Silica-Exposed Lung Fluids Have a Proliferative Activity for Type II Epithelial Cells: A Study on Human and Sheep Alveolar Fluids

Abstract

The type II pneumocyte changes in silicosis are characterized by hyperplasic and hypertrophic epithelial cells, and increased surfactant phospholipids in the bronchoalveolar lavage fluids (BALF). To assess the proliferative activity of alveolar lining fluids, BALF were applied on type II cell cultures. The growth-promoting activity was studied by tritiated thymidine incorporation for 24 h, and the cell number was measured by an electronic counting after a 48-h exposure time. Human BALF from 3 subsets of workers exposed to silica, staged according to ILO classification (silica exposed-workers without disease: hSWD n = 6; workers with simple silicosis: hSS n = 7; workers with confluent silicosis: hCS n = 5), were compared to healthy volunteers (hC n = 6). Sheep BALF from our model of silicosis and control animals (sS and sC) were studied at months 0, 6, and 24 of exposure. A clear enhancement was found in type II cell DNA synthesis under the effect of either normal and silicotic human or sheep BALF, in comparison to the negative control (p < .05). In addition hSWD and hSS BALF as sS BALF at 20% dilution (peak activity) were significantly more stimulating than the normal alveolar fluids from the same species (p < .05). The highest sheep BALF stimulatory activity was found at month 6 (170% of increase vs control, p < .05) and clearly correlated with the high cellularity of BALF. The thymidine incorporation was supported by changes in cell counts. Sheep silicotic BALF run through G50 columns identified at least 3 molecular weight (MW) areas of mitogenic activity between 30 and 5 kDa. Biochemical characteristics of growth factors in the above MW range (PDGF, FGF, TGFα, EGF) were tested. Increased mitogenic activity of type II cells eluted from heparin sepharose columns loaded with silicotic sheep BALF, at 0.5 and 1 M NaCl, corresponded to the removal areas of PDGF- and acidic FGF-like heparin-binding molecules. The high proliferative activity on type II cells of the latter two molecules, alone or in combination with other growth factors, was demonstrated in vitro (> 9 × control). In conclusion, a stimulatory activity for type II cell growth was found in the normal human and sheep alveolar lining fluid. This activity was clearly enhanced in the early stages of human and sheep silicosis. The BALF type II cell growth factors bad biochemical characteristics consistent with the PDGF- and FGF-like molecules