Generation and characterization of closely related epizootic and enzootic infectious cDNA clones for studying interferon sensitivity and emergence mechanisms of Venezuelan equine encephalitis virus

The VLDLR entry receptor is required for the pathogenesis of multiple encephalitic alphaviruses

The very-low-density lipoprotein receptor (VLDLR) has been reported as an entry receptor for Semliki Forest (SFV) and Eastern equine encephalitis (EEEV) alphaviruses in cell cultures. However, the role of VLDLR in alphavirus pathogenesis and the extent to which other alphaviruses can engage VLDLR remains unclear. Here, using a surface protein-targeted CRISPR-Cas9 screen, we identify VLDLR as a receptor for Western equine encephalitis virus (WEEV) and demonstrate that it promotes the infection of multiple viruses in the WEE antigenic complex. In vivo studies show that the pathogenicity of WEEV, EEEV, and SFV, but not the distantly related Venezuelan equine encephalitis virus, is markedly diminished in VLDLR-deficient mice and that mice treated with a soluble VLDLR-Fc decoy molecule are protected against disease. Overall, these results expand our understanding of the role of VLDLR in alphavirus pathogenesis and provide a potential path for developing countermeasures against alphaviruses from different antigenic complexes.

Replication of the Venezuelan Equine Encephalitis Vaccine from a Synthetic PCR Fragment

BACKGROUND/OBJECTIVES

There is no approved human vaccine for Venezuelan equine encephalitis (VEE), a life-threatening disease caused by the VEE virus (VEEV). In previous studies, plasmid DNA encoding the full-length RNA genome of the VEE V4020 vaccine was used for the preparation of experimental live virus VEE vaccines in the plasmid-transfected cell culture.

METHODS

Here, we used the high-fidelity polymerase chain reaction (PCR) to prepare synthetic, transcriptionally active PCR (TAP) fragments encoding the V4020 genome.

RESULTS

TAP fragment initiated the replication of the V4020 live virus vaccine in TAP fragment-transfected cells. A transfection of less than 1 ug of TAP fragment resulted in the replication of the V4020 vaccine virus in CHO cells.

CONCLUSION

We conclude that not only plasmid DNA but also synthetic PCR-generated DNA fragments can be used for the manufacturing of live vaccines for VEEV and, potentially, other viruses.

RNA structures within Venezuelan equine encephalitis virus E1 alter macrophage replication fitness and contribute to viral emergence

Venezuelan equine encephalitis virus (VEEV) is a mosquito-borne +ssRNA virus belonging to the Togaviridae. VEEV is found throughout Central and South America and is responsible for periodic epidemic/epizootic outbreaks of febrile and encephalitic disease in equines and humans. Endemic/enzootic VEEV is transmitted between Culex mosquitoes and sylvatic rodents, whereas epidemic/epizootic VEEV is transmitted between mosquitoes and equids, which serve as amplification hosts during outbreaks. Epizootic VEEV emergence has been shown to arise from mutation of enzootic VEEV strains. Specifically, epizootic VEEV has been shown to acquire amino acid mutations in the E2 viral glycoprotein that facilitate viral entry and equine amplification. However, the abundance of synonymous mutations which accumulate across the epizootic VEEV genome suggests that other viral determinants such as RNA secondary structure may also play a role in VEEV emergence. In this study we identify novel RNA structures in the E1 gene which specifically alter replication fitness of epizootic VEEV in macrophages but not other cell types. We show that SNPs are conserved within epizootic lineages and that RNA structures are conserved across different lineages. We also identified several novel RNA-binding proteins that are necessary for altered macrophage replication. These results suggest that emergence of VEEV in nature requires multiple mutations across the viral genome, some of which alter cell-type specific replication fitness in an RNA structure-dependent manner.

Defining the Cynomolgus Macaque (Macaca fascicularis) Animal Model for Aerosolized Venezuelan Equine Encephalitis: Importance of Challenge Dose and Viral Subtype

Venezuelan equine encephalitis virus (VEEV) outbreaks occur sporadically. Additionally, VEEV has a history of development as a biothreat agent. Yet, no FDA-approved vaccine or therapeutic exists for VEEV disease. The sporadic outbreaks present a challenge for testing medical countermeasures (MCMs) in humans; therefore, well-defined animal models are needed for FDA Animal Rule licensure. The cynomolgus macaque (CM) model has been studied extensively at high challenge doses of the VEEV Trinidad donkey strain (>1.0 × 108 plaque-forming units [PFU]), doses that are too high to be a representative human dose. Based on viremia of two subtypes of VEEV, IC, and IAB, we found the CM infectious dose fifty (ID50) to be low, 12 PFU, and 6.7 PFU, respectively. Additionally, we characterized the pattern of three clinical parameters (viremia, temperature, and lymphopenia) across a range of doses to identify a challenge dose producing consistent signs of infection. Based on these studies, we propose a shift to using a lower challenge dose of 1.0 × 103 PFU in the aerosol CM model of VEEV disease. At this dose, NHPs had the highest viremia, demonstrated a fever response, and had a measurable reduction in complete lymphocyte counts-biomarkers that can demonstrate MCM efficacy.

Entry receptor LDLRAD3 is required for Venezuelan equine encephalitis virus peripheral infection and neurotropism leading to pathogenesis in mice

Venezuelan equine encephalitis virus (VEEV) is an encephalitic alphavirus responsible for epidemics of neurological disease across the Americas. Low-density lipoprotein receptor class A domain-containing 3 (LDLRAD3) is a recently reported entry receptor for VEEV. Here, using wild-type and Ldlrad3-deficient mice, we define a critical role for LDLRAD3 in controlling steps in VEEV infection, pathogenesis, and neurotropism. Our analysis shows that LDLRAD3 is required for efficient VEEV infection and pathogenesis prior to and after central nervous system invasion. Ldlrad3-deficient mice survive intranasal and intracranial VEEV inoculation and show reduced infection of neurons in different brain regions. As LDLRAD3 is a determinant of pathogenesis and an entry receptor required for VEEV infection of neurons of the brain, receptor-targeted therapies may hold promise as countermeasures.

Neutralizing antibodies protect mice against Venezuelan equine encephalitis virus aerosol challenge

Venezuelan equine encephalitis virus (VEEV) remains a risk for epidemic emergence or use as an aerosolized bioweapon. To develop possible countermeasures, we isolated VEEV-specific neutralizing monoclonal antibodies (mAbs) from mice and a human immunized with attenuated VEEV strains. Functional assays and epitope mapping established that potently inhibitory anti-VEEV mAbs bind distinct antigenic sites in the A or B domains of the E2 glycoprotein and block multiple steps in the viral replication cycle including attachment, fusion, and egress. A 3.2-Å cryo-electron microscopy reconstruction of VEEV virus-like particles bound by a human Fab suggests that antibody engagement of the B domain may result in cross-linking of neighboring spikes to prevent conformational requirements for viral fusion. Prophylaxis or postexposure therapy with these mAbs protected mice against lethal aerosol challenge with VEEV. Our study defines functional and structural mechanisms of mAb protection and suggests that multiple antigenic determinants on VEEV can be targeted for vaccine or antibody-based therapeutic development.

Construction and characterization of a full-length infectious clone of Getah virus in vivo

Getah virus (GETV) is a mosquito-borne virus of the genus Alphavirus in the family Togaviridae and, in recent years, it has caused several outbreaks in animals. The molecular basis for GETV pathogenicity is not well understood. Therefore, a reverse genetic system of GETV is needed to produce genetically modified viruses for the study of the viral replication and its pathogenic mechanism. Here, we generated a CMV-driven infectious cDNA clone based on a previously isolated GETV strain, GX201808 (pGETV-GX). Transfection of pGETV-GX into BHK-21 ​cells resulted in the recovery of a recombinant virus (rGETV-GX) which showed similar growth characteristics to its parental virus. Then three-day-old mice were experimentally infected with either the parental or recombinant virus. The recombinant virus showed milder pathogenicity than the parental virus in the mice. Based on the established CMV-driven cDNA clone, subgenomic promoter and two restriction enzyme sites (BamHI and EcoRI) were introduced into the region between E1 protein and 3′UTR. Then the green fluorescent protein (GFP), red fluorescent protein (RFP) and improved light-oxygen-voltage (iLOV) genes were inserted into the restriction enzyme sites. Transfection of the constructs carrying the reporter genes into BHK-21 ​cells proved the rescue of the recombinant reporter viruses. Taken together, the establishment of a reverse genetic system for GETV provides a valuable tool for the study of the virus life cycle, and to aid the development of genetically engineered GETVs as vectors for foreign gene expression.

