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Mechanisms of fibrosis in respiratory disease
S138 Clara cells inhibit alveolar epithelial wound repair through a TRAIL-dependent apoptosis mechanism
  1. K M Akram1,
  2. M Spiteri2,
  3. N R Forsyth1
  1. 1Lung Research Group, Institute of Science and Technology, Keele University, Stoke-on-Trent, UK
  2. 2University Hospital of North Staffordshire, Stoke-on-Trent, UK


Introduction Alveolar bronchiolization, a hyperproliferation of ciliated and non-ciliated (Clara) bronchiolar cells and their extension into the alveolar region, is a common feature of idiopathic pulmonary fibrosis. The role of this bronchiolization process in alveolar wound repair is controversial. Clara cells are also believed to be progenitors for ciliated bronchiolar epithelium but their influence in repair processes is unclear.

Methods Using an in vitro wound repair model we explored the interaction of human Clara cells (H441 cell line) and type II AEC (A549 cell line). A transwell co-culture system was developed to determine the direct contact effect of densely populated Clara cells on wounded AEC monolayers.

Results In serum-free media, lone H441 cell wound repair was higher than equivalent A549 cells, despite the fourfold slower doubling time of H441 cells. Serum-free conditioned media obtained from un-wounded and wounded H441 monolayers did not show any significant influence on A549 wound repair. However, in a direct contact co-culture A549-H441 cell model significant inhibition of A549 wound repair (p<0.005) was observed. Interestingly, H441 migration into the injured A549 layer was seen after 24 h; with a significant proportion of migrated H441 cells found at the wound margins. Coupled to this migration we observed a 50% reduction in A549 cell number at the wound margins. TUNEL assay detected about 40% A549 apoptosis in juxta-wound monolayers in A549-H441 direct contact (p<0.00001). This direct contact-induced apoptosis was significantly blocked by TRAIL-R1 and R2 combined receptor blockers (p<0.00001); whereas, Fas blocker failed to block this apoptosis.

Conclusion In summary, direct contact of H441 cells induces apoptosis in the A549 monolayers through a TRAIL-dependent mechanism which disrupts wound margin integrity, inhibiting wound repair. This novel observation warrants further exploration of the role of Clara cell-alveolar epithelial cell interaction within the context of aberrant wound repair associated with chronic fibrotic lung disorders.

Abstract S138 Figure 1

H441 direct contact induced apoptosis in A549 cells through TRAIL-R1+R2 (10 µg/ml each) blockade. Fas blocker (10 µg/ml) failed to block direct cell contact- induced apoptosis. In positive control, apoptosis was induced with 200 µM H2O2. Negative control was represented by A549 cells cultured alone in monolayer.

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