Role of CD8+ T cells in control of West Nile virus infection

A protective role for dengue virus-specific CD8+ T cells

Infection with one of the four serotypes of dengue virus (DENV1-4) can result in a range of clinical manifestations in humans, from dengue fever to the more serious dengue hemorrhagic fever/dengue shock syndrome. Although T cells have been implicated in the immunopathogenesis of secondary infections with heterologous DENV serotypes, the role of T cells in protection against DENV is unknown. In this study, we used a mouse-passaged DENV2 strain, S221, to investigate the role of CD8(+) T cells in the immune response to primary DENV infection. S221 did not replicate well in wild-type mice, but did induce a CD8(+) T cell response, whereas viral replication and a robust CD8(+) T cell response were observed after infection of IFN-alpha/betaR(-/-) mice. Depletion of CD8(+) T cells from IFN-alpha/betaR(-/-) mice before infection resulted in significantly higher viral loads compared with undepleted mice. Mapping the specificity of the CD8(+) T cell response led to the identification of 12 epitopes derived from 6 of the 10 DENV proteins, with a similar immunodominance hierarchy observed in wild-type and IFN-alpha/betaR(-/-) mice. DENV-specific CD8(+) T cells produced IFN-gamma, TNF-alpha, expressed cell surface CD107a, and exhibited cytotoxic activity in vivo. Finally, immunization with four of the immunodominant CD8(+) T cell epitopes enhanced viral clearance. Collectively, our results reveal an important role for CD8(+) T cells in the host defense against DENV and demonstrate that the anti-DENV CD8(+) T cell response can be enhanced by immunization, providing rationale for designing DENV-specific vaccines that induce cell-mediated immunity.

Persistent West Nile virus infection in the house sparrow (Passer domesticus)

Long-term persistence of West Nile virus (WNV) infection within vertebrate reservoir hosts is a potential mechanism for overwintering of this (and other) arbovirus(es) at temperate latitudes. The house sparrow (Passer domesticus), an established amplifying host for WNV and other arboviruses, was used as a model to confirm chronicity of WNV infection in passerine birds and to evaluate the feasibility of two overwintering mechanisms: blood-borne infection of arthropod vectors (recrudescence) and oral infection of vertebrate reservoir hosts (ingestion of infected tissues through predation). WNV-inoculated sparrows were monitored for persistent infection for up to 2 years. Infectious virus persisted in tissues through 43 days, but not in sera beyond 6 days. Viral RNA persisted in tissues through 65 days. Chronicity of WNV infection in some tissues, but not blood, supports the predation mechanism of WNV overwintering, but not recrudescence. RNA persistence impacts interpretation and etiologic determination of avian mortality.

Inflammasome-activating nanoparticles as modular systems for optimizing vaccine efficacy

Innate immune system activation is a critical step in the initiation of an effective adaptive immune response; therefore, activation of a class of innate pathogen receptors called pattern recognition receptors (PRR) is a central feature of many adjuvant systems. It has recently been shown that one member of an intracellular PRR, the NLRP3 inflammasome, is activated by a number of classical adjuvants including aluminum hydroxide and saponins [Eisenbarth SC, Colegio OR, O'Connor W, Sutterwala FS, Flavell RA. Crucial role for the Nalp3 inflammasome in the immunostimulatory properties of aluminium adjuvants. Nature 2008;453(June (7198)):1122-6; Li H, Willingham SB, Ting JP, Re F. Cutting edge: inflammasome activation by alum and alum's adjuvant effect are mediated by NLRP3. J Immunol 2008;181(July (1)):17-21]. Inflammasome activation in vitro requires signaling of both the Toll-like receptor (TLR) and NLRP3 in antigen-presenting cells. Here we present a class of nanomaterials endowed with these two signals for rapid optimization of vaccine design. We constructed this system using a simple approach that incorporates lipopolysaccharides (LPS) onto the surface of nanoparticles constructed from a biocompatible polyester, poly(lactic-co-glycolic acid) (PLGA), loaded with antigen. We demonstrate that LPS-modified particles are preferentially internalized by dendritic cells compared to uncoated nanoparticles and the system, when administered to mice, elicits potent humoral and cellular immunity against a model antigen, ovalbumin. Wild-type macrophages pulsed with LPS-modified nanoparticles resulted in production of the proinflammatory cytokine IL-1beta consistent with inflammasome activation. In comparison, NLRP3-deficient and caspase-1-deficient macrophages showed negligible production of IL-1beta. Furthermore, when endocytosis and lysosomal destabilization were inhibited, inflammasome activity was diminished, supporting the notion that nanoparticles rupture lysosomal compartments and behave as 'danger signals' [Hornung V, Bauernfeind F, Halle A, Samstad EO, Kono H, Rock KL, et al. Silica crystals and aluminum salts activate the NALP3 inflammasome through phagosomal destabilization. Nat Immunol 2008;9(August (8)):847-56]. The generality of this vaccination approach is tested by encapsulation of a recombinant West Nile envelope protein and demonstrated by protection against a murine model of West Nile encephalitis. The design of such an antigen delivery mechanism with the ability to stimulate two potent innate immune pathways represents a potent new approach to simultaneous antigen and adjuvant delivery.

