The use of chimeric Venezuelan equine encephalitis viruses as an approach for the molecular identification of natural virulence determinants

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.

Identifying the Role of E2 Domains on Alphavirus Neutralization and Protective Immune Responses

BACKGROUND

Chikungunya virus (CHIKV) and other alphaviruses are the etiologic agents of numerous diseases in both humans and animals. Despite this, the viral mediators of protective immunity against alphaviruses are poorly understood, highlighted by the lack of a licensed human vaccine for any member of this virus genus. The alphavirus E2, the receptor-binding envelope protein, is considered to be the predominant target of the protective host immune response. Although envelope protein domains have been studied for vaccine and neutralization in flaviviruses, their role in alphaviruses is less characterized. Here, we describe the role of the alphavirus E2 domains in neutralization and protection through the use of chimeric viruses.

METHODOLOGY/PRINCIPAL FINDINGS

Four chimeric viruses were constructed in which individual E2 domains of CHIKV were replaced with the corresponding domain from Semliki Forest virus (SFV) (ΔDomA/ΔDomB/ΔDomC/ ΔDomA+B). Vaccination studies in mice (both live and inactivated virus) revealed that domain B was the primary determinant of neutralization. Neutralization studies with CHIKV immune serum from humans were consistent with mouse studies, as ΔDomB was poorly neutralized.

CONCLUSIONS/SIGNIFICANCE

Using chimeric viruses, it was determined that the alphavirus E2 domain B was the critical target of neutralizing antibodies in both mice and humans. Therefore, chimeric viruses may have more relevance for vaccine discovery than peptide-based approaches, which only detect linear epitopes. This study provides new insight into the role of alphavirus E2 domains on neutralization determinants and may be useful for the design of novel therapeutic technologies.

IRES-Containing VEEV Vaccine Protects Cynomolgus Macaques from IE Venezuelan Equine Encephalitis Virus Aerosol Challenge

Venezuelan equine encephalitis virus (VEEV) is an arbovirus endemic to the Americas that is responsible for severe, sometimes fatal, disease in humans and horses. We previously described an IRES-based VEE vaccine candidate based up the IE serotype that offers complete protection against a lethal subtype IE VEEV challenge in mice. Here we demonstrate the IRES-based vaccine’s ability to protect against febrile disease in cynomolgus macaques. Vaccination was well tolerated and elicited robust neutralizing antibody titers noticed as early as day 14. Moreover, complete protection from disease characterized by absence of viremia and characteristic fever following aerosolized IE VEEV challenge was observed in all vaccinees compared to control animals, which developed clinical disease. Together, these results highlight the safety and efficacy of IRES-based VEEV vaccine to protect against an endemic, pathogenic VEEV IE serotype.

Venezuelan equine encephalitis virus (VEEV) is a mosquito-borne arbovirus endemic to the Americas that affects a wide range of equids and humans. Vaccination has been one of the strategies to combat spread of disease in areas with high rates incidence of VEEV, although existing vaccines have proven less than effective against genetically diverse serotypes. In addition to being a natural vectorborne threat, VEEV is considered a biological threat agent that could be used as a weapon. We evaluated a new Internal Ribosome Entry Site (IRES)-containing chimeric viral vaccine using an advanced nonhuman primate model of VEEV infection. Vaccinated animals showed robust humoral immune responses to a single prime immunization with IE VEEV/IRES vaccine. The vaccine protected against an aerosolized IE (68U201) challenge, with vaccinees showing no blood viremia or development of febrile disease, including no pyrexia associated with VEEV infection. This vaccine product has shown efficacy against serotype-specific challenge model and provides enabling data as the basis for future clinical development.

