A novel LPS binding /bactericidal permeability-increasing protein (LBP/BPI) from the scallop Argopecten purpuratus plays an essential role in host resistance to Vibrio infection

Lipopolysaccharide binding proteins (LBPs) and bactericidal permeability increasing proteins (BPIs) play significant roles in the immune response of vertebrates against bacterial pathogens. These soluble proteins produced by immune cells, specifically interact with and bind to bacterial lipopolysaccharides (LPS), with BPIs also displaying antibacterial activity. In Argopecten purpuratus scallop larvae resistant to Vibrio bivalvicida VPAP30, we identified a significant overexpression of a transcript displaying molecular features of an LBP/BPI protein, both before and after infection. Therefore, in the present work we aimed to understand the role of this novel LBP/BPI, named ApLBP/BPI3, in the scallop resistance to this Vibrio. The ApLBP/BPI3 open reading frame encodes a putative protein of 506 amino acids, with a molecular weight 56.78 kDa. The protein contains a C-terminal domain of 403-amino acid that, after theorical cleavage, displays a soluble form of 44.99 kDa, featuring two BPI/LBP/CETP domains, an apolar binding pocket, a single disulfide bond and a BPI dimerization interface. Phylogenetic analysis reveals high similarity between ApLBP/BPI3 and other mollusk LBP/BPI proteins. Aplbp/bpi3 transcripts were constitutively and highly expressed in hemocytes, gills, mantle, and digestive gland tissues, and were induced following VPAP30 infection in scallop larvae and adult hemocytes. We characterized ApLBP/BPI3 protein using a polyclonal antibody against a synthetic peptide. ApLBP/BPI3 was secreted to the media by infected cultured hemocytes, detected by ELISA. ApLBP/BPI3 was spotted inside non-infected hemocytes, bound to the cell wall of V. bivalvicida after in vitro hemocyte infection, and coating the gills and mantle epithelial barriers before and after an in vivo immune challenge, with stronger detection after VPAP30 injection, detected by immunofluorescence. Infected scallop larvae showed increased ApLBP/BPI3 levels, with slightly higher production in resistant larvae, determined by Western blot. Finally, silencing the Aplbp/bpi3 transcript through RNA interference and and subsequently infecting scallop juveniles with an LD50 of V. bivalvicida resulted in 100 % mortality. Altogether, results demonstrate the essential role of this immune effector in the resistance of A. purpuratus.