The role of the tryptophan-metabolizing enzyme indoleamine 2,3-dioxygenase (IDO) in down-regulating human alloresponses has recently been controversially debated. We here demonstrate that human monocyte-derived dendritic cells (mDCs) can be endowed with sustained IDO competence in vitro by 48-hour activation with lipopolysaccharide (LPS) and interferon-gamma (IFN-␥). IFN-␥ also amplified proinflammatory cytokine secretion during activation. Yet, on reculture after activation cytokine production ceased, whereas IDO enzymatic activity IntroductionIndoleamine 2,3-dioxygenase (IDO), the rate-limiting enzyme in tryptophan catabolism, has attracted attention for its proposed role in tolerance induction. 1,2 IDO, when expressed in antigen presenting cells (APCs), such as dendritic cells (DCs), establishes a microenvironment that at the DC/T-cell interface is depleted of tryptophan and enriched with tryptophan metabolites. Because of this alteration of the intercellular microenvironment, IDO activity has been suggested to impair T-cell responses. 3,4 The mechanisms of IDO-mediated inhibition include that T cells stimulated under tryptophan-depleted conditions are impaired to undergo full cell cycle progression 3,5 and are susceptible to apoptotic cell death. 6,7 Furthermore, it has been reported that IDO-expressing DCs can expand naturally occurring regulatory T cells. 8 In a murine model Fallarino et al 9 have shown that IDO activity supported the generation of adaptive regulatory T cells. Likewise, human IDOexpressing tumor cells have been reported to induce CD4 ϩ CD25 ϩ regulatory T cells. 10 Most recently, the previously recognized immunoregulatory activity of human plasmacytoid DCs has been related to IDO activity. 11 Thus, IDO-mediated down-regulation of T-cell responses has been suggested to be involved in a multitude of immunoregulatory processes, for example, pregnancy, 12 tumor growth, 13 and the induction of tolerance in transplantation (reviewed in Hainz et al 14 ).IDO expression is not a constitutive feature of human DCs in homeostatic immunologic conditions but requires induction. Among the multiple mediators of IDO induction (reviewed in Puccetti 15 ), interferon-␥ (IFN-␥) plays a prominent role. 16 IFN-␥ has generally been considered a prototypic proinflammatory cytokine (reviewed in Schoenborn and Wilson 17 ); compelling evidence, however, supports the ability of IFN-␥ to promote anti-inflammatory responses. [18][19][20] The ability of IFN-␥ to induce IDO has been suggested as a critical factor linked to this anti-inflammatory activity. 21,22 In our previous studies addressing possible mechanisms of the immunodeficient state after hematopoietic stem cell transplantation, 23 we found that monocytes after hematopoietic stem cell transplantation were particularly sensitive to respond to an exposure to IFN-␥ with an accelerated release of tryptophan metabolite kynurenine, and, thus, to turn into suppressor cells. This finding suggested the possibility that an augmented IDO activity in recipients of a hemat...

Mutations of FoxP3 result in the disturbance of FoxP3 expression and lack of functional CD4 ϩ CD25 high regulatory T cells in humans, causing immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome. The only curative approach for IPEX syndrome, which is fatal within the first year of life in many cases, is allogeneic hematopoietic stem cell transplantation (HSCT). We monitored lineage-specific chimerism in a FoxP3-deficient patient after nonmyeloablative HSCT and graft rejection over 6 years. The patient lost donor chimerism in T cells, B cells, NK cells, monocytes, and granulocytes 1 year after transplantation, but remained clinically healthy without immunosuppression. To elucidate the immunologic basis of his continuing remission, we performed detailed lymphocyte subset analyses and detected FoxP3-expressing CD4 ϩ CD25 high T cells, comprising 1% to 2% of the CD4 ϩ T cells (ϳ 10/ L), which were more than 90% donor-derived. The patient is disease-free and shows no signs of autoimmunity, suggesting that stable, selective engraftment of regulatory T cells is possible and associated with cure from IPEX syndrome.HSCT may be a curative approach for IPEX syndrome. 1-3 However, the required levels of donor chimerism and conditioning intensity are unknown. Myeloablative conditioning is associated with substantial transplantation-related mortality, 2-5 whereas nonmyeloablative conditioning carries an increased risk of rejection because of dysregulated effector T-cell function. 6 Here we report a boy with IPEX syndrome who underwent peripheral blood HSCT at the age of 11 months from an unrelated 10/10-matched donor (2 ϫ 10 7 /kg CD34 ϩ , 10 9 /kg CD3 ϩ cells) after nonmyeloablative conditioning according to the local protocol (fludarabine 6 ϫ 30 mg/m 2 ; melphalan 140 mg/ m 2 ; alemtuzumab 5 ϫ 0.2 mg/kg). The post-HSCT course was uncomplicated with complete engraftment on day ϩ12. All IPEX-linked symptoms resolved until day ϩ30. Acute graftversus-host disease (skin, II°) was treated with steroids until day ϩ56; cyclosporin A was tapered 1 year after transplantation. At this time, in vitro responses to vaccine antigens and phytohemagglutinin were normal. Six years posttransplantation the patient remains insulin-dependent due to irreversible islet cell damage but has no active autoimmunity. The clinically healthy boy shows no increased frequency or severity of infections (Table S1; available on the Blood website; see the Supplemental Materials link at the top of the online article). Chimerism was determined in peripheral blood cell subsets regularly (described elsewhere 6 ). Approval was obtained from the St Anna Children's Hospital (Vienna, Austria) Institutional Review Board for these studies. Informed consent was obtained in accordance with the Declaration of Helsinki.After 100% donor cell engraftment, there was a continuous decline throughout the first year to an overall and subset-specific donor chimerism of 5% to 10% (Ϯ 6%; Figure 1A) without any clinical symptoms of relapse. Five-, 5.5-, and ...

