Infection and injury of neurons by West Nile encephalitis virus

The interferon-inducible gene viperin restricts West Nile virus pathogenesis

Type I interferon (IFN) signaling coordinates an early antiviral program in infected and uninfected cells by inducing IFN-stimulated genes (ISGs) that modulate viral entry, replication, and assembly. However, the specific antiviral functions in vivo of most ISGs remain unknown. Here, we examined the contribution of the ISG viperin to the control of West Nile virus (WNV) in genetically deficient cells and mice. While modest increases in levels of WNV replication were observed for primary viperin(-/-) macrophages and dendritic cells, no appreciable differences were detected in deficient embryonic cortical neurons or fibroblasts. In comparison, viperin(-/-) adult mice infected with WNV via the subcutaneous or intracranial route showed increased lethality and/or enhanced viral replication in central nervous system (CNS) tissues. In the CNS, viperin expression was induced in both WNV-infected and adjacent uninfected cells, including activated leukocytes at the site of infection. Our experiments suggest that viperin restricts the infection of WNV in a tissue- and cell-type-specific manner and may be an important ISG for controlling viral infections that cause CNS disease.

PP2A targeting by viral proteins: a widespread biological strategy from DNA/RNA tumor viruses to HIV-1

Protein phosphatase 2A (PP2A) is a large family of holoenzymes that comprises 1% of total cellular proteins and accounts for the majority of Ser/Thr phosphatase activity in eukaryotic cells. Although initially viewed as constitutive housekeeping enzymes, it is now well established that PP2A proteins represent a family of highly and sophistically regulated phosphatases. The past decade, multiple complementary studies have improved our knowledge about structural and functional regulation of PP2A holoenzymes. In this regard, after summarizing major cellular regulation, this review will mainly focus on discussing a particulate biological strategy, used by various viruses, which is based on the targeting of PP2A enzymes by viral proteins in order to specifically deregulate, for their own benefit, cellular pathways of their hosts. The impact of such PP2A targeting for research in human diseases, and in further therapeutic developments, is also discussed.

West Nile virus: immunity and pathogenesis

West Nile virus (WNV) is a neurotropic, arthropod-borne flavivirus that is maintained in an enzootic cycle between mosquitoes and birds, but can also infect and cause disease in horses and humans. WNV is endemic in parts of Africa, Europe, the Middle East, and Asia, and since 1999 has spread to North America, Mexico, South America, and the Caribbean. WNV infects the central nervous system (CNS) and can cause severe disease in a small minority of infected humans, mostly immunocompromised or the elderly. This review discusses some of the mechanisms by which the immune system can limit dissemination of WNV infection and elaborates on the mechanisms involved in pathogenesis. Reasons for susceptibility to WNV-associated neuroinvasive disease in less than 1% of cases remain unexplained, but one favored hypothesis is that the involvement of the CNS is associated with a weak immune response allowing robust WNV replication in the periphery and spread of the virus to the CNS.

Treatment of spatial memory impairment in hamsters infected with West Nile virus using a humanized monoclonal antibody MGAWN1

In addition to functional disorders of paresis, paralysis, and cardiopulmonary complications, subsets of West Nile virus (WNV) patients may also experience neurocognitive deficits and memory disturbances. A previous hamster study has also demonstrated spatial memory impairment using the Morris water maze (MWM) paradigm. The discovery of an efficacious therapeutic antibody MGAWN1 from pre-clinical rodent studies raises the possibility of preventing or treating WNV-induced memory deficits. In the current study, hamsters were treated intraperitoneally (i.p.) with 32 mg/kg of MGAWN1 at 4.5 days after subcutaneously (s.c.) challenging with WNV. As expected, MGAWN1 prevented mortality, weight loss, and improved food consumption of WNV-infected hamsters. The criteria for entry of surviving hamsters into the study were that they needed to have normal motor function (forelimb grip strength, beam walking) and normal spatial reference memory in the MWM probe task. Twenty-eight days after the acute phase of the disease had passed, MGAWN1- and saline-treated infected hamsters were again trained in the MWM. Spatial memory was evaluated 48 h after this training in which the hamsters searched for the location where a submerged escape platform had been positioned. Only 56% of infected hamsters treated with saline spent more time in the correct quadrant than the other three quadrants, as compared to 92% of MGAWN1-treated hamsters (P⩽0.05). Overall these studies support the possibility that WNV can cause spatial memory impairment and that therapeutic intervention may be considered.

West Nile virus and its emergence in the United States of America

Zoonotic West Nile virus (WNV) circulates in natural transmission cycles involving certain mosquitoes and birds, horses, humans, and a range of other vertebrates are incidental hosts. Clinical infections in humans can range in severity from uncomplicated WNV fever to fatal meningoencephalitis. Since its introduction to the Western Hemisphere in 1999, WNV had spread across North America, Central and South America and the Caribbean, although the vast majority of severe human cases have occurred in the United States of America (USA) and Canada. By 2002-2003, the WNV outbreaks have involved thousands of patients causing severe neurologic disease (meningoencephalitis and poliomyelitis-like syndrome) and hundreds of associated fatalities in USA. The purpose of this review is to present recent information on the epidemiology and pathogenicity of WNV since its emergence in North America.

Recombinant West Nile virus envelope protein E and domain III expressed in insect larvae protects mice against West Nile disease

In this study, West Nile virus (WNV) envelope (rE) protein and its domain III (rDIII) were efficiently expressed in a cost-effective system based on insect larvae as non-fermentative living biofactories. Mice immunized with the partially purified rE or rDIII elicited high antibodies titers that neutralized viral infectivity in cell culture and in suckling mice. All vaccinated animals were fully protected when challenged with neurovirulent WNV NY99. Passive transfer of protective antibodies from immunized mothers to their offspring occurred both by transplacental and lactation routes. These results indicate that the insect-derived antigens tested may constitute potential vaccine candidates to be further evaluated.

Cyclooxygenase-2 inhibitor blocks the production of West Nile virus-induced neuroinflammatory markers in astrocytes

Inflammatory immune responses triggered initially to clear West Nile virus (WNV) infection later become detrimental and contribute to the pathological processes such as blood–brain barrier (BBB) disruption and neuronal death, thus complicating WNV-associated encephalitis (WNVE). It has been demonstrated previously that WNV infection in astrocytes results in induction of multiple matrix metalloproteinases (MMPs), which mediate BBB disruption. Cyclooxygenase (COX) enzymes and their product, prostaglandin E2 (PGE2), modulate neuroinflammation and regulate the production of multiple inflammatory molecules including MMPs. Therefore, this study determined and characterized the pathophysiological consequences of the expression of COX enzymes in human brain cortical astrocytes (HBCAs) following WNV infection. Whilst COX-1 mRNA expression did not change, WNV infection significantly induced RNA and protein expression of COX-2 in HBCAs. Similarly, PGE2 production was also enhanced significantly in infected HBCAs and was blocked in the presence of the COX-2-specific inhibitor NS-398, thus suggesting that COX-2, and not COX-1, was the source of the increased PGE2. Treatment of infected HBCAs with NS-398 attenuated the expression of MMP-1, -3 and -9 in a dose-dependent manner. Similarly, expression of interleukin-1β, -6 and -8, which were markedly elevated in infected HBCAs, exhibited a significant reduction in their levels in the presence of NS-398. These results provide direct evidence that WNV-induced COX-2/PGE2 is involved in modulating the expression of multiple neuroinflammatory mediators, thereby directly linking COX-2 with WNV disease pathogenesis. The ability of COX-2 inhibitors to modulate WNV-induced COX-2 and PGE2 signalling warrants further investigation in an animal model as a potential approach for clinical management of neuroinflammation associated with WNVE.

