Temperature-sensitive steps in the biosynthesis of Venezuelan equine encephalitis virus

Pathogenesis of temperature-sensitive mutants of sindbis virus in the embryonated egg. I. Characterization and kinetics of viral multiplication

Exploratory experiments were performed with temperature-sensitive (ts) mutants of Sindbis virus for studies on viral pathogenesis in the embryonated egg, a host which is immunologically underdeveloped. Parent and mutants were found to be virulent at the permissive temperature (33 C), but only the mutants were attenuated at the nonpermissive temperature (38.5 C). The degree of attenuation varied with the mutant and the route of inoculation. Chicks which survived infection by ts mutants at a nonpermissive temperature weighed the same as controls and showed no obvious abnormalities on gross examination. Whenever death of the embryo at the nonpermissive temperature occurred after inoculation with a mutant, it was apparently due to the selection of a population of temperature-insensitive virulent revertants. Kinetic studies showed that, after inoculation of the chorioallantoic membrane and incubation at the permissive temperature, a number of cycles of virus multiplication and dissemination apparently occurred rapidly. At the nonpermissive temperature, multiplication was undetectable. Certain pathophysiological signs were seen in the slower, less virulent infections by the mutants at the permissive temperature that were apparently masked or obscured in the more virulent, rapid infection by the parent. From these results and those reported in a subsequent paper, it appears that ts mutants of viruses possess potential as valuable tools for analyzing pathogenesis and immunity in the intact animal host that are complementary to more conventional approaches which employ normal (temperature-insensitive) viruses.

Biological, physical, and chemical properties of Eastern equine encephalitis virus. I. Purification and physical properties

A new purification procedure was adopted for Eastern equine encephalitis virus which does not subject the virus to pelleting at any stage. Three- to 4-liter volumes were passed through a diethylaminoethyl cellulose column. The virus-containing fractions were banded on a sucrose cushion and finally concentrated in an isopycnic band in a linear sucrose gradient. This method reduced the volume 1,000-fold with a concomitant increase in viral titer, i.e., better than 90% recovery. Numerous criteria have been used to establish that this viral preparation was essentially free from cellular debris and nonviral material. Physical studies on this purified viral product were initiated. The sedimentation coefficient as determined by band sedimentation was 240S, the buoyant density in sucrose was 1.18 g/cc, and the diameter of the virus was 54 nm. From the diameter and the buoyant density it was possible to calculate the molecular weight of a spherical particle. In this case, the calculated molecular weight for Eastern equine encephalitis virus was 58 x 10(6) daltons.

Interference Among Group A Arboviruses

Interference among group A arboviruses is described which does not involve the mediation of interferon. Interference was observed only if the interfering virus had an advantage over the challenge virus, either in time or in multiplicity of infection. Adsorption, penetration, and uncoating of challenge virus did not appear to be inhibited, but the synthesis of infectious viral ribonucleic acid of the challenge virus was significantly retarded. It was shown with temperature-sensitive viruses or mutants that the replication of viral ribonucleic acid by the interfering virus was required to establish interference. A mechanism of interference based on a competition for replication sites or substrates is compared with other possible explanations.

Interference among group A arboviruses

Interference among group A arboviruses is described which does not involve the mediation of interferon. Interference was observed only if the interfering virus had an advantage over the challenge virus, either in time or in multiplicity of infection. Adsorption, penetration, and uncoating of challenge virus did not appear to be inhibited, but the synthesis of infectious viral ribonucleic acid of the challenge virus was significantly retarded. It was shown with temperature-sensitive viruses or mutants that the replication of viral ribonucleic acid by the interfering virus was required to establish interference. A mechanism of interference based on a competition for replication sites or substrates is compared with other possible explanations.

Fractionation of Eastern equine encephalitis virus by density gradient centrifugation in CsCl

When partially purified Eastern equine encephalitis (EEE) virus was centrifuged to equilibrium in CsCl, three virus specific bands were observed. A hemagglutinin was detected at a buoyant density of 1.18 g/cm(3). Infectious EEE virus banded in two positions; most of the virus banded at 1.20 g/cm(3) and a lesser amount banded at 1.22 to 1.23 g/cm(3). Analysis of radioactive profiles of CsCl-fractionated EEE virus labeled with either (32)PO(4) or (3)H-uridine suggested that the hemagglutinin was stripped from the intact EEE virion. The viral origin of the hemagglutinin was verified by inhibition with specific antiserum. Attempts to differentiate between infectious EEE virus of the different buoyant densities showed that the denser particle was neither a virus contaminant nor a density mutant. No evidence was obtained to indicate that the denser particle was an immature form of EEE virus. The two infectious EEE species obtained after CsCl fractionation were indistinguishable antigenically. Furthermore, unfractionated as well as CsCl-fractionated EEE virus sedimented at about 260S in sucrose gradients. These results together with the results of rebanding experiments suggested that the denser EEE species (1.23 g/cm(3)) results from a salt (CsCl)-induced alteration or breakdown of the EEE virion (1.20 g/cm(3)), and that it arises as the hemagglutinin is stripped from the surface of the EEE virion.