Peptidoglycan recognition protein-S1 acts as a receptor to activate AMP expression through the IMD pathway in the silkworm Bombyx mori.

Peptidoglycan recognition proteins (PGRPs) are the most important pattern recognition receptors (PRRs) in insects. PGRPs can recognize pathogenic microorganism peptidoglycans (PGs) and play an important role in innate immunity. Twelve PGRPs have been identified in silkworms. However, the specific roles played by these PGPRs in the silkworm innate immune system have not been elucidated to date. In this study, we systematically investigated the biological functions of BmPGRP-S1 in silkworms. We observed that BmPGRP-S1 was highly expressed in silkworm immune-related organs and was upregulated in response to bacterial challenges. Furthermore, we determined that BmPGRP-S1 can bind to bacteria or PGs and activate antimicrobial peptide (AMP) expression. Inhibition of the expression of BmPGRP-S1 by siRNA reduced AMP gene expression in silkworms. Further experiments demonstrated that BmPGRP-S1 is involved in IMD pathway activation to induce AMP expression. Taken together, these results demonstrate that BmPGRP-S1 serves as a receptor to activate AMP gene expression through the IMD pathway to address bacterial challenges.