Bruce Beutler was awarded a portion of the 2011 Nobel Prize in Physiology or Medicine for his work demonstrating that TLR4 is the LPS receptor.[20][21] Portions of the LOS from several bacterial strains have been shown to be chemically similar to human host cell surface molecules; the ability of some bacteria to present molecules on their surface which are chemically identical or similar to the surface molecules of some types of host cells is termed molecular mimicry.[22] For example, in Neisseria meningitidis L2,3,5,7,9, the terminal tetrasaccharide portion of the oligosaccharide (lacto-N-neotetraose) is the same tetrasaccharide as that found in paragloboside, a precursor for ABH glycolipid antigens found on human
erythrocytes.[9] In another example, the terminal trisaccharide portion (lactotriaose) of the oligosaccharide from pathogenic Neisseria spp. LOS is also found in
lactoneoseries glycosphingolipids from human cells.[9] Most meningococci from groups B and C, as well as gonococci, have been shown to have this trisaccharide as part of their LOS structure.[9] The presence of these human cell surface ‘mimics’ may, in addition to acting as a ‘camouflage’ from the immune system, play a role in the abolishment of immune tolerance when infecting hosts with certain human leukocyte antigen (HLA) genotypes, such as HLA-B35.[9]
Effect of variability（变异性） on immune response
O-antigens (the outer
carbohydrates) are the most variable portion of the LPS molecule, imparting the antigenic specificity. In contrast, lipid A is the most conserved part. However, lipid A composition also may vary（变异） (e.g., in number and nature of acyl 酰基chains even within or between genera属). Some of these variations may impart antagonistic(对立的)
Toll-like receptors of the innate immune system recognize LPS and trigger an immune response. properties to these LPS. For example Rhodobacter sphaeroides diphosphoryl lipid A (RsDPLA) is a potent antagonist of LPS in human cells, but is an agonist in hamster and equine cells.
It has been speculated that conical Lipid A (e.g., from E. coli) are more agonistic, less conical lipid A like those of Porphyromonas gingivalis may activate a different signal (TLR2 instead of TLR4), and completely cylindrical lipid A like that of Rhodobacter sphaeroides is antagonistic to TLRs.[23][24]
LPS gene clusters are highly variable between different strains, subspecies, species of bacterial pathogens of plants and animals.[25][26]
Normal human blood serum contains anti-LOS antibodies that are bactericidal and patients that have infections caused by serotypically distinct strains possess anti-LOS antibodies that differ in their specificity compared with normal serum.[27] These differences in humoral immune response to different LOS types can be attributed to the structure of the LOS molecule, primarily within the structure of the
oligosaccharide portion of the LOS molecule.[27] In Neisseria gonorrhoeae it has been demonstrated that the antigenicity of LOS molecules can change during an infection due to the ability of these bacteria to synthesize more than one type of LOS,[27] a
characteristic known as phase variation. Additionally, Neisseria gonorrhoeae, as well as Neisseria meningitidis and Haemophilus influenzae,[9] are capable of further modifying their LOS in vitro, for example through sialylation (modification with sialic acid residues), and as a result are able to increase their resistance to
complement-mediated killing [27] or even down-regulate complement activation[9] or evade the effects of bactericidal antibodies.[9] Sialylation may also contribute to
hindered neutrophil attachment and phagocytosis by immune system cells as well as a reduced oxidative burst.[9] Haemophilus somnus, a pathogen of cattle, has also been shown to display LOS phase variation, a characteristic which may help in the evasion of bovine host immune defenses.[28] Taken together, these observations suggest that variations in bacterial surface molecules such as LOS can help the pathogen evade both the humoral (antibody and complement-mediated) and the cell-mediated (killing by neutrophils, for example) host immune defenses.
