Tumor necrosis factor alpha protects against lethal West Nile virus infection by promoting trafficking of mononuclear leukocytes into the central nervous system. J Virol. 2008;82:8956-8964. [PMID: 18632856 DOI: 10.1128/jvi.01118-08]

Deficient IFN signaling by myeloid cells leads to MAVS-dependent virus-induced sepsis

The type I interferon (IFN) signaling response limits infection of many RNA and DNA viruses. To define key cell types that require type I IFN signaling to orchestrate immunity against West Nile virus (WNV), we infected mice with conditional deletions of the type I IFN receptor (IFNAR) gene. Deletion of the Ifnar gene in subsets of myeloid cells resulted in uncontrolled WNV replication, vasoactive cytokine production, sepsis, organ damage, and death that were remarkably similar to infection of Ifnar^−/− mice completely lacking type I IFN signaling. In Mavs^−/−×Ifnar^−/− myeloid cells and mice lacking both Ifnar and the RIG-I-like receptor adaptor gene Mavs, cytokine production was muted despite high levels of WNV infection. Thus, in myeloid cells, viral infection triggers signaling through MAVS to induce proinflammatory cytokines that can result in sepsis and organ damage. Viral pathogenesis was caused in part by massive complement activation, as liver damage was minimized in animals lacking complement components C3 or factor B or treated with neutralizing anti-C5 antibodies. Disease in Ifnar^−/− and CD11c Cre⁺Ifnar^f/f mice also was facilitated by the proinflammatory cytokine TNF-α, as blocking antibodies diminished complement activation and prolonged survival without altering viral burden. Collectively, our findings establish the dominant role of type I IFN signaling in myeloid cells in restricting virus infection and controlling pathological inflammation and tissue injury.

Although it is well established that the interferon (IFN) signaling pathway restricts infection by many viruses, the key cell types in vivo that contribute to this process remain poorly characterized. To address this question in the context of West Nile virus (WNV) pathogenesis, we infected mice that specifically delete the type I IFN receptor gene (Ifnar) in subsets of myeloid cells, including dendritic cells and macrophages. Remarkably, mice lacking Ifnar expression only in myeloid cell subsets rapidly developed a sepsis-like syndrome that was characterized by enhanced WNV infection and visceral organ injury and caused by massive proinflammatory cytokine production and complement activation. By using additional gene targeted deletion mice, we show that WNV infection triggered signaling through the RIG-I like receptor adaptor protein MAVS to cause complement activation, sepsis, and tissue damage. Indeed, liver damage was minimized in animals lacking specific complement components, or treated with neutralizing anti-complement or anti-TNF-α antibodies. Our results establish how type I IFN signaling in dendritic cells and macrophages restricts infection, controls inflammatory cascades, and prevents pathogenesis in vivo.

Etanercept reduces neuroinflammation and lethality in mouse model of Japanese encephalitis

BACKGROUND

Japanese encephalitis virus (JEV) is a neurotropic flavivirus that causes Japanese encephalitis (JE), which leads to high fatality rates in human. Tumor necrosis factor alpha (TNF-α) is a key factor that mediates immunopathology in the central nervous system (CNS) during JE. Etanercept is a safe anti-TNF-α drug that has been commonly used in the treatment of various human autoimmune diseases.

METHODS

The effect of etanercept on JE was investigated with a JEV-infected mouse model. Four groups of mice were assigned to receive injections of phosphate-buffered saline, etanercept, JEV, or JEV plus etanercept. Inflammatory responses in mouse brains and mortality of mice were evaluated within 23 days post infection.

RESULTS

The in vitro assay with mouse neuron/glia cultures showed that etanercept treatment reduced the inflammatory response induced by JEV infection. In vivo experiments further demonstrated that administration of etanercept protected mice from JEV-induced lethality. Neuronal damage, glial activation, and secretion of proinflammatory cytokines were found to be markedly decreased in JEV-infected mice that received etanercept treatment. Additionally, etanercept treatment restored the integrity of the blood-brain barrier and reduced viral load in mouse brains.

CONCLUSIONS

Etanercept effectively reduces the inflammation and provides protection against acute encephalitis in a JEV-infected mouse model.

Integrated analysis of microRNAs and their disease related targets in the brain of mice infected with West Nile virus

To determine whether cellular miRNAs play a role in West Nile virus (WNV) neuropathogenesis, we evaluated WNV-infected mice brain for the expression profile of miRNAs, their potential functions and their correlation with genes involved in inflammatory pathways. A total of 528 miRNAs and 168 mRNA genes were examined. One hundred thirty-nine miRNAs were significantly differentially expressed in WNV-infected mice brain. Ingenuity pathway analysis demonstrated that these miRNAs and their target genes are involved in pathways related to inflammatory response, immune-cell trafficking and cell death. Moreover, we demonstrate an inverse correlation between WNV-modulated miRNAs and their target neuroinflammatory genes in the same mice brain. We demonstrate that miR-196a, miR-202-3p, miR-449c, and miR-125a-3p target multiple genes involving cytokines, chemokines, and apoptotic genes, which belong to different signaling pathways that play critical role in WNV neuropathogenesis. Functional studies targeting specific miRNA are warranted to develop therapeutics for the management of WNV disease.

