Effect of enforced expression of human bcl-2 on Japanese encephalitis virus-induced apoptosis in cultured cells

Inflammatory & Apoptotic Factor Fluctuations Associated with Japanese Encephalitis Virus Infection in Transgenic IFNAR1-/- Mice

Japanese encephalitis virus (JEV) is an orthoflavivirus that causes Japanese encephalitis, a mosquito-borne viral infection that primarily affects humans and animals. JEV is a major cause of encephalitis in many parts of Asia, particularly in rural and agricultural areas. In this study, we used the IFNAR1-/- mice model to investigate alterations in cytokine and apoptotic factor levels in IFNAR1-/- mice upon JEV infection. A 5-week-adult female C57BL/6 IFN-α/β receptor knockout (IFNAR1-/-) transgenic mice were intramuscularly inoculated with several viral titers and monitored within 10 dpi. The weight changes and survival rates were evaluated during the study period. Gene expression analysis was performed using RT-qPCR, targeting genes related to specific cytokines and apoptotic factors, to identify the inflammatory factors fluctuations associated with JEV strain KBPV-VR-27 infection in IFNAR1-/- mice. The expression of cytokine genes was enhanced in IFNAR1-/- mice infected with JEV KBPV-VR-27. Notably, a significant induction of cytokines, such as IL-13, IL-17α, IFN-β, and IFN-γ, was observed in the brain, while upregulation of IL-6, IFN-β, and IFN-γ was exhibited in the lung. In addition, among the targeted apoptotic factors, only significant induction of Bak was observed in the brain. We also found that the spleen exhibited a higher viral load compared to the brain and lungs. In conclusion, the findings of this study shed light on the varying viral loads across targeted organs, with the brain exhibiting a lower viral load but pronounced expression of targeted pro-inflammatory cytokines in IFNAR1-/- mice.

Molecular Mechanisms Associated with Neurodegeneration of Neurotropic Viral Infection

Viral infections of the central nervous system (CNS) cause variable outcomes from acute to severe neurological sequelae with increased morbidity and mortality. Viral neuroinvasion directly or indirectly induces encephalitis via dysregulation of the immune response and contributes to the alteration of neuronal function and the degeneration of neuronal cells. This review provides an overview of the cellular and molecular mechanisms of virus-induced neurodegeneration. Neurotropic viral infections influence many aspects of neuronal dysfunction, including promoting chronic inflammation, inducing cellular oxidative stress, impairing mitophagy, encountering mitochondrial dynamics, enhancing metabolic rewiring, altering neurotransmitter systems, and inducing misfolded and aggregated pathological proteins associated with neurodegenerative diseases. These pathogenetic mechanisms create a multidimensional injury of the brain that leads to specific neuronal and brain dysfunction. The understanding of the molecular mechanisms underlying the neurophathogenesis associated with neurodegeneration of viral infection may emphasize the strategies for prevention, protection, and treatment of virus infection of the CNS.

Mice as an Animal Model for Japanese Encephalitis Virus Research: Mouse Susceptibility, Infection Route, and Viral Pathogenesis

Japanese encephalitis virus (JEV), a zoonotic flavivirus, is principally transmitted by hematophagous mosquitoes, continually between susceptible animals and incidentally from those animals to humans. For almost a century since its discovery, JEV was geographically confined to the Asia-Pacific region with recurrent sizable outbreaks involving wildlife, livestock, and people. However, over the past decade, it has been detected for the first time in Europe (Italy) and Africa (Angola) but has yet to cause any recognizable outbreaks in humans. JEV infection leads to a broad spectrum of clinical outcomes, ranging from asymptomatic conditions to self-limiting febrile illnesses to life-threatening neurological complications, particularly Japanese encephalitis (JE). No clinically proven antiviral drugs are available to treat the development and progression of JE. There are, however, several live and killed vaccines that have been commercialized to prevent the infection and transmission of JEV, yet this virus remains the main cause of acute encephalitis syndrome with high morbidity and mortality among children in the endemic regions. Therefore, significant research efforts have been directed toward understanding the neuropathogenesis of JE to facilitate the development of effective treatments for the disease. Thus far, multiple laboratory animal models have been established for the study of JEV infection. In this review, we focus on mice, the most extensively used animal model for JEV research, and summarize the major findings on mouse susceptibility, infection route, and viral pathogenesis reported in the past and present, and discuss some unanswered key questions for future studies.

Molecular pathogenesis of Japanese encephalitis and possible therapeutic strategies

Japanese encephalitis virus (JEV), a single-stranded, enveloped RNA virus, is a health concern across Asian countries, associated with severe neurological disorders, especially in children. Primarily, pigs, bats, and birds are the natural hosts for JEV, but humans are infected incidentally. JEV requires a few host proteins for its entry and replication inside the mammalian host cell. The endoplasmic reticulum (ER) plays a significant role in JEV genome replication and assembly. During this process, the ER undergoes stress due to its remodelling and accumulation of viral particles and unfolded proteins, leading to an unfolded protein response (UPR). Here, we review the overall strategy used by JEV to infect the host cell and various cytopathic effects caused by JEV infection. We also highlight the role of JEV structural proteins (SPs) and non-structural proteins (NSPs) at various stages of the JEV life cycle that are involved in up- and downregulation of different host proteins and are potentially relevant for developing efficient therapeutic drugs.

