An unusual plaque variant of rubella virus

Improvement of the specific infectivity of the rubella virus (RUB) infectious clone: determinants of cytopathogenicity induced by RUB map to the nonstructural proteins

A plasmid, Robo102, which contains a cDNA copy of the rubella virus (RUB) genomic RNA from which infectious transcripts can be synthesized in vitro, was recently developed (C. Y. Wang, G. Dominguez, and T. K. Frey, J. Virol. 68:3550-3557, 1994). To increase the specific infectivity of Robo102 transcripts (approximately 5 plaques/10 microg of transcripts), a modified reverse transcription-PCR method was used to amplify nearly 90% of the RUB genome in three fragments, which were then used to replace the corresponding fragments in Robo102. Replacement of a fragment covering nucleotides (nt) 5352 to 9759 of the RUB genome yielded a construct, Robo202, which produced highly infectious transcripts (10(4) plaques/microg), indicating the presence of an unrecognized deleterious mutation (or mutations) in this region of the Robo102 cDNA. Robo102 was based on the w-Therien strain of RUB, which forms opaque plaques in Vero cells, while the PCR replacement fragments were generated from a variant, f-Therien, which produces clear plaques in Vero cells. Although Robo202 contains over 4,000 nt from f-Therien, Robo202 virus produces opaque plaques. However, when the other two PCR fragments amplified from f-Therien (nt 1 to 1723 and nt 2800 to 5352) were introduced into Robo202, the resulting construct, Robo302, yielded transcripts that produced a virus that formed clear plaques. This indicates that the determinants of plaque morphology map to the regions of the genome covered by these two fragments, both of which are in the nonstructural open reading frame. Generation of Robo202/302 chimeras indicated that the most 5' terminal fragment (nt. 1 to 1723) had the greatest effect on plaque morphology. The plaque morphology was correlated with the ability of the viruses to kill infected cells. The only difference at the molecular level detected among the viruses was that the more cytopathic viruses produced more nonstructural proteins than did the less cytopathic viruses. This finding, as well as the mapping of the genetic determinants to the region of the genome encoding these proteins, indicates that the nonstructural proteins can mediate cell killing.

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.

Improved potency assay of rubella vaccine: parameters for plaque formation

Parameters for plaque formation by rubella vaccine strains licensed in Japan were studied. Formation of clear and large plaques on RK13 cells depends on several essential parameters. Plaques differed in morphology among five vaccine strains and the distinctiveness of the plaques was affected by pH of the agar overlay medium during incubation at 35 degrees C. Plaques became progressively larger in size as the concentration of sodium bicarbonate in the agar overlay medium increased from 0.04% to 0.15%, but the contrast of plaques to the background cells decreased markedly. The most distinct plaques of all vaccine strains were formed in the agar overlay medium containing 0.07% of sodium bicarbonate, i.e., pH 6.83, incubated in a humidified atmosphere containing 5% CO2. The number of plaques formed by vaccine strains decreased at 37 degrees C. Vaccine strains other than MEQ11 and TCRB19 formed larger and more contrasted plaques with sharp outline at 35 degrees C than at 32 degrees C. MEQ11 and TCRB19 strains yielded higher infective virus titres at 32 degrees C, but they formed distinct plaques at 35 degrees C and 32 degrees C. For the plaque test, the inoculum volume was another critical factor for obtaining an approximate titre that reflected the absolute titre of the sample. A volume of 0.1 ml was feasible for a well with a diameter of 35 mm.