Inhibition by gangliosides of the specific binding of lipopolysaccharide (LPS) to human monocytes prevents LPS-induced interleukin-1 production

The dangerous liaisons in innate immunity involving recombinant proteins and endotoxins: Examples from the literature and the Leptospira field

Endotoxins, also known as lipopolysaccharides (LPS), are essential components of cell walls of diderm bacteria such as Escherichia coli. LPS are microbe-associated molecular patterns that can activate pattern recognition receptors. While trying to investigate the interactions between proteins and host innate immunity, some studies using recombinant proteins expressed in E. coli reported interaction and activation of immune cells. Here, we set out to provide information on endotoxins that are highly toxic to humans and bind to numerous molecules, including recombinant proteins. We begin by outlining the history of the discovery of endotoxins, their receptors and the associated signaling pathways that confer extreme sensitivity to immune cells, acting alone or in synergy with other microbe-associated molecular patterns. We list the various places where endotoxins have been found. Additionally, we warn against the risk of data misinterpretation due to endotoxin contamination in recombinant proteins, which is difficult to estimate with the Limulus amebocyte lysate assay, and cannot be completely neutralized (e.g., treatment with polymyxin B or heating). We further illustrate our point with examples of recombinant heat-shock proteins and viral proteins from severe acute respiratory syndrome coronavirus 2, dengue and HIV, for which endotoxin contamination has eventually been shown to be responsible for the inflammatory roles previously ascribed. We also critically appraised studies on recombinant Leptospira proteins regarding their putative inflammatory roles. Finally, to avoid these issues, we propose alternatives to express recombinant proteins in nonmicrobial systems. Microbiologists wishing to undertake innate immunity studies with their favorite pathogens should be aware of these difficulties.

Application of Sygen® in Diabetic Peripheral Neuropathies—A Review of Biological Interactions

This study investigates the role of Sygen^® in diabetic peripheral neuropathy, a severe disease that affects the peripheral nervous system in diabetic individuals. This disorder often impacts the lower limbs, causing significant discomfort and, if left untreated, progresses into more serious conditions involving chronic ulcers and even amputation in many cases. Although there are management strategies available, peripheral neuropathies are difficult to treat as they often present multiple causes, especially due to metabolic dysfunction in diabetic individuals. Gangliosides, however, have long been studied and appreciated for their role in neurological diseases. The monosialotetrahexosylganglioside (GM1) ganglioside, popularly known as Sygen, provides beneficial effects such as enhanced neuritic sprouting, neurotrophism, neuroprotection, anti-apoptosis, and anti-excitotoxic activity, being particularly useful in the treatment of neurological complications that arise from diabetes. This product mimics the roles displayed by neurotrophins, improving neuronal function and immunomodulation by attenuating exacerbated inflammation in neurons. Furthermore, Sygen assists in axonal stabilization and keeps nodal and paranodal regions of myelin fibers organized. This maintains an adequate propagation of action potentials and restores standard peripheral nerve function. Given the multifactorial nature of this complicated disorder, medical practitioners must carefully screen the patient to avoid confusion and misdiagnosis. There are several studies analyzing the role of Sygen in neurological disorders. However, the medical literature still needs more robust investigations such as randomized clinical trials regarding the administration of this compound for diabetic peripheral neuropathies, specifically.

Anti-inflammatory role of GM1 and other gangliosides on microglia

Background

Gangliosides are glycosphingolipids highly enriched in the brain, with important roles in cell signaling, cell-to-cell communication, and immunomodulation. Genetic defects in the ganglioside biosynthetic pathway result in severe neurodegenerative diseases, while a partial decrease in the levels of specific gangliosides was reported in Parkinson’s disease and Huntington’s disease. In models of both diseases and other conditions, administration of GM1—one of the most abundant gangliosides in the brain—provides neuroprotection. Most studies have focused on the direct neuroprotective effects of gangliosides on neurons, but their role in other brain cells, in particular microglia, is not known. In this study we investigated the effects of exogenous ganglioside administration and modulation of endogenous ganglioside levels on the response of microglia to inflammatory stimuli, which often contributes to initiation or exacerbation of neurodegeneration.

