Tick-borne encephalitis virus is the causative agent of tick-borne encephalitis, 
a potentially fatal neurological infection. Tick-borne encephalitis virus 
belongs to the family of flaviviruses and is transmitted by infected ticks. 
Despite the availability of vaccines, approximately 2000-3000 cases of 
tick-borne encephalitis occur annually in Europe for which no curative therapy 
is available. The antiviral effects of RNA mediated interference by small 
interfering RNA (siRNA) was evaluated in cell culture and organotypic 
hippocampal cultures. Langat virus, a flavivirus highly related to Tick-borne 
encephalitis virus exhibits low pathogenicity for humans but retains 
neurovirulence for rodents. Langat virus was used for the establishment of an in 
vitro model of tick-borne encephalitis. We analyzed the efficacy of 19 siRNA 
sequences targeting different regions of the Langat genome to inhibit virus 
replication in the two in vitro systems. The most efficient suppression of virus 
replication was achieved by siRNA sequences targeting structural genes and the 
3' untranslated region. When siRNA was administered to HeLa cells before the 
infection with Langat virus, a 96.5% reduction of viral RNA and more than 98% 
reduction of infectious virus particles was observed on day 6 post infection, 
while treatment after infection decreased the viral replication by more than 
98%. In organotypic hippocampal cultures the replication of Langat virus was 
reduced by 99.7% by siRNA sequence D3. Organotypic hippocampal cultures 
represent a suitable in vitro model to investigate neuronal infection mechanisms 
and treatment strategies in a preserved three-dimensional tissue architecture. 
Our results demonstrate that siRNA is an efficient approach to limit Langat 
virus replication in vitro.