Japanese encephalitis virus produces a CD4+ Th2 response and associated immunoprotection in an adoptive-transfer murine model

Japanese encephalitis is an acute infection of the central nervous system caused by Japanese encephalitis virus (JEV). The importance of an effective humoral response in preventing JEV infection has already been established, although the contribution of cellular immunity remains unclear. This study used an experimental murine model to understand the protective effects of cell-mediated immunity in JEV infection. Fourteen-day-old mice adoptively transferred with JEV-immune splenocytes were found to be protected from peripheral JEV challenge. The survival rate was reduced when transferred cells were depleted of their CD4+ T-cell population. Correspondingly, increased protection was observed when JEV-primed isolated CD4+ T cells were transferred compared with isolated CD8+ T cells. Mice protected from JEV infection by the adoptive transfer of JEV-immune splenocytes had higher levels of immunomodulatory cytokines and decreased expression of pro-inflammatory cytokines. Concurrent with the increase in Th2 cytokines, JEV-specific IgM and IgG1 antibody titres were found to be elevated in protected mice. Taken together, these data indicate a definite role for CD4+ T cells in protection from lethal JEV infection in naïve 14-day-old mice. Induction of a Th2 cytokine response and IgG1 antibody probably achieves an immunomodulatory effect that results in the enhanced survival of these animals.

A Japanese encephalitis virus vaccine candidate strain is attenuated by decreasing its interferon antagonistic ability

Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus that causes acute encephalitis with high mortality in humans. To understand the virus-host interactions that influence JEV virulence, we determined the lethality of a neurovirulent (RP-9) and an attenuated (RP-2ms) variant of JEV in several immunodeficient mice strains. The attenuated phenotype of RP-2ms was completely lost in Stat-1-deficient mice, but its virulence was only slightly increased in mice lacking the components of adaptive immunity, suggesting an important role of the interferon (IFN) system in controlling JEV infection. Cell-based assays demonstrated that RP-2ms is more sensitive to IFN-alpha treatment; however, the NS5 protein of RP-2ms was still a potent antagonist of IFN, like RP-9 NS5. Using a recombinant infectious clone of RP-9, we found that a single Glu-->Lys mutation at residue 138 of the envelope protein (E-E138K) rendered the mutated RP-9 sensitive to the antiviral effect of IFN-alpha. Furthermore, IFN signaling was blocked earlier in the RP-9-infected cells relative to that in cells infected with RP-2ms or recombinant RP-9 bearing the E-E138K mutation. Thus, the E-E138K mutation of JEV appears to affect the viral growth properties, leading to a reduced efficiency in blocking IFN signaling, which then results in an attenuated phenotype in inoculated animals.

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.

West Nile virus-specific CD4 T cells exhibit direct antiviral cytokine secretion and cytotoxicity and are sufficient for antiviral protection

CD4 T cells have been shown to be necessary for the prevention of encephalitis during West Nile virus (WNV) infection. However, the mechanisms used by Ag-specific CD4 T cells to protect mice from WNV encephalitis remain incompletely understood. Contrary to the belief that CD4 T cells are protective because they merely maintain the CD8 T cell response and improve Ab production, in this study we provide evidence for the direct antiviral activity of CD4 T cells that functions to protect the host from WNV encephalitis. In adoptive transfers, naive CD4 T cells protected a significant number of lethally infected RAG(-/-) mice, demonstrating the protective effect of CD4 T cells independent of B cells and CD8 T cells. To shed light on the mechanism of this protection, we defined the peptide specificities of the CD4 T cells responding to WNV infection in C57BL/6 (H-2(b)) mice, and used these peptides to characterize the in vivo function of antiviral CD4 T cells. WNV-specific CD4 T cells produced IFN-gamma and IL-2, but also showed potential for in vivo and ex vivo cytotoxicity. Furthermore, peptide vaccination using CD4 epitopes conferred protection against lethal WNV infection in immunocompetent mice. These results demonstrate the role of direct effector function of Ag-specific CD4 T cells in preventing severe WNV 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.

Recombinant vesicular stomatitis virus-based west Nile vaccine elicits strong humoral and cellular immune responses and protects mice against lethal challenge with the virulent west Nile virus strain LSU-AR01

Vesicular stomatitis virus (VSV) has been extensively utilized as a viral vector system for the induction of protective immune responses against a variety of pathogens. We constructed recombinant VSVs specifying either the Indiana or Chandipura virus G glycoprotein and expressing the West Nile virus (WNV) envelope (E) glycoprotein. Mice were intranasally vaccinated using a prime (Indiana)-boost (Chandipura) immunization approach and challenged with the virulent WNV-LSU-AR01. Ninety-percent (9 of 10) of the vaccinated mice survived as compared to 10% of the mock-vaccinated mice after WNV lethal challenge. Histopathological examination of brain tissues revealed neuronal necrosis in mock-vaccinated mice but not in vaccinated mice, and vaccinated, but not mock-vaccinated mice developed a strong neutralizing antibody response against WNV. Extensive immunological analysis using polychromatic flow cytometry staining revealed that vaccinated, but not mock-vaccinated mice developed robust cellular immune responses as evidenced by up-regulation of CD4⁺ CD154⁺ IFNγ⁺ T cells in vaccinated, but not mock-vaccinated mice. Similarly, vaccinated mice developed robust E-glycoprotein-specific CD8⁺ T cell immune responses as evidenced by the presence of a high percentage of CD8⁺ CD62L^low IFNγ⁺ cells. In addition, a sizeable population of CD8⁺ CD69⁺ cells was detected indicating E-specific activation of mature T cells and CD4⁺ CD25⁺ CD127^low T regulatory (T reg) cells were down-regulated. These results suggest that VSV-vectored vaccines administered intranasally can efficiently induce protective humoral and cellular immune responses against WNV infections.

