Induction of a 26-kDa membrane-form tumor necrosis factor (TNF)-α in human alveolar macrophages

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Abstract
The expressions of a membrane form of TNF (m-TNF) by human alveolar macrophages (AM) and autologous blood monocytes from healthy donors were examined. Upon lipopolysaccharide (LPS) stimulation, AM produced 26-kDa TNF-α on their cell surface. We designed a bioassay for measuring m-TNF in which macrophages were fixed with paraformaldehyde after stimulation for 18 h, then m-TNF activity was assessed as cytotoxicity of fixed macrophages on L929 cells. This assay was specific to m-TNF because: 1) no soluble factors were contributed to the cytotoxicity of fixed AM, 2) anti-TNF-α monoclonal antibody completely neutralized m-TNF activity, and 3) m-TNF activity was not altered after low- pH or high-salt treatment. On LPS stimulation, AM produced significant amounts of m-TNF earlier than TNF-α secretion. AM also expressed significant amounts of m-TNF when stimulated with other bacterial components and their derivatives. Interleukin (IL)-4 suppressed both m-TNF production and TNF-α secretion. p-Toluene- sulfonyl-L-arginine methyl ester (TAME) inhibited specifically TNF-α secretion and not m-TNF expression. Although blood monocytes produced small amounts of m- TNF, monocyte-derived macrophages showed enhanced m-TNF after cultivation with GM-CSF for 10 days. These findings indicate that m-TNF is expressed as a step in the TNF-α producing system, and suggest that m-TNF may play important roles in exhibition of macrophage function in situ.

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