Downloaded from www.microbiologyresearch.org by IP: 54.70.40.11 On: Fri, 11 Jan 2019 03:52:04 Short Communication Plasmacytoid dendritic cells and Toll-like receptor 7-dependent signalling promote efficient protection of mice against highly virulent influenza A virus Michael M. Kaminski, Annette Ohnemus, Marius Cornitescu and Peter Staeheli Correspondence Peter Staeheli peter.staeheli@uniklinik-freiburg. de Received 23 October 2011 Accepted 12 December 2011 Department of Virology, University of Freiburg, D-79104 Freiburg, Germany Types I and III interferons (IFNs) elicit protective antiviral immune responses during influenza virus infection. Although many cell types can synthesize IFN in response to virus infection, it remains unclear which IFN sources contribute to antiviral protection in vivo. We found that mice carrying functional alleles of the Mx1 influenza virus resistance gene partially lost resistance to infection with a highly pathogenic H7N7 influenza A virus strain if Toll-like receptor 7 (TLR7) signalling was compromised. This effect was achieved by deleting either the TLR7 gene or the gene encoding the TLR7 adaptor molecule MyD88. A similar decrease of influenza virus resistance was observed when animals were deprived of plasmacytoid dendritic cells (pDCs) at day 1 post-infection. Our results provide in vivo proof that pDCs contribute to the protection of the lung against influenza A virus infections, presumably via signals from TLR7. Previous work with mice lacking functional receptors for type I (a/b) or type III (l) interferon (IFN) illustrated the importance of these virus-induced cytokines in host resistance against influenza viruses (Ank et al., 2008; Mordstein et al., 2008; Mu ¨ller et al., 1994). IFN-mediated protection against influenza virus is particularly pro- nounced in mice carrying functional alleles of the Mx1 resistance gene. The Mx1 gene is defective in most standard laboratory mouse strains which, as a consequence, fail to mount a complete antiviral immune response and develop a high degree of resistance against influenza viruses (Staeheli et al., 1988). The Mx1 gene codes for a nuclear 72 kDa protein that interferes with the influenza virus multiplication cycle at a very early step (Haller et al., 2009; Staeheli et al., 1993). Its expression is strongly dependent on activating signals either from type I or type III IFN receptor complex (Holzinger et al., 2007; Mordstein et al., 2008). Although most nucleated cells can synthesize IFN in response to viral infection, the various cell types differ dramatically with regard to the magnitude by which viral infections can trigger the synthesis of IFN. For example, plasmacytoid dendritic cells (pDCs) are professional IFN-producing cells that represent the major source of circulating IFN during infection with viruses such as murine cytomegalovirus (Asselin-Paturel et al., 2001; Cervantes-Barragan et al., 2007; Tabeta et al., 2004), mouse hepatitis virus (Cervantes-Barragan et al., 2007), respiratory syncytial virus (Smit et al., 2006) and vesicular stomatitis virus (Barchet et al., 2002; Diebold et al., 2004; Lund et al., 2004). During some other viral infections, including infections with influenza A virus, the contri- bution of pDCs to IFN synthesis in the infected host appears to be small (Jewell et al., 2007) or even negligible (GeurtsvanKessel et al., 2008; Wolf et al., 2009). pDCs strongly express Toll-like receptors (TLR) 7, 8 and 9 through which these cells can efficiently sense virus- derived nucleic acids. TLR7 recognizes ssRNA in the extracellular space. With the help of the adaptor molecule MyD88, ligand-activated TLR7 can trigger a signalling cascade in pDCs that ultimately results in vigorous activation of type I and type III IFN genes (Akira & Takeda, 2004). Conflicting reports are available regarding the role of pDCs and the TLR7/MyD88 signalling pathway during infection with influenza viruses. Ex vivo studies demonstrated that pDCs use the TLR7 complex for sensing influenza viruses (Diebold et al., 2004). However, MyD88-deficient mice were either found (Seo et al., 2010) or not found (Koyama et al., 2007; Le Goffic et al., 2006) to exhibit enhanced influenza susceptibility compared with wild-type mice. Further, pDC depletion experiments failed to reveal a substantial contribution of these cells to influenza virus resistance (GeurtsvanKessel et al., 2008; Wolf et al., 2009). These studies were all carried out with standard Mx1- negative laboratory mice that do not develop robust resistance to influenza virus infection. We reasoned that a clearer picture might emerge from similar studies in Mx1-positive mice. To test this hypothesis, we first introduced the defective MyD88 allele of Mx1-negative C57BL/6 mice (Adachi et al., 1998) into our Mx1-positive B6.A2G-Mx1 mouse Journal of General Virology (2012), 93, 555–559 DOI 10.1099/vir.0.039065-0 039065 G 2012 SGM Printed in Great Britain 555