Yellow fever 17D virus, a safe and effective live,
attenuated vaccine, was used as a vector for genes encoding the protective
antigenic determinants of a heterologous member of the genus Flavivirus,
Japanese encephalitis (JE) virus, the leading cause of acute viral
central nervous system infection and death throughout Asia. The viral envelope (prM and E) genes of a full-length
cDNA clone of YF 17D virus were replaced with the corresponding genes of JE SA14-14-2, a strain licensed as a live,
attenuated vaccine in China. Full-length
RNA transcripts of the YF/JE chimaera were used to transfect Vero cells. The progeny virus (named '
ChimeriVax-JE'), was used to define safety after intracerebral (i.c.) inoculation of rhesus monkeys. Monkeys (N = 3) inoculated with a high dose (6.6 log10 pfu) developed a brief
viremia, showed no signs of illness, developed high titers of anti-JE
neutralizing antibody, and had minimal brain and spinal cord lesion scores according to criteria specified in the WHO monkey neurovirulence test. A control group of 3 monkeys that received a lower dose (4.2 log10 pfu) of commercial YF 17D
vaccine had slightly higher lesion scores. To develop a lethal monkey model of JE for
vaccine protection tests, we inoculated groups of monkeys i.c. or intranasally (i.n.) with a JE virus strain found to be highly neurovirulent and neuroinvasive for mice. Monkeys inoculated i.c., but not i.n., developed severe
encephalitis after an incubation period of 8-13 days. The
ChimeriVax-JE virus was passed in a cell line acceptable for human use (diploid fetal rhesus lung) and 4.3 or 5.3 log10 pfu were inoculated into groups of 3 monkeys by the subcutaneous route. All 6 animals developed brief
viremias (peak titer < 2.0 log10 pfu/ml) and subsequently had anti-JE but no
yellow fever neutralizing antibodies. On day 64, the monkeys were challenged i.c. with 5.5 log10 pfu of virulent JE virus. The immunized animals had no detectable
viremia post-challenge, whereas 4 unimmunized controls became viremic. Only 1 of 6 (17%) vaccinated monkeys but 4 of 4 (100%) unvaccinated controls developed
encephalitis. Histopathological examination 30 days after challenge confirmed that the protected, immunized animals had no or minimal evidence of
encephalitis. These data demonstrated the ability of the
ChimeriVax-JE to induce a rapid humoral immune response and to protect against a very severe, direct intracerebral virus challenge. Target areas of neuronal damage and
inflammation in monkeys infected IC with wild-type JE, the chimaeric virus and YF 17D were similar, indicating that the histopathological scoring system used for the WHO
yellow fever monkey neurovirulence test will be applicable to control testing of chimaeric seed viruses and
vaccines.