The role of defensins in lung biology and therapy

Innate host defence, involving both cellular and humoral mediators, is a prominent function of the human airways. Cellular mediators of innate immunity include dendritic cells, natural killer cells, cytotoxic T cells, macrophages and neutrophils, while humoral mediators of innate immunity consist of components of the epithelial lining fluid (ELF) covering the airways. Microbicidal substances in the ELF can selectively disrupt bacterial cell walls and membranes, sequester microbial nutrients or act as decoys for microbial attachment. Antimicrobial components of airway secretions include lysozymes, lactoferrin, secretory leukoprotease inhibitor, defensins and cathelicidins. Defensins are the most widely studied family of antimicrobial peptides present in airway fluid. Humans produce at least 10 different defensin molecules, six alpha-defensins and four beta-defensins similar in structure and function. Direct evidence that defensins have central roles in host defense has only recently become available. Some defensins and defensin-like molecules could serve as templates for the development of pulmonary pharmaceuticals. As potential therapeutics, they possess several desirable properties, including the ability to kill a broad spectrum of micro-organisms while permitting little development of microbial resistance. Many peptides can also neutralize effects of lipopolysaccharide on macrophages and other host defense cells and decrease the release of proinflammatory cytokines thereby giving protection against septic shock. Protegrin-1 is a minidefensin isolated from pig leukocytes and has proved to be an attractive template for large-scale development of antibacterials. One such protegrin analog, iseganan is in phase III clinical trials for the treatment of oral mucositis secondary to systemic chemotherapy. Other prospective uses of iseganan include control of respiratory pathogens in patients with cystic fibrosis and reduction of oral bacteria to prevent ventilator-associated pneumonia. However, in order to advance the production and clinical testing of peptide-based therapeutics, technical hurdles of synthesizing large quantities of complexly folded peptides must be first overcome. Strategies to develop potent peptide-based microbicides are promising in the struggle against increasingly resistant pathogens.