•

Generation and recovery of a CMV-driven infectious cDNA clone of GETV isolate, GX201808 (pGETV-GX).

•

The recombinant virus showed milder pathogenicity than the parental virus in a mouse model.

•

The Getah virus infectious clone can be used as a vector for expressing reporter genes.

Pan-protective anti-alphavirus human antibodies target a conserved E1 protein epitope

Alphaviruses are emerging, mosquito-transmitted pathogens that cause musculoskeletal and neurological disease in humans. Although neutralizing antibodies that inhibit individual alphaviruses have been described, broadly reactive antibodies that protect against both arthritogenic and encephalitic alphaviruses have not been reported. Here, we identify DC2.112 and DC2.315, two pan-protective yet poorly neutralizing human monoclonal antibodies (mAbs) that avidly bind to viral antigen on the surface of cells infected with arthritogenic and encephalitic alphaviruses. These mAbs engage a conserved epitope in domain II of the E1 protein proximal to and within the fusion peptide. Treatment with DC2.112 or DC2.315 protects mice against infection by both arthritogenic (chikungunya and Mayaro) and encephalitic (Venezuelan, Eastern, and Western equine encephalitis) alphaviruses through multiple mechanisms, including inhibition of viral egress and monocyte-dependent Fc effector functions. These findings define a conserved epitope recognized by weakly neutralizing yet protective antibodies that could be targeted for pan-alphavirus immunotherapy and vaccine design.

Designing multivalent immunogens for alphavirus vaccine optimization

There is a pressing need for vaccines against mosquito-borne alphaviruses such as Venezualen and eastern equine encephalitis viruses (VEEV, EEEV). We demonstrate an approach to vaccine development based on physicochemical properties (PCP) of amino acids to design a PCP-consensus sequence of the epitope-rich B domain of the VEEV major antigenic E2 protein. The consensus "spike" domain was incorporated into a live-attenuated VEEV vaccine candidate (ZPC/IRESv1). Mice inoculated with either ZPC/IRESv1 or the same virus containing the consensus E2 protein fragment (VEEVconE2) were protected against lethal challenge with VEEV strains ZPC-738 and 3908, and Mucambo virus (MUCV, related to VEEV), and had comparable neutralizing antibody titers against each virus. Both vaccines induced partial protection against Madariaga virus (MADV), a close relative of EEEV, lowering mortality from 60% to 20%. Thus PCP-consensus sequences can be integrated into a replicating virus that could, with further optimization, provide a broad-spectrum vaccine against encephalitic alphaviruses.

LDLRAD3 is a receptor for Venezuelan equine encephalitis virus

Venezuelan equine encephalitis virus (VEEV) is a neurotropic alphavirus transmitted by mosquitoes that causes encephalitis and death in humans¹. VEEV is a biodefence concern because of its potential for aerosol spread and the current lack of sufficient countermeasures. The host factors that are required for VEEV entry and infection remain poorly characterized. Here, using a genome-wide CRISPR-Cas9-based screen, we identify low-density lipoprotein receptor class A domain-containing 3 (LDLRAD3)-a highly conserved yet poorly characterized member of the scavenger receptor superfamily-as a receptor for VEEV. Gene editing of mouse Ldlrad3 or human LDLRAD3 results in markedly reduced viral infection of neuronal cells, which is restored upon complementation with LDLRAD3. LDLRAD3 binds directly to VEEV particles and enhances virus attachment and internalization into host cells. Genetic studies indicate that domain 1 of LDLRAD3 (LDLRAD3(D1)) is necessary and sufficient to support infection by VEEV, and both anti-LDLRAD3 antibodies and an LDLRAD3(D1)-Fc fusion protein block VEEV infection in cell culture. The pathogenesis of VEEV infection is abrogated in mice with deletions in Ldlrad3, and administration of LDLRAD3(D1)-Fc abolishes disease caused by several subtypes of VEEV, including highly virulent strains. The development of a decoy-receptor fusion protein suggests a strategy for the prevention of severe VEEV infection and associated disease in humans.

Rationally Attenuated Vaccines for Venezuelan Equine Encephalitis Protect Against Epidemic Strains with a Single Dose

Venezuelan equine encephalitis virus (VEEV) is a re-emerging virus of human, agriculture, and bioweapon threat importance. No FDA-approved treatment is available to combat Venezuelan equine encephalitis in humans, prompting the need to create a vaccine that is safe, efficacious, and cannot be replicated in the mosquito vector. Here we describe the use of a serotype ID VEEV (ZPC-738) vaccine with an internal ribosome entry site (IRES) to alter gene expression patterns. This ZPC/IRES vaccine was genetically engineered in two ways based on the position of the IRES insertion to create a vaccine that is safe and efficacious. After a single dose, both versions of the ZPC/IRES vaccine elicited neutralizing antibody responses in mice and non-human primates after a single dose, with more robust responses produced by version 2. Further, all mice and primates were protected from viremia following VEEV challenge. These vaccines were also safer in neonatal mice than the current investigational new drug vaccine, TC-83. These results show that IRES-based attenuation of alphavirus genomes consistently produce promising vaccine candidates, with VEEV/IRES version 2 showing promise for further development.

Mutations in Hypervariable Domain of Venezuelan Equine Encephalitis Virus nsP3 Protein Differentially Affect Viral Replication

Venezuelan equine encephalitis virus (VEEV) is one of the important human and animal pathogens. It forms replication enzyme complexes (RCs) containing viral nonstructural proteins (nsPs) that mediate the synthesis of virus-specific RNAs. The assembly and associated functions of RC also depend on the presence of a specific set of host proteins. Our study demonstrates that the hypervariable domain (HVD) of VEEV nsP3 interacts with the members of the FXR family of cellular proteins and also binds the Src homology 3 (SH3) domain-containing proteins CD2AP and SH3KBP1. Interactions with FXR family members are mediated by the C-terminal repeating peptide of HVD. A single short, minimal motif identified in this study is sufficient for driving efficient VEEV replication in the absence of HVD interactions with other host proteins. The SH3 domain-containing proteins bind to another fragment of VEEV HVD. They can promote viral replication in the absence of FXR-HVD interactions albeit less efficiently. VEEV replication can be also switched from an FXR-dependent to a chikungunya virus-specific, G3BP-dependent mode. The described modifications of VEEV HVD have a strong impact on viral replication in vitro and pathogenesis. Their effects on viral pathogenesis depend on mouse age and the genetic background of the virus.IMPORTANCE The replication of alphaviruses is determined by specific sets of cellular proteins, which mediate the assembly of viral replication complexes. Some of these critical host factors interact with the hypervariable domain (HVD) of alphavirus nsP3. In this study, we have explored binding sites of host proteins, which are specific partners of nsP3 HVD of Venezuelan equine encephalitis virus. We also define the roles of these interactions in viral replication both in vitro and in vivo A mechanistic understanding of the binding of CD2AP, SH3KBP1, and FXR protein family members to VEEV HVD uncovers important aspects of alphavirus evolution and determines new targets for the development of alphavirus-specific drugs and directions for viral attenuation and vaccine development.

Macromolecular Synthesis Shutoff Resistance by Myeloid Cells Is Critical to IRF7-Dependent Systemic Interferon Alpha/Beta Induction after Alphavirus Infection

Alphavirus infection of fibroblastic cell types in vitro inhibits host cell translation and transcription, leading to suppression of interferon alpha/beta (IFN-α/β) production. However, the effect of infection upon myeloid cells, which are often the first cells encountered by alphaviruses in vivo, is unclear. Previous studies demonstrated an association of systemic IFN-α/β production with myeloid cell infection efficiency. Murine infection with wild-type Venezuelan equine encephalitis virus (VEEV), a highly myeloid-cell-tropic alphavirus, results in secretion of very high systemic levels of IFN-α/β, suggesting that stress responses in responding cells are active. Here, we infected myeloid cell cultures with VEEV to identify the cellular source of IFN-α/β, the timing and extent of translation and/or transcription inhibition in infected cells, and the transcription factors responsible for IFN-α/β induction. In contrast to fibroblast infection, myeloid cell cultures infected with VEEV secreted IFN-α/β that increased until cell death was observed. VEEV inhibited translation in most cells early after infection (<6 h postinfection [p.i.]), while transcription inhibition occurred later (>6 h p.i.). Furthermore, the interferon regulatory factor 7 (IRF7), but not IRF3, transcription factor was critical for IFN-α/β induction in vitro and in sera of mice. We identified a subset of infected Raw 264.7 myeloid cells that resisted VEEV-induced translation inhibition and secreted IFN-α/β despite virus infection. However, in the absence of IFN receptor signaling, the size of this cell population was diminished. These results indicate that IFN-α/β induction in vivo is IRF7 dependent and arises in part from a subset of myeloid cells that are resistant, in an IFN-α/β-dependent manner, to VEEV-induced macromolecular synthesis inhibition.IMPORTANCE Most previous research exploring the interaction of alphaviruses with host cell antiviral responses has been conducted using fibroblast lineage cell lines. Previous studies have led to the discovery of virus-mediated activities that antagonize host cell antiviral defense pathways, such as host cell translation and transcription inhibition and suppression of STAT1 signaling. However, their relevance and impact upon myeloid lineage cell types, which are key responders during the initial stages of alphavirus infection in vivo, have not been well studied. Here, we demonstrate the different abilities of myeloid cells to resist VEEV infection compared to nonmyeloid cell types and begin to elucidate the mechanisms by which host antiviral responses are upregulated in myeloid cells despite the actions of virus-encoded antagonists.

