Recombinant, chimaeric live, attenuated vaccine (ChimeriVax (TM)) incorporating the envelope genes of Japanese encephalitis (SA14-14-2) virus and thecapsid and nonstructural genes of yellow fever (17D) virus is safe, immunogenic and protective in nonhuman primates

Yellow fever 17D virus, a safe and effective live, attenuated vaccine, was used as a vector for genes encoding the protective antigenic determinants o f a heterologous member of the genus Flavivirus, Japanese encephalitis (JE) virus, the leading cause of acute viral central nervous system infection a nd death throughout Asia. The viral envelope (prM and E) genes of a full-le ngth 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 Ve ro cells. The progeny virus (named 'ChimeriVax(TM)JE'), was used to define safety after intracerebral (IC) inoculation of rhesus monkeys. Monkeys (N = 3) inoculated with a high dose (6.6 log(10) 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 log(10) pfu) of commercial YF 17D vaccine had slightly higher lesion scores. To develop a lethal monkey m odel of JE for vaccine protection tests, we inoculated groups of monkeys IC or intranasally (IN) with a JE virus strain found to be highly neurovirule nt and neuroinvasive for mice. Monkeys inoculated IC, but not IN, developed severe encephalitis after an incubation period of 8-13 days. The ChimeriVa x(TM)-JE virus was passed in a cell line acceptable for human use (diploid fetal rhesus lung) and 4.3 or 5.3 log(10) pfu were inoculated into groups o f 3 monkeys by the subcutaneous route. All 6 animals developed brief viremi as (peak titer < 2.0 log(10) pfu/ml) and subsequently had anti-JE but no ye llow fever neutralizing antibodies, On day 64, the monkeys were challenged IC with 5.5 log(10) pfu of virulent JE virus. The immunized animals had no detectable viremia post-challenge, whereas 4 unimmunized controls became vi remic. Only 1 of 6 (17%) vaccinated monkeys but 4 of 4 (100%) unvaccinated controls developed encephalitis. Histopathological examination 30 days afte r challenge confirmed that the protected, immunized animals had no or minim al evidence of encephalitis, These data demonstrated the ability of the Chi meriVax(TM)-JE: to induce a rapid humoral immune response and to protect ag ainst 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 histopatho logical scoring system used for the WHO yellow fever monkey neurovirulence test will be applicable to control testing of chimaeric seed viruses and va ccines. (C) 1999 Elsevier Science Ltd. All rights reserved.