Dynamic change of polarity in spread through air spaces of pulmonary malignancies.

Spread through air spaces (STAS) is a histological finding of lung tumours where tumour cells exist within the air space of the lung parenchyma beyond the margin of the main tumour. Although STAS is an important prognostic factor, the pathobiology of STAS remains unclear. Here, we investigated the mechanism of STAS by analysing the relationship between STAS and polarity switching in vivo and in vitro. Histopathological analysis revealed that apical membranes were observed outside the STAS lesions around colorectal cancer (CRC) lung metastases and lung adenocarcinomas. When apical-out CRC organoids were administered intratracheally to mice, the organoids had greater metastatic potential than did single cells. To investigate the pathobiology of STAS, we established an in vitro model of STAS in which CRC or lung cancer organoids were co-cultured with 2D-cultured mouse airway epithelial organoids (2D-MAOs). Adhesion of cancer organoids to 2D-MAOs was much less than to type I collagen or endothelial cells, suggesting a protective role of the airway epithelium against adhesion. Loss of the apical membrane of CRC organoids at the contact surface with 2D-MAOs after adhesion was responsible for establishing adhesion. When airway epithelium was stimulated by transforming growth factor beta 1 (TGF-β1), adhesion of CRC organoids was enhanced. Among TGF-β1-induced genes in airway epithelium, follistatin-like protein 1 (FSTL1) increased CRC organoid adhesion by promoting loss of the apical membrane. These results suggested that TGF-β1-induced FSTL1 may promote metastatic progression of STAS by altering the polarity status. Elucidating the mechanism of STAS could contribute to the improvement of survival in patients with pulmonary malignancies associated with STAS. © 2025 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.