Skin keratinocytes provide a first line of defense against invading microorganisms in two ways: (i) by acting as a physical
barrier to pathogen entry and (ii) by initiating a vigorous innate immune response upon sensing danger signals. How keratinocytes
detect virus infections and generate antiviral immune responses is not well understood. Orthopoxviruses are dermatotropic
DNA viruses that cause lethal disease in humans. Virulence in animal models depends on the virus-encoded bifunctional Z-DNA/double-stranded
RNA (dsRNA)-binding protein E3. Here, we report that infection of mouse primary keratinocytes with a vaccinia ΔE3L mutant
virus triggers the production of beta interferon (IFN-β), interleukin-6 (IL-6), CCL4, and CCL5. None of these immune mediators
is produced by keratinocytes infected with wild-type vaccinia virus. The dsRNA-binding domain of E3 suffices to prevent activation
of the innate immune response. ΔE3L induction of IFN-β, IL-6, CCL4, and CCL5 secretion requires mitochondrial antiviral signaling
protein (MAVS; an adaptor for the cytoplasmic viral RNA sensors RIG-I and MDA5) and the transcription factor IRF3. IRF3 phosphorylation
is induced in keratinocytes infected with ΔE3L, an event that depends on MAVS. The response of keratinocytes to ΔE3L is unaffected
by genetic ablation of Toll-like receptor 3 (TLR3), TRIF, TLR9, and MyD88.
Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.
"Alternatively, it may block the activation of its downstream effector(s). We have previously shown that E3 ZBD plays an inhibitory role in TLR9/MyD88/IRF7-mediated myxoma virussensing in pDCs , whereas E3 dsRBD attenuated the cytosolic dsRNA-sensing pathway mediated by MAVS/IRF3 . It has been recently shown that ZBD of E3 antagonizes PKR in primary MEF . "
[Show abstract][Hide abstract]ABSTRACT: Modified vaccinia virus Ankara (MVA) is an attenuated poxvirus that has been engineered as a vaccine against infectious agents and cancers. Our goal is to understand how MVA modulates innate immunity in dendritic cells (DCs), which can provide insights to vaccine design. In this study, using murine bone marrow-derived dendritic cells, we assessed type I interferon (IFN) gene induction and protein secretion in response to MVA infection. We report that MVA infection elicits the production of type I IFN in murine conventional dendritic cells (cDCs), but not in plasmacytoid dendritic cells (pDCs). Transcription factors IRF3 (IFN regulatory factor 3) and IRF7, and the positive feedback loop mediated by IFNAR1 (IFN alpha/beta receptor 1), are required for the induction. MVA induction of type I IFN is fully dependent on STING (stimulator of IFN genes) and the newly discovered cytosolic DNA sensor cGAS (cyclic guanosine monophosphate-adenosine monophosphate synthase). MVA infection of cDCs triggers phosphorylation of TBK1 (Tank-binding kinase 1) and IRF3, which is abolished in the absence of cGAS and STING. Furthermore, intravenous delivery of MVA induces type I IFN in wild-type mice, but not in mice lacking STING or IRF3. Treatment of cDCs with inhibitors of endosomal and lysosomal acidification or the lysosomal enzyme Cathepsin B attenuated MVA-induced type I IFN production, indicating that lysosomal enzymatic processing of virions is important for MVA sensing. Taken together, our results demonstrate a critical role of the cGAS/STING-mediated cytosolic DNA-sensing pathway for type I IFN induction in cDCs by MVA. We present evidence that vaccinia virulence factors E3 and N1 inhibit the activation of IRF3 and the induction of IFNB gene in MVA-infected cDCs.
"Consistently, the attenuated phenotype of E3L-deleted virus following intratracheal infection of wild-type mice was partially reversed upon infection of mice lacking PKR (Rice et al. 2011). The fact that deletion of PKR does not fully rescue the pathogenicity of the E3L-deleted virus (Rice and Kerr 1984; Xiang et al. 2002) indicates that E3L likely targets other components of the cellular antiviral defense, such as the mitochondrial anti-viral signaling (MAVS) pathway for interferon induction (Deng et al. 2008 ). In support of this idea, the E3L-deleted virus was pathogenic in mice lacking both PKR and the anti-viral endoribonuclease RNAse L (Rice et al. 2011). "
[Show abstract][Hide abstract]ABSTRACT: Responding to viral infection, the interferon-induced, double-stranded RNA (dsRNA)-activated protein kinase PKR phosphorylates translation initiation factor eIF2α to inhibit cellular and viral protein synthesis. To overcome this host defense mechanism, many poxviruses express the protein E3L, containing an N-terminal Z-DNA binding (Zα) domain and a C-terminal dsRNA-binding domain (dsRBD). While E3L is thought to inhibit PKR activation by sequestering dsRNA activators and by directly binding the kinase, the role of the Zα domain in PKR inhibition remains unclear. Here, we show that the E3L Zα domain is required to suppress the growth-inhibitory properties associated with expression of human PKR in yeast, to inhibit PKR kinase activity in vitro, and to reverse the inhibitory effects of PKR on reporter gene expression in mammalian cells treated with dsRNA. Whereas previous studies revealed that the Z-DNA binding activity of E3L is critical for viral pathogenesis, we identified point mutations in E3L that functionally uncouple Z-DNA binding and PKR inhibition. Thus, our studies reveal a molecular distinction between the nucleic acid binding and PKR inhibitory functions of the E3L Zα domain, and they support the notion that E3L contributes to viral pathogenesis by targeting PKR and other components of the cellular anti-viral defense pathway.
"In respect to pathogens, several viral components are known to regulate innate immunity. NS1 of the influenza A virus (7), E3 of the vaccinia virus (8), and VP39 of the ebola virus (9) bind to viral dsRNA and interfere the binding with TLR3. NS3-4A (10) of the hepatitis C virus inhibits TLR3 signaling by degrading Toll-interleukin 1 receptor domain-containing adapter inducing interferon-β (TRIF). "
[Show abstract][Hide abstract]ABSTRACT: Toll-like receptor 3 (TLR3) recognizes double-stranded RNA (dsRNA) and induces inflammation. In this study we attempted to ascertain if there are endogenous host molecules controlling the production of cytokines and chemokines. Two candidates, ribosomal protein L19 and L22, were analyzed to determine if they influence cytokine production followed by TLR3 activation. In this study we report that L19 acts upon production of IP-10 or IL-8 differently in glioblastoma cells.
L19 or L22 was transfected into HEK293-TLR3, A549 or A172 cells. After treatment with several inhibitors of NF-kB, PI3K, p38 or ERK, production of IL-8 or IP-10 was measured by ELISA. siRNA was introduced to suppress expression of L19. After Vesicular stomatitis virus infection, viral multiplication was measured by western blot.
L19 increased ERK activation to produce IL-8. In A172 cells, in which TLR3 is expressed at endosomes, L19 inhibited interferon regulatory factor 3 (IRF3) activation and IP-10 production to facilitate viral multiplication, whereas L19 inhibited viral multiplication in A549 cells bearing TLR3 on their cell membrane.
Our results suggest that L19 regulates TLR3 signaling, which is cell type specific and may be involved in pathogenesis of autoimmune diseases and chronic inflammatory diseases.