Interleukin-1 induction by lipopolysaccharides: structural requirements of the 3-deoxy-D-manno-2-octulosonic acid (KDO)

Bacterial endotoxins and pathogenesis of Gram-negative infections: current status and future direction

100 years after the discovery of a bacterial 'endotoxin', 50 years after the introduction of antibiotics and 25 years after the routine use of intensive care units to support septic shock patients, Gram-negative infections continue to account for significant morbidity and mortality. In the coming decade, basic research on the structure/function of LPS, the cytokine cascade, and receptor-mediated intracellular signalling responses to LPS and cytokines will provide a greater understanding of the molecular, cellular and systemic responses to endotoxin and infection. New therapeutic agents now emerging from research, and better designed clinical trials to assess those agents will contribute to the next significant decline in sepsis- and shock-related morbidity and mortality. This article summarizes the findings of a workshop convened at the National Institutes of Health (NIH) to examine current research on endotoxin and Gram-negative septic shock.

Structural analysis of lipopolysaccharides from Gram-negative bacteria

This review covers data on composition and structure of lipid A, core, and O-polysaccharide of the known lipopolysaccharides from Gram-negative bacteria. The relationship between the structure and biological activity of lipid A is discussed. The data on roles of core and O-polysaccharide in biological activities of lipopolysaccharides are presented. The structural homology of some oligosaccharide sequences of lipopolysaccharides to gangliosides of human cell membranes is considered.

Determination of distinctive carbohydrate signatures obtained from the Aeromonas hydrophila (chemotype II) core oligosaccharide pinpointing the presence of the 4-O-phosphorylated 5-O-linked Kdo reducing end group using electrospray ionization quadrupole orthogonal time-of-flight mass spectrometry and tandem mass spectrometry

The electrospray quadrupole orthogonal time-of-flight mass spectrometric (ESI-QqTOF-MS) structural elucidation of the core oligosaccharide of Aeromonas hydrophila (chemotype II) lipopolysaccharide has been investigated and it was demonstrated that it contained an 4-O-phosphorylated Kdo reducing end group, which was glycosylated by the remaining outer core oligosaccharide through its O-5 position. After releasing the core oligosaccharide from the native LPS with acid, the phosphorylated Kdo residue eliminated phosphoric acid, to produce a core oligosaccharide containing a mixture of diastereomeric 4,8- and 4,7-anhydro-alpha-keto acids and an open-chain olefinic Kdo residue. The characteristic glycone sequence was elucidated by collision-induced dissociation tandem mass spectrometry (CID-MS/MS) of the protonated molecule of the native core oligosaccharide. In addition, the analysis of the Hakomori permethylated core oligosaccharide was carried out by electrospray ionization quadrupole orthogonal time-of-flight mass spectrometry (ESI-QqTOF-MS) and matrix-assisted laser desorption/ionization (MALDI)-QqTOF-MS analyses. The presence of more than nine isobaric isomers of this core was detected. The CID-MS/MS analysis of the various protonated permethylated core oligosaccharide molecules showed a similar and diagnostic fragmentation pattern. The over-methylation of the permethylated core oligosaccharide containing either the 4,7- or the 4,8-anhydro-alpha-keto acid unit and the open-chain olefinic Kdo unit was reported. It was realized that the extra minor satellite signals obtained in the ESI-QqTOF-MS and MALDI-TOF-MS analyses were dimethyl sulfoxide (DMSO) stable covalent addition products, which have occurred by a Michael addition on the 4,8-Kdo exocyclic double bond. The occurrence of this series of covalent addition products during the MS analysis of a permethylated core oligosaccharide should be considered as 'carbohydrate-distinctive signatures' establishing and confirming the presence of a 4-O-phosphorylated-5-O-linked Kdo reducing end group.

