/ www.sciencexpress.org / 9 February 2012 / Page 1 / 10.1126/science.1215418 Pathogen-associated molecular patterns decisively influence antiviral immune responses, whereas the contribution of endogenous signals of tissue damage, also known as “damage-associated molecular patterns” or “alarmins” remains ill-defined. We show that interleukin- 33 (IL-33), an alarmin released from necrotic cells, is necessary for potent CD8+ T cell (CTL) responses to replicating, prototypic RNA and DNA viruses in mice. IL- 33 signaled through its receptor on activated CTLs, enhanced clonal expansion in a CTL-intrinsic fashion, determined plurifunctional effector cell differentiation and was necessary for virus control. Moreover, recombinant IL-33 augmented vaccine-induced CTL responses. Radio-resistant cells of the splenic T cell zone produced IL-33, and efficient CTL responses required IL- 33 from radio-resistant cells but not from hematopoietic cells. Thus, alarmin release by radio-resistant cells orchestrates protective antiviral CTL responses. Pathogen-associated molecular patterns (PAMPs) characterize intruding microorganisms and are important for adaptive immune responses to viral infection (1). Conversely, endogenous molecular patterns, which indicate tissue injury, are referred to as “alarmins” and form a second class of “damage-associated molecular patterns” (DAMPs) (2). Unlike PAMPs, the potential contribution of alarmins to antiviral immune defense remains largely elusive. Many viruses are excellent inducers of cytotoxic CD8+ T lymphocytes (CTLs) (3), the basis of which is incompletely understood. To screen for inflammatory signals augmenting antiviral CTL responses, we used lymphocytic choriomeningitis virus (LCMV) infection of mice. We performed genome-wide cDNA expression analysis of total spleen tissue from LCMV-infected mice and compared it to uninfected controls. From a large panel of interleukins and pro-inflammatory cytokines, interferon-γ (IFN-γ) and IL-33 were most upregulated (table S1). The IL-33 receptor ST2, an IL-1 receptor family member also known as T1 and IL1RL1, was also upregulated. IL-33 is expressed in the nucleus of non-hematopoietic cells such as fibroblasts, epithelial and endothelial cells of various tissues (4), but its role in antiviral CTL responses is unknown. Its bioactive pro-inflammatory form is released as a result of necrosis but not apoptosis, classifying IL-33 as an alarmin (5–7). IL-33 mRNA expression peaked at three to five days after infection and grossly paralleled the kinetics of LCMV RNA (Fig. 1A). To test whether IL-33 was important for CTL responses to LCMV, we performed infection experiments in IL-33-deficient (Il33 -/- ) mice (8). Absence of IL-33 reduced the absolute number of CTLs responding to the immunodominant LCMV epitope GP33 by >90%. The frequency of epitope-specific CTLs was reduced by >75% (Fig. 1B). When expressed as a nuclear factor in healthy cells, IL-33 is complexed with chromatin and modulates gene The Alarmin Interleukin-33 Drives Protective Antiviral CD8 + T Cell Responses Weldy V. Bonilla, 1,2 * Anja Fröhlich, 3,4 * Karin Senn, 5 * Sandra Kallert, 1,2 Marylise Fernandez, 1,2 Susan Johnson, 1,2 Mario Kreutzfeldt, 1,6 Ahmed N. Hegazy, 3,4,7 Christina Schrick, 1,6 Padraic G. Fallon, 8 Roman Klemenz, 5 Susumu Nakae, 9 Heiko Adler, 10 Doron Merkler, 1,6,11 Max Löhning, 3,4 † Daniel D. Pinschewer 1,2 † 1 Department of Pathology and Immunology, University of Geneva, 1 rue Michel Servet, 1211 Geneva 4, Switzerland. 2 World Health Organization Collaborating Center for Vaccine Immunology, University of Geneva, Switzerland. 3 Experimental Immunology, Department of Rheumatology and Clinical Immunology, Charité – University Medicine Berlin, Berlin, Germany. 4 German Rheumatism Research Center (DRFZ), a Leibniz Institute, Charitéplatz 1, 10117 Berlin, Germany. 5 Institute for Cancer Research, Department of Pathology, University Hospital of Zurich, Schmelzbergstrasse 12, 8091 Zurich, Switzerland. 6 Division of Clinical Pathology, Geneva University Hospital, 1 rue Michel Servet, 1211 Geneva 4, Switzerland. 7 Department of Gastroenterology, Hepatology and Endocrinology, CCM, Charité – University Medicine Berlin, Berlin, Germany. 8 Institute of Molecular Medicine, St. James's Hospital, Trinity College Dublin, Dublin 8, Ireland. 9 The Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, and Japan Science and Technology Agency, PRESTO 4-1-8 Hncho, Kawaguchi, Saitama 332-0012, Japan. 10 Helmholtz Zentrum München, Institute of Molecular Immunology and Clinical Cooperation Group Hematopoietic Cell Transplantation (CCG HCT), Marchioninistraße 25, 81377 München, Germany. 11 Department of Neuropathology, University Medical Center, Georg August University, Göttingen, Germany. *These authors contributed equally to this work. †These authors contributed equally to this work. To whom correspondence should be addressed. E-mail: loehning@drfz.de (M.L.); daniel.pinschewer@gmx.ch (D.P.P.) on February 13, 2012 www.sciencemag.org Downloaded from