Coexpression of RTI40 with alveolar epithelial type II cell proteins in lungs following injury: identification of alveolar intermediate cell types

Abstract

Injured alveolar epithelial type (AT) I cells are replaced following the proliferation and transformation of ATII cells to new ATI cells. RTI₄₀ is an ATI cell-specific protein required for normal lung development. We hypothesized that intermediate cell types in the ATII-to-ATI cell transformation would coexpress RTI₄₀ and ATII cell-selective proteins. To test this hypothesis, we used a rat model of Staphylococcus aureus-induced acute lung injury and a panel of ATI and ATII cell-specific and -selective antibodies. S. aureus induced an acute inflammatory reaction that was resolving by day 3 postinoculation. At day 3 postinoculation, the alveolar wall was thickened secondary to ATII cell hyperplasia. With the use of confocal microscopy, there was a fivefold increase in the fractional surface area of alveolar walls stained with ATII cell membrane proteins (RTII₇₀ and MMC4) and a decrease in the fractional surface area associated with RTI₄₀-expressing cells. S. aureus-treated lungs also contained unique cell types that coexpressed the RTI₄₀ and ATII markers RTI₄₀/MMC4/RTII₇₀- and RTI₄₀/MMC4-positive cells. These cells were not observed in control lungs. RTI₄₀/MMC4-positive cells were also found in cultured ATII cells before they transformed to an ATI-like phenotype. Our data suggest that RTI₄₀/MMC4/RTII₇₀- and RTI₄₀/MMC4-positive cells are intermediates in the ATII-to-ATI cell transformation. These data also suggest that the coexpression of RTI₄₀ with ATII cell proteins may be used to identify and investigate ATII cell transdifferentiation to ATI cells following injury.