A highly attenuated strain of Japanese encephalitis virus induces a protective immune response in mice

A pair of virulent (RP-9) and attenuated (RP-2ms) mutants of Japanese encephalitis virus (JEV) were generated from a Taiwanese isolate NT109. The mutants differed in several aspects in vitro and in vivo, RP-2ms exhibited smaller plaque than RP-9 on BHK-21 cells, and when intracerebrally injected, RP-2ms was much less neurovirulent than RP-9. As peripherally inoculated, RP-2ms lost neuroinvasiveness while RP-9 penetrated blood-brain barrier, replicated in mouse brain, and killed all the mice. Single RP-2ms immunization completely protected C3H and ICR mice from a lethal challenge with RP-9; the sera from such mice contained antibodies against JEV envelope and nonstructural 1 proteins, indicating RP-2ms had replicated in the mice. Neutralizing activity against NT109 in such sera was further demonstrated by plaque reduction neutralization test. In addition, significant lymphoproliferation was detected in spleen cells from the RP-2ms-immunized mice, and cytotoxic activity in these cells specific for the MHC-matched, JEV-infected cells, but not mock cells, was also observed. Altogether, these results demonstrate that RP-2ms, a highly attenuated JEV strain, can induce a protective immunity in mice.