Role of host cell factors in flavivirus infection: Implications for pathogenesis and development of antiviral drugs

Abstract

The genus Flavivirus contains approximately 70 arthropod-borne enveloped RNA viruses many of which cause severe human and in some cases, animal disease. They include dengue virus, yellow fever virus, West Nile virus, Japanese encephalitis virus, and tick-borne encephalitis virus. Hundreds of thousands of deaths due to flavivirus infections occur each year, many of which are unpreventable due to lack of availability of appropriate vaccines and/or antiviral drugs.

Flaviviruses exploit the cytoplasmic cellular machinery to facilitate propagation of infectious progeny virions. They engage in dynamic and antagonistic interactions with host cell membranes and biochemical processes. Following infection, the cells initiate various antiviral strategies to counteract viral invasion. In its defense, the virus has alternative strategies to suppress these host responses to infection. The fine balance between these interactions determines the outcome of the viral infection and disease progression.

Published studies have revealed specific effects of flaviviruses on cellular processes, but the underlying mechanisms that determine the specific cytopathogenetic changes induced by different flaviviruses have not, as yet, been elucidated. Independently of the suppression of the type I IFN response which has been described in detail elsewhere, this review focuses on recent discoveries relating to alterations of host metabolism following viral infection. Such studies may contribute to new approaches to antiviral drug development. The role of host cellular factors will be examined in the context of protection and/or pathogenesis resulting from flavivirus infection, with particular emphasis on West Nile virus and dengue virus.

Introduction

Flaviviruses are a group of arboviruses belonging to the family Flaviviridae. Several members of this genus, such as dengue virus (DENV), yellow fever virus (YFV), West Nile virus (WNV), Japanese encephalitis virus (JEV) and tick-borne encephalitis virus (TBEV), are highly pathogenic for humans and constitute major international health problems (Gould and Solomon, 2008, Mackenzie et al., 2004). Many of these viruses are transmitted by mosquitoes and/or ticks leading to flaviviral diseases now global in nature. Most infected humans or animals have either asymptomatic or an undifferentiated febrile illness. However, severe and often fatal infections with haemorrhagic or encephalitic manifestation may arise during infection with DENV, or some tick-borne flaviviruses such as Kyasanur Forest disease virus or Omsk haemorrhagic fever virus. Surprisingly, despite the large number of humans that suffer severe flavivirus infections annually, there are no available antiviral therapies and only three specific vaccines are available against YFV, TBEV and JEV. Although these vaccines are very effective, in practice their utilization encounters limitations and difficulties supporting the contention that there is a real need for antiviral drugs to supplement the health control measures currently available from protective immunisation.

A major gap in our knowledge of YF is how to manage and treat sick patients infected via mosquito bites or following vaccine-associated adverse events as recently reported (Staples and Monath, 2008). In fact, treatment of the most clinically severe cases of YF by supportive care in most cases is essentially ineffective, and improvements to the vaccine's safety are being sought. Consequently, there is a clear need for safe and effective drugs to treat patients during all stages of the disease.

Despite the availability of at least four inactivated vaccines, tick-borne encephalitis virus also poses a serious and potentially increasing health problem in Western, Central and Eastern Europe (Khasnatinov et al., 2009). In fact, vaccination campaigns have had varying degrees of success, since they are relatively expensive and require repeated administration at about four yearly intervals in order to maintain protective immunity (Juceviciene et al., 2005). Therefore, with no effective therapy for TBEV, treatment is only supportive including the administration of Paracetamol, Aspirin and other nonsteroidal anti-inflammatory drugs (Mansfield et al., 2009).

Similarly, despite the fact that three different vaccines are available (Paulke-Korinek and Kollaritsch, 2008) and even though JEV is the main cause of encephalitis with about 10,000 fatal cases annually in Asia (WHO, 2007), there is no effective antiviral therapy to treat this infection. However, some reports of systemic and neurological complications have called the live attenuated vaccine's safety into question (Gould et al., 2008). The inactivated vaccines show excellent tolerability but humans in endemic areas need to be boosted every few years to ensure immunity.

In terms of morbidity and mortality, the most important flaviviruses are the four dengue virus serotypes (an estimated 2.5 billion people at risk globally which more than 70% reside in Asia Pacific countries, WHO, 2008). Nevertheless, despite their major impact on World Health, no vaccines or antiviral drugs currently exist. Consequently, a single orally administered antiviral treatment has become a major target for the immediate future.

It is therefore clear that whilst the use of flavivirus vaccines has its merits, there are many situations in which effective antiflaviviral drugs could play an important role in health management. For example, non-immune indigenous infants, tourists, visitors and temporarily employed workers from non-endemic regions are particularly vulnerable to flavivirus infections in endemic countries. Immuno-compromised or elderly and malnourished people also constitute high-risk groups. Consequently, available antiviral medication could offer an effective alternative or adjunct to vaccination, both for prophylactic treatment concerning persons localized in endemic areas and to treat severely ill patients.

Today no antiviral therapies are approved for use against flaviviruses. There is thus an urgent need for new molecules that could reduce viraemia during the early stages of infection, block viral replication in the brain in cases of encephalitis, or modulate host responses (Bray, 2008). Antiviral drug discovery protocols rely heavily on screening libraries of compounds and/or small molecules for inhibitory effects and low human toxicity when tested against viral pathogens. Whilst these methods have proven to be moderately successful to reduce the effects of HIV, and hepatitis viruses, at the moment no antiviral drug totally eradicates the infection. Therefore, new approaches are needed. Moreover, it is known that resistance development is a major obstacle to antiviral therapy, and all active antiviral agents have been shown to select for resistance mutations (Nijhuis et al., 2009). Chemotherapy against viral infections can be developed using two strategies, either by blocking virus encoded functions or by blocking the cellular functions needed for viral multiplication. This second approach has the potential complication that it may hamper normal cellular function, but it also has potential advantages in that the therapy should be active against all viruses within the same genus, and emergence of resistance against this type of chemotherapy should be relatively rare. Thus, it is now justified to consider the host cell components as potential therapeutic targets.

This review will therefore focus on the impact of flaviviruses on host cell proteins with a view to identifying potential targets for the development of antiviral drugs. The potential roles of some of these modified cellular proteins belonging to major metabolic pathways are discussed in relation to pathogenesis and the early host antiviral response. We place particular emphasis on DENV and WNV studies, highlighting how new approaches such as proteomics have improved our understanding of the molecular basis of the complex cellular physiological mechanisms associated with these infections.