A Dedicated Promoter Drives Constitutive Expression of the Cell-Autonomous Immune Resistance GTPase, Irga6 (IIGP1) in Mouse Liver Jia Zeng¤a, Iana Angelova Parvanova¤b, Jonathan C. Howard* Institute for Genetics of the University of Cologne, Cologne, Germany Abstract Background: In general, immune effector molecules are induced by infection. Methodology and Principal Findings: However, strong constitutive expression of the cell-autonomous resistance GTPase, Irga6 (IIGP1), was found in mouse liver, contrasting with previous evidence that expression of this protein is exclusively dependent on induction by IFNc. Constitutive and IFNc-inducible expression of Irga6 in the liver were shown to be dependent on transcription initiated from two independent untranslated 59 exons, which splice alternatively into the long exon encoding the full-length protein sequence. Irga6 is expressed constitutively in freshly isolated hepatocytes and is competent in these cells to accumulate on the parasitophorous vacuole membrane of infecting Toxoplasma gondii tachyzoites. Conclusions and Significance: The role of constitutive hepatocyte expression of Irga6 in resistance to parasites invading from the gut via the hepatic portal system is discussed. Citation: Zeng J, Parvanova IA, Howard JC (2009) A Dedicated Promoter Drives Constitutive Expression of the Cell-Autonomous Immune Resistance GTPase, Irga6 (IIGP1) in Mouse Liver. PLoS ONE 4(8): e6787. doi:10.1371/journal.pone.0006787 Editor: Laurent Re´nia, BMSI-A*STAR, Singapore Received May 16, 2009; Accepted July 16, 2009; Published August 26, 2009 Copyright: 2009 Zeng et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by grants from the Deutsche Forschungsgemeinschaft (SPP1110, SFB670, SFB680). Jia Zeng received a stipend from the International Graduate School for Genetics and Functional Genomics from the Land Nordrhein-Westfalen and the University of Cologne. Iana Parvanova was supported in part by a stipend from the Graduiertenkolleg ‘‘Genetik Zellula¨rer Systeme’’ from the Deutsche Forschungsgemeinschaft. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: j.howard@uni-koeln.de ¤a Current address: Leeds Institute of Molecular Medicine, Section of Experimental Therapeutics, St James’s University Hospital, Leeds, United Kingdom ¤b Current address: Division of Molecular Pathobiology, Department of Clinical Research and VPH, Vetsuisse Faculty Bern, Bern, Switzerland Introduction In general, immune resistance genes show low rates of constitutive transcription, and rely on signals generated via infection for their transcriptional induction. The most powerful mediator of elevated transcription of resistance genes is interferon (IFN) c, secreted early in the response to pathogens by natural killer (NK) and NKT cells and later by T helper 1 (Th1) and cytotoxic T cells (TC1) of the adaptive immune system [1]. The immunity-related GTPase (IRG) family of GTPases, responsible for cell-autonomous immunity against a variety of intracellular pathogens [2,3], has seemed to be no exception to this generalization. In our own report of the identification of several IRG proteins [4] we documented strong induction of these proteins in the liver 24 hours after infection with the strongly immunostimulatory pathogen, Listeria monocytogenes, and showed that this induction critically depended on the presence of the IFNc-receptor. Similarly, IRG proteins were strongly induced by IFNc in tissue culture cell lines [4,5]. Similar conclusions were reached in a histological study of the BALB/c mouse spleen during the course of infection with Listeria monocytogenes, where Irga6 (IIGP1) was seen to be induced in several cell types shortly after infection, but no expression was reported before infection [6]. Nevertheless, there are several reports on constitutive expression of members of the IRG family in mice. Taylor reported constitutive expression of Irgm3 (IGTP) in the thymus [7], with similar results noted for Irgb6 (TGTP), while there have been independent reports of expression of IRG proteins in haemopoietic stem cells [8,9]. In none of these cases was it clear whether the apparently constitutive expression was in fact due to local IFNc expression of unknown cause in these lymphomyeloid tissues, rather than to truly IFNc-independent expression. In this report we show that Irga6, contrary to earlier conclusions, is in fact strongly and constitutively expressed in mouse liver. Furthermore, this expression is independent of IFN signalling and is driven by a typical hepatocyte-specific promoter upstream of a dedicated 59-untranslated exon (exon 1B). Irga6 is however, also strongly induced by IFNc, as correctly reported earlier, and this is due to a second, IFN-specific promoter upstream of a second, dedicated 59 exon (exon 1A). These results provide a rationalization of our earlier observations on the transcript structure of Irga6, where we noted that the EST database contained clones with identical coding sequences but different 59-untranslated (59-UT) sequences consistent with alternative 59 exon usage, and were able to locate the two corresponding exons upstream of the coding exon (exon 2) [10]. PLoS ONE | www.plosone.org 1 August 2009 | Volume 4 | Issue 8 | e6787