Further characterization of an attenuated western equine encephalitis virus: search for in vitro markers

Pathogenicity of an attenuated, temperature-sensitive mutant of western equine encephalitis virus induced by a chemical mutagen

To know the pathogenicity of the chemically induced, temperature-sensitive (ts) mutant of western equine encephalitis virus, designated tsNG39, the lethality for mice injected with tsNG39, virus yield, interferon production, and histological changes in the brains of these mice were examined in parallel with those of mice inoculated with the parent strain (PS). All of the mice injected intracranially with PS died within 3.5 days after injection irrespective of the inoculum size of virus, whereas the lethality of the mice inoculated with tsNG39 varied from 94.3 to 65.5% among groups of mice and this variation seemed to be correlated with the inoculum size of virus rather than with the maximum virus titer in the brain. By histological examination, two types of changes in the brain were distinguished, inflammatory and degenerative ones. Inflammatory changes were more prominent in the brains injected with tsNG39 than in those receiving PS. Degenerative changes were dominant in the brains injected with PS, but they were slight in the earlier phase of infection by tsNG39 became prominent only later. The degree of degenerative change was well correlated with both the virus titer in the mouse brain and the death pattern of mice injected with PS or tsNG39. Since degenerative changes are thought to be caused by the direct effect of injected virus, these results indicated that the factor responsible for the low virulence of tsNG39 was the slow viral growth in the brain.

Gamma ray-induced small plaque mutants of western equine encephalitis virus

Small plaque mutants of Western equine encephalitis virus were obtained from the surviving fractions of wild-type virus which was irradiated with gamma rays. The frequency with which small plaque mutants appeared in the surviving fraction increased with the radiation dose. These mutants were not more resistant to radiation than wild-type virus. The growth rate of a mutant, S127, was lower than that of wild-type. Clonally purified mutant virions presented two peaks in a velocity sedimentation profile; peak 1 corresponded to the peak of wild type and peak 2 moved faster than peak 1. Virions of both peaks were infectious and consistently formed small plaques in chicken embryo cells. Virions reisolated from either peak and grown in chicken embryo cells also revealed two peaks in sedimentation analysis. In the electron microscope examination peak 2 proved to consist of giant form particles, each of which contained more than one nucleoid surrounded with a common envelope. Despite this remarkable morphological difference, densities of the wild-type and S127 mutant virions were similar in cesium chloride gradients. The RNAs and proteins of mutant virions could not be distinguished from those of wild type on the basis of size or charge.