Characterization of Bombyx mori nucleopolyhedrovirus infection in fat body-derived Bombyx mori cultured cells

Bombyx mori nucleopolyhedrovirus (BmNPV) is known to replicate in many tissues of Bombyx mori larvae. However, the cell lines used for BmNPV research are predominantly derived from B. mori ovaries or early embryos. In the present study, we examined the properties of NIAS-Bm-aff3 (aff3), a cell line that was established from B. mori larval fat body, which is one of the major tissues for BmNPV propagation. aff3 is a floating cell line, and cell adhesion was enhanced following the coating of the culture dish with poly -D-lysine. RT-qPCR assays demonstrated that the expression of germ cell markers, Vasa, Siwi, and BmAgo3, was much lower in aff3 cells as compared to the B. mori ovary-derived cell line BmN-4. Conversely, aff3 cells express an adipocyte marker, Fabp1, at higher levels, indicating that this cell line retains the characteristics of fat body cells. BmNPV infection induces unique cell fusion in aff3 cells, which was also observed following infection with Autographa californica multiple nucleopolyhedrovirus, a virus that does not cause productive infection in B. mori cells. Occlusion bodies (OBs) produced in BmNPV-infected aff3 cells exhibit large cuboidal shapes as compared to those produced in BmN-4 cells. Furthermore, extremely large OBs (similar to 25 mu m in side length) were produced in aff3 cells when infected with a cuboidal polyhedrin mutant. Taking into account these unusual properties, we conclude that aff3 could prove to be a useful resource for conducting baculovirus research.