Structural investigation of bacterial lipopolysaccharides by mass spectrometry and tandem mass spectrometry

Mass spectrometric studies are now playing a leading role in the elucidation of lipopolysaccharide (LPS) structures through the characterization of antigenic polysaccharides, core oligosaccharides and lipid A components including LPS genetic modifications. The conventional MS and MS/MS analyses together with CID fragmentation provide additional structural information complementary to the previous analytical experiments, and thus contribute to an integrated strategy for the simultaneous characterization and correct sequencing of the carbohydrate moiety.

Glucosamine found as a substituent of both phosphate groups in Bordetella lipid A backbones: role of a BvgAS-activated ArnT ortholog

Endotoxins are amphipathic lipopolysaccharides (LPSs), major constituents of the outer membrane of gram-negative bacteria. They consist of a lipid region, covalently linked to a core oligosaccharide, to which may be linked a repetitive glycosidic chain carrying antigenic determinants. Most of the biological activities of endotoxins have been associated with the lipid moiety of the molecule: unique to gram-negative bacteria, LPS is a ligand of the mammalian TLR4-MD2-CD14 pathogen recognition receptor complex. Lipid A preparations are often heterogeneous with respect to both the numbers and the lengths of fatty acids and the natures of substituents on the phosphate groups when present. The variants can significantly affect host immune responses. Nine species in the Bordetella genus have been described, and the fine LPS structures of seven of them have been published. In this report, lipids A from Bordetella pertussis Tohama I and B. bronchiseptica strain 4650 were further characterized and revealed to have a glucosamine substituting both lipid A phosphate groups of the diglucosamine backbone. These substitutions have not been previously described for bordetellae. Moreover, a B. pertussis transposon mutation that maps within a gene encoding a Bordetella ArnT (formerly PmrK) glycosyl transferase ortholog does not carry this substitution, thus providing a genetic basis for the modification. Reverse transcriptase PCR of this locus showed that it is Bvg regulated, suggesting that the ability of Bordetella to modify lipid A via this glucosamine modification is a potential virulence trait.

Expression of the lipopolysaccharide-modifying enzymes PagP and PagL modulates the endotoxic activity of Bordetella pertussis

Lipopolysaccharide (LPS) is one of the major constituents of the gram-negative bacterial cell envelope. Its endotoxic activity causes the relatively high reactogenicity of whole-cell vaccines. Several bacteria harbor LPS-modifying enzymes that modulate the endotoxic activity of the LPS. Here we evaluated whether two such enzymes, i.e., PagP and PagL, could be useful tools for the development of an improved and less reactogenic whole-cell pertussis vaccine. We showed that expression of PagP and PagL in Bordetella pertussis leads to increased and decreased endotoxic activity of the LPS, respectively. As expected, PagP activity also resulted in increased endotoxic activity of whole bacterial cells. However, more unexpectedly, this was also the case for PagL. This paradoxical result may be explained, in part, by an increased release of LPS, which we observed in the PagL-expressing cells.

Structure of bacterial lipopolysaccharides

Bacterial lipopolysaccharides are the major components of the outer surface of Gram-negative bacteria They are often of interest in medicine for their immunomodulatory properties. In small amounts they can be beneficial, but in larger amounts they may cause endotoxic shock. Although they share a common architecture, their structural details exert a strong influence on their activity. These molecules comprise: a lipid moiety, called lipid A, which is considered to be the endotoxic component, a glycosidic part consisting of a core of approximately 10 monosaccharides and, in "smooth-type" lipopolysaccharides, a third region, named O-chain, consisting of repetitive subunits of one to eight monosaccharides responsible for much of the immunospecificity of the bacterial cell.

Structural and functional analyses of bacterial lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) are powerful immunomodulators in infected hosts, and may cause endotoxic shock. Most of them share a common architecture but vary considerably in structural motifs from one genus, species, and strain to another. Cells of the innate immune response recognize evolutionarily conserved LPS molecular patterns of endotoxins and structural details thereby greatly influencing their response.

Does the shape of lipid A determine the interaction of LPS with Toll-like receptors?

