Molecular and biological changes associated with HeLa cell attenuation of wild-type yellow fever virus

Six passages of the mosquito-borne flavivirus yellow fever (YF) wild-type s train Asibi in HeLa cells attenuated the virus for monkeys and newborn mice and resulted in loss of mosquito competence. Attenuation after the passage in HeLa cells was not unique to YF virus strain Asibi as demonstrated by t he HeLa passage attenuation of wild-type YF virus strain French viscerotrop ic virus and YF vaccine virus 17D-204 for newborn mice. In contrast, wild-t ype strain Dakar 1279 and the French neurotropic vaccine virus remained vir ulent for newborn mice after six passages in HeLa cells. Thus not all strai ns of YF virus can be attenuated by passage in HeLa cells. Attenuation of Y F virus strains Asibi and French viscerotropic virus was accompanied by alt erations in the antigenic and biological properties of the viruses, includi ng changes to envelope protein epitopes, Attenuation for newborn mice was c oincidental with the acquisition by the HeLa-passaged viruses of the vaccin e-specific envelope protein epitope recognized by monoclonal antibody H5. T his suggests that this conformational change may play a role in the attenua tion process. Wild-type Dakar 1279, which remained virulent for newborn mic e after passage in HeLa cells, retained its wild-type antigenic character. The genome of Asibi HeLa p6 virus differed from wild-type Asibi virus by 29 nucleotides that encoded 10 amino acid substitutions. 5 in the envelope pr otein, 1 in NS2A, 3 in NS4B, and 1 in NS5. The substitution at NS4B-95 is s een in three different attenuation processes of wild-type YF virus, leading us to speculate that it is involved in the attenuation of virulence of wil d-type strain Asibi.