Abstract
Bluetongue (BT) is a disease of ruminants caused by the bluetongue virus (BTV) transmitted by bites of Culicoides midges. Bluetongue has a worldwide prevalence and mortality in sheep varies from 0 to 30%. There are at least 27 BTV serotypes showing no or little cross protection. In 2006, BTV has
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been reported in north-western Europe for the first time, and has caused a huge outbreak with large economic losses. BTV (family Reoviridae, genus Orbivirus) is a non-enveloped virus with a ten-segmented dsRNA genome encoding seven viral proteins (VP1-7) and four non-structural (NS1-4) proteins. VP2 is the outer capsid protein, and is the serotype specific target for neutralizing antibodies. NS3 and its N-terminal truncated form NS3a (Seg-10) enable virus release from infected cells. Currently, live-attenuated and inactivated BT vaccines have been marketed, but these have several shortcomings. Therefore there is a need for next-generation types of BT vaccines with improved vaccine profile. Here, such a novel vaccine platform based on BTV without NS3/NS3a expression is described. BTV based on lab adapted strain BTV1, vaccine related BTV6, and virulent BTV8 were equipped with a deletion in Seg-10 abolishing NS3/NS3a expression and with Seg-2 encoding serotype determining VP2 from serotype 8. All three vaccine candidates were not virulent in sheep and did not show viremia, whereas seroconversion was dependent on replication of vaccine virus. This suggests that BTV without NS3/NS3a replicates only locally. Sheep were completely protected against infection of virulent BTV8. In a second efficacy experiment in sheep, complete serotype specific protection was demonstrated at nine weeks after booster vaccination. A competitive ELISA based on NS3 antibodies enables the differentiation of infected from vaccinated animals (DIVA). Uptake of the NS3/NS3a knockout vaccine candidate by midges is highly unlikely, due to the absence of viremia after vaccination. This vaccine candidate has therefore been named the Disabled Infectious, Single Animal (DISA) vaccine. Additionally, replication of BT DISA vaccine in midges is abolished as shown after injection of Culicoides sonorensis midges. The BT DISA vaccine platform was also adapted for other BTV serotypes by exchange of Seg-2 (VP2) or by use of chimeric Seg-2 originating from two different serotypes. The latter showed a broader application of the vaccine platform and neutralizing responses against both ancestor serotypes. In conclusion, BT DISA vaccine is effective, safe, enables DIVA, and can be used for many serotypes by exchange of VP2.
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