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A UL47 Gene Deletion Mutant of Bovine Herpesvirus Type 1 Exhibits Impaired Growth in Cell Culture and Lack of Virulence in Cattle

ABSTRACT Tegument protein VP8 encoded by the UL47 gene of bovine herpesvirus type 1 (BHV-1) is the most abundant constituent of mature virions. In the present report, we describe the characterization of UL47 gene-deleted BHV-1 in cultured cells and its natural host. The UL47 deletion mutant exhibited reduced plaque size and more than 100-fold decrease in intracellular and extracellular viral titers in cultured cells. Ultrastructural observations of infected cells showed normal maturation of BHV-1 virions in the absence of VP8. There was no evidence for a change in immediate-early gene activator function of VP16 in the UL47 deletion mutant virus-infected cells, since bovine ICP4 mRNA and protein levels were similar to those in the wild-type and revertant virus-infected cells throughout the course of infection. Whereas VP16, glycoprotein C (gC), gB, and VP5 were expressed to wild-type levels in the UL47 deletion mutant-infected cells, the gD and VP22 protein levels were significantly reduced. The reduction in gD protein was associated with increased turnover of the protein. Furthermore, some of the analyzed early and late proteins were expressed with earlier kinetics in the absence of VP8. Extracellular virions of the UL47 deletion mutant contained reduced amounts of gD, gB, gC, and VP22 but similar amounts of VP16 compared to those of wild-type or revertant virus particles. In addition, the UL47 gene product was indispensable for BHV-1 replication in vivo, since no clinical manifestations or viral shedding were detected in the UL47 deletion mutant-infected calves, and the virus failed to induce significant levels of humoral and cellular immunity.