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Impaired Clearance of Apoptotic Cells From Cystic Fibrosis Airways*

R. William Vandivier, MD; Valerie A. Fadok, PhD; Carol Anne Ogden, PhD; Peter R. Hoffmann, BA; Joseph D. Brain, MD; Frank J. Accurso, MD; James H. Fisher, MD, FCCP; Kelly E. Greene, MD; Peter M. Henson, PhD
Author and Funding Information

*From the University of Colorado Health Sciences Center (Drs. Vandivier, Accurso, and Fisher) and the National Jewish Research and Medical Center (Drs. Fadok, Greene, Henson, and Ogden, and Mr. Hoffmann), Denver, CO; and the Harvard School of Public Health (Dr. Brain), Boston, MA.

Correspondence to: R. William Vandivier, MD, National Jewish Research and Medical Center, 1400 Jackson St, Denver, CO 80206



Chest. 2002;121(3_suppl):89S. doi:10.1378/chest.121.3_suppl.89S
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Cystic fibrosis (CF) lung disease is characterized by early, protracted inflammation that is associated with a massive influx of inflammatory cells and the release of intracellular proteases. The regulation of this intense inflammatory response depends on the rapid removal by phagocytes of dying (apoptotic) inflammatory cells prior to their necrotic degeneration. The recognition mechanisms involved in apoptotic cell removal include CD36 and other scavenger receptors, CD14, integrins, the phosphatidylserine receptor, and soluble tethering proteins such as collectins and thrombospondin. Our objective was to determine whether the removal of apoptotic cells was impaired in the airways of CF patients, and whether airway proteases contributed to defective clearance through the cleavage of phagocytosis receptors. Furthermore, since the levels of the lung collectins surfactant protein (SP) A and SP-D are decreased in CF patients, we tested their involvement in the removal of apoptotic cells. The number of apoptotic cells, as assessed by typical morphologic changes, was increased in the sputum of CF patients compared to control subjects who have chronic bronchitis, which supports the hypothesis that the airway clearance of apoptotic cells is defective in CF patients. The role of CF airway proteases in defective apoptotic cell clearance was tested by incubating human monocyte-derived macrophages (HMDMs) with apoptotic Jurkat cells (ie, a T-cell line) in the presence of 10% CF sol with or without a specific neutrophil elastase inhibitor. Incubation with CF sol inhibited HMDM phagocytosis of apoptotic Jurkat cells, but this effect was reversed by an elastase inhibitor. Neutrophil elastase and cathepsin G also inhibited the HMDM phagocytosis of apoptotic Jurkat cells. CF sol cleaved HMDM phosphatidylserine receptor in a neutrophil elastase-dependent manner but had no effect on CD36 or FcγRII. The role of SP-A and SP-D in apoptotic cell removal was tested by preincubating apoptotic neutrophils with SP-A or SP-D and assessing the effect on their ingestion by HMDMs. Opsonization by SP-A or SP-D augmented HMDM ingestion of apoptotic neutrophils in a manner that was dependent on the interaction with surface calreticulin. The importance of SP-D in the clearance of apoptotic cells in vivo was tested in SP-D knockout, wild-type, and overexpressing mice that were challenged intratracheally with apoptotic neutrophils. These experiments showed that the clearance of apoptotic neutrophils was inhibited by SP-D deficiency and was enhanced by SP-D overexpression. Similar experiments are underway in SP-A knockout and overexpressing mice. These results demonstrate that apoptotic cells are increased in the airways of CF patients and suggest that the impaired clearance of apoptotic cells may be due to protease cleavage of key cell surface receptors and a deficiency of SP-A or SP-D. We speculate that severe and sustained inflammation in the airways of CF patients may, in part, be enhanced by the defective clearance of dying inflammatory cells.

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