Virus-Encoded MicroRNAs Facilitate Gammaherpesvirus Latency and Pathogenesis In Vivo

microRNAs (miRNAs). It is predicted that these regulatory RNAs facilitate infection by suppressing host target genes involved in a wide range of key cellular pathways. However, the precise contribution that gammaherpesvirus miRNAsmake to viral life cycle and pathogenesis in vivo is unknown. MHV68 infection of mice provides a highly useful system to dissect the function of specific viral elements in the context of both asymptomatic infection and disease. Here, we report (i) analysis of in vitro and in vivo MHV68miRNA expression, (ii) generation of anMHV68miRNAmutant with reduced expression of all 14 pre-miRNA stem-loops, and (iii) comprehensive phenotypic characterization of the miRNAmutant virus in vivo. The profile of MHV68 miRNAs detected in infected cell lines varied with cell type and did not fully recapitulate the profile from cells latently infected in vivo. The miRNAmutant virus, MHV68.Zt6, underwent normal lytic replication in vitro and in vivo, demonstrating that the MHV68 miRNAs are dispensable for acute replication. During chronic infection, MHV68.Zt6 was attenuated for latency establishment, including a specific defect in memory B cells. Finally, MHV68.Zt6 displayed a striking attenuation in the development of lethal pneumonia in mice deficient in IFN-. These data indicate that the MHV68miRNAsmay facilitate virus-driven maturation of infected B cells and implicate the miRNAs as a critical determinant of gammaherpesvirus-associated disease. IMPORTANCE Gammaherpesviruses such as EBV and KSHV are widespread pathogens that establish lifelong infections and are associated with the development of numerous types of diseases, including cancer. Gammaherpesviruses encode many small non