Insect immulectins are involved in various aspects of the innate immunity, including encapsulation, melanization, and phagocytosis. Although the silkworm Bombyx mori immulectin (BmIML) has been reported previously, the ligand and functions of BmIML have not been investigated. Here, we show the dual roles of BmIML in cuticular melanization and immunity of B. mori. BmIML recognizes carbohydrates in a Ca2+-dependent manner and binds to Gram-negative and Gram-positive bacteria, fungi, and yeast. BmIML was expressed in the fat body during infections and localized to the hemocytes of silkworms. Additionally, BmIML was expressed in the epidermis during the prepupal stage and localized to the cuticle of silkworms. After treatment with E. coli, dopa, dopamine, or tyrosine injections, BmIML production was induced in the fat body but not in the epidermis of silkworms. Treatment with BmIML RNAi resulted in the arrest of pupal cuticular melanization. Therefore, we concluded that BmIML is involved in pupal cuticular melanization and innate immunity responses of silkworms, suggesting dual roles for BmIML.

Insect cuticular melanization is regulated by the prophenoloxidase (proPO)-activating system, which is a component of innate immunity. However, the differentiation between cuticular melanization and innate immunity is not well defined. Here, we demonstrate that the proPO-activating system regulates cuticular melanization in the silkworm pupae using a different mechanism. Our results indicate that the differential and spatial regulation of key components, such as the proPO-activating factor, tyrosine hydroxylase, proPOs, and immulectin, primes the proPO-activating system for either cuticular melanization or innate immunity. This dual strategy for cuticular melanization in development and innate immunity upon infection demonstrates a two-pronged defense mechanism mediated by the priming of the proPO system.

Insect cuticular melanization is regulated by the prophenoloxidase (proPO)- activating system, which is also involved in the innate immune reaction. Here, we demonstrate how the differentiation of the proPO-activating system is regulated toward a cuticular melanization or innate immunity function in silkworm (Bombyx mori) pupae. Our results indicate that the differential and spatial regulation of key components, such as the proPO-activating factor, tyrosine hydroxylase, and porPOs, primes the proPO-activating system for either cuticular melanization or innate immunity. This dual strategy for cuticular melanization in development and innate immunity upon infection demonstrates a two-pronged defense mechanism that is mediated by the priming of the proPO system.