The role of ficolin as a pattern recognition receptor in antibacterial immunity in Eriocheir sinensis

Ficolin, a member of the fibrinogen-related proteins family (FREPs), functions as a pattern recognition receptor (PRR) in vertebrates and in invertebrates as a novel lectin. In this study, we discovered the Ficolin homolog of Chinese mitten crab (Eriocheir sinensis), which we named EsFicolin. The obtained sequence showed that it has a highly conserved C-terminal fibrinogen-related domain (FReD) and a coiled-coil structure for trimer formation. EsFicolin was up-regulated in hemocytes after being stimulated by bacteria. Recombinant EsFicolin protein binds to gram-negative and gram-positive bacteria and agglutinates bacteria through pathogen-associated molecular patterns. In-depth study found that recombinant EsFicolin could effectively remove bacteria and showed direct antibacterial activity. EsFicolin could also promote the phagocytosis of hemocytes to enhance bacterial clear-ance. These findings suggest that EsFicolin plays an important role in the crab antibacterial immune response.