A single von Willebrand factor C-domain protein acts as an extracellular pattern-recognition receptor in the river prawnMacrobrachium nipponense

The single von Willebrand factor C-domain proteins (SVWCs) are mainly found in arthropods. Their expression may be regulated by several environmental stresses, including nutritional status and bacterial and viral infections. However, the underlying regulatory mechanism is unclear. In the present study, we identified a member of the SVWC family from the river prawnMacrobrachium nipponenseas a soluble and bacteria-inducible pattern-recognition receptor (designated MnSVWC).In vitro, recombinant MnSVWC exhibited pronounced binding and Ca2+-dependent agglutinating abilities against diverse microbes, including Gram-negative bacteria (i.e. Escherichia coliandAeromonas victoria), Gram-positive bacteria (Staphylococcus aureusandBacillus subtilis), and yeast (Pichia pastoris). ELISA assays revealed that recombinant MnSVWC recognizes a broad range of various pathogen-associated molecular patterns (PAMPs) and has high affinity to lipopolysaccharide and lysine-type and diaminopimelic acid?type peptidylglycan andd-galactose and low affinity tod-mannan and ?-1,3-glucan. Mutant MnSVWC(P57A)with an impaired Glu-Pro-Asn (EPN) motif displayed reduced affinity to all these PAMPs to varying extent. Moreover, MnSVWC bound to the surface of hemocytes and promoted their phagocytic activity and clearance of invasive bacteria. RNAi-mediated MnSVWC knockdown in prawn reduced the ability to clear invading bacteria, but did not block the activities of the Toll pathway or the arthropod immune deficiency (IMD) pathway, or the expression of antimicrobial peptide genes. These results indicate that MnSVWC functions as an extracellular pattern-recognition receptor inM. nipponensethat mediates cellular immune responses by recognizing PAMPs, agglutinating invasive microbes, and promoting phagocytosis in hemocytes.