Marek's disease virus-encoded microRNA-M6-5p facilitates viral latent infection by targeting histone demethylase KDM2B

Marek's disease virus (MDV), a highly contagious and oncogenic avian alphaherpesvirus, establishes a latent infection primarily in CD4+ T cells. Latent infec tions are necessary for both persistent lifelong MDV infection and viral tumorigene sis. MicroRNAs (miRNAs) play critical roles as post-transcriptional regulators of viral infections. However, the role of miRNAs in regulating MDV latency remains unclear. In this study, we found that an MDV-encoded miRNA, miR-M6-5p, inhibited viral lytic replication in vitro by functional screening and that infection with an MDV mutant lacking miR-M6-5p resulted in impaired MDV latency, proliferation, and tumor formation in vivo. Importantly, we identified lysine-specific demethylase 2b (KDM2B), an important epigenetic factor, as a target of miR-M6-5p. Furthermore, KDM2B knockdown increased the level of the transcriptionally repressive histone mark H3K27me3 on the key lytic gene pp38 promoter, accompanied by suppression of pp38 expression and reduced latentto-lytic switch in MDV-latently infected cells, while treatment of cells with H3K27me3 inhibitors (GSK126 and Tazemetostat) markedly promoted the expression of pp38 and MDV reactivation from latency. Thus, miR-M6-5p facilitates MDV latency by epigenet ically suppressing pp38 expression by targeting KDM2B. These findings unravel the mechanism by which a virus-encoded miRNA plays a critical role in the regulation of latent MDV infection.