Pro-inflammatory cytokines derived from West Nile virus (WNV)-infected SK-N-SH cells mediate neuroinflammatory markers and neuronal death

Background

WNV-associated encephalitis (WNVE) is characterized by increased production of pro-inflammatory mediators, glial cells activation and eventual loss of neurons. WNV infection of neurons is rapidly progressive and destructive whereas infection of non-neuronal brain cells is limited. However, the role of neurons and pathological consequences of pro-inflammatory cytokines released as a result of WNV infection is unclear. Therefore, the objective of this study was to examine the role of key cytokines secreted by WNV-infected neurons in mediating neuroinflammatory markers and neuronal death.

Methods

A transformed human neuroblastoma cell line, SK-N-SH, was infected with WNV at multiplicity of infection (MOI)-1 and -5, and WNV replication kinetics and expression profile of key pro-inflammatory cytokines were analyzed by plaque assay, qRT-PCR, and ELISA. Cell death was measured in SK-N-SH cell line in the presence and absence of neutralizing antibodies against key pro-inflammatory cytokines using cell viability assay, TUNEL and flow cytometry. Further, naïve primary astrocytes were treated with UV-inactivated supernatant from mock- and WNV-infected SK-N-SH cell line and the activation of astrocytes was measured using flow cytometry and ELISA.

Results

WNV-infected SK-N-SH cells induced the expression of IL-1β, -6, -8, and TNF-α in a dose- and time-dependent manner, which coincided with increase in virus-induced cell death. Treatment of cells with anti-IL-1β or -TNF-α resulted in significant reduction of the neurotoxic effects of WNV. Furthermore treatment of naïve astrocytes with UV-inactivated supernatant from WNV-infected SK-N-SH cell line increased expression of glial fibrillary acidic protein and key inflammatory cytokines.

Conclusion

Our results for the first time suggest that neurons are one of the potential sources of pro-inflammatory cytokines in WNV-infected brain and these neuron-derived cytokines contribute to WNV-induced neurotoxicity. Moreover, cytokines released from neurons also mediate the activation of astrocytes. Our data define specific role(s) of WNV-induced pro-inflammatory cytokines and provide a framework for the development of anti-inflammatory drugs as much-needed therapeutic interventions to limit symptoms associated with WNVE.

The innate immune adaptor molecule MyD88 restricts West Nile virus replication and spread in neurons of the central nervous system

Type I interferons (IFN-α/β) control viral infection by triggering the expression of genes that restrict transcription, translation, replication, and assembly. Many viruses induce IFN responses after recognition by cytoplasmic or endosomal RNA sensors (RIG-I-like RNA helicases [RLR] and Toll-like receptors [TLR]), which signal through the cognate adaptor signaling molecules IPS-1, TRIF, and MyD88. Recent studies have demonstrated that IPS-1-dependent induction of IFN-α/β downstream of RLR recognition restricts West Nile virus (WNV) infection in many cell types, whereas TRIF-dependent TLR3 signaling limits WNV replication in neurons. Here, we examined the contribution of MyD88 signaling to the control of WNV by evaluating IFN induction and virus replication in genetically deficient cells and mice. MyD88(-/-) mice showed increased lethality after WNV infection and elevated viral burden primarily in the brain, even though little effect on the systemic type I IFN response was observed. Intracranial inoculation studies corroborated these findings, as WNV spread more rapidly in the central nervous system of MyD88(-/-) mice, and this phenotype preceded the recruitment of inflammatory leukocytes. In vitro, increased WNV replication was observed in MyD88(-/-) macrophages and subsets of neurons but not in myeloid dendritic cells. MyD88 had an independent effect on recruitment of monocyte-derived macrophages and T cells into the brain that was associated with blunted induction of the chemokines that attract leukocytes. Our experiments suggest that MyD88 restricts WNV by inhibiting replication in subsets of cells and modulating expression of chemokines that regulate immune cell migration into the central nervous system.

Caspase-12 controls West Nile virus infection via the viral RNA receptor RIG-I

Caspase-12 has been shown to negatively modulate inflammasome signaling during bacterial infection. Its function in viral immunity, however, has not been characterized. We now report an important role for caspase-12 in controlling viral infection via the pattern-recognition receptor RIG-I. After challenge with West Nile virus (WNV), caspase-12-deficient mice had greater mortality, higher viral burden and defective type I interferon response compared with those of challenged wild-type mice. In vitro studies of primary neurons and mouse embryonic fibroblasts showed that caspase-12 positively modulated the production of type I interferon by regulating E3 ubiquitin ligase TRIM25-mediated ubiquitination of RIG-I, a critical signaling event for the type I interferon response to WNV and other important viral pathogens.

Arboviral encephalitides: transmission, emergence, and pathogenesis

Arthropod-borne viruses (arboviruses) are of paramount concern as a group of pathogens at the forefront of emerging and re-emerging diseases. Although some arboviral infections are asymptomatic or present with a mild influenza-like illness, many are important human and veterinary pathogens causing serious illness ranging from rash and arthritis to encephalitis and hemorrhagic fever. Here, we discuss arboviruses from diverse families (Flaviviruses, Alphaviruses, and the Bunyaviridae) that are causative agents of encephalitis in humans. An understanding of the natural history of these infections as well as shared mechanisms of neuroinvasion and neurovirulence is critical to control the spread of these viruses and for the development of effective vaccines and treatment modalities.

Neurological suppression of diaphragm electromyographs in hamsters infected with West Nile virus

To address the hypothesis that respiratory distress associated with West Nile virus (WNV) is neurologically caused, electromyographs (EMGs) were measured longitudinally from the diaphragms of alert hamsters infected subcutaneously (s.c.) with WNV. The EMG activity in WNV-infected hamsters was consistently and significantly (P Collapse

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MESH Headings

• Afferent Pathways/physiopathology
• Afferent Pathways/virology
• Animals
• Brain Stem/physiopathology
• Brain Stem/virology
• Cervical Vertebrae
• Cricetinae
• Diaphragm/innervation
• Diaphragm/physiopathology
• Electromyography
• Evoked Potentials, Auditory
• Female
• Immunohistochemistry
• Mesocricetus
• Microscopy, Confocal
• Neurons/virology
• Spinal Cord/virology
• West Nile Fever/physiopathology
• West Nile virus

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Grants

• P20 RR020146-08 NCRR NIH HHS
• P20 RR020146 NCRR NIH HHS
• P30 GM110702 NIGMS NIH HHS
• N01 AI050036 NIAID NIH HHS
• N01AI30063 NIAID NIH HHS
• U54 AI065357 NIAID NIH HHS
• RR020146 NCRR NIH HHS
• R01 NS020246 NINDS NIH HHS
• U54 AI065357-05 NIAID NIH HHS
• 1 U54 AI-065357-04 NIAID NIH HHS

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Affiliation(s)

John D Morrey Institute for Antiviral Research, Department of Animal, Dairy, and Veterinary Sciences, Utah State University, Logan, Utah 84341, USA.
Venkatraman Siddharthan
Hong Wang
Jeffery O Hall
Neil E Motter
Robert D Skinner
Ramona T Skirpstunas

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TNF-alpha-dependent regulation of CXCR3 expression modulates neuronal survival during West Nile virus encephalitis

The chemokine CXCL10 exerts antiviral effects within the central nervous system (CNS) through the recruitment of virus-specific T cells. However, elevated levels of CXCL10 may induce neuronal apoptosis given its receptor, CXCR3, is expressed by neurons. Using a murine model of West Nile virus (WNV) encephalitis, we determined that WNV-infected neurons express TNF-alpha, which down-regulates neuronal CXCR3 expression via signaling through TNFR1. Down-regulation of neuronal CXCR3 decreased CXCL10-mediated calcium transients and delayed Caspase 3 activation. Loss of CXCR3 activation, via CXCR3-deficiency or pretreatment with TNF-alpha prevented neuronal apoptosis during in vitro WNV infection. These results suggest that neuronal TNF-alpha expression during WNV encephalitis may be an adaptive response to diminish CXCL10-induced death.