Role of apoptosis in Duck Tembusu virus infection of duckling brains in vivo

The Duck Tembusu virus (DTMUV) is a novel flavivirus that occurs mainly in poultry. DTMUV infection can cause common neurological symptoms in ducklings, but the pathogenesis of DTMUV has not been elucidated yet. In this study, a DTMUV-infected duckling model was constructed to investigate the apoptosis in the duckling brains. After DTMUV infection, apoptotic cells were observed by transmission electron microscopy. It was found that the abundances of apoptosis-related genes and proteins were not obviously changed in the early stage of infection but significantly changed in the middle and late stages of the disease. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling assay staining results were also consistent with the above phenomena. Interestingly, although apoptosis occurred in the duckling brains infected by DTMUV, some antiapoptotic genes in the brain increased in varying degrees. In conclusion, DTMUV infection could induce apoptosis in ducklings' brains, and the occurrence of apoptosis was accompanied by the virus infection process with certain regularity. This study provides a scientific basis for elucidating the apoptotic mechanism of brain lesions induced by DTMUV infection.

Polarization of Type 1 Macrophages Is Associated with the Severity of Viral Encephalitis Caused by Japanese Encephalitis Virus and Dengue Virus

Infection with flaviviruses causes mild to severe diseases, including viral hemorrhagic fever, vascular shock syndrome, and viral encephalitis. Several animal models explore the pathogenesis of viral encephalitis, as shown by neuron destruction due to neurotoxicity after viral infection. While neuronal cells are injuries caused by inflammatory cytokine production following microglial/macrophage activation, the blockade of inflammatory cytokines can reduce neurotoxicity to improve the survival rate. This study investigated the involvement of macrophage phenotypes in facilitating CNS inflammation and neurotoxicity during flavivirus infection, including the Japanese encephalitis virus, dengue virus (DENV), and Zika virus. Mice infected with different flaviviruses presented encephalitis-like symptoms, including limbic seizure and paralysis. Histology indicated that brain lesions were identified in the hippocampus and surrounded by mononuclear cells. In those regions, both the infiltrated macrophages and resident microglia were significantly increased. RNA-seq analysis showed the gene profile shifting toward type 1 macrophage (M1) polarization, while M1 markers validated this phenomenon. Pharmacologically blocking C-C chemokine receptor 2 and tumor necrosis factor-α partly retarded DENV-induced M1 polarization. In summary, flavivirus infection, such as JEV and DENV, promoted type 1 macrophage polarization in the brain associated with encephalitic severity.

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.

The evolutionary dynamics of viruses: virion release strategies, time delays and fitness minima

Viruses exhibit a diverse array of strategies for infecting host cells and for virion release after replication. Cell exit strategies generally involve either budding from the cell membrane or killing the host cell. The conditions under which either is at a selective advantage is a key question in the evolutionary theory of viruses, with the outcome having potentially important impacts on the course of infection and pathogenicity. Although a plethora of external factors will influence the fitness of either strategy; here, we focus just on the effects of the physical properties of the system. We develop theoretical approaches to assess the effects of the time delays between initial infection and virion release. We show that the length of the delay before apoptosis is an important trait in virus evolutionary dynamics. Our results show that for a fixed time to apoptosis, intermediate delays lead to virus fitness that is lower than short times to apoptosis - leading to an apoptotic strategy - and long times to apoptosis - leading to a budding strategy at the between-cell level. At fitness minima, selection is expected to be disruptive and the potential for adaptive radiation in virus strategies is feasible. Hence, the physical properties of the system are sufficient to explain the existence of both budding and virus-induced apoptosis. The fitness functions presented here provide a formal basis for further work focusing on the evolutionary implications of trade-offs between time delays, intracellular replication and resulting mutation rates.

Elevation of Cleaved p18 Bax Levels Associated with the Kinetics of Neuronal Cell Death during Japanese Encephalitis Virus Infection

Japanese encephalitis virus (JEV) infection induces uncontrolled neuronal apoptosis, leading to irreversible brain damage. However, the mechanism of JEV-induced neuronal apoptosis has not been clearly elucidated. This study aimed to investigate both virus replication and neuronal cell apoptosis during JEV infection in human neuroblastoma SH-SY5Y cells. As a result, the kinetic productions of new viral progeny were time- and dose-dependent. The stimulation of SH-SY5Y cell apoptosis was dependent on the multiplicity of infections (MOIs) and infection periods, particularly during the late period of infection. Interestingly, we observed that of full-length Bax (p21 Bax) level started to decrease, which corresponded to the increased level of its cleaved form (p18 Bax). The formation of p18 Bax resulting in cytochrome c release into the cytosol appeared to correlate with JEV-induced apoptotic cell death together with the activation of caspase-3/7 activity, especially during the late stage of a robust viral infection. Therefore, our results suggest another possible mechanism of JEV-induced apoptotic cell death via the induction of the proteolysis of endogenous p21 Bax to generate p18 Bax. This finding could be a new avenue to facilitate novel drug discovery for the further development of therapeutic treatments that could relieve neuronal damage from JEV infection.

Protective effects of selenium against zearalenone-induced apoptosis in chicken spleen lymphocyte via an endoplasmic reticulum stress signaling pathway

Selenium (Se), an antioxidant agent, provides significant protection from reactive oxygen species (ROS)-induced cell damage in vivo and in vitro. However, it is unclear whether Se can protect against zearalenone (ZEN)-induced apoptosis in chicken spleen lymphocyte. In this study, we investigated the underlying mechanism of the apoptosis induced by ZEN in chicken spleen lymphocyte and further evaluated the protective mechanism of Se on ZEN-induced apoptosis. The results show that ZEN induced an increase in ROS generation and lipid peroxidation, and a decrease in levels of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and glutathione (GSH). The results of apoptosis morphologically from acridine orange/ethidium bromide (AO/EB) fluorescent staining and flow cytometry analysis show apparent apoptosis in the ZEN-treated group, and was confirmed by the upregulation of caspase-3, -12 and downregulation of Bcl-2. Meanwhile, ZEN activated the endoplasmic reticulum (ER) stress by upregulating ER stress-related molecular sensors (GRP78, ATF6, ATF4, IRE). However, co-treatment with Se effectively blocked ROS generation, improved antioxdative capacity, and reversed apoptosis and ER stress-related genes and protein expression. Taken together, these data suggest that oxidative stress and ER stress play a vital role in ZEN-induced apoptosis, and Se had a significant preventive effect on ZEN-induced apoptosis in chicken spleen lymphocyte via ameliorating the ER stress signaling pathway.

