Scavenger receptor B8 improves survivability by mediating innate immunity in silkworm, Bombyx mori

Scavenger receptor class B (SR-B) is an extracellular transmembrane glycoprotein that plays a vital role in innate immunity. Although SR-Bs have been widely studied in vertebrates, their functions remained to elucidate in insects. Here, we identified and characterized a scavenger receptor class B member from the silkworm, Bombyx mori (designated as BmSCRB8). BmSCRB8 is broadly expressed in various immune tissues/organs, including fat body, gut, and hemocyte. Its expression is dramatically enhanced after challenge with different types of bacteria or pathogen-associated molecular patterns (PAMPs). The recombinant BmSCRB8 protein can detect different types of bacteria by directly binding to PAMPs and significantly improve the bacterial clearance in vivo. After knockdown of BmSCRB8, the pathogenic bacterial clearance was strongly impaired, and several AMP genes were down-regulated following E. coli challenge. Moreover, pathogenic bacteria's treatment following the depletion of BmSCRB8 remarkably decreased silkworm larvae's survival rate. Taken together, these results demonstrate that BmSCRB8 acts as a pattern recognition protein and plays an essential role in silkworm innate immunity by enhancing bacterial clearance and contributing to the production of AMPs in vivo.