Flavivirus encephalitis: pathological aspects of mouse and other animal models

Encephalitic flaviviruses are important arthropod-borne pathogens of humans and other animals. In particular, the recent emergence of the West Nile virus (WNV) and Japanese encephalitis virus (JEV) in new geographic areas has caused a considerable public health alert and international concern. Among the experimental in vivo models of WNV and JEV infection, mice and other laboratory rodents are the most thoroughly studied and well-characterized systems, having provided data that are important for understanding the infectious process in humans. Macaca monkeys have also been used as a model for WNV and JEV infection, mainly for the evaluation of vaccine efficacy, although a limited number of published studies have addressed pathomorphology. These animal models demonstrate the development of encephalitis with many similarities to the human disease; however, the histological events that occur during infection, especially in peripheral tissues, have not been fully characterized.

IPS-1 is essential for the control of West Nile virus infection and immunity

The innate immune response is essential for controlling West Nile virus (WNV) infection but how this response is propagated and regulates adaptive immunity in vivo are not defined. Herein, we show that IPS-1, the central adaptor protein to RIG-I-like receptor (RLR) signaling, is essential for triggering of innate immunity and for effective development and regulation of adaptive immunity against pathogenic WNV. IPS-1^−/− mice exhibited increased susceptibility to WNV infection marked by enhanced viral replication and dissemination with early viral entry into the CNS. Infection of cultured bone-marrow (BM) derived dendritic cells (DCs), macrophages (Macs), and primary cortical neurons showed that the IPS-1-dependent RLR signaling was essential for triggering IFN defenses and controlling virus replication in these key target cells of infection. Intriguingly, infected IPS-1^−/− mice displayed uncontrolled inflammation that included elevated systemic type I IFN, proinflammatory cytokine and chemokine responses, increased numbers of inflammatory DCs, enhanced humoral responses marked by complete loss of virus neutralization activity, and increased numbers of virus-specific CD8+ T cells and non-specific immune cell proliferation in the periphery and in the CNS. This uncontrolled inflammatory response was associated with a lack of regulatory T cell expansion that normally occurs during acute WNV infection. Thus, the enhanced inflammatory response in the absence of IPS-1 was coupled with a failure to protect against WNV infection. Our data define an innate/adaptive immune interface mediated through IPS-1-dependent RLR signaling that regulates the quantity, quality, and balance of the immune response to WNV infection.

West Nile virus (WNV) is a mosquito-transmitted RNA virus that has emerged in the Western hemisphere and is now the leading cause of arboviral encephalitis in the United States. However, the virus/host interface that controls WNV pathogenesis is not well understood. Previous studies have established that the innate immune response and interferon (IFN) defenses are essential for controlling virus replication and dissemination. In this study, we assessed the importance of the RIG-I like receptor (RLR) signaling pathway in WNV pathogenesis through analysis of mice lacking IPS-1, the central adaptor molecule of RLR signaling. Our studies revealed that IPS-1 is essential for protection against WNV infection and that it regulates processes that control virus replication and triggering of innate immune defenses. We found that IPS-1 plays an important role in establishing adaptive immunity through an innate/adaptive interface that elicits effective antibody responses and controls the expansion of regulatory T cells. Thus, RLRs are essential for pathogen recognition of WNV infection and their signaling programs help orchestrate immune response maturation, regulation of inflammation, and immune homeostasis that define the outcome of WNV infection.

HIV Coreceptors and Their Roles in Leukocyte Trafficking During Neuroinflammatory Diseases

Due to the increasing resistance of HIV-1 to antiretroviral therapies, there has been much emphasis on the discovery and development of alternative therapeutics for HIV-1-infected individuals. The chemokine receptors CXCR4 (Bleul et al. 1996a; Feng et al. 1996; Nagasawa et al. 1996; Oberlin et al. 1996) and CCR5 (Alkhatib et al. 1996; Deng et al. 1996; Dragic et al. 1996) were identified as target molecules from the time their role as coreceptors for HIV-1 entry into leukocytes was first discovered 10 years ago. Initial studies focused on the use of the chemokine ligands, or altered derivatives, of CXCR4 and CCR5 to prevent the entrance of HIV-1 into immune cells (Schols 2006). While these studies showed some initial promise, there was evidence of significant caveats to their use, including selection of alternative coreceptor utilizing strains (Marechal et al. 1999; Mosier et al. 1999) and the potential to cause inflammatory side effects. These data prompted the development and study of small molecule inhibitors of CXCR4 and CCR5, which have also been used to examine the roles of these molecules in a variety of inflammatory and infectious diseases.

The immune adaptor molecule SARM modulates tumor necrosis factor alpha production and microglia activation in the brainstem and restricts West Nile Virus pathogenesis

Sterile alpha and HEAT/Armadillo motif (SARM) is a highly conserved Toll/interleukin-1 receptor (TIR)-containing adaptor protein that is believed to negatively regulate signaling of the pathogen recognition receptors Toll-like receptor 3 (TLR3) and TLR4. To test its physiological function in the context of a microbial infection, we generated SARM(-/-) mice and evaluated the impact of this deficiency on the pathogenesis of West Nile virus (WNV), a neurotropic flavivirus that requires TLR signaling to restrict infection. Although SARM was preferentially expressed in cells of the central nervous system (CNS), studies with primary macrophages, neurons, or astrocytes showed no difference in viral growth kinetics. In contrast, viral replication was increased specifically in the brainstem of SARM(-/-) mice, and this was associated with enhanced mortality after inoculation with a virulent WNV strain. A deficiency of SARM was also linked to reduced levels of tumor necrosis factor alpha (TNF-alpha), decreased microglia activation, and increased neuronal death in the brainstem after WNV infection. Thus, SARM appears to be unique among the TIR adaptor molecules, since it functions to restrict viral infection and neuronal injury in a brain region-specific manner, possibly by modulating the activation of resident CNS inflammatory cells.

Progress on the development of therapeutics against West Nile virus

A decade has passed since the appearance of West Nile virus (WNV) in humans in the Western Hemisphere in New York City. During this interval, WNV spread inexorably throughout North and South America and caused millions of infections ranging from a sub-clinical illness, to a self-limiting febrile syndrome or lethal neuroinvasive disease. Its entry into the United States triggered intensive research into the basic biology of WNV and the elements that comprise a protective host immune response. 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 current state of knowledge on epidemiology, clinical presentation, pathogenesis, and immunobiology of WNV infection, and highlights progress toward an effective therapy.

