Infection with Langat Flavivirus or expression of the envelope protein induces apoptotic cell death

Flaviviruses: Innate Immunity, Inflammasome Activation, Inflammatory Cell Death, and Cytokines

The innate immune system is the host’s first line of defense against the invasion of pathogens including flavivirus. The programmed cell death controlled by genes plays an irreplaceable role in resisting pathogen invasion and preventing pathogen infection. However, the inflammatory cell death, which can trigger the overflow of a large number of pro-inflammatory cytokines and cell contents, will initiate a severe inflammatory response. In this review, we summarized the current understanding of the innate immune response, inflammatory cell death pathway and cytokine secretion regulation during Dengue virus, West Nile virus, Zika virus, Japanese encephalitis virus and other flavivirus infections. We also discussed the impact of these flavivirus and viral proteins on these biological processes. This not only provides a scientific basis for elucidating the pathogenesis of flavivirus, but also lays the foundation for the development of effective antiviral therapies.

The Dual Regulation of Apoptosis by Flavivirus

Apoptosis is a form of programmed cell death, which maintains cellular homeostasis by eliminating pathogen-infected cells. It contains three signaling pathways: death receptor pathway, mitochondria-mediated pathway, and endoplasmic reticulum pathway. Its importance in host defenses is highlighted by the observation that many viruses evade, hinder or destroy apoptosis, thereby weakening the host’s immune response. Flaviviruses such as Dengue virus, Japanese encephalitis virus, and West Nile virus utilize various strategies to activate or inhibit cell apoptosis. This article reviews the research progress of apoptosis mechanism during flaviviruses infection, including flaviviruses proteins and subgenomic flaviviral RNA to regulate apoptosis by interacting with host proteins, as well as various signaling pathways involved in flaviviruses-induced apoptosis, which provides a scientific basis for understanding the pathogenesis of flaviviruses and helps in developing an effective antiviral therapy.

Immunity to TBEV Related Flaviviruses with Reduced Pathogenicity Protects Mice from Disease but Not from TBEV Entry into the CNS

Tick-borne encephalitis virus (TBEV) is a leading cause of vector-borne viral encephalitis with expanding endemic regions across Europe. In this study we tested in mice the efficacy of preinfection with a closely related low-virulent flavivirus, Langat virus (LGTV strain TP21), or a naturally avirulent TBEV strain (TBEV-280) in providing protection against lethal infection with the highly virulent TBEV strain (referred to as TBEV-Hypr). We show that prior infection with TP21 or TBEV-280 is efficient in protecting mice from lethal TBEV-Hypr challenge. Histopathological analysis of brains from nonimmunized mice revealed neuronal TBEV infection and necrosis. Neuroinflammation, gliosis, and neuronal necrosis was however also observed in some of the TP21 and TBEV-280 preinfected mice although at reduced frequency as compared to the nonimmunized TBEV-Hypr infected mice. qPCR detected the presence of viral RNA in the CNS of both TP21 and TBEV-280 immunized mice after TBEV-Hypr challenge, but significantly reduced compared to mock-immunized mice. Our results indicate that although TBEV-Hypr infection is effectively controlled in the periphery upon immunization with low-virulent LGTV or naturally avirulent TBEV 280, it may still enter the CNS of these animals. These findings contribute to our understanding of causes for vaccine failure in individuals vaccinated with TBE vaccines.

Exposure to Dengue Envelope Protein Domain III Induces Nlrp3 Inflammasome-Dependent Endothelial Dysfunction and Hemorrhage in Mice

Typically occurring during secondary dengue virus (DENV) infections, dengue hemorrhagic fever (DHF) causes abnormal immune responses, as well as endothelial vascular dysfunction, for which the responsible viral factor remains unclear. During peak viremia, the plasma levels of virion-associated envelope protein domain III (EIII) increases to a point at which cell death is sufficiently induced in megakaryocytes in vitro. Thus, EIII may constitute a virulence factor for endothelial damage. In this study, we examined endothelial cell death induced by treatment with DENV and EIII in vitro. Notably, pyroptosis, the major type of endothelial cell death observed, was attenuated through treatment with Nlrp3 inflammasome inhibitors. EIII injection effectively induced endothelial abnormalities, and sequential injection of EIII and DENV-NS1 autoantibodies induced further vascular damage, liver dysfunction, thrombocytopenia, and hemorrhage, which are typical manifestations in DHF. Under the same treatments, pathophysiological changes in the Nlrp3 inflammasome–deficient mice were notably reduced compared with those in the wild-type mice. These results suggest that the Nlrp3 inflammasome constitutes a potential therapeutic target for treating DENV-induced hemorrhage in DHF.

Mosquito Cell-Derived Japanese Encephalitis Virus-Like Particles Induce Specific Humoral and Cellular Immune Responses in Mice

The Japanese encephalitis virus (JEV) is the major cause of an acute encephalitis syndrome in many Asian countries, despite the fact that an effective vaccine has been developed. Virus-like particles (VLPs) are self-assembled multi-subunit protein structures which possess specific epitope antigenicities related to corresponding native viruses. These properties mean that VLPs are considered safe antigens that can be used in clinical applications. In this study, we developed a novel baculovirus/mosquito (BacMos) expression system which potentially enables the scalable production of JEV genotype III (GIII) VLPs (which are secreted from mosquito cells). The mosquito-cell-derived JEV VLPs comprised 30-nm spherical particles as well as precursor membrane protein (prM) and envelope (E) proteins with densities that ranged from 30% to 55% across a sucrose gradient. We used IgM antibody-capture enzyme-linked immunosorbent assays to assess the resemblance between VLPs and authentic virions and thereby characterized the epitope specific antigenicity of VLPs. VLP immunization was found to elicit a specific immune response toward a balanced IgG2a/IgG1 ratio. This response effectively neutralized both JEV GI and GIII and elicited a mixed Th1/Th2 response in mice. This study supports the development of mosquito cell-derived JEV VLPs to serve as candidate vaccines against JEV.

