Cathelicidins: An Ancient Family of Fish Antimicrobial Peptides

Cathelicidins are an important family of antimicrobial peptide effectors of innate immunity in vertebrates. While their mammalian counterparts have been extensively characterized in the last two decades, data on fish cathelicidins have only recently been gathered. This review covers the available recent insights on fish cathelicidins regarding their evolution, protein structures, gene expression, antimicrobial, and immunomodulatory properties, highlighting the similarity and the differences with the mammalian ones. The picture that emerges is that of a group of immune effectors that has been subjected to intense pressure for variation in single species of salmonids, cods, and a few other families of bony fish to address the great variety of associated microorganisms and pathogens. Genes of fish cathelicidins have been shown to be highly responsive to bacteria; moreover, experimental data suggest both direct antibacterial activity and modulation of other immune effectors. These functions, which still need to be fully characterized, indicate cathelicidins as an important and effective defense line in fish.