Pathogenesis of Venezuelan equine encephalitis virus infection in mice and hamsters

Syrian Hamsters Model Does Not Reflect Human-like Disease after Aerosol Exposure to Encephalitic Alphaviruses

Venezuelan (VEE), eastern (EEE), and western (WEE) equine encephalitis viruses are encephalitic New World alphaviruses that cause periodic epizootic and epidemic outbreaks in horses and humans that may cause severe morbidity and mortality. Currently there are no FDA-licensed vaccines or effective antiviral therapies. Each year, there are a limited number of human cases of encephalitic alphaviruses; thus, licensure of a vaccine or therapeutic would require approval under the FDA animal rule. Approval under the FDA animal rule requires the disease observed in the animal model to recapitulate what is observed in humans. Currently, initial testing of vaccines and therapeutics is performed in the mouse model. Unfortunately, alphavirus disease manifestations in a mouse do not faithfully recapitulate human disease; the VEEV mouse model is lethal whereas in humans VEEV is rarely lethal. In an effort to identify a more appropriate small animal model, we evaluated hamsters in an aerosol exposure model of encephalitic alphavirus infection. The pathology, lethality, and viremia observed in the infected hamsters was inconsistent with what is observed in NHP models and humans. These data suggest that hamsters are not an appropriate model for encephalitic alphaviruses to test vaccines or potential antiviral therapies.

Marmosets as models of infectious diseases

Animal models of infectious disease often serve a crucial purpose in obtaining licensure of therapeutics and medical countermeasures, particularly in situations where human trials are not feasible, i.e., for those diseases that occur infrequently in the human population. The common marmoset (Callithrix jacchus), a Neotropical new-world (platyrrhines) non-human primate, has gained increasing attention as an animal model for a number of diseases given its small size, availability and evolutionary proximity to humans. This review aims to (i) discuss the pros and cons of the common marmoset as an animal model by providing a brief snapshot of how marmosets are currently utilized in biomedical research, (ii) summarize and evaluate relevant aspects of the marmoset immune system to the study of infectious diseases, (iii) provide a historical backdrop, outlining the significance of infectious diseases and the importance of developing reliable animal models to test novel therapeutics, and (iv) provide a summary of infectious diseases for which a marmoset model exists, followed by an in-depth discussion of the marmoset models of two studied bacterial infectious diseases (tularemia and melioidosis) and one viral infectious disease (viral hepatitis C).

Molecular characterization and pathology of field isolates of foot-and-mouth virus in Swiss albino mice

Abstract Foot-and-mouth disease is responsible for severe economic losses to the livestock industry of Pakistan. This study aimed to use Swiss albino mice as a cost-effective experimental animal model to study different immunological and histopathological aspects of FMDV instead of natural targeted species like cattle. After isolation of field isolates FMDV on BHK-21 cell line, biological titer of the virus and mice infectious dose50 was calculated. Virus was injected in 45 Swiss albino mice (group A) through intraperitoneal route. The gross, histopathological and immunopathological lesions in heart, trachea and lungs were recorded at different day’s intervals. Histopathologically, the heart showed congestion, hemorrhages and necrosis of cardiac muscles. Trachea showed deciliated epithelium and lungs showed hemorrhages, bronchial edema and alveolar emphysema. Immunohistochemical studies revealed the presence of virus in cardiac muscles, tracheal and bronchial epithelium and alveolar lumen. The findings evoked a thought that laboratory animals could be an alternative to large animals to meet budget limitations for further research on foot-and-mouth-disease.

Developing a Prototype Pathogen Plan and Research Priorities for the Alphaviruses

The Togaviridae family, genus, Alphavirus, includes several mosquito-borne human pathogens with the potential to spread to near pandemic proportions. Most of these are zoonotic, with spillover infections of humans and domestic animals, but a few such as chikungunya virus (CHIKV) have the ability to use humans as amplification hosts for transmission in urban settings and explosive outbreaks. Most alphaviruses cause nonspecific acute febrile illness, with pathogenesis sometimes leading to either encephalitis or arthralgic manifestations with severe and chronic morbidity and occasional mortality. The development of countermeasures, especially against CHIKV and Venezuelan equine encephalitis virus that are major threats, has included vaccines and antibody-based therapeutics that are likely to also be successful for rapid responses with other members of the family. However, further work with these prototypes and other alphavirus pathogens should target better understanding of human tropism and pathogenesis, more comprehensive identification of cellular receptors and entry, and better understanding of structural mechanisms of neutralization.

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.

The life cycle of the alphaviruses: From an antiviral perspective

The alphaviruses are a widely distributed group of positive-sense, single stranded, RNA viruses. These viruses are largely arthropod-borne and can be found on all populated continents. These viruses cause significant human disease, and recently have begun to spread into new populations, such as the expansion of Chikungunya virus into southern Europe and the Caribbean, where it has established itself as endemic. The study of alphaviruses is an active and expanding field, due to their impacts on human health, their effects on agriculture, and the threat that some pose as potential agents of biological warfare and terrorism. In this systematic review we will summarize both historic knowledge in the field as well as recently published data that has potential to shift current theories in how alphaviruses are able to function. This review is comprehensive, covering all parts of the alphaviral life cycle as well as a brief overview of their pathology and the current state of research in regards to vaccines and therapeutics for alphaviral disease.

In Vitro and In Vivo Phenotypes of Venezuelan, Eastern and Western Equine Encephalitis Viruses Derived from cDNA Clones of Human Isolates

The Department of Defense recently began an effort to improve and standardize virus challenge materials and efficacy determination strategies for testing therapeutics and vaccines. This includes stabilization of virus genome sequences in cDNA form where appropriate, use of human-derived virus isolates, and noninvasive strategies for determination of challenge virus replication. Eventually, it is desired that these approaches will satisfy the FDA "Animal Rule" for licensure, which substitutes animal efficacy data when human data are unlikely to be available. To this end, we created and examined the virulence phenotype of cDNA clones of prototypic human infection-derived strains of the alphaviruses, Venezuelan (VEEV INH9813), eastern (EEEV V105) and western (WEEV Fleming) equine encephalitis viruses, and created fluorescent and luminescent reporter expression vectors for evaluation of replication characteristics in vitro and in vivo. Sequences of minimally passaged isolates of each virus were used to synthesize full-length cDNA clones along with a T7 transcription promoter-based bacterial propagation vector. Viruses generated from the cDNA clones were compared with other "wild type" strains derived from cDNA clones and GenBank sequences to identify and eliminate putative tissue culture artifacts accumulated in the cell passaged biological stocks. This was followed by examination of aerosol and subcutaneous infection and disease in mouse models. A mutation that increased heparan sulfate binding was identified in the VEEV INH9813 biological isolate sequence and eliminated from the cDNA clone. Viruses derived from the new human isolate cDNA clones showed similar mouse virulence to existing clone-derived viruses after aerosol or subcutaneous inoculation.

