D3214: Melanization and Toll share regulatory serine proteases in eggs of Manduca sexta

The tobacco hornworm, Manduca sexta, is a lepidopteran model widely used to study insect physiological processes including innate immunity. Sequential activation of the extra-cellular serine protease system controls immune responses, such as melanization and antimicrobial peptide synthesis by activating prophenoloxidase and Toll pathways, respectively. After accomplishing their functions, key serine proteases are down-regulated by serpins. So far, most studies are focused on adult or larval antimicrobial responses. While embryos are covered by eggshell and underlying vitelline membrane, other protective mechanisms may exist. However, little is known on how immunity is mediated or regulated in insect eggs. Here, we provide evidences that 3 serine proteases (HP14b, SP144 and GP6) are substantially expressed only in the embryonic stage of M. sexta. Sequential activation of these 3 serine proteases (HP14b-SP144-GP6) leading to the activation of prophenoloxidase and the cytokine Spätzle (the ligand for Toll receptor). In addition, HP14b, SP144 and GP6 are inhibited by serpin-11/12, serpin-1J/4/5 and serpin-1A/1J/6/11, respectively. In summary, we have elucidated a serine protease network mediates immune responses in eggs of M. sexta.