Subverting the mechanisms of cell death: flavivirus manipulation of host cell responses to infection

Viruses exploit host metabolic and defence machinery for their own replication. The flaviviruses, which include Dengue (DENV), Yellow Fever (YFV), Japanese Encephalitis (JEV), West Nile (WNV) and Zika (ZIKV) viruses, infect a broad range of hosts, cells and tissues. Flaviviruses are largely transmitted by mosquito bites and humans are usually incidental, dead-end hosts, with the notable exceptions of YFV, DENV and ZIKV. Infection by flaviviruses elicits cellular responses including cell death via necrosis, pyroptosis (involving inflammation) or apoptosis (which avoids inflammation). Flaviviruses exploit these mechanisms and subvert them to prolong viral replication. The different effects induced by DENV, WNV, JEV and ZIKV are reviewed. Host cell surface proteoglycans (PGs) bearing glycosaminoglycan (GAG) polysaccharides — heparan/chondroitin sulfate (HS/CS) — are involved in initial flavivirus attachment and during the expression of non-structural viral proteins play a role in disease aetiology. Recent work has shown that ZIKV-infected cells are protected from cell death by exogenous heparin (a GAG structurally similar to host cell surface HS), raising the possibility of further subtle involvement of HS PGs in flavivirus disease processes. The aim of this review is to synthesize information regarding DENV, WNV, JEV and ZIKV from two areas that are usually treated separately: the response of host cells to infection by flaviviruses and the involvement of cell surface GAGs in response to those infections.

Zika Virus Promotes Neuronal Cell Death in a Non-Cell Autonomous Manner by Triggering the Release of Neurotoxic Factors

Zika virus (ZIKV) has recently caused a worldwide outbreak of infections associated with severe neurological complications, including microcephaly in infants born from infected mothers. ZIKV exhibits high neurotropism and promotes neuroinflammation and neuronal cell death. We have recently demonstrated that N-methyl-d-aspartate receptor (NMDAR) blockade by memantine prevents ZIKV-induced neuronal cell death. Here, we show that ZIKV induces apoptosis in a non-cell autonomous manner, triggering cell death of uninfected neurons by releasing cytotoxic factors. Neuronal cultures infected with ZIKV exhibit increased levels of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and glutamate. Moreover, infected neurons exhibit increased expression of GluN2B and augmented intracellular Ca²⁺ concentration. Blockade of GluN2B-containing NMDAR by ifenprodil normalizes Ca²⁺ levels and rescues neuronal cell death. Notably, TNF-α and IL-1β blockade decreases ZIKV-induced Ca²⁺ flux through GluN2B-containing NMDARs and reduces neuronal cell death, indicating that these cytokines might contribute to NMDAR sensitization and neurotoxicity. In addition, ZIKV-infected cultures treated with ifenprodil exhibits increased activation of the neuroprotective pathway including extracellular signal-regulated kinase and cAMP response element-binding protein, which may underlie ifenprodil-mediated neuroprotection. Together, our data shed some light on the neurotoxic mechanisms triggered by ZIKV and begin to elucidate how GluN2B-containing NMDAR blockade can prevent neurotoxicity.

Tissue tropism of Saint Louis encephalitis virus: Histopathology triggered by epidemic and non-epidemic strains isolated in Argentina

Saint Louis encephalitis virus (SLEV) reemerged in South America, and caused encephalitis outbreaks at the beginning of the 21st century. To enhance our knowledge about SLEV virulence, we performed comparative pathogenesis studies in Swiss albino mice inoculated with two different variants, the epidemic strain CbaAr-4005 and the non-epidemic strain CorAn-9275. Only the infection of mice with SLEV strain CbaAr-4005 resulted in high viremia, invasion of peripheral tissues including the lungs, kidney, and spleen, and viral neuroinvasion. This was associated with inflammatory pathology in the lungs, spleen, and brain as well as morbidity and mortality. In contrast, neither signs of desease nor viral replication were observed in mice infected with strain CorAn-9275. Interestingly, important loss of B cells and development of altered germinal centers (GC) were detected in the spleen of mice infected with strain CbaAr-4005, whereas mice infected with SLEV CorAn-9275 developed prominent GC with conserved follicular architecture, and neutralizing antibodies.

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.

Japanese encephalitis virus invasion of cell: allies and alleys

The mosquito-borne flavivirus, Japanese encephalitis virus (JEV), is the leading cause of virus-induced encephalitis globally and a major public health concern of several countries in Southeast Asia, with the potential to become a global pathogen. The virus is neurotropic, and the disease ranges from mild fever to severe hemorrhagic and encephalitic manifestations and death. The early steps of the virus life cycle, binding, and entry into the cell are crucial determinants of infection and are potential targets for the development of antiviral therapies. JEV can infect multiple cell types; however, the key receptor molecule(s) still remains elusive. JEV also has the capacity to utilize multiple endocytic pathways for entry into cells of different lineages. This review not only gives a comprehensive update on what is known about the virus attachment and receptor system (allies) and the endocytic pathways (alleys) exploited by the virus to gain entry into the cell and establish infection but also discusses crucial unresolved issues. We also highlight common themes and key differences between JEV and other flaviviruses in these contexts.

