THE GENOMIC SEQUENCE OF DEFECTIVE INTERFERING SEMLIKI-FOREST-VIRUS (SFV) DETERMINES ITS ABILITY TO BE REPLICATED IN MOUSE-BRAIN AND TO PROTECT AGAINST A LETHAL SFV INFECTION IN-VIVO

We have recently cloned and sequenced two genomes of defective interfe ring (DI) Semliki Forest virus (SFV), DI-6 (2146 nt), and DI-19 (1244 nt). These are similar in that both contain two large central deletion s (encompassing the 5' part of the nsP1 gene and the 3' part of the ns P2 gene and all of the structural genes), and all the sequence of the latter is represented in the genome of SFV DI-6. RNA was transcribed f rom both and transfected into SN-infected BHK-21 cells. RT-PCR analysi s of tissue culture fluid harvested 18 h after transfection suggested that SFV DI virions had been rescued from the cloned genomes. Unlike t he genomes of noncloned DI SN, these genomes bred true for at least 7 serial passages. Cloned DI-6 and DI-19 Viruses interfered to a similar extent with the multiplication of SFV in cultured cells, but only DI- 19 protected mice from a lethal intranasal dose of SFV. Further invest igation by RT-PCR analysis showed that DI-19 but not DI-6 genomes were replicated in mouse brain after direct intracerebral injection of DI virus together with an excess of infectious helper SN. Thus the replic ation and hence antiviral activity of two closely related DI SN genome s appears to be exquisitely sequence specific and cell specific. These findings mark a significant step on the way to using DI genomes as an tivirals and also may explain why so few animal-protecting DI viruses have been identified. (C) 1998 Academic Press.