase expression, has no effect on L-tryptophan influx. In explants both BCH and 1-methyl-tryptophan inhibit indoleamine 2,3-dioxygenase-mediated L-tryptophan degradation. This also applies when L-tryptophan degradation has been stimulated by interferon-gamma. These findings show transport of L-tryptophan into the trophoblast to be a rate-limiting step for indoleamine 2,3-dioxygenase-mediated L-tryptophan degradation and therefore for the normal physiology of mammalian pregnancy.

Th type 1-stimulating activity of lung macrophages inhibits Th2-mediated allergic airway inflammation by an IFN-gamma-dependent mechanism

In the mucosal immune system, resident dendritic cells are specialized for priming Th2-polarized immunity, whereas the Ag-presenting activity of macrophages has been linked with the development of Th1 phenotype. As an immune switch toward Th1 can protect against Th2-mediated allergic response, this study investigated the capacity of lung macrophages to stimulate Th1 responses during the secondary exposure to inhaled allergen, thereby suppressing Th2-mediated allergic airway inflammation in a murine model of allergic asthma. Following airway macrophage depletion in OVA-sensitized mice, lung T cells defaulted to a phenotype that produced less Th1 (IFN-gamma) and more Th2 (IL-4 and IL-5) cytokines, leading to more severe airway hyperreactivity and inflammation after intranasal Ag challenge. After OVA pulsing and adoptive transfer, lung macrophages selectively promoted a Th1 response in Ag-sensitized recipients and did not induce pulmonary eosinophilia. By contrast, OVA pulsing and adoptive transfer of a lung cell preparation, consisting of dendritic cells, B cells, and macrophages, promoted a Th2 response with an associated inflammatory response that was suppressed when macrophages were present and pretreated with IFN-gamma, but exacerbated when macrophages were depleted before IFN-gamma treatment. In addition, Th1-promoting activity of lung macrophages was not related to the autocrine production of IL-12p40. These results suggest that the Th1-promoting APC activity may be an inherent property of the lung macrophage population, and may play an important role, upon stimulation by IFN-gamma, in antagonizing an ongoing Th2 immunity and Th2-dependent allergic responses.

Amino acid transport regulation and early embryo development

Amino acids are essential components of media utilized to culture fertilized human eggs to the blastocyst stage in vitro. Use of such media has led to a significant increase in the proportion of embryos that implant upon transfer to the uterus and to a decrease in the number that need to be transferred to achieve pregnancy. Little is known about the mechanisms by which amino acids foster development of healthy human blastocysts. Indications are, however, that many of these mechanisms are the same in human and mouse embryos. Both essential and nonessential amino acid transport benefit preimplantation mouse embryo development, albeit at different stages. Nonessential amino acid transport improves development primarily during cleavage, whereas essential amino acid transport supports development of more viable embryos, especially subsequent to the eight-cell stage. This review discusses likely mechanisms for these beneficial effects.

CD40 ligation ablates the tolerogenic potential of lymphoid dendritic cells

The outcome of dendritic cell (DC) presentation of P815AB, a tolerogenic tumor/self peptide, depends on a balance between the respective immunogenic and tolerogenic properties of myeloid (CD8 alpha(-)) and lymphoid (CD8 alpha(+)) DC. We have previously shown that CD8(-) DC can be primed by IL-12 to overcome inhibition by the CD8(+) subset and initiate immunogenic presentation in vivo when the two types of peptide-pulsed DC are cotransferred into recipient hosts. IFN-gamma enhances the inhibitory activity of CD8(+) DC on Ag presentation by the other subset, blocking the ability of IL-12-treated CD8(-) DC to overcome suppression. We report here that CD40 ligation on lymphoid DC ablated their inhibitory function on Ag presentation as well as IFN-gamma potentiation of the effect. CD40 modulation of IFN-gamma action on lymphoid DC involved a reduction in IFN-gamma R expression and tryptophan-degrading ability. This effect was accompanied in vitro by an impaired capacity of the CD40-modulated and IFN-gamma-treated DC to initiate T cell apoptosis. In vivo, not only did CD40 triggering on lymphoid DC abrogate their tolerogenic activity, but it also induced the potential for immunogenic presentation of P815AB. Importantly, a pattern similar to P815AB as well as CD40 modulation of lymphoid DC function were observed on testing reactivity to NRP, a synthetic peptide mimotope recognized by diabetogenic CD8(+) T cells in nonobese diabetic mice.