The C Terminus of the Core β-Ladder Domain in Japanese Encephalitis Virus Nonstructural Protein 1 Is Flexible for Accommodation of Heterologous Epitope Fusion

NS1 is the only nonstructural protein that enters the lumen of the endoplasmic reticulum (ER), where NS1 is glycosylated, forms a dimer, and is subsequently secreted during flavivirus replication as dimers or hexamers, which appear to be highly immunogenic to the infected host, as protective immunity can be elicited against homologous flavivirus infections. Here, by using a trans-complementation assay, we identified the C-terminal end of NS1 derived from Japanese encephalitis virus (JEV), which was more flexible than other regions in terms of housing foreign epitopes without a significant impact on virus replication. This mapped flexible region is located in the conserved tip of the core β-ladder domain of the multimeric NS1 structure and is also known to contain certain linear epitopes, readily triggering specific antibody responses from the host. Despite becoming attenuated, recombinant JEV with insertion of a neutralizing epitope derived from enterovirus 71 (EV71) into the C-terminal end of NS1 not only could be normally released from infected cells, but also induced dual protective immunity for the host to counteract lethal challenge with either JEV or EV71 in neonatal mice. These results indicated that the secreted multimeric NS1 of flaviviruses may serve as a natural protein carrier to render epitopes of interest more immunogenic in the C terminus of the core β-ladder domain.

IMPORTANCE

The positive-sense RNA genomes of mosquito-borne flaviviruses appear to be flexible in terms of accommodating extra insertions of short heterologous antigens into their virus genes. Here, we illustrate that the newly identified C terminus of the core β-ladder domain in NS1 could be readily inserted into entities such as EV71 epitopes, and the resulting NS1-epitope fusion proteins appeared to maintain normal virus replication, secretion ability, and multimeric formation from infected cells. Nonetheless, such an insertion attenuated the recombinant JEV in mice, despite having retained the brain replication ability observed in wild-type JEV. Mother dams immunized with recombinant JEV expressing EV71 epitope-NS1 fused proteins elicited neutralizing antibodies that protected the newborn mice against lethal EV71 challenge. Together, our results implied a potential application of JEV NS1 as a viral carrier protein to express a heterologous epitope to stimulate dual/multiple protective immunity concurrently against several pathogens.

Constant up-regulation of BiP/GRP78 expression prevents virus-induced apoptosis in BHK-21 cells with Japanese encephalitis virus persistent infection

Background

Persistent infection of the Japanese Encephalitis Virus (JEV) has been reported in clinical cases, experimental animals, and various cell culture systems. We previously reported the establishment of spontaneous JEV persistent infection, assisted by defective interfering particle accumulation and/or attenuated helper viruses, in BHK-21 cells devoid of virus-induced apoptosis, cBS6-2 and cBS6-3. However, cell-specific factors may play important roles in controlling JEV replication and have never been assessed for this specific phenomenon. Recent evidence suggests that viruses have evolved various mechanisms to cope with endoplasmic reticulum stress signaling pathways for their efficient amplification and transmission, including the unfolded protein response (UPR).

Results

To identify the host cell factors that affect JEV persistence, we investigated the expression of essential UPR factors in cBS6-2 and cBS6-3 cells. Of the selected UPR factors tested, the most noticeable deviations from those of the normal BHK-21 cells with JEV acute infection were as follows: the suppression of C/EBP homologous binding protein (CHOP) and the constant up-regulation of immunoglobulin binding protein (BiP) expression in cBS6-2 and cBS6-3 cells. In JEV acute infection on normal BHK-21 cells, silencing CHOP expression through specific siRNA blocked cell death almost completely. Meanwhile, depletion of BiP by specific siRNA unlocked CHOP expression in cBS6-2 and cBS6-3 cells, resulting in massive cell death. Fulminant apoptotic cell death for both cell clones on tunicamycin treatment revealed that the JEV persistently infected cells still contained functional arms for cell fate decisions.

Conclusions

BHK-21 cells with JEV persistent infection strive against virus-induced apoptosis through constant up-regulation of BiP expression, resulting in the complete depletion of CHOP even with apparent virus amplification in the cells. Accordingly, the attenuation of virus replication as well as the modifications to cell metabolism could be additional factors contributing to the development of JEV persistent infection in mammalian cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-015-0269-5) contains supplementary material, which is available to authorized users.

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.

Immune evasion strategies of flaviviruses

Flavivirus is a genus of the family Flaviviridae. It includes West Nile virus (WNV), dengue virus (DENV), yellow fever virus (YFV), Japanese encephalitis virus (JEV), tick-borne encephalitis virus (TBEV), and several other viruses which lead to extensive morbidity and mortality in humans. To establish infection and replication in the hosts, flaviviruses have evolved a variety of strategies to modulate the host's immune responses. In this review, the strategies employed by flaviviruses to evade the innate and adaptive immunity of host are summarized based on current studies, with a major focus on the inhibition of interferon, complement, natural killer (NK) cell, B cell, and T cell responses. This review aims to provide an overview of the current understanding for the mechanisms used by flaviviruses to escape the host's immune response, which will facilitate the future studies on flavivirus pathogenesis and the development of anti-flavivirus therapeutics.

The contribution of rodent models to the pathological assessment of flaviviral infections of the central nervous system

Members of the genus Flavivirus are responsible for a spectrum of important neurological syndromes in humans and animals. Rodent models have been used extensively to model flavivirus neurological disease, to discover host-pathogen interactions that influence disease outcome, and as surrogates to determine the efficacy and safety of vaccines and therapeutics. In this review, we discuss the current understanding of flavivirus neuroinvasive disease and outline the host, viral and experimental factors that influence the outcome and reliability of virus infection of small-animal models.