β₂-adrenergic agonists modulate TNF-α induced astrocytic inflammatory gene expression and brain inflammatory cell populations

BACKGROUND

The NF-κB signaling pathway orchestrates many of the intricate aspects of neuroinflammation. Astrocytic β₂-adrenergic receptors have emerged as potential regulators in central nervous system inflammation and are potential targets for pharmacological modulation. The aim of this study was to elucidate the crosstalk between astrocytic β₂-adrenergic receptors and the TNF-α induced inflammatory gene program.

METHODS

Proinflammatory conditions were generated by the administration of TNF-α. Genes that are susceptible to astrocytic crosstalk between β₂-adrenergic receptors (stimulated by clenbuterol) and TNF-α were identified by qPCR-macroarray-based gene expression analysis in a human 1321 N1 astrocytoma cell line. Transcriptional patterns of the identified genes in vitro were validated by RT-PCR on the 1321 N1 cell line as well as on primary rat astrocytes. In vivo expression patterns were examined by intracerebroventricular administration of clenbuterol and/or TNF-α in rats. To examine the impact on the inflammatory cell content of the brain we performed extensive FACS analysis of rat brain immune cells after intracerebroventricular clenbuterol and/or TNF-α administration.

RESULTS

Parallel transcriptional patterns in vivo and in vitro confirmed the relevance of astrocytic β₂-adrenergic receptors as modulators of brain inflammatory responses. Importantly, we observed pronounced effects of β2-adrenergic receptor agonists and TNF-α on IL-6, CXCL2, CXCL3, VCAM1, and ICAM1 expression, suggesting a role in inflammatory brain cell homeostasis. Extensive FACS-analysis of inflammatory cell content in the brain demonstrated that clenbuterol/TNF-α co-administration skewed the T cell population towards a double negative phenotype and induced a shift in the myeloid brain cell population towards a neutrophilic predominance.

CONCLUSIONS

Our results show that astrocytic β₂-adrenergic receptors are potent regulators of astrocytic TNF-α-activated genes in vitro and in vivo, and ultimately modulate the molecular network involved in the homeostasis of inflammatory cells in the central nervous system. Astrocytic β₂-adrenergic receptors and their downstream signaling pathway may serve as potential targets to modulate neuroinflammatory responses.

Increased pathogenicity of West Nile virus (WNV) by glycosylation of envelope protein and seroprevalence of WNV in wild birds in Far Eastern Russia

In this review, we discuss the possibility that the glycosylation of West Nile (WN) virus E-protein may be associated with enhanced pathogenicity and higher replication of WN virus. The results indicate that E-protein glycosylation allows the virus to multiply in a heat-stable manner and therefore, has a critical role in enhanced viremic levels and virulence of WN virus in young-chick infection model. The effect of the glycosylation of the E protein on the pathogenicity of WN virus in young chicks was further investigated. The results indicate that glycosylation of the WN virus E protein is important for viral multiplication in peripheral organs and that it is associated with the strong pathogenicity of WN virus in birds. The micro-focus reduction neutralization test (FRNT) in which a large number of serum samples can be handled at once with a small volume (15 μL) of serum was useful for differential diagnosis between Japanese encephalitis and WN virus infections in infected chicks. Serological investigation was performed among wild birds in the Far Eastern region of Russia using the FRNT. Antibodies specific to WN virus were detected in 21 samples of resident and migratory birds out of 145 wild bird samples in the region.

Novel approaches and challenges to treatment of central nervous system viral infections

Existing and emerging viral central nervous system (CNS) infections are major sources of human morbidity and mortality. Treatments of proven efficacy are currently limited predominantly to herpesviruses and human immunodeficiency virus (HIV). Development of new therapies has been hampered by the lack of appropriate animal model systems for some important viruses and by the difficulty in conducting human clinical trials for diseases that may be rare, or in the case of arboviral infections, often have variable seasonal and geographic incidence. Nonetheless, many novel approaches to antiviral therapy are available, including candidate thiazolide and pyrazinecarboxamide derivatives with potential broad‐spectrum antiviral efficacy. New herpesvirus drugs include viral helicase‐primase and terminase inhibitors. The use of antisense oligonucleotides and other strategies to interfere with viral RNA translation has shown efficacy in experimental models of CNS viral disease. Identifying specific molecular targets within viral replication cycles has led to many existing antiviral agents and will undoubtedly continue to be the basis of future drug design. A promising new area of research involves therapies based on enhanced understanding of host antiviral immune responses. Toll‐like receptor agonists and drugs that inhibit specific cytokines as well as interferon preparations have all shown potential therapeutic efficacy. Passive transfer of virus‐specific cytotoxic T lymphocytes has been used in humans and may provide an effective therapy for some herpesvirus infections and potentially for progressive multifocal leukoencephalopathy. Humanized monoclonal antibodies directed against specific viral proteins have been developed and in several cases evaluated in humans in settings including West Nile virus and HIV infection and in pre‐exposure prophylaxis for rabies. Ann Neurol 2013;74:412–422

