No abstract

Several clinical varicella-zoster virus isolates obtained during testing of a live varicella vaccine had DNA restriction fragment patterns resembling neither vaccine nor wild-type virus [Gelb et al., J Infect. Dis. 155, 633-640, 1987]. One explanation for these isolates was recombination in vivo. To determine if such recombination is likely, two strains of varicella-zoster virus, distinguishable by restriction endonuclease fragment size differences (wild-type strain EF and the OKA vaccine strain), were grown together in tissue culture. After three passages, the mixed infection virus was plaque-purified. DNA from about 13% of the plaque-purified isolates had one or more BglI fragments found in neither parental virus. Hybridization studies showed that isolates containing one of the new BglI fragments were recombinants of the two parental strains. The BglI restriction fragment pattern of these recombinants resembled those of the unusual varicella isolates from individuals either vaccinated with the live attenuated OKA varicella vaccine and later exposed to natural varicella, or simultaneously exposed to both a recent recipient of the vaccine and natural varicella.

The Oka vaccine strains of varicella-zoster virus (VZV) have a significantly different BgII DNA restriction pattern from that of American wild-type isolates of VZV. This difference consists primarily of an additional BgII site, which lies within the BamHI "D" fragment. In conjunction with a study of the efficacy of an experimental Merck/Oka VZV vaccine, the area of the genome from which the most marked restriction pattern alteration arises was studied more closely to determine if there are other significant differences between the Oka strains and American wild-type strains. BamHI "D" fragments from the DNA of the Oka parent strain (the progenitor of the vaccine strain), the RIT/Oka vaccine strain (a derivative of the Oka parent strain), the Merck/Oka vaccine strain, and the EF strain (an American wild type), were submitted to extensive endonuclease digestion studies to ascertain if additional unique restriction sites are present in the Oka parent or vaccine strains. The extra BgII restriction site characteristic of the Merck/Oka vaccine strain is also present in the DNA of the parent virus as well as its derivatives and was therefore not produced by the "attenuation" process. No other novel sites were found in the Oka parent or Oka-derived strains in this section of the genome. The Merck/Oka vaccine strain of VZV, despite its Japanese origin, is therefore quite similar to circulating American varicella-zoster virus strains. Varicella-zoster virus DNA, at least in the area of the BamHI D fragment, also appears to be remarkably stable from strain to strain.

SUMMARYA persistent infection with varicella-zoster virus was established in the Mewo human melanoma cell line. This persistently infected cell line went through periodic crises of virus-induced cell killing and then recovery. Analyses of viral DNA derived from the persistently infected cultures revealed that novel viral nucleic acid rearrangements had been generated. These viral DNA sequences were derived from a specific region of the inverted repeat sequence of the genome flanking the short unique genome segment. The novel DNA was of various lengths, each generated by tandem duplication of an approximately 2760 base pair sub-sequence of the normal viral inverted repeat. These novel sequences were inserted into an otherwise apparently normal genome.