Pirahy virus: Identification of a new and potential emerging arbovirus in South Brazil

Genomic and epidemiological surveillance are paramount for the discovery of new viruses with the potential to cross species barriers. Here, we present a new member of the genus Alphavirus found in Trichoprosopon and Wyeomia mosquitoes, tentatively named Pirahy virus (PIRAV). PIRAV was isolated from mosquito pools collected in a rural area of Piraí do Sul, South Brazil. In vitro assays revealed that PIRAV replicates and causes cytopathic effects in vertebrate cell lines such as Vero E6, SH-SY5Y, BHK-21 and UMNSAH/DF-1. Genomic signature analysis supports these results showing a dinucleotide and codon usage balance compatible with several hosts. Phylogenetic analyses placed PIRAV basal to the Venezuelan equine encephalitis complex. Genome analyses, electron microscopy, and biological characterization show findings that may alert for the emergence of a new arbovirus in South America.

Toll-like receptor 4 mediates blood-brain barrier permeability and disease in C3H mice during Venezuelan equine encephalitis virus infection

Venezuelan equine encephalitis virus (VEEV) is an encephalitic alphavirus that can cause debilitating, acute febrile illness and potentially result in encephalitis. Currently, there are no FDA-licensed vaccines or specific therapeutics for VEEV. Previous studies have demonstrated that VEEV infection results in increased blood-brain barrier (BBB) permeability that is mediated by matrix metalloproteinases (MMPs). Furthermore, after subarachnoid hemorrhage in mice, MMP-9 is upregulated in the brain and mediates BBB permeability in a toll-like receptor 4 (TLR4)-dependent manner. Here, we demonstrate that disease in C3H mice during VEEV TC-83 infection is dependent on TLR4 because intranasal infection of C3H/HeN (TLR4^WT) mice with VEEV TC-83 resulted in mortality as opposed to survival of TLR4-defective C3H/HeJ (TLR4^mut) mice. In addition, BBB permeability was induced to a lesser extent in TLR4^mut mice compared with TLR4^WT mice during VEEV TC-83 infection as determined by sodium fluorescein and fluorescently-conjugated dextran extravasation. Moreover, MMP-9, MMP-2, ICAM-1, CCL2 and IFN-γ were all induced to significantly lower levels in the brains of infected TLR4^mut mice compared with infected TLR4^WT mice despite the absence of significantly different viral titers or immune cell populations in the brains of infected TLR4^WT and TLR4^mut mice. These data demonstrate the critical role of TLR4 in mediating BBB permeability and disease in C3H mice during VEEV TC-83 infection, which suggests that TLR4 is a potential target for the development of therapeutics for VEEV.

Immunopathogenesis of alphaviruses

Alphaviruses, members of the enveloped, positive-sense, single-stranded RNA Togaviridae family, represent a reemerging public health threat as mosquito vectors expand into new geographic territories. The Old World alphaviruses, which include chikungunya virus, Ross River virus, and Sindbis virus, tend to cause a clinical syndrome characterized by fever, rash, and arthritis, whereas the New World alphaviruses, which consist of Venezuelan equine encephalitis virus, eastern equine encephalitis virus, and western equine encephalitis virus, induce encephalomyelitis. Following recovery from the acute phase of infection, many patients are left with debilitating persistent joint and neurological complications that can last for years. Clues from human cases and studies using animal models strongly suggest that much of the disease and pathology induced by alphavirus infection, particularly atypical and chronic manifestations, is mediated by the immune system rather than directly by the virus. This review discusses the current understanding of the immunopathogenesis of the arthritogenic and neurotropic alphaviruses accumulated through both natural infection of humans and experimental infection of animals, particularly mice. As treatment following alphavirus infection is currently limited to supportive care, understanding the contribution of the immune system to the disease process is critical to developing safe and effective therapies.

Vaccine Advances against Venezuelan, Eastern, and Western Equine Encephalitis Viruses

Vaccinations are a crucial intervention in combating infectious diseases. The three neurotropic Alphaviruses, Eastern (EEEV), Venezuelan (VEEV), and Western (WEEV) equine encephalitis viruses, are pathogens of interest for animal health, public health, and biological defense. In both equines and humans, these viruses can cause febrile illness that may progress to encephalitis. Currently, there are no licensed treatments or vaccines available for these viruses in humans. Experimental vaccines have shown variable efficacy and may cause severe adverse effects. Here, we outline recent strategies used to generate vaccines against EEEV, VEEV, and WEEV with an emphasis on virus-vectored and plasmid DNA delivery. Despite candidate vaccines protecting against one of the three viruses, few studies have demonstrated an effective trivalent vaccine. We evaluated the potential of published vaccines to generate cross-reactive protective responses by comparing DNA vaccine sequences to a set of EEEV, VEEV, and WEEV genomes and determining the vaccine coverages of potential epitopes. Finally, we discuss future directions in the development of vaccines to combat EEEV, VEEV, and WEEV.

Syrian Hamster as an Animal Model for the Study on Infectious Diseases

Infectious diseases still remain one of the biggest challenges for human health. In order to gain a better understanding of the pathogenesis of infectious diseases and develop effective diagnostic tools, therapeutic agents, and preventive vaccines, a suitable animal model which can represent the characteristics of infectious is required. The Syrian hamster immune responses to infectious pathogens are similar to humans and as such, this model is advantageous for studying pathogenesis of infection including post-bacterial, viral and parasitic pathogens, along with assessing the efficacy and interactions of medications and vaccines for those pathogens. This review summarizes the current status of Syrian hamster models and their use for understanding the underlying mechanisms of pathogen infection, in addition to their use as a drug discovery platform and provides a strong rationale for the selection of Syrian hamster as animal models in biomedical research. The challenges of using Syrian hamster as an alternative animal model for the research of infectious diseases are also addressed.

Current Understanding of the Molecular Basis of Venezuelan Equine Encephalitis Virus Pathogenesis and Vaccine Development

Dedication

This review is dedicated in the memory of Dr Radha K. Maheshwari, a great mentor and colleague, whose passion for research and student training has left a lasting effect on this manuscript and many other works.

Abstract

Venezuelan equine encephalitis virus (VEEV) is an alphavirus in the family Togaviridae. VEEV is highly infectious in aerosol form and a known bio-warfare agent that can cause severe encephalitis in humans. Periodic outbreaks of VEEV occur predominantly in Central and South America. Increased interest in VEEV has resulted in a more thorough understanding of the pathogenesis of this disease. Inflammation plays a paradoxical role of antiviral response as well as development of lethal encephalitis through an interplay between the host and viral factors that dictate virus replication. VEEV has efficient replication machinery that adapts to overcome deleterious mutations in the viral genome or improve interactions with host factors. In the last few decades there has been ongoing development of various VEEV vaccine candidates addressing the shortcomings of the current investigational new drugs or approved vaccines. We review the current understanding of the molecular basis of VEEV pathogenesis and discuss various types of vaccine candidates.

