Defensins, antimicrobial and cytotoxic peptides of neutrophils, bind to and are inactivated by blood proteins. We identified defensin interactions with alpha 1-proteinase inhibitor (alpha 1-PI), alpha 1-antichymotrypsin (alpha 1-ACT), alpha 2-antiplasmin (alpha 2-AP), and antithrombin III (AT III) and examined defensin binding to alpha 1-PI and alpha 1-ACT in more detail. Defensin interactions with either alpha 1-PI or alpha 1-ACT were not affected by iodoacetamide or high salt concentration. Preincubation of alpha 1-ACT or alpha 1-PI with increasing concentrations of defensin resulted in a progressive decrease of antiprotease activity of both inhibitors against cathepsin G and antiprotease activity of alpha 1-PI against human neutrophil elastase. At higher concentrations, defensin also ablated the inhibitory effect of normal human serum on cathepsin G and human neutrophil elastase. Both alpha 1-PI and alpha 1-ACT inhibited defensin cytotoxicity toward the human lung carcinoma cell line A549, whereas the elastase inhibitor antileukoprotease did not. Complex interactions between serpins and defensin may have a role in regulating inflammatory processes.

Antimicrobial peptides of several structural classes have been found in phagocytes and epithelial cells of many animals. The broadly microbicidal protegrins (PG1,-2, and-3) were originally isolated as 16 to 18-amino-acid peptides from pig neutrophil lysates, but the corresponding cDNA sequences encoded much larger precursors that belonged to the cathelicidin family of antimicrobial peptides. We explored the storage, secretion, and microbicidal activation of protegrins in porcine neutrophils and in a model system consisting of recombinant proprotegrin 3 (pPG3) and various serine proteases and their inhibitors. Protegrins were stored in neutrophils as inactive proforms that were cleaved by neutrophil elastase to mature protegrins during the preparation of granule lysate and during phorbol myristate acetate-stimulated granule secretion from intact neutrophils. Recombinant pPG3 was efficiently cleaved by trace amounts of human neutrophil elastase or equivalent amounts of elastase activity from porcine neutrophils, but pPG3 was relatively resistant to porcine pancreatic elastase or human neutrophil cathepsin G. The recombinant pPG3 and neutrophil proprotegrins lacked microbicidal activity, but the mature protegrins generated in the elastase-mediated cleavage reaction were as active against Listeria monocytogenes as the chemically synthesized protegrin. The secretion and elastase-mediated activation of proprotegrins accounted for much of the stable microbicidal activity of porcine neutrophil secretions against L. monocytogenes. Secreted proprotegrins and trace amounts of elastase constitute a binary microbicidal system that is likely to contribute to the antimicrobial activity of porcine inflammatory fluids.

Defensins are highly abundant and variably cationic peptides that possess antimicrobial, cytotoxic, and chemoattractant properties and equip mammalian phagocytes for participation in host defense and inflammatory processes. We studied the binding of the human defensin HNP-1 by proteins in plasma and serum and identified activated (F-form) alpha 2-macroglobulin (alpha 2M) as a principal binding protein for HNP-1. In contrast, native (S-form) alpha 2M bound little HNP-1. The binding of HNP-1 by F-form alpha 2M was resistant to salt and boiling in 2% sodium dodecyl sulfate but was ablated by dithiothreitol. Pretreatment of methylamine-activated serum or plasma by iodoacetamide substantially decreased the binding of HNP-1 to alpha 2M, suggesting that thiol groups in activated alpha 2M play a role in defensin binding, possibly by covalently trapping defensins via thiol-disulfide exchange. Western blots of conventionally collected sera showed endogenous defensins complexed with the F-form of alpha 2M, indicating that the generation of defensin-alpha 2M complexes was not limited to the in vitro model of methylamine-activated serum or plasma and radiolabeled HNP-1. Previous studies indicated that native alpha 2M can be converted to its F-form by many proteases, including those released by neutrophils and platelets, and that the F-form is recognized and internalized by specific receptors on macrophages and hepatocytes. Our findings suggest that the alpha 2M system may function as a scavenger of defensins and other peptide mediators in inflamed tissues and may constitute an important mechanism for the regulation and containment of inflammation.