Free iron ions decrease indoleamine 2,3-dioxygenase expression and reduce IFNg-induced inhibition of Chlamydia trachomatis infection Birgit Krausse-Opatz a, * , Ulrike Wittkop a , Frank M. Gutzki c , Cornelia Schmidt a , Barbara Ju ¨ rgens-Saathoff a , Sabine Meier a , Bibiana Beckmann c , Osamu Takikawa d , Michael A. Morgan b , Dimitrios Tsikas c , Dirk O. Stichtenoth c , Annette D. Wagner a , Henning Zeidler a , Lars Ko ¨ hler a a Department of Rheumatology, Hannover Medical School, Carl-Neuberg-Str.1, D-30625 Hannover, Germany b Department of Haematology, Haemostasis, Oncology and Stem Cell Transplantation, Hannover Medical School, Carl-Neuberg-Str.1, D-30625 Hannover, Germany c Department of Clinical Pharmacology, Hannover Medical School, Carl-Neuberg-Str.1, D-30625 Hannover, Germany d Laboratory of Radiation Safety, National Institute for Longevity Sciences, National Center for Geriatrics and Gerontology, Obu, Aichi 474-8522, Japan article info Article history: Received 30 November 2008 Received in revised form 28 February 2009 Accepted 12 March 2009 Available online 21 March 2009 Keywords: Indoleamine 2,3-dioxygenase Interferon-g Iron Chlamydia trachomatis abstract Interferon-g (IFNg)-mediated indoleamine 2,3-dioxygenase (IDO) expression, important in innate immunity, immune suppression, and tolerance, can be counteracted by ferrous iron (FeSO 4 ). Elevation of intracellular iron levels during stimulation with IFNg impeded IFNg-induced IDO mRNA and protein expression in HEp-2 cells. Decreased IDO expression was accompanied by decreased tryptophan degradation. Accordingly, IFNg-mediated suppressing effects on Chlamydia trachomatis (CT) infection were reduced or even abolished in the presence of FeSO 4 . Conversely, lowering intracellular iron levels by deferoxamine (DFO) did not increase IFNg-induced IDO expression but potentiated Chlamydia-sup- pressing effects by lowering intracellular iron availability. Additionally, DFO led to a CT-induced IDO expression in HEp-2 cells not treated with IFNg. In summary, this study demonstrates that iron acts as a regulatory element for modulating IDO expression, in addition to its function as an essential element for chlamydial growth. This may represent an important control mechanism of IDO expression at the transcriptional level. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction L-Tryptophan represents the least available essential amino acid for humans and some microorganisms. If environmental trypto- phan is available, it is used even by bacteria that are capable of tryptophan synthesis by an energy-consuming de novo synthesis pathway. Human cells infected by intracellular pathogens utilize the tryptophan dependence in an innate immune mechanism to inhibit growth of intracellular bacteria and viruses by expressing the enzyme indoleamine 2,3-dioxygenase (IDO), which catalyzes the initial step of L-tryptophan catabolism. Degradation of L-tryp- tophan via the kynurenine pathway reduces tryptophan availability and thus controls the growth of intracellular pathogens. Trypto- phan degradation leads to production of kynurenine pathway metabolites, some of which account for immunosuppressive T cell responses and T cell tolerance. Some kynurenine metabolites are significantly involved in prevention of T cell responses against tumor cells as well as against fetal rejection in utero [1–3]. There is currently much discussion regarding the far-reaching implications of IDO expression in the course and outcome of infectious and oncologic diseases as well as in pregnancy. Better understanding of the factors influencing IDO expression will lead to discovery of means by which this system can be manipulated, and is highly relevant to improving therapeutic benefits. The proinflammatory cytokine IFNg induces IDO expression in different cells and cell lines in vivo and in vitro. Monocytes, macrophages and specialized antigen presenting cells are relevant IDO expressing cells in inflammatory processes. In human periph- eral monocytes, IDO was expressed after infection with Chlamydia trachomatis (CT) [4]. This obligate intracellular bacterium causes a variety of clinically important diseases in humans such as blind- ing trachoma, acute and chronic urogenital tract infections and reactive arthritis. CT establishes productive infections in epithelial cells, which represent the natural entry port of CT into the human host. Productive infections are characterized by a unique develop- mental cycle consisting of inclusion body formation, multiplication * Corresponding author. Present address: Hannover Medical School, Department of Haematology, Haemostaseology, Oncology and Stem Cell Transplantation, Carl- Neuberg-Str.1, 30625 Hannover, Germany. Tel.: þ49 511532 9495; fax: þ49 511 532 3611. E-mail addresses: birgit.krausse@freenet.de, krausse-opatz.birgit@mh-hannover.de (B. Krausse-Opatz). Contents lists available at ScienceDirect Microbial Pathogenesis journal homepage: www.elsevier.com/locate/micpath 0882-4010/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.micpath.2009.03.001 Microbial Pathogenesis 46 (2009) 289–297