Lipopolysaccharide (LPS) triggers the activation of the immune system through the induction of cytokine release. Although it was assumed initially that LPS molecules from different bacteria are similar, recent evidence suggests that structural and functional differences between LPS species are the rule rather than the exception. It has been proposed that the shape of the lipid A component determines the bioactivity of LPS, with lipid A that adopts a conical conformation being more active than lipid A that adopts a cylindrical shape. The mechanism linking the molecular conformation with the biological activity of LPS has not been elucidated. We propose that LPS with a conical shape (e.g. from Escherichia coli) induces cytokine production through Toll-like receptor 4 (TLR4), whereas more cylindrical LPS (e.g. from Porphyromonas gingivalis) induces expression of a different set of cytokines through TLR2. Strictly cylindrical LPS molecules (e.g. the lipid A precursor Ia or from Rhodobacter sphaeroides) have antagonistic properties at the level of TLRs.

Isolement et caractérisation chimique des lipopolysaccharides de type R dans une souche hypovirulente de Yersinia pestis

The lipopolysaccharides LPS I and LPS II, isolated from the hypovirulent EV40 strain of Yersinia pestis, are composed only of type R lipopolysaccharides. This type consists of two forms, a and b, depending on their solubility pattern in a solvent mixture containing varying proportions of chloroform, methanol, hexane, and hydrochloric acid. LPS I consists of one subtype, RIb, while LPS II consists of two subtypes, RIIa and RIIb. Analysis by gel electrophoresis shows that the mass of these lipopolysaccharide forms are in the vicinity of 2000-3000 Da. The RIb and RIIb subtypes, which are found in the majority of lipopolysaccharide I and II fractions, are composed of ketoses and amines that are similar to those occurring in LPS I and LPS II. In contrast, the two subtypes RIIa and RIIb are different both in terms of the composition of lipid A and the extent of its substitution. Certain fractions of RIIa contain only lipid A and 3-deoxy-D-manno-octulosonic acid (KDO), while other fractions of RIIb possess a lipid A, which is not substituted by arabinose. The whole set of these R-type lipopolysaccharide forms are excellent models for the study of the role of the primary structure of the polysaccharide region, and for the effect of lipid A substitution on the biological activity of bacterial lipopolysaccharides.Key words: Yersinia pestis, hypovirulence, lipopolysaccharides, R type.[Journal translation]

Bacterial modulins: a novel class of virulence factors which cause host tissue pathology by inducing cytokine synthesis

Cytokines are a diverse group of proteins and glycoproteins which have potent and wide-ranging effects on eukaryotic cell function and are now recognized as important mediators of tissue pathology in infectious diseases. It is increasingly recognized that for many bacterial species, cytokine induction is a major virulence mechanism. Until recent years, the only bacterial component known to stimulate cytokine synthesis was lipopolysaccharide (LPS). It is only within the past decade that it has been clearly shown that many components associated with the bacterial cell wall, including proteins, glycoproteins, lipoproteins, carbohydrates, and lipids, have the capacity to stimulate mammalian cells to produce a diverse array of cytokines. It has been established that many of these cytokine-inducing molecules act by mechanisms distinct from that of LPS, and thus their activities are not due to LPS contamination. Bacteria produce a wide range of virulence factors which cause host tissue pathology, and these diverse factors have been grouped into four families: adhesins, aggressins, impedins, and invasins. We suggest that the array of bacterial cytokine-inducing molecules represents a new class of bacterial virulence factor, and, by analogy with the known virulence families, we suggest the term "modulin" to describe these molecules, because the action of cytokines is to modulate eukaryotic cell behavior. This review summarizes our current understanding of cytokine biology in relation to tissue homeostasis and disease and concisely reviews the current literature on the cytokine-inducing molecules produced by gram-negative and gram-positive bacteria, with an emphasis on the cellular mechanisms responsible for cytokine induction. We propose that modulins, by controlling the host immune and inflammatory responses, maintain the large commensal flora that all multicellular organisms support.

