Venezuelan equine encephalomyelitis virus (VEEV):strain differentiation and specification of virulence markers

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.

Neuronal cell death in alphavirus encephalomyelitis

Alphaviruses are mosquito-borne, enveloped, plus-strand RNA viruses that cause a spectrum of diseases in humans that include fever, rash, arthritis, meningitis, and encephalomyelitis. Sindbis virus (SINV) is the prototype alphavirus, causes encephalomyelitis in mice, and provides a model system for studying the pathogenesis of alphavirus-induced neurological disease. Major target cells for SINV infection in the central nervous system (CNS) are neurons, and both host and viral factors determine the fate of infected neurons. Young animals are most susceptible to fatal disease. This correlates with the ability of SINV to induce apoptosis in immature neurons. In vitro, apoptotic death of neuroblastoma cells can be induced by fusion of the virus envelope with the endosomal membrane and does not require infectious virus. This fusion process activates acid sphingomyelinase that cleaves sphingomyelin to release ceramide, an initiator of apoptosis. Within an hour, poly(ADP-ribose) polymerase is activated, and this is followed by release of cytochrome c and activation of effector caspases. SINV-induced cell death can be delayed or prevented by treatment with antioxidants or caspase inhibitors and by intracellular expression of Bcl-2, Beclin-1, or protease inhibitors. Older animals survive infection unless infected with a neurovirulent strain of SINV. In these mice, anterior horn motor neurons die by a primarily necrotic process that is influenced by excitotoxic amino acids and inflammation, whereas hippocampal neurons can be either apoptotic or necrotic. Death also occurs in uninfected neurons in the vicinity of infected neurons and can be delayed or prevented by treatment with glutamate receptor antagonists.

Replication of alphaviruses in cultures of donkey monocytes

Representative strains of Venezuelan equine encephalitis virus (VEEV) and Eastern equine encephalitis virus (EEEV) were compared for their ability to grow in cultures of unstimulated leucocytes and monocytes derived from donkey peripheral blood. Replication of epizootic and vaccine strains of VEEV, but not of enzootic strains was observed in this system. Only a minority of monocytes supported virus replication as detected by immunofluorescence, electron microscopy and infectious center assays. EEEV did not appear to replicate in this cell system although virus attached to and was internalized by monocytes.

Influence of Venezuelan equine encephalomyelitis virus on catecholamine metabolism in mouse CNS: early changes in turnover rates and content

The influence of two strains of Venezuelan equine encephalomyelitis virus on catecholamine metabolism in the central nervous system (CNS) of the mouse was studied by the intracranial inoculation of NMRI-IVIC mice, 24 days old. Doses of 300 LD50 of the moderately virulent Pixuna strain and 10(4) LD50 of the nonvirulent TC-83 strain were used. The animals were sacrificed 4, 7, 12, and 18 days after the inoculation, and the CNS contents of dopamine, norepinephrine, and adrenaline were determined by a radioenzymatic method. The turnover of these compounds was studied with the aid of the enzymatic inhibitors pargyline and alpha-methyl tyrosine. With the Pixuna strain a decrease in the level and turnover of catecholamines was observed in all eight brain areas studied: cerebellum, pons-medulla, hypothalamus, mesencephalon, striatum, olfactory bulb-tuberculum, hippocampus, and cerebral cortex. In some regions the levels returned to normal in 12-18 days, but in the striatum, mesencephalon, hypothalamus, and olfactory bulb-tuberculum, recovery was slower and required at least 18 days after inoculation. The levels of catecholamines in the brain of mice inoculated with the nonvirulent TC-83 strain were not modified, but there was a decrease in the turnover.

Virion polypeptide heterogeneity among virulent and avirulent strains of eastern equine encephalitis (EEE) virus

Comparative analysis of structural virion polypeptides of 24 selected EEE virus strains, representing North and South American types, was performed by one-dimensional discontinuous sodium dodecyl sulfate (SDS)-polyacrylamide-gel electrophoresis (PAGE). The structural proteins of different EEE virus isolates, resolved by this method, exhibited mol.wts. values in the range of 57-60 X 10(3) for (E-1), 51-54 X 10(3) for (E-2) and 35-38 X 10(3) daltons for the core (NP) nucleocapsid. The exception was the South American human lethal virus, TRVL-89287 strain, which was shown to possess only a single envelope glycoprotein. The high molecular weight envelope (E-1) glycoprotein species was absent or co-migrated adjacent to the smaller envelope (E-2) glycoprotein. Results indicated similarities in the core (NP) proteins, however greater variability in the envelope (E-/ and/or E-2) glycoproteins. Based on these variations seven distinct profiles could be observed among the EEE virus strain studied. The classification based on the patterns of structural polypeptides obtained by SDS-PAGE of these strains does not correlate well with any other previously reported in vitro characteristics (antigenic subtypes, HTP elution profiles) nor with the in vivo virulence markers.

Kinetics of heat inactivation of Venezuelan equine encephalomyelitis virus

Thermal inactivation of Venezuelan Equine Encephalomyelitis Virus (VEEV) was studied at temperatures from 26 degrees to 55 degrees C. Inactivation of infectivity took place by two thermodynamically different reactions, one of which predominated at temperatures below 44 degrees C and the other at higher temperatures. The presence of 1 or 2 M NaCl stabilized the VEE virus at low temperatures but enhanced the inactivation at high temperatures. This latter effect at temperatures higher than 50 degrees C, is associated with the occurrence of two-component survival curves. The different effects of hypertonic NaCl concentrations at the two ranges of temperature, are related to different mechanisms of inactivation operating at each range (protein denaturation and nucleic acid-RNA breakdown). Different kinetics of thermal inactivation at 55 degrees C were observed between virus strains with different virulence. However, no significant correlations was found between the virulence of the eleven VEE virus strains studied and their thermostability at 37 degres and 55 degrees C.