Prevention of T cell-driven complement activation and inflammation by tryptophan catabolism during pregnancy

Indoleamine 2,3 dioxygenase (IDO) activity during pregnancy protects developing fetuses from maternal immune responses in CBA mice. We show here that fetal allografts were rejected only in mating combinations where paternally inherited tissue antigens elicited potent maternal T cell responses after exposure to IDO inhibitor. IDO inhibitor treatment triggered extensive inflammation at the maternal-fetal interface in susceptible mating combinations, which was characterized by complement deposition and hemorrhagic necrosis. Identical inflammatory responses occurred in B cell-deficient (RAG-I-/-) mothers that carried a monoclonal cohort of CD8+ T cells specific for a single paternally inherited fetal major histocompatibility complex antigen. Thus, fetal allograft rejection was accompanied by a unique form of inflammation that was characterized by T cell-dependent, antibody-independent activation of complement. In contrast, no inflammation, complement deposition or T cell infiltration was elicited when mice carrying syngeneic fetuses were exposed to IDO inhibitor. These data show that IDO activity protects the fetus by suppressing T cell-driven local inflammatory responses to fetal alloantigens.

Immortalized myeloid suppressor cells trigger apoptosis in antigen-activated T lymphocytes

We described a generalized suppression of CTL anamnestic responses that occurred in mice bearing large tumor nodules or immunized with powerful recombinant viral immunogens. Immune suppression entirely depended on GM-CSF-driven accumulation of CD11b(+)/Gr-1(+) myeloid suppressor cells (MSC) in secondary lymphoid organs. To further investigate the nature and properties of MSC, we immortalized CD11b(+)/Gr-1(+) cells isolated from the spleens of immunosuppressed mice, using a retrovirus encoding the v-myc and v-raf oncogenes. Immortalized cells expressed monocyte/macrophage markers (CD11b, F4/80, CD86, CD11c), but they differed from previously characterized macrophage lines in their capacities to inhibit T lymphocyte activation. Two MSC lines, MSC-1 and MSC-2, were selected based upon their abilities to inhibit Ag-specific proliferative and functional CTL responses. MSC-1 line was constitutively inhibitory, while suppressive functions of MSC-2 line were stimulated by exposure to the cytokine IL-4. Both MSC lines triggered the apoptotic cascade in Ag-activated T lymphocytes by a mechanism requiring cell-cell contact. Some well-known membrane molecules involved in the activation of apoptotic pathways (e.g., TNF-related apoptosis-inducing ligand, Fas ligand, TNF-alpha) were ruled out as candidate effectors for the suppression mechanism. The immortalized myeloid lines represent a novel, useful tool to shed light on the molecules involved in the differentiation of myeloid-related suppressors as well as in the inhibitory pathway they use to control T lymphocyte activation.

Identification of the indoleamine 2,3-dioxygenase nucleotide sequence in a rat liver transplant model

A tryptophan catabolizer, indoleamine 2,3-dioxygenase (IDO) is highly expressed in the placenta and plays an essential role in maternal tolerance. Recent data have shown that the administration of an IDO inhibitor blocked not only maternal tolerance but also liver allograft tolerance. However, little is known about the induction of IDO in liver allografts, although a gene specific for tryptophan 2,3-dioxygenase (TDO) is believed to be expressed in the liver. In the present study, we investigated whether IDO is induced in liver allografts. Synthetic oligonucleotide primers based on the mouse IDO cDNA sequence were used to amplify RNA derived from livers of donor, syngeneic or allogeneic OLT rats. RNA encoding IDO was induced in the rat allogeneic liver after orthotopic liver transplantation (OLT), but not in syngeneic OLT. The rat nucleotide sequence of the RT-PCR products obtained from OLT livers revealed identities of 89% homology to the mouse IDO and of 68% to the human IDO. This study demonstrated the presence of RNA encoding IDO in allogeneic OLT livers, which may be involved in the immune response after liver transplantation.

Dendritic cells and prospects for transplantation tolerance

The past year has witnessed the resolution of some long-standing enigmas surrounding the immunobiology of dendritic cells, illuminating their opposing roles in peripheral tolerance and allograft rejection. Nevertheless these advances have posed many new questions, the answers to which may subtly influence our approach to the treatment of rejection while bringing ever closer the prospect of donor-specific transplanation tolerance.

