MODE OF ACTION OF ANTI-LIPOPOLYSACCHARIDE-BINDING PROTEIN ANTIBODIES FOR PREVENTION OF ENDOTOXEMIC SHOCK IN MICE

Lipopolysaccharide (LPS)-binding protein (LBP) has been shown to regul ate the response of monocytes to LPS in vitro. In a previous study, po lyclonal anti-LBP IgGs were found to protect D galactosamine-sensitize d mice against a lethal endotoxemic shock induced by a low challenge o f LPS or lipid A when administered simultaneously with endotoxin. In t he present study, we investigated the mode of action of these anti-LBP IgGs. In vitro, we demonstrated that they interfere with LPS binding to monocytes or polymorphonuclear cells in different ways: by the mere prevention of binding of LPS to LBP thus preventing the binding of LP S to CD14, or by reacting with LPS-LBP complexes thus mediating their binding to complement or Fc receptors an monocytes and on polymorphonu clear cells. In vivo, we demonstrated that anti-LBP IgGs afforded prot ection against lethal endotoxemic shock by one of two mechanisms. Firs t, LBP blockade by pretreatment with anti-LBP IgG allowed protection a gainst a low dose of LPS (100 ng). This protection occurred despite LP S levels in blood similar to those in control mice but in the absence of detectable tumor necrosis factor (TNF). This demonstrated that anti -LBP IgG could block the LBP-mediated TNF release upon LPS challenge. In contrast, anti-LBP IgG did not afford protection in mice not sensit ized with D-galactosamine and challenged with high-dose LPS (1 mg), co nfirming that LPS at high concentrations could stimulate cells indepen dently of the LBP pathway. Second, anti-LBP treatment administered sim ultaneously with LPS challenge protected mice against both low and hig h doses of LPS. Unlike after pretreatment with anti-LBP IgG, this prot ection was accompanied by a decrease of circulating LPS, suggesting th at anti-LBP IgG in these conditions facilitated clearance of LPS proba bly by clearing LPS-LBP complexes. These data and the fact that LBP bi nds to all LPS through lipid A suggest that antibody directed to LBP c ould be a candidate for therapeutic strategies in endotoxemic shock.