MicrobiologyBytes

In response to invasion with bacterial or viral pathogens, cells are able to mount an immediate immune response through their ability to use specialized cellular molecules, referred to as pattern recognition receptors or PRRs, to detect unusual DNA, ssRNA or dsRNA structures. This leads to induction of the interferons and pro-inflammatory cytokines that are involved in the innate immune response.

Hepatitis C virus (HCV) is one of the RNA helicase RIG-I-activating viruses, because of its 5′ppp-structured RNA, 3′-structured genomic RNA and replicative RNA duplexes. In contrast to other RIG-I activating viruses such as Sendai virus, influenza, or vesicular stomatitis virus, HCV is a poor inducer of IFN and pro-inflammatory cytokines in cell culture systems. One reason for this is that the HCV NS3/4A protease cleaves cellular proteins (MAVS), resulting in a rapid disruption of the IFN induction pathway.

A recent paper shows that HCV infection can stimulate the IFN induction pathway up to 12 hrs post-infection, whereas detection of MAVS cleavage begins at 18 hrs post-infection and is maximal at 24 hrs. The data reveal that 12 hrs post-infection, HCV promotes a rapid inhibition of IFN induction at the level of translation, indicating a new mechanism of regulation. This regulation was linked to activation of the dsRNA-activated eIF2α kinase PKR. Altogether, the results show that HCV uses PKR to control the translation of newly transcribed IFN mRNAs before sufficient NS3/4A protein can be synthesized to efficiently restrain transcription of IFN.

Although PKR is also recognized in several virus infections for its antiviral proprieties, it may be more suitable to control its action in the case of HCV infection. Therefore, a carefully-controlled use of PKR inhibitors, in conjunction with IFN/ribavirin, might be beneficial for the treatment of chronically HCV-infected patients, since it would lead to a boost in the induction of innate immunity and a sustained inhibition of the virus propagation.

Hepatitis C Virus Controls Interferon Production through PKR Activation. 2010 PLoS ONE 5(5): e10575. doi:10.1371/journal.pone.0010575

Related:

• Hepatitis C Virus: a mountain to climb
• The Interferon Antiviral Response
• Update on HBV and HCV Therapy

Lipid droplets consist of a core of neutral lipids surrounded by an outer phospholipid monolayer and associated proteins. These organelles are thought to be generated when neutral lipids accumulate in the endoplasmic reticulum (ER) bilayer. Recently, lipid droplets have been shown to play a role in several cellular processes, including lipid storage, lipid trafficking, and protein storage and degradation. The importance of this organelle is underscored by the fact that lipid droplets have been linked to several metabolic diseases, most notably diabetes and obesity. Lipid droplets have also been shown to play a critical role in the replication of several pathogens. One of the most well characterized examples is hepatitis C virus (HCV).

Viperin is an interferon (IFN)-induced antiviral protein that is induced upon HCV infection and inhibits HCV replication. Like the HCV NS proteins, viperin has been shown to localize to the cytosolic face of the ER through an N-terminal amphipathic
alpha-helix. This N-terminal amphipathic
alpha-helix is essential for viperin to inhibit HCV and influenza, as mutants lacking this domain have greatly reduced antiviral activity. Although the precise mechanism by which viperin inhibits HCV is still unknown, viperin was previously shown to inhibit influenza virus budding by disrupting plasma membrane lipid raft microdomains, which are sites of influenza virion assembly and budding. Independent of viral infection, the N-terminal amphipathic
alpha-helix of viperin inhibits protein secretion and appears to induce ER membrane curvature.

Numerous questions remain about how lipid droplets are generated and used by viruses. This paper shows that the IFN-induced antiviral protein viperin, which localizes to the cytosolic face of the ER and inhibits HCV, localizes to lipid droplets. This paper shows that the N-terminal amphipathic alpha-helix of viperin that is responsible for ER localization is also necessary and sufficient to localize both viperin and the fluorescent protein dsRed to lipid droplets. Point mutations in the alpha-helix that prevent ER association also disrupt lipid droplet association, and sequential deletion mutants indicate that the same number of helical turns are necessary for ER and lipid droplet association. The N-terminal amphipathic alpha-helix of the hepatitis C viral protein NS5A can localize dsRed and viperin to lipid droplets. These findings indicate that the amphipathic alpha-helices of viperin and NS5A are lipid droplet-targeting domains and suggest that viperin inhibits HCV by localizing to lipid droplets using a domain and mechanism similar to that used by HCV itself.

