RNA Interference-Mediated Knockdown of Bombyx mori Haemocyte-Specific Cathepsin L ( Cat L )-Like Cysteine Protease Gene Increases Bacillus thuringiensis kurstaki Toxicity and Reproduction in Insect Cadavers.

The silkworm's Cat L -like gene, which encodes a lysosomal cathepsin L-like cysteine protease, is thought to be part of the insect's innate immunity via an as-yet-undetermined mechanism. Assuming that the primary function of Cat L-like is microbial degradation in mature phagosomes, we hypothesise that the suppression of the Cat L -like gene expression would increase Bacillus thuringiensis ( Bt ) bacteraemia and toxicity in knockdown insects. Here, we performed a functional analysis of Cat L -like in larvae that were fed mulberry leaves contaminated with a commercial biopesticide formulation based on Bt kurstaki ( Btk ) (i.e., Dipel) to investigate its role in insect defence against a known entomopathogen. Exposure to sublethal doses of Dipel resulted in overexpression of the Cat L -like gene in insect haemolymph 24 and 48 h after exposure. RNA interference (RNAi)-mediated suppression of Cat L -like expression significantly increased the toxicity of Dipel to exposed larvae. Moreover, Btk replication was higher in RNAi insects, suggesting that Cat L-like cathepsin may be involved in a bacterial killing mechanism of haemocytes. Finally, our results confirm that Cat L-like protease is part of the antimicrobial defence of insects and suggest that it could be used as a target to increase the insecticidal efficacy of Bt -based biopesticides.