Virulence determinants of New World alphaviruses and broad-acting therapeutic strategies

ABSTRACT 

New World alphaviruses of eastern equine encephalitis virus (EEEV), western equine encephalitis virus (WEEV) and Venezuelan equine encephalitis virus (VEEV) are endemic in North and South America, and infect humans and equine through mosquitoes. In addition, these viruses are highly infectious when aerosolized, making them potential biowarfare and bioterrorism agents. Currently, no approved vaccines or drugs are available for prevention and treatment. Extensive studies have been carried out to understand molecular mechanisms of virulence among New World alphaviruses. This review will focus on virus-encoded, interferon antagonizing proteins which play major role in determination of virulence of New World alphaviruses. Understanding of molecular mechanism of these proteins will shed light on development of broad-acting antivirals against New World alphaviruses.

O'nyong nyong virus molecular determinants of unique vector specificity reside in non-structural protein 3

O'nyong nyong virus (ONNV) and Chikungunya virus (CHIKV) are two closely related alphaviruses with very different infection patterns in the mosquito, Anopheles gambiae. ONNV is the only alphavirus transmitted by anopheline mosquitoes, but specific molecular determinants of infection of this unique vector specificity remain unidentified. Fifteen distinct chimeric viruses were constructed to evaluate both structural and non-structural regions of the genome and infection patterns were determined through artificial infectious feeds in An. gambiae with each of these chimeras. Only one region, non-structural protein 3 (nsP3), was sufficient to up-regulate infection to rates similar to those seen with parental ONNV. When ONNV non-structural protein 3 (nsP3) replaced nsP3 from CHIKV virus in one of the chimeric viruses, infection rates in An. gambiae went from 0% to 63.5%. No other single gene or viral region addition was able to restore infection rates. Thus, we have shown that a non-structural genome element involved in viral replication is a major element involved in ONNV's unique vector specificity.

IRES-based Venezuelan equine encephalitis vaccine candidate elicits protective immunity in mice

Venezuelan equine encephalitis virus (VEEV) is an arbovirus that causes periodic outbreaks that impact equine and human populations in the Americas. One of the VEEV subtypes located in Mexico and Central America (IE) has recently been recognized as an important cause of equine disease and death, and human exposure also appears to be widespread. Here, we describe the use of an Internal Ribosome Entry Site (IRES) from encephalomyocarditis virus to stably attenuate VEEV, creating a vaccine candidate independent of unstable point mutations. Mice infected with this virus produced antibodies and were protected against lethal VEEV challenge. This IRES-based vaccine was unable to establish productive infection in mosquito cell cultures or in intrathoracically injected Aedes taeniorhynchus, demonstrating that it cannot be transmitted from a vaccinee. These attenuation, efficacy and safety results justify further development for humans or equids of this new VEEV vaccine candidate.

Genetic and anatomic determinants of enzootic Venezuelan equine encephalitis virus infection of Culex (Melanoconion) taeniopus

Venezuelan equine encephalitis (VEE) is a re-emerging, mosquito-borne viral disease with the potential to cause fatal encephalitis in both humans and equids. Recently, detection of endemic VEE caused by enzootic strains has escalated in Mexico, Peru, Bolivia, Colombia and Ecuador, emphasizing the importance of understanding the enzootic transmission cycle of the etiologic agent, VEE virus (VEEV). The majority of work examining the viral determinants of vector infection has been performed in the epizootic mosquito vector, Aedes (Ochlerotatus) taeniorhynchus. Based on the fundamental differences between the epizootic and enzootic cycles, we hypothesized that the virus-vector interaction of the enzootic cycle is fundamentally different from that of the epizootic model. We therefore examined the determinants for VEEV IE infection in the enzootic vector, Culex (Melanoconion) taeniopus, and determined the number and susceptibility of midgut epithelial cells initially infected and their distribution compared to the epizootic virus-vector interaction. Using chimeric viruses, we demonstrated that the determinants of infection for the enzootic vector are different than those observed for the epizootic vector. Similarly, we showed that, unlike A. taeniorhynchus infection with subtype IC VEEV, C. taeniopus does not have a limited subpopulation of midgut cells susceptible to subtype IE VEEV. These findings support the hypothesis that the enzootic VEEV relationship with C. taeniopus differs from the epizootic virus-vector interaction in that the determinants appear to be found in both the nonstructural and structural regions, and initial midgut infection is not limited to a small population of susceptible cells.