Depressed immune response in malnourished rats correlates with increased thymic noradrenaline level

Depressed immune response is well documented in protein-calorie malnutrition (PCM). Also, central and peripheral noradrenaline (NA) activities have been reported to be increased in malnourished animals. Since increases in central and peripheral NA may inhibit immune function, it is possible that malnutrition-induced immunodepression could be mediated by noradrenergic hyperactivity. To address this hypothesis the effect of malnutrition on cell-mediated immune response, as well as on NA levels of the median eminence, spleen and thymus was studied in PCM rats. Decreased lymphoproliferative response and IL-1 production by mononuclear macrophages was observed in PCM. Besides, increased NA concentration was detected in thymuses of PCM rats, while unchanged levels of this neurotransmitter were observed in median eminence and spleen. These data suggest a positive correlation between malnutrition-induced immunodepression and sympathetic noradrenergic activity in thymus, an organ implicated in immune cell differentiation during early development.

CD14 and CD11b mediate serum-independent binding to human monocytes of an acylpolygalactoside isolated from Klebsiella pneumoniae

A water-soluble acylpolygalactosyl (APG) of 34 kDa was obtained from the Klebsiella pneumoniae membrane by alkaline hydrolysis and delipidation. APG comprises a poly(1,3)galactose chain, a core, and a lipid moiety made of a glucosamine disaccharide with two N-linked beta OH-myristates. The monocyte binding sites for APG were investigated by flow cytometry. Biotin-labelled APG (Biot-APG) bound to monocytes at 4 degrees C in the absence of serum, calcium, and magnesium. The binding was dose dependent, saturable, and displaced by unlabelled APG. Neither the polysaccharide chain present in APG-related molecules nor the PPi group or additional ester-linked myristates and palmitates were required for APG binding. The role of CD11b and CD14 was demonstrated by competitive inhibition with monoclonal antibodies and by the uptake of APG by these solubilized proteins. APG was rapidly internalized into monocytes at 37 degrees C while CD14 and CD11b/CD18 molecules were partially down-modulated. Lipopolysaccharides (LPS) from the same K. pneumoniae strain and from Escherichia coli and Salmonella minnesota partially competed for Biot-APG binding in the absence but not in the presence of serum. When altered by alkaline hydrolysis, those LPS became strong competitors for APG binding. It was concluded that alkaline hydrolysis of the K. pneumoniae membrane yielded molecules structurally related to LPS which bind to LPS membrane receptors in the absence of serum.

Complement activation and cytokine production as consequences of immunological bioincompatibility of extracorporeal circuits

The use of devices which result in exposure of blood to artificial surface has gained increasing importance in routine medical and surgical practice. In the field of biocompatibility, attention has long been directed at the mechanisms of thrombus formation of surfaces. In recent years however, a special interest has emerged for the study of the immunological consequences of blood-artificial surface interactions, thus broadening the concept of hemocompatibility. The contact of blood with artificial devices results in the activation of a number of humoral and cellular processes involved in natural and in specific immunological recognition of foreign surfaces by the host, and in the secondary occurrence of acute and chronic adverse reactions in patients undergoing extracorporeal circulation. The purpose of this review is to discuss the mechanisms involved in immunological bioincompatibility of extracorporeal circuits, with particular emphasis on the molecular basis of the activation of the complement system, the role of endotoxins, and the induction of cytokine production by activated monocytes.

A localized absence of interleukin-4 triggers periodontal disease activity: a novel hypothesis

Pathogenic bacteria constitute the primary extrinsic agent in the etiology of Adult periodontitis. In addition to direct toxic effects, bacteria induce destructive immunologic and other inflammatory reactions in the host, leading to the observed pathologic alteration in the tissue. The risk to develop periodontal disease is not equal for all individuals, suggesting host factors are important in determining an individuals disease susceptibility. Regulation of immune response is important in maintaining the equilibrium between periodontal health and disease. We hypothesize that, in the case of Adult periodontitis, a localized lack of the regulatory cytokine interleukin-4 (IL-4) in the gingival tissues predisposes susceptible individuals to progress from gingivitis to periodontitis.

