Endotoxin·albumin complexes transfer endotoxin monomers to MD-2 resulting in activation of TLR4

Abstract

Response to Gram-negative bacteria (GNB) is partially mediated by the recognition of GNB-derived endotoxin by host cells. Potent host response to endotoxin depends on the sequential interaction of endotoxin with lipopolysaccharide binding protein (LBP), CD14, MD-2 and TLR4. While CD14 facilitates the efficient transfer of endotoxin monomers to MD-2 and MD-2·TLR4, activation of MD-2·TLR4 can occur in the absence of CD14 through an unknown mechanism. Here, we show that incubation of purified endotoxin (E) aggregates (E_agg, M _r  ≥ 20 million) in PBS with ≥ 0.1% albumin in the absence of divalent cations Ca²⁺ and Mg²⁺, yields E·albumin complexes (M _r ∼70,000). E·albumin transfers E monomers to sMD-2 or sMD-2·TLR4 ectodomain (TLR4_ecd) with a ‘K_d’ of ∼4 nM and induces MD-2·TLR4-dependent, CD14-independent cell activation with a potency only 10-fold less than that of monomeric E·CD14 complexes. Our findings demonstrate, for the first time, a mechanistic basis for delivery of endotoxin monomers to MD-2 and for activation of TLR4 that is independent of CD14.