<i>Penaeus monodon</i> fibrinogen-related lectin interacts with lipopolysaccharide and β-1,3-glucan binding protein to activate the innate immune system

The black tiger shrimp, Penaeus monodon, is cultivated commercially in many countries, including Thailand. However, diseases are major limiting factors in shrimp production. Understanding the shrimp immune responses to pathogens will be essential for health management and disease control. Fibrinogen-related proteins (FREPs) act as pattern recognition receptors by recognizing carbohydrate residues on pathogen surfaces. Here, we report that P. monodon FREP (PmFREP) interacts with the P. monodon lipopolysaccharide and β-1,3-glucan binding protein (PmLGBP) to co-ordinate the innate immune response in shrimp. Like PmLGBP, PmFREP participates in the prophenoloxidase (proPO)-activating cascade upon the Vibrio parahaemolyticus acute hepatopancreatic necrosis disease-causing strain infection. PmFREP knockdown results in a significant decrease in phenoloxidase (PO) activity, whereas recombinant PmFREP injection noticeably increases the enzyme activity. The coiled-coil (CC) region at the N-terminus of PmFREP mediates the direct binding between PmFREP and PmLGBP to enhance PO activity. Our results suggest that PmFREP recognizes the pathogens to form an immunological complex with PmLGBP through the CC region, subsequently enhancing the proPO-activating cascade to eliminate the pathogens. Therefore, PmFREP serves as a key component for signaling transduction in the shrimp immune defense.