The French neurotropic vaccine strain of yellow fever virus accumulates mutations slowly during passage in cell culture

This study of the yellow fever French neurotropic vaccine strain from the I nstitut Pasteur (FNV-IP) demonstrates that this viral genome is not as stab le as that of the 17D-204 vaccine virus. FNV-IP was plaque-purified three t imes and then passaged eight times in Vero cells. Viral populations from th e second and eighth passage post purification were sequenced and compared t o the published sequences of FNV-IP. The passage-2 viral population had 31 nucleotide and nine amino acid changes compared to the parental virus while the passage-8 virus had six additional nucleotide changes encoding a singl e amino acid substitution. The plaque-purified virus also had two sequence deletions in the 3'-noncoding region. The plaque purification resulted in s election of a passage-2 virus that had a mouse LD50 of 20 pfu/ml, 67-fold g reater than parental FNV-IP which had an LD50 of 0.3 pfu/ml. Subsequent pas sage in Vero cells resulted in a passage-8 virus which had increased neurov irulence with an LD,, of 3.2 pfu/ml. The only amino acid difference between the passage-2 and passage-8 viruses was at amino acid 638 of NS5 which lie s within domain V of the RNA-dependent-RNA polymerase. Overall, these data indicate that FNV-IP virus has an inherently less stable genome than 17D va ccine virus and a variable viral population. (C) 2000 Elsevier Science B.V. All rights reserved.