Analysis and phylogenetic comparisons of full-length VP2 genes of the 24 bluetongue virus serotypes

Abstract

The outer capsid protein VP2 of Bluetongue virus (BTV) is a target for the protective immune response generated by the mammalian host. VP2 contains the majority of epitopes that are recognized by neutralizing antibodies and is therefore also the primary determinant of BTV serotype. Full-length cDNA copies of genome segment 2 (Seg-2, which encodes VP2) from the reference strains of each of the 24 BTV serotypes were synthesized, cloned and sequenced. This represents the first complete set of full-length BTV VP2 genes (from the 24 serotypes) that has been analysed. Each Seg-2 has a single open reading frame, with short inverted repeats adjacent to conserved terminal hexanucleotide sequences. These data demonstrated overall inter-serotype variations in Seg-2 of 29 % (BTV-8 and BTV-18) to 59 % (BTV-16 and BTV-22), while the deduced amino acid sequence of VP2 varied from 22.4 % (BTV-4 and BTV-20) to 73 % (BTV-6 and BTV-22). Ten distinct Seg-2 lineages (nucleotypes) were detected, with greatest sequence similarities between those serotypes that had previously been reported as serologically ‘related’. Fewer similarities were observed between different serotypes in regions of VP2 that have been reported as antigenically important, suggesting that they may play a role in the neutralizing antibody response. The data presented form an initial basis for BTV serotype identification by sequence analyses and comparison of Seg-2, and for development of molecular diagnostic assays for individual BTV serotypes (by RT-PCR).