Venezuelan equine encephalitis virus (VEEV) is transmitted to humans and horses by mosquitoes in Mexico, Central and South America. These infections can lead to fatal encephalitis in humans as well as horses, donkeys and mules, and there are no licensed vaccines or treatments available for humans. VEEV circulates in two distinct transmission cycles (epizootic and enzootic), which are differentiated by the ecological niche that each virus inhabits. Epizootic strains, those that cause major outbreaks in humans and equids, have been studied extensively and have been used primarily to develop and test several vaccine candidates. In this study, we demonstrate some important differences in the roles of different viral genes between enzootic/endemic versus epizootic VEEV strains that affect mosquito infection as well as differences in the way that enzootic VEEV more efficiently infects the mosquito initially. Our findings have important implications for designing vaccines and for understanding the evolution of VEEV-mosquito interactions.

Mutations in nsP1 and PE2 are critical determinants of Ross River virus-induced musculoskeletal inflammatory disease in a mouse model

The viral determinants of alphavirus-induced rheumatic disease have not been elucidated. We identified an RRV strain (DC5692) which, in contrast to the T48 strain, does not induce musculoskeletal inflammation in a mouse model of RRV disease. Substitution of the RRV T48 strain nonstructural protein 1 (nsP1) coding sequence with that from strain DC5692 generated a virus that was attenuated in vivo despite similar viral loads in tissues. In contrast, substitution of the T48 PE2 coding region with the PE2 coding region from DC5692 resulted in attenuation in vivo and reduced viral loads in tissues. In gain of virulence experiments, substitution of the DC5692 strain nsP1 and PE2 coding regions with those from the T48 strain was sufficient to restore full virulence to the DC5692 strain. These findings indicate that determinants in both nsP1 and PE2 have critical and distinct roles in the pathogenesis of RRV-induced musculoskeletal inflammatory disease in mice.

Vector competence of Culex (Melanoconion) taeniopus for equine-virulent subtype IE strains of Venezuelan equine encephalitis virus

The mosquito Culex (Melanoconion) taeniopus is a proven vector of enzootic Venezuelan equine encephalitis virus (VEEV) subtype IE in Central America. It has been shown to be highly susceptible to infection by this subtype, and conversely to be highly refractory to infection by other VEEV subtypes. During the 1990s in southern coastal Mexico, two VEE epizootics in horses were attributed to subtype IE VEEV. These outbreaks were associated with VEEV strains with an altered infection phenotype for the epizootic mosquito vector, Aedes (Ochlerotatus) taeniorhynchus. To determine the infectivity for the enzootic vector, Culex taeniopus, mosquitoes from a recently established colony were orally exposed to VEEV strains from the outbreak. The equine-virulent strains exhibited high infectivity and transmission potential comparable to a traditional enzootic subtype IE VEEV strain. Thus, subtype IE VEEV strains in Chiapas are able to efficiently infect enzootic and epizootic vectors and cause morbidity and mortality in horses.

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.

Venezuelan equine encephalitis in Panama: fatal endemic disease and genetic diversity of etiologic viral strains

Venezuelan equine encephalitis (VEE) is a reemerging, mosquito-borne viral disease of the neotropics that is severely debilitating and sometimes fatal to humans. Periodic epidemics mediated by equine amplification have been recognized since the 1920s, but interepidemic disease is rarely recognized. We report here clinical findings and genetic characterization of 42 cases of endemic VEE detected in Panama from 1961–2004. Recent clusters of cases occurred in Darien (eastern Panama) and Panama provinces (central Panama) near rainforest and swamp habitats. Patients ranged from 10 months to 48 years of age, and the more severe cases with neurological complications, including one fatal infection, were observed in children. The VEE virus strains isolated from these cases all belonged to an enzootic, subtype ID lineage known to circulate among sylvatic vectors and rodent reservoir hosts in Panama and Peru. These findings underscore endemic VEE as an important but usually neglected arboviral disease of Latin America.