Pattern recognition receptor MDA5 modulates CD8+ T cell-dependent clearance of West Nile virus from the central nervous system

Many viruses induce type I interferon responses by activating cytoplasmic RNA sensors, including the RIG-I-like receptors (RLRs). Although two members of the RLR family, RIG-I and MDA5, have been implicated in host control of virus infection, the relative role of each RLR in restricting pathogenesis in vivo remains unclear. Recent studies have demonstrated that MAVS, the adaptor central to RLR signaling, is required to trigger innate immune defenses and program adaptive immune responses, which together restrict West Nile virus (WNV) infection in vivo. In this study, we examined the specific contribution of MDA5 in controlling WNV in animals. MDA5(-/-) mice exhibited enhanced susceptibility, as characterized by reduced survival and elevated viral burden in the central nervous system (CNS) at late times after infection, even though small effects on systemic type I interferon response or viral replication were observed in peripheral tissues. Intracranial inoculation studies and infection experiments with primary neurons ex vivo revealed that an absence of MDA5 did not impact viral infection in neurons directly. Rather, subtle defects were observed in CNS-specific CD8(+) T cells in MDA5(-/-) mice. Adoptive transfer into recipient MDA5(+/+) mice established that a non-cell-autonomous deficiency of MDA5 was associated with functional defects in CD8(+) T cells, which resulted in a failure to clear WNV efficiently from CNS tissues. Our studies suggest that MDA5 in the immune priming environment shapes optimal CD8(+) T cell activation and subsequent clearance of WNV from the CNS.

TNF-α acts as an immunoregulator in the mouse brain by reducing the incidence of severe disease following Japanese encephalitis virus infection

Japanese encephalitis virus (JEV) causes acute central nervous system (CNS) disease in humans, in whom the clinical symptoms vary from febrile illness to meningitis and encephalitis. However, the mechanism of severe encephalitis has not been fully elucidated. In this study, using a mouse model, we investigated the pathogenetic mechanisms that correlate with fatal JEV infection. Following extraneural infection with the JaOArS982 strain of JEV, infected mice exhibited clinical signs ranging from mild to fatal outcome. Comparison of the pathogenetic response between severe and mild cases of JaOArS982-infected mice revealed increased levels of TNF-α in the brains of severe cases. However, unexpectedly, the mortality rate of TNF-α KO mice was significantly increased compared with that of WT mice, indicating that TNF-α plays a protective role against fatal infection. Interestingly, there were no significant differences of viral load in the CNS between WT and TNF-α KO mice. However, exaggerated inflammatory responses were observed in the CNS of TNF-α KO mice. Although these observations were also obtained in IL-10 KO mice, the mortality and enhanced inflammatory responses were more pronounced in TNF-α KO mice. Our findings therefore provide the first evidence that TNF-α has an immunoregulatory effect on pro-inflammatory cytokines in the CNS during JEV infection and consequently protects the animals from fatal disease. Thus, we propose that the increased level of TNF-α in severe cases was the result of severe disease, and secondly that immunopathological effects contribute to severe neuronal degeneration resulting in fatal disease. In future, further elucidation of the immunoregulatory mechanism of TNF-α will be an important priority to enable the development of effective treatment strategies for Japanese encephalitis.

The neuroimmune response to West Nile virus

The recent introduction of highly pathogenic strains of West Nile virus (WNV) into naïve populations in Europe, Israel, and the USA has resulted in a marked increase in both the number of reported cases and the severity of disease compared to previous outbreaks. The impact of the increased virulence of recently emerged strains of WNV is exacerbated by the fact that antiviral therapies and vaccines are not currently available for use in humans. A greater understanding of the viral and host factors involved in WNV-mediated neuropathology is necessary to facilitate the development of novel therapeutic approaches. This review summarizes the current state of knowledge of the role of the cell-intrinsic innate immune responses as well as the cell-mediated innate and adaptive immune responses in promoting the detection and clearance of WNV from the CNS.

