A synergistic interferon-c production is induced by mouse hepatitis virus in interleukin-12 (IL-12)/IL-18-activated natural killer cells and modulated by carcinoembryonic antigen-related cell adhesion molecules (CEACAM) 1a receptor Introduction Natural killer (NK) cells are an important arm of innate immunity against virus-infected cells, bacteria and tumour cells, and exert direct cytotoxicity functions as well as indirect antiviral functions through the secretion of interferon-c (IFN-c). 1 The role of intrahepatic NK cells in the outcome of human viral hepatitis is not fully understood. NK cells are known to act as a first line to control many viral infections such as herpes simplex virus type 1, Epstein–Barr virus, human herpesvirus 6 and murine cytomegalovirus. 2,3 However, human hepatitis C virus has developed strategies to evade the detection and elimination by NK cells, 4 as is also suggested by the increased numbers of NK cells in the livers of patients during the immunotolerance phase of chronic hepatitis B virus infection. 5 The liver is enriched in NK and NKT cells, which play major roles in the con- trol of viral hepatitis infections by limiting viral replication via an IFN-c-dependent mechanism. Recently, several studies have demonstrated the importance of the cytotoxic activities of NK cells in preventing the establishment of chronic human hepatitis C virus infections. 6,7 Natural killer cells are rapidly recruited from the bone marrow and spleen during viral infections. 8 Interleukin-12 (IL-12), IL-15, IL-18 and IFN-c enhance the cytotoxic activity of NK cells and further increase the production of IFN-c by these cells. 1,2 Moreover, the production of IFN-c by NK cells stimulated with IL-12 and IL-18 is dependent on the activation of the immunoreceptor tyrosine-based activation motifs (ITAMs) and is regulated through the p38 and extracellular signal-regulated kinase-1/2 mitogen- activated protein kinase (ERK-1/2 MAPK) pathways. 9,10 However, little is known about the mechanism involved in the efficient production of antiviral IFN-c by intrahepatic NK cells during the acute phase of viral hepatitis. Mouse hepatitis virus (MHV) is an excellent model for studying the immunological disorders associated with viral Alexandre Jacques, 1 Christian Bleau, 1 Claire Turbide, 2 Nicole Beauchemin 2 and Lucie Lamontagne 1 1 De´partement des Sciences Biologiques, Universite ´ du Que ´bec a ` Montre ´al, and 2 McGill Cancer Centre, McGill University, Montre ´al, QC, Canada doi:10.1111/j.1365-2567.2008.03030.x Received 23 October 2008; revised 19 November 2008; accepted 1 December 2008. Correspondence: Dr L. Lamontagne, De ´partement des Sciences Biologiques, Universite ´ du Que ´bec a ` Montre ´al, C.P. 8888 Succ. Centre-Ville, Montre ´al, QC, Canada H3C 3P8 Email: lamontagne.lucie@uqam.ca Senior author: Lucie Lamontagne Summary The production of interferon-c (IFN-c) by infiltrating natural killer (NK) cells in liver is involved in the control of mouse hepatitis virus (MHV) infection. The objectives of this study were to identify the mechanisms used by MHV type 3 to modulate the production of IFN-c by NK cells during the acute hepatitis in susceptible C57BL/6 mice. Ex vivo and in vitro experiments revealed that NK cells, expressing carcinoembryonic antigen-related cell adhesion molecules (CEACAM) 1a (the MHV recep- tor), can produce a higher level of IFN-c in the presence of both L2- MHV3 and interleukin-12 (IL-12)/IL-18. The synergistic production of IFN-c by NK cells depends on viral replication rather than viral fixation only, because it is inhibited or not induced in cells infected with ultra- violet-inactivated viruses and in cells from Ceacam1a )/) mice infected with virulent viruses. The synergistic IFN-c production involves the p38 mitogen-activated protein kinase (MAPK) rather than the extracellular signal-regulated kinase-1/2 MAPK signalling pathway. However, the signal triggered through the engagement of CEACAM1a decreases the produc- tion of IFN-c, when these molecules are cross-linked using specific mono- clonal antibodies. These results suggest that control of acute hepatitis by IFN-c-producing NK cells may depend on both production of IL-12 and IL-18 in the liver environment and viral infection of NK cells. Keywords: CEACAM1a; coronavirus; hepatitis; interferon-c; interleukin-12/ interleukin-18; mitogen-activated protein kinase; natural killer cells Please cite this article in press as: Jacques A. et al. A synergistic interferon-c production is induced by mouse hepatitis virus in interleukin-12 (IL-12)/IL-18-activated natural killer cells and modulated by carcinoembryonic antigen-related cell adhesion molecules (CEACAM) 1a receptor, Immunology (2009) doi: 10.1111/j.1365-2567.2008.03030.x Ó 2009 Blackwell Publishing Ltd, Immunology, 128, e551–e561 e551 IMMUNOLOGY ORIGINAL ARTICLE