Venezuelan equine encephalitis (VEE) is a mosquito-borne viral disease that has caused major epidemics in many parts of Latin America and has even spread into Texas on one occasion. These epidemics result from spillover to humans of a horse-mosquito-horse amplification cycle that has occurred periodically since the 1920s. However, between these equine-mediated epidemics, little attempt is typically made to detect VEE in humans. Here, we show that VEE virus strains that typically circulate in a mosquito-rodent cycle, termed enzootic strains, also produce many cases of severe and sometimes fatal disease in Panama in the absence of apparent epidemics. These endemic infections are probably rarely detected because they are difficult to distinguish clinically from dengue fever, another mosquito-borne viral illness common in the tropics. Our findings underscore endemic VEE as an important but usually neglected arboviral disease of Latin America.

Common marmosets (Callithrix jacchus) as a nonhuman primate model to assess the virulence of eastern equine encephalitis virus strains

Eastern equine encephalitis virus (EEEV) produces the most severe human arboviral disease in North America (NA) and is a potential biological weapon. However, genetically and antigenically distinct strains from South America (SA) have seldom been associated with human disease or mortality despite serological evidence of infection. Because mice and other small rodents do not respond differently to the NA versus SA viruses like humans, we tested common marmosets (Callithrix jacchus) by using intranasal infection and monitoring for weight loss, fever, anorexia, depression, and neurologic signs. The NA EEEV-infected animals either died or were euthanized on day 4 or 5 after infection due to anorexia and neurologic signs, but the SA EEEV-infected animals remained healthy and survived. The SA EEEV-infected animals developed peak viremia titers of 2.8 to 3.1 log(10) PFU/ml on day 2 or 4 after infection, but there was no detectable viremia in the NA EEEV-infected animals. In contrast, virus was detected in the brain, liver, and muscle of the NA EEEV-infected animals at the time of euthanasia or death. Similar to the brain lesions described for human EEE, the NA EEEV-infected animals developed meningoencephalitis in the cerebral cortex with some perivascular hemorrhages. The findings of this study identify the common marmoset as a useful model of human EEE for testing antiviral drugs and vaccine candidates and highlight their potential for corroborating epidemiological evidence that some, if not all, SA EEEV strains are attenuated for humans.

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.

Venezuelan equine encephalitis virus infection of cotton rats

VEEV killed 2 allopatric populations of cotton rats but not a sympatric population from Florida.

Venezuelan equine encephalitis virus (VEEV) is an emerging pathogen of equids and humans, but infection of its rodent reservoir hosts has received little study. To determine whether responses to infection vary among geographic populations, we inoculated 3 populations of cotton rats with 2 enzootic VEEV strains (Co97-0054 [enzootic ID subtype] and 68U201 [enzootic IE subtype]). The 3 populations were offspring from wild-caught cotton rats collected in a VEE-enzootic area of south Florida, USA; wild-caught cotton rats from a non–VEE-enzootic area of Texas, USA; and commercially available (Harlan) colony-reared cotton rats from a non–VEE-enzootic region. Although each population had similar early viremia titers, no detectable disease developed in the VEE-sympatric Florida animals, but severe disease and death affected the Texas and Harlan animals. Our findings suggest that the geographic origins of cotton rats are important determinants of the outcome of VEE infection and reservoir potential of these rodents.

Venezuelan equine encephalitis virus in the mosquito vector Aedes taeniorhynchus: infection initiated by a small number of susceptible epithelial cells and a population bottleneck

We evaluated infection of Aedes taeniorhynchus mosquitoes, vectors of Venezuelan equine encephalitis virus (VEEV), using radiolabeled virus and replicon particles expressing green (GFP) or cherry fluorescent protein (CFP). More epidemic VEEV bound to and infected mosquito midguts compared to an enzootic strain, and a small number of midgut cells was preferentially infected. Chimeric replicons infected midgut cells at rates comparable to those of the structural gene donor. The numbers of midgut cells infected averaged 28, and many infections were initiated in only 1-5 cells. Infection by a mixture of GFP- and CFP-expressing replicons indicated that only about 100 midgut cells were susceptible. Intrathoracic injections yielded similar patterns of replication with both VEEV strains, suggesting that midgut infection is the primary limitation to transmission. These results indicate that the structural proteins determine initial infection of a small number of midgut cells, and that VEEV undergoes population bottlenecks during vector infection.