Effects of cytomegalovirus infection on embryogenesis and brain development

Congenital cytomegalovirus (CMV) infection is a significant cause of brain disorders, such as microcephaly, mental retardation, hearing loss and visual disorders in humans. The type and severity of brain disorder may be dependent on the stage of embryonic development when the congenital infection occurs. Developmental disorders may be associated with the type of embryonic cells to which CMV is susceptible and the effects of the infection on the cellular functions of these cells. Early murine embryos, including embryonic stem (ES) cells, are not susceptible to CMV infection. A part of the embryonic cells acquire susceptibility during early development. Mesenchymal cells are the targets of infection at midgestation, affecting organogenesis of the brain, eyes and oral-facial regions. In contrast to ES cells, neural stem progenitor cells (NSPC) from fetal brains are susceptible to murine CMV (MCMV) infection. The viral infection inhibits proliferation and differentiation of the NSPC to neuronal and glial cells in addition to induction of neuronal cell loss. These cellular events may cause brain malformations, such as microcephaly and polymicrogyria. Furthermore, MCMV persists in neuronal cells in developing brains, presumably resulting in neuronal dysfunction.

The structural immunology of antibody protection against West Nile virus

SUMMARY

Recent investigations of the interaction between the West Nile virus (WNV) envelope protein (E) and monoclonal antibodies (mAbs) have elucidated fundamental insights into the molecular mechanisms of neutralization. Structural studies have defined an epitope on the lateral ridge of domain III (DIII-lr) of the WNV E protein that is recognized by antibodies with the strongest neutralizing activity in vitro and in vivo. Antibodies that bind this epitope are highly potent because they efficiently block at a post-entry step of viral infection with relatively low virion occupancy requirements. In this review, we discuss the structural, molecular, and immunologic basis for antibody-mediated protection against WNV, and its implications for novel therapeutic or vaccine strategies.

Mouse embryonic stem cells are not susceptible to cytomegalovirus but acquire susceptibility during differentiation

BACKGROUND

Cytomegalovirus (CMV) is the most significant infectious cause of congenital anomalies of the central nervous system caused by intrauterine infection in humans. The timing of infection and the susceptibility of cells in early gestational stages are not well understood. In this study we investigated the susceptibility of embryonic stem (ES) cells to CMV infection during differentiation.

METHODS

ES cell lines were established from transgenic mice integrated with the murine CMV (MCMV) immediate-early (IE) promoter connected with a reporter lacZ gene. The susceptibility of the ES cells was analyzed in terms of viral gene expression and viral replication after induction of differentiation.

RESULTS

ES cells were nonpermissive to MCMV infection in the undifferentiated state. Upon differentiation, permissive cells appeared approximately 2 weeks after the leukemia inhibitory factor was removed. Upon neural differentiation by retinoic acid (RA), glial cells showed specific susceptibility in terms of expression of the viral antigen. The MCMV IE promoter was not activated in ES cells from the transgenic mice. Activation of the IE promoter was detected approximately 2 weeks after induction of differentiation and observed predominantly in glial cells. Upon MCMV infection of the ES cells, viral infection was correlated with the activation of the IE promoter.

CONCLUSIONS

ES cells are nonpermissive to MCMV infection and acquire permissiveness about 2 weeks after induction of differentiation, especially in glial cells. Acquisition of permissiveness in differentiated ES cells may be associated with activation of the IE promoter.

Persistent West Nile virus associated with a neurological sequela in hamsters identified by motor unit number estimation

To investigate the hypothesis that neurological sequelae are associated with persistent West Nile virus (WNV) and neuropathology, we developed an electrophysiological motor unit number estimation (MUNE) assay to measure the health of motor neurons temporally in hamsters. The MUNE assay was successful in identifying chronic neuropathology in the spinal cords of infected hamsters. MUNE was suppressed at days 9 to 92 in hamsters injected subcutaneously with WNV, thereby establishing that a long-term neurological sequela does occur in the hamster model. MUNE suppression at day 10 correlated with the loss of neuronal function as indicated by reduced choline acetyltransferase staining (R(2) = 0.91). Between days 10 and 26, some alpha-motor neurons had died, but further neuronal death was not detected beyond day 26. MUNE correlated with disease phenotype, because the lowest MUNE values were detected in paralyzed limbs. Persistent WNV RNA and foci of WNV envelope-positive cells were identified in the central nervous systems of all hamsters tested from 28 to 86 days. WNV-positive staining colocalized with the neuropathology, which suggested that persistent WNV or its products contributed to neuropathogenesis. These results established that persistent WNV product or its proteins cause dysfunction, that WNV is associated with chronic neuropathological lesions, and that this neurological sequela is effectively detected by MUNE. Inasmuch as WNV-infected humans can also experience a poliomyelitis-like disease where motor neurons are damaged, MUNE may also be a sensitive clinical or therapeutic marker for those patients.

Apoptosis in animal models of virus-induced disease

Apoptosis is associated with virus-induced human diseases of the central nervous system, heart and liver, and causes substantial morbidity and mortality. Although virus-induced apoptosis is well characterized in individual cells in cell culture, virus-induced apoptosis in vivo and the role of apoptosis in virus-induced disease is not well established. This review focuses on animal models of virus-induced diseases of the central nervous system, heart and liver that provide insights into the role of apoptosis in pathogenesis, the pathways involved and the potential therapeutic implications.

West Nile virus infection modulates human brain microvascular endothelial cells tight junction proteins and cell adhesion molecules: Transmigration across the in vitro blood-brain barrier

Neurological complications such as inflammation, failure of the blood-brain barrier (BBB), and neuronal death contribute to the mortality and morbidity associated with WNV-induced meningitis. Compromised BBB indicates the ability of the virus to gain entry into the CNS via the BBB, however, the underlying mechanisms, and the specific cell types associated with WNV-CNS trafficking are not well understood. Brain microvascular endothelial cells, the main component of the BBB, represent a barrier to virus dissemination into the CNS and could play key role in WNV spread via hematogenous route. To investigate WNV entry into the CNS, we infected primary human brain microvascular endothelial (HBMVE) cells with the neurovirulent strain of WNV (NY99) and examined WNV replication kinetics together with the changes in the expressions of key tight junction proteins (TJP) and cell adhesion molecules (CAM). WNV infection of HBMVE cells was productive as analyzed by plaque assay and qRT-PCR, and did not induce cytopathic effect. Increased mRNA and protein expressions of TJP (claudin-1) and CAM (vascular cell adhesion molecule and E-selectin) were observed at days 2 and 3 after infection, respectively, which coincided with the peak in WNV replication. Further, using an in vitro BBB model comprised of HBMVE cells, we demonstrate that cell-free WNV can cross the BBB, without compromising the BBB integrity. These data suggest that infection of HBMVE cells can facilitate entry of cell-free virus into the CNS without disturbing the BBB, and increased CAM may assist in the trafficking of WNV-infected immune cells into the CNS, via 'Trojan horse' mechanism, thereby contributing to WNV dissemination in the CNS and associated pathology.

