This report evaluates airway inflammatory responses and expression of the enzyme neprilysin in response to diesel exhaust particle exposure. Dr. Wong and colleagues hypothesized that components of diesel exhaust decrease neprilysin levels in airways, leading to airway function disorders and heightened responses to diesel exhaust. They exposed normal mice, mice genetically deficient in neprilysin, human subjects, and human airway epithelial cells to diesel exhaust particles and measured airway inflammation, neprilysin expression, and any unique responses in neprilysin-deficient mice.