Construction and characterization of a single-cycle chimeric flavivirus vaccine candidate that protects mice against lethal challenge with dengue virus type 2

We have previously described a novel flavivirus vaccine technology based on a single-cycle, capsid (C) gene-deleted flavivirus called RepliVAX. RepliVAX can be propagated in cells that express high levels of C but undergoes only a single cycle of infection in vaccinated hosts. Here we report that we have adapted our RepliVAX technology to produce a dengue vaccine by replacing the prM/E genes of RepliVAX WN (a West Nile virus [WNV] RepliVAX) with the same genes of dengue virus type 2 (DENV2). Our first RepliVAX construct for dengue virus (RepliVAX D2) replicated poorly in WNV C-expressing cells. However, addition of mutations in prM and E that were selected during blind passage of a RepliVAX D2 derivative was used to produce a second-generation RepliVAX D2 (designated D2.2) that displayed acceptable growth in WNV C-expressing cells. RepliVAX D2.2 grew better in DENV2 C-expressing cells than WNV C-expressing cells, but after several passages in DENV2 C-expressing cells it acquired further mutations that permitted efficient growth in WNV C-expressing cells. We tested the potency and efficacy of RepliVAX D2.2 in a well-described immunodeficient mouse model for dengue (strain AG129; lacking the receptors for both type I and type II interferons). These mice produced dose-dependent DENV2-neutralizing antibody responses when vaccinated with RepliVAX D2.2. When challenged with 240 50% lethal doses of DENV2, mice given a single inoculation of RepliVAX D2.2 survived significantly longer than sham-vaccinated animals, although some of these severely immunocompromised mice eventually died from the challenge. Taken together these studies indicate that the RepliVAX technology shows promise for use in the development of vaccines that can be used to prevent dengue.

North American encephalitic arboviruses

Arboviruses continue to be a major cause of encephalitis in North America, and West Nile virus neuroinvasive disease is now the dominant cause of encephalitis. Transmission to humans of North American arboviruses occurs by infected mosquitoes or ticks. Most infections are asymptomatic or produce a flulike illness. Rapid serum or cerebrospinal fluid IgM antibody capture ELISA assays are available to diagnosis the acute infection for all North American arboviruses. Unfortunately, no antiviral drugs are approved for the treatment of arbovirus infection and current therapy is supportive.

The pathogenesis of poliomyelitis: what we don't know

Poliomyelitis has long served as a model for studies of viral pathogenesis, but there remain many important gaps in our understanding of this disease. It is the intent of this review to highlight these residual but important questions, in light of a possible future moratorium on research with polioviruses. Salient questions include: (1) What cells in the gastrointestinal tract are initially infected and act as the source of excreted virus? (2) What is the receptor used by mouse-adapted strains of poliovirus and how can some polioviruses use both mouse and primate receptors? (3) What determines species differences in susceptibility of the gastrointestinal tract to polioviruses? Why cannot PVR transgenic mice be infected by the natural enteric route? (4) Why are neuroadapted polioviruses unable to infect nonneural cells? (5) What is the role of postentry blocks in replication as determinants of neurovirulence? (6) What route(s) does poliovirus take to enter the central nervous system and how does it cross the blood-brain barrier? (7) Why does poliovirus preferentially attack lower motor neurons in contrast to many other neuronal types within the central nervous system? (8) Does cellular immunity play any role in recovery from acute infection or in vaccine-induced protection? (9) In which cells does poliovirus persist in patients with gamma-globulin deficiencies? (10) Is there any evidence that poliovirus genomes can persist in immunocompetent hosts? (11) Why has type 2 poliovirus been eradicated while types 1 and 3 have not? (12) Can transmission of vaccine-derived polioviruses be prevented with inactivated poliovirus vaccine? (13) What is the best strategy to control and eliminate vaccine-derived polioviruses?

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.

The memory T cell response to West Nile virus in symptomatic humans following natural infection is not influenced by age and is dominated by a restricted set of CD8+ T cell epitopes

We examined the West Nile virus (WNV)-specific T cell response in a cohort of 52 patients with symptomatic WNV infections, including neuroinvasive and non-invasive disease. Although all virus proteins were shown to contain T cell epitopes, certain proteins, such as E, were more commonly targeted by the T cell response. Most patients exhibited reactivity toward 3-4 individual WNV peptides; however, several patients exhibited reactivity toward >10 individual peptides. The relative hierarchy of T cell reactivities in all patients showed a fixed pattern that was sustained throughout the 12-mo period of the current study. Surprisingly, we did not observe any relationship between age and either the breadth or magnitude of the T cell response following infection. We also did not observe a relationship between disease severity and either the breadth or magnitude of the T cell response. The T cell epitopes were distributed in a non-random fashion across the viral polyprotein and a limited number of epitopes appeared to dominate the CD8(+) T cell response within our cohort. These data provide important new insight into the T cell response against WNV in humans.

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.

Dynamics of immune cell recruitment during West Nile encephalitis and identification of a new CD19+B220-BST-2+ leukocyte population

West Nile virus (WNV) is an emerging neurotropic flavivirus. We investigated the dynamics of immune cell recruitment in peripheral tissues and in the CNS during WNV encephalitis in an immunocompetent mouse model. In the periphery, immune cell expansion can successfully limit viremia and lymphoid tissue infection. However, viral clearance in the periphery is too late to prevent viral invasion of the CNS. In the CNS, innate immune cells, including microglia/macrophages, NK cells, and plasmacytoid dendritic cells, greatly expand as the virus invades the brain, whereas B and T cells are recruited after viral invasion, and fail to control the spread of the virus. Thus, the onset of WNV encephalitis was correlated both with CNS viral infection and with a large local increase of innate immune cells. Interestingly, we identify a new immune cell type: CD19(+)B220(-) BST-2(+), which we name G8-ICs. These cells appear during peripheral infection and enter the CNS. G8-ICs express high levels of MHC class II, stain for viral Ag, and are localized in the paracortical zone of lymph nodes, strongly suggesting they are previously unidentified APCs that appear in response to viral infection.

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.