Innate host responses to West Nile virus: Implications for central nervous system immunopathology

West Nile virus (WNV) is an emerging neurotropic flavivirus that has recently spread to America and Southern Europe via an enzootic/epizootic bird-mosquito-bird transmission cycle. The virus can occasionally infect humans through mosquito bites, and man-to-man transmission has also been reported via infected blood or organ donation. In the human host, WNV causes asymptomatic infection in about 70%-80% of cases, while < 1% of clinical cases progress to severe neuroinvasive disease; long-term neurological sequelae are common in more than 50% of these severe cases. The pathogenesis of the neuroinvasive form of WNV infection remains incompletely understood, and risk factors for developing severe clinical illness are largely unknown. The innate immune response plays a major role in the control of WNV replication, which is supported by the fact that the virus has developed numerous mechanisms to escape the control of antiviral interferons. However, exaggerated inflammatory responses lead to pathology, mainly involving the central nervous system. This brief review presents the salient features of innate host responses, WNV immunoevasion strategies, and WNV-induced immunopathology.

Differential replication of pathogenic and nonpathogenic strains of West Nile virus within astrocytes

The severity of West Nile virus (WNV) infection in immunocompetent animals is highly strain dependent, ranging from avirulent to highly neuropathogenic. Here, we investigate the nature of this strain-specific restriction by analyzing the replication of avirulent (WNV-MAD78) and highly virulent (WNV-NY) strains in neurons, astrocytes, and microvascular endothelial cells, which comprise the neurovascular unit within the central nervous system (CNS). We demonstrate that WNV-MAD78 replicated in and traversed brain microvascular endothelial cells as efficiently as WNV-NY. Likewise, similar levels of replication were detected in neurons. Thus, WNV-MAD78's nonneuropathogenic phenotype is not due to an intrinsic inability to replicate in key target cells within the CNS. In contrast, replication of WNV-MAD78 was delayed and reduced compared to that of WNV-NY in astrocytes. The reduced susceptibility of astrocytes to WNV-MAD78 was due to a delay in viral genome replication and an interferon-independent reduction in cell-to-cell spread. Together, our data suggest that astrocytes regulate WNV spread within the CNS and therefore are an attractive target for ameliorating WNV-induced neuropathology.

West Nile Virus: biology, transmission, and human infection

West Nile Virus was introduced into the Western Hemisphere during the late summer of 1999 and has been causing significant and sometimes severe human diseases since that time. This article briefly touches upon the biology of the virus and provides a comprehensive review regarding recent discoveries about virus transmission, virus acquisition, and human infection and disease.

Inflammasome adaptor protein Apoptosis-associated speck-like protein containing CARD (ASC) is critical for the immune response and survival in west Nile virus encephalitis

West Nile virus (WNV) is a neurotropic flavivirus that has emerged globally as a significant cause of viral encephalitis in humans. The WNV-induced innate immune response, including production of antiviral cytokines, is critical for controlling virus infection. The adaptor protein ASC mediates a critical step in innate immune signaling by bridging the interaction between the pathogen recognition receptors and caspase 1 in inflammasome complexes, but its role in WNV immunopathogenesis is not defined. Here, we demonstrate that ASC is essential for interleukin-1β (IL-1β) production and development of effective host immunity against WNV. ASC-deficient mice exhibited increased susceptibility to WNV infection, and reduced survival was associated with enhanced virus replication in the peripheral tissues and central nervous system (CNS). Infection of cultured bone marrow-derived dendritic cells showed that ASC was essential for the activation of caspase 1, a key component of inflammasome assembly. ASC(-/-) mice exhibited attenuated levels of proinflammatory cytokines in the serum. Intriguingly, infected ASC(-/-) mice also displayed reduced levels of alpha interferon (IFN-α) and IgM in the serum, indicating the overall protective role of ASC in restricting WNV infection. However, brains from ASC(-/-) mice displayed unrestrained inflammation, including elevated levels of proinflammatory cytokines and chemokines, such as IFN-γ, CCL2, and CCL5, which correlated with more pronounced activation of the astrocytes, enhanced infiltration of peripheral immune cells in the CNS, and increased neuronal cell death. Collectively, our data provide new insights into the role of ASC as an essential modulator of inflammasome-dependent and -independent immune responses to effectively control WNV infection.

Immune responses to West Nile virus infection in the central nervous system

West Nile virus (WNV) continues to cause outbreaks of severe neuroinvasive disease in humans and other vertebrate animals in the United States, Europe, and other regions of the world. This review discusses our understanding of the interactions between virus and host that occur in the central nervous system (CNS), the outcome of which can be protection, viral pathogenesis, or immunopathogenesis. We will focus on defining the current state of knowledge of WNV entry, tropism, and host immune response in the CNS, all of which affect the balance between injury and successful clearance.

