Sequence variation and biological activity of rubella virus isolates.

Haemagglutination (HA) by rubella virus is mediated by the E1 glycoprotein. Rubella isolates which haemagglutinate with different avidity have been characterised. A significant reduction of HA titre at pH 6.0 was observed in one isolate in which isoleucine is substituted for threonine at rubella E1 residue 280. This residue is located in an epitope (EP1) which we have previously identified and shown to bind HA inhibiting (HA1) monoclonal antibodies. The isolates studied are also distinguishable by plaque size but no sequence variations in the immunogenic region of E1 were identified which might account for this difference. No correlation was observed between infectivity and binding affinity of neutralising monoclonal antibodies for different rubella virus strains.