Hyperoxia inhibits alveolar epithelial repair by inhibiting the transdifferentiation of alveolar epithelial type II cells into type I cells

Introduction

The alveolar epithelium is comprised of type I (ATI) and type II (ATII) cells. ATI cells are incapable of cell division, with epithelial repair achieved by proliferation of ATII cells and transdifferentiation of ATII cells into ATI cells. We have previously shown that hyperoxia inhibits the transdifferentiation of ATII cells in vitro [1]. The objective of these studies was to determine the effect of hyperoxia on the transdifferentiation of ATII cells in vivo.

Methods

Rats (n = 9) were anaesthetised and Staphylococcus aureus or saline (controls) instilled into the distal airways. Animals recovered in air for 72 hours and were then randomised to normoxia (air) or hyperoxia (FiO₂ ~0.9) for 48 hours. Lung sections were stained with combinations of cell-selective antibodies, immunofluorescent images obtained using confocal microscopy and the proportion of the alveolar surface covered with ATII (RTII₇₀/MMC4-positive), ATI (RTI₄₀-positive) and transitional (RTI₄₀/MMC4-positive) cell-staining membrane determined by quantification of binary masks.

Results

In control lungs, 94 ± 2% of the alveolar surface was lined by ATI, 2 ± 1% by ATII and <1% by transitional cell-staining membrane. In S. aureus-instilled lungs exposed to normoxia alone, there was a decrease in ATI cell-staining membrane (84 ± 3%) with an increase in ATII (8 ± 1%) and transitional (12 ± 4%) cell-staining membrane consistent with ATI cell necrosis, ATII cell hyperplasia and transdifferentiation of ATII cells into ATI cells. In S. aureus-instilled lungs exposed to hyperoxia, there was a decrease in ATI cell-staining membrane (73 ± 5%, P < 0.05) with a marked increase in ATII cell-staining membrane (16 ± 1%, P < 0.001) and less transitional cell-staining membrane (3 ± 1%, P < 0.05). As hyperoxia is proapoptotic and inhibits ATII proliferation [2, 3], these data suggest persistent ATII cell hyperplasia and reduced ATII cell transdifferentiation.

Conclusion

Hyperoxia impairs epithelial repair by inhibiting the transdifferentiation of ATII cells into ATI cells in a model of resolving S. aureus-induced pneumonia.