GENETIC-DETERMINANTS OF SINDBIS-VIRUS NEUROINVASIVENESS

After peripheral inoculation of mice, Sindbis virus replicates in a va riety of tissues, leading to viremia. In some cases, the virus can ent er the central nervous system (CNS) and cause lethal encephalitis. The outcome of infection is age and virus strain dependent. Recently, two pairs of Sindbis virus variants differing in neurovirulence and neuro invasiveness were derived by limited serial passaging in mouse brain. Two early passage isolates (SVA and SVB) were neurotropic but did not cause lethal encephalitis. SVB, but not SVA, was neuroinvasive. A seco nd independent pair of isolates (SVN and SVNI), which had undergone mo re extensive mouse brain passaging, were highly neurotropic and caused lethal encephalitis. Only SVNI could reach the brain after peripheral inoculation. From these isolates, virion RNAs were obtained and used to construct full-length cDNA clones from which infectious RNA transcr ipts could be recovered, The strains recovered from these clones were shown to retain the appropriate phenotypes in weanling mice. Construct ion and analysis of recombinant viruses were used to define the geneti c loci determining neuroinvasion. For SVB, neuroinvasiveness was deter mined by a single residue in the E2 glycoprotein (Gln-55). For SVNI, n euroinvasive loci were identified in both the 5' noncoding region (pos ition 8) and the E2 glycoprotein (Met-190). Either of these changes on the SVN background was sufficient to confer a neuroinvasive phenotype , although these recombinants were less virulent. To completely mimic the SVNI phenotype, three SVNI-specific substitutions on the SVN backg round were required: G at position 8, E2 Met-190, and Lys-260, which b y itself had no effect on neuroinvasion. These genetically defined str ains should be useful for dissecting the molecular mechanisms leading to Sindbis virus invasion of the CNS.