Host defense against many invading Gram-negative bacteria (GNB) depends on innate immune recognition of endotoxin (lipopolysaccharides, LPS), unique surface glycolipids of GNB. Host responses to endotoxin must be highly sensitive but self-limited. In mammals, optimal sensitivity is achieved by ordered interactions of endotoxin with several different extracellular and cell surface proteins-the LPS-binding protein (LBP), CD14, MD-2, and Toll-like receptor (TLR) 4-reflecting the requirement for specific protein-endotoxin and protein-protein interactions. This complex reaction pathway also provides many ways to attenuate endotoxin-driven inflammation and can explain how differences in endotoxin structure, either intrinsic among GNB or induced by metabolic remodeling, can alter host responsiveness and thus the outcome of host-GNB interactions. Major goals of our research are to better understand: (1) the structural bases of specific host-endotoxin interactions; (2) functional diversity among host endotoxin-binding proteins; and (3) how the actions of various endotoxin-binding proteins are regulated to permit optimal host responses to GNB infection. In addition, the identification of a water-soluble endotoxin:MD-2 complex that, depending on the structure of endotoxin or MD-2, has potent TLR4 agonist or antagonist properties suggests novel pharmacologic approaches to immuno-modulation.