Localization of anti-lipopolysaccharide factor (ALFPm3) in tissues of the black tiger shrimp, Penaeus monodon, and characterization of its binding properties.

Anti-lipopolysaccharide factor (ALF) is an antimicrobial peptide originally identified from horseshoe crabs and recently found in several shrimp species. ALFPm3, the most abundant isoform in the black tiger shrimp, Penaeus monodon, has been shown to possess a broad spectrum of antimicrobial activity against Gram-negative and Gram-positive bacteria, and filamentous fungi. In this study, a potential role for ALFPm3 in the shrimp innate immunity was revealed by examining the distribution of the protein in shrimp tissues in response to Vibrio harveyi challenge. Immunohistochemistry using anti-ALFPm3 antibody showed that the ALFPm3 protein is primarily localized in hemocytes and the positive cells observed at the injection site and in the cephalothorax are infiltrating hemocytes that migrate into shrimp tissues after bacterial injection. A rapid increase in the number of hemocytes producing ALFPm3 observed in V. harveyi-injected shrimp suggests a likely important function of the protein in defense against invading pathogens. ALFPm3 was shown to be able to bind to Gram-negative and Gram-positive bacterial cells and their major cell wall components, lipopolysaccharide (LPS) and lipoteichoic acid (LTA), respectively. The results suggested that ALFPm3 performs its antibacterial activity by binding to component(s) of the bacterial cell wall.