The innate immune response to lipopolysaccharide is essential for host defense against Gram-negative bacteria. In response to bacterial infection, the TLR4/MD-2 complex that is expressed on the surface of macrophages, monocytes, dendritic, and epithelial cells senses picomolar concentrations of endotoxic LPS and triggers the production of various pro-inflammatory mediators. In addition, LPS from extracellular bacteria which is either endocytosed or transfected into the cytosol of host cells or cytosolic LPS produced by intracellular bacteria is recognized by cytosolic proteases caspase-4/11 and hosts guanylate binding proteins that are involved in the assembly and activation of the NLRP3 inflammasome. All these events result in the initiation of pro-inflammatory signaling cascades directed at bacterial eradication. However, TLR4-mediated signaling and caspase-4/11-induced pyroptosis are largely involved in the pathogenesis of chronic and acute inflammation. Both extra- and intracellular LPS receptors—TLR4/MD-2 complex and caspase-4/11, respectively—are able to directly bind the lipid A motif of LPS. Whereas the structural basis of lipid A recognition by the TLR4 complex is profoundly studied and well understood, the atomic mechanism of LPS/lipid A interaction with caspase-4/11 is largely unknown. Here we describe the LPS-induced TLR4 and caspase-4/11 mediated signaling pathways and their cross-talk and scrutinize specific structural features of the lipid A motif of diverse LPS variants that have been reported to activate caspase-4/11 or to induce caspase-4/11 mediated activation of NLRP3 inflammasome (either upon transfection of LPS in vitro or upon infection of cell cultures with intracellular bacteria or by LPS as a component of the outer membrane vesicles). Generally, inflammatory caspases show rather similar structural requirements as the TLR4/MD-2 complex, so that a “basic” hexaacylated bisphosphorylated lipid A architecture is sufficient for activation. However, caspase-4/11 can sense and respond to much broader variety of lipid A variants compared to the very “narrow” specificity of TLR4/MD-2 complex as far as the number and the length of lipid chains attached at the diglucosamine backbone of lipid A is concerned. Besides, modification of the lipid A phosphate groups with positively charged appendages such as phosphoethanolamine or aminoarabinose could be essential for the interaction of lipid A/LPS with inflammatory caspases and related proteins.

对脂多糖的先天免疫反应对于宿主防御革兰氏阴性菌至关重要。在响应细菌感染时，在巨噬细胞、单核细胞、树突状细胞和上皮细胞表面表达的 TLR4/MD-2 复合物会感知皮摩尔浓度的内毒素 LPS，并触发各种促炎介质的产生。此外，胞外细菌的 LPS 被内吞或转染到宿主细胞的胞质溶胶或胞内细菌产生的胞质 LPS 被胞质蛋白酶 caspase-4/11 和宿主鸟苷酸结合蛋白识别，这些蛋白参与组装和激活NLRP3 炎症小体。所有这些事件导致针对细菌根除的促炎信号级联反应的启动。然而，TLR4 介导的信号传导和 caspase-4/11 诱导的细胞焦亡在很大程度上参与了慢性和急性炎症的发病机制。细胞外和细胞内 LPS 受体——分别是 TLR4/MD-2 复合物和 caspase-4/11——都能够直接结合 LPS 的脂质 A 基序。尽管 TLR4 复合物对脂质 A 识别的结构基础进行了深入研究和充分理解，但 LPS/脂质 A 与 caspase-4/11 相互作用的原子机制在很大程度上是未知的。在这里，我们描述了 LPS 诱导的 TLR4 和 caspase-4/11 介导的信号通路及其串扰，并仔细检查了不同 LPS 变体的脂质 A 基序的特定结构特征，这些变体已被报道可激活 caspase-4/11 或诱导caspase-4/11 介导 NLRP3 炎症小体的激活（LPS 转染后）体外或在细胞培养物被细胞内细菌或 LPS 作为外膜囊泡的组成部分感染时）。通常，炎性半胱天冬酶显示出与 TLR4/MD-2 复合物相当相似的结构要求，因此“基本”六酰化双磷酸化脂质 A 结构足以激活。然而，就连接在二葡糖胺骨架上的脂链的数量和长度而言，与 TLR4/MD-2 复合物非常“窄”的特异性相比，caspase-4/11 可以感知和响应更广泛的脂质 A 变体脂质 A 的问题。此外，用带正电荷的附属物（如磷酸乙醇胺或氨基阿拉伯糖）修饰脂质 A 磷酸基团对于脂质 A/LPS 与炎性半胱天冬酶和相关蛋白质的相互作用可能是必不可少的。