The bluetongue virus (BTV) core particle contains 2 major polypeptides, P3 and P7, and is surrounded by an outer capsid layer that is composed of the 2 major polypeptides, P2 and P5. Analysis of the immune precipitates from soluble ¹⁴C-labelled BTV polypeptides and hyper-immune rabbit and guinea-pig sera indicated that polypeptide P2 precipitates only with homologous BTV sera.
This would indicate that P2 is the main determinant of serotype specificity. It was also found that in sheep infected with BTV the P2-precipitating antibodies in the serum correlate with the neutralizing antibody titres, whereas the appearance and subsequent decline of P7-precipitating antibodies correspond well with those of the complement fixing antibodies. This suggests that BTV group specificity, as measured by a complement fixation test, is determined by the core protein P7. This result was supported by the observation that mouse ascitic fluid, which contains a high titre of BTV specific complement fixing antibodies and a very low titre of neutralizing antibodies, contains almost exclusively antibodies that precipitate P7.
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