Neurovirulent flavivirus can be attenuated in mice by incorporation of neuron-specific microRNA recognition elements into viral genome

Engineering viruses by inserting microRNA (miRNA) recognition elements (MREs) into the 3'-untranslated region (3'-UTR) of viral RNA can efficiently restrict viral tissue tropism. We used the mosquito-borne Japanese encephalitis virus (JEV) to investigate whether endogenous neuron-specific microRNA-124 (miR-124) could be used to restrict viral neurotropism and, consequently, diminish the neurovirulence of JEV in mice. To recover a neuron-restricted JEV, we inserted 2 copies of a perfectly matched MRE specific to miR-124 into the 3'-UTR to create infectious JEV recombinant RP-124PT (rRP-124PT). The effect of rRP-124PT was attenuated in infected mice as compared with MRE mutant and parental strains, both of which were lethal to challenged mice. Immunization with rRP-124PT appeared to elicit full protective immunity against subsequent JEV lethal challenge. We found neurons of the central nervous system critical targets for infection by JEV, which directly causes lethal encephalitis. The silencing of JEV rRP-124PT in mice by miR-124 illustrates that endogenous miRNA can readily recognize and interact with the 3'-UTR of naturally occurring genomic/mRNAs lacking a polyadenylated tail. Inserting MREs into viral RNA may facilitate further study of flaviviral pathogenesis involving tissue tropism and suggest an additional layer of biosafety for the rational design of safe flavivirus vaccines.

Autophagy is involved in the early step of Japanese encephalitis virus infection

Japanese encephalitis virus (JEV), an enveloped Flavivirus with a positive-sense RNA genome, causes acute encephalitis with high mortality in humans. We used a virulent (RP-9) and an attenuated (RP-2ms) JEV strain to assess the role of autophagy in JEV infection. By monitoring the levels of lipidated LC3, we found that autophagy was induced in human NT-2 cells infected with RP-2ms, especially at the late stage, and to a lesser extent with RP-9. The induction of autophagy by rapamycin increased viral production, whereas the inhibition of autophagy by 3-methyladenine reduced viral yields for both RP-9 and RP-2ms. The viral replication of RP-9 and RP-2ms was also reduced in cells with downregulated ATG5 or Beclin 1 expression, suggesting a proviral role of autophagy in JEV replication. To determine the step of JEV life cycle affected by autophagy, we used an mCherry-LC3 fusion protein as the autophagosome marker. Little of no colocalization of LC3 puncta with dsRNA was noted, whereas the input JEV particles were targeted to autophagosomes stained positive for early endosome marker. Overall, we show for the first time that the cellular autophagy process is involved in JEV infection and the inoculated viral particles traffic to autophagosomes for subsequent steps of viral infection.

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

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.

Immunomodulatory cytokines determine the outcome of Japanese encephalitis virus infection in mice

Japanese encephalitis virus (JEV) induces an acute infection of the central nervous system, the pathogenic mechanism of which is not fully understood. To investigate host response to JEV infection, 14-day-old mice were infected via the extraneural route, which resulted in encephalitis and death. Mice that received JEV immune splenocyte transfer were protected from extraneural JEV infection. Pathology and gene expression profiles were then compared in brains of mice that either succumbed to JEV infection or were protected from infection by JEV immune cell transfer. Mice undergoing progressive JEV infection had increased expression of proinflammatory cytokines, chemokines, and signal transducers associated with the interferon (IFN) pathway. In contrast, mice receiving immune cell transfer had increased production of the Th2 cytokine IL-4, and of IL-10, with subdued expression of IFN-gamma. We observed IL-10 to be an important factor in determining clinical outcome in JEV infection. Data obtained by microarray analysis were further confirmed by quantitative RT-PCR. Together, these data suggest that JEV infection causes an unregulated inflammatory response that can be countered by the expression of immunomodulatory cytokines in mice that survive lethal infection.

A study of motor activity and catecholamine levels in different brain regions following Japanese encephalitis virus infection in rats

Japanese encephalitis (JE) is associated with a variety of movement disorders including transient form of pakinsonian features, dystonia and miscellaneous movement disorders. The neurotransmitters have important role in movement disorders. However their role in different brain regions in relation to behavioral activities in animal model of JE is not understood. The present study was aimed to investigate the behavioral parameters, the levels of catecholamine in brain regions--thalamus, midbrain, corpus striatum and frontal cortex on 0, 10 and 20 days post inoculation (dpi) with histopathological observations. Twelve day old Wistar strain rats were inoculated intracerebrally with a dose of 3 x 10(6) pfu of JE virus. Spontaneous locomotor activity (SLA) and grip strength were monitored. The levels of catecholamine were estimated using HPLC-ECD and histopathological changes were observed using haematoxylin and eosine staining. A significant decrease in SLA and grip strength was observed in JEV infected rats as compared to controls on 10 and 20 dpi. The levels of norepinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, and serotonin were significantly decreased in all the brain regions studied with respect to controls. We did not find significant recovery in catecholamine levels and locomotor activities up to 20 dpi and any significant correlation between behavioral changes and neurotransmitter levels. However histopathological studies revealed mild reduction in degree of damage on 20 dpi. The present study demonstrates the involvement of different brain regions in altered locomotor activity which may be associated with reduction in catecholamine levels in rat model of JE.

Morphological changes in human neural cells following tick-borne encephalitis virus infection

Tick-borne encephalitis (TBE) is one of the leading and most dangerous human viral neuroinfections in Europe and north-eastern Asia. The clinical manifestations include asymptomatic infections, fevers and debilitating encephalitis that might progress into chronic disease or fatal infection. To understand TBE pathology further in host nervous systems, three human neural cell lines, neuroblastoma, medulloblastoma and glioblastoma, were infected with TBE virus (TBEV). The susceptibility and virus-mediated cytopathic effect, including ultrastructural and apoptotic changes of the cells, were examined. All the neural cell lines tested were susceptible to TBEV infection. Interestingly, the neural cells produced about 100- to 10 000-fold higher virus titres than the conventional cell lines of extraneural origin, indicating the highly susceptible nature of neural cells to TBEV infection. The infection of medulloblastoma and glioblastoma cells was associated with a number of major morphological changes, including proliferation of membranes of the rough endoplasmic reticulum and extensive rearrangement of cytoskeletal structures. The TBEV-infected cells exhibited either necrotic or apoptotic morphological features. We observed ultrastructural apoptotic signs (condensation, margination and fragmentation of chromatin) and other alterations, such as vacuolation of the cytoplasm, dilatation of the endoplasmic reticulum cisternae and shrinkage of cells, accompanied by a high density of the cytoplasm. On the other hand, infected neuroblastoma cells did not exhibit proliferation of membranous structures. The virions were present in both the endoplasmic reticulum and the cytoplasm. Cells were dying preferentially by necrotic mechanisms rather than apoptosis. The neuropathological significance of these observations is discussed.

