High incidence of meningeal infiltration by leukemic cells after infection of chimeric virus between neuropathogenic and non-neuropathogenic retroviruses.

Friend murine leukemia virus (F-MuLV) clone A8, previously shown to cause spongiform degeneration in rat brain, induced leukemia within 10 weeks after infection when inoculated into newborn rats. The chimeric virus Rec2, which contains the pol and env genes of 57 virus on the background of A8 and does not cause spongiform degeneration in the central nervous system (CNS), induced leukemic cell infiltration of the CNS, mainly of the meningeal region, in 58.3% of infected rats. In contrast, A8 induced little or no leukemic cell infiltration of the CNS. Other chimeric viruses containing the LTR and the 5' half of the 5' leader sequence of A8 induced aggressive leukemia, and after infection of these viruses, leukemic cell infiltration of the CNS was only observed in less than 20.0% of the rats. These results indicate that the fragment containing the LTR and the 5' half of the 5' leader sequence of A8 is essential for induction of aggressive leukemia in rats but is not sufficient to cause CNS infiltration. We found that leukemic cell infiltration of the CNS is dependent on the sequence of the virus.