Genetically engineered, live, attenuated vaccines protect nonhuman primates against aerosol challenge with a virulent IE strain of Venezuelan equine encephalitis virus

Two live, attenuated strains of Venezuelan equine encephalitis virus (VEE), IE1150K and V3526, were administered to macaques to determine if they could elicit protection against an aerosol challenge with virulent VEE virus of the IE variety (VEEV-IE). These viruses were rescued from full-length cDNA clones of 68U201 (VEEV-IE variety) and Trinidad donkey (VEEV-IA/B variety), respectively, and both have a furin cleavage site deletion mutation and a second-site resuscitating mutation. Both vaccines elicited neutralizing antibodies to viruses of the homologous variety but not to viruses of the heterologous variety. Eight weeks after vaccination, the macaques were challenged by aerosol exposure to virulent 68U201. Macaques vaccinated with V3526 were protected as well as macaques inoculated with IE1009, the wild-type infectious clone of 68U201. However, IE1150K failed to significantly protect macaques relative to controls. V3526 has now been shown to protect macaques against both IA/B [Pratt WD, Davis NL, Johnston RE, Smith JF. Genetically engineered, live attenuated vaccines for Venezuelan equine encephalitis: testing in animal models. Vaccine 2003;21(25-26):3854-62] and IE strains of VEE viruses.

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.

Venezuelan equine encephalitis emergence: enhanced vector infection from a single amino acid substitution in the envelope glycoprotein

In 1993 and 1996, subtype IE Venezuelan equine encephalitis (VEE) virus caused epizootics in the Mexican states of Chiapas and Oaxaca. Previously, only subtype IAB and IC VEE virus strains had been associated with major outbreaks of equine and human disease. The IAB and IC epizootics are believed to emerge via adaptation of enzootic (sylvatic, equine-avirulent) strains for high titer equine viremia that results in efficient infection of mosquito vectors. However, experimental equine infections with subtype IE equine isolates from the Mexican outbreaks demonstrated neuro-virulence but little viremia, inconsistent with typical VEE emergence mechanisms. Therefore, we hypothesized that changes in the mosquito vector host range might have contributed to the Mexican emergence. To test this hypothesis, we evaluated the susceptibility of the most abundant mosquito in the deforested Pacific coastal locations of the VEE outbreaks and a proven epizootic vector, Ochlerotatus taeniorhynchus. The Mexican epizootic equine isolates exhibited significantly greater infectivity compared with closely related enzootic strains, supporting the hypothesis that adaptation to an efficient epizootic vector contributed to disease emergence. Reverse genetic studies implicated a Ser --> Asn substitution in the E2 envelope glycoprotein as the major determinant of the increased vector infectivity phenotype. Our findings underscore the capacity of RNA viruses to alter their vector host range through minor genetic changes, resulting in the potential for disease emergence.

Genetic determinants of Venezuelan equine encephalitis emergence

Following a period of inactivity from 1973-1991, Venezuelan equine encephalitis (VEE) reemerged during the past decade in South America and Mexico. Experimental studies of VEE virus (VEEV) infection of horses with virus strains isolated during these outbreaks have revealed considerable variation in the ability of equine-virulent, epizootic strains to exploit horses as efficient amplification hosts. Subtype IC strains from recent outbreaks in Venezuela and Colombia amplify efficiently in equines, with a correlation between maximum viremia titers and the extent of the outbreak from which the virus strain was isolated. Studies of enzootic VEEV strains that are believed to represent progenitors of the epizootic subtypes support the hypothesis that adaptation to efficient replication in equines is a major determinant of emergence and the ability of VEEV to spread geographically. Correlations between the ability of enzootic and epizootic VEEV strains to infect abundant, equiphilic mosquitoes, and the location and extent of these outbreaks, also suggest that specific adaptation to Ochlerotatus taeniorhynchus mosquitoes is a determinant of some but not all emergence events. Genetic studies imply that mutations in the E2 envelope glycoprotein gene are major determinants of adaptation to both equines and mosquito vectors.

