ABSTRACT
Human and murine receptors for tumor necrosis factor alpha (TNF-alpha) are present on most somatic cells and have been characterized and cloned. In contrast, very little is currently known about whether TNF-alpha can bind to pathogens and whether such binding results in important biological consequences for the infected host. We now report that a number of gram-negative bacteria have receptors for TNF-alpha. Using 125I-labeled TNF-alpha, we show that Shigella flexneri has 276 receptors for TNF-alpha, with a Kd of 2.5 nM. The binding of labeled TNF-alpha to these bacterial receptors can be inhibited by cold TNF-alpha but not by cold TNF-beta. Binding of 125I-TNF-alpha to S. flexneri was inhibited by trypsin treatment of bacterial cells or incubation at 52 degrees C for 3 min. Monoclonal antibody to either the 55-kDa or the 75-kDa TNF-alpha receptor, which are present on different eukaryotic cells, had no effect on 125I-TNF-alpha binding to bacteria. A number of gram-negative bacteria were capable of binding 125I-TNF-alpha. Gram-positive bacteria bound significantly less 125I-TNF-alpha than gram-negative bacteria. Pretreatment of S. flexneri with TNF-alpha resulted in enhanced bacterial invasion of HeLa cells and enhanced uptake by human and murine macrophages. Pretreatment of HeLa cells with antibody to the 55-kDa TNF-alpha receptor abrogated enhanced invasion of HeLa cells by TNF-alpha-bacterium complexes. These results suggest that TNF-alpha-bacterium complexes can interact with TNF-alpha receptors present on eukaryotic cells. This report shows that gram-negative bacteria have receptors for TNF-alpha and that a virulence property of a bacterium is altered as a consequence of cytokine binding.
