Early mesothelial cell proliferation after asbestos exposure: in vivo and in vitro studies

There is some evidence that proliferation of pleural mesothelial cells (MC) occurs soon after deposition of asbestos fibers. To study this effect, we instilled a single dose of 0.1 mg crocidolite into the lungs of mice for 1 and 6 weeks and counted labeled nuclei after 3H-thymidine (3HT) injection. Short fibers (< 1 micron) induced little change in the lung; they were mostly phagocytized and had a minimal effect on MC labeling. Long fibers up to 20 microns damaged the bronchiolar epithelium and were incorporated into connective tissue. Increased 3HT uptake was seen in fibroblasts and epithelial cells and also in MC, which peaked at 2% labeled at 1 week compared to near 0% labeling in controls. No fibers were found in or near labeled MC, which suggested that a cytokine generated in the lung during the early response phase might induce MC proliferation. To look for a cytokine effect in vitro, we instilled asbestos into rat lungs and, after 1 and 6 weeks, bronchoalveolar and pleural lavage fluids as well as macrophages were collected. Alveolar macrophages contained fibers, but pleural macrophages (PM) did not. After short-term culture, macrophage supernatants and the lavage fluids were tested on rat lung MC in culture. At 1 week, PM secreted growth factor(s) for MC, and the mitogenic effect was more pronounced with lavage fluids. No effects on MC were found using material prepared 6 weeks after asbestos. The early MC growth increase was not blocked by antibodies to cytokines, such as platelet-derived growth factor, fibroblast growth factors, or tumor necrosis factor, but was inhibited by anti-keratinocyte growth factor (anti-KGF). The results show that an early growth phase of MC after asbestos exposure appears unrelated to particle translocation to the pleura but is associated with cytokine release, most likely KGF, by lung cells.