Infection study of Bombyx mori macula-like virus (BmMLV) using a BmMLV-negative cell line and an infectious cDNA clone.

Previously, a novel macula-like virus was identified from Bombyx mori cultured cell line BmN and termed B. mori macula-like virus (BmMLV). BmMLV encodes a 6.5-kb-long positive, single-strand RNA genome, which contains putative RNA-dependent RNA polymerase (RdRp), coat protein (cp) and p15 genes. In this study, CP expression in several B. mori-derived cell lines was examined by using the CP antibody. Surprisingly, Western blot analysis revealed that all of the cell lines tested have already been infected with BmMLV. To perform reverse genetic studies in BmMLV, a new BmMLV-negative cell line, designated as BmVF from the embryos of B. mori was established. Infection studies showed that BmVF cells were permissive to BmMLV persistent infection. In addition, a full-length infectious cDNA clone of BmMLV, termed pHMLV was developed. Upon transfection of pHMLV into BmMLV-negative BmVF cells, viral CP was detected in both cells and conditioned medium. When the cDNA-derived virus in conditioned medium was inoculated onto BmVF cells, efficient propagation of BmMLV was observed. Collectively, these results indicate that the new BmMLV-negative cell line and the infectious cDNA clone of BmMLV will be useful for elucidation of the mechanism of BmMLV replication and the functional roles of BmMLV genes.