Approach to Strain Selection and the Propagation of Viral Stocks for Venezuelan Equine Encephalitis Virus Vaccine Efficacy Testing under the Animal Rule

Licensure of a vaccine to protect against aerosolized Venezuelan equine encephalitis virus (VEEV) requires use of the U.S. Food and Drug Administration (FDA) Animal Rule to assess vaccine efficacy as human studies are not feasible or ethical. An approach to selecting VEEV challenge strains for use under the Animal Rule was developed, taking into account Department of Defense (DOD) vaccine requirements, FDA Animal Rule guidelines, strain availability, and lessons learned from the generation of filovirus challenge agents within the Filovirus Animal Nonclinical Group (FANG). Initial down-selection to VEEV IAB and IC epizootic varieties was based on the DOD objective for vaccine protection in a bioterrorism event. The subsequent down-selection of VEEV IAB and IC isolates was based on isolate availability, origin, virulence, culture and animal passage history, known disease progression in animal models, relevancy to human disease, and ability to generate sufficient challenge material. Methods for the propagation of viral stocks (use of uncloned (wild-type), plaque-cloned, versus cDNA-cloned virus) to minimize variability in the potency of the resulting challenge materials were also reviewed. The presented processes for VEEV strain selection and the propagation of viral stocks may serve as a template for animal model development product testing under the Animal Rule to other viral vaccine programs. This manuscript is based on the culmination of work presented at the “Alphavirus Workshop” organized and hosted by the Joint Vaccine Acquisition Program (JVAP) on 15 December 2014 at Fort Detrick, Maryland, USA.

Intestinal Dysmotility Syndromes following Systemic Infection by Flaviviruses

Although chronic gastrointestinal dysmotility syndromes are a common worldwide health problem, underlying causes for these disorders are poorly understood. We show that flavivirus infection of enteric neurons leads to acute neuronal injury and cell death, inflammation, bowel dilation, and slowing of intestinal transit in mice. Flavivirus-primed CD8⁺ T cells promote these phenotypes, as their absence diminished enteric neuron injury and intestinal transit delays, and their adoptive transfer reestablished dysmotility after flavivirus infection. Remarkably, mice surviving acute flavivirus infection developed chronic gastrointestinal dysmotility that was exacerbated by immunization with an unrelated alphavirus vaccine or exposure to a non-infectious inflammatory stimulus. This model of chronic post-infectious gastrointestinal dysmotility in mice suggests that viral infections with tropism for enteric neurons and the ensuing immune response might contribute to the development of bowel motility disorders in humans. These results suggest an opportunity for unique approaches to diagnosis and therapy of gastrointestinal dysmotility syndromes.

The Interferon-Induced Exonuclease ISG20 Exerts Antiviral Activity through Upregulation of Type I Interferon Response Proteins

The host immune responses to infection lead to the production of type I interferon (IFN), and the upregulation of interferon-stimulated genes (ISGs) reduces virus replication and virus dissemination within a host. Ectopic expression of the interferon-induced 20-kDa exonuclease ISG20 suppressed replication of chikungunya virus and Venezuelan equine encephalitis virus, two mosquito-vectored RNA alphaviruses. Since the replication of alphavirus genomes occurs exclusively in the cytoplasm, the mechanism of nucleus-localized ISG20 inhibition of replication is unclear. In this study, we determined that ISG20 acts as a master regulator of over 100 genes, many of which are ISGs. Specifically, ISG20 upregulated IFIT1 genes and inhibited translation of the alphavirus genome. Furthermore, IFIT1-sensitive alphavirus replication was increased in Isg20^−/− mice compared to the replication of wild-type viruses but not in cells ectopically expressing ISG20. We propose that ISG20 acts as an indirect regulator of RNA virus replication in the cytoplasm through the upregulation of many other ISGs.

Type I interferon (IFN)-stimulated genes (ISGs) have critical roles in inhibiting virus replication and dissemination. Despite advances in understanding the molecular basis of ISG restriction, the antiviral mechanisms of many remain unclear. The 20-kDa ISG ISG20 is a nuclear 3′–5′ exonuclease with preference for single-stranded RNA (ssRNA) and has been implicated in the IFN-mediated restriction of several RNA viruses. Although the exonuclease activity of ISG20 has been shown to degrade viral RNA in vitro, evidence has yet to be presented that virus inhibition in cells requires this activity. Here, we utilized a combination of an inducible, ectopic expression system and newly generated Isg20^−/− mice to investigate mechanisms and consequences of ISG20-mediated restriction. Ectopically expressed ISG20 localized primarily to Cajal bodies in the nucleus and restricted replication of chikungunya and Venezuelan equine encephalitis viruses. Although restriction by ISG20 was associated with inhibition of translation of infecting genomic RNA, degradation of viral RNAs was not observed. Instead, translation inhibition of viral RNA was associated with ISG20-induced upregulation of over 100 other genes, many of which encode known antiviral effectors. ISG20 modulated the production of IFIT1, an ISG that suppresses translation of alphavirus RNAs. Consistent with this observation, the pathogenicity of IFIT1-sensitive alphaviruses was increased in Isg20^−/− mice compared to that of wild-type viruses but not in cells ectopically expressing ISG20. Our findings establish an indirect role for ISG20 in the early restriction of RNA virus replication by regulating expression of other ISGs that inhibit translation and possibly other activities in the replication cycle.

IMPORTANCE The host immune responses to infection lead to the production of type I interferon (IFN), and the upregulation of interferon-stimulated genes (ISGs) reduces virus replication and virus dissemination within a host. Ectopic expression of the interferon-induced 20-kDa exonuclease ISG20 suppressed replication of chikungunya virus and Venezuelan equine encephalitis virus, two mosquito-vectored RNA alphaviruses. Since the replication of alphavirus genomes occurs exclusively in the cytoplasm, the mechanism of nucleus-localized ISG20 inhibition of replication is unclear. In this study, we determined that ISG20 acts as a master regulator of over 100 genes, many of which are ISGs. Specifically, ISG20 upregulated IFIT1 genes and inhibited translation of the alphavirus genome. Furthermore, IFIT1-sensitive alphavirus replication was increased in Isg20^−/− mice compared to the replication of wild-type viruses but not in cells ectopically expressing ISG20. We propose that ISG20 acts as an indirect regulator of RNA virus replication in the cytoplasm through the upregulation of many other ISGs.

The Efficacy of the Interferon Alpha/Beta Response versus Arboviruses Is Temperature Dependent

Interferon alpha/beta (IFN-α/β) is a critical mediator of protection against most viruses, with host survival frequently impossible in its absence. Many studies have investigated the pathways involved in the induction of IFN-α/β after virus infection and the resultant upregulation of antiviral IFN-stimulated genes (ISGs) through IFN-α/β receptor complex signaling. However, other than examining the effects of genetic deletion of induction or effector pathway components, little is known regarding the functionality of these responses in intact hosts and whether host genetic or environmental factors might influence their potency. Here, we demonstrate that the IFN-α/β response against multiple arthropod-vectored viruses, which replicate over a wide temperature range, is extremely sensitive to fluctuations in temperature, exhibiting reduced antiviral efficacy at subnormal cellular temperatures and increased efficacy at supranormal temperatures. The effect involves both IFN-α/β and ISG upregulation pathways with a major aspect of altered potency reflecting highly temperature-dependent transcription of IFN response genes that leads to altered IFN-α/β and ISG protein levels. Discordantly, signaling steps prior to transcription that were examined showed the opposite effect from gene transcription, with potentiation at low temperature and inhibition at high temperature. Finally, we demonstrate that by lowering the temperature of mice, chikungunya arbovirus replication and disease are exacerbated in an IFN-α/β-dependent manner. This finding raises the potential for use of hyperthermia as a therapeutic modality for viral infections and in other contexts such as antitumor therapy. The increased IFN-α/β efficacy at high temperatures may also reflect an innate immune-relevant aspect of the febrile response.