West Nile 25A virus infection of B-cell-deficient ((micro)MT) mice: characterization of neuroinvasiveness and pseudoreversion of the viral envelope protein

The attenuated West Nile virus 25A strain (WN25A) was investigated for its neuroinvasive properties in B-cell-deficient (microMT) mice. After peripheral inoculation, WN25A caused fatal encephalitis in the majority of 6-8-week-old mice, characterized by a systemic infection with viraemia, moderate virus burdens in peripheral tissues and a high titre of brain-associated virus. Mice generally succumbed to infection within a few weeks of infection. However, others survived for as long as 10 weeks, and some for even longer. Normal age-matched C57BL/6 mice showed no signs of illness after inoculation with WN25A virus. Nucleotide sequencing of WN25A viruses recovered from the brains of B-cell-deficient mice revealed that the conserved N-linked glycosylation site in the viral envelope protein was abolished by substitution of a serine residue at position 155. This was found to be a pseudoreversion relative to the wild-type WN-Israel strain, based on virulence testing of one such brain-associated virus in both B-cell-deficient and normal C57BL/6 mice. This study provides further characterization of the mouse virulence properties of the attenuated WN25A virus in the context of B-cell deficiency. Replication in these mice does not involve rapid neuroadaptation or reversion of WN25A virus to a neuroinvasive phenotype. Molecular modelling studies suggest a difference in local structure of the E protein associated with either an asparagine or serine residue at position 155 compared with the tyrosine found in the virulent parental WN-Israel virus.

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.

NKG2D receptor signaling enhances cytolytic activity by virus-specific CD8+ T cells: evidence for a protective role in virus-induced encephalitis

Inoculation with the neurotropic JHM strain of mouse hepatitis virus (JHMV) into the central nervous system (CNS) of mice results in an acute encephalitis associated with an immune-mediated demyelinating disease. During acute disease, infiltrating CD8(+) T cells secrete gamma interferon (IFN-gamma) that controls replication in oligodendrocytes, while infected astrocytes and microglia are susceptible to perforin-mediated lysis. The present study was undertaken to reveal the functional contributions of the activating NKG2D receptor in host defense and disease following JHMV infection. NKG2D ligands RAE-1, MULT1, and H60 were expressed within the CNS following JHMV infection. The immunophenotyping of infiltrating cells revealed that NKG2D was expressed on approximately 90% of infiltrating CD8(+) T cells during acute and chronic disease. Blocking NKG2D following JHMV infection resulted in increased mortality that correlated with increased viral titers within the CNS. Anti-NKG2D treatment did not alter T-cell infiltration into the CNS or the generation of virus-specific CD8(+) T cells, and the expression of IFN-gamma was not affected. However, cytotoxic T-lymphocyte (CTL) activity was dependent on NKG2D expression, because anti-NKG2D treatment resulted in a dramatic reduction in lytic activity by virus-specific CD8(+) T cells. Blocking NKG2D during chronic disease did not affect either T-cell or macrophage infiltration or the severity of demyelination, indicating that NKG2D does not contribute to virus-induced demyelination. These findings demonstrate a functional role for NKG2D in host defense during acute viral encephalitis by selectively enhancing CTL activity by infiltrating virus-specific CD8(+) T cells.

Epitope discovery in West Nile virus infection: Identification and immune recognition of viral epitopes

Cytotoxic T lymphocytes (CTL) play an important role in the control and elimination of infection by West Nile virus (WNV), yet the class I human leukocyte antigen (HLA)-presented peptide epitopes that enable CTL recognition of WNV-infected cells remain uncharacterized. The goals of this work were first to discover the peptide epitopes that distinguish the class I HLA of WNV-infected cells and then to test the T cell reactivity of newly discovered WNV epitopes. To discover WNV-immune epitopes, class I HLA was harvested from WNV (NY99 strain)-infected and uninfected HeLa cells. Then peptide epitopes were eluted from affinity-purified HLA, and peptide epitopes from infected and uninfected cells were comparatively mapped by mass spectroscopy. Six virus-derived peptides from five different viral proteins (E, NS2b, NS3, NS4b, and NS5) were discovered as unique to HLA-A*0201 of infected cells, demonstrating that the peptides sampled by class I HLA are distributed widely throughout the WNV proteome. When tested with CTL from infected individuals, one dominant WNV target was apparent, two epitopes were subdominant, and three demonstrated little CTL reactivity. Finally, a sequence comparison of these epitopes with the hundreds of viral isolates shows that HLA-A*0201 presents epitopes derived from conserved regions of the virus. Detection and recovery from WNV infection are therefore functions of the ability of class I HLA molecules to reveal conserved WNV epitopes to an intact cellular immune system that subsequently recognizes infected cells.

The relative contribution of antibody and CD8+ T cells to vaccine immunity against West Nile encephalitis virus

West Nile virus (WNV) is a mosquito borne, neurotropic flavivirus that causes a severe central nervous system (CNS) infection in humans and animals. Although commercial vaccines are available for horses, none is currently approved for human use. In this study, we evaluated the efficacy and mechanism of immune protection of two candidate WNV vaccines in mice. A formalin-inactivated WNV vaccine induced higher levels of specific and neutralizing antibodies compared to a DNA plasmid vaccine that produces virus-like particles. Accordingly, partial and almost complete protection against a highly stringent lethal intracranial WNV challenge were observed in mice 60 days after single dose immunization with the DNA plasmid and inactivated virus vaccines, respectively. In mice immunized with a single dose of DNA plasmid or inactivated vaccine, antigen-specific CD8(+) T cells were induced and contributed to protective immunity as acquired or genetic deficiencies of CD8(+) T cells lowered the survival rates. In contrast, in boosted animals, WNV-specific antibody titers were higher, survival rates after challenge were greater, and an absence of CD8(+) T cells did not appreciably affect mortality. Overall, our experiments suggest that in mice, both inactivated WNV and DNA plasmid vaccines are protective after two doses, and the specific contribution of antibody and CD8(+) T cells to vaccine immunity against WNV is modulated by the prime-boost strategy.

