Varicella-zoster virus gE escape mutant VZV-MSP exhibits an accelerated cell-to-cell spread phenotype in both infected cell cultures and SCID-hu mice

Varicella-zoster virus is considered to have one of the most stable genomes of all human herpesviruses. In 1998, we reported the unanticipated discove ry of a wild-type virus that had lost an immunodominant B-cell epitope on t he gE ectodomain (VZV-MSP): the gE escape mutant virus exhibited an unusual pattern of egress. Further studies have now documented a markedly enhanced cell-to-cell spread by the mutant virus in cell culture. This property was investigated by laser scanning confocal microscopy combined with a softwar e program that allows the measurement of pixel intensity of the fluorescent signal. For this new application of imaging technology, the VZV immediate early protein 62 (IE 62) was selected as the fluoresceinated marker. By 48 h postinfection, the number of IE 62-positive pixels in the VN-MSP-infected culture was nearly fourfold greater than the number of pixels in a culture infected with a low-passage laboratory strain. Titrations by infectious ce nter assays supported the above image analysis data. Confirmatory studies i n the SCID-hu mouse documented that VZV-MSP spread more rapidly than other VZV strains in human fetal skin implants. Generally the cytopathology and v esicle formation produced by other strains at 21 days postinfection were de monstrable with VN-MSP at 14 days. To assess whether additional genes were contributing to the unusual VZV-MSP phenotype, similar to 20 kb of the VZV- MSP genome was sequenced, including ORFs 31 (gB), 37 (gH), 47, 60 (gL), 61, 62 (IE 62), 66, 67 (gl), and 68 (gE). Except for a few polymorphisms, as w ell as the previously discovered mutation within gE, the nucleotide sequenc es within most open reading frames were identical to the prototype VN-Dumas strain. In short, VN-MSP represents a novel variant virus with a distingui shable phenotype demonstrable in both infected cell cultures and SCID-hu mi ce. (C) 2000 Academic Press.