The interferon alpha/beta (IFN-α/β) response is a first-line innate defense against arthropod-borne viruses (arboviruses). Arboviruses, such as chikungunya virus (CHIKV), can infect cells and replicate across a wide temperature range due to their replication in both mammalian/avian and arthropod hosts. Accordingly, these viruses can cause human disease in tissues regularly exposed to temperatures below the normal mammalian core temperature, 37°C. We questioned whether temperature variation could affect the efficacy of IFN-α/β responses against these viruses and help to explain some aspects of human disease manifestations. We observed that IFN-α/β efficacy was dramatically lower at subnormal temperatures and modestly enhanced at febrile temperatures, with the effects involving altered IFN-α/β response gene transcription but not IFN-α/β pathway signaling. These results provide insight into the functioning of the IFN-α/β response in vivo and suggest that temperature elevation may represent an immune-enhancing therapeutic modality for a wide variety of IFN-α/β-sensitive infections and pathologies.

Current landscape and future prospects of antiviral drugs derived from microbial products

Viral infections are a major global health threat. Over the last 50 years, significant efforts have been devoted to the development of antiviral drugs and great success has been achieved for some viruses. However, other virus infections, such as epidemic influenza, still spread globally and new threats continue to arise from emerging and re-emerging viruses and drug-resistant viruses. In this review, the contributions of microbial products isolated in Institute of Microbial Chemistry for antiviral research are summarized. In addition, the current state of development of antiviral drugs that target influenza virus and hepatitis B virus, and the future prospects for antivirals from natural products are described and discussed.The Journal of Antibiotics advance online publication, 11 October 2017; doi:10.1038/ja.2017.115.

Host oxidative folding pathways offer novel anti-chikungunya virus drug targets with broad spectrum potential

Alphaviruses require conserved cysteine residues for proper folding and assembly of the E1 and E2 envelope glycoproteins, and likely depend on host protein disulfide isomerase-family enzymes (PDI) to aid in facilitating disulfide bond formation and isomerization in these proteins. Here, we show that in human HEK293 cells, commercially available inhibitors of PDI or modulators thereof (thioredoxin reductase, TRX-R; endoplasmic reticulum oxidoreductin-1, ERO-1) inhibit the replication of CHIKV chikungunya virus (CHIKV) in vitro in a dose-dependent manner. Further, the TRX-R inhibitor auranofin inhibited Venezuelan equine encephalitis virus and the flavivirus Zika virus replication in vitro, while PDI inhibitor 16F16 reduced replication but demonstrated notable toxicity. 16F16 significantly altered the viral genome: plaque-forming unit (PFU) ratio of CHIKV in vitro without affecting relative intracellular viral RNA quantities and inhibited CHIKV E1-induced cell-cell fusion, suggesting that PDI inhibitors alter progeny virion infectivity through altered envelope function. Auranofin also increased the extracellular genome:PFU ratio but decreased the amount of intracellular CHIKV RNA, suggesting an alternative mechanism of action. Finally, auranofin reduced footpad swelling and viremia in the C57BL/6 murine model of CHIKV infection. Our results suggest that targeting oxidative folding pathways represents a potential new anti-alphavirus therapeutic strategy.

Recombinant Isfahan Virus and Vesicular Stomatitis Virus Vaccine Vectors Provide Durable, Multivalent, Single-Dose Protection against Lethal Alphavirus Challenge

The demonstrated clinical efficacy of a recombinant vesicular stomatitis virus (rVSV) vaccine vector has stimulated the investigation of additional serologically distinct Vesiculovirus vectors as therapeutic and/or prophylactic vaccine vectors to combat emerging viral diseases. Among these viral threats are the encephalitic alphaviruses Venezuelan equine encephalitis virus (VEEV) and Eastern equine encephalitis virus (EEEV), which have demonstrated potential for natural disease outbreaks, yet no licensed vaccines are available in the event of an epidemic. Here we report the rescue of recombinant Isfahan virus (rISFV) from genomic cDNA as a potential new vaccine vector platform. The rISFV genome was modified to attenuate virulence and express the VEEV and EEEV E2/E1 surface glycoproteins as vaccine antigens. A single dose of the rISFV vaccine vectors elicited neutralizing antibody responses and protected mice from lethal VEEV and EEEV challenges at 1 month postvaccination as well as lethal VEEV challenge at 8 months postvaccination. A mixture of rISFV vectors expressing the VEEV and EEEV E2/E1 glycoproteins also provided durable, single-dose protection from lethal VEEV and EEEV challenges, demonstrating the potential for a multivalent vaccine formulation. These findings were paralleled in studies with an attenuated form of rVSV expressing the VEEV E2/E1 glycoproteins. Both the rVSV and rISFV vectors were attenuated by using an approach that has demonstrated safety in human trials of an rVSV/HIV-1 vaccine. Vaccines based on either of these vaccine vector platforms may present a safe and effective approach to prevent alphavirus-induced disease in humans.IMPORTANCE This work introduces rISFV as a novel vaccine vector platform that is serologically distinct and phylogenetically distant from VSV. The rISFV vector has been attenuated by an approach used for an rVSV vector that has demonstrated safety in clinical studies. The vaccine potential of the rISFV vector was investigated in a well-established alphavirus disease model. The findings indicate the feasibility of producing a safe, efficacious, multivalent vaccine against the encephalitic alphaviruses VEEV and EEEV, both of which can cause fatal disease. This work also demonstrates the efficacy of an attenuated rVSV vector that has already demonstrated safety and immunogenicity in multiple HIV-1 phase I clinical studies. The absence of serological cross-reactivity between rVSV and rISFV and their phylogenetic divergence within the Vesiculovirus genus indicate potential for two stand-alone vaccine vector platforms that could be used to target multiple bacterial and/or viral agents in successive immunization campaigns or as heterologous prime-boost agents.

Host translation shutoff mediated by non-structural protein 2 is a critical factor in the antiviral state resistance of Venezuelan equine encephalitis virus

Most previous studies of interferon-alpha/beta (IFN-α/β) response antagonism by alphaviruses have focused upon interruption of IFN-α/β induction and/or receptor signaling cascades. Infection of mice with Venezuelan equine encephalitis alphavirus (VEEV) or Sindbis virus (SINV) induces serum IFN-α/β, that elicits a systemic antiviral state in uninfected cells successfully controlling SINV but not VEEV replication. Furthermore, VEEV replication is more resistant than that of SINV to a pre-existing antiviral state in vitro. While host macromolecular shutoff is proposed as a major antagonist of IFN-α/β induction, the underlying mechanisms of alphavirus resistance to a pre-existing antiviral state are not fully defined, nor is the mechanism for the greater resistance of VEEV. Here, we have separated viral transcription and translation shutoff with multiple alphaviruses, identified the viral proteins that induce each activity, and demonstrated that VEEV nonstructural protein 2-induced translation shutoff is likely a critical factor in enhanced antiviral state resistance of this alphavirus.

New World and Old World Alphaviruses Have Evolved to Exploit Different Components of Stress Granules, FXR and G3BP Proteins, for Assembly of Viral Replication Complexes

The positive-strand RNA viruses initiate their amplification in the cell from a single genome delivered by virion. This single RNA molecule needs to become involved in replication process before it is recognized and degraded by cellular machinery. In this study, we show that distantly related New World and Old World alphaviruses have independently evolved to utilize different cellular stress granule-related proteins for assembly of complexes, which recruit viral genomic RNA and facilitate formation of viral replication complexes (vRCs). Venezuelan equine encephalitis virus (VEEV) utilizes all members of the Fragile X syndrome (FXR) family, while chikungunya and Sindbis viruses exploit both members of the G3BP family. Despite being in different families, these proteins share common characteristics, which determine their role in alphavirus replication, namely, the abilities for RNA-binding and for self-assembly into large structures. Both FXR and G3BP proteins interact with virus-specific, repeating amino acid sequences located in the C-termini of hypervariable, intrinsically disordered domains (HVDs) of viral nonstructural protein nsP3. We demonstrate that these host factors orchestrate assembly of vRCs and play key roles in RNA and virus replication. Only knockout of all of the homologs results in either pronounced or complete inhibition of replication of different alphaviruses. The use of multiple homologous proteins with redundant functions mediates highly efficient recruitment of viral RNA into the replication process. This independently evolved acquisition of different families of cellular proteins by the disordered protein fragment to support alphavirus replication suggests that other RNA viruses may utilize a similar mechanism of host factor recruitment for vRC assembly. The use of different host factors by alphavirus species may be one of the important determinants of their pathogenesis.