Health effects
Endotoxemia（内毒素血症）
The presence of endotoxins in the blood is called endotoxemia. It can lead to septic shock, if the immune response is severely pronounced.[29]
Moreover, endotoxemia of intestinal origin, especially, at the host-pathogen interface, is considered to be an important factor in the development of
alcoholic hepatitis,[30] which is likely to develop on the basis of the small bowel bacterial overgrowth syndrome and an increased intestinal permeability.[31]
Lipid A may cause uncontrolled activation of mammalian immune systems with production of inflammatory mediators that may lead to septic shock.[10] This inflammatory reaction is mediated by Toll-like receptor 4 which is responsible for immune system cell activation.[10] Damage to the endothelial layer of blood vessels caused by these inflammatory mediators can lead to capillary leak syndrome, dilation of blood vessels and a decrease in cardiac function and can lead to septic shock.[32] Pronounced complement activation can also be observed later in the course as the bacteria multiply in the blood.[32] High bacterial proliferation triggering destructive endothelial damage can also lead to disseminated intravascular coagulation (DIC) with loss of function of certain internal organs such as the kidneys, adrenal glands and lungs due to
compromised blood supply. The skin can show the effects of vascular damage often coupled with depletion of coagulation factors in the form of petechiae, purpura and ecchymoses. The limbs can also be affected, sometimes with devastating consequences such as the development of gangrene, requiring subsequent amputation.[32] Loss of function of the adrenal glands can cause adrenal insufficiency and additional hemorrhage into the adrenals causes Waterhouse-Friderichsen syndrome, both of which can be life threatening. It has also been reported that gonococcal LOS can cause damage to human fallopian tubes.[27]
Auto-immune disease
The molecular mimicry of some LOS molecules is thought to cause
autoimmune-based host responses, such as flareups of multiple sclerosis.[9][22] Other examples of bacterial mimicry of host structures via LOS are found with the bacteria Helicobacter pylori and Campylobacter jejuni, organisms which cause gastrointestinal disease in humans, and Haemophilus ducreyi which causes chancroid. Certain C. jejuni LPS serotypes (attributed to certain tetra- and pentasaccharide moieties of the core oligosaccharide) have also been implicated with Guillain-Barré syndrome and a variant of Guillain-Barré called Miller-Fisher syndrome.[9]
Link to obesity
Epidemiological studies have previously shown that increased endotoxin load, which can be a result of increased populations of endotoxin producing bacteria in the intestinal tract, is associated with certain obesity-related patient groups.[33][34][35] Other studies have shown that purified endotoxin from
Escherichia coli can induce obesity and insulin-resistance phenotypes when injected into germ-free mouse models.[36] A more recent study has uncovered a potentially contributing role for Enterobacter cloacae B29 toward obesity and insulin resistance in a human patient.[37] The presumed mechanism for the association of endotoxin with obesity is that endotoxin induces an
inflammation-mediated pathway accounting for the observed obesity and insulin resistance.[36]
Bacterial genera associated with endotoxin-related obesity effects: Escherichia,
Enterobacter
Laboratory research and biotechnology production systems
Lipopolysaccharides are frequent contaminants in plasmid DNA prepared from bacteria or proteins expressed from bacteria, and must be removed from the DNA or protein to avoid contaminating experiments and to avoid toxicity of products manufactured using industrial fermentation.
Also, ovalbumin is frequently contaminated with endotoxins. Ovalbumin is one of the extensively studied proteins in animal models and also an established model allergen for airway hyper-responsiveness (AHR). Commercially available ovalbumin that is contaminated with LPS can fully activate endothelial cells in an in-vitro assay of the first step of inflammation, and it falsifies research results, as it does not accurately reflect the effect of sole protein antigen on animal physiology.
In pharmaceutical production, it is necessary to remove all traces of endotoxin from drug product containers, as even small amounts of endotoxin will cause illness in humans. A depyrogenation oven is used for this purpose. Temperatures in excess of 300°C are required to break down this substance. A defined endotoxin reduction rate is a correlation between time and temperature. Based on primary packaging material as syringes or vials, a glass temperature of 250°C and a holding time of 30 minutes is typical to achieve a reduction of endotoxin levels by a factor of 1000.
The standard assay for detecting presence of endotoxin is the Limulus Amebocyte Lysate (LAL) assay, utilizing blood from the Horseshoe crab.[38] Very low levels of LPS can cause coagulation of the limulus lysate due to a powerful amplification through an enzymatic cascade. However, due to the dwindling population of
horseshoe crabs, and the fact that there are factors that interfere with the LAL assay, efforts have been made to develop alternative assays, with the most promising ones being ELISA tests using a recombinant version of a protein in the LAL assay, Factor C.[39]