Keratinocyte growth factor-induced hyperplasia of rat alveolar type II cells in vivo is resolved by differentiation into type I cells and by apoptosis

Keratinocyte growth factor (KGF) is a potent mitogen of alveolar epithelial type II cells (AEII). AEII hyperplasia is resolved within several days fol lowing intratracheal instillation of KGF by unknown mechanism(s). AEII hyperplasia was induced in rat lungs by intrabronchial instillation of 5 mg recombinant human (rh)KGFkg body weight(-1) or an equivalent amount o f diluent. Epithelial architecture, cell proliferation, transformation of A EII into type I cells (AEI) and apoptosis were investigated by means of imm unohistochemistry, stereology, double immunofluorescence microscopy, electr on microscopy and the terminal deoxynucleotidyl transferase-mediated deoxyu ridine triphosphate nick end-labelling (TUNEL) technique in lungs fixed 1, 2, 3 and 7 days after treatment. After 1 day of rhKGF instillation, an increase was observed in the nuclear antigen Ki-67, a proliferation marker detected by the antibody MIB-5-expres sing surfactant protein (SP)-B, -C, -D-positive AEII. The incidence of mito sis was increased by day 2, resulting in AEII micropapillae with intense ba solateral expression of the exon 6 containing isoform (v6) of CD446 (CD44v6 ), a marker for AEII. By day 3, monolayers of AEII exhibiting lateral CD44v 6 covered 45% of the alveolar surface. After 7 days, there were numerous in termediate AEII/AEI cells characterized by a flat elongated shape, staining for SP-D, apical appearance of AEI marker Lycopersicon esculentum lectin a nd lateral staining for AEII marker CD44v6. Increased numbers of TUNEL-posi tive epithelial cells were seen at days 2-7. In conclusion, restoration of normal alveolar epithelium after instillation of recombinant human keratinocyte growth factor is accomplished by termina l differentiation and apoptosis of hyperplastic alveolar epithelial type II cells in vivo.