IL-1β signaling promotes CNS-intrinsic immune control of West Nile virus infection

West Nile virus (WNV) is an emerging flavivirus capable of infecting the central nervous system (CNS) and mediating neuronal cell death and tissue destruction. The processes that promote inflammation and encephalitis within the CNS are important for control of WNV disease but, how inflammatory signaling pathways operate to control CNS infection is not defined. Here, we identify IL-1β signaling and the NLRP3 inflammasome as key host restriction factors involved in viral control and CNS disease associated with WNV infection. Individuals presenting with acute WNV infection displayed elevated levels of IL-1β in their plasma over the course of infection, suggesting a role for IL-1β in WNV immunity. Indeed, we found that in a mouse model of infection, WNV induced the acute production of IL-1β in vivo, and that animals lacking the IL-1 receptor or components involved in inflammasome signaling complex exhibited increased susceptibility to WNV pathogenesis. This outcome associated with increased accumulation of virus within the CNS but not peripheral tissues and was further associated with altered kinetics and magnitude of inflammation, reduced quality of the effector CD8⁺ T cell response and reduced anti-viral activity within the CNS. Importantly, we found that WNV infection triggers production of IL-1β from cortical neurons. Furthermore, we found that IL-1β signaling synergizes with type I IFN to suppress WNV replication in neurons, thus implicating antiviral activity of IL-1β within neurons and control of virus replication within the CNS. Our studies thus define the NLRP3 inflammasome pathway and IL-1β signaling as key features controlling WNV infection and immunity in the CNS, and reveal a novel role for IL-1β in antiviral action that restricts virus replication in neurons.

Since its introduction into North America in 1999, West Nile virus (WNV) has emerged as a leading cause of viral encephalitic disease in the United States. While low level inflammation is important for clearance of WNV, high levels of inflammation are associated with increased disease. The goal of this study was to identify host signaling pathways that control the balance of inflammation and protective immunity to WNV. Using a mouse model of infection, we identified a central nervous system (CNS)-intrinsic requirement for the NLRP3 inflammasome and IL-1β signaling in limiting WNV associated disease within the CNS. First, IL-1β signaling was essential for regulating the magnitude and kinetics of inflammation within CNS. Secondly, the absence of IL-1β signaling disrupted the quality of the effector T lymphocyte response against the virus. Finally, these dysregulated immune responses were linked to a direct ability for IL-1β signaling to synergize with type I IFN signaling and limit virus replication within cortical neurons, key target cells of WNV infection within the CNS. Together this study identifies the NLRP3 inflammasome and IL-1β signaling as key restriction factors that act to regulate viral load and the quality of inflammatory responses within the CNS to impart protective immunity against WNV infection.

Concordance of preclinical and clinical pharmacology and toxicology of monoclonal antibodies and fusion proteins: soluble targets

Monoclonal antibodies (mAbs) and fusion proteins directed towards soluble targets make an important contribution to the treatment of disease. The purpose of this review was to correlate the clinical and preclinical data on the 14 currently approved mAbs and fusion proteins targeted to soluble targets. The principal sources used to gather data were: the peer reviewed Literature; European Medicines Agency 'Scientific Discussions' and United States Food and Drug Administration 'Pharmacology/Toxicology Reviews' and package inserts (United States Prescribing Information). Data on the following approved biopharmaceuticals were included: adalimumab, anakinra, bevacizumab, canakinumab, certolizumab pegol, denosumab, eculizumab, etanercept, golimumab, infliximab, omalizumab, ranibizumab, rilonacept and ustekinumab. Some related biopharmaceuticals in late-stage development were also included for comparison. Good concordance with human pharmacodynamics was found for both non-human primates (NHPs) receiving the human biopharmaceutical and mice receiving rodent homologues (surrogates). In contrast, there was limited concordance for human adverse effects in genetically deficient mice, mice receiving surrogates or NHPs receiving the human pharmaceutical. In summary, the results of this survey show that although both mice and NHPs have good predictive value for human pharmacodynamics, neither species have good predictive value for human adverse effects. No evidence that NHPs have superior predictive value was found.

