Antigenic Variation among 173 Strains of Type 3 Poliovirus Isolated in Finland during the 1984 to 1985 Outbreak

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Abstract
Antigenic properties of 128 clinical type 3 poliovirus isolates of the 1984 to 1985 Finland outbreak from 95 persons and 45 strains from sewage water specimens were evaluated using five neutralizing monoclonal antibodies (MAbs) directed against an antigenic site (designated site 1) on VP1 at amino acids 89 to 100. All five MAbs neutralized the type 3 poliovirus strains used in the vaccines, P3/Saukett and P3/Sabin, but none of them neutralized the prototype strain of the outbreak (P3/Finland/23174/84). Forty-six percent of the clinical isolates resembled the prototype strain (class A) while the rest of the isolates were neutralized by one or more of the MAbs (classes B to D). Although an antigenic drift from A to one of the other classes was observed in sequential specimens from several individuals, no clear-cut overall change in the class distribution was found within the 3 months time span of the outbreak. Homogeneous virus populations were isolated from the sewage specimens using a microtitre endpoint dilution method. The last positive sewage specimens which were obtained in January to February 1985 still had a majority of the class A strain. Some of the clinical isolates were also tested using MAbs directed against distinct antigenic sites. These studies showed that strains that gave the same pattern of reactivity with site 1 MAbs could be differentiated using antibodies directed against other sites. Fifteen strains belonging to different antigenic subclasses were subjected to partial RNA sequencing of the genome region coding for antigenic site 1. The antigenic variation was usually, but not always associated with corresponding amino acid substitutions in antigenic site 1. These results indicate that the antigenic sites of type 3 poliovirus vary extensively within a given outbreak and even during replication in a given host. This variation may have both pathogenetic and epidemiological significance.