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Cigarette Smoke Alters Respiratory Syncytial Virus–Induced Apoptosis and Replication

¹ Division of Pulmonary, Critical Care, and Occupational Medicine, and ² Department of Microbiology and Interdisciplinary Graduate Program in Immunology, University of Iowa Roy J. and Lucille A. Carver College of Medicine and Veterans Administration Medical Center, Iowa City, Iowa

Correspondence and requests for reprints should be addressed to Dayna Groskreutz, M.D., Division of Pulmonary, Critical Care, and Occupational Medicine, 100 EMRB, Iowa City, IA 52242. E-mail: Dayna-Groskreutz{at}uiowa.edu

Individuals exposed to cigarette smoke have a greater number and severity of viral infections, including respiratory syncytial virus (RSV) infections, than do nonsmokers, but the cellular mechanism is unknown. Our objective was to determine the mechanism by which cigarette smoke augments viral infection. We hypothesize that cigarette smoke causes necrosis and prevents virus-induced cellular apoptosis, and that this is associated with increased inflammation and viral replication. Primary airway epithelial cells were exposed to cigarette smoke extract for 2 days, followed by 1 day of RSV exposure. Western blot detection of cleaved caspases 3 and 7 showed less apoptosis when cells were treated with cigarette smoke before viral infection. This finding was confirmed with ELISA and TUNEL detection of apoptosis. Measures of cell viability, including propidium iodide staining, ATP assay, and cell counts, indicated that cigarette smoke causes necrosis rather than virus-induced apoptosis. Using plaque assay and fluorescently-labeled RSV, we showed that although there were less live cells in the cigarette smoke–pretreated group, viral load was increased. The effect was inhibited by pretreatment of cells with N-acetylcysteine and aldehyde dehydrogenase, suggesting that the effect was primarily mediated by reactive aldehydes. Cigarette smoke causes necrosis rather than apoptosis in viral infection, resulting in increased inflammation and enhanced viral replication.

Key Words: respiratory syncytial virus • apoptosis • necrosis • cigarette smoke

CLINICAL RELEVANCE Individuals exposed to cigarette smoke have a greater number and severity of viral infections, but the cellular mechanism is unknown. Our study shows that cigarette smoke causes necrosis rather than apoptosis in viral infection, resulting in increased inflammation and enhanced viral replication.

 

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