Mitochondrial-Directed Antioxidant Reduces Microglial-Induced Inflammation in Murine In Vitro Model of TC-83 Infection

Venezuelan equine encephalitis virus (VEEV) is an arbovirus that is associated with robust inflammation that contributes to neurodegenerative phenotypes. In addition to triggering central nervous system (CNS) inflammation, VEEV will also induce mitochondrial dysfunction, resulting in increased cellular apoptosis. In this study, we utilize the TC-83 strain of VEEV to determine the role of mitochondrial oxidative stress in mediating inflammation elicited by murine brain microglial cells. Using an in vitro model, we show that murine microglia are susceptible to TC-83 infection, and that these cells undergo mitochondrial stress as the result of infection. We also indicate that bystander microglia contribute more significantly to the overall inflammatory load than directly infected microglia. Use of a mitochondrial targeted antioxidant, mitoquinone mesylate, greatly reduced the pro-inflammatory cytokines released by both direct infected and bystander microglia. Our data suggest that release of interleukin-1β, a key instigator of neuroinflammation during VEEV infection, may be the direct result of accumulating mitochondrial stress. This data improves our understanding inflammation elicited by murine microglia and will aid in the development of more accurate in vitro and in vivo murine model of VEEV-induced neuroinflammation.

Comparison of Aerosol- and Percutaneous-acquired Venezuelan Equine Encephalitis in Humans and Nonhuman Primates for Suitability in Predicting Clinical Efficacy under the Animal Rule

Licensure of medical countermeasure vaccines to protect against aerosolized Venezuelan equine encephalitis virus (VEEV) requires the use of the Animal Rule to assess vaccine efficacy, because human studies are not feasible or ethical. We therefore performed a retrospective study of VEE cases that occurred in at-risk laboratory workers and support personnel during the United States Biowarfare Program (1943-1969) to better define percutaneous- and aerosol-acquired VEE in humans and to compare these results with those described for the NHP model (in which high-dose aerosol VEEV challenge led to more severe encephalitis than parenteral challenge). Record review and analysis of 17 aerosol- and 23 percutaneous-acquired human cases of VEE included incubation period, symptoms, physical examination findings, and markers of infection. Human VEE disease by both exposure routes presented as acute febrile illness, typically with fever, chills, headache, back pain, malaise, myalgia, anorexia, and nausea. Aerosol exposure more commonly led to upper respiratory tract-associated findings of sore throat (59% compared with 26%), pharyngeal erythema (76% compared with 52%), neck pain (29% compared with 4%), and cervical lymphadenopathy (29% compared with 4%). Other disease manifestations, including encephalitis, were similar between the 2 exposure groups. The increase in upper respiratory tract findings in aerosol-acquired VEE in humans has not previously been reported but is supported by the mouse model, which showed nasal mucosal necrosis, necrotizing rhinitis, and an increase in upper respiratory tract viral burden associated with aerosol VEEV challenge. Fever, viremia, and lymphopenia were common markers of VEE disease in both humans and NHP, regardless of the exposure route. Taken collectively, our findings provide support for use of the nonlethal NHP model for advanced development of medical countermeasures against aerosol- or percutaneous-acquired VEE.

Differential host gene responses from infection with neurovirulent and partially-neurovirulent strains of Venezuelan equine encephalitis virus

Background

Venezuelan equine encephalitis virus (VEEV) is an alphavirus in the family Togaviridae. VEEV causes a bi-phasic illness in mice where primary replication in lymphoid organs is followed by entry into the central nervous system (CNS). The CNS phase of infection is marked by encephalitis and large scale neuronal death ultimately resulting in death. Molecular determinants of VEEV neurovirulence are not well understood. In this study, host gene expression response to highly neurovirulent VEEV (V3000 strain) infection was compared with that of a partially neurovirulent VEEV (V3034 strain) to identify host factors associated with VEEV neurovirulence.

Methods

Whole genome microarrays were performed to identify the significantly modulated genes. Microarray observations were classified into three categories i.e., genes that were similarly modulated against both V3000 and V3034 infections, and genes that were uniquely modulated in infection with V3034 or V3000. Histologic sections of spleen and brain were evaluated by hematoxylin and eosin stains from all the mice.

Results

V3000 infection induced a greater degree of pathology in both the spleen and brain tissue of infected mice compared to V3034 infection. Genes commonly modulated in the spleens after V3000 or V3034 infection were associated with innate immune responses, inflammation and antigen presentation, however, V3000 induced a gene response profile that suggests a stronger inflammatory and apoptotic response compared to V3034. In the brain, both the strains of VEEV induced an innate immune response reflected by an upregulation of the genes involved in antigen presentation, interferon response, and inflammation. Similar to the spleen, V3000 was found to induce a stronger inflammatory response than V3034 in terms of induction of pro-inflammatory genes and associated pathways. Ccl2, Ccl5, Ccl6, and Ly6 were uniquely upregulated in V3000 infected mouse brains and correlated with the extensive inflammation observed in the brain.

Conclusion

The common gene profile identified from V3000 and V3034 exposure can help in understanding a generalized host response to VEEV infection. Inflammatory genes that were uniquely identified in mouse brains with V3000 infection will help in better understanding the lethal neurovirulence of VEEV. Future studies are needed to explore the roles played by the genes identified in VEEV induced encephalitis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-017-2355-3) contains supplementary material, which is available to authorized users.

Mouse models of alphavirus-induced inflammatory disease

Part of the Togaviridae family, alphaviruses are arthropod-borne viruses that are widely distributed throughout the globe. Alphaviruses are able to infect a variety of vertebrate hosts, but in humans, infection can result in extensive morbidity and mortality. Symptomatic infection can manifest as fever, an erythematous rash and/or significant inflammatory pathologies such as arthritis and encephalitis. Recent overwhelming outbreaks of alphaviral disease have highlighted the void in our understanding of alphavirus pathogenesis and the re-emergence of alphaviruses has given new impetus to anti-alphaviral drug design. In this review, the development of viable mouse models of Old Word and New World alphaviruses is examined. How mouse models that best replicate human disease have been used to elucidate the immunopathology of alphavirus pathogenesis and trial novel therapeutic discoveries is also discussed.

Modulation of GSK-3β activity in Venezuelan equine encephalitis virus infection

Alphaviruses, including Venezuelan Equine Encephalitis Virus (VEEV), cause disease in both equine and humans that exhibit overt encephalitis in a significant percentage of cases. Features of the host immune response and tissue-specific responses may contribute to fatal outcomes as well as the development of encephalitis. It has previously been shown that VEEV infection of mice induces transcription of pro-inflammatory cytokines genes (e.g., IFN-γ, IL-6, IL-12, iNOS and TNF-α) within 6 h. GSK-3β is a host protein that is known to modulate pro-inflammatory gene expression and has been a therapeutic target in neurodegenerative disorders such as Alzheimer's. Hence inhibition of GSK-3β in the context of encephalitic viral infections has been useful in a neuroprotective capacity. Small molecule GSK-3β inhibitors and GSK-3β siRNA experiments indicated that GSK-3β was important for VEEV replication. Thirty-eight second generation BIO derivatives were tested and BIOder was found to be the most potent inhibitor, with an IC₅₀ of ∼0.5 µM and a CC₅₀ of >100 µM. BIOder was a more potent inhibitor of GSK-3β than BIO, as demonstrated through in vitro kinase assays from uninfected and infected cells. Size exclusion chromatography experiments demonstrated that GSK-3β is found in three distinct complexes in VEEV infected cells, whereas GSK-3β is only present in one complex in uninfected cells. Cells treated with BIOder demonstrated an increase in the anti-apoptotic gene, survivin, and a decrease in the pro-apoptotic gene, BID, suggesting that modulation of pro- and anti-apoptotic genes contributes to the protective effect of BIOder treatment. Finally, BIOder partially protected mice from VEEV induced mortality. Our studies demonstrate the utility of GSK-3β inhibitors for modulating VEEV infection.