Impaired virus clearance, compromised immune response and increased mortality in type 2 diabetic mice infected with West Nile virus

Clinicoepidemiological data suggest that type 2 diabetes is associated with increased risk of West Nile virus encephalitis (WNVE). However, no experimental studies have elucidated the role of diabetes in WNV neuropathogenesis. Herein, we employed the db/db mouse model to understand WNV immunopathogenesis in diabetics. Nine-week old C57BL/6 WT and db/db mice were inoculated with WNV and mortality, virus burden in the periphery and brain, and antiviral defense responses were analyzed. db/db mice were highly susceptible to WNV disease, exhibited increased tissue tropism and mortality than the wild-type mice, and were unable to clear the infection. Increased and sustained WNV replication was observed in the serum, peripheral tissues and brain of db/db mice, and heightened virus replication in the periphery was correlated with enhanced neuroinvasion and replication of WNV in the brain. WNV infection in db/db mice was associated with enhanced inflammatory response and compromised antiviral immune response characterized by delayed induction of IFN-α, and significantly reduced concentrations of WNV-specific IgM and IgG antibodies. The compromised immune response in db/db mice correlated with increased viremia. These data suggest that delayed immune response coupled with failure to clear the virus leads to increased mortality in db/db mice. In conclusion, this study provides unique mechanistic insight into the immunopathogenesis of WNVE observed in diabetics and can be used to develop therapeutics for the management of WNVE among diabetic patients.

IL-22 signaling contributes to West Nile encephalitis pathogenesis

The Th17 cytokine, IL-22, regulates host immune responses to extracellular pathogens. Whether IL-22 plays a role in viral infection, however, is poorly understood. We report here that Il22(-/-) mice were more resistant to lethal West Nile virus (WNV) encephalitis, but had similar viral loads in the periphery compared to wild type (WT) mice. Viral loads, leukocyte infiltrates, proinflammatory cytokines and apoptotic cells in the central nervous system (CNS) of Il22(-/-) mice were also strikingly reduced. Further examination showed that Cxcr2, a chemokine receptor that plays a non-redundant role in mediating neutrophil migration, was significantly reduced in Il22(-/-) compared to WT leukocytes. Expression of Cxcr2 ligands, cxcl1 and cxcl5, was lower in Il22(-/-) brains than wild type mice. Correspondingly, neutrophil migration from the blood into the brain was attenuated following lethal WNV infection of Il22(-/-) mice. Our results suggest that IL-22 signaling exacerbates lethal WNV encephalitis likely by promoting WNV neuroinvasion.

CD8+ T cells use TRAIL to restrict West Nile virus pathogenesis by controlling infection in neurons

Previous studies of mice have demonstrated that an orchestrated sequence of innate and adaptive immune responses is required to control West Nile virus (WNV) infection in peripheral and central nervous system (CNS) tissues. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL; also known as CD253) has been reported to inhibit infection with dengue virus, a closely related flavivirus, in cell culture. To determine the physiological function of TRAIL in the context of flavivirus infection, we compared the pathogenesis of WNV in wild-type and TRAIL(-/-) mice. Mice lacking TRAIL showed increased vulnerability and death after subcutaneous WNV infection. Although no difference in viral burden was detected in peripheral tissues, greater viral infection was detected in the brain and spinal cord at late times after infection, and this was associated with delayed viral clearance in the few surviving TRAIL(-/-) mice. While priming of adaptive B and T cell responses and trafficking of immune and antigen-specific cells to the brain were undistinguishable from those in normal mice, in TRAIL(-/-) mice, CD8(+) T cells showed qualitative defects in the ability to clear WNV infection. Adoptive transfer of WNV-primed wild-type but not TRAIL(-/-) CD8(+) T cells to recipient CD8(-/-) mice efficiently limited infection in the brain and spinal cord, and analogous results were obtained when wild-type or TRAIL(-/-) CD8(+) T cells were added to WNV-infected primary cortical neuron cultures ex vivo. Collectively, our results suggest that TRAIL produced by CD8(+) T cells contributes to disease resolution by helping to clear WNV infection from neurons in the central nervous system.

Cell-intrinsic innate immune control of West Nile virus infection

West Nile virus (WNV) is an enveloped positive-stranded RNA virus that has emerged over the past decade in North America to cause epidemics of meningitis, encephalitis, and acute flaccid paralysis in humans. WNV has broad species specificity, and replicates efficiently in many cell types, including those of the innate immune and central nervous systems. Recent studies have defined the pathogen recognition receptor (PRR) and signaling pathways by which WNV is detected, and several effector mechanisms that contribute to protective cell-intrinsic immunity. This review focuses on recent advances in identifying the host sensors that detect WNV, the adaptor molecules and signaling pathways that regulate the induction of interferon (IFN)-dependent defenses, and the proteins that limit WNV replication, spread, and disease pathogenesis.

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.

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.

