Lipopolysaccharide (LPS)-reactive monoclonal antibodies fail to inhibit LPS-induced tumor necrosis factor secretion by mouse-derived macrophages.

Murine monoclonal antibodies (MAbs) reactive with epitopes on the O-side chain, core oligosaccharide, or lipid A of Escherichia coli and Salmonella minnesota lipopolysaccharide (LPS) were evaluated for their ability to inhibit LPS-induced tumor necrosis factor (TNF) secretion by mouse-derived RAW 264.7 macrophages. As little as 50 ng of purified LPS or lipid A stimulated macrophages to produce TNF detectable as cytotoxic activity in an L-929 fibroblast assay. None of 13 MAbs (concentration range, 0.1-1,000 micrograms/mL) blocked LPS- or lipid A (0.025-0.1 micrograms/mL)-induced TNF secretion by RAW 264.7 cells. Rabbit antiserum to synthetic lipid A also failed to block lipid A-induced TNF activity. Similar negative results were obtained when intact bacteria or membrane vesicles were used as TNF inducers. In contrast, polymyxin B, but not the less hydrophobic polymyxin B nonapeptide, produced almost complete inhibition of macrophage TNF secretion induced by LPS, lipid A, membrane vesicles, and intact bacteria. Thus, antibody reactivity with predominantly hydrophilic elements of LPS or lipid A may not affect hydrophobic interactions between lipid A and target cell membranes necessary and sufficient for the induction of TNF. These findings raise doubts concerning the existence of true endotoxin-neutralizing antibodies.