Suppression Of Antiviral Responses Research Articles

Encephalomyocarditis Virus Structural Protein VP3 Interacts with MAVS and Promotes its Autophagic Degradation to Interfere with the Type I Interferon Signaling Pathway.

Understanding the mechanisms through which interferon (IFN) signaling is negatively regulated is crucial for preserving the equilibrium of innate immune reactions, as the innate immune system functions, such as the original barrier, combat threats to the host. Although the function of the encephalomyocarditis virus (EMCV) viral proteins in antagonizing innate immunity has been related to earlier studies, the precise mechanism underlying the role of viral protein 3 (VP3) in type I IFN has yet to be fully illuminated. VP3 expression and many other adaptor molecules belonging to type I IFN pathway expression levels were evaluated using Western blotting. The IFN and other antiviral genes, such as interferon-stimulated genes (ISGs) 15 and 56, were assessed by real-time quantitative polymerase chain reaction (RT-qPCR). A 50% tissue culture infectious dose (TCID50) assay was utilized to explore the effect of VP3 on EMCV proliferation in human embryonic kidney (HEK293) cells. Co-immunoprecipitation (Co-IP) assays and confocal microscope analysis were used to investigate the underlying mechanisms mediated by VP3. We discovered that the VP3 of EMCV acts as a suppressor of innate immune reactions. Increased levels of VP3 enhance viral reproduction through modulation of innate immune signaling pathways and suppression of antiviral responses. Additional information indicated that during viral infection, the VP3 of EMCV enhances autophagy and interacts specifically with mitochondrial antiviral signaling protein (MAVS), leading to its degradation in an autophagy pathway that relies on p62. Our findings showed that EMCV developed a tactic to combat host antiviral defenses by using autophagy to break down a protein that controls the innate immune response following a viral infection of the host. Notably, VP3 plays an important role in this process. Overall, these discoveries may provide a novel therapeutic target for EMCV.

First description of natural concomitant infection of avian nephritis virus and infectious bronchitis virus reveals exacerbated inflammatory response and renal damage in broiler chicks

We describe the first report on spontaneous Avian Nephritis Virus (ANV) and Infectious Bronchitis Virus (IBV) concurrent infection in broiler chicks. On necropsy, the kidneys were found swollen with its parenchyma and ureters stuffed with urate flakes. Histopathologically, the renal tubular damage and inflammatory response were severe in concurrently infected birds compared to the cases infected only with ANV, which had direct correlation with significantly (p < 0.001) increased expression of IL-1 β, IL-4, IL-12, IL-13, iNOS and IFN-γ transcripts in the kidneys of concurrently infected birds. Relative decrease in IFN-β transcript levels in the concurrently infected birds indicates suppression of antiviral response; the iNOS level was manifold increased which can be attributed to the enhanced macrophage response. Nucleotide sequencing of S1-spike glycoprotein gene of IBV and RNA dependent RNA polymerase gene of ANV confirmed etiologies as Igacovirus of Gammacoronavirus and ANV-2 of Avastrovirus 2, respectively. Both ANV and IBV virus affect kidneys. Our findings suggested that concurrent infections of these two viruses might have enhanced the transcripts of Th1, Th2 and proinflammatory cytokines with reduced IFN-β transcripts resulting in decreased host innate antiviral mechanisms leading to exacerbated renal lesions. Future experimental co-infection studies could throw more lights on pathology and pathogenesis during concurrent infections of ANV and IBV in poultry.

Inhibition of anti-viral responses in intestinal epithelial cells by epigenetic modifying drugs is mediated by a reduction in viral pattern recognition receptor expression and activity

Background: Pattern recognition receptors form an essential part of the host defenses against pathogens, in particular in the intestinal epithelium. However, despite their importance relatively little is understood about the regulation of their expression. Increasing evidence suggesting that epigenetic mechanisms such as DNA methylation and histone acetylation have substantial effects on gene expression and regulation. Epigenetic modifying drugs are now used to treat certain cancers but not a lot is known about their effects on the innate immune system. Thus, we set out to examine the role of such drugs in the expression and function of Toll-like receptors. Methods: Using the HCT116 epithelial cell line, we determined the effects of genetic knockout of the DNA methyltransferases enzymes (DNMTs), as well as pharmacological inhibition of the DNMTs and histone deacetylase complexes (HDACs) on TLR responses to their ligands.Results: Our initial results showed that anti-viral responses were affected by changes in the epigenome, with TLR3 responses showing the most dramatic differences. We determined that inhibition of methylation and acetylation inhibited poly I:C induced increases in signaling protein phosphorylation, as well as increases in cytokine mRNA expression and release. We also observed that treatment with epigenetic modifying drugs were leading to large increases in IRF8 expression, a protein that is a known negative regulator of TLR3. When we overexpressed IRF8 in our WT cells we noticed inhibition of poly I:C responses. Conclusion: This research highlighted the potential immunoregulatory role of epigenetic modifying drugs specifically in response to viral stimulation.