Maturation of human monocyte-derived dendritic cells studied by microarray hybridization

We compared the transcript profiles of human myeloid immature dendritic (IDC) cells and mature dendritic cells (MDC) by hybridization of cell-derived cDNA to DNA probes immobilized on microarrays. The microarrays contained probes for 4110 known genes. We report maturation-dependent changes in transcription of clusters of differentiation, cytokines, cytokine receptors, chemokines, chemokine receptors, neuropeptides, adhesion molecules, and other genes. We identified 1124 transcripts expressed in IDC and 1556 transcripts expressed in MDC. Maturation increased the levels of 291 transcripts twofold or more and reduced the levels of 78 transcripts to one-half or less than in IDC. We identified a concerted maturation-stage-dependent transcription of the variable chains of the members of the gamma-chain-cytokine receptor family IL-4R, IL-7R, and IL-15R. Also, we found the reversal of the ratio of transcripts for galectin-3 and galectin-9 upon maturation. We identified maturation-dependent changes in the levels of transcripts for numerous genes encoding proteins previously undetected in dendritic cells such as indoleamine 2,3-deoxygenase, Epstein-Barr virus induced protein 3 and kinesin-2. Moreover, MDC transcribed and translated insulin like growth factor-1 receptor, transforming growth factor alpha, and neuropeptide Y.

IFN-gamma inhibits presentation of a tumor/self peptide by CD8 alpha- dendritic cells via potentiation of the CD8 alpha+ subset

Using an in vivo model of tumor/self peptide presentation for induction of class I-restricted skin test reactivity, we have previously shown that a minority population of CD8+ dendritic cells (DC) negatively regulates the induction of T cell reactivity by peptide-loaded CD8- DC in DBA/2 mice. However, the CD8- fraction can be primed by IL-12 to overcome inhibition by the CD8+ subset when the two types of DC are cotransferred into recipient hosts. We report here that exposure of CD8+ DC to IFN-gamma greatly enhances their inhibitory activity on Ag presentation by the other subset, blocking the ability of IL-12-treated CD8- DC to overcome suppression. In contrast, IFN-gamma has no direct effects on the APC function of the latter cells and does not interfere with IL-12 signaling. The negative regulatory effect triggered by IFN-gamma in CD8+ DC appears to involve interference with tryptophan metabolism in vivo. Through tryptophan depletion affecting T cell responses, IFN-gamma acting on CD8+ DC may thus contribute to regulation of immunity to tumor/self peptides presented by the CD8- subset.

Immune reaction links disease progression in cancer patients with depression

Mood disturbances and depression are supposed to have a negative impact on patients' outcome in malignant tumour disease. On the other hand, poor prognosis in cancer patients is associated with chronic immune challenge which is paralleled by enhanced degradation of the essential amino acid tryptophan and thus decreased plasma tryptophan concentrations. Because tryptophan is precursor for the biosynthesis of the neurotransmitter serotonin (= 5-hydroxytryptamine, 5HT), low tryptophan concentrations will lead to decreased availability of serotonin which finally increases the susceptibility for the development of mood disturbances and depression in the patients. Thus, the development of depression in cancer patients may result from chronic cellular immune stimulation. In conclusion, a more aggressive tumour rather than depression will be responsible for worse outcome of cancer patients and will be associated with a more drastic challenge of the immune system, as a side effect leading to neurotransmitter disturbances.

Immunology at the maternal-fetal interface: lessons for T cell tolerance and suppression

Mammalian reproduction poses an immunological paradox because fetal alloantigens encoded by genes inherited from the father should provoke responses by maternal T cells leading to fetal loss. Current understanding of T cell immunobiology and the critical role of inflammatory processes during pregnancy is reviewed and discussed. Lessons derived from studies on the regulation of T cell responsiveness during mammalian gestation are considered in the wider context of T cell tolerance toward some microbial infections and tumors, avoidance of autoimmunity, and tissue allograft rejection.

