In summary, ethanol exposure compromises chloride ion secretion which is pivotal to maintain ASL volume and composition. Such an effect may alter the properties of airway host defenses predisposing ethanol abusers to an increased risk of infection in the lung. Restoration of cellular cAMP level with phosphodiesterase inhibitors could potentially ameliorate mucociliary clearance by improving epithelial ion secretion and hence lung host defense. Figure 6 . Phosphodiesterase inhibitors restore the ethanol suppression of adenosine-induced transepithelial chloride conductance. The air-liquid interface cultures of Calu-3 cells were exposed to 0 and 100 mM of ethanol for 24 hours. Adenosinestimulated I SC was measured, showing that ethanol-exposed Calu-3 epithelia had significantly lower adenosine-stimulated I SC than the no ethanol control. However, phosphodiesterase inhibitors, IBMX (100 mM) or papaverine (50 mM), fully restored the ethanol-suppressed adenosine-stimulated I SC . Asterisks indicate statistically significant differences between groups (p,0.05, n = 4 for each group). doi:10.1371/journal.pone.0032112.g006 High Fidelity Processing and Activation of the Human α-Defensin HNP1 Precursor by Neutrophil Elastase and Proteinase 3 The azurophilic granules of human neutrophils contain four α-defensins called human neutrophil peptides (HNPs 1–4). HNPs are tridisulfide-linked antimicrobial peptides involved in the intracellular killing of organisms phagocytosed by neutrophils. The peptides are produced as inactive precursors (proHNPs) which are processed to active microbicides by as yet unidentified convertases. ProHNP1 was expressed in E. coli and the affinity-purified propeptide isolated as two species, one containing mature HNP1 sequence with native disulfide linkages (“folded proHNP1”) and the other containing non-native disulfide linked proHNP1 conformers (misfolded proHNP1). Native HNP1, liberated by CNBr treatment of folded proHNP1, was microbicidal against Staphylococcus aureus, but the peptide derived from misfolded proHNP1 was inactive. We hypothesized that neutrophil elastase (NE), proteinase 3 (PR3) or cathepsin G (CG), serine proteases that co-localize with HNPs in azurophil granules, are proHNP1 activating convertases. Folded proHNP1 was converted to mature HNP1 by both NE and PR3, but CG generated an HNP1 variant with an N-terminal dipeptide extension. NE and PR3 cleaved folded proHNP1 to produce a peptide indistinguishable from native HNP1 purified from neutrophils, and the microbicidal activities of in vitro derived and natural HNP1 peptides were equivalent. In contrast, misfolded proHNP1 conformers were degraded extensively under the same conditions. Thus, NE and PR3 possess proHNP1 convertase activity that requires the presence of the native HNP1 disulfide motif for high fidelity activation of the precursor in vitro. Antimicrobial peptides (AMPs) produced by animal cells provide a first line of defense against potentially infectious microorganisms. In mammals, defensins and cathelicidins are the primary AMPs expressed in professional phagocytes such as neutrophils and macrophages [1] . Mammalian defensins comprise three structural subfamilies termed a-, band h-defensins. While all defensins are arginine rich, tridisulfide containing peptides, they are distinguished from each other by the presence of unique disulfide motifs [2] . In addition, h-defensins, which are approximately half the size of aand b-defensins, are macrocyclic molecules which have only been isolated from leukocytes of Old World monkeys (reviewed in [2, 3] ). Defensins have antimicrobial activities against diverse microbial targets that include bacteria [4] , fungi [5, 6] , viruses [7, 8, 9] , and protozoa [10] . Certain a-, b-, and h-defensins function as regulators of adaptive immune cells and thus function to modulate inflammation [11, 12, 13] and bridge innate and adaptive immunity [14] .