Abstract
Bluetongue (BT) is a severe and economically important disease of ruminants that is widely distributed around the world, caused by the bluetongue virus (BTV). More than 28 different BTV serotypes have been identified in serum neutralisation tests (SNT), which, along with geographic variants (topotypes) within each serotype, reflect differences in BTV outer-capsid protein VP2. VP2 is the primary target for neutralising antibodies, although the basis for cross-reactions and serological variations between and within BTV serotypes is poorly understood. Recombinant BTV VP2 proteins (rVP2) were expressed in Nicotiana benthamiana, based on sequence data for isolates of thirteen BTV serotypes (primarily from Europe), including three ‘novel’ serotypes (BTV-25, -26 and -27) and alternative topotypes of four serotypes. Cross-reactions within and between these viruses were explored using rabbit anti-rVP2 sera and post BTV-infection sheep reference-antisera, in I-ELISA (with rVP2 target antigens) and SNT (with reference strains of BTV-1 to -24, -26 and -27). Strong reactions were generally detected with homologous rVP2 proteins or virus strains/serotypes. The sheep antisera were largely serotype-specific in SNT, but more cross-reactive by ELISA. Rabbit antisera were more cross-reactive in SNT, and showed widespread, high titre cross-reactions against homologous and heterologous rVP2 proteins in ELISA. Results were analysed and visualised by antigenic cartography, showing closer relationships in some, but not all cases, between VP2 topotypes within the same serotype, and between serotypes belonging to the same ‘VP2 nucleotype’.
Highlights
Bluetongue virus is the ‘type’ species and most intensively studied member of the genus Orbivirus, within the family Reoviridae, order Reovirales [1,2,3,4]
The neutralising antibodies (nAbs) detected in the rabbit and sheep antisera, were more serotype-specific, in the sheep reference antisera that have been used for many years to identify the serotype of novel bluetongue virus (BTV) field isolates
This difference may again reflect the use of more recent BTV strains from Europe and elsewhere, as a source of the Seg-2 sequences used to generate the Recombinant BTV VP2 proteins (rVP2) proteins, while the sheep reference sera were generated by infections with the same reference strains used in the serum neutralisation tests (SNT)
Summary
Bluetongue virus is the ‘type’ species and most intensively studied member of the genus Orbivirus, within the family Reoviridae, order Reovirales [1,2,3,4]. Some of the most recently discovered BTV serotypes [9,10,11,12,13] do not appear to infect midges and are thought to be transmitted by direct contact between individual hosts [7,14]. Bluetongue has spread into new habitats, including 12 serotypes detected in Europe since 1998. This movement has been linked to increased human travel and trade, and the effects of climate change on vector-insect activity and distribution in the region [15,16,17]
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