Monocyte cytokine secretion induced by chemically-defined derivatives of Klebsiella pneumoniae

The capacity of a K. pneumoniae membrane proteoglycan (Kp-MPG) and four of its chemically defined derivatives to activate human monocytes was studied by measuring immunoreactive IL-1 beta, IL-6 and tumour necrosis factor-alpha (TNF-alpha) in culture supernatants. Monocyte culture supernatants were also tested for their comitogenic activity on concanavalin A-stimulated thymocytes and for their cytotoxic activity on the mouse fibroblastic L929 cell line. The four Kp-MPG derivatives were: (i) an acylpoly(1-3)galactoside (APG); (ii) an APG preparation submitted to acid hydrolysis which removed all fatty acids but left intact the galactose chain of APG (GC-APG); (iii) a preparation obtained by mild alkaline hydrolysis, containing additional ester-linked C14 and C16 fatty acids bound to the APG molecule (EFA-APG); and (iv) a polymer of the latter compound (APG pol). Kp-MPG induced the synthesis of IL-1 beta, IL-6 and TNF-alpha with dose-responses and kinetics similar to those of Salmonella minnesota lipopolysaccharide (Sm-Re-LPS). APG pol and EFA-APG induced the secretion of the three cytokines with lower potency than Kp-MPG or Sm-Re-LPS. APG did not trigger any detectable cytokine production and GC-APG induced only borderline and inconsistent responses. Our data demonstrate the critical role of ester-linked C14 and C16 fatty acids in the triggering of monocyte response to Kp-MPG derivatives.

Binding of a bacterial acylpoly(1,3)galactoside to human blood leucocytes

The binding of a 34-kDa (mol. wt.) acylpoly(1,3)galactoside (APG) extracted from a membrane proteoglycan of Klebsiella pneumoniae to human blood leucocytes was investigated. APG is made of a long poly(1,3)galactose chain, a core-like region and a lipid moiety which comprises two glucosamine residues bound to a phosphate group and two beta OH myristic acids. Fluoresceinated APG was shown to bind preferentially to monocytes and to a lesser extent to polymorphonuclear neutrophils, as determined by flow cytometry. Binding of fluoresceinated APG was inhibited by unlabelled APG; it was concentration dependent, but not saturable, with rapid kinetics. It occurred at +4 degrees C but was markedly increased at 37 degrees C. It involved trypsin-sensitive molecules on the membrane of monocytes. Neither the parent proteoglycan nor lipopolysaccharide from K. pneumoniae or Salmonella minnesota competed for APG binding. A minor non-specific binding to lymphocytes, occurring predominantly on B cells, was observed. Unlike that of lipopolysaccharide, the APG binding was not blocked by polymyxin B sulphate. Interaction between the galactose chain of APG and the galactose receptor does not account for the binding of APG to monocytes because the galactose receptor (Mac-2) is expressed at high density on activated macrophages but not on monocytes. Despite its strong binding to human blood monocytes, APG displayed a much weaker activity than K. pneumoniae membrane proteoglycan with respect to induction of monocyte cytokine synthesis. When administered as a Technetium 99 conjugate, APG was shown to label inflammatory foci in experimental animals, and its property as a marker of macrophages is currently being evaluated in clinical trials.

