Genetic and phenotypic changes accompanying the emergence of epizootic subtype IC Venezuelan equine encephalitis viruses from an enzootic subtype ID progenitor.
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Recent studies have indicated that epizootic Venezuelan equine encephalitis (VEE) viruses can evolve from enzootic, subtype ID strains that circulate continuously in lowland tropical forests (A. M. Powers, M. S. Oberste, A. C. Brault, R. Rico-Hesse, S. M. Schmura, J. F. Smith, W. Kang, W. P. Sweeney, and S. C. Weaver, J. Virol. 71:6697-6705, 1997). To identify mutations associated with the phenotypic changes leading to epizootics, we sequenced the entire genomes of two subtype IC epizootic VEE virus strains isolated during a 1992-1993 Venezuelan outbreak and four sympatric, subtype ID enzootic strains closely related to the predicted epizootic progenitor. Analysis by maximum-parsimony phylogenetic methods revealed 25 nucleotide differences which were predicted to have accompanied the 1992 epizootic emergence; 7 of these encoded amino acid changes in the nsP1, nsP3, capsid, and E2 envelope glycoprotein, and 2 were mutations in the 3' untranslated genome region. Comparisons with the genomic sequences of IAB and other IC epizootic VEE virus strains revealed that only one of the seven amino acid changes associated with the 1992 emergence, a threonine-to-methionine change at position 360 of the nsP3 protein, accompanied another VEE virus emergence event. Two changes in the E2 envelope glycoprotein region believed to include the major antigenic determinants, both involving replacement of uncharged residues with arginine, are also candidates for epizootic determinants. (+info)
Role of interferon and interferon regulatory factors in early protection against Venezuelan equine encephalitis virus infection.
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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. (+info)
Geographic distribution of Venezuelan equine encephalitis virus subtype IE genotypes in Central America and Mexico.
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Phylogenetic analysis of 20 strains of Venezuelan equine encephalitis (VEE) virus subtype IE isolated from 1961 to 1996 in Mexico and throughout Central America showed that VEE virus subtype IE was monophyletic with respect to other VEE virus subtypes. Nonetheless, there were at least three distinct geographically separated VEE virus IE genotypes: northwestern Panama, Pacific coast (Mexico/Guatemala), and Gulf/Caribbean coast (Mexico/Belize). Strains from the Caribbean coast of Guatemala, Honduras, and Nicaragua may cluster with the Gulf/Caribbean genotype, but additional isolates from the region between Guatemala and Panama will be required to firmly establish their phylogenetic position. Viruses associated with two separate equine epizootics in Mexico in the 1990s were phylogenetically related to nonepizootic viruses from neighboring Guatemala and may represent the emergence or re-emergence of equine-virulent VEE virus subtype IE in Middle America. (+info)
Limited potential for mosquito transmission of genetically engineered, live-attenuated Venezuelan equine encephalitis virus vaccine candidates.
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In an attempt to improve the current live-attenuated vaccine (TC-83) for Venezuelan equine encephalitis (VEE), specific mutations associated with attenuation of VEE virus in rodent models were identified. These mutations were inserted into full-length cDNA clones of the Trinidad donkey strain of VEE virus by site-directed mutagenesis, and isogenic virus strains with these mutations were recovered after transfection of baby hamster kidney cells with infectious RNA. We evaluated 10 of these strains for their ability to replicate in and be transmitted by Aedes taeniorhynchus, a natural vector of epizootic VEE virus. Two vaccine candidates, one containing a deletion of the PE2 furin cleavage site, the other a combination of three separate point mutations in the E2 glycoprotein, replicated in mosquitoes and were transmitted to hamsters significantly less efficiently than was either parental (wild type) VEE virus or TC-83 virus. Although the attenuated strains were transmitted to hamsters by mosquitoes, after intrathoracic inoculation, there was no evidence of reversion to a virulent phenotype. The mutations that resulted in less efficient replication in, or transmission by, mosquitoes should enhance vaccine safety and reduce the possibility of environmental spread to unintentional hosts. (+info)
Genetic evidence for the origins of Venezuelan equine encephalitis virus subtype IAB outbreaks.
