Antimicrobial activity of rabbit CAP18-derived peptides.

A cationic antimicrobial protein of 18 kDa (CAP18) was originally isolated from rabbit granulocytes by using as an assay the agglutination of Re-lipopolysaccharide-coated erythrocytes. The C-terminal 37 amino acids of CAP18 (CAP18(106-142)) make up the lipopolysaccharide-binding domain. Synthetic CAP18(106-142) has broad antimicrobial activity against both gram-positive (50% inhibitory concentration, 130 to 200 nM) and gram-negative (50% inhibitory concentration, 20 to 100 nM) bacteria. Susceptible strains include Staphylococcus aureus, Streptococcus pneumoniae, Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhimurium. Antimicrobial activity is highly dependent on peptide structure. Although a 32-amino-acid peptide resulting from the truncation of 5 amino acids from the C terminus of CAP18(106-142) is highly active, other fragments of CAP18(106-142), including CAP18(106-142) with a truncated N terminus, do not exhibit antimicrobial activity. Unlike previously characterized antimicrobial peptides derived from granulocyte proteins, CAP18(106-142) is active in serum. CAP18(106-142) or a derivative peptide may have therapeutic potential for bacterial sepsis.