A single-chain antibody fragment that identifies a neutralizing epitope on the 
envelope protein of louping ill and some other tick-borne flaviviruses was 
previously expressed in soluble form from bacteria and shown to be functionally 
active in vitro. To see whether or not the single-chain antibody could bind and 
inactivate infectious virus in vivo, we have used recombinant Sindbis virus as a 
delivery vehicle for intracellular expression of the antibody fragment. The 
variable genes and interchain linker encoding the single-chain antibody were 
cloned into a double subgenomic Sindbis virus expression vector to generate 
recombinant Sindbis virus. Infection with this recombinant Sindbis virus 
provided high-level cytoplasmic expression of the antibody fragment in mammalian 
cells. We demonstrate (i) that the antibody fragment was antigen binding and 
(ii) that louping ill virus infectivity was significantly reduced in the 
presence of intracellular antibody expressed by the superinfecting recombinant 
Sindbis virus.