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.

Japanese encephalitis virus infection activates caspase-8 and -9 in a FADD-independent and mitochondrion-dependent manner

Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, replicates primarily at the endoplasmic reticulum and thereby triggers apoptosis of infected cells. This study investigated the hierarchical activation of the caspase network induced by JEV infection. It was found that JEV activated the initiators caspase-8 and -9, as well as effector caspase-3, in infected baby hamster kidney and mouse neuroblastoma (N18) cells. In neuronal N18 cells, JEV infection triggered cytochrome c release from mitochondria, which in turn activated caspase-9 and -3. Treatment of JEV-infected N18 cells with cyclosporin A or ruthenium red, which attenuate mitochondrial injuries, blocked activation of caspase-9 or -3, typifying that, in neuronal cells, this apoptosis involves the mitochondrial pathway. Alternatively, in caspase-3-deficient MCF-7 cells, JEV persisted and readily triggered a typical apoptotic response, including cytochrome c release and full activation of caspase-9 and -8 along with caspase-6, indicating that JEV did not require caspase-3 to manifest caspase-8 activation and apoptosis. Interestingly, a Fas-associated death-domain-containing protein (FADD) dominant-negative mutant, which interfered with transmission of the extracellular death signals into cells through the Fas/tumour necrosis factor (TNF) receptor, failed to block JEV-induced apoptosis and caspase-8 activation, implying that receptor oligomerization of the Fas/TNF pathway might not participate in JEV-induced apoptosis. Taken together, these results illustrate that JEV infection triggers caspase cascades involving the initiators caspase-8 and -9, probably through FADD-independent but mitochondrion-dependent pathways.

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.

Japanese encephalitis revisited

Japanese encephalitis (JE) is a major public health problem in Southeast Asia with around 50,000 cases and 10,000 deaths per year affecting essentially children below 10 years of age. The JE virus has shown a tendency to extend to other geographic regions. JE may cause severe encephalitis and survivors are frequently left with serious neurological lesions. In the absence of an effective antiviral treatment, prevention constitutes the best defense against this disease. Besides the implementation of vector control, immunization by vaccination is available. Vector control is expensive and difficult to implement on a large scale, leaving vaccination as the best means for mass campaigns. Recent progress in the development of live attenuated vaccines has given hope of an effective vaccine, which is both easy to use and inexpensive for large scale programs.

Tumor necrosis factor receptor-1-induced neuronal death by TRADD contributes to the pathogenesis of Japanese encephalitis

While a number of studies have documented the neurotropism of Japanese encephalitis virus (JEV), little is known regarding the molecular mechanism of neuronal death following viral infection. The tumor necrosis factor receptor (TNFR)-associated death domain (TRADD) has been suggested to be the crucial signal adaptor that mediates all intracellular responses from TNFR-1. Using mouse (Neuro2a) and human (SK-N-SH) neuroblastoma cell lines, we have shown that the altered expression of TNFR-1 and TRADD following JEV infection regulates the downstream apoptotic cascades. Activation of TRADD led to mitochondria-mediated neuronal apoptosis. As TRADD-knockout animals or deficient cell lines are unavailable, it has been difficult to definitively address the physiological role of TRADD in diseases pathology following JEV infection. We circumvented this problem by silencing TRADD expression with small-interfering RNA (siRNA) and have found that TRADD is required for TNFR-1-initiated neuronal apoptosis following in vitro infection with JEV. Interestingly, siRNA against TRADD also decreased the viral load in Neuro2a cells. Furthermore, siRNA against TRADD increased the survival of JEV-infected mice by altering the expression of pro apoptotic versus antiapoptotic molecules. These studies show that the engagement of TNFR-1 and TRADD following JEV infection plays a crucial role in neuronal apoptosis.

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.

Apoptosis in mosquito midgut epithelia associated with West Nile virus infection

The mosquito Culex pipiens pipiens is a documented vector of West Nile virus (WNV, Flaviviridae, Flavivirus). Our laboratory colony of C. p. pipiens, however, was repeatedly refractory to experimental transmission of WNV. Our goal was to identify if a cellular process was inhibiting virus infection of the midgut. We examined midguts of mosquitoes fed control and WNV-infected blood meals. Three days after feeding, epithelial cells from abdominal midguts of mosquitoes fed on WNV fluoresced under an FITC filter following Acridine Orange staining, indicating apoptosis in this region. Epithelial cells from experimental samples examined by TEM exhibited ultrastructural changes consistent with apoptosis, including shrinkage and detachment from neighbors, heterochromatin condensation, nuclear degranulation, and engulfment of apoptotic bodies by adjacent cells. Virions were present in cytoplasm and within cytoplasmic vacuoles of apoptotic cells. No apoptosis was detected by TEM in control samples. In parallel, we used Vero cell plaque assays to quantify infection after 7 and 10 day extrinsic incubation periods and found that none of the mosquitoes (0/55 and 0/10) which imbibed infective blood were infected. We propose that programmed cell death limits the number of WNV-infected epithelial cells and inhibits disseminated viral infections from the mosquito midgut.

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.

