Neuroattenuated bunyavirus variant: Derivation, characterization, and revertant clones

A neuroattenuated variants bunyavirus, designated RFC/25B.5 (B.5), was selected by serial passage of a reassortant clone (RFC virus) of a California serogroup virus in BHK-21 cells, followed by plaque purification of that passaged stock. Based on its virulence index (ratio of PFU/50% lethal dose), clone B5 was over 40,000-fold less virulent than its unpassaged RFC parent after intracerebral (i.c.) inoculation into adult mice. Clone B.5 also exhibited markedly reduced neuroinvasiveness after subcutaneous injection into neonatal mice, although it retained its ability to replicate and kill suckling mice after i.c. injection. A murine neuroblastoma line (NA cells) can be used as an in vitro surrogate for the adult mouse brain, since clone B.5 replicated to at least 1,000-fold-lower titres in NA cells than did several neurovirulent California serogroup viruses. Clone B.5 replicated in BHK-21 cells at 37°C to titers similar to those achieved by other California serogroup viruses but was temperature sensitive (ts) since its replication was markedly restricted at 38.9°C. Ten ts revertant clones of B.5 virus were selected at 38.9°C, and all of them lost their ts phenotype and regained the ability to replicate to high titer in NA cells and to kill adult mice after i.c. injection. Clone B.5 is the first described California serogroup virus which is truly attenuated after i.c. inoculation, and its availability will permit genetic analysis of bunyavirus neurovirulence.