Antimicrobial peptides are abundant components of the mammalian innate immune system providing for the recognition of microbial organisms and their rapid inactivation. These molecular agents may play a key role in neonatal lung immunity based on: (1) the demonstration that their inactivation in airway secretions leads to loss of antimicrobial activity against P. aeruginosa (Cell (1997) 88:553-560); and (2) their absence in the developing human fetus (at least through the second trimester, Am J Resp Cell Mol Biol(1997) 16:343-349). Within the mammalian lung, two distinct classes of broad-spectrum antimicrobial peptides are present: the cationic, salt-sensitive, cys-rich defensins (PNAS (1991) 88:3952-3956) and the anionic, salt-insensitive, asp-rich peptides (PNAS (1996) 93:412-416). Contrasting markedly in structure, these two elements of innate immunity may function synergistically in host defense. We have isolated and characterized representatives of both classes from sheep as a model for human lung immunity. Sheep β-defensin 1 is a 42 amino acid cationic peptide containing the 6 conserved defensin-specific cysteine residues (Reviewed in Curr Opin in Immunol (1994) 6:584-589). Sheep heptapeptides H-DDDDDDD-OH, H-GDDDDDD-OH, and H-GADDDDD-OH are anionic, Zn-dependent antimicrobial agents. Immunohistochemical localization using peptide-specific polyclonal antisera demonstrates that both classes are present in airway columnar epithelium. Our studies identify specific interactions between these two host defense elements which may alter the antimicrobial milieu and change the pattern of pulmonary pathogen clearance. These results may help explain the recurrence of lung infections in patients subjected to mechanical bypass of airway defenses (e.g. during intubation) or affected by airway epithelial dysplasia (e.g. bronchopulmonary dysplasia). In addition, we have found a microbicidal synergy between the endogenous antimicrobial peptides and gentamicin, offering a novel therapeutic approach to enhancing the efficacy of conventional antibiotics.