Messias Andresa, Santos Denys E S, Pontes Frederico J S, Soares Thereza A
Department of Fundamental Chemistry, Universidade Federal de Pernambuco, 50740-560 Recife, Brazil.
Phys Chem Chem Phys. 2021 Jul 21;23(28):15127-15137. doi: 10.1039/d1cp02173g.
Cations play a critical role in the stability and morphology of lipid-A aggregates by neutralizing, hydrating and cross-linking these glycolipid molecules. Monophosphorylated lipid-A is the major immunostimulatory principle in commercially available adjuvants containing Al3+ such as adjuvant system 04 (AS04). The antagonist/agonist immunomodulatory properties of lipid-A are associated with chemical variations (e.g. the number of acyl chains and phosphate groups) and their aggregate arrangements (e.g. lamellar, nonlamellar or mixed). Therefore, the identification of the active form of lipid-A can provide valuable guidance in the development of vaccine adjuvants capable of boosting the immune system with decreased reactogenicity. Although the effect of mono and divalent cations on the structural polymorphism and endotoxicity of LPS has been previously investigated, much less is known about the effect of trivalent cations. We have investigated the effect of NaCl and AlCl3 salt solutions on the structural dynamics and stability of mono and diphosphorylated lipid-A membranes via atomistic MD simulations. The Al3+ ion exerts two major effects on the structural dynamics of lipid-A membranes. It acts as an efficient cross-linker of mono or diphosphorylated lipid-A molecules, thus stabilizing the lamellar arrangement of these glycolipids. It also alters the lipid-A packing and membrane fluidity, inducing disorder → order structural transitions of the membrane. This effect is promptly reversed upon the addition of NaCl solution, which promotes a nearly threefold increase in the amount of water in the carbohydrate moiety of the Al3+-containing lipid-A membranes. The exchange dynamics and residence times of cation-coordinated water molecules in these membranes provide insights into the molecular mechanism for the Na+-induced transition from a densely packed ordered phase to a disordered one. Al3+ counter-ions favor ordered lamellar aggregates, which has been previously associated with the lack of endotoxic activity and cytokine-inducing action. The resulting microscopic understanding of the structure and dynamics of lipid-A aggregates in the presence of Al3+ and Na+ salts can provide valuable guidance in the development of vaccine adjuvants capable of boosting the immune system with decreased reactogenicity.
阳离子通过中和、水合和交联这些糖脂分子，在脂多糖A聚集体的稳定性和形态中发挥关键作用。单磷酸化脂多糖A是市售含Al3+佐剂（如佐剂系统04，AS04）中的主要免疫刺激成分。脂多糖A的拮抗剂/激动剂免疫调节特性与化学变化（如酰基链和磷酸基团的数量）及其聚集排列（如层状、非层状或混合排列）有关。因此，确定脂多糖A的活性形式可为开发能够增强免疫系统同时降低反应原性的疫苗佐剂提供有价值的指导。尽管此前已经研究了单价和二价阳离子对脂多糖结构多态性和内毒素毒性的影响，但对于三价阳离子的影响了解较少。我们通过原子分子动力学模拟研究了NaCl和AlCl3盐溶液对单磷酸化和双磷酸化脂多糖A膜的结构动力学和稳定性的影响。Al3+离子对脂多糖A膜的结构动力学有两个主要影响。它作为单磷酸化或双磷酸化脂多糖A分子的有效交联剂，从而稳定这些糖脂的层状排列。它还改变脂多糖A的堆积和膜流动性，诱导膜的无序→有序结构转变。加入NaCl溶液后，这种效应迅速逆转，这促进了含Al3+脂多糖A膜碳水化合物部分的水量增加近三倍。这些膜中阳离子配位水分子的交换动力学和停留时间为Na+诱导的从紧密堆积的有序相到无序相转变的分子机制提供了见解。Al3+反离子有利于形成有序的层状聚集体，这与缺乏内毒素活性和细胞因子诱导作用有关。由此获得的关于在Al3+和Na+盐存在下脂多糖A聚集体的结构和动力学的微观认识，可为开发能够增强免疫系统同时降低反应原性的疫苗佐剂提供有价值的指导。