Treatment of mice with human monoclonal antibody 24h after lethal aerosol challenge with virulent Venezuelan equine encephalitis virus prevents disease but not infection

We recently described a Venezuelan equine encephalitis virus (VEEV)-specific human monoclonal antibody (MAb), F5 nIgG, that recognizes a new neutralization epitope on the VEEV E2 envelope glycoprotein. In this study, we investigated the ability of F5 nIgG given prophylactically or therapeutically to protect mice from subcutaneous or aerosolized VEEV infection. F5 nIgG had potent ability to protect mice from infection by either route when administered 24h before exposure; however, mice treated 24h after aerosol exposure developed central nervous system infections but exhibited no clinical signs of disease. Infectious virus, viral antigen and RNA were detected in brains of both treated and untreated mice 2-6 days after aerosol exposure but were cleared from the brains of treated animals by 14-28 days after infection. This fully human MAb could be useful for prophylaxis or immediate therapy for individuals exposed to VEEV accidentally in the laboratory or during a deliberate release.

Validation of assays to monitor immune responses in the Syrian golden hamster (Mesocricetus auratus)

The Syrian golden hamster (Mesocricetus auratus) is a valuable but under-utilized animal model for studies of human viral pathogens such as bunyaviruses, arenaviruses, flaviviruses, henipaviruses, and SARS-coronavirus. A lack of suitable reagents and specific assays for monitoring host responses has limited the use of this animal model to clinical observations, pathology and humoral immune responses. The objective of this study was to establish and validate assays to monitor host immune responses in the hamster including important pro-inflammatory, anti-inflammatory and innate immune responses, as well as markers of apoptosis, cell proliferation, cell junction integrity and coagulation. Commercially available mouse and rat ELISA and luminex panels were screened for potential cross-reactivity, but were found to be of limited value for studying host responses in hamsters. Subsequently, quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) assays for the detection of 51 immune-related and four internal reference genes were developed. To validate the immune-related assays, hamsters were infected with vesicular stomatitis virus (VSV), Indiana species, or treated with lipopolysaccharide (LPS) and host immune responses were monitored in selected organs. Ribosomal protein L18 was identified as the most stable internal reference gene. In conclusion, these new assays will greatly improve the use of the hamster as an important small animal model in infectious disease research.

A recombinant humanized monoclonal antibody completely protects mice against lethal challenge with Venezuelan equine encephalitis virus

A recombinant humanized antibody to Venezuelan equine encephalitis virus (VEEV) was constructed in a monocistronic adenoviral expression vector with a foot-and-mouth-disease virus-derived 2A self-cleavage oligopeptide inserted between the antibody heavy and light chains. After expression in mammalian cells, the heavy and light chains of the humanized antibody (hu1A4A1IgG1-2A) were completely cleaved and properly dimerized. The purified hu1A4A1IgG1-2A retained VEEV binding affinity and neutralizing activity similar to its parental murine antibody. The half-life of hu1A4A1IgG1-2A in mice was approximately 2 days. Passive immunization of hu1A4A1IgG1-2A in mice (50 microg/mouse) 24 h before or after virulent VEEV challenge provided complete protection, indicating that hu1A4A1IgG1-2A has potent prophylactic and therapeutic effects against VEEV infection.

REVIEW PAPER

The encephalitides caused by Venezuelan (VEEV), eastern (EEEV), and western (WEEV) equine encephalitis viruses are important natural diseases of horses and humans and potential agents of biowarfare or bioterrorism. No licensed vaccines or specific therapies exist to prevent or treat human infections with VEEV, EEEV, or WEEV. Well-characterized animal models are needed to support the development of such medical countermeasures under the United States Food and Drug Administration’s “Animal Rule.” This review focuses on the pathological features and pathogenetic mechanisms of these alphaviral encephalitides in animal models, with an emphasis on aerosol infections. Infection of mice, nonhuman primates, and other species with VEEV, EEEV, and WEEV causes encephalitis and often death. There is great variability in the specific manifestations of disease in the different models, however. Many aspects of the disease in animal models and in humans remain to be characterized using modern methods. Especially needed is a better understanding of the fundamental mechanisms involved in 3 key phases of the pathogenesis of alphavirus encephalitis. These are the early extraneural phase, the process of neuroinvasion itself, and virus and host factors related to neurovirulence. A greater understanding of these aspects could provide avenues for the development of medical countermeasures and better establish suitable animal models of alphavirus encephalitis for testing them under the Animal Rule.

Swine influenza vaccines: current status and future perspectives

Swine influenza is an important contagious disease in pigs caused by influenza A viruses. Although only three subtypes of influenza A viruses, H1N1, H1N2 and H3N2, predominantly infect pigs worldwide, it is still a big challenge for vaccine manufacturers to produce efficacious vaccines for the prevention and control of swine influenza. Swine influenza viruses not only cause significant economic losses for the swine industry, but are also important zoonotic pathogens. Vaccination is still one of the most important and effective strategies to prevent and control influenza for both the animal and human population. In this review, we will discuss the current status of swine influenza worldwide as well as current and future options to control this economically important swine disease.

Alphavirus antiviral drug development: scientific gap analysis and prospective research areas

The New World alphaviruses Venezuelan equine encephalitis virus (VEEV), eastern equine encephalitis virus (EEEV), and western equine encephalitis virus (WEEV) pose a significant threat to human health as the etiological agents of serious viral encephalitis through natural infection as well as through their potential use as a biological weapon. At present, there is no FDA-approved medical treatment for infection with these viruses. The Defense Threat Reduction Agency, Joint Science and Technology Office for Chemical and Biological Defense (DTRA/JSTO), is currently funding research aimed at developing antiviral drugs and vaccines against VEEV, EEEV, and WEEV. A review of antiviral drug discovery efforts for these viruses revealed significant gaps in the data, assays, and models required for successful drug development. This review provides a description of these gaps and highlights specific critical research areas for the development of a target-based drug discovery program for the VEEV, EEEV, and WEEV nonstructural proteins. These efforts will increase the probability of the successful development of a pharmaceutical intervention against these viral threat agents.