Exome-wide search and functional annotation of genes associated in patients with severe tick-borne encephalitis in a Russian population

Background

Tick-borne encephalitis (TBE) is a viral infectious disease caused by tick-borne encephalitis virus (TBEV). TBEV infection is responsible for a variety of clinical manifestations ranging from mild fever to severe neurological illness. Genetic factors involved in the host response to TBEV that may potentially play a role in the severity of the disease are still poorly understood. In this study, using whole-exome sequencing, we aimed to identify genetic variants and genes associated with severe forms of TBE as well as biological pathways through which the identified variants may influence the severity of the disease.

Results

Whole-exome sequencing data analysis was performed on 22 Russian patients with severe forms of TBE and 17 Russian individuals from the control group. We identified 2407 candidate genes harboring rare, potentially pathogenic variants in exomes of patients with TBE and not containing any rare, potentially pathogenic variants in exomes of individuals from the control group. According to DAVID tool, this set of 2407 genes was enriched with genes involved in extracellular matrix proteoglycans pathway and genes encoding proteins located at the cell periphery. A total of 154 genes/proteins from these functional groups have been shown to be involved in protein-protein interactions (PPIs) with the known candidate genes/proteins extracted from TBEVHostDB database. By ranking these genes according to the number of rare harmful minor alleles, we identified two genes (MSR1 and LMO7), harboring five minor alleles, and three genes (FLNA, PALLD, PKD1) harboring four minor alleles.

When considering genes harboring genetic variants associated with severe forms of TBE at the suggestive P-value < 0.01, 46 genes containing harmful variants were identified. Out of these 46 genes, eight (MAP4, WDFY4, ACTRT2, KLHL25, MAP2K3, MBD1, OR10J1, and OR2T34) were additionally found among genes containing rare pathogenic variants identified in patients with TBE; and five genes (WDFY4,ALK, MAP4, BNIPL, EPPK1) were found to encode proteins that are involved in PPIs with proteins encoded by genes from TBEVHostDB. Three genes out of five (MAP4, EPPK1, ALK) were found to encode proteins located at cell periphery.

Conclusions

Whole-exome sequencing followed by systems biology approach enabled to identify eight candidate genes (MAP4, WDFY4, ACTRT2, KLHL25, MAP2K3, MBD1, OR10J1, and OR2T34) that can potentially determine predisposition to severe forms of TBE. Analyses of the genetic risk factors for severe forms of TBE revealed a significant enrichment with genes controlling extracellular matrix proteoglycans pathway as well as genes encoding components of cell periphery.

Electronic supplementary material

The online version of this article (10.1186/s12920-019-0503-x) contains supplementary material, which is available to authorized users.

The distribution of important sero-complexes of flaviviruses in Malaysia

Flaviviruses (FVs) are arthropod-borne viruses of medical and veterinary importance. Numerous species of FVs have been isolated from various host; mainly humans, animals, ticks, and mosquitoes. Certain FVs are extremely host-specific; at the same time, some FVs can infect an extensive range of species. Based on published literatures, 11 species of FVs have been detected from diverse host species in Malaysia. In humans, dengue virus and Japanese encephalitis virus have been reported since 1901 and 1942. In animals, the Batu Cave virus, Sitiawan virus, Carey Island, Tembusu virus, Duck Tembusu virus, and Japanese encephalitis viruses were isolated from various species. In mosquitoes, Japanese encephalitis virus and Kunjin virus were isolated from Culex spp., while Zika virus and Jugra virus were isolated from Aedes spp. In ticks, the Langat virus was isolated from Ixodes spp. One of the major challenges in the diagnosis of FVs is the presence of sero-complexes as a result of cross-reactivity with one or more FV species. Subsequently, the distribution of specific FVs among humans and animals in a specific population is problematic to assess and often require comprehensive and thorough analyses. Molecular assays such as quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and digital droplet RT-PCR (ddRT-PCR) have been used for the differentiation of flavivirus infections to increase the accuracy of epidemiological data for disease surveillance, monitoring, and control. In situations where sero-complexes are common in FVs, even sensitive assays such as qRT-pCR can produce false positive results. In this write up, an overview of the various FV sero-complexes reported in Malaysia to date and the challenges faced in diagnosis of FV infections are presented.

Subgenomic flaviviral RNAs: What do we know after the first decade of research

The common feature of flaviviral infection is the accumulation of abundant virus-derived noncoding RNA, named flaviviral subgenomic RNA (sfRNA) in infected cells. This RNA represents a product of incomplete degradation of viral genomic RNA by the cellular 5'-3' exoribonuclease XRN1 that stalls at the conserved highly structured elements in the 3' untranslated region (UTR). This mechanism of sfRNA generation was discovered a decade ago and since then sfRNA has been a focus of intense research. The ability of flaviviruses to produce sfRNA was shown to be evolutionary conserved in all members of Flavivirus genus. Mutations in the 3'UTR that affect production of sfRNAs and their interactions with host factors showed that sfRNAs are responsible for viral pathogenicity, host adaptation, and emergence of new pathogenic strains. RNA structural elements required for XRN1 stalling have been elucidated and the role of sfRNAs in inhibiting host antiviral responses in arthropod and vertebrate hosts has been demonstrated. Some molecular mechanisms determining these properties of sfRNA have been recently characterized, while other aspects of sfRNA functions remain an open avenue for future research. In this review we summarise the current state of knowledge on the mechanisms of generation and functional roles of sfRNAs in the life cycle of flaviviruses and highlight the gaps in our knowledge to be addressed in the future.

Viral Determinants of Virulence in Tick-Borne Flaviviruses

Tick-borne flaviviruses have a global distribution and cause significant human disease, including encephalitis and hemorrhagic fever, and often result in neurologic sequelae. There are two distinct properties that determine the neuropathogenesis of a virus. The ability to invade the central nervous system (CNS) is referred to as the neuroinvasiveness of the agent, while the ability to infect and damage cells within the CNS is referred to as its neurovirulence. Examination of laboratory variants, cDNA clones, natural isolates with varying pathogenicity, and virally encoded immune evasion strategies have contributed extensively to our understanding of these properties. Here we will review the major viral determinants of virulence that contribute to pathogenesis and influence both neuroinvasiveness and neurovirulence properties of tick-borne flaviviruses, focusing particularly on the envelope protein (E), nonstructural protein 5 (NS5), and the 3′ untranslated region (UTR).