Summary Indoleamine 2,3‐dioxygenase (IDO), by enzymatic tryptophan degradation, has recently been proposed to have profound immunoregulatory activity. By most recent findings IDO induction follows reverse signaling of cytotoxic T‐lymphocyte antigen‐4 (CTLA‐4) to its ligands CD80/86 and acts as a counter‐regulatory mechanism to T‐cell stimulation. With regard to transplantation, experimental evidence suggests that IDO has the potential to down‐regulate allo‐responses of T cells in vitro and to promote tolerance in murine models of pancreatic islet transplantation and of allogeneic T‐cell transfer in vivo. However, the physiologic role of IDO in human organ transplantation still is to be elucidated. Experiments that clearly identify a significance of IDO in tolerance induction to vascularized organ allografts or in effecting costimulation blockade are required. In this review we provide a conceptual view of the current knowledge of IDO in the context of transplantation and, in light of its particular biological features, speculate about its potential application in novel therapeutic approaches for tolerance induction.

Prostaglandin E2 (PGE 2 ), an abundantly produced lipid messenger in mammalian organisms, has been attributed to possess potent albeit ambivalent immunological functions. Recently, PGE 2 has been reported to stimulate the commonly believed immunosuppressive indoleamine 2,3-dioxygenase (IDO) pathway in human dendritic cells (DCs), but without promoting DC immunosuppressive activity. Here, we report that PGE 2 used as a DC maturation agent apparently has more diverse functions. PGE 2 -matured DCs acquired powerful IDO activity, which was sustained even after removing PGE 2 . These IDO-competent DCs were able to stimulate allogeneic T-cell proliferation, but achieved inhibitory activity as their content in DC/T-cell co-cultures increased. The DC inhibitory activity was reversed upon blockade of IDO activity, confirming that the suppressive effect was in fact mediated by IDO and occurred in a dose-dependent fashion. IDO-mediated T-cell suppression was restored upon re-stimulation of T cells in the absence of IDO activity, confirming its reversibility. T cells stimulated by PGE 2 -matured IDO-competent DCs were sensitized to produce multiple cytokines, comprising Th1, Th2, and Th17 phenotypes. Collectively, these data suggest that T cells stimulated by PGE 2 -matured DCs are not terminally differentiated and their ultimate type of response may be formed by microenvironmental conditions. Keywords: Human dendritic cells r IL-23 r Immunosuppression r Indoleamine 2,3-dioxygenase (IDO) r Prostaglandin E2 Supporting Information available online IntroductionThe intracellular enzyme indoleamine 2,3-dioxygenase (IDO) has a key function in tryptophan metabolism along the kynurenine pathway [1]. IDO's metabolic activity has been perceived to play a significant role in immune regulation. On the cellular level, the complementary effects of IDO-mediated metabolism, tryptophan starvation, and accumulation of immunomodulatory kynurenines Correspondence: Dr. Andreas Heitger e-mail: andreas.heitger@ccri.at at the antigen-presenting cell (APC)/T-cell synapse were proposed to effect regulation of T-cell activation and proliferation [1,2]. In fact, in the last decade, multiple studies corroborated the role of IDO in immune regulation and induction of tolerance (reviewed in [3,4]). IDO competence, i.e. IDO expression and activity [5,6], has been suggested to play a critical role in numerous clinical conditions entailing immune regulation, including tolerance induction in pregnancy [7], transplantation [8,9], and tumor immune evasion [10]. * These authors contributed equally to the work. Eur. J. Immunol. 2012Immunol. . 42: 1117Immunol. -1128 Effective induction of IDO competence is, by and large, restricted to cells of the monocyte/macrophage lineage and, in humans is predominantly found in dendritic cells (DCs) [4,11]. Numerous agents, most prominently interferon (IFN)-γ, have been described to induce IDO activity in DCs [12]. IDO induction is generally believed to represent a feedback mechanism of APC activation [8]. This understanding...