Vaccines for Venezuelan equine encephalitis

Arboviruses are capable of causing encephalitis in animals and human population when transmitted by the vector or potentially via infectious aerosol. Recent re-emergence of Venezuelan equine encephalitis virus (VEEV) in South America emphasizes the importance of this pathogen to public health and veterinary medicine. Despite its importance no antivirals or vaccines against VEEV are currently available in the USA. Here we review some of the older and newer approaches aimed at generating a safe and immunogenic vaccine as well as most recent data about the mechanistic of protection in animal models of infection.

Pathogenesis of aerosolized Eastern Equine Encephalitis virus infection in guinea pigs

Mice and guinea pigs were experimentally exposed to aerosols containing regionally-distinct strains (NJ1959 or ArgM) of eastern equine encephalitis virus (EEEV) at two exclusive particle size distributions. Mice were more susceptible to either strain of aerosolized EEEV than were guinea pigs; however, clinical signs indicating encephalitis were more readily observed in the guinea pigs. Lower lethality was observed in both species when EEEV was presented at the larger aerosol distribution (> 6 μm), although the differences in the median lethal dose (LD₅₀) were not significant. Virus isolation and immunohistochemistry indicated that virus invaded the brains of guinea pigs within one day postexposure, regardless of viral strain or particle size distribution. Immunohistochemistry further demonstrated that neuroinvasion occurred through the olfactory system, followed by transneuronal spread to all regions of the brain. Olfactory bipolar neurons and neurons throughout the brain were the key viral targets. The main microscopic lesions in infected guinea pigs were neuronal necrosis, inflammation of the meninges and neuropil of the brain, and vasculitis in the brain. These results indicate that guinea pigs experimentally infected by aerosolized EEEV recapitulate several key features of fatal human infection and thus should serve as a suitable animal model for aerosol exposure to EEEV.

Oligonucleotide array analysis of Toll-like receptors and associated signalling genes in Venezuelan equine encephalitis virus-infected mouse brain

Venezuelan equine encephalitis (VEE) is an emerging infectious disease. VEE virus (VEEV) may cause lethal infection of the central nervous system in horses and humans. The mechanisms underlying the host immune response to VEEV infection in the brain are not fully understood. Toll-like receptors (TLRs) recognize conserved microbial sequences and induce specific biological responses in the form of proinflammatory cytokine induction. TLR expression in blood following VEEV infection has been reported in non-human primates and TLRs are also upregulated in the brains of mice infected with other alphaviruses. In this study, mice (3–5 weeks old) were infected with V3000, a neurovirulent strain of VEEV, and gene expression of TLRs and their associated signalling molecules was evaluated. VEEV infection resulted in upregulation of TLR 1, 2, 3, 7 and 9, chemokines, inflammatory cytokines, interferon (IFN), IFN regulatory factors and genes involved in signal transduction such as Mcp1, Cxcl10, IL12α/β, IFN-β, IRF-1, IRF-7, Jun, Fos, MyD88, Nfkb, Cd14 and Cd86. These results demonstrate the upregulation of TLRs and associated signalling genes following VEEV infection of the brain, with important implications for how VEEV induces inflammation and neurodegeneration.

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.

Venezuelan equine encephalitis virus infection causes modulation of inflammatory and immune response genes in mouse brain

BACKGROUND

Neurovirulent Venezuelan equine encephalitis virus (VEEV) causes lethal encephalitis in equines and is transmitted to humans by mosquitoes. VEEV is highly infectious when transmitted by aerosol and has been developed as a bio-warfare agent, making it an important pathogen to study from a military and civilian standpoint. Molecular mechanisms of VEE pathogenesis are poorly understood. To study these, the gene expression profile of VEEV infected mouse brains was investigated. Changes in gene expression were correlated with histological changes in the brain. In addition, a molecular framework of changes in gene expression associated with progression of the disease was studied.

RESULTS

Our results demonstrate that genes related to important immune pathways such as antigen presentation, inflammation, apoptosis and response to virus (Cxcl10, CxCl11, Ccl5, Ifr7, Ifi27 Oas1b, Fcerg1,Mif, Clusterin and MHC class II) were upregulated as a result of virus infection. The number of over-expressed genes (>1.5-fold level) increased as the disease progressed (from 197, 296, 400, to 1086 at 24, 48, 72 and 96 hours post infection, respectively).

CONCLUSION

Identification of differentially expressed genes in brain will help in the understanding of VEEV-induced pathogenesis and selection of biomarkers for diagnosis and targeted therapy of VEEV-induced neurodegeneration.

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.

C3H/HeN mouse model for the evaluation of antiviral agents for the treatment of Venezuelan equine encephalitis virus infection

The TC-83 vaccine strain of Venezuelan equine encephalitis virus (VEEV) causes encephalitis and death in C3H/HeN mice infected by intranasal (i.n.) instillation. Since TC-83 is exempt as a select agent, this mouse model was used in the evaluation of antiviral therapies. Virus titers in the brains of infected mice peaked on 4 dpi and persisted at high levels until death at 9.4+/-0.5 dpi. Mouse brains appeared histologically normal on 2 dpi, but developed meningoencephalitis, neuropil vacuolation, and gliosis by 8 dpi. Results from a protein cytokine array showed significant elevations over time in interleukin (IL)-1alpha, IL-1beta, IL-6, IL-12, MCP-1, IFNgamma, TNFalpha, MIP-1alpha, and RANTES in homogenized brain samples of infected mice. Immunohistochemical staining showed a colocalization of viral antigen with neuron markers. Treatment with interferon-alpha B/D or ampligen significantly improved survival, brain virus titer and cytokine levels, mean day-to-death, and weight change in infected mice. The time-course of infection and disease parameters of mice infected with TC-83 VEEV were similar in many ways to disease parameters in mice infected with other VEEV strains. Thus, infection of C3H/HeN mice with TC-83 VEEV may serve as a suitable model for the evaluation of antiviral compounds for the treatment of this viral disease.

Venezuelan equine encephalitis virus capsid protein inhibits nuclear import in Mammalian but not in mosquito cells

Venezuelan equine encephalitis virus (VEEV) represents a continuous public health threat in the United States. It has the ability to cause fatal disease in humans and in horses and other domestic animals. We recently demonstrated that replicating VEEV interferes with cellular transcription and uses this phenomenon as a means of downregulating a cellular antiviral response. VEEV capsid protein was found to play a critical role in this process, and its approximately 35-amino-acid-long peptide, fused with green fluorescent protein, functioned as efficiently as did the entire capsid. We detected a significant fraction of VEEV capsid associated with nuclear envelope, which suggested that this protein might regulate nucleocytoplasmic trafficking. In this study, we demonstrate that VEEV capsid and its N-terminal sequence efficiently inhibit multiple receptor-mediated nuclear import pathways but have no effect on the passive diffusion of small proteins. The capsid protein of the Old World alphavirus Sindbis virus and the VEEV capsid, with a previously defined frameshift mutation, were found to have no detectable effect on nuclear import. Importantly, the VEEV capsid did not noticeably interfere with nuclear import in mosquito cells, and this might play a critical role in the ability of the virus to develop a persistent, life-long infection in mosquito vectors. These findings demonstrate a new aspect of VEEV-host cell interactions, and the results of this study are likely applicable to other New World alphaviruses, such as eastern and western equine encephalitis viruses.

