Immunology Program, University of Iowa Graduate College, Iowa City, Iowa, USA.
Innate Immun. 2012 Jun;18(3):478-91. doi: 10.1177/1753425911422723. Epub 2011 Oct 12.
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 Ca(2+) and Mg(2+), 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.
宿主细胞通过识别革兰氏阴性菌（GNB）来源的内毒素部分地对其产生反应。宿主对内毒素的强烈反应取决于内毒素与脂多糖结合蛋白（LBP）、CD14、MD-2 和 TLR4 的顺序相互作用。虽然 CD14 有助于内毒素单体有效地转移到 MD-2 和 MD-2·TLR4，但通过未知机制，MD-2·TLR4 的激活可以在没有 CD14 的情况下发生。在这里，我们表明，在不存在二价阳离子 Ca(2+)和 Mg(2+)的情况下，将纯化的内毒素（E）聚集体（E(agg)，M ( r ) ≥ 2000 万）在 PBS 中与 ≥ 0.1%白蛋白孵育，会产生 E·白蛋白复合物（M ( r ) ∼70000）。E·白蛋白以“K (d)”约为 4 nM 的亲和力将 E 单体转移到 sMD-2 或 sMD-2·TLR4 外域（TLR4(ecd)），并诱导 MD-2·TLR4 依赖性、不依赖 CD14 的细胞激活，其效力仅比单体 E·CD14 复合物低 10 倍。我们的发现首次证明了将内毒素单体递送到 MD-2 并激活 TLR4 的机制基础，该机制独立于 CD14。