Regulation of Apoptosis during Flavivirus Infection

Apoptosis is a type of programmed cell death that regulates cellular homeostasis by removing damaged or unnecessary cells. Its importance in host defenses is highlighted by the observation that many viruses evade, obstruct, or subvert apoptosis, thereby blunting the host immune response. Infection with Flaviviruses such as Japanese encephalitis virus (JEV), Dengue virus (DENV) and West Nile virus (WNV) has been shown to activate several signaling pathways such as endoplasmic reticulum (ER)-stress and AKT/PI3K pathway, resulting in activation or suppression of apoptosis in virus-infected cells. On the other hands, expression of some viral proteins induces or protects apoptosis. There is a discrepancy between induction and suppression of apoptosis during flavivirus infection because the experimental situation may be different, and strong links between apoptosis and other types of cell death such as necrosis may make it more difficult. In this paper, we review the effects of apoptosis on viral propagation and pathogenesis during infection with flaviviruses.

Dual Function of Ccr5 during Langat Virus Encephalitis: Reduction in Neutrophil-Mediated Central Nervous System Inflammation and Increase in T Cell-Mediated Viral Clearance

Tick-borne encephalitis virus (TBEV) is a vector-transmitted flavivirus that causes potentially fatal neurologic infection. There are thousands of cases reported annually, and despite the availability of an effective vaccine, the incidence of TBEV is increasing worldwide. Importantly, up to 30% of affected individuals develop long-term neurologic sequelae. We investigated the role of chemokine receptor Ccr5 in a mouse model of TBEV infection using the naturally attenuated tick-borne flavivirus Langat virus (LGTV). Ccr5-deficient mice presented with an increase in viral replication within the CNS and decreased survival during LGTV encephalitis compared with wild-type controls. This enhanced susceptibility was due to the temporal lag in lymphocyte migration into the CNS. Adoptive transfer of wild-type T cells, but not Ccr5-deficient T cells, significantly improved survival outcome in LGTV-infected Ccr5-deficient mice. Concomitantly, a significant increase in neutrophil migration into the CNS in LGTV-infected Ccr5(-/-) mice was documented at the late stage of infection. Ab-mediated depletion of neutrophils in Ccr5(-/-) mice resulted in a significant improvement in mortality, a decrease in viral load, and a decrease in overall tissue damage in the CNS compared with isotype control-treated mice. Ccr5 is crucial in directing T cells toward the LGTV-infected brain, as well as in suppressing neutrophil-mediated inflammation within the CNS.

Type I Interferon response in olfactory bulb, the site of tick-borne flavivirus accumulation, is primarily regulated by IPS-1

BACKGROUND

Although type I interferons (IFNs)-key effectors of antiviral innate immunity are known to be induced via different pattern recognition receptors (PRRs), the cellular source and the relative contribution of different PRRs in host protection against viral infection is often unclear. IPS-1 is a downstream adaptor for retinoid-inducible gene I (RIG-I)-like receptor signaling. In this study, we investigate the relative contribution of IPS-1 in the innate immune response in the different brain regions during infection with tick-borne encephalitis virus (TBEV), a flavivirus that causes a variety of severe symptoms like hemorrhagic fevers, encephalitis, and meningitis in the human host.

METHODS

IPS-1 knockout mice were infected with TBEV/Langat virus (LGTV), and viral burden in the peripheral and the central nervous systems, type I IFN induction, brain infiltrating cells, and inflammatory response was analyzed.

RESULTS

We show that IPS-1 is indispensable for controlling TBEV and LGTV infections in the peripheral and central nervous system. Our data indicate that IPS-1 regulates neuropathogenicity in mice. IFN response is differentially regulated in distinct regions of the central nervous system (CNS) influencing viral tropism, as LGTV replication was mainly restricted to olfactory bulb in wild-type (WT) mice. In contrast to the other brain regions, IFN upregulation in the olfactory bulb was dependent on IPS-1 signaling. IPS-1 regulates basal levels of antiviral interferon-stimulated genes (ISGs) like viperin and IRF-1 which contributes to the establishment of early viral replication which inhibits STAT1 activation. This diminishes the antiviral response even in the presence of high IFN-β levels. Consequently, the absence of IPS-1 causes uncontrolled virus replication, in turn resulting in apoptosis, activation of microglia and astrocytes, elevated proinflammatory response, and recruitment of inflammatory cells into the CNS.

CONCLUSIONS

We show that LGTV replication is restricted to the olfactory bulb and that IPS-1 is a very important player in the olfactory bulb in shaping the innate immune response by inhibiting early viral replication and viral spread throughout the central nervous system. In the absence of IPS-1, higher viral replication leads to the evasion of antiviral response by inhibiting interferon signaling. Our data suggest that the local microenvironment of distinct brain regions is critical to determine virus permissiveness.

The role of viral persistence in flavivirus biology

In nature, vector borne flaviviruses are persistently cycled between either the tick or mosquito vector and small mammals such as rodents, skunks, and swine. These viruses account for considerable human morbidity and mortality worldwide. Increasing and substantial evidence of viral persistence in humans, which includes the isolation of RNA by RT PCR and infectious virus by culture, continues to be reported. Viral persistence can also be established in vitro in various human, animal, arachnid, and insect cell lines in culture. Although some research has focused on the potential roles of defective virus particles, evasion of the immune response through the manipulation of autophagy and/or apoptosis, the precise mechanism of flavivirus persistence is still not well understood. We propose additional research for further understanding of how viral persistence is established in different systems. Avenues for additional studies include determining whether the multifunctional flavivirus protein NS5 has a role in viral persistence, the development of relevant animal models of viral persistence, and investigating the host responses that allow vector borne flavivirus replication without detrimental effects on infected cells. Such studies might shed more light on the viral–host relationships and could be used to unravel the mechanisms for establishment of persistence.