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.

Prior exposure to uninfected mosquitoes enhances mortality in naturally-transmitted West Nile virus infection

BACKGROUND

The global emergence of West Nile virus (WNV) has highlighted the importance of mosquito-borne viruses. These are inoculated in vector saliva into the vertebrate skin and circulatory system. Arthropod-borne (arbo)viruses such as WNV are transmitted to vertebrates as an infectious mosquito probes the skin for blood, depositing the virus and saliva into the skin and circulation. Growing evidence has demonstrated that arthropod, and recently mosquito, saliva can have a profound effect on pathogen transmission efficiency, pathogenesis, and disease course. A potentially important aspect of natural infections that has been ignored is that in nature vertebrates are typically exposed to the feeding of uninfected mosquitoes prior to the mosquito that transmits WNV. The possibility that pre-exposure to mosquito saliva might modulate WNV infection was explored.

PRINCIPAL FINDINGS

Here we report that sensitization to mosquito saliva exacerbates viral infection. Prior exposure of mice to mosquito feeding resulted in increased mortality following WNV infection. This aggravated disease course was associated with enhanced early viral replication, increased interleukin-10 expression, and elevated influx of WNV-susceptible cell types to the inoculation site. This exacerbated disease course was mimicked by passive transfer of mosquito-sensitized serum.

SIGNIFICANCE

This is the first report that sensitization to arthropod saliva can exacerbate arthropod-borne infection, contrary to previous studies with parasite and bacteria infections. This research suggests that in addition to the seroreactivity of the host to virus, it is important to take into account the immune response to vector feeding.

Antigen-specific cytotoxic T lymphocytes protect against lethal West Nile virus encephalitis

Infection with West Nile virus (WNV) causes fatal encephalitis in immunocompromised animals. Previous studies in mice have established that T cell protection is required for clearance of WNV infection from tissues and preventing viral persistence. The current study assessed whether specific WNV peptide epitopes could elicit a cytotoxic T lymphocyte (CTL) response capable of protecting against virus infection. Hidden Markov model analysis was used to identify WNV-encoded peptides that bound the MHC class I proteins K(b) or D(b). Of the 35 peptides predicted to bind MHC class I molecules, one immunodominant CTL recognition peptide was identified in each of the envelope and non-structural protein 4B genes. Addition of these but not control peptides to CD8(+) T cells from WNV-infected mice induced IFN-gamma production. CTL clones that were generated ex vivo lysed peptide-pulsed or WNV-infected target cells in an antigen-specific manner. Finally, adoptive transfer of a mixture of envelope- and non-structural protein 4B-specific CTL to recipient mice protected against lethal WNV challenge. Based on this, we conclude that CTL responses against immundominant WNV epitopes confer protective immunity and thus should be targets for inclusion in new vaccines.

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.

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.

Comparative efficacies of three commercially available vaccines against West Nile Virus (WNV) in a short-duration challenge trial involving an equine WNV encephalitis model

We used a severe challenge model that produces clinical West Nile virus (WNV) disease to test the efficacy of three commercially available equine WNV vaccines in horses. Twenty-four healthy, WNV-seronegative horses of varying ages and genders were placed, in random and blind manner, into three trial groups consisting of eight horses each; two horses in each group received (i) an inactivated WNV vaccine (K-WN), (ii) a modified-live vaccine (CP-WN) containing the WNV prM and E proteins expressed by a canarypox vector, (iii) a live-chimera vaccine (WN-FV) containing WNV prM and E proteins expressed in a YF17D vector, or (iv) a diluent control. Challenge by this model caused grave neurological signs, viremia, moderate to severe histopathologic lesions in the brain and spinal cord, and an outcome of 0% survivorship in all six control horses. In contrast, challenge in horses at between 28 days postvaccination with the chimera vaccine and 56 days postvaccination with the commercial inactivated or modified-live vaccine resulted in 100% survivorship (protection from the onset of WNV encephalitis and viremia). Horses vaccinated with the live-chimera vaccine showed significantly fewer clinical signs than did the control horses (P </= 0.01) and the horses vaccinated with inactivated vaccine (P = 0.035). Mild residual inflammatory lesions were seen in a few of the vaccinated horses.

Complex adenovirus-mediated expression of West Nile virus C, PreM, E, and NS1 proteins induces both humoral and cellular immune responses

West Nile Virus (WNV), a member of the family Flaviviridae, was first identified in Africa in 1937. In recent years, it has spread into Europe and North America. The clinical manifestations of WNV infection range from mild febrile symptoms to fatal encephalitis. Two genetic lineages (lineages I and II) are recognized; lineage II is associated with mild disease, while lineage I has been associated with severe disease, including encephalitis. WNV has now spread across North America, significantly affecting both public and veterinary health. In the efforts to develop an effective vaccine against all genetic variants of WNV, we have studied the feasibility of inducing both neutralizing and cellular immune responses by de novo synthesis of WNV antigens using a complex adenoviral vaccine (CAdVax) vector. By expressing multiple WNV proteins from a single vaccine vector, we were able to induce both humoral and cellular immune responses in vaccinated mice. Neutralization assays demonstrated that the antibodies were broadly neutralizing against both lineages of WNV, with a significant preference for the homologous lineage II virus. The results from this study show that multiple antigens synthesized de novo from a CAdVax vector are capable of inducing both humoral and cellular immune responses against WNV and that a multiantigen approach may provide broad protection against multiple genetic variants of WNV.

