Rta of murine gammaherpesvirus 68 reactivates the complete lytic cycle from latency

Herpesviruses are characterized as having two distinct life cycle phases: l ytic replication and latency. The mechanisms of latency establishment and m aintenance, as well as the switch from latency to lytic replication, are po orly understood. Human gammaherpesviruses, including Epstein-Barr virus (EB V) and human herpesvirus-8 (HHV-8), also known as Kaposi's sarcoma-associat ed herpesvirus (KSHV), are associated with lymphoproliferative diseases and several human tumors, Unfortunately, the lack of cell lines to support eff icient de novo product;ive infection and restricted host ranges of EBV and HHV-8 make it difficult to explore certain important biological questions. Murine gammaherpesvirus 68 (MHV-68, or gamma HV68) can establish de novo ly tic infection in a variety of cell lines and is also able to infect laborat ory mice, offering an ideal model with which to study various aspects of ga mmaherpesvirus infection, Here we describe in vitro studies of the mechanis ms of the switch from latency to lytic replication of MHV-68, An MHV-68 gen e, rta (replication and transcription activator), encoded primarily by open reading frame 50 (ORF50), is homologous to the rta genes of other gammaher pesviruses, including HHV-8 and EBV, HHV-8 and EBV Rta have been shown to p lay central roles in viral reactivation from latency. We first studied the kinetics of HHV-68 rta gene transcription during de novo lytic infection. M HV-68 rta was predominantly expressed as a 2-kb immediate-early transcript. Sequence analysis of MHV-68 rta cDNA revealed that an 866-nucleotide intro n 5' of ORF50 was removed to create the Rta ORF of 583 amino acids. To test the functions of MHV-68 Rta in reactivation, a plasmid expressing Rta was transfected into a latently infected cell line, S11E, which was established from a B-cell lymphoma in an MHV-68-infected mouse. Rta induced expression of viral early and late genes, lytic replication of viral DNA, and product ion of infectious viral particles. We conclude that Rta alone is able to di srupt latency, activate viral lytic replication, and drive the lytic cycle to completion, This study indicates that MHV-68 provides a valuable model f or investigating regulation of the balance between latency and lytic replic ation in vitro and in vivo.