OVEREXPRESSION OF GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR INDUCES PULMONARY GRANULATION-TISSUE FORMATION AND FIBROSIS BY INDUCTIONOF TRANSFORMING GROWTH-FACTOR-BETA-1 AND MYOFIBROBLAST ACCUMULATION

We have previously reported that transfer to rat lung of the granulocy te-macrophage colong-stimulating factor (GM-CSF) gene leads to high ex pression of GM-CSF between days 1 and 4 and granulation tissue formati on followed by an irreversible fibrotic response starting from day 12 onward. In the current study, we investigated the underlying mechanism s. We found that GM-CSF overexpression did not enhance production of t umor necrosis factor-alpha in a significant manner at any time after G M-CSF gene transfer. However, the content of transforming growth facto r-beta 1 in bronchoalveolar lavage fluid was markedly induced at day 4 and appeared to be maximal around day 7 and remained high at day 12. Macrophages purified from bronchoalveolar lavage fluid 7 days after GM -CSF gene transfer spontaneously released significant quantities of tr ansforming growth factor-beta 1 protein in vitro. After peak transform ing growth factor-beta 1 production was the emergence of alpha-smooth muscle actin-rich myofibroblasts. Accumulation of these cells was most prominent at day 12 within the granulation tissues and they were stil l present in fibrotic areas between days 12 and 24 and diminished mark edly afterward. Thus, markedly afterward. Thus, we provide the first i n vivo evidence that tumor necrosis factor-alpha may be dissociated fr om participation in a fibrotic process in the lung and GM-CSF may play a more direct role in pulmonary fibrogenesis at least in part through its capability to induce transforming growth factor-beta 1 in macroph ages and the subsequent emergence of myofibroblast phenotypes. This GM -CSF transgene lung model is useful for a stepwise dissection of both cellular and molecular events involved in pulmonary fibrosis.