A cluster of virus-encoded microRNAs accelerates acute systemic Epstein-Barr virus infection but does not significantly enhance virus-induced oncogenesis in vivo.

Over 90% of the adult human population is chronically infected with the Epstein-Barr virus (EBV), an oncogenic herpesvirus. EBV primarily infects naive human B cells and persists latently in memory B cells. Most individuals experience an asymptomatic infection that is effectively controlled by the adaptive immune response. However, EBV-associated lymphomas can develop in immunocompromised individuals. These tumors typically express all nine EBV latent proteins (latency III). Latency III is also associated with the expression of three precursor microRNAs (miRNAs) located within the EBV BHRF1 gene locus. The role of these BHRF1 miRNAs was unclear until recent in vitro studies demonstrated that they cooperate to enhance virus-induced B cell transformation and decrease the antigenic load of virus-infected cells, indicating that the BHRF1 miRNA cluster may serve as a novel therapeutic target for the treatment of latency III EBV-associated malignancies. However, to date, it is not known if BHRF1 miRNAs enhance virus-induced oncogenesis and/or immune evasion of EBV in vivo. To understand the in vivo contribution of the BHRF1 miRNA cluster to EBV infection and EBV-associated tumorigenesis, we monitored EBV infection and assessed tumor formation in humanized mice exposed to wild-type virus and a viral mutant (Δ123) that lacks all three BHRF1 miRNAs. Our results demonstrate that while the BHRF1 miRNAs facilitate the development of acute systemic EBV infection, they do not enhance the overall oncogenic potential of EBV in vivo.