EXPRESSION OF TNF AND THE NECESSITY OF TNF RECEPTORS IN BLEOMYCIN-INDUCED LUNG INJURY IN MICE

Bleomycin (BLM) induction of lung fibrosis in mire is an established m odel to study the mechanism of pulmonary fibrosis. Cytokine secretion has been implicated as a fundamental component of the lung fibrotic pr ocess observed in response to BLM. Among the cytokines implicated in l ung fibrosis, Tumor necrosis factor (TNF) alpha has been considered to play a fundamental role. In the present study, we characterized the c ellular sources of TNF during BLM-induced lung injury and examined the importance of TNF receptors in this process. To characterize the expr ession of TNF, we utilized two strains of mice, one sensitive (C57BL/6 ) and one resistant (BALB/c) to BLM-induced lung injury. Mice received BLM (120 mg/kg total) or saline, as control, by multiple subcutaneous injections. BLM induced the development of inflammation in subpleural areas only in the lungs of BLM-sensitive mice. These subpleural areas were characterized by infiltration of CD68-positive macrophages and i ncreased collagen deposition. BLM enhanced the expression of TNF mRNA in BLM-sensitive, but not in BLM-resistant, mice. In situ hybridizatio n studies localized the expression of TNF in the areas of BLM-induced inflammation in 6% and 27% of macrophages at 14 and 21 days post BLM t reatment. In addition to TNF, BLM exposure resulted in the upregulated expression of transforming growth factor (IGF)-beta 1, but not interl eukin (IL)-1, mRNA in the lungs of both murine strains at 14 and 21 da ys. This upregulated expression of TGF-beta 1 mRNA was greater in the lungs of BLM-sensitive mice. In separate experiments, double TNF recep tor knockout mice were exposed to BLM. These animals demonstrated an i ncreased expression of TNF, but not TGF-beta 1, mRNA in response to BL M and did not exhibit histologic evidence of lung injury following BLM exposure. In summary, the upregulation of TNF mRNA in macrophages cor related with the appearance of inflammation following BLM exposure and was limited to the BLM-sensitive strain. Furthermore, in addition to the release of the TNF ligand, ii appears that the presence of TNF rec eptors ir necessary for the development of BLM-induced lung injury, an d signaling through these receptors may contribute to the regulation o f the TGF-beta 1 mRNA expression observed in response to bleomycin. Th ese results provide further support for a role of macrophages and TNF in the induction of lung inflammation.