Venezuelan equine encephalitis

Venezuelan equine encephalitis virus (VEEV) remains a naturally emerging disease threat as well as a highly developed biological weapon. Recently, progress has been made in understanding the complex ecological and viral genetic mechanisms that coincide in time and space to generate outbreaks. Enzootic, equine avirulent, serotype ID VEEV strains appear to alter their serotype to IAB or IC, and their vertebrate and mosquito host range, to mediate repeated VEE emergence via mutations in the E2 envelope glycoprotein that represent convergent evolution. Adaptation to equines results in highly efficient amplification, which results in human disease. Although epizootic VEEV strains are opportunistic in their use of mosquito vectors, the most widespread outbreaks appear to involve specific adaptation to Ochlerotatus taeniorhynchus, the most common vector in many coastal areas. In contrast, enzootic VEEV strains are highly specialized and appear to utilize vectors exclusively in the Spissipes section of the Culex (Melanoconion) subgenus.

Glycosaminoglycan binding properties of natural venezuelan equine encephalitis virus isolates

Equine-virulent, epidemic/epizootic strains of Venezuelan equine encephalitis (VEE) virus (VEEV) arise via mutation of progenitor enzootic strains that replicate poorly in equines. Sequencing studies have implicated positively charged amino acids on the surface of the E2 envelope glycoprotein in the acquisition of equine virulence and viremia potential, suggesting that changes in binding to cell surface glycosaminoglycans (GAGs) may mediate VEE emergence. Therefore, we evaluated the binding of natural enzootic and epizootic VEEV isolates to Chinese hamster ovary (CHO) cells expressing normal, high levels of GAGs as well as to mutant CHO cells lacking GAG expression. Binding to GAGs was not consistently associated with the epizootic phenotype, and cell culture passages resulted in increased GAG binding. The low levels of GAG binding exhibited by some low-passage, equine-virulent subtype IC VEEV strains indicate that the positive-charge E2 mutations implicated in VEE subtype IC emergence are not artifacts of laboratory passage and suggest that GAG binding does not play a major role in mediating VEE emergence. The increased GAG binding exhibited by VEEV strain CPA201 from the 1993 Mexican epizootic, when compared to that of closely related enzootic subtype IE strains, was shown to result from a Glu-to-Lys mutation at position 117 of the E2 envelope glycoprotein.

Vector infection determinants of Venezuelan equine encephalitis virus reside within the E2 envelope glycoprotein

Epizootic subtype IAB and IC Venezuelan equine encephalitis viruses (VEEV) readily infect the epizootic mosquito vector Aedes taeniorhynchus. The inability of enzootic subtype IE viruses to infect this mosquito species provides a model system for the identification of natural viral determinants of vector infectivity. To map mosquito infection determinants, reciprocal chimeric viruses generated from epizootic subtype IAB and enzootic IE VEEV were tested for mosquito infectivity. Chimeras containing the IAB epizootic structural gene region and, more specifically, the IAB PE2 envelope glycoprotein E2 precursor gene demonstrated an efficient infection phenotype. Introduction of the PE2 gene from an enzootic subtype ID virus into an epizootic IAB or IC genetic backbone resulted in lower infection rates than those of the epizootic parent. The finding that the E2 envelope glycoprotein, the site of epitopes that define the enzootic and epizootic subtypes, also encodes mosquito infection determinants suggests that selection for efficient infection of epizootic mosquito vectors may mediate VEE emergence.

