[Persistent virus in the silkworm cell lines: Bombyx mori latent virus]

Bombyx mori latent virus (BmLV) is a positive, single-stranded insect RNA virus with a close relationship to plant tymoviruses and currently classified as an "unclassified" tymovirus. BmLV is accumulated at extremely high levels only in cell lines derived from the silkworm, Bombyx mori, but it does not lead to lethality and establishes persistent infections. It was unknown whether BmLV affects the Baculovirus Expression Vector System using Bombyx mori nucleopolyhedrovirus, and how BmLV replicates and establishes persistent infections in insect cell lines. In this review, I introduce the discovery of BmLV, the establishment of virus-free cultured cells and the safety aspect of this virus. I also describe that two distinct small RNA-mediated pathways maintain the virus level in BmLV-infected cells, thereby allowing the virus to establish persistent infection. Virus-derived small interfering RNAs (vsiRNAs) and PIWI-interacting RNAs (vpiRNAs) are both produced as the BmLV infection progressed. We revealed that while siRNA pathway functions in both acute and persistent infection of BmLV, piRNA pathway functions only in the persistent infection of this virus.