Persistent infections by vector borne flaviviruses are an important, but inadequately studied topic.

The NS3 protease and helicase domains of Japanese encephalitis virus trigger cell death via caspase‑dependent and ‑independent pathways

Japanese encephalitis virus (JEV), a mosquito‑borne flavivirus, causes acute encephalitis and nervous damage. Previous studies have demonstrated that JEV induces apoptosis in infected cells. However, to date the mechanisms of JEV‑induced apoptosis are unclear. In order to identify the viral proteins associated with JEV‑induced apoptosis, pEGFP‑non‑structural protein 3 (NS3) 1‑619 (expressing the JEV NS3 intact protein, including the protease and helicase domains), pEGFP‑NS3 1‑180 (expressing the protease domain) and pEGFP‑NS3 163‑619 (expressing the helicase domain) were transfected into target cells to study cell death. Results demonstrate that the JEV NS3 intact protein and protease and helicase domains induce cell death. In addition, cell death was identified to be significantly higher in cells transfected with the NS3 protease domain compared with the intact protein and helicase domain. Caspase activation was also analyzed in the current study. NS3 intact protein and NS3 protease and helicase domains activated caspase‑9/‑3‑dependent and ‑independent pathways. However, caspase‑8 activity was not found to be significantly different in NS3‑transfected cells compared with control. In summary, the present study demonstrates that the NS3 helicase and protease domains of JEV activate caspase‑9/‑3‑dependent and ‑independent cascades and trigger cell death.

Molecular characterization of Drosophila cells persistently infected with Flock House virus

Little is known about the molecular determinants causing and sustaining viral persistent infections at the cellular level. We found that Drosophila cells persistently infected (PI) with Flock House virus (FHV) invariably harbor defective viral RNAs, which are replicated by the FHV RNA-dependent RNA polymerase. Some defective RNAs encoded a functional B2 protein, the FHV suppressor of RNA interference, which might contribute to maintenance of virus persistence. Viral small interfering RNAs (vsiRNAs) of both polarities were detected in PI cells and primarily mapped to regions of the viral genome that were preserved in the isolated defective RNAs. This indicated that defective RNAs could represent major sources of vsiRNAs. Immunofluorescence analysis revealed that mitochondria and viral proteins are differentially distributed in PI cells and lytically infected cells, which may partly explain the reduction in infectious viral progeny. Our results provide a basis for further investigations of the molecular mechanisms underlying persistent infections.

Identification of cellular proteome modifications in response to West Nile virus infection

Flaviviruses are positive-stranded RNA viruses that are a public health problem because of their widespread distribution and their ability to cause a variety of diseases in humans. West Nile virus is a mosquito-borne member of this genus and is the etiologic agent of West Nile encephalitis. Clinical manifestations of West Nile virus infection are diverse, and their pathogenic mechanisms depend on complex virus-cell interactions. In the present work, we used proteomics technology to analyze early Vero cell response to West Nile infection. The differential proteomes were resolved 24 h postinfection using two-dimensional DIGE followed by mass spectrometry identification. Quantitative analysis (at least 2-fold quantitative alteration, p < 0.05) revealed 127 differentially expressed proteins with 68 up-regulated proteins and 59 down-regulated proteins of which 93 were successfully identified. The implication for mammalian cellular responses to this neurotropic flavivirus infection was analyzed and made possible more comprehensive characterization of the virus-host interactions involved in pathogenesis. The present study thus provides large scale protein-related information that should be useful for understanding how the host metabolism is modified by West Nile infection and for identifying new potential targets for antiviral therapy.

Persistent infections of mammals and mammalian cell cultures with West Nile virus

Before 1990, West Nile virus (WNV) was considered to be one of many arthropod-borne viruses that caused mild febrile illness in man. However, in the 1990s, the virus was associated with severe CNS disease that produced mortality in horses and man in Europe. In 1999, WNV was identified as the etiologic agent of an outbreak of human and avian encephalitis in New York City (NY, USA). Like many other Flaviviridae family members, WNV is generally considered to cause acute infections, however, persistent WNV infections have been observed in laboratory-infected animals and in human patients. These persistent infections could be facilitated by changes to the viral genome that allow the virus to evade detection by the host cell, a property that has been studied in cell culture. This review highlights our current knowledge of persistent WNV infections in vitro and in vivo, and speculates on how persistence could influence virus transmission.

Caspase 3-dependent cell death of neurons contributes to the pathogenesis of West Nile virus encephalitis

West Nile virus (WNV) is a neurotropic, arthropod-borne flavivirus that has become a significant global cause of viral encephalitis. To examine the mechanisms of WNV-induced neuronal death and the importance of apoptosis in pathogenesis, we evaluated the role of a key apoptotic regulator, caspase 3. WNV infection induced caspase 3 activation and apoptosis in the brains of wild-type mice. Notably, congenic caspase 3(-/-) mice were more resistant to lethal WNV infection, although there were no significant differences in the tissue viral burdens or the kinetics of viral spread. Instead, decreased neuronal death was observed in the cerebral cortices, brain stems, and cerebella of caspase 3(-/-) mice. Analogously, primary central nervous system (CNS)-derived neurons demonstrated caspase 3 activation and apoptosis after WNV infection, and treatment with caspase inhibitors or a genetic deficiency in caspase 3 significantly decreased virus-induced death. These studies establish that caspase 3-dependent apoptosis contributes to the pathogenesis of lethal WNV encephalitis and suggest possible novel therapeutic targets to restrict CNS injury.