Positively charged amino acid substitutions in the e2 envelope glycoprotein are associated with the emergence of venezuelan equine encephalitis virus

Epidemic-epizootic Venezuelan equine encephalitis (VEE) viruses (VEEV) have emerged repeatedly via convergent evolution from enzootic predecessors. However, previous sequence analyses have failed to identify common sets of nucleotide or amino acid substitutions associated with all emergence events. During 1993 and 1996, VEEV subtype IE epizootics occurred on the Pacific Coast of the states of Chiapas and Oaxaca in southern Mexico. Like other epizootic VEEV strains, when inoculated into guinea pigs and mice, the Mexican isolates were no more virulent than closely related enzootic strains, complicating genetic studies of VEE emergence. Complete genomic sequences of 4 of the Mexican strains were determined and compared to those of closely related enzootic subtype IE isolates from Guatemala. The epizootic viruses were less than 2% different at the nucleotide sequence level, and phylogenetic relationships confirmed that the equine-virulent Mexican strains probably evolved from enzootic progenitors on the Pacific Coast of Mexico or Guatemala. Of 35 amino acids that varied among the Guatemalan and Mexican isolates, only 8 were predicted phylogenetically to have accompanied the phenotypic change. One mutation at position 117 of the E2 envelope glycoprotein, involving replacement of Glu by Lys, resulted in a small-plaque phenotype characteristic of epizootic VEEV strains. Analysis of additional E2 sequences from representative enzootic and epizootic VEEV isolates implicated similar surface charge changes in the emergence of previous South American epizootic phenotypes, indicating that E2 mutations are probably important determinants of the equine-virulent phenotype and of VEE emergence. Maximum-likelihood analysis indicated that one change at E2 position 213 has been influenced by positive selection and convergent evolution of the epizootic phenotype.

Role of alpha/beta interferon in Venezuelan equine encephalitis virus pathogenesis: effect of an attenuating mutation in the 5' untranslated region

Venezuelan equine encephalitis virus (VEE) is an important equine and human pathogen of the Americas. In the adult mouse model, cDNA-derived, virulent V3000 inoculated subcutaneously (s.c.) causes high-titer peripheral replication followed by neuroinvasion and lethal encephalitis. A single change (G to A) at nucleotide 3 (nt 3) of the 5' untranslated region (UTR) of the V3000 genome resulted in a virus (V3043) that was avirulent in mice. The mechanism of attenuation by the V3043 mutation was studied in vivo and in vitro. Kinetic studies of virus spread in adult mice following s.c. inoculation showed that V3043 replication was reduced in peripheral organs compared to that of V3000, titers in serum also were lower, and V3043 was cleared more rapidly from the periphery than V3000. Because clearance of V3043 from serum began 1 to 2 days prior to clearance of V3000, we examined the involvement of alpha/beta interferon (IFN-alpha/beta) activity in VEE pathogenesis. In IFN-alpha/betaR(-/-) mice, the course of the wild-type disease was extremely rapid, with all animals dying within 48 h (average survival time of 30 h compared to 7.7 days in the wild-type mice). The mutant V3043 was as virulent as the wild type (100% mortality, average survival time of 30 h). Virus titers in serum, peripheral organs, and the brain were similar in V3000- and V3043-infected IFN-alpha/betaR(-/-) mice at all time points up until the death of the animals. Consistent with the in vivo data, the mutant virus exhibited reduced growth in vitro in several cell types except in cells that lacked a functional IFN-alpha/beta pathway. In cells derived from IFN-alpha/betaR(-/-) mice, the mutant virus showed no growth disadvantage compared to the wild-type virus, suggesting that IFN-alpha/beta plays a major role in the attenuation of V3043 compared to V3000. There were no differences in the induction of IFN-alpha/beta between V3000 and V3043, but the mutant virus was more sensitive than V3000 to the antiviral actions of IFN-alpha/beta in two separate in vitro assays, suggesting that the increased sensitivity to IFN-alpha/beta plays a major role in the in vivo attenuation of V3043.