Lung tissue sections were deparaffinized, washed in xylene, rehydrated, permeabilized with proteinase K, and incubated with the TUNEL reaction mixture according to manufacturer instructions (In Situ Cell Death Detection kit, AP, Roche Applied Science, Indianapolis, IN). Negative controls were treated with labeling solution without terminal transferase. Immediately after TUNEL labeling the slides were washed three times in PBS, blocked with Protein Block (Dako, Carpinteria CA) and incubated for 2 hr in the dark at 37°C with the mouse monoclonal pan-cytokeratin antibody C11 (Abcam, Cambridge, UK) previously labeled with Alexa Fluor 555 (Invitrogen, Eugene, OR). Negative control slides were incubated with an isotype control mouse IgG1k labeled with Alexa Fluor 555 (BD Pharmingen, San Diego, CA). The slides were washed and immediately visualized with a fluorescent microscope.