Defective interfering RNAs of Japanese encephalitis virus found in mosquito cells and correlation with persistent infection

Defective interfering (DI) RNAs are deletion mutants of viral genomes that are known in many cases to contribute to persistent infection and modification of viral pathogenesis. Cell type also plays a critical role in the establishment of viral persistence. In this study we have identified for the first time the generation of DI RNAs of Japanese encephalitis virus in C6/36 mosquito cells. A persistent infection was established by replacing growth medium on surviving cells and continued cell passaging. Persistent infection was demonstrated by a continual release of infectious virus, fluorescent antibody staining, and Northern analysis. A population of DI RNAs of approximately 8.2-9.7 kb, not detectable in acutely infected cells, became apparent in the persistently infected cells by 25 days postinfection. Sequence analyses revealed a population of DI RNAs that contained in-frame deletions of 1.3-2.8 kb covering the region of the E gene and some flanking C or prM and NS1 gene sequences. Transcripts from one cDNA clone of a DI RNA replicated in uninfected mosquito cells as demonstrated by RT-PCR. DI RNA-containing virions in supernatant fluids from persistently infected mosquito cells could be used to establish persistent infection in BHK-21 cells. The correlation of DI RNA presence with cell survival suggests that DI RNAs are contributing mechanistically to the establishment of persistent infection in both the mosquito and mammalian cells.

Programmed cell death induced by Japanese encephalitis virus YL vaccine strain or its recombinant envelope protein in varied cultured cells

OBJECTIVE

The Japanese encephalitis virus YL vaccine strain (JEV-YL) was investigated as regards its organ tropism and the role of recombinant envelope glycoprotein in the induction of apoptosis was explored.

METHODS

Threevaried cell lines (HepG2, Vero and C6) were infected with JEV-YL or transfected with eukaryotic expression plasmids (pcE, pcF1R2, pcF1R1 and pcF2R2) which contain different parts of the envelope gene and phenotypic properties were examined by flow cytometry and DNA fragmentation analysis.

RESULTS

After JEV-YL infection, smaller plaque was produced on HepG2 cells than on Vero cells, whereas no cytopathic effect was observed on C6 cells; moreover, by apoptosis and DNA fragmentation assays, the hallmark cytopathic effects were detected in HepG2 and Vero cells but not in C6 cells. Furthermore, cells used in our study transfected with recombinant core plasmid, pcE, which include full-length E gene but the deleted forms (pcF1R2, pcF1R1 and pcF2R2) did not have similar results as JEV-YLs.

CONCLUSIONS

The JEV-YL vaccine strain had changed cell tropism to liver cells different from other virulent strains which have neural tropism, and in this study we proved that the transient-expressed entire E protein of JEV-YL could induce apoptosis and the mutations of E protein may change the organ tropism of JEV-YL.

Development of effective therapies against West Nile virus infection

Since its entry into North America in 1999, West Nile virus has spread throughout the USA and Canada, and now annually causes a clinical spectrum of human disease ranging from a self-limiting acute febrile illness to potentially lethal encephalitis. Although no therapy is currently approved for use in humans, several strategies are being pursued to develop effective prophylaxis and treatments. This review describes the epidemiology, clinical presentation and pathogenesis of West Nile virus infection, and highlights recent progress towards an effective therapy.

Host cell killing by the West Nile Virus NS2B-NS3 proteolytic complex: NS3 alone is sufficient to recruit caspase-8-based apoptotic pathway

The West Nile Virus (WNV) non-structural proteins 2B and 3 (NS2B-NS3) constitute the proteolytic complex that mediates the cleavage and processing of the viral polyprotein. NS3 recruits NS2B and NS5 proteins to direct protease and replication activities. In an effort to investigate the biology of the viral protease, we cloned cDNA encoding the NS2B-NS3 proteolytic complex from brain tissue of a WNV-infected dead crow, collected from the Lower Merion area (Merion strain). Expression of the NS2B-NS3 gene cassette induced apoptosis within 48 h of transfection. Electron microscopic analysis of NS2B-NS3-transfected cells revealed ultra-structural changes that are typical of apoptotic cells including membrane blebbing, nuclear disintegration and cytoplasmic vacuolations. The role of NS3 or NS2B in contributing to host cell apoptosis was examined. NS3 alone triggers the apoptotic pathways involving caspases-8 and -3. Experimental results from the use of caspase-specific inhibitors and caspase-8 siRNA demonstrated that the activation of caspase-8 was essential to initiate apoptotic signaling in NS3-expressing cells. Downstream of caspase-3 activation, we observed nuclear membrane ruptures and cleavage of the DNA-repair enzyme, PARP in NS3-expressing cells. Nuclear herniations due to NS3 expression were absent in the cells treated with a caspase-3 inhibitor. Expression of protease and helicase domains themselves was sufficient to trigger apoptosis generating insight into the apoptotic pathways triggered by NS3 from WNV.

Adaptation of West Nile virus replicons to cells in culture and use of replicon-bearing cells to probe antiviral action

Flaviviruses are emerging threats to public health worldwide. Recently, one flavivirus, West Nile virus (WNV), has caused the largest epidemic of viral encephalitis in US history. Like other flaviviruses, WNV is thought to cause a persistent infection in insect cells, but an acute cytopathic infection of mammalian cells. To study adaptation of WNV to persistently replicate in cell culture and generate a system capable of detecting antiviral compounds in the absence of live virus, we generated subgenomic replicons of WNV and adapted these to persistently replicate in mammalian cells. Here we report that adaptation of these replicons to cell culture results in a reduction of genome copy number, and demonstrate that hamster, monkey, and human cells that stably carry the replicons can be used as surrogates to detect the activity of anti-WNV compounds. Additionally, we have used these cells to investigate the interaction of WNV genomes with interferon (IFN). These studies demonstrated that IFN can cure cells of replicons and that replicon-bearing cells display lower responses to IFN than their IFN-cured derivatives.

The NS3 protein of hepatitis C virus induces caspase-8-mediated apoptosis independent of its protease or helicase activities

Apoptosis has been implicated in the pathogenesis of hepatitis C virus (HCV)-related disease. Here, we show that expression of HCV NS3, or the NS2/NS3 precursor protein, in mammalian cells results in induction of apoptosis and activation of caspases. HCV NS3-induced apoptosis was blocked by a caspase-8, but not a caspase-9-specific inhibitor. HCV NS3 coimmunoprecipitated with caspase-8, but not with other caspases or with FADD. Coexpression of HCV NS3 and caspase-8 resulted in aggregation of the caspase in punctate structures that colocalized with HCV NS3. Cell lines stably expressing low levels HCV NS3 showed increased sensitivity to Fas-induced cell death. Point mutations of NS3 showed that the pro-apoptotic function of the protein is distinct from its protease and helicase activities. These findings suggest that HCV NS3 promotes caspase-8 induced apoptosis at a pathway site distal to FADD, and that flavivirus NS3 may represent a new class of pro-apoptotic proteins.