Lipopolysaccharide receptors and signal transduction pathways in mononuclear phagocytes

There is little question but that bacterial lipopolysaccharides (LPS) remain one of the most potent stimuli which can affect macrophage activation. Although the precise biochemical mechanisms responsible for this remain to be fully defined, there is now evidence accumulating from a number of laboratories that functional receptors for these bacterial products do exist and may contribute to the initial triggering event. Unfortunately, there is currently no consensus as to which of the candidate receptors identified to date serves as the primary binding target for LPS, and it is possible that the difference in macrophage cell types, LPS probes, and detection systems will all influence the nature of the binding. At the present time, therefore, macromolecules of 96-kDa, 95-kDa (adhesion beta chain), 80-kDa, 65-kDa, and 55-kDa may be considered as possible LPS targets. With the exception of the 96-kDa protein identified by Hampton and his co-workers, there exists some experimental evidence for a functional role for each of the molecules so far identified. It is apparent that the molecular cloning and sequencing and subsequent biochemical characterization of these LPS receptors will be required to determine unequivocally their role in LPS-mediated triggering events. Such information will be invaluable in sorting out the relevant biochemical second signals involved in macrophage activation. Although much new information has recently been accumulated on potential signaling pathways for LPS, the definitive events remain far from unequivocally established. In view of the obvious importance of LPS-macrophage interactions in the overall capacity of the mammalian host to respond appropriately to the potentially hostile prokaryotic environment, a precise delineation of LPS-mediated macrophage activation is critical to our understanding of this important inflammatory mediator cell.

A novel hypothesis concerning the mechanisms of activation, and of control, of periodontal bone loss

Chronic inflammatory periodontitis fulfills the classical definition of an infectious disease in that it is a disease of the host caused by the activities of one or more parasites. Typically, the etiology of an infectious disease has been defined as the specific microbe which incites the disease process, even though the quality and nature of host responses to the pathogen may underlie much of the pathology seen. This approach in the study of the etiology of chronic periodontitis has not resulted in the identification of a single 'periodontopathogen', but rather is leading to the realization that multiple sets of microbes may induce the same endpoint, albeit some possibly more efficiently than others. The premise of this paper is that a different view of the literature in the area, with the primary emphasis on the mechanisms of damage and resistance to periodontitis, reveals a probable commonality, rather than a plethora of diseases. The concept of a mechanism-based etiology, rather than of a microbe-based one, deserves consideration for this complex, host-parasite interaction. The novel hypothesis presented here is that the common virulence factor of chronic periodontitis is lipopolysaccharide (LPS), that the central damaging mediator is a cyclooxygenase product of arachidonic acid (probably prostaglandin E2), and that the critical resistance mechanism that limits disease activity is the effective, peripheral neutralization of LPS by emigrated polymorphonuclear neutrophils.

Dissociation between the interleukin 1-inducing capacity and Limulus reactivity of lipopolysaccharides from gram-negative bacteria

In this study we compared the interleukin 1 (IL 1)-inducing capacity and the reactivity in the Limulus amoebocyte assay (LAL) of purified lipopolysaccharides (LPSs) from various bacterial strains. LPSs differed greatly in their capacities (on a weight basis) to induce IL 1 release from serum-free cultured human monocytes. LPS species that induced high levels of IL 1 release from human monocytes exhibited a high thiobarbiturate-reactive 2-keto-3-deoxy-octonic acid (KDO) content. No relationship was found between the IL 1-inducing activity and the LAL reactivity of purified LPSs. Filtration experiments in which membranes of decreasing size-exclusion limits were used demonstrated that molecular species of LPS with an apparent Mr below 3,000 may induce IL 1, whereas only species with an apparent Mr above 8,000 are recognized in the LAL assay. The latter observation suggests that the reaction with LAL requires an aggregated form of LPS. These results indicate that biologically active LPS species can cross dialysis membranes in vivo although no LAL reactive material is detected in the blood compartment. The Limulus assay is an insufficient criterion for the absence of LPS in biological fluids.

Interleukin-1 beta in human colostrum

The two forms of interleukin-1, IL-1 alpha and IL-1 beta respectively, and tumour necrosis factor (TNF) are polypeptides sharing different biological activities which are often associated with host defence mechanisms. Because of the well-recognized benefits of breast feeding for newborns, colostrum from 9 healthy lactating women was analysed for the presence of these 3 cytokines. Specific radioimmunoassay revealed that colostrum contains a significant amount of IL-1 beta (mean +/- SEM values of 1,130 +/- 259 pg/ml). The concentrations of IL-1 alpha and TNF were negligible. Colostral leukocytes are able to produce IL-1 since high activity was found after stimulation with Staphylococcus epidermidis. In addition, these cells produced IL-1 spontaneously in vitro, in contrast to resting maternal blood monocytes. As IL-1 increases resistance to infection, the presence of this cytokine represent a beneficial aspect of breast feeding.

