Involvement of a versatile pattern recognition receptor, apolipophorin-III in prophenoloxidase activation and antibacterial defense of the Chinese oak silkworm, Antheraea pernyi.

Apolipophorin-III (apoLp-III) is an exchangeable apolipoprotein found in many insect species and functions as a lipid transport vehicle. Recent studies have shown that apoLp-III is a multifunctional molecule involved in not only lipid transportation but also innate immune responses. In the present study, the pattern recognition properties of Antheraea pernyi apoLp-III were investigated. Recombinant Ap-apoLp-III was bound to different species of microbes and further study showed the rAp-apoLp-III is capable of interacting with pathogen associated molecular patterns (PAMPs) on the microbial cell surface. In addition, an Ap-apoLp-III/PAMP mixture stimulated the prophenoloxidase (PPO) activation of A. pernyi hemolymph in vitro, to a greater extent than PAMP alone while Ap-apoLp-III itself failed to activate the PPO system, indicating that Ap-apoLp-III up-regulates PPO activation by combining with PAMP. After pathogen invasion following an injection of Staphylococcus aureus, RNAi-mediated silencing of apoLp-III decreased the transcriptional abundance of three antimicrobial peptide genes. These data suggest that apoLp-III is a versatile pattern recognition receptor and may play important roles in the innate immune responses of Antheraea pernyi.