Flaviviridae Viruses and Oxidative Stress: Implications for Viral Pathogenesis.

Oxidative stress is induced once the balance of generation and neutralization of reactive oxygen species (ROS) is broken in the cell, and it plays crucial roles in a variety of natural and diseased processes. Infections of Flaviviridae viruses trigger oxidative stress, which affects both the cellular metabolism and the life cycle of the viruses. Oxidative stress associated with specific viral proteins, experimental culture systems, and patient infections, as well as its correlations with the viral pathogenesis attracts much research attention. In this review, we primarily focus on hepatitis C virus (HCV), dengue virus (DENV), Zika virus (ZIKV), Japanese encephalitis virus (JEV), West Nile virus (WNV), and tick-borne encephalitis virus (TBEV) as representatives of Flaviviridae viruses and we summarize the mechanisms involved in the relevance of oxidative stress for virus-associated pathogenesis. We discuss the current understanding of the pathogenic mechanisms of oxidative stress induced by Flaviviridae viruses and highlight the relevance of autophagy and DNA damage in the life cycle of viruses. Understanding the crosstalk between viral infection and oxidative stress-induced molecular events may offer new avenues for antiviral therapeutics.

Regulated and liver-specific tamarin alpha interferon gene delivery by a helper-dependent adenoviral vector.

Gene therapy approaches based on liver-restricted and regulated alpha interferon (IFN-alpha) expression, recently shown to be effective in different murine hepatitis models, appear promising alternatives to inhibit hepatitis C virus (HCV) replication in patients and minimize side effects. Tamarins (Saguinus species) infected by GB virus B (GBV-B) are considered a valid surrogate model for hepatitis C to study the biology of HCV infection and the development of new antiviral drugs. To test the efficacy of local delivery and expression of IFN-alpha in this model, we have developed HD-TET-tIFN, a helper-dependent adenovirus vector expressing tamarin IFN-alpha (tIFN) under the control of the tetracycline-inducible transactivator rtTA2s-S2. Expression of tIFN was successfully induced both in vitro and in vivo in rodents by doxycycline administration with consequent activation of IFN-responsive genes. More importantly, tIFN efficiently inhibited GBV-B replicon in a Huh-7 hepatoma cell line at low HD-TET-tIFN doses. A certain degree of transcriptional control of tIFN was achieved in tamarins injected with HD-TET-tIFN, but under the conditions used in this study, infection and replication of GBV-B were only delayed and not totally abrogated upon virus challenge. Hepatic delivery and regulated expression of IFN-alpha appear to be a possible approach for the cure of hepatitis, but this approach requires more studies to increase its efficacy. To our knowledge, this is the first report showing a regulated gene expression in a nonhuman primate hepatitis model.

Hepatitis C virus infection and the brain.

While HCV was initially believed to uniformly cause liver inflammation with the consequence of liver cirrhosis in most of the infected patients, prospective studies have shown a much lower than expected rate of cirrhosis in patients infected for more than 20 years. However, a new problem associated with hepatitis C virus infection is emerging. This is the development of sometimes disabling fatigue. While many other viruses of the flaviviridae cause encephalitis, the most closely related virus to HCV in humans, the GB Virus C seems not to be associated with fatigue. Thus the mechanism for the development of fatigue in HCV infection seems specific for HCV. Delineating the mechanism will be a first step to develop treatment option for this currently untreatable impairment.

Hepatitis C virus: clades and properties.

Hepatitis C virus (HCV) is a member of the virus family Flaviviridae. At present HCV is classified into a discrete hepacivirus genus and is represented by six clades according to genome sequencing. Each clade is further divisible into subtypes, which may prove important for the study of clinical differences and epidemiological studies. Limited homology also exists with hepatitis G/GB viruses, despite the fact that the hepatotropic nature of the latter agents remains contentious. The variability amongst the six HCV clades is less than that observed between the four serotypes of dengue, suggesting that each clade may represent a distinct virus were tests such as plaque neutralization to become available for delineating HCV isolates. The distribution worldwide varies, with Clades 1 and 2 predominating in most regions-an important consideration for the development of any vaccine. In addition, the clade distribution among cohorts may vary according to age. Point source outbreaks of HCV, for example in large numbers of women inadvertently infected with HCV-contaminated anti-D globulin, offers an opportunity to study the evolution of HCV genotypes over several decades. Parallel studies in chimpanzees have shown that the hypervariable region of E2 may play a role in HCV immunity, with quasispecies rapidly replacing the predominant subtype as immunity develops to the initiating virus strain. There is some evidence that an IFN-sensitive motif exists in the NS5 gene which may have some predictive value in determining the likely outcome of IFN treatment. A database is available for all HCV sequences, together with information about their properties and guidance for the evaluation of new isolates (http://s2as02.genes.nig.ac.jp).

Perspectives for the treatment of infections with Flaviviridae.

The family Flaviviridae contains three genera: Hepacivirus, Flavivirus, and Pestivirus. Worldwide, more than 170 million people are chronically infected with Hepatitis C virus and are at risk of developing cirrhosis and/or liver cancer. In addition, infections with arthropod-borne flaviviruses (such as dengue fever, Japanese encephalitis, tick-borne encephalitis, St. Louis encephalitis, Murray Valley encephalitis, West Nile, and yellow fever viruses) are emerging throughout the world. The pestiviruses have a serious impact on livestock. Unfortunately, no specific antiviral therapy is available for the treatment or the prevention of infections with members of the Flaviviridae. Ongoing research has identified possible targets for inhibition, including binding of the virus to the cell, uptake of the virus into the cell, the internal ribosome entry site of hepaciviruses and pestiviruses, the capping mechanism of flaviviruses, the viral proteases, the viral RNA-dependent RNA polymerase, and the viral helicase. In light of recent developments, the prevalence of infections caused by these viruses, the disease spectrum, and the impact of infections, different strategies that could be pursued to specifically inhibit viral targets and animal models that are available to study the pathogenesis and antiviral strategies are reviewed.

The first Flaviviridae symposium.

The first annual Flaviviridae Symposium, sponsored by ICN Pharmaceuticals Inc., was held in Lyon, France, on 9 October, 1997, to communicate current understanding on the Flaviviridae. This multidisciplinary symposium attracted over 300 international delegates and presentations covered virology, viral pathogenesis, potential therapies and strategies for vaccine development. The symposium reviewed the research area that may lead to the discovery and design of human and veterinary medicines against members of this virus family.