Many viruses encode proteins containing intrinsically disordered domains, whose functions are as yet unknown. Here we show that such a domain (HVD) in the alphavirus nsP3 protein orchestrates assembly of viral replication complexes through interaction with RNA-binding cellular factors. Surprisingly, geographically isolated viruses have evolved to utilize different cellular proteins: the nsP3 HVD of Venezuelan equine encephalitis virus (VEEV) binds all members of the FXR family, while nsP3 HVDs of Sindbis and chikungunya viruses interact with G3BP proteins. Despite being in different families, G3BPs and FXRs have similar domain organization, and assemble into higher order complexes, such as stress granules. Alphaviruses exploit their abilities for complex self-assembly and RNA binding to build RNA-containing pre-replication complexes. Using CRISPR/Cas9 mediated knockouts, we show that deletion of all homologs strongly affects virus replication, while knockout of a single FXR or G3BP homolog has no or mild effect. Our data suggest that an alphavirus HVD serves as a hub to recruit host factors for replication complex assembly and may determine virus adaptation to distinct cellular environments. Notably, the improved understanding of HVD interactions allows alphavirus replication to be switched from an FXR- to G3BP-dependent mode and opens new possibilities for development of antiviral therapeutics.

IFIT1 Differentially Interferes with Translation and Replication of Alphavirus Genomes and Promotes Induction of Type I Interferon

Alphaviruses are a group of widely distributed human and animal pathogens. It is well established that their replication is sensitive to type I IFN treatment, but the mechanism of IFN inhibitory function remains poorly understood. Using a new experimental system, we demonstrate that in the presence of IFN-β, activation of interferon-stimulated genes (ISGs) does not interfere with either attachment of alphavirus virions to the cells, or their entry and nucleocapsid disassembly. However, it strongly affects translation of the virion-delivered virus-specific RNAs. One of the ISG products, IFIT1 protein, plays a major role in this translation block, although an IFIT1-independent mechanism is also involved. The 5'UTRs of the alphavirus genomes were found to differ significantly in their ability to drive translation in the presence of increased concentration of IFIT1. Prior studies have shown that adaptation of naturally circulating alphaviruses to replication in tissue culture results in accumulation of mutations in the 5'UTR, which increase the efficiency of the promoter located in the 5'end of the genome. Here, we show that these mutations also decrease resistance of viral RNA to IFIT1-induced translation inhibition. In the presence of higher levels of IFIT1, alphaviruses with wt 5'UTRs became potent inducers of type I IFN, suggesting a new mechanism of type I IFN induction. We applied this knowledge of IFIT1 interaction with alphaviruses to develop new attenuated variants of Venezuelan equine encephalitis and chikungunya viruses that are more sensitive to the antiviral effects of IFIT1, and thus could serve as novel vaccine candidates.

A viral RNA structural element alters host recognition of nonself RNA

Although interferon (IFN) signaling induces genes that limit viral infection, many pathogenic viruses overcome this host response. As an example, 2'-O methylation of the 5' cap of viral RNA subverts mammalian antiviral responses by evading restriction of Ifit1, an IFN-stimulated gene that regulates protein synthesis. However, alphaviruses replicate efficiently in cells expressing Ifit1 even though their genomic RNA has a 5' cap lacking 2'-O methylation. We show that pathogenic alphaviruses use secondary structural motifs within the 5' untranslated region (UTR) of their RNA to alter Ifit1 binding and function. Mutations within the 5'-UTR affecting RNA structural elements enabled restriction by or antagonism of Ifit1 in vitro and in vivo. These results identify an evasion mechanism by which viruses use RNA structural motifs to avoid immune restriction.

Chikungunya virus induces IPS-1-dependent innate immune activation and protein kinase R-independent translational shutoff

Chikungunya virus (CHIKV) is an arthritogenic mosquito-transmitted alphavirus that is undergoing reemergence in areas around the Indian Ocean. Despite the current and potential danger posed by this virus, we know surprisingly little about the induction and evasion of CHIKV-associated antiviral immune responses. With this in mind we investigated innate immune reactions to CHIKV in human fibroblasts, a demonstrable in vivo target of virus replication and spread. We show that CHIKV infection leads to activation of the transcription factor interferon regulatory factor 3 (IRF3) and subsequent transcription of IRF3-dependent antiviral genes, including beta interferon (IFN-β). IRF3 activation occurs by way of a virus-induced innate immune signaling pathway that includes the adaptor molecule interferon promoter stimulator 1 (IPS-1). Despite strong transcriptional upregulation of these genes, however, translation of the corresponding proteins is not observed. We further demonstrate that translation of cellular (but not viral) genes is blocked during infection and that although CHIKV is found to trigger inactivation of the translational molecule eukaryotic initiation factor subunit 2α by way of the double-stranded RNA sensor protein kinase R, this response is not required for the block to protein synthesis. Furthermore, overall diminution of cellular RNA synthesis is also observed in the presence of CHIKV and transcription of IRF3-dependent antiviral genes appears specifically blocked late in infection. We hypothesize that the observed absence of IFN-β and antiviral proteins during infection results from an evasion mechanism exhibited by CHIKV that is dependent on widespread shutoff of cellular protein synthesis and a targeted block to late synthesis of antiviral mRNA transcripts.

Acute infection with venezuelan equine encephalitis virus replicon particles catalyzes a systemic antiviral state and protects from lethal virus challenge

The host innate immune response provides a critical first line of defense against invading pathogens, inducing an antiviral state to impede the spread of infection. While numerous studies have documented antiviral responses within actively infected tissues, few have described the earliest innate response induced systemically by infection. Here, utilizing Venezuelan equine encephalitis virus (VEE) replicon particles (VRP) to limit infection to the initially infected cells in vivo, a rapid activation of the antiviral response was demonstrated not only within the murine draining lymph node, where replication was confined, but also within distal tissues. In the liver and brain, expression of interferon-stimulated genes was detected by 1 to 3 h following VRP footpad inoculation, reaching peak expression of >100-fold over that in mock-infected animals. Moreover, mice receiving a VRP footpad inoculation 6, 12, or 24 h prior to an otherwise lethal VEE footpad challenge were completely protected from death, including a drastic reduction in challenge virus titers. VRP pretreatment also provided protection from intranasal VEE challenge and extended the average survival time following intracranial challenge. Signaling through the interferon receptor was necessary for antiviral gene induction and protection from VEE challenge. However, VRP pretreatment failed to protect mice from a heterologous, lethal challenge with vesicular stomatitis virus, yet conferred protection following challenge with influenza virus. Collectively, these results document a rapid modulation of the host innate response within hours of infection, capable of rapidly alerting the entire animal to pathogen invasion and leading to protection from viral disease.

Characteristics of alpha/beta interferon induction after infection of murine fibroblasts with wild-type and mutant alphaviruses

We examined the characteristics of interferon alpha/beta (IFN-alpha/beta) induction after alphavirus or control Sendai virus (SeV) infection of murine fibroblasts (MEFs). As expected, SeV infection of wild-type (wt) MEFs resulted in strong dimerization of IRF3 and the production of high levels of IFN-alpha/beta. In contrast, infection of MEFs with multiple alphaviruses failed to elicit detectable IFN-alpha/beta. In more detailed studies, Sindbis virus (SINV) infection caused dimerization and nuclear migration of IRF3, but minimal IFN-beta promoter activity, although surprisingly, the infected cells were competent for IFN production by other stimuli early after infection. A SINV mutant defective in host macromolecular synthesis shutoff induced IFN-alpha/beta in the MEF cultures dependent upon the activities of the TBK1 IRF3 activating kinase and host pattern recognition receptors (PRRs) PKR and MDA5 but not RIG-I. These results suggest that wild-type alphaviruses antagonize IFN induction after IRF3 activation but also may avoid detection by host PRRs early after infection.

