Antigenic variability of porcine reproductive and respiratory syndrome (PRRS) virus isolates. Influence of virus passage in pig.

In order to study the antigenic variability of porcine reproductive and respiratory syndrome virus (PRRSV), 18 European and Canadian field isolates were analysed with a panel of 15 monoclonal antibodies (MAbs) raised against five different European and one Canadian PRRSV isolates. The antigenic pattern of these isolates was used to infer their phylogenetic relationships. Isolates which had the same reactivity pattern were gathered together so that five antigenic profiles were analysed. The pairwise distances between these groups were defined based on antigenic pattern differences. Two main antigenic groups were obtained, discriminating between the European and the Canadian populations, as illustrated by the great distance observed between European and Canadian isolates (D = 0.5619 +/- 0.0625) compared to the distance between European isolates (D = 0.1524 +/- 0.0735). The distance matrix allowed also the construction of a tree diagram. Bootstrap analysis was performed to test the confidence in the branching. The tree diagram confirmed the distinction between the European and the Canadian PRRSV populations. Antigenic variability between an isolate and its progeny recovered after one or two passages in vivo was examined on six isolates. It was restricted to the GP3 protein of the virus.