NLRX1 Mediates MAVS Degradation To Attenuate the Hepatitis C Virus-Induced Innate Immune Response through PCBP2.

Activation of innate immunity is essential for host cells to restrict the spread of invading viruses and other pathogens. However, attenuation or termination of signaling is also necessary for preventing immune-mediated tissue damage and spontaneous autoimmunity. Here, we identify nucleotide binding oligomerization domain (NOD)-like receptor X1 (NLRX1) as a negative regulator of the mitochondrial antiviral signaling protein (MAVS)-mediated signaling pathway during hepatitis C virus (HCV) infection. The depletion of NLRX1 enhances the HCV-triggered activation of interferon (IFN) signaling and causes the suppression of HCV propagation in hepatocytes. NLRX1, a HCV-inducible protein, interacts with MAVS and mediates the K48-linked polyubiquitination and subsequent degradation of MAVS via the proteasomal pathway. Moreover, poly(rC) binding protein 2 (PCBP2) interacts with NLRX1 to participate in the NLRX1-induced degradation of MAVS and the inhibition of antiviral responses during HCV infection. Mutagenic analyses further revealed that the NOD of NLRX1 is essential for NLRX1 to interact with PCBP2 and subsequently induce MAVS degradation. Our study unlocks a key mechanism of the fine-tuning of innate immunity by which NLRX1 restrains the retinoic acid-inducible gene I-like receptor (RLR)-MAVS signaling cascade by recruiting PCBP2 to MAVS for inducing MAVS degradation through the proteasomal pathway. NLRX1, a negative regulator of innate immunity, is a pivotal host factor for HCV to establish persistent infection.IMPORTANCE Innate immunity needs to be tightly regulated to maximize the antiviral response and minimize immune-mediated pathology, but the underlying mechanisms are poorly understood. In this study, we report that NLRX1 is a proviral host factor for HCV infection and functions as a negative regulator of the HCV-triggered innate immune response. NLRX1 recruits PCBP2 to MAVS and induces the K48-linked polyubiquitination and degradation of MAVS, leading to the negative regulation of the IFN signaling pathway and promoting HCV infection. Overall, this study provides intriguing insights into how innate immunity is regulated during viral infection.

Alphavirus Infection: Host Cell Shut-Off and Inhibition of Antiviral Responses.

Alphaviruses cause debilitating disease in humans and animals and are transmitted by blood-feeding arthropods, typically mosquitoes. With a traditional focus on two models, Sindbis virus and Semliki Forest virus, alphavirus research has significantly intensified in the last decade partly due to the re-emergence and dramatic expansion of chikungunya virus in Asia, Europe, and the Americas. As a consequence, alphavirus–host interactions are now understood in much more molecular detail, and important novel mechanisms have been elucidated. It has become clear that alphaviruses not only cause a general host shut-off in infected vertebrate cells, but also specifically suppress different host antiviral pathways using their viral nonstructural proteins, nsP2 and nsP3. Here we review the current state of the art of alphavirus host cell shut-off of viral transcription and translation, and describe recent insights in viral subversion of interferon induction and signaling, the unfolded protein response, and stress granule assembly.

Current perspectives on dengue episode in Malaysia.

Prevalence of dengue transmission has been alarmed by an estimate of 390 million infections per annum. Urban encroachment, ecological disruption and poor sanitation are all contributory factors of increased epidemiology. Complication however arises from the fact that dengue virus inherently exists as four different serotypes. Secondary infection is often manifested in the more severe form, such that antibody-dependent enhancement (ADE) could aggravate ailment by allowing pre-existing antibodies to form complexes with infecting viruses as means of intrusion. Consequently, increased viraemic titter and suppression of antiviral response are observed. Deep concerns are thus expressed in regards to escalating trend of hospitalisation and mortality rates. In Malaysia, situation is exacerbated by improper clinical management and pending vector control operations. As a preparedness strategy against the potential deadly dengue pandemic, the call for development of a durable and cost-effective dengue vaccine against all infecting serotypes is intensified. Even though several vaccine candidates are currently being evaluated in clinical trials, uncertainties in regards to serotypes interference, incomplete protection and dose adequacy have been raised. Instead of sole reliance on outsourcing, production of local vaccine should be considered in coherent to government's efforts to combat against dengue.