Methods

In vitro studies were performed using BV2 cells, mouse, rat, and human primary microglia cultures. Modulation of microglial ganglioside levels was achieved by administration of exogenous gangliosides, or by treatment with GENZ-123346 and L–t-PDMP, an inhibitor and an activator of glycolipid biosynthesis, respectively. Response of microglia to inflammatory stimuli (LPS, IL-1β, phagocytosis of latex beads) was measured by analysis of gene expression and/or secretion of pro-inflammatory cytokines. The effects of GM1 administration on microglia activation were also assessed in vivo in C57Bl/6 mice, following intraperitoneal injection of LPS.

Results

GM1 decreased inflammatory microglia responses in vitro and in vivo, even when administered after microglia activation. These anti-inflammatory effects depended on the presence of the sialic acid residue in the GM1 glycan headgroup and the presence of a lipid tail. Other gangliosides shared similar anti-inflammatory effects in in vitro models, including GD3, GD1a, GD1b, and GT1b. Conversely, GM3 and GQ1b displayed pro-inflammatory activity. The anti-inflammatory effects of GM1 and other gangliosides were partially reproduced by increasing endogenous ganglioside levels with L–t-PDMP, whereas inhibition of glycolipid biosynthesis exacerbated microglial activation in response to LPS stimulation.

Conclusions

Our data suggest that gangliosides are important modulators of microglia inflammatory responses and reveal that administration of GM1 and other complex gangliosides exerts anti-inflammatory effects on microglia that could be exploited therapeutically.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02374-x.

Sphingolipid metabolism during Toll-like receptor 4 (TLR4)-mediated macrophage activation

Macrophage activation in response to stimulation of Toll-like receptor 4 (TLR4) provides a paradigm for investigating energy metabolism that regulates the inflammatory response. TLR4-mediated pro-inflammatory macrophage activation is characterized by increased glycolysis and altered mitochondrial metabolism, supported by selective amino acid uptake and/or usage. Fatty acid metabolism remains as a highly complex rewiring that accompanies classical macrophage activation. TLR4 activation leads to de novo synthesis of fatty acids, which flux into sphingolipids, complex lipids that form the building blocks of eukaryotic cell membranes and regulate cell function. Here, we review the importance of TLR4-mediated de novo synthesis of membrane sphingolipids in macrophages. We first highlight fatty acid metabolism during TLR4-driven macrophage immunometabolism. We then focus on the temporal dynamics of sphingolipid biosynthesis and emphasize the modulatory role of some sphingolipid species (i.e. sphingomyelins, ceramides and glycosphingolipids) on the pro-inflammatory and pro-resolution phases of LPS/TLR4 activation in macrophages.

Glycan-Glycan Interaction Determines Shigella Tropism toward Human T Lymphocytes

Direct interactions between bacterial and host glycans have been recently reported to be involved in the binding of pathogenic bacteria to host cells. In the case of Shigella, the Gram-negative enteroinvasive bacterium responsible for acute rectocolitis, such interactions contribute to bacterial adherence to epithelial cells. However, the role of glycans in the tropism of Shigella for immune cells whose glycosylation pattern varies depending on their activation state is unknown. We previously reported that Shigella targets activated, but not nonactivated, human CD4⁺ T lymphocytes. Here, we show that nonactivated CD4⁺ T lymphocytes can be turned into Shigella-targetable cells upon loading of their plasma membrane with sialylated glycosphingolipids (also termed gangliosides). The Shigella targeting profile of ganglioside-loaded nonactivated T cells is similar to that of activated T cells, with a predominance of injection of effectors from the type III secretion system (T3SS) not resulting in cell invasion. We demonstrate that gangliosides interact with the O-antigen polysaccharide moiety of lipopolysaccharide (LPS), the major bacterial surface antigen, thus promoting Shigella binding to CD4⁺ T cells. This binding step is critical for the subsequent injection of T3SS effectors, a step which we univocally demonstrate to be dependent on actin polymerization. Altogether, these findings highlight the critical role of glycan-glycan interactions in Shigella pathogenesis.

