An inhibitor of NF-kB encoded in Cotesia plutella bracovirus inhibits expression of antimicrobial peptides and enhances pathogenicity of Bacillus thuringiensis

Abstract

Upon parasitization by an endoparasitoid wasp, Cotesia plutellae, the diamondback moth, Plutella xylostella, exhibits significant immunosuppression. A bracovirus (CpBV) symbiotic C. plutellae, has been regarded as a main parasitic factor due to acute and persistent expression of various encoded genes. Inhibitor-kB genes (CpBV-IkB) are homologous to cactus gene of Drosophila and are found in CpBV genome. However, their function in parasitism was unknown. Here, we tested hypothesis that CpBV-IkB may interrupt nuclear factor kappa B (NF-kB) to inhibit its translocation into the nucleus, resulting in the suppression of antimicrobial peptide synthesis. A CpBV-IkB was cloned into an expression vector and micro-injected into nonparasitized larvae. The transiently expressed CpBV-IkB in P. xylostella inhibited the expression of hemolin, but did not inhibit the expression of lysozyme or cecropin. This inhibitory activity of CpBV-IkB was more evident in a non-natural host, Spodoptera exigua, where both lysozyme and cecropin were inhibited. A recombinant Autographa californica multiple nucleopolyhedrosis virus (AcMNPV) was constructed by recombining CpBV-IkB gene under an early expression promoter. The budded form of the recombinant virus was injected into the hemocoel, while polyhedral form of the recombinant virus was orally administered to the P. xylostella larvae. In both treatments, the expression of CpBV-IkB encoded in the baculovirus was confirmed by reverse transcriptase (RT)-PCR. When the recombinant virus was mixed with Bacillus thuringiensis subsp. kurstaki (Bt), the bacterial pathogenicity was significantly enhanced in a dose-dependent manner, compared to a Bt mixture with an AcMNPV recombined with an enhanced green fluorescence protein gene. These results suggest that the inhibitory effect of CpBV-IkB on the production of antimicrobial peptide results in the enhanced pathogenicity of Bt.