MAVS-Mediated Apoptosis and Its Inhibition by Viral Proteins

BACKGROUND: Host responses to viral infection include both immune activation and programmed cell death. The mitochondrial antiviral signaling adaptor, MAVS (IPS-1, VISA or Cardif) is critical for host defenses to viral infection by inducing type-1 interferons (IFN-I), however its role in virus-induced apoptotic responses has not been elucidated. PRINCIPAL FINDINGS: We show that MAVS causes apoptosis independent of its function in initiating IFN-I production. MAVS-induced cell death requires mitochondrial localization, is caspase dependent, and displays hallmarks of apoptosis. Furthermore, MAVS(-/-) fibroblasts are resistant to Sendai virus-induced apoptosis. A functional screen identifies the hepatitis C virus NS3/4A and the Severe Acute Respiratory Syndrome coronavirus (SARS-CoV) nonstructural protein (NSP15) as inhibitors of MAVS-induced apoptosis, possibly as a method of immune evasion. SIGNIFICANCE: This study describes a novel role for MAVS in controlling viral infections through the induction of apoptosis, and identifies viral proteins which inhibit this host response

The IRF-3/Bax-Mediated Apoptotic Pathway, Activated by Viral Cytoplasmic RNA and DNA, Inhibits Virus Replication ▿ †

Induction of apoptosis in cells infected by Sendai virus (SeV), which triggers the cytosolic RIG-I pathway, requires the presence of interferon regulatory factor 3 (IRF-3). Independent of IRF-3's transcriptional role, a novel IRF-3 activation pathway causes its interaction with the proapoptotic protein Bax and its mitochondrial translocation to induce apoptosis. Here we report that two other RNA viruses, vesicular stomatitis virus (VSV) and encephalomyocarditis virus (EMCV), may also activate the same pathway. Moreover, cytosolic DNA, produced by adenovirus or introduced by transfection, activated the pathway in an RNA polymerase III-dependent fashion. To evaluate the contribution of this newly discovered apoptotic pathway to the host's overall antiviral response, we measured the efficiencies of replication of various viruses in vitro and viral pathogenesis in vivo, using cells and mice that are selectively deficient in components required for the apoptotic pathway of IRF-3. Our results clearly demonstrate that the IRF-3/Bax-mediated apoptotic signaling branch contributes significantly to the host's protection from viral infection and consequent pathogenesis

Interferon regulatory factor-3 is an in vivo target of DNA-PK

Eukaryotic cells have evolved complex signaling networks to sense environmental stress and to repair stress-induced damage. IFN regulatory factor-3 (IRF-3) is a transcription factor that plays a central role in the host response to viral infection. Although the main activity of IRF-3 characterized to date has been its role in the induction of IFN-α and -β after virus infection, recent evidence indicates additional roles for IRF-3 in the response to DNA damage and in virus-induced apoptosis. Here we identify IRF-3 as the first in vivo target for DNA-dependent protein kinase (DNA-PK). Phosphorylation of IRF-3 by DNA-PK after virus infection results in its nuclear retention and delayed proteolysis. These results expand the known roles of DNA-PK and provide a functional link between the cellular machineries that regulate the innate immune response and that sense and respond to DNA damage. As such this study contributes to a more integrated view of the cellular responses to various cellular stress signals

IRF-3-dependent, NFκB- and JNK-independent activation of the 561 and IFN-β genes in response to double-stranded RNA

Double-stranded (ds) RNA induces transcription of the 561 gene by activating IFN regulatory factor (IRF) transcription factors, whereas similar induction of the IFN-β gene is thought to require additional activation of NFκB and AP-1. In mutant P2.1 cells, dsRNA failed to activate NFκB, IRF-3, p38, or c-Jun N-terminal kinase, and transcription of neither 561 mRNA nor IFN-β mRNA was induced. The defect in the IRF-3 pathway was traced to a low cellular level of this protein because of its higher rate of degradation in P2.1 cells. As anticipated, in several clonal derivatives of P2.1 cells expressing different levels of transfected IRF-3, activation of IRF-3 and induction of 561 mRNA by dsRNA was restored fully, although the defects in other responses to dsRNA persisted. Surprisingly, IFN-β mRNA also was induced strongly in these cells in response to dsRNA, demonstrating that the activation of NFκB and AP-1 is not required. This conclusion was confirmed in wild-type cells overexpressing IRF-3 by blocking NFκB activation with the IκB superrepressor and AP-1 activation with a p38 inhibitor. Therefore, IRF-3 activation by dsRNA is sufficient to induce the transcription of genes with simple promoters such as 561 as well as complex promoters such as IFN-β