Two separate envelope regions influence induction of brain disease by a polytropic murine retrovirus (FMCF98).

The major determinants involved in neurological disease induction by polytropic murine leukemia virus FMCF98 are encoded by the envelope gene. To map these determinants further, we produced four chimeras which contained neurovirulent FMCF98 envelope sequences combined with envelope sequences from the closely related nonneurovirulent polytropic virus FMCF54. Surprisingly, two chimeric viruses containing completely separate envelope regions from FMCF98 could both induce neurological disease. Clinical signs caused by both neurovirulent chimeras appeared to be indistinguishable from those caused by FMCF98, although the incubation periods were longer. One neurovirulence determinant mapped to the N-terminal portion of gp7O, which contains the VRA and VRB receptor-binding regions, while the other determinant mapped downstream of both of the variable regions. Western blot (immunoblot) analyses and immunohistochemical staining of tissue sections indicated that the variations in neurovirulence of these viruses could not be explained by differences in either the quantitative level or the location of virus expression in the brain.