Animals persistently infected with BVDV are important in the epizootiology of the Bovine Viral Diarrhea (BVD) because they are a permanent source of contamination within a herd. These animals produce large quantities of virus and have, therefore, been proposed as responsible for generating antigenic variability. However, limited studies have failed to detect antigenic or genetic changes in viruses isolated at different time from persistently infected animals. One hypothesis to account for this stability is that the immunotolerance is accompanied by a selection against antigenic change. The presence of an immunotolerant persistently infected (IPI) animal in a herd would in turn lead to herd specific strains. To verify this hypothesis, we compared 17 BVDV strains isolated from IPI animals from 3 herds of Eastern Belgium. The comparison was based on the sequence of a 389 bp fragment of E2--a gene encoding for a highly variable glycoprotein. Sequences were strongly conserved within herds but were quite different between herds, indicating that BVDV herd-specific strains do exist and are associated with the presence of IPI animals.
The putative gene encoding the cytopathic bovine viral diarrhoea virus (BVDV) Osloss strain p80 protein was amplified by PCR and inserted into a T7 promoterbased vector for expression in Escherichia coli. Bacterial expression led to cytoplasmic insoluble inclusion bodies which were denatured by urea treatment and renatured by dialysis. Rabbit antisera were raised against this p80 recombinant antigen and assayed for the immunoprecipitation of either pl20 or p80 protein from cytopathic or non-cytopathic BVDV biotype-infected bovine cells. The p80 gene sequence was also integrated into a baculovirus genome for its expression in Spodoptera frugiperda insect cells. The recombinant proteins isolated from bacteria or insect cells showed distinct antigenic properties when analysed by ELISA. Their ability to detect anti-BVDV specific antibodies was examined in a monoclonal antibody-based competitive ELISA performed on a series of field cattle sera. This comparative assay revealed the superiority of the insect cell-mediated expression to mimic the natural BVDV antigen produced by cell culture. The baculovirus/insect cell recombinant antigen gave the highest correlation between the ELISA-detected antibodies and the corresponding virus neutralization data.
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