Viral Mediated Redirection of NEMO/IKKγ to Autophagosomes Curtails the Inflammatory Cascade

et al. (2012) Viral Mediated Redirection of NEMO/IKKc to Autophagosomes Curtails the Inflammatory Cascade. PLoS Pathog 8(2): e1002517. doi:10.1371/journal.ppat.1002517 Viral Mediated Redirection of NEMO/IKKc to Autophagosomes Curtails the Inflammatory Cascade Patricia M. Fliss 0 1 Tali Pechenick Jowers 0 1 Melanie M. Brinkmann 0 1 Barbara Holstermann 0 1 Claudia Mack 0 1 Paul Dickinson 0 1 Heinrich Hohenberg 0 1 Peter Ghazal 0 1 Wolfram Brune 0 1 Blossom Damania, University of North Carolina at Chapel Hill, United States of America 0 Current address: Department of Dermatology, University Medical Center Freiburg , Freiburg , Germany 1 1 Heinrich Pette Institute, Leibniz Institute for Experimental Virology , Hamburg, Germany , 2 Division of Viral Infections, Robert Koch Institute , Berlin, Germany , 3 Division of Pathway Medicine, Centre for Infectious Diseases, University of Edinburgh Medical School , Edinburgh , United Kingdom , 4 Helmholtz Center for Infection Research , Braunschweig , Germany The early host response to viral infections involves transient activation of pattern recognition receptors leading to an induction of inflammatory cytokines such as interleukin-1b (IL-1b) and tumor necrosis factor a (TNFa). Subsequent activation of cytokine receptors in an autocrine and paracrine manner results in an inflammatory cascade. The precise mechanisms by which viruses avert an inflammatory cascade are incompletely understood. Nuclear factor (NF)-kB is a central regulator of the inflammatory signaling cascade that is controlled by inhibitor of NF-kB (IkB) proteins and the IkB kinase (IKK) complex. In this study we show that murine cytomegalovirus inhibits the inflammatory cascade by blocking Toll-like receptor (TLR) and IL-1 receptor-dependent NF-kB activation. Inhibition occurs through an interaction of the viral M45 protein with the NF-kB essential modulator (NEMO), the regulatory subunit of the IKK complex. M45 induces proteasome-independent degradation of NEMO by targeting NEMO to autophagosomes for subsequent degradation in lysosomes. We propose that the selective and irreversible degradation of a central regulatory protein by autophagy represents a new viral strategy to dampen the inflammatory response. - Transcription factor NF-kB activates the expression of numerous target genes, most of which are involved in regulating innate and adaptive immune responses [1,2]. It is activated in response to a variety of stimuli, which include pathogen-associated molecular patterns (PAMPs) and proinflammatory cytokines, such as TNFa and IL-1b. While TNFa and IL-1b activate their specific receptors at the cell surface, PAMPs are recognized by so-called patternrecognition receptors (PRRs) located at the cell surface, within endosomal membranes, or the cytosol [3]. The best characterized PRRs are the TLRs, a family of transmembrane proteins that recognize PAMPs at the cell surface or within endosomes [4]. They detect a broad range of PAMPs originating from viruses, bacteria or fungi. For instance, TLR2 and 4 are typically activated by bacterial peptidoglycans and lipopolysacharide (LPS), respectively. However, they can also be activated by certain viral glycoproteins [5]. Other TLRs, such as TLR3, 7, and 9, recognize double- or single-stranded RNA or unmethylated DNA of viral or bacterial origin [6]. The NF-kB activation pathways emanating from IL-1 receptor (IL-1R), TNF receptor 1 (TNFR1), and PRRs such as the TLRs are similar and overlapping (Figure S1). In all these pathways, NFkB activity is controlled by inhibitory IkB proteins, of which IkBa is the best-characterized, and by the IkB kinase (IKK) complex. The IKK complex consists of two catalytic subunits, IKKa and b [7], and the essential regulatory subunit, IKKc, which is more commonly referred to as NEMO (NF-kB essential modulator) [8]. NEMO acts as a scaffold protein for the IKK complex and mediates interactions with upstream signaling molecules such as RIP1 and IRAK1 [9,10]. Upon activation, the IKK complex phosphorylates IkBa, resulting in a rapid ubiquitylation and proteasomal degradation of IkBa. By this means, NF-kB is released from its inhibitor, translocates to the nucleus, and activates transcription of proinflammatory cytokines, chemokines, and antiapoptotic and antimicrobial proteins [11,12]. During viral infection the first wave of proinflammatory cytokine production is induced by PRRs upon virus recognition [5,13]. This immediate and transient response is sustained and further potentiated by cytokines, such as TNFa and IL-1b, which activate their cognate receptors in an autocrine and paracrine manner (Figure S1). This allows for local signal amplification as well as systemic signal broadcasting beyond the original site of infection [14,15]. The ensuing inflammatory cascade further promotes vascular endothelial permeability and infiltration of leukocytes to the site of infection, and is key to both pathogen elimination and tissue healing [16]. Cytomegaloviruses are large DNA viruses of the herpesvirus family that are highly prevalent and cause lifelong infections in their respective host populations [17]. They induce a broad Upon viral infection cells immediately induce an innate immune response which involves the production of inflammatory cytokines. These cytokines activate specific receptors on infected and surrounding cells leading to local signal amplification as well as signal broadcasting beyond the original site of infection. Inflammatory cytokine production depends on transcription factor NFkB, whose activity is controlled by a kinase complex that includes the NF-kB essential modulator (NEMO). In order to replicate and spread in their hosts, viruses have evolved numerous strategies to counteract innate immune defenses. In this study we identify a highly effective viral strategy to blunt the host inflammatory response: The murine cytomegalovirus M45 protein binds to NEMO and redirects it to autophagosomes, vesicular structures that deliver cytoplasmic constituents to lysosomes for degradation and recycling. By this means, the virus installs a sustained block to all classical NF-kB activation pathways, which include signaling cascades originating from pattern recognition receptors and inflammatory cytokine receptors. Redirection of an essential component of the host cell defense machinery to the autophagic degradation pathway is a previously unrecognized viral immune evasion strategy whose principle is likely shared by other pathogens. spectrum of innate and adaptive immune responses including the production of cytokines, induction of programmed cell death, and priming of T lymphocytes, but have also evolved various strategies to modulate these antiviral host responses. Co-evolution of these viruses with their hosts resulted in a dynamic equilibrium between the host immune response and viral immune evasion strategies [18]. However, when this equilibrium gets out of balance, as it is the case (...truncated)