In human airways, epithelial cells lining the lumen and intraluminal cells (e.g., polymorphonuclear cells) participate in the innate immune response. These cells secrete or express on their surfaces arginine-specific ADP ribosyltransferases. Defensins, antimicrobial proteins secreted by immune cells, are arginine-rich, leading us to hypothesize that ADP ribosylation could modify their biological activities. We found that an arginine-specific ADP ribosyltransferase-1 present on airway epithelial cells modifies Arg-14 of ␣ defensin-1. ADP-ribosylated defensin-1 had decreased antimicrobial and cytotoxic activities but still stimulated T cell chemotaxis and IL-8 release from A549 cells. Further, ADP-ribosylated defensin-1 inhibited cytotoxic and antimicrobial activities of unmodified defensin-1. We identified ADP-ribosylated defensin-1 in bronchoalveolar lavage fluid from smokers but not from nonsmokers, confirming its existence in vivo. Thus, airway mono-ADP-ribosyltransferases could have an important regulatory role in the innate immune response through modification of ␣ defensin-1 and perhaps other basic molecules, with alteration of their biological properties.
In the human airway, epithelial cells, in concert with intraluminal cells (e.g., macrophages), contribute to the innate immune response. Activation of airway epithelial cells releases proinflammatory mediators and chemotaxins, leading to the recruitment of inflammatory cells (e.g., neutrophils) (1, 2). Activated neutrophils, in turn, release azurophilic granules that contain human neutrophil peptides (HNP), small cationic peptides ranging in size from 29-33 aa (3). HNPs are characterized by a high arginine content and contain three pairs of disulfidelinked cysteines (4). They have a broad spectrum of antimicrobial and cytotoxic activities and play an important role in the innate immune response against both Gram-positive and -negative bacteria, fungi, and certain enveloped viruses (5-8). High HNP levels have been found in airway secretions of patients with inflammatory lung diseases (9-11).A family of eukaryotic and prokaryotic mono-ADP ribosyltransferases (ARTs) catalyze the transfer of ADP ribose from NAD ϩ to arginine residues in proteins (12). Among the monoARTs are bacterial products (e.g., cholera toxin), which are responsible for the clinical syndrome in disease (13). A family of mammalian ARTs that are secreted or localized on the cell surface through glycosylphosphatidylinositol anchors (14-19) are expressed preferentially on epithelial and inflammatory cells such as lymphocytes and neutrophils (17,20). Substrates of the five known mammalian ARTs (ART-1-5) include proteins that are involved in critical cellular events (e.g., lymphocyte activation, neutrophil chemotaxis) (21-23). Two of these transferases, ART-1 and -5, specifically modify arginine residues in proteins. The finding of ARTs on airway epithelial and inflammatory cells prompted us to evaluate potential substrates for ADP ribosylation in the airway (24). Because of the high arginine c...