Animal models of highly pathogenic RNA viral infections: encephalitis viruses

The highly pathogenic RNA viruses that cause encephalitis include a significant number of emerging or re-emerging viruses that are also considered potential bioweapons. Many of these viruses, including members of the family Flaviviridae, the genus Alphavirus in the family Togaviridae, and the genus Henipavirus in the family Paramyxoviridae, circulate widely in their endemic areas, where they are transmitted by mosquitoes or ticks. They use a variety of vertebrate hosts, ranging from birds to bats, in their natural life cycle. As was discovered in the United States, the introduction of a mosquito-borne encephalitis virus such as West Nile virus can cause significant health and societal concerns. There are no effective therapeutics for treating diseases caused by any of these viruses and there is limited, if any, vaccine availability for most. In this review we provide a brief summary of the current status of animal models used to study highly pathogenic encephalitic RNA viruses for the development of antiviral therapeutics and vaccines.

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.

Foot-and-mouth disease virus (FMDV) causes an acute disease that can be lethal for adult laboratory mice

Foot-and-mouth disease virus (FMDV) is a picornavirus that causes an acute vesicular disease of cloven-hoofed animals. This virus continues to be threat to livestock worldwide with outbreaks causing severe economic losses. However, very little is known about FMDV pathogenesis, partially due to the inconveniences of working with cattle and swine, the main natural hosts of the virus. Here we demonstrate that C57BL/6 and BALB/C adult mice are highly susceptible to FMDV infection when the virus is administered subcutaneously or intraperitoneally. The first clinical signs are ruffled fur, apathy, humped posture, and wasting, which are followed by neurological signs such as hind-limb paralysis. Within 2-3 days of disease onset, the animals die. Virus is found in all major organs, indicating a systemic infection. Mice developed microvesicles near the basal layer of the epithelium, event that precedes the vesiculation characteristics of FMD. In addition, a lymphoid depletion in spleen and thymus and severe lymphopenia is observed in the infected mice. When these mice were immunized with conventional inactivated FMDV vaccine, they were protected (100% of vaccinated animals) against challenge with a lethal dose of FMDV. The data indicate that this mouse model may facilitate the study of FMDV pathogenesis, and the development of new effective vaccines for FMD.

Recombinant sindbis/Venezuelan equine encephalitis virus is highly attenuated and immunogenic

Venezuelan equine encephalitis virus (VEEV) is an important, naturally emerging zoonotic virus. VEEV was a significant human and equine pathogen for much of the past century, and recent outbreaks in Venezuela and Colombia (1995), with about 100,000 human cases, indicate that this virus still poses a serious public health threat. The live attenuated TC-83 vaccine strain of VEEV was developed in the 1960s using a traditional approach of serial passaging in tissue culture of the virulent Trinidad donkey (TrD) strain. This vaccine presents several problems, including adverse, sometimes severe reactions in many human vaccinees. The TC-83 strain also retains residual murine virulence and is lethal for suckling mice after intracerebral (i.c.) or subcutaneous (s.c.) inoculation. To overcome these negative effects, we developed a recombinant, chimeric Sindbis/VEE virus (SIN-83) that is more highly attenuated. The genome of this virus encoded the replicative enzymes and the cis-acting RNA elements derived from Sindbis virus (SINV), one of the least human-pathogenic alphaviruses. The structural proteins were derived from VEEV TC-83. The SIN-83 virus, which contained an additional adaptive mutation in the nsP2 gene, replicated efficiently in common cell lines and did not cause detectable disease in adult or suckling mice after either i.c. or s.c. inoculation. However, SIN-83-vaccinated mice were efficiently protected against challenge with pathogenic strains of VEEV. Our findings suggest that the use of the SINV genome as a vector for expression of structural proteins derived from more pathogenic, encephalitic alphaviruses is a promising strategy for alphavirus vaccine development.

Alphavirus vectors and vaccination

Alphaviruses are positive-stranded RNA viruses that have a broad host range and therefore are capable of replicating in many vertebrate and invertebrate cells. The single-stranded alphavirus genome is divided into two ORFs. The first ORF encodes the nonstructural proteins that are translated upon entry of the virus into the cytoplasm and are responsible for transcription and replication of viral RNA. The second ORF is under the control of a subgenomic promoter and normally encodes the structural proteins, which are responsible for encapsidation of viral RNA and final assembly into enveloped particles. Expression vectors have been engineered from at least three alphaviruses in which the structural protein gene region has been replaced by heterologous genes and have been shown to express high levels of the heterologous protein in cultured cells. These RNA vectors, known as replicons, are capable of replicating on their own but are not packaged into virus-like particles unless the structural proteins are provided in trans. Thus, replicons are single cycle vectors incapable of spreading from infected to noninfected cells. Because of these features, alphavirus replicon vectors are being developed as a platform vaccine technology for numerous viral, bacterial, protozoan and tumour antigens where they have been shown to be efficient inducers of both humoral and T cell responses. In addition, as the alphavirus structural proteins are not expressed in vaccine recipients, antivector immune responses are generally minimal, allowing for multiple effective immunisations of the same individual.

Role for mucosal immune responses and cell-mediated immune functions in protection from airborne challenge with Venezuelan equine encephalitis virus

Venezuelan equine encephalitis virus (VEEV) replicates in lymphoid tissues following peripheral inoculation and a high titre viraemia develops. Encephalitis develops after the virus enters the central nervous system from the blood, with the earliest neuronal involvement being via the olfactory nerve. Following aerosol challenge with virulent VEEV, the virus is thought to replicate in the nasal mucosa and there could be direct entry into the olfactory nerve via infected neuroepithelial cells. Protection from VEEV infection is believed to be primarily mediated by virus specific antibody. The correlation between protection and neutralising serum antibody titres is, however, inconsistent when the virulent virus is administered by the airborne route. This study demonstrates a link between antibody in serum and the nasal mucosa and protection by means of passive immunisation studies. Intra-nasal administration of antibody increased protection against airborne virus in Balb/c mice. Vaccination of mu MT strain mice that do not have functional B cells and cannot produce antibody revealed normal proliferation of spleen cells in vitro and robust cytokine production. Aerosol challenge of mu MT mice demonstrated that complete protection was only achieved when passive immunisation with antibody was supplemented with active immunisation with the TC-83 vaccine strain of the virus. This implies that cell-mediated immune functions are required for protection against airborne challenge with virulent VEEV.

