Development of new vaccine strategies against Marek's disease virus with increased virulence

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The aim of this research is molecular characterization of recent isolates of Marek's disease virus (MDV) which can cause vaccine breaks in the field, in order to develop new vaccine strategies against theses very virulent MDV (vvMDV) by using a new MDV vaccine with the modified L-meq gene.Strains of vvMDV were isolated from wild birds such as white-fronted geese or from domestic chickens with vaccine breaks, and their nucleotide sequences were compared with those of known vvMDVs. The nucleotide sequences of the meq gene (a candidate oncogene of MDV) of recent isolates of MDVs in Japan showed high homologies with those of known vvMDVs, but some mutations previously unidentified were also found in these isolates. In addition, an additional EcoRI site was identified in the glycoprotein B gene of an isolate, strain Hokkaido, while a small deletion in the glycoprotein L gene, which had been frequently observed in vvMDV, was found in several isolates from wild birds and from chickens. In the … More next experiment, we analyzed the effects of these mutations on the transcriptional activation by the meq gene product, MEQ. MDV strains with increased virulence showed higher transcriptional activation by their MEQ, and mutations in the transcriptional activation domain of MEQ could be a determinant for the virulence of MDV strains. However, MEQ of strain Hokkaido, with newly identified mutations, did not display higher transcriptional activation by their MEQ. The L-meq gene product (L-MEQ), present in vaccine strains of apathogenic MDV, was shown to suppress both transcriptional activation by MEQ and replication in vitro of vvMDV, suggesting important roles of L-MEQ on the maintenance of latent phase. We also identified a new transcription variant of the meq gene, termed Δmeq, whose expression is up-regulated in Marek's disease tumor cells when apoptosis is induced. Δmeq inhibited the transcriptional activation by MEQ and L-MEQ, showing that Δmeq could play roles as a negative regulator of MEQ on apoptosis or reactivation of MDV from latent phase. Thus, Δmeq can be used to develop a new vaccine which induces programmed cell death only in transformed cells by vvMDV. Less