| Summary: | <i>Gallid alphaherpesvirus</i> 2 (GaHV-2), commonly known as Marek’s disease virus type 1 (MDV-1), is an oncogenic avian alphaherpesvirus, and along with its close relatives—<i>Gallid alphaherpesvirus</i> 3 (GaHV-3) or MDV-2 and <i>Meleagrid alphaherpesvirus</i> 1 (MeHV-1) or turkey herpesvirus (HVT)—belongs to the <i>Mardivirus</i> genus. We and others previously showed that MDV-1 U<sub>S</sub>3 protein kinase plays an important role in viral replication and pathogenesis, which could be partially compensated by MDV-2 and HVT U<sub>S</sub>3. In this study, we further studied the differential roles of MDV-1, MDV-2 and HVT U<sub>S</sub>3 in regulating viral gene expression and replication. Our results showed that MDV-2 and HVT U<sub>S</sub>3 could differentially compensate MDV-1 U<sub>S</sub>3 regulation of viral gene expression in vitro. MDV-2 and HVT U<sub>S</sub>3 could also partially rescue the replication deficiency of MDV-1 U<sub>S</sub>3 null virus in the spleen and thymus, as determined by immunohistochemistry analysis of MDV-1 pp38 protein. Importantly, using immunohistochemistry and dual immunofluorescence assays, we found that MDV-2 U<sub>S</sub>3, but not HVT U<sub>S</sub>3, fully compensated MDV-1 U<sub>S</sub>3 regulation of MDV-1 replication in bursal B lymphocytes. In conclusion, our study provides the first comparative analysis of U<sub>S</sub>3 from MDV-1, MDV-2 and HVT in regulating viral gene expression in cell culture and MDV-1 replication in lymphocytes.
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