THE SIGNIFICANCE OF THE HYDROPHILIC BACKBONE AND THE HYDROPHOBIC FATTY-ACID REGIONS OF LIPID-A FOR MACROPHAGE BINDING AND CYTOKINE INDUCTION

Natural partial structures of lipopolysaccharide (LPS) as well as synt hetic analogues and derivatives of lipid A were compared with respect to inhibit the binding of I-125-labelled Re-chemotype LPS to mouse mac rophage-like J774.1 cells and to induce cytokine-release in J774.1 cel ls. LPS, synthetic Escherichia coli-type lipid A (compound 506) and te traacyl precursor Ia (compound 406) inhibited the binding of I-125-LPS to macrophage-like J774.1 cells and induced the release of tumor necr osis factor alpha (TNF alpha) and interleukin 6 (IL6). Deacylated R-ch emotype LPS preparations were completely inactive in inhibiting bindin g and in inducing cytokine-release. Among tetraacyl compounds, the inh ibition-capacity of LPS-binding was in decreasing order: PE-4 (alpha-p hosphonooxyethyl analogue of 406)> 406 >> 404 (4'-monophosphoryl parti al structure of 406)> 405 (1-monophosphoryl partial structure of 406). In the case of hexaacyl preparations, compounds 506, PE-1 (alpha-phos phonooxyethyl analogue of 506) and PE-2 (differing from PE-1 in having 14:0 at positions 2 and 3 of the reducing GlcN) inhibited LPS-binding and induced cytokine release equally well, whereas preparation PE-3 ( differing from PE-2 in containing a beta-phosphonooxyethyl group) show ed a substantially lower capacity in binding-inhibition and cytokine-i nduction. The conclusion is that chemical changes in the hydrophilic l ipid A backbone reduce the capacity of lipid A to bind to cells, where as the number of fatty acids determines the capacity of lipid A to act ivate cells. These results indicate that the bisphosphorylated hexosam ine backbone of lipid A is essential for specific binding of LPS to ma crophages and that the acylation pattern plays a critical role for LPS -promoted cell activation, i.e. cytokine induction.