Expression and purification of recombinant human indoleamine 2, 3-dioxygenase

Indoleamine 2,3-dioxygenase, the first and rate-limiting enzyme in human tryptophan metabolism, has been implicated in the pathogenesis of many diseases. The human enzyme was expressed in Escherichia coli EC538 (pREP4) as a fusion protein to a hexahistidyl tag and purified to homogeneity in terms of electrophoretic and mass spectroscopic analysis, by a combination of phosphocellulose and nickel-agarose affinity chromatography. The yield of the fusion protein was 1.4 mg per liter of bacterial culture with an overall recovery of 56% from the crude extract. When the culture medium was supplemented with 7 microM hemin, the purified protein contained 0.8 mol of heme per mole of enzyme and exhibited an absorption spectrum consistent with the ferric form of hemoprotein. The pI value of the recombinant enzyme was 7.09 compared with 6.9 for the native enzyme. This was as expected from the addition of the hexahistidyl tag. Similar to the native enzyme, the recombinant enzyme required methylene blue and ascorbic acid for enzyme activity and oxidized not only l-tryptophan but also d-tryptophan and 5-hydroxy-l-tryptophan. The molecular activities for these substrates and their K(m) values were similar to those of the native enzyme, indicating that the addition of the hexahistidyl tag did not significantly affect catalytic activity. The recombinant protein can therefore be used to investigate properties of the native enzyme. This will aid the development of specific inhibitors of indoleamine 2,3-dioxygenase, which may be effective in halting disease progression.

Redox reactions related to indoleamine 2,3-dioxygenase and tryptophan metabolism along the kynurenine pathway

The heme enzyme indoleamine 2,3-dioxygenase (IDO) oxidizes the pyrrole moiety of L-tryptophan (Trp) and other indoleamines and represents the initial and rate-limiting enzyme of the kynurenine (Kyn) pathway. IDO is a unique enzyme in that it can utilize superoxide anion radical (O2*- ) as both a substrate and a co-factor. The latter role is due to the ability of O2*- to reduce inactive ferric-IDO to the active ferrous form. Nitrogen monoxide (*NO) and H2O2 inhibit the dioxygenase and various inter-relationships between the nitric oxide synthase- and IDO-initiated amino acid degradative pathways exist. Induction of IDO and metabolism of Trp along the Kyn pathway is implicated in a variety of physiological and pathophysiological processes, including anti-microbial and anti-tumor defense, neuropathology, immunoregulation and antioxidant activity. Antioxidant activity may arise from O2*- scavenging by IDO and formation of the potent radical scavengers and Kyn pathway metabolites, 3-hydroxyanthranilic acid and 3-hydroxykynurenine. Under certain conditions, these aminophenols and other Kyn pathway metabolites may exhibit pro-oxidant activities. This article reviews findings indicating that redox reactions are involved in the regulation of IDO and Trp metabolism along the Kyn pathway and also participate in the biological activities exhibited by Kyn pathway metabolites.

Na+-dependent high-affinity glutamate transport in macrophages

Excessive accumulation of glutamate in the CNS leads to excitotoxic neuronal damage. However, glutamate clearance is essentially mediated by astrocytes through Na+-dependent high-affinity glutamate transporters (excitatory amino acid transporters (EAATs)). Nevertheless, EAAT function was recently shown to be developmentally restricted in astrocytes and undetectable in mature astrocytes. This suggests a need for other cell types for clearing glutamate in the brain. As blood monocytes infiltrate the CNS in traumatic or inflammatory conditions, we addressed the question of whether macrophages expressed EAATs and were involved in glutamate clearance. We found that macrophages derived from human blood monocytes express both the cystine/glutamate antiporter and EAATs. Kinetic parameters were similar to those determined for neonatal astrocytes and embryonic neurons. Freshly sorted tissue macrophages did not possess EAATs, whereas cultured human spleen macrophages and cultured neonatal murine microglia did. Moreover, blood monocytes did not transport glutamate, but their stimulation with TNF-alpha led to functional transport. This suggests that the acquisition of these transporters by macrophages could be under the control of inflammatory molecules. Also, monocyte-derived macrophages overcame glutamate toxicity in neuron cultures by clearing this molecule. This suggests that brain-infiltrated macrophages and resident microglia may acquire EAATs and, along with astrocytes, regulate extracellular glutamate concentration. Moreover, we showed that EAATs are involved in the regulation of glutathione synthesis by providing intracellular glutamate. These observations thus offer new insight into the role of macrophages in excitotoxicity and in their response to oxidative stress.