Independent regulation of reovirus membrane penetration and apoptosis by the mu1 phi domain

Apoptosis plays an important role in the pathogenesis of reovirus encephalitis. Reovirus outer-capsid protein mu1, which functions to penetrate host cell membranes during viral entry, is the primary regulator of apoptosis following reovirus infection. Ectopic expression of full-length and truncated forms of mu1 indicates that the mu1 phi domain is sufficient to elicit a cell death response. To evaluate the contribution of the mu1 phi domain to the induction of apoptosis following reovirus infection, phi mutant viruses were generated by reverse genetics and analyzed for the capacity to penetrate cell membranes and elicit apoptosis. We found that mutations in phi diminish reovirus membrane penetration efficiency by preventing conformational changes that lead to generation of key reovirus entry intermediates. Independent of effects on membrane penetration, amino acid substitutions in phi affect the apoptotic potential of reovirus, suggesting that phi initiates apoptosis subsequent to cytosolic delivery. In comparison to wild-type virus, apoptosis-defective phi mutant viruses display diminished neurovirulence following intracranial inoculation of newborn mice. These results indicate that the phi domain of mu1 plays an important regulatory role in reovirus-induced apoptosis and disease.

CD8+ T-cells mediate immunopathology in tick-borne encephalitis

Epidemics of tick-borne encephalitis involving thousands of humans occur annually in the forested regions of Europe and Asia. Despite the importance of this disease, the underlying basis for the development of encephalitis remains undefined. Here, we prove the key role of CD8(+) T-cells in the immunopathology of tick-borne encephalitis, as demonstrated by prolonged survival of SCID or CD8(-/-) mice, following infection, when compared with immunocompetent mice or mice with adoptively transferred CD8(+) T-cells. The results imply that tick-borne encephalitis is an immunopathological disease and that the inflammatory reaction significantly contributes to the fatal outcome of the infection.

Axonal degeneration as a self-destructive defense mechanism against neurotropic virus infection

Theiler's murine encephalomyelitis virus (TMEV) and other neurotropic virus infections result in degeneration of each component of the neuron: apoptosis of the cell body, axonal (Wallerian) degeneration, and dendritic and synaptic pathology. In general, axonal degeneration is detrimental for hosts. However, axonal degeneration can be beneficial in the case of infection with neurotropic viruses that spread in the CNS using axonal transport. C57BL/Wld(S) (Wld(S), Wallerian degeneration slow mutant) mice are protected from axonal degeneration. Wld(S) mice infected with the neurovirulent GDVII strain of TMEV are more resistant to virus infection than wild-type mice, suggesting that axonal preservation contributes to the resistance. By contrast, infection with the less virulent Daniels strain of TMEV results in high levels of virus propagation in the CNS, suggesting that prolonged survival of axons in Wld(S) mice favors virus spread. Thus, axonal degeneration might be a beneficial self-destruct mechanism that limits the spread of neurotropic viruses, in the case of less virulent virus infection. We hypothesize that neurons use 'built-in' self-destruct protection machinery (compartmental neurodegeneration) against neurotropic virus infection, since the CNS is an immunologically privileged site. Early induction of apoptosis in the neuronal cell body limits virus replication. Wallerian degeneration of the axon prevents axonal transport of virus. Dendritic and synaptic degeneration blocks virus transmission at synapses. Thus, the balance between neurodegeneration and virus propagation may be taken into account in the future design of neuroprotective therapy.

Drak2 contributes to West Nile virus entry into the brain and lethal encephalitis

Death-associated protein kinase-related apoptosis-inducing kinase-2 (Drak2), a member of the death-associated protein family of serine/threonine kinases, is specifically expressed in T and B cells. In the absence of Drak2, mice are resistant to experimental autoimmune encephalomyelitis due to a decrease in the number of cells infiltrating the CNS. In the present study, we investigated the role of Drak2 in West Nile virus (WNV)-induced encephalitis and found that Drak2(-/-) mice were also more resistant to lethal WNV infection than wild-type mice. Although Drak2(-/-) mice had an increase in the number of IFN-gamma-producing T cells in the spleen after infection, viral levels in the peripheral tissues were not significantly different between these two groups of mice. In contrast, there was a reduced viral load in the brains of Drak2(-/-) mice, which was accompanied by a decrease in the number of Drak2(-/-) CD4(+) and CD8(+) T cells in the brain following WNV infection. Moreover, we detected viral Ags in T cells isolated from the spleen or brain of WNV-infected mice. These results suggest that following a systemic infection, WNV might cross the blood brain barrier and enter the CNS by being carried by infected infiltrating T cells.

CXCR4 antagonism increases T cell trafficking in the central nervous system and improves survival from West Nile virus encephalitis

The migration of lymphocytes into the CNS during viral encephalitis is hindered by the blood-brain barrier (BBB) such that most infiltrating cells remain localized to perivascular spaces. This sequestration of leukocytes away from the parenchyma is believed to protect the CNS from immunopathologic injury. Infections of the CNS with highly cytopathic neurotropic viruses, such as West Nile virus (WNV), however, require the parenchymal penetration of T lymphocytes for virus clearance and survival, suggesting that perivascular localization might hinder antiviral immune responses during WNV encephalitis. Using human and murine brain specimens from individuals with WNV encephalitis, we evaluated the expression of CXCL12 and its receptor, CXCR4, at the BBB and tested the hypothesis that inhibition of CXCR4 would promote T lymphocyte entry into the CNS parenchyma and increase viral clearance. Antagonism of CXCR4 significantly improved survival from lethal infection through enhanced intraparenchymal migration of WNV-specific CD8(+) T cells within the brain, leading to reduced viral loads and, surprisingly, decreased immunopathology at this site. The benefits of enhanced CD8(+) T cell infiltration suggest that pharmacologic targeting of CXCR4 may have therapeutic utility for the treatment of acute viral infections of the CNS.

Tumor necrosis factor alpha protects against lethal West Nile virus infection by promoting trafficking of mononuclear leukocytes into the central nervous system

West Nile virus (WNV) is a neurotropic flavivirus that has emerged globally as a significant cause of viral encephalitis in humans, especially in immunocompromised individuals. Previous studies have shown essential protective roles for antiviral cytokines (e.g., alpha interferon [IFN-alpha] and IFN-gamma) against WNV in mice. However, studies using cell culture offer conflicting answers regarding whether tumor necrosis factor alpha (TNF-alpha) has an anti-WNV function. To test the biological significance of TNF-alpha against WNV in vivo, experiments were performed with TNF receptor-1 (TNF-R1)-deficient and TNF-alpha-depleted C57BL/6 mice. TNF-R1(-/-) mice had enhanced mortality and decreased survival time after WNV infection compared to congenic wild-type mice. Consistent with this, administration of a neutralizing anti-TNF-alpha monoclonal antibody also decreased survival after WNV infection. Relatively small differences in viral burdens in peripheral tissues of TNF-R1(-/-) mice were observed, and this occurrence correlated with a modest antiviral effect of TNF-alpha on primary macrophages but not dendritic cells. In contrast, the viral titers detected in the central nervous systems of TNF-R1(-/-) mice were significantly increased compared to those of wild-type mice, although TNF-alpha did not have a direct antiviral effect in primary neuron cultures. Whereas no defect in priming of adaptive B- and T-cell responses in TNF-R1(-/-) mice was observed, there were significant reductions in accumulations of CD8+ T cells and macrophages in the brain. Our data are most consistent with a model in which interaction of TNF-alpha with TNF-R1 protects against WNV infection by regulating migration of protective inflammatory cells into the brain during acute infection.

