African horsesickness virus (AHSV) is a virus species in the genus Orbivirus of the family Reoviridae. There are nine serotypes of AHSV showing different levels of cross neutralization. AHSV is transmitted by species of Culicoides biting midges and causes African Horsesickness (AHS) in equids with a mortality rate of up to 95% in naïve horses. AHS has become a serious threat for countries outside Africa, since endemic Culicoides species in moderate climate conditions appear to be competent vectors for the related bluetongue virus (BTV). To control AHS, live-attenuated vaccines (LAVs) are used in Africa. We used reverse genetics to generate ‘synthetic' reassortants of AHSV for all nine serotypes by exchange of genome segment 2 (Seg-2). This segment encodes VP2 which is the serotype determining protein and the dominant target for neutralizing antibodies. Single Seg-2 AHSV reassortants showed similar cytopathogenic effect in mammalian cells, but displayed different growth kinetics. Reverse genetics for AHSV was also used to study Seg-10 expressing NS3/NS3a proteins. We demonstrated that NS3/NS3a proteins are not essential for AHSV replication in vitro. NS3/NS3a of AHSV is however involved in cytopathogenic effect in mammalian cells, and is very important for virus release from cultured insect cells in particular. Similar to the concept of BT Disabled Infectious Single Animal (DISA) vaccine platform, an AHS DISA vaccine platform lacking NS3/NS3a expression was developed. Using Seg-2[lsqb]VP2[rsqb] exchange we will be able to develop AHS DISA vaccine candidates for all current AHSV serotypes
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