IFN-a Is Constitutively Expressed in the Human Thymus, but Not in Peripheral Lymphoid Organs Arnaud D. Colantonio1¤, Marta Epeldegui1, Maria Jesiak1, Loes Jachimowski5, Bianca Blom5, Christel H. Uittenbogaart1,2,3,4* 1 Department of Microbiology, Immunology, and Molecular Genetics, David E. Geffen School of Medicine, University of California Los Angeles, Los Angeles, California, United States of America, 2 Department of Pediatrics, David E. Geffen School of Medicine, University of California Los Angeles, Los Angeles, California, United States of America, 3 UCLA AIDS Institute, David E. Geffen School of Medicine, University of California Los Angeles, Los Angeles, California, United States of America, 4 Jonsson Comprehensive Cancer Center, David E. Geffen School of Medicine, University of California Los Angeles, Los Angeles, California, United States of America, 5 Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands Abstract Type I interferons have been typically studied for their effects in the context of bacterial or viral infections. However in this report, we provide evidence that Interferon-alpha (IFN-a) expressing cells are present in the thymus in the absence of infection. We show that pDC express the highest level of IFN-a and that MxA, which is exclusively expressed after engagement of the type I IFN receptor by IFN-a/b, is expressed in normal fetal and post-natal thymus, but not in the periphery. The highest level of MxA is expressed in mature thymocytes and pDC located in the medulla and at the cortico- medullary junction. The anti-microbial peptide LL-37, which is expressed in the thymus, when complexed with eukaryotic nucleic acids, induces the secretion of IFN-a by thymic pDC. This results in the upregulation of MxA expression in responsive thymocytes. We propose that the secretion of IFN-a in the thymus may function to regulate the rate of T cell development and modulate the requirements for the selection of developing T cells. Citation: Colantonio AD, Epeldegui M, Jesiak M, Jachimowski L, Blom B, et al. (2011) IFN-a Is Constitutively Expressed in the Human Thymus, but Not in Peripheral Lymphoid Organs. PLoS ONE 6(8): e24252. doi:10.1371/journal.pone.0024252 Editor: Derya Unutmaz, New York University, United States of America Received July 4, 2011; Accepted August 5, 2011; Published August 31, 2011 Copyright: 2011 Colantonio 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 National Institutes of Health (AI 080564, AI 52002, and AI 28697 (University of California Los Angeles, Center for AIDS Research), a Dissertation Award from the University of California: Universitywide AIDS Research Program (UARP) currently known as California HIV/ AIDS Research Program (to AC), and National Institutes of Health Training grants T32-AR-053463 and T32-CA-009120 (to ME). 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: uittenbo@ucla.edu ¤ Current address: Department of Microbiology and Molecular Genetics, Harvard Medical School, New England Primate Research Center, Southborough, Massachusetts Introduction Type I interferons (IFN) are immunomodulatory cytokines that function to alert cells to the presence of pathogens [1]. Antiviral activity of type I interferons is mediated by the expression of interferon stimulated genes (ISG), which is dependent on signaling through the IFN-a receptor (reviewed in[2]). IFN-a receptor signaling leads to phosphorylation of STAT1/2 and results in the expression of interferon regulatory factor 7 (IRF-7), required for the transcription of downstream ISG. Upregulation of ISG prevents the spread of viral infection through several mechanisms including the specific degradation of viral gene products, inhibition of protein translation, and ultimately apoptotic cell death. One ISG, Myxovirus resistance A (MxA) has been linked with resistance to viral infection [3,4,5]. MxA protein inhibits the viral life cycle at three distinct steps, including nucleocapsid transport to the nucleus [6], transcription of viral gene products [7], or viral assembly [8]. Expression of this particular ISG is tightly regulated and only expressed when IFN-a is secreted [9]. Expression of MxA has been widely utilized as a bio-marker for secreted IFN-a/b in both viral and bacterial infections [10,11,12]. In addition to antiviral effects, type I interferons are known to have immunoregulatory activities, such as suppression of murine T and B cell development in vivo [13,14,15]. We previously showed that IFN-a suppresses the development of human T cells in vitro by inhibiting early steps of T cell development [16]. Thus, in addition to its antiviral effects, IFN-a may play a regulatory role in the thymus. We previously identified IFN-a positive cells in normal thymus tissue in the SCID-hu mouse model [17]. However, both the nature of the IFN-a expressing cells and the stimulus that induced IFN-a remained elusive. The purpose of the current study is to further characterize IFN-a expressing cells in normal thymus tissue, compare these cells to those found in peripheral lymphoid tissues, and to examine the trigger for IFN-a production in the absence of infection. Although every white blood cell has the ability to produce IFN- a, plasmacytoid dendritic cells (pDC) are the highest producers of type I IFN. In the thymus pDC are located in the thymus medulla [18,19] and play a role in the induction of regulatory T cells [20,21]. The primary function of peripheral blood and peripheral lymphoid pDC is to secrete large amounts of IFN-a/b in response to viral and bacterial infection [18,22,23,24,25,26,27,28]. pDC sense infection via the expression of the Toll like receptors (TLR) - 7 and -9, which bind single stranded RNA and hypomethylated CpG DNA motifs, respectively. While thymic pDC show some immunophenotypic differences compared to peripheral pDC [19], PLoS ONE | www.plosone.org 1 August 2011 | Volume 6 | Issue 8 | e24252