Tumour necrosis factor alpha (TNF-alpha) stimulation of cells with established dengue virus type 2 infection induces cell death that is accompanied by a reduced ability of TNF-alpha to activate nuclear factor kappaB and reduced sphingosine kinase-1 activity

Tumor necrosis factor alpha (TNF-α) has an antiviral role in some infections but in dengue virus (DENV) infection it is linked to severe pathology. We have previously shown that TNF-α stimulation cannot activate nuclear factor κB (NF-κB) to the fullest extent in DENV-2-infected cells. Here, we investigate further responses of DENV-2-infected cells to TNF-α, focussing particularly on cell death and pro-survival signals. TNF-α stimulation of productively DENV-2-infected monocyte-derived macrophages or HEK-293 cells induced caspase-3-mediated cell death. While TNF-α induced comparable degradation of the inhibitor of NF-κB alpha (IκB-α) and NF-κB activation in mock-infected and DENV-2-infected cells early in infection, later in infection and coinciding with TNF-α-induced cell death, TNF-α-stimulated IκB-α degradation and NF-κB activation was reduced. This was associated with reduced levels of sphingosine kinase-1 (SphK1) activity in DENV-2-infected cells; SphK1 being a known mediator of TNF-α-stimulated survival signals. Transfection experiments demonstrated inhibition of TNF-α-stimulated NF-κB activation by expression of DENV-2 capsid (CA) but enhancement by DENV-2 NS5 protein. DENV-2 CA alone, however, did not induce TNF-α-stimulated cell death or inhibit SphK1 activity. Thus, productively DENV-2-infected cells have compromised TNF-α-stimulated survival pathways and show enhanced susceptibility to TNF-α-stimulated cell death, suggesting a role for TNF-α in the killing of healthy productively DENV-2-infected cells. Additionally, the altered ability of TNF-α to activate NF-κB as infection progresses is reflected by the opposing actions of DENV-2 CA and NS5 proteins on TNF-α-stimulated NF-κB activation and could have important consequences for NF-κB-driven release of inflammatory cytokines.

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.

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.

Altered effector functions of virus-specific and virus cross-reactive CD8+ T cells in mice immunized with related flaviviruses

Memory cross-reactive CD8+ T-cell responses may induce protection or immunopathology upon secondary viral challenge. To elucidate the potential role of T cells in sequential flavivirus infection, we characterized cross-reactive CD4+ and CD8+ T-cell responses between attenuated and pathogenic Japanese encephalitis virus (JEV) and pathogenic West Nile virus (WNV). A previously reported WNV NS4b CD8+ T-cell epitope and its JEV variant elicited CD8+ T-cell responses in both JEV- and WNV-infected mice. The peptide variant homologous to the immunizing virus induced greater cytokine secretion and activated higher frequencies of epitope-specific CD8+ T cells. However, there was a virus-dependent, peptide variant-independent pattern of cytokine secretion; the IFNgamma+-to-IFNgamma+TNFalpha+ CD8+ T-cell ratio was greater in JEV- than in WNV-infected mice. Despite similarities in viral burden for pathogenic WNV and JEV viruses, CD8+ T cells from pathogenic JEV-immunized mice exhibited functional and phenotypic profiles similar to those seen for the attenuated JEV strain. Patterns of killer cell lectin-like receptor G1 (KLRG1) and CD127 expression differed by virus type, with a rapid expansion and contraction of short-lived effector cells in JEV infection and persistence of high levels of short-lived effector cells in WNV infection. Such cross-reactive T-cell responses to primary infection may affect the outcomes of sequential flavivirus infections.

Effector functions of camelid heavy-chain antibodies in immunity to West Nile virus

Three classes of IgG have been described for camelids. IgG1 has a conventional four-chain structure, while IgG2 and IgG3 do not incorporate light chains. The structures and antigen-binding affinities of the so-called heavy-chain classes have been studied in detail; however, their regulation and effector functions are largely undefined. The aim of this study was to examine the participation of conventional and heavy-chain IgG antibodies in the camelid immune defense directed against West Nile virus (WNV). We found that natural infection or vaccination with killed WNV induced IgG1 and IgG3. Vaccination also induced IgG1 and IgG3; IgG2 was produced during the anamnestic response to vaccination. When purified IgGs were tested in plaque-reduction neutralization titer (PRNT) tests, IgG3 demonstrated PRNT activities comparable to those of conventional IgG1. In contrast, IgG2 demonstrated only suboptimal activity at the highest concentrations tested. Flow cytometric analysis revealed that macrophages bound IgG1, IgG2, and IgG3. Furthermore, subneutralizing concentrations of all three isotypes enhanced WNV infection of cultured macrophages. Our results document distinctions in regulation and function between camelid heavy-chain isotypes. The reduced size and distinct structure of IgG3 did not negatively impact its capacity to neutralize virus. In contrast, IgG2 appeared to be less efficient in neutralization. This information advances our understanding of these unusual antibodies in ways that can be applied in the development of effective vaccines for camelids.