Expression of hepatitis C virus envelope protein 2 induces apoptosis in cultured mammalian cells

AIM: To explore the role of hepatitis C virus (HCV) envelope protein 2 (E2) in the induction of apoptosis.

METHODS: A carboxyterminal truncated E2 (E2-661) was transiently expressed in several cultured mammalian cell lines or stably expressed in Chinese hamster ovary (CHO) cell line. Cell proliferation was assessed by ³H thymidine uptake. Apoptosis was examined by Hoechst 33258 staining, flow cytometry and DNA fragmentation analysis.

RESULTS: Reduced proliferation was readily observed in the E2-661 expressing cells. These cells manifested the typical features of apoptosis, including cell shrinkage, chromatin condensation and hypodiploid genomic DNA content. Similar apoptotic cell death was observed in an E2-661 stably expressing cell line.

CONCLUSION: HCV E2 can induce apoptosis in cultured mammalian cells.

Tick-borne flaviviruses

Tick-borne encephalitis (TBE), one of the most dangerous neuroinfections in Europe and Asia, is caused by tick-borne encephalitis virus (TBEV) and currently involves approximately 11,000 human cases annually, mostly in Russia. This chapter describes the main problems associated with the epidemiology, ecology, pathogenesis, and control of this disease. We have attempted to review the factors that influence the incidence and distribution of TBE, and to discuss possible reasons for the different clinical manifestations including most commonly observed asymptomatic infections, fever forms, acute encephalitis, and the less frequently registered biphasic milk fever and chronic encephalitis. Epidemiologic data concerning the other tick-borne flaviviruses, namely Louping ill virus, Langat virus, and Powassan virus that also produce encephalitis on a smaller scale, are also presented. Here we describe the history and current epidemiological role of Omsk hemorrhagic fever virus and Kyasanur forest disease virus, two viruses that are genetically closely related to TBEV, but produce hemorrhagic fever instead of encephalitis, and provide possible explanations for these differences. The other viruses in the tick-borne flavivirus group are also included despite the fact that they do not play an essential epidemiologic role in humans. This chapter contains a brief history of vaccination against TBE including the trials with live attenuated vaccine and reviews the modern trends in development of vaccine virus strains.

Mechanisms of dengue virus-induced cell death

The outcome of virus infection depends on viral and host factors. The interactions between flaviviruses and their target cells must be investigated if we are to understood the pathogenicity of these RNA viruses. Host cells are thought to respond to viral infection by initiation of apoptotic cell death. Apoptosis is an active process of cellular self-destruction with distinctive morphological and biochemical features. There is mounting evidence that dengue (DEN) virus can trigger the host cell to undergo apoptosis in a cell-dependent manner. Virally induced apoptosis contributes directly to the cytopathogenic effects of DEN virus in cultured cells. The induction of apoptosis involves the activation of intracellular signaling systems. Although the underlying molecular processes that trigger apoptosis are not well characterized, our knowledge regarding the cellular mechanisms and viral determinants of the outcome of DEN virus infection of target cells is improving. The cellular factors that regulate cell death, such as Bcl-2 family members, can modulate the outcome of DEN virus infection in cultured cells. Apoptosis inhibitors delay DEN virus-induced apoptosis, thereby providing a suitable environment for the virus. During DEN virus infection, cell death is also modulated by the virulence of the infecting strains. The purpose of this review is to present recent information on the cellular mechanisms and viral proteins associated with apoptosis in response to DEN virus. This knowledge may provide new insights into the viral pathogenicity.

Avian encephalomyelitis virus nonstructural protein 2C induces apoptosis by activating cytochrome c /caspase-9 pathway

The nonstructural protein 2C is highly conserved among picornaviruses and plays an important role in the assembly of mature virions, membrane association, and viral RNA synthesis. The investigation of other potential functions of nonstructural protein 2C from avian encephalomyelitis virus (AEV) resulted in identifying for the first time that the protein 2C is involved in apoptosis. Expression of the protein 2C on chick embryo brain (CEB) and Cos-7 cells produced TUNEL-positive cells characterized by a cleavage of cellular DNA and the formation of membrane-enclosed apoptotic bodies. Analysis of the protein 2C showed that the N-terminal domain containing 35 amino acid (aa) residues (between 46 and 80 aa) is associated with apoptotic function. Transfection of the deletion mutant lacking this 35 aa's into CEB and Cos-7 cells failed to induce apoptosis. Furthermore, the protein 2C induced apoptosis in the transfected CEB and Cos-7 cells through activation of caspase-9 rather than caspase-8 followed by activation of caspase-3 pathway. Analysis of the Western blots of caspase-3 and caspase-9 showed the characteristics of active caspase-3 and -9 in the 2C-transfected CEB and Cos-7 cells as seen in the AEV-infected CEB cells while they were in the form of procaspase-3 and procaspase-9 in the 2C mutant-transfected cells. To further elucidate the mechanism of the 2C-induced apoptosis, the 2C-transfected CEB and Cos-7 cells were fractionated into mitochondria and cytosol and subjected for Western blotting, located cytochrome c in the mitochondria as well as the cytosol fractions, while it was only sequestered in the mitochondrial fraction in the mutant 2C-transfected cells. The protein 2C was located in the mitochondria and cytosol of the transfected/infected CEB and transfected Cos-7 cells, but the mutant lost its ability to localize to the mitochondria. Altogether, the results demonstrate that the protein 2C localized to the mitochondria of the transfected cells triggered the efflux of cytochrome c into the cytosol in turn activating the upstream caspase-9 and then the downstream caspase-3, thus leading to apoptosis in the cells.

