A Toll-Spätzle Pathway in the Immune Response of Bombyx mori

SIMPLE SUMMARY: The Toll-Spätzle pathway is a crucial defense mechanism in insect innate immunity through inducing the expression of antimicrobial peptides (AMPs) against pathogens. As an important economical insect, Bombyx mori (B. mori) has been used as an insect model of Lepidoptera. However, the recognition of the Toll-Spätzle pathway in silkworm is very limited. In this study, we reported that B. mori Spätzle2 (BmSpz2) could be activated by pathogens. The activated BmSpz2 could bind with B. mori Toll11 (BmToll11) or B. mori Toll9-1 (BmToll9–1). Further investigations showed that overexpression of BmToll11 or BmToll9–1 could significantly upregulate AMPs expression. The findings of this study confirmed that a Toll-Spz pathway regulated immune response to microbial infection in B. mori and may help improve the understanding of the signaling pathways involved in the innate immune responses of Lepidoptera insect. ABSTRACT: The Toll-Spätzle pathway is a crucial defense mechanism in insect innate immunity, it plays an important role in fighting against pathogens through the regulation of antimicrobial peptide gene expression. Although Toll and Spätzle (Spz) genes have been identified in Bombyx mori, little is known regarding the specific Spz and Toll genes members involved in innate immunity. There is also limited direct evidence of the interaction between Spz and Toll. In this study, the dual-luciferase reporter assay results showed that BmToll11 and BmToll9–1 could activate both drosomycin and diptericin promoters in S2 cells. Furthermore, BmToll11, BmToll9–1, and five BmSpzs genes were found to be significantly upregulated in B. mori infected by Escherichia coli and Staphylococcus aureus. Additionally, the yeast two-hybrid assay results confirmed that BmSpz2, but not other BmSpzs, could interact with both BmToll11 and BmToll9–1. These findings suggest that the activated BmSpz2 can bind with BmToll11 and BmToll9–1 to induce the expression of AMPs after the silkworm is infected by pathogens.