Detection of somatotropin receptors on human monocytes

The expression of somatotropin receptors on human peripheral blood monocytes was investigated. Binding of [125I]somatotropin to human monocytes was found to be specific and saturable. The binding was rapid and time-dependent, and was abolished by pretreatment of monocytes with trypsin. Scatchard plot analysis revealed that each cell bore more than 8 x 10(3) binding sites with an affinity constant of 1 x 10(8) M-1.

Induction of cell-associated interleukin 1 through stimulation of the adhesion-promoting proteins LFA-1 (CD11a/CD18) and CR3 (CD11b/CD18) of human monocytes

Serum-free culture of human monocytes in the presence of monoclonal antibodies to the LFA-1 alpha chain (CD11a), CR3 alpha chain (CD11b) or beta chain (CD18) bound to Sepharose induced the dose-dependent production of cell-associated interleukin (IL) 1 activity and of IL 1 alpha and IL 1 beta antigens, but no release of extracellular IL 1 activity or antigen in the culture medium. Triggering of IL 1 production was also observed with insolubilized anti-CD11/CD18 F(ab')2 antibodies. Two cross-linked antibodies recognizing distinct epitopes on the CD11b molecule induced cell-associated IL 1. Soluble antibodies did not induce IL 1 production. The kinetics of induction of IL 1 by stimulation of adhesion-promoting proteins differed from those of IL 1 induction by adhesion to plastic. The lack of induction of IL 1 release by stimulation of the CD11/CD18 molecules resembled the intracellular accumulation of IL 1 induced by lipid A. Induction of IL 1 by adhesive processes may be a mechanism by which T cells trigger IL 1 production by monocytes during antigen presentation.

Lipopolysaccharide/lipid A receptors on lymphocytes and macrophages

Significant advances have been realized during the past five years in the understanding of the mechanism(s) by which endotoxic LPS interactions with mammalian lymphoreticular cells leads to characteristic cellular responses. There is now strong experimental evidence to support the concept that specific receptors for the lipid A component of LPS do, in fact, exist and are functional on these cells. While the available data do not rule out a potential contribution of nonspecific hydrophobic interactions of lipid A with the membrane bilayer in the cellular activation process, it would appear that interaction with the LPS receptor alone is sufficient to initiate triggering. Whether there exist more than one molecular entity which might function on mammalian cell membranes as a specific receptor for LPS, or whether different cell types may manifest different LPS receptors remains as an interesting area for future research. Further, the concept that molecular complexes of LPS with mammalian host proteins, such as the acute phase LPS binding protein, might trigger additional novel pathways for cell activation is both exciting and of potential importance. The precise mechanism or mechanisms by which LPS-receptor ligand interactions translate into appropriate transmembrane signalling events is currently uncertain. Clearly there exists evidence for contribution of many of the traditional second signals, although at present, the data are incomplete and not always consistent between laboratories. Of potential concern in this respect are the sometimes rather striking differences noted between lipid A and intact polysaccharide containing S-LPS. While such differences may be significant and important, it should be remembered that S-LPS itself is a potent stimulus for many lymphoreticular cell subpopulations, and any postulated pathways must encompass S-LPS as well as lipid A. In any case, it is likely that the further molecular-biochemical characterization of LPS receptors will yield crucial information for the eventual elucidation of the precise pathways for LPS transmembrane signalling. Such information will be invaluable in the future harnessing of the immunostimulatory potential of LPS as well as the abrogation of its profound deleterious pathophysiological effects in endotoxin shock.