The and potent that even a bloodstream infection can

The role of bacterial LPS is to incite a
substantial innate immune system in mammalian hosts.  The lipid moiety, lipid A, is regarded to be
endotoxic and is important to the endotoxin activity of LPS. Additionally, it
is a bioactive element of LPS and is recognised by the innate immune system
even when present at low levels. Immune detection of lipid A is highly
sensitive and potent that even a bloodstream infection can cause endotoxic
shock. LPS stimulates an immune response by interacting with some of the active
receptors –  cluster of differentiation
14 (CD14) and lymphocyte antigen 96 (MD2) receptor complex, all of which have
been found to trigger the production of pro-inflammatory cytokines such as
tumour necrosis factor-? (TNF), interleukin-6 (IL-6) and interleukin-1 (IL-1).  LPS can also bind to Toll receptors, which
are intracellular signalling domain, and LPS signalling has also been
specifically linked to Toll-like receptor 4 (TLR4). LPS activates proximal
signalling pathways in a similar way to those used by IL-1, IL-1
receptor-associated kinase (IRAK) and TNFR-associated factor 6 (TRAF 6).
Moreover, LPS exposure has been associated with the activation of a number of
signalling cascades, hence stimulating the secretion of inflammatory mediators.
LPS are recognised by their host cells because TLR4 and myeloid differentiation
factor 2 (MD-2) form a heterodimer, and are able to pick out certain common
characteristics in structurally diverse LPS molecules. Determining the crystal
structure of the TLR4-MD-2-LPS complex helps to simplify their ligand
specificity and receptor activation mechanism. Two copies of the TLR4-MD-2-LPS
complex are arranged symmetrically and this comes about from the formation of
an m-shaped receptor multimer resulting from LPS binding. LPS interacts with a large
hydrophobic pocket in MD-2, with one of the six lipid chains of LPS forming a
hydrophobic interaction with the conserved phenylalanines of TLR4.



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