Alterations in the immunohistochemical distribution patterns of vascular endothelial growth factor receptors Flk1 and Flt1 in bleomycin-induced rat lung fibrosis

To investigate the role of vascular endothelial growth factor (VEGF) in fib rogenesis, the distribution patterns of the VEGF receptors Flt1 and Flk1 we re studied by immunohistochemistry, double immunofluorescence, and immunoel ectron microscopy in normal (n=2) and bleomycin-treated (n=21) adult rats. Lungs were studied at 5, 24, 28, 35, and 42 days after treatment (p.t.). Fl t1, Flk1, and VEGF immunoreactivity localised predominantly to the pulmonar y epithelium. In control lungs, Flt1 immunoreactivity was present in ciliat ed bronchial epithelium and type 2 pneumocytes, Flk1 in Clara cells, and VE GF in Clara cells and type 2 pneumocytes. Flk1 localised to mast cells, pre sent in the peribronchovascular and pleural interstitium only. Flt1- and Fl k1-mRNAs were observed in Clara cells and type 2 pneumocytes. Bleomycin-ind uced fibrogenesis was characterised by a decrease in Flk1 immunoreactivity of Clara cells, and an increase in VEGF-immunoreactive myofibroblasts and t ype 2 pneumocytes by day 5 p.t., followed by a progressive accumulation of Flk1-immunoreactive mast cells by day 24 p.t. in fibrotic lesions containin g VEGF-immunoreactive myofibroblasts. After 42 days, fibrotic regions were densely populated by mast cells. Since mast cells are known to be chemotact ically attracted by VEGF, we suggest that VEGF/Flk1 represents the molecula r link between proliferation of myofibroblasts, accumulation of mast cells, and the burst of fibrosis at sites of initial lesions in bleomycin-induced fibrosis.