Antiviral effect of dehydroepiandrosterone on Japanese encephalitis virus infection

Japanese encephalitis virus (JEV), which causes neurological disorders, completes its life cycle and triggers apoptotic cell death in infected cells. Dehydroepiandrosterone (DHEA), an adrenal-derived steroid, has been implicated in protection against neurotoxicity and protection of animals from viral-induced encephalitis, resulting in an increased survival rate of the animals. Currently, the mechanisms underlying the beneficial effects of DHEA against the virus are largely unknown. In this study, DHEA suppression of JEV replication and virus-induced apoptosis in murine neuroblastoma (N18) cells was investigated. It was found that DHEA suppressed JEV-induced cytopathic effects, JEV-induced apoptotic cell death and JEV propagation in a concentration-dependent manner. Antiviral activity was more efficient in cultures treated with DHEA immediately after viral adsorption compared with that in cultures receiving delayed administration after adsorption or transient exposure before adsorption. JEV-induced cytotoxicity was accompanied by the inactivation of extracellular signal-regulated protein kinase (ERK). Inactivation of ERK by JEV infection was reversed by DHEA. When cells were treated with the ERK inhibitor U0126, DHEA lost its antiviral effect. Activation of ERK by anisomycin mimicked the action of DHEA in suppressing JEV-induced cytotoxicity. DHEA-related compounds, such as its sulfate ester (DHEAS) and pregnenolone, were unable to suppress JEV-induced cytotoxicity and ERK inactivation. The hormone-receptor antagonists ICI 182780 and flutamide failed to abrogate the antiviral effect of DHEA. These findings suggest that the antiviral effect of DHEA is not linked directly to the genomic steroid-receptor pathways and suggest that the signalling pathways of ERK play a role in the antiviral action of DHEA.

Flavivirus activates phosphatidylinositol 3-kinase signaling to block caspase-dependent apoptotic cell death at the early stage of virus infection

Flaviviruses such as dengue virus (DEN) and Japanese encephalitis virus (JEV) are medically important in humans. The lipid kinase, phosphatidylinositol 3-kinase (PI3K) and its downstream target Akt have been implicated in the regulation of diverse cellular functions such as proliferation, and apoptosis. Since JEV and DEN appear to trigger apoptosis in cultured cells at a rather late stage of infection, we evaluated the possible roles of the PI3K/Akt signaling pathway in flavivirus-infected cells. We found that Akt phosphorylation was noticeable in the JEV- and DEN serotype 2 (DEN-2)-infected neuronal N18 cells in an early, transient, PI3K- and lipid raft-dependent manner. Blocking of PI3K activation by its specific inhibitor LY294002 or wortmannin greatly enhanced virus-induced cytopathic effects (CPEs), even at an early stage of infection, but had no effect on virus production. This severe CPE was characterized as apoptotic cell death as evidenced by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) staining and cleavage of caspase-3 and poly(ADP-ribose) polymerase (PARP). Mechanically, the initiator and effector caspases involved are mainly caspase-9 and caspase-6, since only a pan-caspase inhibitor and the inhibitors preferentially target caspase-9 and -6, but not the ones antagonizing caspase-8, -3, or -7 alleviated the levels of PARP cleavage after virus infection and PI3K blockage. Furthermore, Bcl-2 appears to be a crucial mediator downstream of PI3K/Akt signaling, since overexpression of Bcl-2 reduced virus-induced apoptosis even when PI3K activation was repressed. Collectively, our results suggest an anti-apoptotic role for the PI3K/Akt pathway triggered by JEV and DEN-2 to protect infected cells from early apoptotic cell death.

Ultrastructural study of West Nile virus pathogenesis in Culex pipiens quinquefasciatus (Diptera: Culicidae)

The ultrastructural features of West Nile virus (WNV) replication and dissemination in orally infected Culex pipiens quinquefasciatus Say were analyzed over a 25-d infection period. To investigate the effects of virus replication on membrane induction, cellular organization, and cell viability in midgut and salivary gland tissues, midguts were dissected on days 3, 7, 14, and 21, and salivary glands were collected on days 7, 14, 21, and 25 postinfection (d.p.i.) for examination by transmission electron microscopy (TEM). Whole mosquito heads were embedded for TEM analysis 14 d.p.i. to localize WNV particles and to investigate the effects of replication on nervous tissues of the brain. Membrane proliferation was induced by WNV in the midgut epithelium, midgut muscles, and salivary glands, although extensive endoplasmic reticulum swelling was a unique feature of salivary gland infection. TEM revealed WNV-induced pathology in salivary glands at 14, 21, and 25 d.p.i., and we hypothesize that long-term virus infection of this tissue results in severe cellular degeneration and apoptotic-like cell death. This finding indicates that the efficiency of WNV transmission may decrease with mosquito age postinfection.

Activated peripheral blood mononuclear cells induce p44/42 mitogen-activated protein kinase phosphorylation in trophoblast-like JAR cells

Mammalian pregnancy bears many similarities to transplantation, since the fetus is semi-allogenic to mother. Thus, mammals have developed numerous mechanisms to protect the developing fetus from maternal immunologic recognition and attack. We have previously shown that human choriocarcinoma JAR cells, which resemble first trimester trophoblasts, regulate several important mRNAs in activated peripheral blood mononuclear cells (PBMC). We now provide further evidence that communication between maternal and fetal tissues is bi-directional, and that activation of PBMC leads to activation of specific signaling pathways in JAR cells. Activated PBMC were co-cultured with JAR cells for specific time intervals, after which JAR cells were lysed and subjected to western blotting for activated forms of the JNK, Erk 1-2, and p38 mitogen-activated protein kinases (MAPK). Phosphorylation of Erk 1-2, but not JNK or p38, was induced in co-cultures of PBMC and JAR cells. These results were also obtained when JAR cells were incubated with conditioned medium from activated, but not resting, PBMC. Results were confirmed using specific MAPK reporter constructs, using luciferase activity as a measure of Elk-1 phosphorylation. Erk 1-2 phosphorylation was not required for JAR cells to inhibit IL-2 production in activated PBMC. Addition of the specific MAPK inhibitor UO126 to JAR cells prior to the addition of activated PBMC to the cultures did not abolish the capacity of JAR cells to inhibit IL-2 mRNA expression in PBMC. We conclude that there is likely to be significant bi-directional signaling between leukocytes and trophoblasts at the maternal-fetal interface. We propose the existence of a delicate maternal-fetal immunologic homeostasis based on these experimental results.