Infection and injury of neurons by West Nile encephalitis virus

West Nile virus (WNV) infects neurons and leads to encephalitis, paralysis, and death in humans, animals, and birds. We investigated the mechanism by which neuronal injury occurs after WNV infection. Neurons in the anterior horn of the spinal cords of paralyzed mice exhibited a high degree of WNV infection, leukocyte infiltration, and degeneration. Because it was difficult to distinguish whether neuronal injury was caused by viral infection or by the immune system response, a novel tissue culture model for WNV infection was established in neurons derived from embryonic stem (ES) cells. Undifferentiated ES cells were relatively resistant to WNV infection. After differentiation, ES cells expressed neural antigens, acquired a neuronal phenotype, and became permissive for WNV infection. Within 48 h of exposure to an exceedingly low multiplicity of infection (5 x 10(-4)), 50% of ES cell-derived neurons became infected, producing nearly 10(7) PFU of infectious virus per ml, and began to die by an apoptotic mechanism. The establishment of a tractable virus infection model in ES cell-derived neurons facilitates the study of the molecular basis of neurotropism and the mechanisms of viral and immune-mediated neuronal injury after infection by WNV or other neurotropic pathogens.

Dengue virus M protein contains a proapoptotic sequence referred to as ApoptoM

The induction of apoptotic cell death is a prominent cytopathic effect of dengue (DEN) viruses. One of the key questions to be addressed is which viral components induce apoptosis in DEN virus-infected cells. This study investigated whether the small membrane (M) protein was involved in the induction of apoptosis by DEN virus. This was addressed by using a series of enhanced green fluorescent protein-fused DEN proteins. Evidence is provided that intracellular production of the M ectodomains (residues M-1 to M-40) of all four DEN serotypes triggered apoptosis in host cells such as mouse neuroblastoma Neuro 2a and human hepatoma HepG2 cells. The M ectodomains of the wild-type strains of Japanese encephalitis, West Nile and yellow fever viruses also had proapoptotic properties. The export of the M ectodomain from the Golgi apparatus to the plasma membrane appeared to be essential for the initiation of apoptosis. The study found that anti-apoptosis protein Bcl-2 protected HepG2 cells against the death-promoting activity of the DEN M ectodomain. This suggests that the M ectodomain exerts its cytotoxic effects by activating a mitochondrial apoptotic pathway. The cytotoxicity of the DEN M ectodomain reflected the intrinsic proapoptotic properties of the nine carboxy-terminal amino acids (residues M-32 to M-40) designated ApoptoM: Residue M-36 was unique in that it modulated the death-promoting activity of the M ectodomain. Defining the ApoptoM-activated signalling pathways leading to apoptosis will provide the basis for studying how the M protein might play a key role in the fate of the flavivirus-infected cells.

High-neurovirulence GDVII virus induces apoptosis in murine astrocytes through tumor necrosis factor (TNF)-receptor and TNF-related apoptosis-inducing ligand

We carried out a study to determine if the high-neurovirulence GDVII strain of Theiler's murine encephalomyelitis virus (TMEV) and the demyelinating, low-neurovirulence BeAn strain induced apoptosis in murine astrocytes. Astrocytes, the major glial cell population of the central nervous system, were semipermissive for GDVII virus replication. Programmed cell death, demonstrated by apoptosis-specific caspase-3 protease activity, was maximal 8 h after GDVII infection at an m.o.i. of 1. Purified TMEV capsid proteins VP1, VP2, and VP3 did not induce apoptosis but antibodies to VP1 and VP2 inhibited it. Antibody inhibition of caspase-3 activity as well as flow cytometry experiments implicated TNF-related apoptosis-inducing ligand (TRAIL) and TNF-alpha-receptor (TNF-R) in apoptosis signaling. Conversely, TNF-alpha and the TRAIL-receptor were not upregulated. Furthermore, the number of functional TNF-alpha receptors, but not their affinity, was increased in apoptotic GDVII virus-infected astrocytes, as confirmed in binding experiments with 125I-labeled recombinant murine TNF-alpha. In vivo studies showed that most of the cells loaded with the virus when injected in the brains of SJL mice were neurons but very few showed TUNEL costaining. Conversely, many of the apoptotic cells found were also positive for GFAP staining.

Feline calicivirus replication induces apoptosis in cultured cells

Infection of Crandell-Rees feline kidney (CRFK) cells by feline calicivirus (FCV) causes rapid cytopathic effects followed by cell death. In this study, we observed that FCV replication in cells results in the induction of changes characteristic of apoptosis, including translocation of phosphatidyl serine to the cell outer membrane, chromatin condensation, and oligonucleosomal DNA fragmentation. FCV infection was associated with increases in the activities of caspase-3, -8, and -9, with the level of activation of caspase-3 higher than those of caspases-8 and -9. Caspase activation in CRFK cells was not observed when cells were inoculated with UV-inactivated FCV or when cycloheximide was present during virus infection, indicating that FCV replication and de novo synthesis of virus proteins are critical for induction of apoptosis.

Induction of caspase-dependent apoptosis by betanodaviruses GGNNV and demonstration of protein alpha as an apoptosis inducer

Betanodaviruses, members of the Nodaviridae family, are the causative agents of viral nervous necrosis in fish and infection by which cause high mortality in larvae and juveniles in a wide range of marine fish species in Asia, Europe, Australia, Martinique, and Tahit. Greasy grouper (Epinephelus tauvina) nervous necrosis viruses (GGNNV) were investigated for their apoptotic activity in culture cells. GGNNV infection of sea bass (SB) cells appeared to induce a typical cytopathic effect (CPE), i.e., cytoplasmic vacuolation, thinning, rounding up, detachment of infected cells from the cultured dish, and eventually cell lysis and death. The infected SB cells underwent DNA fragmentation and stained positive in terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) assay, suggesting that GGNNV infection induced apoptosis in SB cells. In addition, GGNNV-infected SB cells showed an increased activity of caspase-8-like proteases (IETDase) and caspase-3-like proteases (IETDase), whereas inhibitor of caspase-8 and caspase-3 reduced GGNNV-induced apoptosis. This suggests that GGNNV may promote apoptosis via the extrinsic pathway in SB cells. Protein alpha, the precursor of GGNNV capsid proteins, was transiently expressed in SB and Cos-7 cells. The DNA fragmentation and TUNEL positive signal were apparent in SB and Cos-7 cells expressing protein alpha, suggesting that protein alpha may serve as an apoptotic inducer in these cells. Moreover, expression of protein alpha resulted in the activation of caspase-3-like proteases in both cells, which could be inhibited by a caspase-3-like protease specific inhibitor DEVD-CHO peptide. These results suggest that fish caspases are important elements in GGNNV-meditated apoptosis.