CD40-CD40 ligand interactions promote trafficking of CD8+ T cells into the brain and protection against West Nile virus encephalitis

Recent studies have established a protective role for T cells during primary West Nile virus (WNV) infection. Binding of CD40 by CD40 ligand (CD40L) on activated CD4+ T cells provides an important costimulatory signal for immunoglobulin class switching, antibody affinity maturation, and priming of CD8+ T-cell responses. We examined here the function of CD40-dependent interactions in limiting primary WNV infection. Compared to congenic wild-type mice, CD40(-/-) mice uniformly succumbed to WNV infection. Although CD40(-/-) mice produced low levels of WNV-specific immunoglobulin M (IgM) and IgG, viral clearance from the spleen and serum was not altered, and CD8+ T-cell priming in peripheral lymphoid tissues was normal. Unexpectedly, CD8+ T-cell trafficking to the central nervous system (CNS) was markedly impaired in CD40(-/-) mice, and this correlated with elevated WNV titers in the CNS and death. In the brains of CD40(-/-) mice, T cells were retained in the perivascular space and did not migrate into the parenchyma, the predominant site of WNV infection. In contrast, in wild-type mice, T cells trafficked to the site of infection in neurons. Beside its role in maturation of antibody responses, our experiments suggest a novel function of CD40-CD40L interactions: to facilitate T-cell migration across the blood-brain barrier to control WNV infection.

Protective capacity and epitope specificity of CD8(+) T cells responding to lethal West Nile virus infection

West Nile virus (WNV) is a small, positive-strand RNA virus belonging to the Flaviviridae genus, which causes lethal encephalitis in a subset of infected birds and mammals. In humans, WNV exhibits pronounced age-related morbidity and mortality, but the basis of this effect is unclear, and the molecular and cellular parameters of the host-WNV infection are just beginning to be elucidated. Indeed, numerous mechanisms were implicated in protection in vivo against WNV (IFN-I and IFN-gamma, antibody, C', CD8 and CD4 T cells), but the individual importance of each one of these remains unclear. Here, we show that transfer of highly enriched naïve CD8(+ )T cells protects the majority of alymphoid mice against lethal WNV infection. To substantiate and expand this finding, we defined the peptide specificity of the CD8 response in H-2b mice and used a panel of identified peptides to map one dominant (NS4b (2248-2256)) and several subdominant epitopes. The hierarchy of these epitopes was stably maintained in the memory responses. Most importantly, CTL lines directed against these peptides conferred protection against lethal WNV infection in direct proportion to the epitope immunodominance. These results provide a springboard for future characterization of T cell responses against WNV and demonstrate, for the first time, that CD8 T cells can single-handedly protect from this disease.

The molecular basis of antibody-mediated neutralization of West Nile virus

The study of the interaction between the West Nile virus envelope protein and monoclonal antibodies has provided insight into the molecular mechanisms of neutralization. Structural studies have identified an epitope on the lateral ridge of domain III of the West Nile virus E protein that is recognized by antibodies with the strongest neutralizing activity in vitro and in vivo. Antibodies that bind to this epitope are particularly inhibitory because they block infection at a post-attachment step and at concentrations that result in a low occupancy of the available sites on the virion.

Pneumococcal capsular polysaccharide vaccine-mediated protection against serotype 3 Streptococcus pneumoniae in immunodeficient mice

Pneumococcal capsular polysaccharide (PPS) vaccines are less immunogenic in immunocompromised than immunocompetent individuals. However, neither the efficacy of PPS vaccines in immunocompromised individuals nor the host cellular subsets required for vaccine efficacy against pneumococcal disease have been directly investigated. In this study, we vaccinated CD4-deficient (CD4(-/-)), CD8-deficient (CD8(-/-)), and secretory immunoglobulin M-deficient (sIgM(-/-)) mice and wild-type C57BL/6 (Wt) mice with a conjugate of PPS of serotype 3 and tetanus toxoid (PPS3-TT) and determined the antibody response and efficacy of vaccination against systemic and pulmonary challenge with serotype 3 pneumococcus in immunized and control mice. Our results showed that the isotype and predominant IgG subclass of the PPS3 response differed between immunodeficient mouse strains and between immunodeficient and Wt mice, with CD8(-/-) mice having the most robust response. Vaccination protected Wt, CD4(-/-), and sIgM(-/-) mice from death resulting from both systemic and pulmonary challenge, whereas CD8(-/-) mice were protected only from systemic and not from pulmonary challenge. Passive vaccination with PPS3-TT-induced sera from Wt, CD4(-/-), CD8(-/-), and sIgM(-/-) mice protected naïve Wt mice from death due to pulmonary challenge; however, CD8(-/-) mice were not protected by sera from Wt or CD8(-/-) mice. Our findings suggest that PPS-based vaccines can be effective in the setting of CD4 T-cell deficiency but that CD8 T cells could be required for vaccine-mediated protection against pulmonary challenge with serotype 3 pneumococcus.

West Nile virus: recent trends in diagnosis and vaccine development

West Nile virus (WNV) is a mosquito-borne flavivirus, native to Africa, Europe, and Western Asia. In many respects, WNV is an outstanding example of a zoonotic pathogen that has leaped geographical barriers and can cause severe disease in human and horse. Before the emergence of WNV in the USA, only few methods of diagnosis were available. Recently, many changes in the fields of WN diagnosis and prevention have happened. This paper will review all these new tools. After a description of the main concerns in WNV and West Nile (WN) disease in humans and animals, this review will present the main available tests for serology and virology detection, from gold standard tests to more recently developed methods. Finally, licensed vaccines and candidate vaccines developed in humans, horses and birds will also been described.

