Direct and indirect bacterial killing functions of neutrophil defensins in lung explants

Studies of the antimicrobial activity of neutrophil defensins have mostly been carried out in microbiological media, and their effects on the host defense in physiological conditions are unclear. We examined 1) the antibacterial activity of defensins in physiological media with and without lung tissue present, 2) the effect of defensins on hydrogen peroxide (H₂O₂) production by lung tissue that had been exposed to bacteria, and 3) the effect of diphenyleneiodonium (DPI), an inhibitor of reactive oxygen species formation, on the antibacterial activity of defensins in the presence of lung tissue. Defensins were incubated with Escherichia coli or Pseudomonas aeruginosa in the absence or presence of primary cultured mouse lung explants. Defensins reduced bacterial counts by ∼65-fold and ∼25-fold, respectively, at 48 h; bacterial counts were further decreased by ∼600-fold and ∼12,000-fold, respectively, in the presence of lung tissue. Defensins induced H₂O₂ production by lung tissue, and the rate of killing of E. coli by defensins was reduced by ∼2,500-fold in the presence of 10 μM DPI. We conclude that defensins exert a significant antimicrobial effect under physiological conditions and that this effect is enhanced in the presence of lung tissue by a mechanism that involves the production of reactive oxygen species.