Synthetic lipid A mimetics for exploration of LPS recognition by the proteins

Recognition of parasites, especially of bacterial origin, by the immune system of the mammalian host belongs to the intensively studied field in biochemical and biomedical research. The innate immune system is responsible for the control of infectious disease during the first day of infection, until the adaptive immune system starts to combat the invaders. An important role in defencing the host from pathogenic bacteria belongs to the proteins of the innate immune system. Recognition of lipopolysaccharide (LPS), a major constituent of the cell-wall of Gramnegative bacteria, proceeds through the pattern recognition receptor Toll-like Receptor 4 (TLR4) and an intracellular enzyme Caspace-4/11, which triggers a series of intracellular events leading to induction of the pro- inflammatory signaling cascade. Liberation of LPS from the bacterial cell wall into the blood circulation of the host results in the activation of immune cells and expression of cytokines, such as TNF-a (tumor necrosis factor) and interleukins, which generally contributes to the clearance of infection. Though, systemic over-production of inflammatory mediators can cause sepsis syndrome characterized by fever, hypotension and multiple organ failure that eventually results in endotoxic septic shock, a leading cause of death in intensive care units worldwide. Additionally, activation of TLR4 bridges the innate and adaptive immunity, which highlights stimulation of TLR4 complex by non-pyrogenic ligands as a useful approach for development of vaccine adjuvants. The activation of TLR4 proceeds through the binding of endotoxic portion of LPS, a glycophospholipid Lipid A, to the central hydrophobic pocket of the co-receptor protein Myeloid Differentiation factor 2 (MD-2), followed by dimerization of two ligand-receptor complexes. Different bacterial species entailing Lipid A of different chemical structure induce dissimilar degree of receptor complex dimerization and ensuing induction of inflammatory signaling. To explore the molecular basis for Lipid A induced activation of the immune receptors, the chemical synthesis and immuno- functional studies of innovative conformationally confined Lipid A mimetics is intended. Lipid A is composed of a flexible carbohydrate-based polar head group which carries a variable number of lipid chains. The inherent flexibility of the diglucosamine backbone of Lipid A was recently shown to count to the major factors responsible for the endotoxic potential of LPS. Synthesis of Lipid A mimetics wherein the flexible linkage is exchanged for a rigid 1,1-glycosidic linkage will provide potentially immuno-modulating Caspase-4/11 and TLR4 ligands. Evaluation of the tetriary structure of synthetic ligands in respect to immuno-stimulating activity will provide reliable structure-activity relationships and contribute to elucidation of molecular basis of the ligand-driven oligomerization of the receptor proteins. Activation of TLR4- and Caspase-4/11 mediated innate immune response was demonstrated to be immunoprotective against infection, highlighting the potential of addressing TLR4 complex as versatile therapeutic target. Understanding of molecular basis of the activation of receptor complexes by Lipid A would ensure the development of advanced anti-inflammatory and immuno-regulatory therapeutics and novel vaccine adjuvants.