West Nile virus-induced acute flaccid paralysis is prevented by monoclonal antibody treatment when administered after infection of spinal cord neurons

Acute flaccid polio-like paralysis occurs during natural West Nile virus (WNV) infection in a subset of cases in animals and humans. To evaluate the pathology and the possibility for therapeutic intervention, the authors developed a model of acute flaccid paralysis by injecting WNV directly into the sciatic nerve or spinal cord of hamsters. By directly injecting selected sites of the nervous system with WNV, the authors mapped the lesions responsible for hind limb paralysis to the lumbar spinal cord. Immunohistochemical analysis of spinal cord sections from paralyzed hamsters revealed that WNV-infected neurons localized primarily to the ventral motor horn of the gray matter, consistent with the polio-like clinical presentation. Neuronal apoptosis and diminished cell function were identified by TUNEL (terminal deoxynucleotidyl transferase-mediated BrdUTP nick end labeling) and choline acetyltransferase staining, respectively. Administration of hE16, a potently neutralizing humanized anti-WNV monoclonal antibody, 2 to 3 days after direct WNV infection of the spinal cord, significantly reduced paralysis and mortality. Additionally, a single injection of hE16 as late as 5 days after WNV inoculation of the sciatic nerve also prevented paralysis. Overall, these experiments establish that WNV-induced acute flaccid paralysis in hamsters is due to neuronal infection and injury in the lumbar spinal cord and that treatment with a therapeutic antibody prevents paralysis when administered after WNV infection of spinal cord neurons.

Immunological headgear: antiviral immune responses protect against neuroinvasive West Nile virus

With the emergence of epidemic strains of West Nile virus (WNV) in North America, there has been a surge in new research and knowledge regarding the peripheral immune responses that prevent neuroinvasion, the routes of WNV entry into the central nervous system (CNS) and the critical CNS immune responses that promote viral clearance and recovery at this anatomic site. WNV infection induces archetypal antiviral immune responses that, in most cases, lead to elimination of the virus with relatively few immunopathological consequences. Here, we present our current understanding of the innate and adaptive immune responses that limit dissemination to the CNS from WNV infection and the antiviral immune responses within the CNS that intervene when they fail.

Experimental reovirus-induced acute flaccid paralysis and spinal motor neuron cell death

Acute flaccid paralysis (AFP) describes the loss of motor function in 1 or more limbs commonly associated with viral infection and destruction of motor neurons in the anterior horns of the spinal cord. Therapy is limited, and the development of effective treatments is hampered by a lack of experimental models. Reovirus infection of neonatal mice provides a model for the study of CNS viral infection pathogenesis. Injection of the Reovirus serot Type 3 strains Abney (T3A) or Dearing (T3D) into the hindlimb of 1-day-old mice resulted in the development of AFP in more than 90% of infected mice. Acute flaccid paralysis began in the ipsilateral hindlimb at 8 to 10 days postinfection and progressed to paraplegia 24 hours later. Paralysis correlated with injury, neuron loss, and spread of viral antigen first to the ipsilateral and then to the contralateral anterior horns. As demonstrated by the activation of caspase 3 and its colocalization with viral antigen in the anterior horn and concomitant cleavage of poly-(adenosine diphosphate-ribose) polymerase, AFP was associated with apoptosis. Calpain activity and inducible nitric oxide synthase expression were both elevated in the spinal cords of paralyzed animals. This study represents the first detailed characterization of a novel and highly efficient experimental model of virus-induced AFP that will facilitate evaluation of therapeutic strategies targeting virus-induced paralysis.

CXCR3 mediates region-specific antiviral T cell trafficking within the central nervous system during West Nile virus encephalitis

Regional differences in inflammation during viral infections of the CNS suggest viruses differentially induce patterns of chemoattractant expression, depending on their cellular targets. Previous studies have shown that expression of the chemokine CXCL10 by West Nile virus (WNV)-infected neurons is essential for the recruitment of CD8 T cells for the purpose of viral clearance within the CNS. In the current study we used mice deficient for the CXCL10 receptor, CXCR3, to evaluate its role in leukocyte-mediated viral clearance of WNV infection within various CNS compartments. WNV-infected CXCR3-deficient mice exhibited significantly enhanced mortality compared with wild-type controls. Immunologic and virologic analyses revealed that CXCR3 was dispensable for control of viral infection in the periphery and in most CNS compartments but, surprisingly, was required for CD8 T cell-mediated antiviral responses specifically within the cerebellum. WNV-specific, CXCR3-expressing T cells preferentially migrated into the cerebellum, and WNV-infected cerebellar granule cell neurons expressed higher levels of CXCL10 compared with similarly infected cortical neurons. These results indicate that WNV differentially induces CXCL10 within neuronal populations and suggest a novel model for nonredundancy in chemokine-mediated inflammation among CNS compartments.

Increased blood-brain barrier permeability is not a primary determinant for lethality of West Nile virus infection in rodents

Blood-brain barrier (BBB) permeability was evaluated in mice and hamsters infected with West Nile virus (WNV, flavivirus) as compared to those infected with Semliki Forest (alphavirus) and Banzi (flavivirus) viruses. BBB permeability was determined by measurement of fluorescence in brain homogenates or cerebrospinal fluid (CSF) after intraperitoneal (i.p.) injection of sodium fluorescein, by macroscopic examination of brains after i.p. injection of Evans blue, or by measurement of total protein in CSF compared to serum. Lethal infection of BALB/c mice with Semliki Forest virus and Banzi virus caused the brain : serum fluorescence ratios to increase from a baseline of 2-4% to as high as 11 and 15%, respectively. Lethal infection of BALB/c mice with WNV did not increase BBB permeability. When C57BL/6 mice were used, BBB permeability was increased in some, but not all, of the WNV-infected animals. A procedure was developed to measure BBB permeability in live WNV-infected hamsters by comparing the fluorescence in the CSF, aspirated from the cisterna magnum, with the fluorescence in the serum. Despite a time-dependent tendency towards increased BBB permeability in some WNV-infected hamsters, the highest BBB permeability values did not correlate with mortality. These data indicated that a measurable increase in BBB permeability was not a primary determinant for lethality of WNV infection in rodents. The lack of a consistent increase in BBB permeability in WNV-infected rodents has implications for the understanding of viral entry, viral pathogenesis and accessibility of the CNS of rodents to drugs or effector molecules.

The molecular basis of antibody protection against West Nile virus

West Nile virus (WNV) infection of mosquitoes, birds, and vertebrates continues to spread in the Western Hemisphere. In humans, WNV infects the central nervous system and causes severe disease, primarily in the immunocompromised and elderly. In this review we discuss the mechanisms by which antibody controls WNV infection. Recent virologic, immunologic, and structural experiments have enhanced our understanding on how antibodies neutralize WNV and protect against disease. These advances have significant implications for the development of novel antibody-based therapies and targeted vaccines.

