Molecular Analysis of the Glycoprotein C-Negative Phenotype of Attenuated Marek's Disease Virus

High-Level Expression of Marek's Disease Virus Glycoprotein C Is Detrimental to Virus Growth In Vitro

“…One explanation of this effect is that soluble gC in cell culture can bind to an unknown receptor on the surfaces of neighboring cells, which may block interaction of membrane-bound gC that could play a role for cell-to-cell spread of MDV. This interpretation is supported by the fact that gC expression is reduced after only a few passages in vitro of highly virulent MDV strains (73). Irrespective of the mode of action of the secreted form of gC, our findings indicate a strong pressure for gC down regulation in cell culture, although some attenuated and passaged strains, such as CVI988-Rispens, still express significant amounts of gC and yet produce relatively large plaques in cultured cells (B. K. Tischer and N. Osterrieder, unpublished observations).…”

“…We have demonstrated that gC expression is clearly detrimental to virus growth in vitro. Further experiments will focus on the function of gC in chickens to elucidate whether there is a correlation of the level of gC expression and MDV pathogenesis as discussed by other groups (31,73) or whether the reduction of the U L 44 expression in vitro is more a cell culture effect that is independent of virulence. In addition, future studies will aim at dissecting the effects of the soluble form versus the membrane-bound form of the glycoprotein by the introduction of targeted mutations in a predicted common splice acceptor site that would abrogate expression of two secreted gC products that result from two putative spliced mRNAs.…”

“…Besides the observation that expression levels of the U L 44 gene negatively correlate with attenuation of MDV and that gC expression is reduced by serial passage of MDV strains in cell culture, knowledge on the function of gC in highly cell-associated herpesviruses, such as MDV, has been sparse (73). Alignment of the gC amino acid sequence of MDV to that of its HSV-1 counterpart demonstrated that the carboxy-terminal domains of MDV and HSV-1 gC share high similarity, whereas the amino termini are completely different.…”

“…Most of the gC produced by MDV-infected cells is secreted into the supernatant, which has been attributed to unstable membrane anchoring that would be caused by the short predicted cytoplasmic domain (31,73). Preliminary data obtained from reverse transcriptase PCR studies performed in our laboratory, however, indicate alternate splicing of gC that would result in the production of two gC species that lack a transmembrane domain.…”

“…Coinciding with reduced gC production, virulence of MDV decreases. Despite the greatly reduced gC expression levels after passage, no difference in the nucleotide sequence of U L 44 or its promoter region between viruses with low and high gC expression levels were detectable, and the role of viral transcription factors has been discussed in the shutdown of gC expression (73). Although there is an apparent correlation between gC expression and virulence, there is no convincing evidence that gC is actually involved in virulence of MDV, because the glycoprotein has been found to be expressed at higher levels in some avirulent virus strains (4).…”