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

Dendritic cells (DCs) play a key role in the activation and regulation of B and T lymphocytes. Production of indoleamine 2, 3-dioxygenase (IDO) by macrophages has recently been described to result in inhibition of T cell proliferation through tryptophan degradation. Since DCs can be derived from monocytes, we sought to determine whether DCs could produce IDO which could potentially regulate T cell proliferation. Northern blot analysis of RNA from cultured monocyte-derived human DC revealed that IDO mRNA was induced upon activation with CD40 ligand and IFN-gamma. IDO produced from activated DCs was functionally active and capable of metabolizing tryptophan to kynurenine. Activated T cells were also capable of inducing IDO production by DCs, which was inhibited by a neutralizing Ab against IFN-gamma. DC production of IDO resulted in inhibition of T cell proliferation, which could be prevented using the IDO inhibitor 1-methyl-dl -tryptophan. These results suggest that activation of DCs induces the production of functional IDO, which causes depletion of tryptophan and subsequent inhibition of T cell proliferation. This may represent a potential mechanism for DCs to regulate the immune response.

Enhanced tryptophan degradation in systemic lupus erythematosus

In vitro and in vivo, tryptophan degradation was found to be associated with T cell functional loss and tolerance induction. In systemic lupus erythematosus (SLE) besides the Th2-type cytokine interleukin-10, Th1-type cytokines including interferon-gamma (IFN-gamma) are expressed especially during exacerbation of the disease. IFN-gamma stimulates the enzyme indoleamine (2,3)-dioxygenase (IDO) converting tryptophan to the metabolite kynurenine which in macrophages is subsequently degraded to other, partly neurotoxic compounds like quinolinic acid, and finally to nicrotinamides. We measured kynurenine and tryptophan concentrations in the sera of 55 SLE patients. In these patients, the concentrations of tryptophan (median, interquartile range: 53.9, 45.7-64.1 microM) were lower (p < 0.0001), and the kynurenine concentrations (2.45, 1.75-3.40 microM) were increased (p < 0.0005) compared to healthy blood donors (70.0, 63.8-80.6; 1.80, 1.45-2.27 microM, respectively). Also the kynurenine per tryptophan quotients (K/T), which allow to estimate IDO activity, were significantly higher in patients than in normals (0.043, 0.033-0.062 vs. 0.027, 0.021-0.030; p < 0.0001), indicating enhanced IDO-induced tryptophan degradation in SLE. There was no significant relationship between tryptophan, kynurenine and the SLEDAI, and also the correlation of K/T with SLEDAI was rather weak (rs = 0.243, p < 0.05). Higher K/T was found in patients presenting with serositis (p = 0.01), decrease of complement (c3, c4; p < 0.01) and blood count change (anemia, leucopenia, lymphopenia; p = 0.032) than in patients without such disease manifestations. The significant correlation found between K/T and neopterin (rs = 0.808, p < 0.001), a marker of immune activation, points to a role of immune activation to be responsible for tryptophan degradation in SLE patients.

The tryptophan catabolite picolinic acid selectively induces the chemokines macrophage inflammatory protein-1 alpha and -1 beta in macrophages

We previously found that the tryptophan catabolite picolinic acid (PA) is a costimulus for the activation of macrophage effector functions. In this study, we have investigated the ability of PA to modulate the expression of chemokines in macrophages. We demonstrate that PA is a potent activator of the inflammatory chemokines MIP (macrophage inflammatory protein)-1 alpha and MIP-1 beta (MIPs) mRNA expression in mouse macrophages in a dose- and time-dependent fashion and through a de novo protein synthesis-dependent process. The induction by PA occurred within 3 h of treatment and reached a peak in 12 h. The stimulatory effects of PA were selective for MIPs because other chemokines, including monocyte chemoattractant protein-1, RANTES, IFN-gamma-inducible protein-10, MIP-2, and macrophage-derived chemokine, were not induced under the same experimental conditions and were not an epiphenomenon of macrophage activation because IFN-gamma did not affect MIPs expression. Induction of both MIP-1 alpha and MIP-1 beta by PA was associated with transcriptional activation and mRNA stabilization, suggesting a dual molecular mechanism of control. Iron chelation could be involved in MIPs induction by PA because iron sulfate inhibited the process and the iron-chelating agent, desferrioxamine, induced MIPs expression. We propose the existence of a new pathway leading to inflammation initiated by tryptophan catabolism that can communicate with the immune system through the production of PA, followed by secretion of chemokines by macrophages. These results establish the importance of PA as an activator of macrophage proinflammatory functions, providing the first evidence that this molecule can be biologically active without the need for a costimulatory agent.