Venezuelan equine encephalitis virus disrupts STAT1 signaling by distinct mechanisms independent of host shutoff

Venezuelan equine encephalitis virus (VEEV) is an important human and veterinary pathogen causing sporadic epizootic outbreaks of potentially fatal encephalitis. The type I interferon (IFN) system plays a central role in controlling VEEV and other alphavirus infections, and IFN evasion is likely an important determinant of whether these viruses disseminate and cause disease within their hosts. Alphaviruses are thought to limit the induction of type I IFNs and IFN-stimulated genes by shutting off host cell macromolecular synthesis, which in the case of VEEV is partially mediated by the viral capsid protein. However, more specific strategies by which alphaviruses inhibit type I IFN signaling have not been characterized. Analyses of cells infected with VEEV and VEEV replicon particles (VRP) demonstrate that viral infection rapidly disrupts tyrosine phosphorylation and nuclear translocation of the transcription factor STAT1 in response to both IFN-beta and IFN-gamma. This effect was independent of host shutoff and expression of viral capsid, suggesting that VEEV uses novel mechanisms to interfere with type I and type II IFN signaling. Furthermore, at times when STAT1 activation was efficiently inhibited, VRP infection did not limit tyrosine phosphorylation of Jak1, Tyk2, or STAT2 after IFN-beta treatment but did inhibit Jak1 and Jak2 activation in response to IFN-gamma, suggesting that VEEV interferes with STAT1 activation by the type I and II receptor complexes through distinct mechanisms. Identification of the viral requirements for this novel STAT1 inhibition will further our understanding of alphavirus molecular pathogenesis and may provide insights into effective alphavirus-based vaccine design.

Similarities and differences in antagonism of neuron alpha/beta interferon responses by Venezuelan equine encephalitis and Sindbis alphaviruses

Venezuelan equine encephalitis virus (VEEV) is highly virulent in adult laboratory mice, while Sindbis virus (SINV) is avirulent regardless of dose or inoculation route, dependent upon functioning alpha/beta interferon (IFN-alpha/beta) responses. We have examined each virus' resistance to and/or antagonism of IFN-alpha/beta responses in neurons, a cell type targeted by both viruses in mice, by infecting IFN-alpha/beta-treated or untreated primary cultures with viruses or virus-derived replicons that lacked the structural proteins. Priming with IFN-alpha/beta prior to infection revealed that VEEV replication and progeny virion production were resistant to an established antiviral state while those of SINV were more sensitive. Postinfection IFN-alpha/beta treatment revealed that phosphorylation of STAT1 and STAT2 was partially blocked by infection with either virus, dependent upon expression of nonstructural proteins (nsP), but not structural proteins (sP). However, inhibition of STAT phosphorylation by VEEV replicons was not correlated with inhibition of IFN-stimulated gene (ISG) mRNA induction, yet ISG induction was inhibited when sP were present. Host translation was inhibited by VEEV nsP even when cells were pretreated with IFN-alpha/beta. SINV blocked ISG induction and translation, associated with nsP-mediated shutoff of macromolecular synthesis, but both activities were sensitive to IFN-alpha/beta pretreatment. We conclude that both VEEV and SINV limit ISG induction in infected neurons through shutoff of host transcription and translation but that inhibition by VEEV is more resistant to IFN-alpha/beta priming. Likewise, both viruses inhibit IFN receptor-initiated signaling, although the effect upon host responses is not clear. Finally, VEEV appears to be more resistant to effectors of the preestablished antiviral state.

Type I interferon induction is correlated with attenuation of a South American eastern equine encephalitis virus strain in mice

North American eastern equine encephalitis virus (NA-EEEV) strains cause high mortality in humans, whereas South American strains (SA-EEEV) are typically avirulent. To clarify mechanisms of SA-EEEV attenuation, we compared mouse-attenuated BeAr436087 SA-EEEV, considered an EEEV vaccine candidate, with mouse-virulent NA-EEEV strain, FL93-939. Although attenuated, BeAr436087 initially replicated more efficiently than FL93-939 in lymphoid and other tissues, inducing systemic IFN-alpha/beta release, whereas FL93-939 induced little. BeAr436087 was more virulent than FL93-939 in IFN-alpha/beta-deficient mice, confirming that type I IFN responses determined attenuation, but the viruses were similarly sensitive to IFN-alpha/beta priming in vitro. Infection with BeAr436087 protected against FL93-939 disease/death, even when given 8 h afterward, suggesting that the environment produced by BeAr436087 infection attenuated FL93-939. We conclude that avoidance of IFN-alpha/beta induction is a major virulence factor for FL93-939. Furthermore, BeAr436087 could be used for vaccination and therapeutic treatment in the event of exposure to NA-EEEV during a bioterrorism attack.

Susceptibility of Ae. aegypti (Diptera: Culicidae) to infection with epidemic (subtype IC) and enzootic (subtypes ID, IIIC, IIID) Venezuelan equine encephalitis complex alphaviruses

To test the hypothesis that enzootic and epidemic Venezuelan equine encephalitis (VEE) complex alphaviruses can infect and be transmitted by Ae. aegypti, we conducted a series of experimental infection studies. One set of experiments tested the susceptibility of geographic strains of Ae. aegypti from Peru and Texas (U.S.A.) for epidemic (subtype IC) and enzootic (subtype ID) strains from Colombia/Venezuela, whereas the second set of experiments tested the susceptibility of Ae. aegypti from Iquitos, Peru, to enzootic VEE complex strains (subtypes ID, IIIC, and IIID) isolated in the same region, at different infectious doses. Experimental infections using artificial bloodmeals suggested that Ae. aegypti mosquitoes, particularly the strain from Iquitos, Peru, is moderately to highly susceptible to all of these VEE complex alphaviruses. The occurrence of enzootic VEE complex viruses circulating endemically in Iquitos suggests the possibility of a dengue-like transmission cycle among humans in tropical cities.

Eastern and Venezuelan equine encephalitis viruses differ in their ability to infect dendritic cells and macrophages: impact of altered cell tropism on pathogenesis

Eastern and Venezuelan equine encephalitis viruses (EEEV and VEEV, respectively) cause severe morbidity and mortality in equines and humans. Like other mosquito-borne viruses, VEEV infects dendritic cells (DCs) and macrophages in lymphoid tissues, fueling a serum viremia and facilitating neuroinvasion. In contrast, EEEV replicates poorly in lymphoid tissues, preferentially infecting osteoblasts. Here, we demonstrate that infectivity of EEEV for myeloid lineage cells including DCs and macrophages was dramatically reduced compared to that of VEEV, whereas both viruses replicated efficiently in mesenchymal lineage cells such as osteoblasts and fibroblasts. We determined that EEEV infection of myeloid lineage cells was restricted after attachment, entry, and uncoating of the genome. Using replicon particles and translation reporter RNAs, we found that translation of incoming EEEV genomes was almost completely inhibited in myeloid, but not mesenchymal, lineage cells. Alpha/beta interferon (IFN-alpha/beta) responses did not mediate the restriction, as infectivity was not restored in the absence of double-stranded RNA-dependent protein kinase, RNase L, or IFN-alpha/beta receptor-mediated signaling. We confirmed these observations in vivo, demonstrating that EEEV is compromised in its ability to replicate within lymphoid tissues, whereas VEEV does so efficiently. The altered tropism of EEEV correlated with an almost complete avoidance of serum IFN-alpha/beta induction in vivo, which may allow EEEV to evade the host's innate immune responses and thereby enhance neurovirulence. Taken together, our data indicate that inhibition of genome translation restricts EEEV infectivity for myeloid but not mesenchymal lineage cells in vitro and in vivo. In this regard, the tropisms of EEEV and VEEV differ dramatically, likely contributing to observed differences in disease etiology.

A five-amino-acid deletion of the eastern equine encephalitis virus capsid protein attenuates replication in mammalian systems but not in mosquito cells

Eastern equine encephalitis virus (EEEV) is a human and veterinary pathogen that causes sporadic cases of fatal neurological disease. We previously demonstrated that the capsid protein of EEEV is a potent inhibitor of host cell gene expression and that this function maps to the amino terminus of the protein. We now identify amino acids 55 to 75, within the N terminus of the capsid, as critical for the inhibition of host cell gene expression. An analysis of stable EEEV replicons expressing mutant capsid proteins corroborated these mapping data. When deletions of 5 to 20 amino acids within this region of the capsid were introduced into infectious EEEV, the mutants exhibited delayed replication in Vero cells. However, the replication of the 5-amino-acid deletion mutant in C710 mosquito cells was not affected, suggesting that virus replication and assembly were affected in a cell-specific manner. Both 5- and 20-amino-acid deletion mutant viruses exhibited increased sensitivity to interferon (IFN) in cell culture and impaired replication and complete attenuation in mice. In summary, we have identified a region within the capsid protein of EEEV that contributes to the inhibition of host gene expression and to the protection of EEEV from the antiviral effects of IFNs. This region is also critical for EEEV pathogenesis.

