Bacterial DNA (bDNA) contains hypomethylated "CpG" repeats that can be recognized by Toll-like receptor 9 (TLR-9) as a pathogen-associated molecular pattern. The ability of bDNA to initiate lung injury via TLR-9 has been inferred on the basis of studies using artificial CpG DNA. But the role of authentic bDNA in lung injury is still unknown. Moreover, the mechanisms by which CpG DNA species can lead to pulmonary injury are unknown, although neutrophils (PMNs) are thought to play a key role in the genesis of septic acute lung injury. We evaluated the effects of bDNA on PMN-endothelial cell (EC) interactions thought critical for initiation of acute lung injury. Using a biocapacitance system to monitor real-time changes in endothelial permeability, we demonstrate here that bDNA causes EC permeability in a dose-dependent manner uniquely in the presence of PMNs. These permeability changes are inhibited by chloroquine, suggesting TLR-9 dependency. When PMNs were preincubated with bDNA and applied to ECs or when bDNA was applied to ECs without PMNs, no permeability changes were detected. To study the underlying mechanisms, we evaluated the effects of bDNA on PMN-EC adherence. Bacterial DNA significantly increased PMN adherence to ECs in association with upregulated adhesion molecules in both cell types. Taken together, our results strongly support the conclusion that bDNA can initiate lung injury by stimulating PMN-EC adhesive interactions predisposing to endothelial permeability. Bacterial DNA stimulation of TLR-9 appears to promote enhanced gene expression of adhesion molecules in both cell types. This leads to PMN-EC cross-talk, which is required for injury to occur.