Monath TP, Soike K, Levenbook I, Zhang ZX, Arroyo J, Delagrave S, Myers G,
Barrett AD, Shope RE, Ratterree M, Chambers TJ, Guirakhoo F
OraVax Inc., Cambridge, MA 02139, USA. [email protected]
Vaccine 1999 Apr 9;17(15-16):1869-82

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.