A Kazal-type serine protease inhibitor mediates innate immunity in wild silkworm Actias selene Hübner

Abstract

The prophenoloxidase (PPO) activation system is an important innate immune defense mechanism in arthropods. Actias selene is a rare and important wild silk insect that can spin high-quality cocoon silk, but, other than its morphology, its molecular mechanism is rarely reported. Here, we report the purification and characterization of a novel KSPI gene from A. selene (AsKSPI, which can negatively regulate PPO activation. Its open reading frame (ORF) was 291 bp, encoding 96 amino acids. Real-time quantitative PCR (RT-qPCR) showed that AsKSPI mRNA was significantly expressed in the fat body. Immunostimulatory tests showed that the mRNA levels of AsKSPI in the fat body were up-regulated following injection of Micrococcus luteus, Escherichia coli, Beauveria bassiana, and nuclear polyhedrosis virus (NPV). Enzyme activity experiments showed that the purified recombinant AsKSPI could inhibit the activation of PPO in hemolymph of A. selene, but did not affect phenoloxidase (PO) activity after PPO had been activated. So, AsKSPI could regulate the innate immunity of A. selene through the PPO cascade. These findings will contribute to the understanding of the immune mechanism of wild silkworm and provide a basis for better protection and utilization of special economic insect resources.