Bidirectional modulation of TNF-alpha production by alveolar macrophages in asbestos-induced pulmonary fibrosis.

Bronchoalveolar lavage (BAL) cells were isolated from rats 1, 3, and 6 weeks after a single intratracheal instillation of saline, UICC chrysotile asbestos (5 mg), or silica (5 mg). In asbestos-exposed rats, the pulmonary response was characterized by a significant increase in the number of alveolar macrophages (AMs) and the appearance of fibrotic lesions within 1 week. By contrast, mixed macrophage and neutrophil accumulations were observed in the silica group without evidence of fibrosis. Tumor necrosis factor-alpha (TNF-alpha) production by lipopolysaccharide (LPS)-stimulated BAL cells from asbestos-treated rats was significantly lower than controls 1 and 3 weeks after exposure. However, by 6 weeks higher levels of TNF-alpha production were noticeable in this group. Decreases in LPS-induced TNF-alpha production were also observed with BAL cells from silica-treated animals at all time points studied. Lower levels of TNF-alpha were not related to decreased BAL cell viability or the presence of a significant proportion of neutrophils in the silica group. Furthermore, biphasic changes in TNF-alpha production seen in the asbestos group were correlated with concomitant decreases (3 weeks) and increases (6 weeks) in levels of TNF-alpha mRNA in AMs. These data indicate that lower levels of TNF-alpha resulted from inhibition at the gene expression level and provide evidence for bidirectional modulation of TNF-alpha production by AMs during inflammatory reactions.