Role of nonspecific cytotoxic cells in bacterial resistance: Expression of a novel pattern recognition receptor with antimicrobial activity

Pattern recognition receptors (PRR) recognize invariant bacterial, viral, protozoan and certain synthetic ligands. PRR may be expressed as outer membrane (or endosomal) or cytosolic proteins and function to signal cell activation processes during inflammation responses. In the present study, a novel membrane receptor, NCC cationic antimicrobial protein-1 (NCAMP-1), is described that is expressed on nonspecific cytotoxic cell (NCC) membranes and is found in granule extracts from these cells. In recombinant form, full-length (amino acids 1–203) and truncated N (NT; amino acids 1–60) and C (CT; amino acids 116–203) terminal forms of NCAMP-1 had antibacterial activity against bovine, avian and lab strain Escherichia coli. Recombinant NCAMP-1-NT also killed the gram-negative fish pathogen Edwardsiella ictaluri. Maximal bacterial killing of a representative avian E. coli, APEC 3721, occurred at 60 min post-treatment with 2 μg/ml of rNCAMP-1-NT. Killing occurred by NCAMP-1-NT-induced alterations in the permeability of the bacterial cell wall. Polyclonal antibody anti-NCAMP-1 specifically neutralized the antimicrobial activity of recombinant NCAMP-1-NT against E. coli APEC 3751. Expression of NCAMP-1 as a NCC membrane protein was analyzed by flow cytometry using anti-NCAMP-1 monoclonal antibody 9C9. Merged images from immunofluorescence microscopy showed that NCAMP-1 and the NCC receptor protein (NCCRP-1) are co-expressed on NCC membranes. NCAMP-1 was identified in acetic acid granule extracts of NCC by Western blot analysis using polyclonal anti-NCAMP-1 and killing of E. coli by these extracts was specifically inhibited by this polyclonal. These data suggested that NCAMP-1 is a membrane protein and may participate in antibacterial innate immunity by granule exocytosis during inflammatory responses in teleosts.