The C-Terminal Domain of Chikungunya Virus nsP2 Independently Governs Viral RNA Replication, Cytopathicity, and Inhibition of Interferon Signaling

Alphavirus nonstructural protein 2 (nsP2) has pivotal roles in viral RNA replication, host cell shutoff, and inhibition of antiviral responses. Mutations that individually rendered other alphaviruses noncytopathic were introduced into chikungunya virus nsP2. Results show that (i) nsP2 mutation P718S only in combination with KR649AA or adaptive mutation D711G allowed noncytopathic replicon RNA replication, (ii) prohibiting nsP2 nuclear localization abrogates inhibition of antiviral interferon-induced JAK-STAT signaling, and (iii) nsP2 independently affects RNA replication, cytopathicity, and JAK-STAT signaling.

Influenza A(H1N1)pdm09 Virus Suppresses RIG-I Initiated Innate Antiviral Responses in the Human Lung

Influenza infection is a major cause of morbidity and mortality. Retinoic acid-inducible gene I (RIG-I) is believed to play an important role in the recognition of, and response to, influenza virus and other RNA viruses. Our study focuses on the hypothesis that pandemic H1N1/09 influenza virus alters the influenza-induced proinflammatory response and suppresses host antiviral activity. We first compared the innate response to a clinical isolate of influenza A(H1N1)pdm09 virus, OK/09, a clinical isolate of seasonal H3N2 virus, OK/06, and to a laboratory adapted seasonal H1N1 virus, PR8, using a unique human lung organ culture model. Exposure of human lung tissue to either pandemic or seasonal influenza virus resulted in infection and replication in alveolar epithelial cells. Pandemic virus induces a diminished RIG-I mRNA and antiviral cytokine response than seasonal virus in human lung. The suppression of antiviral response and RIG-I mRNA expression was confirmed at the protein level by ELISA and western blot. We performed a time course of RIG-I and interferon-β (IFN-β) mRNA induction by the two viruses. RIG-I and IFN-β induction by OK/09 was of lower amplitude and shorter duration than that caused by PR8. In contrast, the pandemic virus OK/09 caused similar induction of proinflammatory cytokines, IL-8 and IL-6, at both the transcriptional and translational level as PR8 in human lung. Differential antiviral responses did not appear to be due to a difference in cellular infectivity as immunohistochemistry showed that both viruses infected alveolar macrophages and epithelial cells. These findings show that influenza A(H1N1)pdm09 virus suppresses anti-viral immune responses in infected human lung through inhibition of viral-mediated induction of the pattern recognition receptor, RIG-I, though proinflammatory cytokine induction was unaltered. This immunosuppression of the host antiviral response by pandemic virus may have contributed to the more serious lung infections that occurred in the H1N1 pandemic of 2009.

Double-Stranded RNA Binding of Influenza B Virus Nonstructural NS1 Protein Inhibits Protein Kinase R but Is Not Essential To Antagonize Production of Alpha/Beta Interferon

Expression of alpha/beta interferon (IFN-alpha/beta) in virus-infected vertebrate cells is a key event in the establishment of a sustained antiviral response, which is triggered by double-stranded RNA (dsRNA) produced during viral replication. These antiviral cytokines initiate the expression of cellular proteins with activities that limit the replication and spread of the invading viruses. Within this response, the dsRNA-dependent protein kinase R (PKR) that is expressed at constitutive levels and upregulated by IFN-alpha/beta acts as an important antiviral effector that can block the cellular translational machinery. We previously demonstrated that efficient replication of influenza B virus depends on the viral dsRNA-binding NS1 protein that inhibits the transcriptional activation of IFN-alpha/beta genes. Here we tested the postulate that the viral NS1 protein counteracts antiviral responses through sequestering intracellular dsRNA by analyzing a collection of recombinant influenza B viruses. As expected, viruses expressing dsRNA-binding-defective NS1 proteins were strongly attenuated for replication in IFN-competent hosts. Interestingly, these virus mutants failed to prevent activation of PKR but could effectively limit IFN induction. Conversely, a mutant virus expressing the N-terminal dsRNA-binding domain of NS1 prevented PKR activation, but not IFN induction, suggesting an important role for the NS1 C-terminal part in silencing the activation route of IFN-alpha/beta genes. Thus, our findings indicate an unexpected mechanistic dichotomy of the influenza B virus NS1 protein in the suppression of antiviral responses, which involves at least one activity that is largely separable from dsRNA binding.

Suppression of antiviral responses by antibody-dependent enhancement of macrophage infection

Antibody-dependent enhancement (ADE) of macrophage and monocyte infection has been demonstrated in vitro for some of the most deadly RNA viruses known. Recent evidence suggests that ADE-mediated ligation of Fc receptors might suppress host-cell antiviral gene expression by promoting early interleukin-10 (IL-10) secretion, resulting in the expression of suppressor-of-cytokine-signalling (SOCS) proteins and a Th2 bias. These findings provide potential new insights into how ADE might enhance viral infections and exacerbate disease.