IL-10 signaling blockade controls murine West Nile virus infection

West Nile virus (WNV), a mosquito-borne single-stranded RNA flavivirus, can cause significant human morbidity and mortality. Our data show that interleukin-10 (IL-10) is dramatically elevated both in vitro and in vivo following WNV infection. Consistent with an etiologic role of IL-10 in WNV pathogenesis, we find that WNV infection is markedly diminished in IL-10 deficient (IL-10^−/−) mice, and pharmacologic blockade of IL-10 signaling by IL-10 neutralizing antibody increases survival of WNV-infected mice. Increased production of antiviral cytokines in IL-10^−/− mice is associated with more efficient control of WNV infection. Moreover, CD4⁺ T cells produce copious amounts of IL-10, and may be an important cellular source of IL-10 during WNV infection in vivo. In conclusion, IL-10 signaling plays a negative role in immunity against WNV infection, and blockade of IL-10 signaling by genetic or pharmacologic means helps to control viral infection, suggesting a novel anti-WNV therapeutic strategy.

West Nile virus (WNV), a mosquito-transmitted RNA virus, is a worldwide cause of severe human and animal infection. Mammalian host immune responses to WNV infection are not completely understood and a vaccine or specific therapy is unavailable for use in humans. In the present study, we investigated the putative regulatory role of interleukin-10 (IL-10) during WNV infection in mice. We found that IL-10 signaling facilitates WNV infection and suppresses antiviral cytokine production in response to viral infection. Interestingly, blockade of IL-10 signaling by IL-10 neutralizing antibody increases survival of WNV-infected mice, suggesting a potentially novel therapeutic strategy to combat WNV infection. In addition, we found that CD4⁺ T cells produce a significant amount of IL-10 during WNV infection, providing a more accurate cellular target for IL-10 signaling inhibition. IL-10 also plays a critical role in suppression of excessive inflammation and immunopathology caused by autoimmune diseases or host immune system responses to infections; therefore, safety and efficacy of IL-10 signaling blockade as a therapeutic strategy against WNV infection deserves consideration.

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.

Mouse adenovirus type 1-induced breakdown of the blood-brain barrier

Infection with mouse adenovirus type 1 (MAV-1) results in fatal acute encephalomyelitis in susceptible mouse strains via infection of brain endothelial cells. Wild-type (wt) MAV-1 causes less brain inflammation than an early region 3 (E3) null virus in C57BL/6 mice. A mouse brain microvascular endothelial cell line infected with wt MAV-1 had higher expression of mRNAs for the proinflammatory chemokines CCL2 and CCL5 than mock- and E3 null virus-infected cells. Primary mouse brain endothelial cells infected with wt virus had elevated levels of CCL2 compared to mock- or E3 null virus-infected cells. Infection of C57BL/6 mice with wt MAV-1 or the E3 null virus caused a dose-dependent breakdown of the blood-brain barrier, primarily due to direct effects of virus infection rather than inflammation. The tight junction proteins claudin-5 and occludin showed reduced surface expression on primary mouse brain endothelial cells following infection with either wt MAV-1 or the E3 null virus. mRNAs and protein for claudin-5, occludin, and zona occludens 2 were also reduced in infected cells. MAV-1 infection caused a loss of transendothelial electrical resistance in primary mouse brain endothelial cells that was not dependent on E3 or on MAV-1-induced CCL2 expression. Taken together, these results demonstrate that MAV-1 infection caused breakdown of the blood-brain barrier accompanied by decreased surface expression of tight junction proteins. Furthermore, while the MAV-1-induced pathogenesis and inflammation were dependent on E3, MAV-1-induced breakdown of the blood-brain barrier and alteration of endothelial cell function were not dependent on E3 or CCL2.

Toll-like receptor 7 mitigates lethal West Nile encephalitis via interleukin 23-dependent immune cell infiltration and homing

West Nile virus (WNV), a mosquito-transmitted single-stranded RNA (ssRNA) flavivirus, causes human disease of variable severity. We investigated Toll-like receptor 7-deficient (Tlr7(-/-)) and myeloid differentiation factor 88-deficient (Myd88(-/-)) mice, which both have defective recognition of ssRNA, and found increased viremia and susceptibility to lethal WNV infection. Despite increased tissue concentrations of most innate cytokines, CD45(+) leukocytes and CD11b(+) macrophages failed to home to WNV-infected cells and infiltrate into target organs of Tlr7(-/-) mice. Tlr7(-/-) mice and macrophages had reduced interleukin-12 (IL-12) and IL-23 responses after WNV infection, and mice deficient in IL-12 p40 and IL-23 p40 (Il12b(-/-)) or IL-23 p19 (Il23a(-/-)), but not IL-12 p35 (Il12a(-/-)), responded similarly to Tlr7(-/-) mice, with increased susceptibility to lethal WNV encephalitis. Collectively, these results demonstrate that TLR7 and IL-23-dependent WNV responses represent a vital host defense mechanism that operates by affecting immune cell homing to infected target cells.