| Description | Humans as well as other vertebrates are often exposed to lipopolysaccharide (LPS), for instance via enterobacteria. LPS responses are mediated via Toll-like receptor 4 (TLR4). TLRs are conserved pattern recognition receptors which recognize and respond to molecules derived from bacterial, viral and fungal pathogens, such as LPS from the outer membrane of Gram negative bacteria. Recognition of LPS occurs largely by the TLR4/MD2/CD14 complex, expressed among others by macrophages and dendritic cells. The acute phase LPS-binding protein (LBP) recognizes the lipid A part of LPS and catalyses the monomeric LPS transfer to CD14. This facilitates the LPS transfer to TLR4/MD2. All immunological activity of LPS is exclusively dependent upon the presence of TLR4 as determined by the usage of the corresponding control cells, where TLR4 is missing. Recognition of LPS triggers a cascade of adverse systemic responses and organ failure (septic shock). LPS is a key component of the cell wall of gram negative bacteria (S-form LPS). The molecule consist of three structural regions: the O-polysaccharide chain made up of repeating oligosaccharide units, the core oligosaccharide and- Lipid A. The latter is responsible for the endotoxic activity of the entire molecule. R-form LPS synthesized by the so-called rough (R) mutants of gram-negative bacteria lacks the O-specific chain. Furthermore, the core-oligosaccharide may be present in different degrees of completion, depending on the class (Ra to Re) to which the mutant belongs. LPS from wild type bacteria are always a highly heterogeneous mixtures of S-form LPS molecules containing 1 to over 50 repeating oligosaccharide units and contain a varying proportion of R-form molecules. R-form LPS and lipid A, but not S-form LPS, are capable of inducing TNF-; responses also in the absence of CD14. S- and R-form LPS show marked differences in the kinetics of their blood clearance and cellular uptake as well as in the ability to induce oxidative burst in human granulocytes and to activate the host complement system. |
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