Blocking of the alpha interferon-induced Jak-Stat signaling pathway by Japanese encephalitis virus infection

The induction of alpha/beta interferon (IFN-alpha/beta) is a powerful host defense mechanism against viral infection, and many viruses have evolved strategies to overcome the antiviral effects of IFN. In this study, we found that IFN-alpha had only some degree of antiviral activity against Japanese encephalitis virus (JEV) infection, in contrast to another flavivirus, dengue virus serotype 2, which was highly sensitive to IFN-alpha in the cultured cell system. JEV infection appeared to render cells resistant to IFN-alpha since the IFN-alpha-induced luciferase reporter activity driven by the IFN-stimulated response element (ISRE) was gradually reduced as the JEV infection progressed. Since the biological activities of IFNs are triggered by the Janus kinase (Jak) signal transducer and activation of transcription (Stat) signaling cascade, we then studied the activation of Jak-Stat pathway in the virus-infected cells. The IFN-alpha-stimulated tyrosine phosphorylation of Stat1, Stat2, and Stat3 was suppressed by JEV in a virus replication and de novo protein synthesis-dependent manner. Furthermore, JEV infection blocked the tyrosine phosphorylation of IFN receptor-associated Jak kinase, Tyk2, without affecting the expression of IFN-alpha/beta receptor on the cell surface. Consequently, expression of several IFN-stimulated genes in response to IFN-alpha stimulation was also reduced in the JEV-infected cells. Overall, our findings suggest that JEV counteracts the effect of IFN-alpha/beta by blocking Tyk2 activation, thereby resulting in inhibition of Jak-Stat signaling pathway.

Replication-incompetent virions of Japanese encephalitis virus trigger neuronal cell death by oxidative stress in a culture system

It has been shown that replication of the Japanese encephalitis virus (JEV) can trigger infected cells to undergo apoptosis. In the present study, it is further demonstrated that replication-incompetent virions of JEV, obtained by short-wavelength ultraviolet (UV) irradiation, could also induce host-cell death. It was found that UV-inactivated JEV (UV-JEV) caused cell death in neuronal cells such as mouse neuroblastoma N18 and human neuronal NT-2 cells, but not in non-neuronal baby hamster kidney BHK-21 fibroblast or human cervical HeLa cells. Only actively growing, but not growth-arrested, cells were susceptible to the cytotoxic effects of UV-JEV. Killing of UV-JEV-infected N18 cells could be antagonized by co-infection with live, infectious JEV, suggesting that virions of UV-JEV might engage an as-yet-unidentified receptor-mediated death-signalling pathway. Characteristically, mitochondrial alterations were evident in UV-JEV-infected N18 cells, as revealed by electron microscopy and a loss of membrane potential. N18 cells infected by UV-JEV induced generation of reactive oxygen species (ROS) as well as the activation of nuclear factor kappa B (NF-kappaB), and the addition of anti-oxidants or specific NF-kappaB inhibitors to the media greatly reduced the cytotoxicity of UV-JEV. Together, the results presented here suggest that replication-incompetent UV-JEV damages actively growing neuronal cells through a ROS-mediated pathway.

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.

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.

Influenza A virus replication is dependent on an antioxidant pathway that involves GSH and Bcl-2

Growing evidence indicates that viral replication is regulated by the redox state of the host cell. We demonstrate that cells of different origins display differential permissivity for influenza A virus replication, depending on their intracellular redox power as reflected by Bcl-2 expression and glutathione (GSH) content. Bcl-2 expressing cells were found to have higher intracellular levels of GSH and to produce lower amounts of virus than Bcl-2 negative cells. Two different steps in the virus life-cycle were involved in Bcl-2/GSH mediated viral inhibition: 1) expression of late viral proteins (in particular hemagglutinin and matrix); and 2) nuclear-cytoplasmic translocation of viral ribonucleoproteins (vRNPs). Buthionine-sulfoximine-induced inhibition of GSH synthesis in Bcl-2 expressing cells caused an increase in the expression of late viral proteins but did not restore vRNP export to the cytoplasm. Collectively, our findings show that both Bcl-2 expression and GSH content contribute to the host cell's ability to down-regulate influenza virus replication, although their effects are exerted at different stages of the viral life-cycle. In certain cell populations, this form of down-regulation might conceivably favor the establishment of persistent viral infection.

Recent advances in Japanese encephalitis

Japanese encephalitis (JE), the most important cause of epidemic encephalitis worldwide, is confined to Asia, but its geographical area is spreading. West Nile virus, and other closely related flaviviruses, cause similar disease elsewhere. Recent cryoelectron microscopic studies have characterized the flavivirus envelope protein as a new class of viral fusion protein (class II), and examined its arrangement on the virion surface. Changes in the envelope protein's hinge region, or its putative receptor-binding domain, are associated with changes in neurovirulence in animal models of JE. Clinically, JE causes a wide range of presentations, including a poliolike flaccid paralysis. Seizures and raised intracranial pressure are associated with a poor outcome, and may be potentially treatable. A safe efficacious formalin-inactivated vaccine against JE has been available for many years, but is too expensive for use in most Asian countries. A newer live attenuated vaccine has been used in China, but its use elsewhere has been restricted by regulatory concerns. A chimeric vaccine in which JE structural proteins are inserted into the 17D yellow fever vaccine backbone is one of several vaccines in development. There are no established antiviral treatments against JE. Interferon alpha was the most promising drug in small open trials, but a recent double-blind placebo controlled trial showed that it did not affect the outcome in children with JE.