Abstract
Six passages of the mosquito borne flavivirus, yellow fever (YF) wild-type strain Asibi in HeLa cells attenuates the virus for monkeys and newborn mice. Attenuation following passage in HeLa cells was not unique to YF virus strain Asibi as demonstrated by the attenuation of another wild-type YF virus strain, the French viscerotropic virus, and three strains of the mosquito borne West Nile (WN) virus. In comparison, two tick-borne flaviviruses (louping-ill and Langat) and a member of the dengue virus subgroup (dengue-2 virus) remained virulent for mice following six passages in HeLa cells. Attenuation of YF and WN viruses was accompanied by alterations in the antigenic and biological properties of the viruses including changes to surface epitopes, appearence of temperature sensitivity and loss of haemagglutination (HA) ability following six passages in HeLa cells. Some of the antigenic changes have been proposed to have a bearing on the attenuation observed, however, it is believed that loss of HA ability has no function in the attenuation of flavivirus virulence. Moreover, attenuation of YF viruses coincided with the loss of binding of virus with monkey liver cell receptors. The lack of binding of virus to the receptors of the liver may explain the loss of viscerotropism that is observed for attenuated YF virus strains. It is proposed that attenuation of flaviviruses following passage in HeLa cells is dependant on the tropism of the virus. Viruses of low neurotropic potential/invasiveness (YF and WN viruses) are attenuated to significant degrees after very few passages in HeLa cells, whereas viruses of enhanced neurotropism/invasiveness (louping-ill, Langat, and neuroadapted dengue-2 virus) are refractive to attenuation after very few passages in HeLa cells. Therefore, it is suggested that the attenuation of flaviviruses following passage in HeLa cells provides a model system to examine the molecular basis of attenuation of non-neurotropic flaviviruses.