Monoclonal antibody protects mice against infection and disease when given either before or up to 24 h after airborne challenge with virulent Venezuelan equine encephalitis virus

Airborne infection with Venezuelan equine encephalitis virus (VEEV) is a significant hazard for laboratory workers, who may not be immunised against VEEV infection as there is no vaccine currently available suitable for human use. We describe a potential alternative strategy that could protect workers exposed to VEEV or similar viruses. VEEV-specific murine monoclonal antibodies (MAB), given by intraperitoneal (i.p.) injection to mice as a single dose of 100 microg, have a half-life of 6-10 days in serum and spread by transudation to respiratory secretions. Administration of MAB (approximately 4 mg/kg) to mice 24h before challenge with approximately 100LD50 of virulent VEEV protected up to 100% animals. The same dose of MAB delivered up to 24h after challenge protected approximately 50%. Two MAB that were synergistic in vitro in plaque reduction neutralisation tests were not synergistic in vivo in protection assays. An examination of virus multiplication, in the blood and internal organs (brain, spleen, lung) of MAB-treated mice infected by the airborne route with VEEV, suggested that therapeutic activity depended both upon the prevention of virus infection of the brain, and the rapid clearance of virus from the periphery. Antiviral therapy with VEEV-specific human or "humanised" MAB, providing that they are administered early, may offer an alternative means of specific medical intervention for those with a known exposure to VEEV.

Clinical, pathologic, immunohistochemical, and virologic findings of eastern equine encephalomyelitis in two horses

Natural eastern equine encephalitis alphavirus (EEEV) infection was diagnosed in two adult horses with anorexia and colic, changes in sensorium, hyperexcitability, and terminal severe depression. Myocardium, tunica muscularis of stomach, intestine, urinary bladder, and spleen capsule had coagulative necrosis and perivascular lymphocytic infiltrate. Central nervous system (CNS) lesions were diffuse polioencephalomyelitis with leptomeningitis characterized by perivascular T lymphocyte cuffing, marked gliosis, neuronophagia, and multifocal microabscesses. Lesions were more prominent within cerebral cortex, thalamus, hypothalamus, and mesencephalon. EEEV was identified in the cytoplasm of cardiac myocytes and smooth muscle cells of spleen, stomach, intestine, urinary bladder, blood vessels, and dendritic cells. In the CNS, EEEV-positive cells included neurons, astrocytes, oligodendrocytes, microglia, and neutrophils. EEEV was isolated from the CNS of both horses. The detailed description of the encephalic and spinal EEEV localization and the findings of EEEV in extraneural tissues contribute to the understanding of this important multisystemic zoonotic disease.

Immunopathogenesis and immune modulation of Venezuelan equine encephalitis virus-induced disease in the mouse

The course of Venezuelan equine encephalitis (VEE) disease in immunodeficient and immunologically normal mice was compared to define the role of the immune system in this disease process. Immunocompetent mice infected with VEE exhibited a biphasic illness characterized by an early self-limiting lymphoid phase and a fatal CNS phase. The lymphoid phase of the illness was characterized by extensive viral replication within spleen, thymus, Peyer's patches, and lymph nodes, was accompanied by a high-titered serum viremia, and resolved with the production of VEE-specific IgM class antibody at 72 h postinfection (p.i.). Immunocompetent animals survived an average of 6.8 +/- 1.2 days before succumbing to fulminant encephalitis. In contrast, SCID mice infected with VEE showed a persistent replication of virus throughout all organs tested beginning at 24 h p.i. VEE-infected SCID mice exhibited a severe spongiform encephalopathy with 100% mortality and an average survival time of 8.9 +/- 0.9 days. These studies indicated that the characteristic organ tropism of VEE in the mouse is due in large part to an early anti-viral state, the establishment of which is dependent upon the presence of an intact immune system. Finally, the CNS pathology in a VEE-infected mouse had a significant immunologic component. However, in contrast to other neurovirulent alphaviruses, VEE was directly cytopathic for the cells of the CNS, even in the absence of an immune response.

Inflammation is a component of neurodegeneration in response to Venezuelan equine encephalitis virus infection in mice

Infection with the mosquito-transmitted Venezuelan equine encephalitis virus (VEE) causes an acute systemic febrile illness followed by meningoencephalitis. In this communication we characterize the cytokine profile induced in the central nervous system (CNS) in response to virulent or attenuated strains of VEE using RNase Protection Assays. Virulent VEE causes an upregulation of multiple pro-inflammatory genes including inducible nitric oxide synthase (iNOS) and tumor necrosis factor alpha (TNF-alpha). To determine if iNOS and TNF-alpha contribute to the neuropathogenesis of VEE infection, iNOS and TNF receptor knockout mice were used in VEE mortality studies and exhibited extended survival times. Finally, CNS tissue sections labeled for VEE antigen, and adjacent sections double-labeled for an astrocyte marker and apoptosis, revealed that apoptosis of neurons occurs not only in areas of the brain positive for VEE-antigen, but also in areas of astrogliosis. These findings suggest that the inflammatory response, which is in part mediated by iNOS and TNF-alpha, may contribute to neurodegeneration following encephalitic virus infection.

An immunological profile of Balb/c mice protected from airborne challenge following vaccination with a live attenuated Venezuelan equine encephalitis virus vaccine

The live attenuated vaccine strain of Venezuelan equine encephalitis virus (VEEV), TC-83, protects mice against challenge (subcutaneous and aerosol) with virulent VEEV but is not suitable for widescale human use. Elucidation of the immune response profile of protected mice should assist in the development of an improved vaccine. We determined the optimum dose of TC-83 required to consistently protect Balb/c mice from airborne challenge with the virulent Trinidad Donkey strain of VEEV and studied the development of humoral and cellular immune responses in protected mice between 6 h and 21 days post-vaccination. The most dramatic immune responses occurred in draining lymph nodes 24 h following vaccination with increased levels of activated B cells and T cells of both CD4(+) and CD8(+) subtypes. Activated monocyte/macrophages and natural killer cells were also seen between 6 h and 7 days post-vaccination. Serum contained detectable VEEV-specific IgG on day 5 post-vaccination with titres continuing to rise on days 7, 14 and 21. Isotypes of IgG measured on days 7 and 21 were predominantly of the IgG2a subclass, indicating that the immune response was Th1-mediated. Cytokine mRNA was quantified by RT-PCR and revealed production of the Th1 cytokine IFN-gamma and the inflammatory cytokine TNF-alpha, whereas the Th2 cytokine IL4 was not detected above control levels at any of the time points studied. This data describes key cellular immune responses at early times post-vaccination and is consistent with previous data demonstrating protection against aerosol challenge with VEEV in the absence of detectable levels of specific IgG or IgA antibody.

