Kinetics of apoptosis in the lung of mice with allergic airway inflammation.

Background: Apoptosis has been suggested as a means to facilitate the resolution of eosinophilic inflammation in bronchial asthma. However, the natural course of apoptosis has not been elucidated in vivo, and there is no direct evidence for eosinophilic apoptosis within lung tissue.

Objective: The purpose of this study is to clarify whether the apoptosis occurs within the lung tissue, and to define the time-course of change in apoptosis ratio during the resolution of pulmonary eosinophilic inflammation.

Methods: Ovalbumin (OVA)-sensitized Balb/c mice were challenged with aerosolized OVA. We studied apoptotic cells in the lung of OVA-sensitized mice at 1, 3, 7 and 14 days after OVA challenge by in situ detection of DNA fragmentation with deoxynucleotidyl transferase deoxyuridyl triphosphatase nick endlabelling (TUNEL) technique. Apoptotic cells also were identified by electron microscopic analysis in the lung 7 days after OVA challenge.

Results: The TUNEL-method revealed that eosinophils localized in the subepithelium of bronchi undergo apoptosis following OVA challenge. Electron microscopy confirmed the presence of apoptotic cells, apoptotic bodies, and macrophages ingesting apoptotic bodies within the lung tissue. The number of apoptotic cells increased concomitantly with the increase in eosinophilic infiltration for 3 days post-challenge. However, both the apoptotic cell counts and the apoptotic ratio continued to increase even after the eosinophil count peaked, indicating rather late induction of apoptosis in the lung. In addition, TUNEL-positive cells were localized in the lung for 14 days post-challenge, indicating prolonged induction of apoptosis after the OVA challenge.

Conclusion: Our findings constitute direct evidence of eosinophilic apoptosis in situ, and display the kinetics of apoptosis in the lung of the allergic inflammation.