The antiviral protein, viperin, localizes to lipid droplets via its N-terminal amphipathic αalpha-helix. PNAS USA November 17 2009. doi: 10.1073/pnas.0911679106

Related:

• Viperin (cig5), an IFN-inducible antiviral protein directly induced by human cytomegalovirus. 2001 PNAS USA 98(26): 15125-15130
• Viroporins
• Hepatitis C Virus: a mountain to climb
• The Interferon Antiviral Response

Chronic hepatitis B virus (HBV) infection affects about 400 million people around the globe, being one of the most common infectious diseases and among the world’s leading causes of death. Antiviral therapy of chronic hepatitis B (CHB) aims to improve quality of life and survival chance of the patients by preventing progression of liver damage to cirrhosis, end-stage liver disease and liver cancer (HCC), thus preventing anticipated liver-related death. This goal is achieved by suppression of HBV replication in a sustained or maintained manner, either by short-term “curative” treatment with standard (IFN) and pegylated interferon (Peg-IFN) or long-term “suppressive” therapy with nucleos(t)ide analogues, like lamivudine, adefovir, entecavir, telbivudine and tenofovir. Since both strategies have advantages and disadvantages, the wise treatment of a patient with CHB requires careful balance between prediction of the natural history of HBV and of the potential benefit of anti- HBV therapy. Recent data on the long-term efficacy of third generation of nucleos(t)ide analogues entecavir and tenofovir have tipped the balance towards long-term suppression therapy as the first-line option for most patients with CHB, independent of the HBeAg status.

Chronic hepatitis C is a major worldwide health problem with an estimated prevalence of 1.6-2%. In Europe, more than 9 million chronic carriers and approximately 86,000 deaths per year are estimated due to the late complications of hepatitis C virus (HCV). The prognosis of chronic hepatitis C depends on the rate of fibrosis progression, which over a 20-30 year time span, may determine the risk of developing cirrhosis and its complications, namely HCC, liver decompensation, hepatic encephalopathy and oesophageal variceal bleeding. The only therapeutic intervention able to halt this progressive process is eradication of HCV by Interferon (IFN)-based therapies. Since the empirical choice to use IFN in 1986, therapy for chronic hepatitis C has constantly evolved over the past decade, with the attainable sustained virological response (SVR) rates increasing through the years. The addition of the guanosine nucleoside analogue ribavirin (Rbv) to IFN can be considered the major breakthrough in the treatment of chronic hepatitis C. Through mechanisms of action that still remain largely unknown, Rbv has determined a greater number of patients to ultimately achieve a SVR by increasing the rates of on-treatment response and reducing the rates of post-treatment relapse. In the large phase III clinical trials designed to assess its efficacy and safety, the combination of IFN and Rbv resulted in SVR rates of 30-35% in HCV genotype 1 patients and 75-80% in HCV-2 and 3 patients. These figures exceeded by far those obtained by IFN monotherapy, effectively leading the way for combination therapy to become the standard of care in the late 1990’s. The latest innovation in the treatment of chronic hepatitis C has been the pegylation of the IFN molecule (PegIFN) through the attachment of one or more polyethylene glycols to the IFN, a process that is able to modify the immunological, pharmacokinetic and pharmacodynamic properties of the drug. Standard IFN was in fact characterized by a number of limitations, such as poor stability, short elimination half-life and potential immunogenicity, that ultimately determined its small antiviral effect. Moreover, due to the increase in elimination half-life obtained by the pegylation process, it has been possible to lengthen the dosing interval from the unpractical three times a week schedule required by standard IFN, to the more “user friendly” once a week administration, a feature that has increased convenience whilst facilitating adherence to the recommended treatment schedule. Following the demonstration of a more potent antiviral effect in terms of SVR rates in phase III randomized trials, PegIFN has become the standard of care for chronic hepatitis C.

One year of interferon therapy inhibits HBV replication in one third of the patients whereas long-term administration of oral nucleos(t)ide analogues is efficient in most of them, as long as early treatment adaptation in patients with partial virological response and resistance is provided. Following the demonstration of a more potent antiviral effect in terms of sustained virological response (SVR) rates, Pegylated-IFN coupled with Ribavirin has become the standard treatment for chronic hepatitis C, with nearly 65% of all treated patients achieving a SVR. Long-term suppression of HBV and eradication of HCV would halt the progression of chronic hepatitis to cirrhosis, hepatocellular carcinoma and liver decompensation.

