Effect of nucleopolyhedrovirus infection on antibacterial activity and antibacterial peptide production in larvae of the silkworm, Bombyx mori (Lepidoptera: Bombycidae)

Abstract

Effects of nucleopolyhedrovirus (NPV) on antibacterial peptide gene expression and peptide production in Bombyx mori larvae remain unclear. Antibacterial activity was first examined with the hemolymph from B. mori larvae infected with B. mori NPV (BmNPV) and injected with bacterial lipopolysaccharide (LPS) 72 h postinfection (p.i.). No antibacterial activity was detected. Expression of four antibacterial peptide genes were next analyzed in B. mori larvae infected with BmNPV. Although antibacterial peptide genes were activated upon injection of LPS in the larvae infected with BmNPV, none were triggered by BmNPV alone. Production of the antibacterial peptide cecropin B was next examined by Western blotting. Cecropin B was not detected from 72 to 96 h p.i. The absence of cecropin B was coincident with an increase in BmNPV cysteine proteinase (CP) activity. Cecropin B production was confirmed at 96 h p.i. in the larvae infected with CP-deficient BmNPV, indicating that the antibacterial peptides are degraded by BmNPV-CP at a very late stage of viral infection. These results suggest that BmNPV neither activates nor suppresses gene expression of antibacterial peptides and that absence of antibacterial activity is due to BmNPV-CP in B. mori larvae.