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Epizootics of Venezuelan equine encephalitis (VEE) involving subtype IAB viruses occurred sporadically in South, Central and North America from 1938 to 1973. Incompletely inactivated vaccines have long been suspected as a source of the later epizootics. We tested this hypothesis by sequencing the PE2 glycoprotein precursor (1,677 nucleotides) or 26S/nonstructural protein 4 (nsP4) genome regions (4,490 nucleotides) for isolates representing most major outbreaks. Two distinct IAB genotypes were identified: 1) 1940s Peruvian strains and 2) 1938-1973 isolates from South, Central, and North America. Nucleotide sequences of these two genotypes differed by 1.1%, while the latter group showed only 0.6% sequence diversity. Early VEE virus IAB strains that were used for inactivated vaccine preparation had sequences identical to those predicted by phylogenetic analyses to be ancestors of the 1960s-1970s outbreaks. These data support the hypothesis of a vaccine origin for many VEE outbreaks. However, continuous, cryptic circulation of IAB viruses cannot be ruled out as a source of epizootic emergence. (+info)
Vaccination of macaques against pathogenic simian immunodeficiency virus with Venezuelan equine encephalitis virus replicon particles.
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Vaccine vectors derived from Venezuelan equine encephalitis virus (VEE) that expressed simian immunodeficiency virus (SIV) immunogens were tested in rhesus macaques as part of the effort to design a safe and effective vaccine for human immunodeficiency virus. Immunization with VEE replicon particles induced both humoral and cellular immune responses. Four of four vaccinated animals were protected against disease for at least 16 months following intravenous challenge with a pathogenic SIV swarm, while two of four controls required euthanasia at 10 and 11 weeks. Vaccination reduced the mean peak viral load 100-fold. The plasma viral load was reduced to below the limit of detection (1,500 genome copies/ml) in one vaccinated animal between 6 and 16 weeks postchallenge and in another from week 6 through the last sampling time (40 weeks postchallenge). The extent of reduction in challenge virus replication was directly correlated with the strength of the immune response induced by the vectors, which suggests that vaccination was effective. (+info)
Role of dendritic cell targeting in Venezuelan equine encephalitis virus pathogenesis.
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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. (+info)
Development of reverse transcription-PCR assays specific for detection of equine encephalitis viruses.
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Specific and sensitive reverse transcription-PCR (RT-PCR) assays were developed for the detection of eastern, western, and Venezuelan equine encephalitis viruses (EEE, WEE, and VEE, respectively). Tests for specificity included all known alphavirus species. The EEE-specific RT-PCR amplified a 464-bp region of the E2 gene exclusively from 10 different EEE strains from South and North America with a sensitivity of about 3,000 RNA molecules. In a subsequent nested PCR, the specificity was confirmed by the amplification of a 262-bp fragment, increasing the sensitivity of this assay to approximately 30 RNA molecules. The RT-PCR for WEE amplified a fragment of 354 bp from as few as 2,000 RNA molecules. Babanki virus, as well as Mucambo and Pixuna viruses (VEE subtypes IIIA and IV), were also amplified. However, the latter viruses showed slightly smaller fragments of about 290 and 310 bp, respectively. A subsequent seminested PCR amplified a 195-bp fragment only from the 10 tested strains of WEE from North and South America, rendering this assay virus specific and increasing its sensitivity to approximately 20 RNA molecules. Because the 12 VEE subtypes showed too much divergence in their 26S RNA nucleotide sequences to detect all of them by the use of nondegenerate primers, this assay was confined to the medically important and closely related VEE subtypes IAB, IC, ID, IE, and II. The RT-PCR-seminested PCR combination specifically amplified 342- and 194-bp fragments of the region covering the 6K gene in VEE. The sensitivity was 20 RNA molecules for subtype IAB virus and 70 RNA molecules for subtype IE virus. In addition to the subtypes mentioned above, three of the enzootic VEE (subtypes IIIB, IIIC, and IV) showed the specific amplicon in the seminested PCR. The practicability of the latter assay was tested with human sera gathered as part of the febrile illness surveillance in the Amazon River Basin of Peru near the city of Iquitos. All of the nine tested VEE-positive sera showed the expected 194-bp amplicon of the VEE-specific RT-PCR-seminested PCR. (+info)