Axonal transport mediates West Nile virus entry into the central nervous system and induces acute flaccid paralysis

West Nile virus (WNV) has emerged as a significant cause of epidemic viral encephalitis and flaccid limb paralysis, yet the mechanism by which it enters the CNS remains uncertain. We used compartmentalized neuron cultures to demonstrate that WNV spreads in both retrograde and anterograde directions via axonal transport. Transneuronal spread of WNV required axonal release of viral particles and was blocked by addition of a therapeutic neutralizing antibody. To test the physiologic significance of axonal transport in vivo, we directly inoculated the sciatic nerve of hamsters with WNV. Intrasciatic infection resulted in paralysis of the hind limb ipsilateral but not contralateral to the injection site. Limb paralysis was blocked either by surgical transection of the sciatic nerve or treatment with the therapeutic neutralizing antibody. Collectively, these studies establish that WNV undergoes bidirectional spread in neurons and that axonal transport promotes viral entry into the CNS and acute limb paralysis. Moreover, antibody therapeutics directly inhibit transneuronal spread of WNV infection and prevent the development of paralysis in vivo.

Cell-specific IRF-3 responses protect against West Nile virus infection by interferon-dependent and -independent mechanisms

Interferon regulatory factor (IRF)-3 is a master transcription factor that activates host antiviral defense programs. Although cell culture studies suggest that IRF-3 promotes antiviral control by inducing interferon (IFN)-beta, near normal levels of IFN-alpha and IFN-beta were observed in IRF-3(-/-) mice after infection by several RNA and DNA viruses. Thus, the specific mechanisms by which IRF-3 modulates viral infection remain controversial. Some of this disparity could reflect direct IRF-3-dependent antiviral responses in specific cell types to control infection. To address this and determine how IRF-3 coordinates an antiviral response, we infected IRF-3(-/-) mice and two primary cells relevant for West Nile virus (WNV) pathogenesis, macrophages and cortical neurons. IRF-3(-/-) mice were uniformly vulnerable to infection and developed elevated WNV burdens in peripheral and central nervous system tissues, though peripheral IFN responses were largely normal. Whereas wild-type macrophages basally expressed key host defense molecules, including RIG-I, MDA5, ISG54, and ISG56, and restricted WNV infection, IRF-3(-/-) macrophages lacked basal expression of these host defense genes and supported increased WNV infection and IFN-alpha and IFN-beta production. In contrast, wild-type cortical neurons were highly permissive to WNV and did not basally express RIG-I, MDA5, ISG54, and ISG56. IRF-3(-/-) neurons lacked induction of host defense genes and had blunted IFN-alpha and IFN-beta production, yet exhibited only modestly increased viral titers. Collectively, our data suggest that cell-specific IRF-3 responses protect against WNV infection through both IFN-dependent and -independent programs.

Interactions between the West Nile virus capsid protein and the host cell-encoded phosphatase inhibitor, I2PP2A

The West Nile virus (WNV) capsid protein functions in virus assembly to package genomic RNA into nucleocapsid structures. It is becoming clear, that in addition to their structural roles, capsid proteins of RNA viruses have non-structural functions. For example, the WNV capsid protein has been implicated as a pathogenic determinant. Presumably, many, if not all, of the non-structural functions of this protein involve interactions with host cell-encoded proteins. In the present study, we used affinity purification to isolate human proteins that bind to the WNV capsid protein. One of the capsid binding proteins is I(2)(PP2A), a previously characterized inhibitor of the serine/threonine phosphatase PP2A. Mapping studies revealed that capsid binding site overlaps with the region of I(2)(PP2A) that is required for inhibition of PP2A activity. Moreover, expression of the WNV capsid protein resulted in significantly increased PP2A activity and expected downstream events, such as inhibition of AP1-dependent transcription. Infected cells treated with I(2)(PP2A)-specific siRNAs produced less infectious virus than control siRNA-transfected cells, but this difference was minimal. Together, our data indicate that interactions between WNV capsid and I(2)(PP2A) result in increased PP2A activity. Given the central role of this phosphatase in cellular physiology, capsid/I(2)(PP2A) interactions may yet prove to be important for viral pathogenesis.

Fas ligand interactions contribute to CD8+ T-cell-mediated control of West Nile virus infection in the central nervous system

West Nile virus (WNV) is a neurotropic flavivirus that causes encephalitis, most frequently in elderly and immunocompromised humans. Previous studies demonstrated that CD8+ T cells utilize perforin-dependent cytolytic mechanisms to limit WNV infection. Nonetheless, the phenotype of perforin-deficient CD8+ T cells was not as severe as that of an absence of CD8+ T cells, suggesting additional effector control mechanisms. In this study, we evaluated the contribution of Fas-Fas ligand (FasL) interactions to CD8+ T-cell-mediated control of WNV infection. Notably, the cell death receptor Fas was strongly upregulated on neurons in culture and in vivo after WNV infection. gld mice that were functionally deficient in FasL expression showed increased susceptibility to lethal WNV infection. Although antigen-specific priming of CD8+ T cells in peripheral lymphoid tissues was normal in gld mice, increased central nervous system (CNS) viral burdens and delayed clearance were observed. Moreover, the adoptive transfer of WNV-primed wild-type but not gld CD8+ T cells to recipient CD8(-/-) or gld mice efficiently limited infection in the CNS and enhanced survival rates. Overall, our data suggest that CD8+ T cells also utilize FasL effector mechanisms to contain WNV infection in Fas-expressing neurons in the CNS.

Role of IFN-gamma in an experimental murine model of West Nile virus-induced seizures

Seizures are a major complication of viral encephalitis. However, the mechanisms of seizure-associated neuronal dysfunction remain poorly understood. We report that intranasal inoculation with West Nile virus (WNV) (Sarafend) causes limbic seizures in C57BL/6 mice, but not in interferon (IFN)-gamma-deficient (IFN-gamma-/-) mice. Both strains showed similar levels of virus in the brain, as well as similar concentrations of the cytokines, tumor necrosis factor and interleukin-6, both of which can alter neuronal excitability. Experiments in chimeric IFN-gamma-/- mice reconstituted with IFN-gamma-producing leukocytes showed that IFN-gamma is not required during central nervous system infection for limbic seizure development, suggesting a role for IFN-gamma in the developing brain. This was supported responses to pentylenetetrazole, kainic acid (KA), and N-methyl-d-aspartate (NMDA). Both strains of mice exhibited similar behavior after pentylenetetrazole challenge. However, while NMDA and KA treatment resulted in characteristic seizures in C57BL/6 mice, these responses were diminished (NMDA treatment) or absent (KA treatment) in IFN-gamma-/- mice. Furthermore, NMDA-receptor blockade with MK-801 in WNV-infected C57BL/6 mice abrogated seizures and prolonged survival. Our data show that IFN-gamma plays an important role in the development of the excitatory seizure pathways in the brain and that these cascades become pathogenic in encephalitic WNV infection.

Identification of novel small-molecule inhibitors of West Nile virus infection

West Nile virus (WNV) has spread throughout the United States and Canada and now annually causes a clinical spectrum of human disease ranging from a self-limiting acute febrile illness to acute flaccid paralysis and lethal encephalitis. No therapy or vaccine is currently approved for use in humans. Using high-throughput screening assays that included a luciferase expressing WNV subgenomic replicon and an NS1 capture enzyme-linked immunosorbent assay, we evaluated a chemical library of over 80,000 compounds for their capacity to inhibit WNV replication. We identified 10 compounds with strong inhibitory activity against genetically diverse WNV and Kunjin virus isolates. Many of the inhibitory compounds belonged to a chemical family of secondary sulfonamides and have not been described previously to inhibit WNV or other related or unrelated viruses. Several of these compounds inhibited WNV infection in the submicromolar range, had selectivity indices of greater than 10, and inhibited replication of other flaviviruses, including dengue and yellow fever viruses. One of the most promising compounds, AP30451, specifically blocked translation of a yellow fever virus replicon but not a Sindbis virus replicon or an internal ribosome entry site containing mRNA. Overall, these compounds comprise a novel class of promising inhibitors for therapy against WNV and other flavivirus infections in humans.

