Inhibitory role of the Bm8 protein in the propagation of Bombyx mori nucleopolyhedrovirus

Lepidopteran nucleopolyhedroviruses have distinct viral tissue tropisms in host larvae. We previously identified the Bm8 gene of Bombyx mori nucleopolyhedrovirus (BmNPV), the product of which inhibits viral propagation in the middle silk gland (MSG). However, it is unknown whether this inhibitory function of the Bm8 protein is specific to MSGs. Here we generated a Bm8-disrupted recombinant BmNPV expressing green fluorescent protein (GFP) and examined viral propagation in B. mori cultured cells and larvae. We found that Bm8-disrupted BmNPV produced fewer budded viruses and more occlusion bodies (OBs) than the wild-type virus in both cultured cells and larvae. Microscopic observation of OB production and GFP expression revealed that Bm8 disruption accelerated the progression of viral infection in various larval tissues. Furthermore, quantitative reverse transcription-polymerase chain reaction experiments showed that the loss of Bm8 enhanced viral gene expression in BmNPV-infected larval tissues. These results indicate that the Bm8 protein suppresses viral propagation to varying degrees in each larval tissue, which may establish BmNPV tissue tropisms in B. mori larvae.