Involvement of decidual Valpha14 NKT cells in abortion

The immunological mechanisms that regulate abortion are largely unknown. Here, we found that a distinct subset of lymphocytes, Valpha14 NKT cells expressing an invariant antigen receptor encoded by Valpha14/Jalpha281 and Vbeta7 segments, accumulated in the decidua during pregnancy and provoked abortion upon stimulation with alpha-galactosylceramide (alpha-GalCer), a specific ligand for Valpha14 NKT cells. The alpha-GalCer-mediated abortion was not observed in Valpha14 NKT-, IFN-gamma-, tumor necrosis factor alpha-, or perforin-knock-out mice and appeared to be due to the degeneration of embryonic trophoblasts mediated by the activated Valpha14 NKT cells whose perforin-dependent killing and production of IFN-gamma and tumor necrosis factor alpha were essential. The possible role of the decidual Valpha14 NKT cells in the pathogenesis of abortion is discussed.

Selective metabolism of kynurenine in the spleen in the absence of indoleamine 2,3-dioxygenase induction

The kynurenine pathway of L-tryptophan degradation is differentially regulated dependent on the level of immune system activation. During inflammation and disease, activity of the hepatocellular enzyme tryptophan 2,3-dioxygenase (TDO) decreases and a second enzyme, indoleamine 2,3-dioxygenase (IDO), is induced in extrahepatic sites. Substantial formation of a metabolise downstream of this step, quinolinic acid (Quin), subsequently occurs only in select regions of the lymphoid tissues, such as spleen, in a temporally restricted manner. The goal of this study was to determine the localization of Quin in unstimulated mice under conditions where rate-limiting control of the pathway by both TDO and IDO was by-passed. Supplementation of drinking water with L-kynurenine, a pathway intermediate that lies between tryptophan and Quin, resulted in a dose-dependent increase in Quin immunoreactivity in the follicles and discontinuous regions of the marginal zones of the spleen. Strongly immunoreactive cells in the periarteriole lymphoid sheaths adopted a highly reactive morphology despite the lack of immunostimulation and IDO induction. In contrast, a patchy to diffuse pallor of staining was observed in the liver parenchyma with 1 and 10 mM L-kynurenine ingestion, respectively. These data show that selective tryptophan metabolism can occur in discrete subcompartments of the lymphoid tissues beyond the level of IDO. In vivo manipulation of Quin synthesis in the absence of IDO induction may serve as a model for studying regulation and function of the kynurenine pathway activation in the immune system.

The initiation of autoimmune diabetes

It is speculated that presentation of self-peptides to autoreactive T cells normally results in T cell tolerance. In autoimmune conditions, breakdown in the tolerization process results in activation of self-reactive T cells and an immune attack on host tissues. Our understanding of the immune cells and signaling pathways that contribute to this breakdown in T cell tolerization mechanisms is beginning to be deciphered. In particular, the elucidation of the mechanisms that contribute to the release of host antigen, the identification of the antigen-presenting cells that present the host peptides to self-reactive T cells and the role of members of the tumor necrosis factor receptor/ligand families that contribute to inappropriate activation of self-reactive T cells is advancing. The accumulating data from these studies will hopefully provide new ideas for combating autoimmunity.

Tryptophan catabolism and T-cell tolerance: immunosuppression by starvation?

Some macrophages inhibit microbial infections by producing indoleamine 2,3 dioxygenase (IDO), which catabolizes tryptophan. Here, Andrew Mellor and David Munn discuss evidence that cells that synthesize IDO protect the mammalian fetus from maternal T-cell attack and argue that this mechanism might have wider implications for the control of T-cell responses.

HLA-G transgenic mice

We have generated a number of transgenic mice using DNA segments derived from the HLA-G gene. Using these mice we have examined the pattern of expression dictated by HLA-G promoter elements in mice and shown that HLA-G functions both as a restriction element and a transplantation antigen recognized by murine T cells. In addition, we have shown that trophoblast cells expressing H-2Kb under the control of HLA-G promoter elements affect maternal T cell phenotype and responsiveness during pregnancy. Using these same HLA-G/H-2Kb transgenic mice we have shown that trophoblast cells, expressing an inducible enzyme that degrades tryptophan, protects allogeneic conceptus expressing paternally-inherited transgenes from attack by maternal T cells that leads to fetal rejection.