ELIMINATION OF NEUTROPHILS BY APOPTOSIS DURING THE RESOLUTION OF ACUTE PULMONARY INFLAMMATION IN RATS

We evaluated the apoptosis of neutrophils during the resolution of acu te pulmonary inflammation induced by exposure to ozone. The inflammato ry response was assessed in rat lungs 0, 1, 3, and 7 days after 4-h ex posure to air or 2 ppm ozone. Analysis of bronchoalveolar lavage fluid demonstrated significant increases in albumin concentrations on days 0 and 1 and in the number of lavageable neutrophils on days 0, 1, and 3, indicating the presence of acute pulmonary inflammation. These para meters returned to control values on day 7, which suggests that the ac ute pulmonary inflammation induced by ozone was reversible. On days 1 and 3, but not on day 0, the neutrophils showed morphologic evidence o f apoptosis. Based on morphologic analysis, the proportion of apoptoti c neutrophils was 23.3 +/- 2.2% on day 1 and 55.7 +/- 3.2% on day 3. T erminal deoxynucleotidyl transferase-mediated dUTP end labeling (TUNEL ), in contrast, revealed that the proportion of apoptotic cells was 59 .7 +/- 9.1% on day 1 and 68.0 +/- 4.3% on day 3. On day 3, light micro scopy and electron microscopy demonstrated engulfment of the neutrophi ls by macrophages. These findings indicate that the apoptosis of neutr ophils followed by their engulfment by macrophages contributes to the clearance of neutrophils from the sites of inflammation. Moreover, TUN EL detected apoptotic neutrophils with greater sensitivity compared wi th morphologic assessment.