BmSerpin-7 negatively regulates prophenoloxidase activation and antimicrobial peptide synthesis in Bombyx mori

The prophenoloxidase (PPO) activation pathway and the Toll-mediated antimicrobial peptide (AMP) synthesis pathway constitute critical immune responses within the insect immune system. The activation of these pathways is driven by extracellular serine protease cascades, a process counterbalanced by the negative regulation of serpins. In this study, we identified a typical serpin, BmSerpin-7, and characterized its function in the silkworm, Bombyx mori. BmSerpin-7 expression was up-regulated in the hemocytes but down-regulated in the fat body after Micrococcus luteus and Yersinia pseudotuberculosis challenge. Melanization, phenoloxidase (PO) activity and the activation of PPO were markedly suppressed by the recombinant BmSerpin-7. Meanwhile, the antibacterial activity of the hemolymph and bacteria-induced expression of AMP genes, including gloverin-2, gloverin-3, cecropin-D, and cecropin-E, were also remarkably decreased. Furthermore, based on sequence homology and established serpin-protease relationships in Manduca sexta, B. mori hemolymph protease 1 (BmHP1) and hemolymph protease 6 (BmHP6) are proposed as potential proteases involved in BmSerpin-7-mediated immune regulation. Together, our findings support BmSerpin-7 as a dual negative regulator of PPO activation and bacteria-induced AMP production, thereby providing insights into the regulation of immune responses in silkworm.