A two-phase innate host response to alphavirus infection identified by mRNP-tagging in vivo

A concept fundamental to viral pathogenesis is that infection induces specific changes within the host cell, within specific tissues, or within the entire animal. These changes are reflected in a cascade of altered transcription patterns evident during infection. However, elucidation of this cascade in vivo has been limited by a general inability to distinguish changes occurring in the minority of infected cells from those in surrounding uninfected cells. To circumvent this inherent limitation of traditional gene expression profiling methods, an innovative mRNP-tagging technique was implemented to isolate host mRNA specifically from infected cells in vitro as well as in vivo following Venezuelan equine encephalitis virus (VEE) infection. This technique facilitated a direct characterization of the host defense response specifically within the first cells infected with VEE, while simultaneous total RNA analysis assessed the collective response of both the infected and uninfected cells. The result was a unique, multifaceted profile of the early response to VEE infection in primary dendritic cells, as well as in the draining lymph node, the initially targeted tissue in the mouse model. A dynamic environment of complex interactions was revealed, and suggested a two-step innate response in which activation of a subset of host genes in infected cells subsequently leads to activation of the surrounding uninfected cells. Our findings suggest that the application of viral mRNP-tagging systems, as introduced here, will facilitate a much more detailed understanding of the highly coordinated host response to infectious agents.

Structural and nonstructural protein genome regions of eastern equine encephalitis virus are determinants of interferon sensitivity and murine virulence

Eastern equine encephalitis virus (EEEV) causes sporadic epidemics of human and equine disease in North America, but South American strains have seldom been associated with human neurologic disease or mortality, despite serological evidence of infection. In mice, most North American and South American strains of EEEV produce neurologic disease that resembles that associated with human and equine infections. We identified a South American strain that is unable to replicate efficiently in the brain or cause fatal disease in mice yet produces 10-fold higher viremia than virulent EEEV strains. The avirulent South American strain was also sensitive to human interferon (IFN)-alpha, -beta, and -gamma, like most South American strains, in contrast to North American strains that were highly resistant. To identify genes associated with IFN sensitivity and virulence, infectious cDNA clones of a virulent North American strain and the avirulent South American strain were constructed. Two reciprocal chimeric viruses containing swapped structural and nonstructural protein gene regions of the North American and South American strains were also constructed and found to replicate efficiently in vitro. Both chimeras produced fatal disease in mice, similar to that caused by the virulent North American strain. Both chimeric viruses also exhibited intermediate sensitivity to human IFN-alpha, -beta, and -gamma compared to that of the North American and South American strains. Virulence 50% lethal dose assays and serial sacrifice experiments further demonstrated that both structural and nonstructural proteins are important contributors to neurovirulence and viral tissue tropism. Together, the results of this study emphasize the complex and important influences of structural and nonstructural protein gene regions on EEEV virulence.

Gene expression profiling of nonhuman primates exposed to aerosolized Venezuelan equine encephalitis virus

Host responses to Venezuelan equine encephalitis viruses (VEEV) were studied in cynomolgus macaques after aerosol exposure to the epizootic virus. Changes in global gene expression were assessed for the brain, lungs, and spleen. In the brain, major histocompatibility complex (MHC) class I transcripts were induced, while the expression of S100b, a factor associated with brain injury, was inhibited, as was expression of the encephalitogenic gene MOG. Cytokine-mediated signals were affected by infection, including those involving IFN-mediated antiviral activity (IRF-7, OAS, and Mx transcripts), and the increased transcription of caspases. Induction of a few immunologically relevant genes (e.g. IFITM1 and STAT1) was common to all tested tissues. Herein, both tissue-specific and nontissue specific transcriptional changes in response to VEEV are described, including induction of IFN-regulated transcripts and cytokine-induced apoptotic factors, in addition to cellular factors in the brain that may be descriptive of the health status of the brain during the infectious process. Altogether, this work provides novel information on common and tissue-specific host responses against VEEV in a nonhuman primate model of aerosol exposure.

Identification and characterization of interferon-induced proteins that inhibit alphavirus replication

Alpha/beta interferon (IFN-alpha/beta) produces antiviral effects through upregulation of many interferon-stimulated genes (ISGs) whose protein products are effectors of the antiviral state. Previous data from our laboratory have shown that IFN-alpha/beta can limit Sindbis virus (SB) replication through protein kinase R (PKR)-dependent and PKR-independent mechanisms and that one PKR-independent mechanism inhibits translation of the infecting virus genome (K. D. Ryman et al., J. Virol. 79:1487-1499, 2005). Further, using Affymetrix microarray technology, we identified 44 genes as candidates for PKR/RNase L-independent IFN-induced antiviral activities. In the current studies, we have begun analyzing these gene products for antialphavirus activity using three techniques: (i) overexpression of the protein from SB vectors and assessment of virulence attenuation in mice; (ii) overexpression of the proteins in a stable tetracycline-inducible murine fibroblast culture system and assessment of effects upon SB replication; and (iii) small interfering RNA-mediated knockdown of gene mRNA in fibroblast cultures followed by SB replication assessment as above. Tested proteins included those we hypothesized had potential to affect virus genome translation and included murine ISG20, ISG15, the zinc finger antiviral protein (ZAP), viperin, p56, p54, and p49. Interestingly, the pattern of antiviral activity for some gene products was different between in vitro and in vivo assays. Viperin and ZAP attenuated virulence most profoundly in mice. However, ISG20 and ZAP potently inhibited SB replication in vitro, whereas and viperin, p56, and ISG15 exhibited modest replication inhibition in vitro. In contrast, p54 and p49 had little to no effect in any assay.

Synergistic roles of antibody and interferon in noncytolytic clearance of Sindbis virus from different regions of the central nervous system

Sindbis virus (SINV) is an alphavirus that causes infection of neurons and encephalomyelitis in adult immunocompetent mice. Recovery can occur without apparent neurological damage. To better define the factors facilitating noncytolytic clearance of SINV in different regions of the central nervous system (CNS) and the roles of innate and adaptive immune responses at different times during infection, we have characterized SINV infection and clearance in the brain, brain stem, and spinal cords of severe combined immunodeficiency (SCID) and C57BL/6 (wild-type [WT]) mice and mice deficient in beta interferon (IFN-beta) (BKO), antibody (muMT), IFN-gamma (GKO), IFN-gamma receptor (GRKO), and both antibody and IFN-gamma (muMT/GKO). WT mice cleared infectious virus by day 8, while SCID mice had persistent virus replication at all sites. For 3 days after infection, BKO mice had higher titers at all sites than WT mice, despite similar IFN-alpha production, but cleared virus similarly. GKO and GRKO mice cleared infectious virus from all sites by days 8 to 10 and, like WT mice, displayed transient reactivation at 12 to 22 days. muMT mice did not clear virus from the brain, and clearance from the brain stem and lumbar spinal cord was delayed, followed by reactivation. Eighty-one days after infection, muMT/GKO mice had not cleared virus from any site, but titers were lower than for SCID mice. These studies show that IFN-beta is independently important for early control of CNS virus replication, that antiviral antibody is critical for clearance from the brain, and that both antibody and IFN-gamma contribute to prevention of reactivation after initial clearance.

Capsid protein of eastern equine encephalitis virus inhibits host cell gene expression

Eastern equine encephalitis virus (EEEV) causes sporadic but often severe cases of human and equine neurological disease in North America. To determine how EEEV may evade innate immune responses, we screened individual EEEV proteins for the ability to rescue the growth of a Newcastle disease virus expressing green fluorescent protein (NDV-GFP) from the antiviral effects of interferon (IFN). Only expression of the EEEV capsid facilitated NDV-GFP replication. Inhibition of the antiviral effects of IFN by the capsid appears to occur through a general inhibition of cellular gene expression. For example, the capsid inhibited the expression of several reporter genes under the control of RNA polymerase II promoters. In contrast, capsid did not inhibit expression from a T7 RNA polymerase promoter construct, suggesting that the inhibition of gene expression is specific and is not a simple manifestation of toxicity. The inhibition correlated both with capsid-induced phosphorylation of eukaryotic initiation factor 2 alpha and with capsid-mediated inhibition of cellular mRNA accumulation. Mapping analysis identified the N terminus as the region important for the inhibition of host gene expression, suggesting that this inhibition is independent of capsid protease activity. Finally, when cell lines containing EEEV replicons encoding capsid were selected, replicons consistently acquired mutations that deleted all or part of the capsid, for example, amino acids 18 to 135. Given that the amino terminus of the capsid is required to inhibit host cell gene expression, these data suggest that capsid expression from the replicons is ultimately toxic to host cells, presumably because of its ability to inhibit gene expression.

