Inhibition of endotoxin-induced interleukin-6 production by synthetic lipid A partial structures in human peripheral blood mononuclear cells

The effect of two synthetic lipid A partial structures, compound 406 (or LA-14-PP, identical in structure to the lipid A precursor, known as Ia or IVa) and compound 401 (lipid X), on the in vitro modulation of endotoxin (lipopolysaccharide)-induced interleukin-6 production by human blood mononuclear cells was investigated. Lipopolysaccharide of Salmonella abortus equi and synthetic Escherichia coli-type lipid A (compound 506, or LA-15-PP) had potent interleukin-6-inducing capacities. The maximum release of interleukin-6 was found after stimulation with 1 to 10 ng of lipopolysaccharide or 10 to 100 ng of synthetic E. coli-type lipid A per ml. Both synthetic lipid A partial structures (compounds 406 and 401) failed to induce interleukin-6 release. However, they inhibited lipopolysaccharide- or lipid A-induced interleukin-6 production in a dose-dependent manner. Inhibition was found not only in mononuclear cells but also in purified monocytes and was not due to a shift in the kinetics of cytokine production. Suppression was manifested in the early stage of interleukin-6 production. Inhibition was also found in the presence of recombinant gamma interferon, indicating that compound 406 and recombinant gamma interferon act in different, independent pathways. Our data, therefore, indicate that the inhibition of interleukin-6 production by lipid A partial structures may help elucidate the mechanism of interaction of the lipid A component of lipopolysaccharide with immune cells in the inflammatory reaction during gram-negative infection.