Indoleamine 2,3-dioxygenase (IDO), an enzyme involved in the catabolism of tryptophan, is expressed in certain cells and tissues, particularly in antigen-presenting cells of lymphoid organs and in the placenta. It was shown that IDO prevents rejection of the fetus during pregnancy, probably by inhibiting alloreactive T cells, and it was suggested that IDO-expression in antigen-presenting cells may control autoreactive immune responses. Degradation of tryptophan, an essential amino acid required for cell proliferation, was reported to be the mechanism of IDO-induced T cell suppression. Because we wanted to study the action of IDO-expressing dendritic cells (DCs) on allogeneic T cells, the human IDO gene was inserted into an adenoviral vector and expressed in DCs. Transgenic DCs decreased the concentration of tryptophan, increased the concentration of kynurenine, the main tryptophan metabolite, and suppressed allogeneic T cell proliferation in vitro. Kynurenine, 3-hydroxykynurenine, and 3-hydroxyanthranilic acid, but no other IDO-induced tryptophan metabolites, suppressed the T cell response, the suppressive effects being additive. T cells, once stopped in their proliferation, could not be restimulated. Inhibition of proliferation was likely due to T cell death because suppressive tryptophan catabolites exerted a cytotoxic action on CD3+ cells. This action preferentially affected activated T cells and increased gradually with exposure time. In addition to T cells, B and natural killer (NK) cells were also killed, whereas DCs were not affected. Our findings shed light on suppressive mechanisms mediated by DCs and provide an explanation for important biological processes in which IDO activity apparently is increased, such as protection of the fetus from rejection during pregnancy and possibly T cell death in HIV-infected patients.

Indoleamine-2,3-dioxygenase (IDO) is an immunosuppressive enzyme capable of inhibiting a destructive maternal T cell response against allogeneic fetuses. Expression of IDO is evident in tumours and is thought to enable escape from immunologically mediated rejection. Consequently, clinical trials using an inhibitor of IDO, 1-methyltryptophan (1MT), have been initiated. However, a review of the current literature indicates that we are far from understanding the biological relevance of IDO expression during tumorigenesis. A better understanding of IDO biology is needed to comprehend the effect of IDO inhibitors and to provide a rationale for their therapeutic application in cancer.

Although IDO2 is expressed in human tumors, tryptophan degradation is entirely provided by IDO1. Importantly, D-1MT does not inhibit the IDO activity of malignant cells. If ongoing clinical studies show a therapeutic effect of D-1MT, this cannot be attributed to inhibition of IDO in tumor cells.

IntroductionIDO is an enzyme that catabolizes tryptophan and was shown to protect the fetus from rejection mediated by maternal T cells. 1 Such a basic mechanism that evolved for the perpetuation of species might be expected to be used for the regulation of other biologic processes. Researchers were therefore motivated to look for a role of IDO in the regulation of immune responses in health and disease. An objective that has challenged the scientific community for decades is the unraveling of immunologic conditions that enable the development of tumors and, based on this, the design of novel therapeutic strategies for tumor prevention or treatment.Tumors are the consequence of genetic mutations leading to the expression of modified proteins. 2 Despite their potential to activate the immune response, these proteins are not capable of inducing tumor-protective immunity. Many mechanisms have been made responsible for the lack of an effective immune defense against tumors. 3 If IDO has the potential to prevent rejection of the fetus during pregnancy-so went the reasoning-it might also be implicated in induction of fatal tolerance to tumors. Tolerogenic mechanisms can operate in the tumor itself or in key sites where the immune system encounters tumor antigens, such as tumor-draining lymph nodes. 4 Emerging evidence suggests that in lymph nodes, IDO-expressing dendritic cells (DCs) directly suppress and anergize tumor-reactive effector T cells responding to antigens presented by these DCs. 5,6 In addition, through bystander suppression, IDO can inhibit T-cell responses to antigens presented by neighboring DCs. 5 It was therefore appealing to trigger antitumor immunity by inhibiting the activity of IDO. Pioneering work in this field was performed by van den Eynde and colleagues. In a series of elegant experiments, they showed that the IDO-blocking agent 1-methyltryptophan (1MT) significantly reduces the volume of tumors in preimmunized mice (Uyttenhove et al 7 ). But 1MT exists in 2 forms, the levo and the dextro isomer. In the study of van den Eynde's group the levo isoform (L-1MT) was used. Other authors found that, in animal models, the dextro stereoisomer (D-1MT) has superior antitumor activity 8 and therefore chose it for phase 1 clinical trials in humans. This was a surprising choice since, on a biochemical level, in contrast to the L isomer the D form did not properly inhibit IDO. 8 In recent studies, the contradiction was apparently resolved by describing, in addition to the classic IDO (dubbed IDO1), a closely related variant called IDO2, 9 which is strongly inhibited by D-1MT but unaffected by L-1MT. 10 Natural IDO2 was found in several tissues, including a predendritic mouse cell line. 10 Taking into consideration the potential of D-1MT for cancer therapy and the already running clinical trials, it is important to know whether in humans the effect of D-1MT can be mediated by blocking the IDO activity of DCs. We tried to find an answer to this question by studying a subset of human DCs that were claimed to ...

Summary 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 noneffective. 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.