The synthesis of AGPs has been described elsewhere (55) . Briefly, the AGPs were prepared by a highly convergent method, which allowed chemical differentiation of the hydroxyl and amino groups and sequential introduction of the (R)-3-n-alkanoyloxytetrahexanoyl residues. The AGPs were purified by flash chromatography on silica gel (to >95% purity) and analyzed as a triethylammonium salt by standard analytical methods. For stimulation in vitro, the AGPs were formulated in water containing 216 μg/ml dipalmitoylphosphatidyl choline [aqueous formulation (AF)], 0.2% triethanolamine (pH 7.4), or in 2% glycerol (i.v. formulation). (53) . The structure of E5531, an analog of the lipid A of Rhodobacter capsulatus, is presented elsewhere (56) . The structure of E5564 is depicted in Fig. 5 . The crystal structure of the TLR4-MD2 complex with bound E5564 was recently published (57) suggesting that the mechanism of action of E5564 is binding through a large internal pocket in MD-2.