Antigenic fractions of bluetongue virus were separated by ultracentrifugation in Tris-buffered CsCl gradients at pH 6, 7 or 8 and the bluetongue virus polypeptide composition of the bands isolated from these gradients was monitored by polyacrylamide gel slab electrophoresis. The immunological response to these fractions in mice was determined by a haemolytic plaque-forming cell assay, using sheep erythrocytes onto which intact bluetongue virus was adsorbed as lytic indicator cells. Isolated outer layer bluetongue virus polypeptide 2, from gradients at pH 6, and polypeptides 2 and 5, from gradients at pH 7, produced a strong primary JgM plaque-forming cell response. The subviral particles of density 1,39 g.cmֿ ֿֿֿ³ and the bluetongue virus core particles of density 1,42 g.cmֿ ֿ³ also stimulated an IgM response at least as strong as that to intact bluetongue virus of density 1,38 g.cmֿֿ ֿ³. The isolated bluetongue virus fractions therefore appear to maintain their immunogenic integrity as effectively as those of intact bluetongue virus. The pattern of the immune response to bluetongue virus type 4 is similar to that of type 10
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