Differential effects of mutations in NS4B on West Nile virus replication and inhibition of interferon signaling

West Nile virus (WNV) is a human pathogen that can cause symptomatic infections associated with meningitis and encephalitis. Previously, we demonstrated that replication of WNV inhibits the interferon (IFN) signal transduction pathway by preventing the accumulation of phosphorylated Janus kinase 1 (JAK1) and tyrosine kinase 2 (Tyk2) (J. T. Guo et al., J. Virol. 79:1343-1350, 2005). Through a genetic analysis, we have now identified a determinant on the nonstructural protein 4B (NS4B) that controls IFN resistance in HeLa cells expressing subgenomic WNV replicons lacking the structural genes. However, in the context of infectious genomes, the same determinant did not influence IFN signaling. Thus, our results indicate that NS4B may be sufficient to inhibit the IFN response in replicon cells and suggest a role for structural genes, or as yet unknown interactions, in the inhibition of the IFN signaling pathway during WNV infections.

West Nile virus infection activates the unfolded protein response, leading to CHOP induction and apoptosis

West Nile virus (WNV)-mediated neuronal death is a hallmark of WNV meningitis and encephalitis. However, the mechanisms of WNV-induced neuronal damage are not well understood. We investigated WNV neuropathogenesis by using human neuroblastoma cells and primary rat hippocampal neurons. We observed that WNV activates multiple unfolded protein response (UPR) pathways, leading to transcriptional and translational induction of UPR target genes. We evaluated the role of the three major UPR pathways, namely, inositol-requiring enzyme 1-dependent splicing of X box binding protein 1 (XBP1) mRNA, activation of activating transcription factor 6 (ATF6), and protein kinase R-like endoplasmic reticulum (ER) kinase-dependent eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation, in WNV-infected cells. We show that XBP1 is nonessential or can be replaced by other UPR pathways in WNV replication. ATF6 was rapidly degraded by proteasomes, consistent with induction of ER stress by WNV. We further observed a transient phosphorylation of eIF2alpha and induction of the proapoptotic cyclic AMP response element-binding transcription factor homologous protein (CHOP). WNV-infected cells exhibited a number of apoptotic phenotypes, such as (i) induction of growth arrest and DNA damage-inducible gene 34, (ii) activation of caspase-3, and (iii) cleavage of poly(ADP-ribose) polymerase. The expression of WNV nonstructural proteins alone was sufficient to induce CHOP expression. Importantly, WNV grew to significantly higher viral titers in chop(-)(/)(-) mouse embryonic fibroblasts (MEFs) than in wild-type MEFs, suggesting that CHOP-dependent premature cell death represents a host defense mechanism to limit viral replication that might also be responsible for the widespread neuronal loss observed in WNV-infected neuronal tissue.

West Nile virus-induced neuroinflammation: glial infection and capsid protein-mediated neurovirulence

West Nile virus (WNV) infection causes neurological disease at all levels of the neural axis, accompanied by neuroinflammation and neuronal loss, although the underlying mechanisms remain uncertain. Given the substantial activation of neuroinflammatory pathways observed in WNV infection, we hypothesized that WNV-mediated neuroinflammation and cell death occurred through WNV infection of both glia and neurons, which was driven in part by WNV capsid protein expression. Analysis of autopsied neural tissues from humans with WNV encephalomyelitis (WNVE) revealed WNV infection of both neurons and glia. Upregulation of proinflammatory genes, CXCL10, interleukin-1beta, and indolamine-2',3'-deoxygenase with concurrent suppression of the protective astrocyte-specific endoplasmic reticulum stress sensor gene, OASIS (for old astrocyte specifically induced substance), was evident in WNVE patients compared to non-WNVE controls. These findings were supported by increased ex vivo expression of these proinflammatory genes in glia infected by WNV-NY99. WNV infection caused endoplasmic reticulum stress gene induction and apoptosis in neurons but did not affect glial viability. WNV-infected astrocytic cells secreted cytotoxic factors, which caused neuronal apoptosis. The expression of the WNV-NY99 capsid protein in neurons and glia by a Sindbis virus-derived vector (SINrep5-WNVc) caused neuronal death and the release of neurotoxic factors by infected astrocytes, coupled with proinflammatory gene induction and suppression of OASIS. Striatal implantation of SINrep5-WNV(C) induced neuroinflammation in rats, together with the induction of CXCL10 and diminished OASIS expression, compared to controls. Moreover, magnetic resonance neuroimaging showed edema and tissue injury in the vicinity of the SINrep5-WNVc implantation site compared to controls, which was complemented by neurobehavioral abnormalities in the SINrep5-WNVc-implanted animals. These studies underscore the important interactions between the WNV capsid protein and neuroinflammation in the pathogenesis of WNV-induced neurological disorders.

West Nile virus encephalitis: sequential histopathological and immunological events in a murine model of infection

West Nile virus (WNV) has emerged as an important cause of encephalitis in humans and horses in North America. Although there is significant knowledge about the pathogenesis of disease caused by this flavivirus and about the immunity against it, no reports exist describing the sequence of pathological changes and their correlation to the immune response in the brain following infection with WNV. In this report the authors describe the major histopathological changes, as well as changes in cytokine and chemokine expression, in brains from WNV-infected C57Bl/6 mice. During the course of infection skin, spleen and kidney were all sites of WNV replication before virus reached the brain. In brain, increased expression of the chemokines monocyte chemoattractant protein (MCP)-5 (CCL12), interferon gamma inducible protein 10 (IP-10; CXCL10), and monokine induced by gamma interferon (MIG; CXCL9) preceded the expression of interferon gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha), which have previously been considered to be key early cytokines in the pathogenesis and immune response of WNV encephalitis. These results suggest that the chemokines MCP-5, IP-10, and MIG are important triggers of inflammation in brain due to their early up-regulation following WNV infection.

Inhibition of apoptosis prevents West Nile virus induced cell death

Background

West Nile virus (WNV) infection can cause severe meningitis and encephalitis in humans. Apoptosis was recently shown to contribute to the pathogenesis of WNV encephalitis. Here, we used WNV-infected glioma cells to study WNV-replication and WNV-induced apoptosis in human brain-derived cells.

Results

T98G cells are highly permissive for lytic WNV-infection as demonstrated by the production of infectious virus titre and the development of a characteristic cytopathic effect. WNV replication decreased cell viability and induced apoptosis as indicated by the activation of the effector caspase-3, the initiator caspases-8 and -9, poly(ADP-ribose)polymerase (PARP) cleavage and the release of cytochrome c from the mitochondria. Truncation of BID indicated cross-talk between the extrinsic and intrinsic apoptotic pathways. Inhibition of the caspases-8 or -9 inhibited PARP cleavage, demonstrating that both caspases are involved in WNV-induced apoptosis. Pan-caspase inhibition prevented WNV-induced apoptosis without affecting virus replication.

Conclusion

We found that WNV infection induces cell death in the brain-derived tumour cell line T98G by apoptosis under involvement of constituents of the extrinsic as well as the intrinsic apoptotic pathways. Our results illuminate the molecular mechanism of WNV-induced neural cell death.