Replication and clearance of Venezuelan equine encephalitis virus from the brains of animals vaccinated with chimeric SIN/VEE viruses

Venezuelan equine encephalitis virus (VEEV) is an important, naturally emerging zoonotic pathogen. Recent outbreaks in Venezuela and Colombia in 1995, involving an estimated 100,000 human cases, indicate that VEEV still poses a serious public health threat. To develop a safe, efficient vaccine that protects against disease resulting from VEEV infection, we generated chimeric Sindbis (SIN) viruses expressing structural proteins of different strains of VEEV and analyzed their replication in vitro and in vivo, as well as the characteristics of the induced immune responses. None of the chimeric SIN/VEE viruses caused any detectable disease in adult mice after either intracerebral (i.c.) or subcutaneous (s.c.) inoculation, and all chimeras were more attenuated than the vaccine strain, VEEV TC83, in 6-day-old mice after i.c. infection. All vaccinated mice were protected against lethal encephalitis following i.c., s.c., or intranasal (i.n.) challenge with the virulent VEEV ZPC738 strain (ZPC738). In spite of the absence of clinical encephalitis in vaccinated mice challenged with ZPC738 via i.n. or i.c. route, we regularly detected high levels of infectious challenge virus in the central nervous system (CNS). However, infectious virus was undetectable in the brains of all immunized animals at 28 days after challenge. Hamsters vaccinated with chimeric SIN/VEE viruses were also protected against s.c. challenge with ZPC738. Taken together, our findings suggest that these chimeric SIN/VEE viruses are safe and efficacious in adult mice and hamsters and are potentially useful as VEEV vaccines. In addition, immunized animals provide a useful model for studying the mechanisms of the anti-VEEV neuroinflammatory response, leading to the reduction of viral titers in the CNS and survival of animals.

Venezuelan encephalitis emergence mediated by a phylogenetically predicted viral mutation

RNA viruses are notorious for their genetic plasticity and propensity to exploit new host-range opportunities, which can lead to the emergence of human disease epidemics such as severe acute respiratory syndrome, AIDS, dengue, and influenza. However, the mechanisms of host-range change involved in most of these viral emergences, particularly the genetic mechanisms of adaptation to new hosts, remain poorly understood. We studied the emergence of Venezuelan equine encephalitis virus (VEEV), an alphavirus pathogen of people and equines that has had severe health and economic effects in the Americas since the early 20th century. Between epidemics, VEE disappears for periods up to decades, and the viral source of outbreaks has remained enigmatic. Combined with phylogenetic analyses to predict mutations associated with a 1992-1993 epidemic, we used reverse genetic studies to identify an envelope glycoprotein gene mutation that mediated emergence. This mutation allowed an enzootic, equine-avirulent VEEV strain, which circulates among rodents in nearby forests to adapt for equine amplification. RNA viruses including alphaviruses exhibit high mutation frequencies. Therefore, ecological and epidemiological factors probably constrain the frequency of VEE epidemics more than the generation, via mutation, of amplification-competent (high equine viremia) virus strains. These results underscore the ability of RNA viruses to alter their host range, virulence, and epidemic potential via minor genetic changes. VEE also demonstrates the unpredictable risks to human health of anthropogenic changes such as the introduction of equines and humans into habitats that harbor zoonotic RNA viruses.

Variation in interferon sensitivity and induction among strains of eastern equine encephalitis virus

Eastern equine encephalitis virus (EEEV) causes human encephalitis in North America (NA), but in South America (SA) it has rarely been associated with human disease, suggesting that SA strains are less virulent. To evaluate the hypothesis that this virulence difference is due to a greater ability of NA strains to evade innate immunity, we compared replication of NA and SA strains in Vero cells pretreated with interferon (IFN). Human IFN-alpha, -beta, and -gamma generally exhibited less effect on replication of NA than SA strains, supporting this hypothesis. In the murine model, no consistent difference in IFN induction was observed between NA and SA strains. After infection with most EEEV strains, higher viremia levels and shorter survival times were observed in mice deficient in IFN-alpha/beta receptors than in wild-type mice, suggesting that IFN-alpha/beta is important in controlling replication. In contrast, IFN-gamma receptor-deficient mice infected with NA and SA strains had similar viremia levels and mortality rates to those of wild-type mice, suggesting that IFN-gamma does not play a major role in murine protection. Mice pretreated with poly(I-C), a nonspecific IFN inducer, exhibited dose-dependent protection against fatal eastern equine encephalitis, further evidence that IFN is important in controlling disease. Overall, our in vivo results did not support the hypothesis that NA strains are more virulent in humans due to their greater ability to counteract the IFN response. However, further studies using a better model of human disease are needed to confirm the results of differential human IFN sensitivity obtained in our in vitro experiments.

Susceptibility of Psorophora confinnis (Diptera: Culicidae) to infection with epizootic (subtype IC) and enzootic (subtype ID) Venezuelan Equine encephalitis viruses

To test the hypothesis that adaptation to epizootic mosquito vectors mediates the emergence of Venezuelan equine encephalitis virus (family Togaviridae, genus Alphavirus, VEEV) from enzootic progenitors, the susceptibility of the epizootic vector Psorophora confinnis (Lynch-Arribalzaga) to epizootic versus enzootic strains was evaluated. Artificial bloodmeals containing subtype IC strains isolated during the 1962-1964, 1992-1993, and 1995 Venezuelan/Colombian epizootics and closely related Venezuelan enzootic subtype ID strains were used to compare mosquito infectivity and transmission potential. Strains from the smaller 1992-1993 epizootic showed lower or equal infectivity and replication compared with enzootic viruses and to strains isolated during the larger 1962-1964 and 1995 epizootics. These experiments failed to provide evidence that Ps. confinnis selects for epizootic VEEV viruses with higher infectivity, as has been shown for Aedes (Ochlerotatus) taeniorhynchus (Wiedemann). Nonetheless, its high susceptibility, abundance in enzootic and epizootic regions, and feeding behavior suggest that Ps. confinnis is an important bridge vector for both enzootic and epizootic VEEV.

Envelope glycoprotein mutations mediate equine amplification and virulence of epizootic venezuelan equine encephalitis virus

Epidemics of Venezuelan equine encephalitis (VEE) result from high-titer equine viremia of IAB and IC subtype viruses that mediate increased mosquito transmission and spillover to humans. Previous genetic studies suggest that mutations in the E2 envelope glycoprotein allow relatively viremia-incompetent, enzootic subtype ID strains to adapt for equine replication, leading to VEE emergence. To test this hypothesis directly, chimeric VEEV strains containing the genetic backbone of enzootic subtype ID strains and the partial envelope glycoprotein genes of epizootic subtype IC and IAB strains, as well as reciprocal chimeras, were used for experimental infections of horses. Insertion of envelope genes from two different, closely related enzootic subtype ID strains into the epizootic backbones resulted in attenuation, demonstrating that the epizootic envelope genes are necessary for the equine-virulent and viremia-competent phenotypes. The partial epizootic envelope genes introduced into an enzootic ID backbone were sufficient to generate the virulent, viremia-competent equine phenotype. These results indicate that a small number of envelope gene mutations can generate an equine amplification-competent, epizootic VEEV from an enzootic progenitor and underscore the limitations of small animal models for evaluating and predicting the epizootic phenotype.

Susceptibility of Ochlerotatus taeniorhynchus (Diptera: Culicidae) to infection with epizootic (subtype IC) and enzootic (subtype ID) Venezuelan equine encephalitis viruses: evidence for epizootic strain adaptation

To test the hypothesis that adaptation to epizootic mosquito vectors mediates emergence of Venezuelan equine encephalitis virus (VEEV) from enzootic progenitors, experimental infection studies were conducted to determine the susceptibility of Ochlerotatus taeniorhynchus (Wiedemann) to epizootic and enzootic strains. Artificial blood meals containing epizootic subtype IC strains isolated during the 1962-1964, 1992-1993, and 1995 Venezuelan/Colombian epizootics and closely related Venezuelan enzootic subtype ID strains were used to compare infectivity and transmission potential. Their greater infectivity and replication suggested that adaptation of epizootic strains to Oc. taeniorhynchus may have enhanced epizootic transmission during the 1962-1964 and 1995 IC coastal epizootics. However, strains from the small 1992-1993 Venezuelan outbreak that did not extend to coastal regions do not seem to infect this species better than closely related subtype ID strains. Adaptation of VEEV to epizootic vectors such as Oc. taeniorhynchus mosquitoes may be a determinant of some but not all VEE emergence events and may influence spread into coastal regions.