CD8(+) T cell-mediated immune responses in West Nile virus (Sarafend strain) encephalitis are independent of gamma interferon

The flavivirus West Nile virus (WNV) can cause fatal encephalitis in humans and mice. It has recently been demonstrated, in an experimental model using WNV strain Sarafend and C57BL/6 mice, that both virus- and immune-mediated pathology is involved in WNV encephalitis, with CD8(+) T cells being the dominant subpopulation of lymphocyte infiltrates in the brain. Here, the role of activated WNV-immune CD8(+) T cells in mouse WNV encephalitis was investigated further. Passive transfer of WNV-immune CD8(+) T cells reduced mortality significantly and prolonged survival times of mice infected with WNV. Early infiltration of WNV-immune CD8(+) T cells into infected brains is shown, suggesting a beneficial contribution of these lymphocytes to recovery from encephalitis. This antiviral function was not markedly mediated by gamma interferon (IFN-gamma), as a deficiency in IFN-gamma did not affect mortality to two strains of WNV (Sarafend and Kunjin) or brain virus titres significantly. The cytolytic potential, as well as precursor frequency, of WNV-immune CD8(+) T cells were not altered by the absence of IFN-gamma. This was reflected in transfer experiments of WNV-immune CD8(+) T cells from IFN-gamma(-/-) mice into WNV-infected wild-type mice, which showed that IFN-gamma-deficient T cells were as effective as those from WNV-immune wild-type mice in ameliorating disease outcome. It is speculated here that one of the pleiotropic functions of IFN-gamma is mimicked by WNV-Sarafend-mediated upregulation of cell-surface expression of major histocompatibility complex antigens, which may explain the lack of phenotype of IFN-gamma(-/-) mice in response to WNV.

Immunopathology of flavivirus infections

With the recent emergence of the flavivirus, West Nile virus (WNV), in particular, the New York strain of Lineage I WNV in North America in 1999, there has been a significant increase in activity in neurotropic flavivirus research. These viruses cause encephalitis that can result in permanent neurological sequelae or death. Attempts to develop vaccines have made progress, but have been variably successful, despite considerable commercial underwriting. Thus, the discovery of ways and means to combat disease is no less urgent. As such, most recent work has been directed towards dissecting and understanding the pathogenesis of disease, as a way of informing possible approaches to abrogation or amelioration of illness. Whether inherent to flaviviruses or because humans are incidental, dead-end hosts, it is clear that these viruses interact with their human hosts in extremely complex ways. This occurs from the cellular level, at which infection must be established to produce disease, to its interaction with the adaptive immune response, which may result in its eradication, with or without immunopathological and consequent neurological sequelae. As human proximity to and contact with flavivirus insect vectors and amplifying hosts cannot practically be eliminated, our understanding of the pathogenesis of flavivirus-induced diseases, especially with regard to possible targets for treatment, is imperative.

West Nile virus neuroinvasive disease

Since 1999, there have been nearly 20,000 cases of confirmed symptomatic West Nile virus (WNV) infection in the United States, and it is likely that more than 1 million people have been infected by the virus. WNV is now the most common cause of epidemic viral encephalitis in the United States, and it will likely remain an important cause of neurological disease for the foreseeable future. Clinical syndromes produced by WNV infection include asymptomatic infection, West Nile Fever, and West Nile neuroinvasive disease (WNND). WNND includes syndromes of meningitis, encephalitis, and acute flaccid paralysis/poliomyelitis. The clinical, laboratory, and diagnostic features of these syndromes are reviewed here. Many patients with WNND have normal neuroimaging studies, but abnormalities may be present in areas including the basal ganglia, thalamus, cerebellum, and brainstem. Cerebrospinal fluid invariably shows a pleocytosis, with a predominance of neutrophils in up to half the patients. Diagnosis of WNND depends predominantly on demonstration of WNV-specific IgM antibodies in cerebrospinal fluid. Recent studies suggest that some WNV-infected patients have persistent WNV IgM serum and/or cerebrospinal fluid antibody responses, and this may require revision of current serodiagnostic criteria. Although there is no proven therapy for WNND, several vaccines and antiviral therapy with antibodies, antisense oligonucleotides, and interferon preparations are currently undergoing human clinical trials. Recovery from neurological sequelae of WNV infection including cognitive deficits and weakness may be prolonged and incomplete.

CD4+ T-cell responses are required for clearance of West Nile virus from the central nervous system

Although studies have established that innate and adaptive immune responses are important in controlling West Nile virus (WNV) infection, the function of CD4(+) T lymphocytes in modulating viral pathogenesis is less well characterized. Using a mouse model, we examined the role of CD4(+) T cells in coordinating protection against WNV infection. A genetic or acquired deficiency of CD4(+) T cells resulted in a protracted WNV infection in the central nervous system (CNS) that culminated in uniform lethality by 50 days after infection. Mice surviving past day 10 had high-level persistent WNV infection in the CNS compared to wild-type mice, even 45 days following infection. The absence of CD4(+) T-cell help did not affect the kinetics of WNV infection in the spleen and serum, suggesting a role for CD4-independent clearance mechanisms in peripheral tissues. WNV-specific immunoglobulin M (IgM) levels were similar to those of wild-type mice in CD4-deficient mice early during infection but dropped approximately 20-fold at day 15 postinfection, whereas IgG levels in CD4-deficient mice were approximately 100- to 1,000-fold lower than in wild-type mice throughout the course of infection. WNV-specific CD8(+) T-cell activation and trafficking to the CNS were unaffected by the absence of CD4(+) T cells at day 9 postinfection but were markedly compromised at day 15. Our experiments suggest that the dominant protective role of CD4(+) T cells during primary WNV infection is to provide help for antibody responses and sustain WNV-specific CD8(+) T-cell responses in the CNS that enable viral clearance.