Pegylated alpha interferon is an effective treatment for virulent venezuelan equine encephalitis virus and has profound effects on the host immune response to infection

Venezuelan equine encephalitis virus (VEEV) is a highly infectious alphavirus endemic in parts of Central and South America. The disease is transmitted by mosquitoes, and the natural reservoir is the small rodent population, with epidemics occurring in horses and occasionally humans. Following infection, VEEV replicates in lymphoid tissues prior to invasion of the central nervous system. Treatment of VEEV-infected BALB/c mice with polyethylene glycol-conjugated alpha interferon (PEG IFN-alpha) results in a greatly enhanced survival from either a subcutaneous or an aerosol infection. Virus is undetectable within PEG IFN-alpha-treated individuals by day 30 postinfection (p.i.). Treatment results in a number of changes to the immune response characteristics normally associated with VEEV infection. Increased macrophage activation occurs in PEG IFN-alpha-treated BALB/c mice infected with VEEV. The rapid activation of splenic CD4, CD8, and B cells by day 2 p.i. normally associated with VEEV infection is absent in PEG IFN-alpha-treated mice. The high tumor necrosis factor alpha production by macrophages from untreated mice is greatly diminished in PEG IFN-alpha-treated mice. These results suggest key immunological mechanisms targeted by this lethal alphavirus that can be modulated by prolonged exposure to IFN-alpha.

A single-site mutant and revertants arising in vivo define early steps in the pathogenesis of Venezuelan equine encephalitis virus

The early stages of Venezuelan equine encephalitis virus (VEE) pathogenesis in the mouse model have been examined using a genetic approach. Disease progression of a molecularly cloned single-site mutant was compared with that of the parental virus to determine the step in the VEE pathogenetic sequence at which the mutant was blocked. Assuming that such a block constitutes a genetic screen, isolates from different tissues thought to be distal to the block in the VEE pathogenetic sequence were analyzed to determine the pathogenetic step at which revertants of the mutant were selected. Directed mutation and analysis of reversion in vivo provide two powerful genetic tools for the dissection of the wild-type VEE pathogenetic sequence. Virus from the parental virulent clone, V3000, first replicated in the draining lymph node after subcutaneous inoculation in the left rear footpad. Movement of a cloned avirulent mutant, V3010 (E2 76 Glu to Lys), to the draining lymph node was impaired, replication in the node was delayed, and spread beyond the draining lymph node was sporadic. Serum, contralateral lymph node, spleen, and brain isolates from V3010 inoculated animals were invariably revertant with respect to sequence at E2 76 and/or virulence in mice. Revertants isolated from serum and contralateral lymph node retained the V3010 E2 Lys 76 mutation but also contained a second-site mutation, Glu to Lys at E2 116. Modification of the V3010 clone by addition of the second-site mutation at E2 116 produced a virus that bypassed the V3010 block at the draining lymph node but that did not possess full wild-type capacity for replication in the central nervous system or for induction of mortality. A control construct containing only the E2 116 reverting mutation on the V3000 background was identical to V3000 in terms of early pathogenetic steps and virulence. Therefore, analysis of mutant replication and reversion in vivo suggested (1) that the earliest steps in VEE pathogenesis are transit to the draining lymph node and replication at that site, (2) that the mutation in V3010 impairs transit to the draining lymph node and blocks dissemination to other tissues, and (3) that reversion can overcome the block without restoring full virulence.

Early expression of IFN-alpha/beta and iNOS in the brains of Venezuelan equine encephalitis virus-infected mice

To investigate the roles of type I interferon (IFN-alpha/beta) and other mediators of innate immune responses (e.g., inducible nitric oxide synthase [iNOS]) in early dissemination of Venezuelan equine encephalitis virus (VEE) infection, we used mice with targeted deletions in either their IFN-alpha/beta-receptor (IFNAR-1-/-) or interferon regulatory factor 2 (IRF-2-/-) genes. Following footpad infection, both IFNAR-1-/- and IRF-2-/- mice were more susceptible than control mice to VEE. The IFNAR-1-/- mice also exhibit accelerated VEE dissemination to serum, spleen, and brain, and compared with control mice, they evidenced faster kinetics in the upregulation of proinflammatory genes. In contrast, in IRF-2-/- mice, iNOS gene induction was completely absent following peripheral virulent VEE infection. In evaluating the role of cells involved in iNOS production, primary microglial cell cultures were found to be highly permissive to VEE infection. Moreover, VEE infection increased levels of nitric oxide (NO) in resting microglial cultures but decreased NO production in IFN-gamma-stimulated microglia. Thus, these findings suggest that reactive nitrogen species play an important contributory role in VEE dissemination and survival of the host. Our results further suggest the necessity for a carefully balanced host response that follows a middle course between immunopathology and insufficient inflammatory response to VEE infection.

Role of dendritic cell targeting in Venezuelan equine encephalitis virus pathogenesis

The initial steps of Venezuelan equine encephalitis virus (VEE) spread from inoculation in the skin to the draining lymph node have been characterized. By using green fluorescent protein and immunocytochemistry, dendritic cells in the draining lymph node were determined to be the primary target of VEE infection in the first 48 h following inoculation. VEE viral replicon particles, which can undergo only one round of infection, identified Langerhans cells to be the initial set of cells infected by VEE directly following inoculation. These cells are resident dendritic cells in the skin, which migrate to the draining lymph node following activation. A point mutation in the E2 glycoprotein gene of VEE that renders the virus avirulent and compromises its ability to spread beyond the draining lymph blocked the appearance of virally infected dendritic cells in the lymph node in vivo. A second-site suppressor mutation that restores viral spread to lymphoid tissues and partially restore virulence likewise restored the ability of VEE to infect dendritic cells in vivo.

Role of interferon and interferon regulatory factors in early protection against Venezuelan equine encephalitis virus infection

To investigate the role of type I interferon (IFN) and its regulatory transacting proteins, interferon regulatory factors (IRF-1 and IRF-2), in early protection against infection with virulent Venezuelan equine encephalitis virus (VEE), we utilized mice with targeted mutations in the IFN-alpha/beta receptor, IRF-1, or IRF-2 genes. IFN-alpha/beta-receptor knockout mice are highly susceptible to peripheral infection with virulent or attenuated VEE, resulting in their death within 24 and 48 h, respectively. Treatment of normal macrophages with anti-IFN-alpha/beta antibody prior to and during infection with molecularly cloned virulent VEE resulted in increased VEE replication. However, treatment with high doses of IFN or IFN-inducing agents failed to alter percentage mortality or average survival times in mice challenged with a low dose of virulent VEE. In IRF-1 and IRF-2 knockout mice (IRF-1(-/-) and IRF-2(-/-)), the 100% protection against virulent VEE that is conferred by attenuated VEE within 24 h in control C57BL/6 mice was completely absent in IRF-2(-/-) mice, whereas 50% of IRF-1(-/-) mice were protected. IRF-2(-/-) mice were deficient in clearing VEE virus from the spleen and the brain compared to the heterozygous IRF-2(+/-) knockout or C57BL/6 (+/+) mice. Furthermore, a distinct pattern of histopathological changes was observed in brains of IRF-2(-/-) mice after VEE exposure. Taken together, these findings imply that the altered immune response in IRF-1 and IRF-2 knockout mice results in altered virus dissemination, altered virus clearance, and altered virus-induced pathology. Thus, type I interferon, as well as IRF-1 and IRF-2, appears to play an important and necessary role in the pathogenesis of, and protection against, VEE infection.