Glycoproteins E and I of Marek's disease virus serotype 1 are essential for virus growth in cultured cells

The role of glycoprotein E (gE) and gI of Marek's disease virus serotype 1 (MDV-1) for growth in cultured cells was Investigated. MDV-1 mutants lackin g either gE (20 Delta gE), gI (20 Delta gI), or both gE and gI (20 Delta gE 1) were constructed by recE/T-mediated mutagenesis of a recently establishe d infectious bacterial artificial chromosome (BAC) clone of MDV-1 (D. Schum acher, B. K. Tischer, W. Fuchs, and N. Osterrieder, J. Virol. 74:11088-1109 8, 2000). Deletion of either gE or gI, which form a complex in MDV-1-infect ed cells, resulted in the production of virus progeny that were unable to s pread from cell to cell in either chicken embryo fibroblasts or quail muscl e cells. This was reflected by the absence of virus plaques and the detecti on of only single infected cells after transfection, even after coseeding o f transfected cells with uninfected cells. In contrast, growth of rescuant viruses, in which the deleted glycoprotein genes were reinserted by homolog ous recombination, was indistinguishable from that of parental BAC20 virus. In addition, the 20 Delta gE mutant virus was able to spread from cell to cell when cotransfected into chicken embryo fibroblasts with an expression plasmid encoding MDV-1 gE, and the 20 Delta gI mutant virus exhibited cell- to-cell spread capability after cotransfection with a gI expression plasmid . The 20 Delta gEI mutant virus, however, was not able to spread in the pre sence of either a gE or gl expression plasmid, and only single infected cel ls were detected by indirect immunofluorescence. The results reported here demonstrate for the first time that both gE and gI are absolutely essential for cell-to-cell spread of a member of the Alphaherpesvirinae.