Antimicrobial Peptides of the Respiratory Tract

Abstract

Antimicrobial peptides form an essential element of innate immunity. They are key effector molecules in host defense against infection due to their broad-spectrum antimicrobial activity. These small, cationic peptides contribute to the antimicrobial activity of phagocytes and epithelial cells, and thus form an essential part of the host defense system of the respiratory mucosa. Neutrophils and epithelial cells are the main cellular sources of these peptides, but also monocytes, macrophages, dendritic cells and lymphocytes produce antimicrobial peptides. The two main families of antimicrobial peptides that are best characterized and expressed in the human lung are the defensins and cathelicidins, mainly produced by epithelial cells and neutrophils respectively. The members of these families are structurally different, but display partly overlapping activities. A plethora of studies now show that antimicrobial peptides not only act as endogenous antibiotics, but also display a range of activities involved in inflammation, immunity and wound repair. Therefore, the use of the more broader term cationic host defense peptides instead of antimicrobial peptides is becoming increasingly relevant. Studies in animal models and human disease strongly support an important role of these peptides in vivo. In addition, genetic studies show associations between polymorphisms in for example defensin genes and inflammatory lung disease. Finally, antimicrobial peptides are being explored in the development of novel (antimicrobial) therapies.