PcSRB cooperates with TLR2 and promotes phagocytosis and antimicrobial peptides expression

Class B scavenger receptors (SRBs) are type III transmembrane proteins that participate in immune responses. However, the antibacterial mechanisms of SRBs in invertebrates have not yet been revealed. In this study, we analyzed the antibacterial activity of SRB in Procambarus clarkii (crayfish). The expression of PcSRB was upregulated after the Aeromonas hydrophila and Staphylococcus aureus challenge. A bacterial binding assay showed that PcSRB could bind to several bacteria. PcSRB can bind to polysaccharides, such as lipopolysaccharides, lipoteichoic acid, and peptidoglycan. After PcSRB interference, the survival rate of crayfish significantly decreased during the bacterial challenge. The bacterial cleaning efficiency of crayfish was also impaired by PcSRB interference. Further exploration showed that PcSRB promoted the phagocytosis of A. hydrophila by hemocytes. The expression levels of several antimicrobial peptides (AMPs) were also detected. The mRNA levels of Crustin1, Crustin2, LYZ, and SWD were down-regulated after PcSRB knockdown. In addition, PcSRB binds to TLR2, rather than to TLR4, and influences AMPs expression. Therefore, PcSRB cooperates with TLR2 to facilitate bacterial elimination by promoting hemocyte phagocytosis and AMPs expression.