West Nile virus meningoencephalitis

Since its first appearance in the US in 1999, West Nile virus (WNV) has emerged as the most common cause of epidemic meningoencephalitis in North America. In the 6 years following the 1999 outbreak, the geographic range and burden of the disease in birds, mosquitoes and humans has greatly expanded to include the 48 contiguous US and 7 Canadian provinces, as well as Mexico, the Caribbean islands and Colombia. WNV has shown an increasing propensity for neuroinvasive disease over the past decade, with varied presentations including meningitis, encephalitis and acute flaccid paralysis. Although neuroinvasive disease occurs in less than 1% of infected individuals, it is associated with high mortality. From 1999-2005, more than 8,000 cases of neuroinvasive WNV disease were reported in the US, resulting in over 780 deaths. In this review, we discuss epidemiology, risk factors, clinical features, diagnosis and prognosis of WNV meningoencephalitis, along with potential treatments.

Gamma delta T cells facilitate adaptive immunity against West Nile virus infection in mice

West Nile (WN) virus causes fatal meningoencephalitis in laboratory mice, and gammadelta T cells are involved in the protective immune response against viral challenge. We have now examined whether gammadelta T cells contribute to the development of adaptive immune responses that help control WN virus infection. Approximately 15% of TCRdelta(-/-) mice survived primary infection with WN virus compared with 80-85% of the wild-type mice. These mice were more susceptible to secondary challenge with WN virus than the wild-type mice that survived primary challenge with the virus. Depletion of gammadelta T cells in wild-type mice that survived the primary infection, however, does not affect host susceptibility during secondary challenge with WN virus. Furthermore, gammadelta T cells do not influence the development of Ab responses during primary and at the early stages of secondary infection with WN virus. Adoptive transfer of CD8(+) T cells from wild-type mice that survived primary infection with WN virus to naive mice afforded partial protection from lethal infection. In contrast, transfer of CD8(+) T cells from TCRdelta(-/-) mice that survived primary challenge with WN virus failed to alter infection in naive mice. This difference in survival correlated with the numeric and functional reduction of CD8 memory T cells in these mice. These data demonstrate that gammadelta T cells directly link innate and adaptive immunity during WN virus infection.

PKR and RNase L contribute to protection against lethal West Nile Virus infection by controlling early viral spread in the periphery and replication in neurons

West Nile virus (WNV) is a neurotropic, mosquito-borne flavivirus that can cause lethal meningoencephalitis. Type I interferon (IFN) plays a critical role in controlling WNV replication, spread, and tropism. In this study, we begin to examine the effector mechanisms by which type I IFN inhibits WNV infection. Mice lacking both the interferon-induced, double-stranded-RNA-activated protein kinase (PKR) and the endoribonuclease of the 2',5'-oligoadenylate synthetase-RNase L system (PKR(-/-) x RL(-/-)) were highly susceptible to subcutaneous WNV infection, with a 90% mortality rate compared to the 30% mortality rate observed in congenic wild-type mice. PKR(-/-) x RL(-/-) mice had increased viral loads in their draining lymph nodes, sera, and spleens, which led to early viral entry into the central nervous system (CNS) and higher viral burden in neuronal tissues. Although mice lacking RNase L showed a higher CNS viral burden and an increased mortality, they were less susceptible than the PKR(-/-) x RL(-/-) mice; thus, we also infer an antiviral role for PKR in the control of WNV infection. Notably, a deficiency in both PKR and RNase L resulted in a decreased ability of type I IFN to inhibit WNV in primary macrophages and cortical neurons. In contrast, the peripheral neurons of the superior cervical ganglia of PKR(-/-) x RL(-/-) mice showed no deficiency in the IFN-mediated inhibition of WNV. Our data suggest that PKR and RNase L contribute to IFN-mediated protection in a cell-restricted manner and control WNV infection in peripheral tissues and some neuronal subtypes.

Temporal analyses of the neuropathogenesis of a West Nile virus infection in mice

A West Nile virus (WNV) infection in humans can produce neurological symptoms including acute flaccid paralysis, encephalitis, meningitis and myelitis. To investigate the pathogenesis of WNV in the peripheral and the central nervous system (PNS and CNS), the authors used a murine footpad inoculation model of WNV infection. Survival curves of virus-infected animals of ages 4- to 6-weeks-old demonstrated age-dependent mortality where older animals (6-weeks-old) had a higher mortality rate compared to younger animals (4- and 5-weeks-old). The mice that survived the virus infection formed WNV-reactive antibodies, confirming viral infection and clearance. The localization of viral RNA (vRNA) and antigen in infected murine tissues was investigated using TaqMan and immunohistochemistry (IHC) respectively. During a nine day infection, vRNA levels in the spinal cord and brainstem fluctuated, suggesting early viral clearance from these tissues by days 3-4 p.i. with later re-introduction. Viral antigens detected using IHC were primarily observed in three main regions of the brain: cortex, hippocampus and brainstem. Additionally, the dorsal root ganglion neurons of the PNS stained positive for viral antigens. These data are consistent with multiple routes of neuroinvasion following a peripheral inoculation of virus and do not preclude the previous observation that virus-infected peripheral neurons can introduce virus into the CNS by a retrograde transport mechanism.

Development of effective therapies against West Nile virus infection

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

CCR5: no longer a ‘good for nothing’ gene – chemokine control of West Nile virus infection

The chemokine receptor CCR5 was identified in 1996 as a crucial host factor exploited by HIV for cell entry. CCR5 presumably functions normally in antimicrobial host defense because it generally mediates leukocyte chemotactic responses; however, evidence of antimicrobial functions for CCR5 in humans has been elusive. Recently, genetic analyses in mice and humans have provided strong evidence for the CCR5 control of infection by West Nile virus (WNV), a re-emerging pathogen capable of causing fatal encephalitis. Thus, the same receptor can benefit or harm the host, depending on the virus. Although CCR5 might be a logical target for new drug development in HIV/AIDS, the benefits of blocking CCR5 could carry the cost of an increased risk of WNV disease in co-infected patients.