Objective: To determine whether monoclonal antibodies (mAbs) directed against lipopolysaccharide (LPS, endotoxin) act by promoting LPS neutralization, LPS uptake by macrophages, or both processes, the authors assessed the effects of these agents on LPS-induced cytokine secretion and cellular uptake of LPS. Summary Background Data: MAbs directed against LPS have been shown to attenuate LPS-induced macrophage tumor necrosis factor-α (TNF-α) secretion, a process that may contribute to protective capacity. The mechanisms by which this process occurs have not been established. Methods: MAbs directed against LPS were evaluated in vitro for their capacity to (1) inhibit TNF-α secretion, and (2) alter fluorescein isothiocyanate-labeled LPS uptake (employing flow cytometry analysis and fluorescence microscopy) by the macrophage-like cell line RAW 264.7. Results: MAb 8G9, an IgG3 directed against the O-antigen polysaccharide region of Escherichia coli 0111:B4 LPS, significantly reduced LPS-induced TNF-α secretion and promoted a more than 40-fold increase in LPS uptake by macrophages. The authors established that this was mediated by a Fc receptor-mediated process because 8G9 F(ab')2 fragments that lack the Fc portion of the IgG molecule were capable of inhibiting TNF-α secretion, but did not promote increased LPS uptake to the same degree. Cross-reactive, anti-deep core/lipid A mAb 1B6, an IgG(2a), also promoted uptake of E. coli 0111:B4 LPS and O-antigen polysaccharide-deficient E. coli J5 LPS, but only inhibited TNF-α secretion induced by E. coli J5 LPS to which it binds most efficiently MAb 3D10, an IgM also directed against the O-antigen polysaccharide region of E. coli 0111:B4 LPS, inhibited TNF-α secretion but did not increase cellular uptake of LPS, presumably acting solely due to LPS neutralization. Polymyxin B, an antibiotic that binds stoichiometrically to the lipid A portion of LPS, inhibited TNF-α secretion and prevented cellular LPS uptake. Conclusions: These results suggest that IgG and IgM anti-LPS mAbs exert protective capacity by extracellular neutralization of LPS, while IgG Fc receptor-mediated cellular uptake also may serve to bypass macrophage activation and TNF-α secretion by promoting internalization and intracellular neutralization.
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