HBV and HCV Therapy. Viruses 2009, 1(3), 484-509. doi:10.3390/v1030484

Related:

• Hepatitis B and C viruses
• Hepatitis C Virus: a mountain to climb
• Does the outcome of HCV infection vary with the infecting virus type?
• Plugging the holes in hepatitis C virus antiviral therapy

http://www.nhs.uk/hepatitisc/Pages/default.aspx

Related:

• Hepatitis C Virus: a mountain to climb
• A breakthrough in understanding hepatitis C virus
• Does the outcome of HCV infection vary with the infecting virus type?
• Plugging the holes in hepatitis C virus antiviral therapy

Persistent viruses such as hepatitis C virus (HCV) or HIV can defeat the body’s defense system and cause devastating epidemics worldwide. Recent attempts at vaccinating against HIV have relied on the induction of specific antiviral killer T lymphocytes but have failed to confer protection on the host. Better knowledge about how a successful defense should operate is therefore essential for developing and refining new vaccines. Researchers have recently used a mouse model to investigate basic defense mechanisms required to eliminate persisting viruses. Experiments in several genetically engineered mouse models show that contrary to common belief, not only antiviral killer T cells, but also antibodies (produced by B cells), are needed to prevent a virus from persisting in its host. These findings suggest that induction of antibodies, along with antiviral killer T lymphocytes, should be envisaged when devising new strategies for vaccinating against HIV or HCV.

Impaired antibody response causes persistence of prototypic T cell–contained virus. 2009 PLoS Biol 7(4): e1000080
CD8 T cells are recognized key players in control of persistent virus infections, but increasing evidence suggests that assistance from other immune mediators is also needed. Here, we investigated whether specific antibody responses contribute to control of lymphocytic choriomeningitis virus (LCMV), a prototypic mouse model of systemic persistent infection. Mice expressing transgenic B cell receptors of LCMV-unrelated specificity, and mice unable to produce soluble immunoglobulin M (IgM) exhibited protracted viremia or failed to resolve LCMV. Virus control depended on immunoglobulin class switch, but neither on complement cascades nor on Fc receptor c chain or Fc c receptor IIB. Cessation of viremia concurred with the emergence of viral envelope-specific antibodies, rather than with neutralizing serum activity, and even early nonneutralizing IgM impeded viral persistence. This important role for virus-specific antibodies may be similarly underappreciated in other primarily T cell–controlled infections such as HIV and hepatitis C virus, and we suggest this contribution of antibodies be given consideration in future strategies for vaccination and immunotherapy.

Related:

• Natural Regulatory T Cells and Persistent Virus Infection
• Presence of Antibodies Signals Healthier Teeth and Gums
• New Insights Could Lead to a Better Pneumococcal Vaccine

Hepatitis C virus (HCV) infection is a major cause of chronic liver disease that can lead to appreciable morbidity including cirrhosis and hepatocellular carcinoma (HCC). HCV can be classified into several genotypes based on variations in the nucleotide sequence of its genome. The infecting HCV type has been shown to be clinically important because it predicts response to antiviral therapy, with infection by type 1 being associated with the most resistance to treatment. There is no consensus as to whether differences in the clinical and histological severity of liver disease can be explained by differences in the infecting type. Several studies suggest genotype 1b to be associated with more severe disease, but most have found little or no influence of genotype on disease progression. Similar ambiguity surrounds the study of the role of HCV genotypes in spontaneous viral clearance. Some studies have found no association between the viral type and the spontaneous clearance of HCV RNA, with host factors like sex seeming to be more important, while other studies have suggested that infection by genotype 1 or 1b might be less likely to clear spontaneously when compared with infection by other genotypes.
The aim of this study was to investigate whether the HCV type might influence the clinical outcome of infection. Study serum samples were assembled from 749 individuals enrolled into the UK HCV National Register from which data on clinical outcomes were determined. The prevalence of HCV type 1 among those who cleared infection was 69% and among those who remained HCV RNA positive was 51%: type 1 infections were more likely to be HCV RNA negative than non-1 types. Type 1 infections were also more likely to be associated with histological stage scores above the median when compared with non-1 types. In conclusion, HCV type 1 infection was more often HCV RNA negative, suggesting that spontaneous clearance may occur more commonly with this type. Among the RNA-positive infections, type 1 infection may be more aggressive than types 2/3.

Does the clinical outcome of hepatitis C infection vary with the infecting hepatitis C virus type?
J Viral Hepat. 2007 14: 213-220