Serine proteases and serine protease homologs are involved in the prophenoloxidase (proPO)-activating system leadingto melanization.The Bombyx mori serine protease homolog BmSPH-1 regulates nodule melanization. Here, we show the dualrole of BmSPH-1 in the development and immunity of B. mori. BmSPH-1 was expressed in hemocytes after molting andduring the larval-pupal transformation in normal development. In contrast, following infection, BmSPH-1 was expressed inhemocytes and activated in the hemolymph, which resulted in the induction of PO activity. Moreover, BmSPH-1 was activatedin the cuticle during the larval-pupal transformation and early pupal stages. In BmSPH-1 RNAi-treated silkworms, the reducedBmSPH-1 mRNA levels during the spinning stage or the prepupal stage resulted in the arrest of pupation or pupal cuticularmelanization, respectively. The binding assays revealed that BmSPH-1 interacts with B. mori immulectin, proPO, andproPO-activating enzyme. Our findings demonstrate that BmSPH-1 is responsible for the larval-pupal transformation, pupalcuticular melanization and innate immunity of silkworms, illustrating the dual role of BmSPH-1 in development and immunity.

In insects, serine proteases are involved in a variety of physiological processes including digestion, development, and immunity. Bombyx mori serine protease homolog BmSPH-1 regulates nodule melanization and is recruited into nodules from the hemolymph by B. mori lipopolysaccharide-binding protein. Here, we show the dual role of BmSPH-1 in development and immunity of B. mori. BmSPH-1 was expressed in the hemocytes during larval-pupal transformation and localized to the cuticle of silkworms, which indicates that BmSPH-1 is secreted from hemocytes and then transported to the cuticle via the hemolymph. BmSPH-1 was proteolytically activated in the cuticle during larval-pupal transformation and the early pupal stage. BmSPH-1 RNAi resulted in the arrest of larval-pupal transformation and pupal cuticular melanization. Furthermore, the expression of BmSPH-1 was up-regulated in the hemocytes during infection. Taken together, we found that BmSPH-1 is involved in larval-pupal transformation and pupal cuticular melanization as well as the innate immunity of silkworms, which indicates that BmSPH-1 is responsible for either development or immunity.