Characterization of a siberian virus isolated from a patient with progressive chronic tick-borne encephalitis

A strain of Tick-borne encephalitis virus designated Zausaev (Za) was isolated in Siberia from a patient who died of a progressive (2-year) form of tick-borne encephalitis 10 years after being bitten by a tick. The complete genomic sequence of this virus was determined, and an attempt was made to correlate the sequence with the biological characteristics of the virus. Phylogenetic analysis demonstrated that this virus belongs to the Siberian subtype of Tick-borne encephalitis virus. Comparison of Za virus with two related viruses, a Far Eastern isolate, Sofjin, and a Siberian isolate, Vasilchenko, revealed differences among the three viruses in pathogenicity for Syrian hamsters, cytopathogenicity for PS cells, plaque morphology, and the electrophoretic profiles of virus-specific nonstructural proteins. Comparative amino acid alignments revealed 10 individual amino acid substitutions in the Za virus polyprotein sequence that were different from those of other tick-borne flaviviruses. Notably, the dimeric form of the Za virus NS1 protein migrated in polyacrylamide gels as a heterogeneous group of molecules with a significantly higher electrophoretic mobility than those of the Sofjin and Vasilchenko viruses. Two amino acid substitutions, T(277)-->V and E(279)-->G, within the NS1 dimerization domain are probably responsible for the altered oligomerization of Za virus NS1. These studies suggest that the patient from whom Za virus was isolated died due to increased pathogenicity of the latent virus following spontaneous mutagenesis.

Pathogenesis of flavivirus encephalitis

Within the flavivirus family, viruses that cause natural infections of the central nervous system (CNS) principally include members of the Japanese encephalitis virus (JEV) serogroup and the tick-borne encephalitis virus (TBEV) serocomplex. The pathogenesis of diseases involves complex interactions of viruses, which differ in neurovirulence potential, and a number of host factors, which govern susceptibility to infection and the capacity to mount effective antiviral immune responses both in the periphery and within the CNS. This chapter summarizes progress in the field of flavivirus neuropathogenesis. Mosquito-borne and tickborne viruses are considered together. Flavivirus neuropathogenesis involves both neuroinvasiveness (capacity to enter the CNS) and neurovirulence (replication within the CNS), both of which can be manipulated experimentally. Neuronal injury as a result of bystander effects may be a factor during flavivirus neuropathogenesis given that microglial activation and elaboration of inflammatory mediators, including IL-1β and TNF-α, occur in the CNS during these infections and may accompany the production of nitric oxide and peroxynitrite, which can cause neurotoxicity.

Avian encephalomyelitis virus induces apoptosis via major structural protein VP3

Avian encephalomyelitis virus (AEV) strain L(2)Z was investigated for its apoptotic activity in specific-pathogen-free chick embryo brain tissue. DNA fragmentation analysis and electron microscopy observation demonstrated that AEV could induce apoptosis in chick embryo brain tissues characterized by chromatin condensation, plasma membrane blebbing, cell shrinkage, and nucleosomal DNA fragmentation after 4 days postinfection. AEV structural protein genes VP1, VP2, and VP3 were transfected into Cos-7 and chick embryo brain (CEB) cells, respectively. The results showed that only VP3 protein was an apoptotic inducer, as demonstrated by DNA fragmentation analysis and TUNEL assay at 24 and 48 h posttransfection. Furthermore, expression of VP3 protein resulted in the activation of caspase-3-like proteases in both cells, which could be inhibited by a caspase-3-like protease-specific inhibitor Ac-DEVD-CHO peptide, suggesting that AEV VP3 protein induces apoptosis through a caspase-3-like protease pathway. In addition, VP3 protein localized to mitochondria in the Cos-7 and CEB cells at 24 h posttransfection observed by confocal microscopy, indicating that mitochondria may play an important role in VP3-induced apoptosis. Taken together, our results show that AEV could induce apoptosis in chick embryo brain tissue, structural protein VP3 could serve as an apoptotic inducer resulting in apoptosis in cell culture through a caspase-3-like protease pathway, which may be related to its localization to mitochondria.

Langat flavivirus protease NS3 binds caspase-8 and induces apoptosis

The flavivirus NS3 protein plays an important role in the cleavage and processing of the viral polyprotein and in the synthesis of the viral RNA. NS3 recruits NS2B and NS5 proteins to form complexes possessing protease and replicase activities through protease and nucleoside triphosphatase/helicase domains. We have found that NS3 also induces apoptosis. Expression of the Langat (LGT) virus NS3 protein resulted in a cleavage of cellular DNA and reduced the viability of cells. Coexpression of NS3 with apoptotic inhibitors (CrmA and P35) and addition of caspase peptide substrates (Z-VAD-FMK and Z-IETD-FMK) to NS3-transfected cells blocked NS3-induced apoptosis. In cotransfection experiments, NS3 bound to caspase-8 and enhanced caspase-8-mediated apoptosis. NS3 and caspase-8 colocalized in the cytoplasm of transfected cells. Deletion analysis demonstrated that at least two regions of NS3 contribute to its apoptotic activities. The protease and helicase domains are each able to bind to caspase-8, while the protease domain alone induces apoptosis. The protease domain and tetrahelix region of the helicase domain are required for NS3 to augment caspase-8-mediated apoptosis. Thus, the LGT virus NS3 protein is a multifunctional protein that binds to caspase-8 and induces apoptosis.