Glycosylation of host cell surface varies with species and location in the body, thus contributing to species specificity and tropism of microorganisms. Cross talk by Shigella, the Gram-negative enteroinvasive bacterium responsible for bacillary dysentery, with its exclusively human host has been extensively studied. However, the molecular determinants of the step of binding to host cells are poorly defined. Taking advantage of the observation that human-activated CD4⁺ T lymphocytes, but not nonactivated cells, are targets of Shigella, we succeeded in rendering the refractory cells susceptible to targeting upon loading of their plasma membrane with sialylated glycosphingolipids (gangliosides) that are abundantly present on activated cells. We show that interactions between the sugar polar part of gangliosides and the polysaccharide moiety of Shigella lipopolysaccharide (LPS) promote bacterial binding, which results in the injection of effectors via the type III secretion system. Whereas LPS interaction with gangliosides was proposed long ago and recently extended to a large variety of glycans, our findings reveal that such glycan-glycan interactions are critical for Shigella pathogenesis by driving selective interactions with host cells, including immune cells.

CD14/LPS receptor exhibits lectin-like properties

We had previously shown that the specific binding of endotoxin (lipopolysaccharide, LPS) to human monocytes in the presence of serum was mediated by the polysaccharide moiety of the LPS molecule. CD14 has been identified as the main receptor for endotoxin on monocytes/macrophages. In the present report we demonstrate that the CD14 molecule exhibits lectin-like properties. Anti-CD14 monoclonal antibodies inhibited the binding of [3H]-radiolabeled Neisseria meningitidis LPS as efficiently as the homologous unlabeled LPS. Rough Escherichia coli LPS (Rc- and Re-types) could also inhibit the binding of [3H]-LPS to a similar extent, whereas lipid A had no or very weak inhibitory activity. This suggests a major contribution of the inner-core region within the LPS and particularly the Kdo sugars. The lectin-like nature of CD14 was assessed with polyanionic sugars as well as with uncharged polysaccharides. The relative efficiencies in competition were dextran sulfate > fucoidan > mannan > polygalacturonic acid = heparan sulfate ≥ heparin ≥ chondroitin sulfate. Candida albicans phospholipomannan was far more active in the competition experiment than the mannan, indicating that, besides the osidic residues, anionic charges and/or fatty acids may contribute to the interaction with the CD14 molecule. Binding of polysaccharide to CD14 was not sufficient to trigger TNFα and IL-6 production since phospholipomannan and dextran sulfate were unable to induce cytokine release. Taken together, these results demonstrate that the binding of [3H]-LPS to CD14 involves the contribution of sugars and suggest that the signals for cytokine production require additional interactions.

Gangliosides protect bowel in an infant model of necrotizing enterocolitis by suppressing proinflammatory signals

OBJECTIVES

Necrotizing enterocolitis (NEC) has high morbidity in premature infants. Hypoxia-ischemia, infection, and enteral feeding are risk factors associated with NEC, whereas feeding human milk is protective. Vasoactive and inflammatory mediators in NEC remain elusive. Gangliosides are found in human milk and enterocyte membranes. An infant bowel model of NEC was developed to test the hypothesis that gangliosides modulate the inflammatory response to infection and hypoxia.

PATIENTS AND METHODS

Viable, noninflamed bowel was obtained from 9 infants between 26 and 40 weeks' gestational age. Infant bowel was treated in culture with Escherichia coli lipopolysaccharide (LPS) and hypoxia in the presence or absence of preexposure to gangliosides. Bowel necrosis and production of nitric oxide, endothelin-1, serotonin, eicosanoids, hydrogen peroxide, and proinflammatory cytokines were measured.

RESULTS

Ganglioside preexposure reduced bowel necrosis and endothelin-1 production in response to LPS. Gangliosides suppressed infant bowel production of nitric oxide, leukotriene B4, prostaglandin E2, hydrogen peroxide, interleukin-1beta, interleukin-6, and interleukin-8 in response to LPS exposure and hypoxia.

CONCLUSIONS

A bowel protective effect of gangliosides is indicated by modulation of vasoactive mediators and proinflammatory signal suppression.

Gangliosides inhibit flagellin signaling in the absence of an effect on flagellin binding to toll-like receptor 5

A recent study (Ogushi, K., Wada, A., Niidome, T., Okuda, T., Llanes, R., Nakayama, M., Nishi, Y., Kurazono, H., Smith, K. D., Aderem, A., Moss, J., and Hirayama, T. (2004) J. Biol. Chem. 279, 12213-12219) concluded that gangliosides serve as co-receptors for flagellin signaling via toll-like receptor 5 (TLR5). In view of several findings in this study that were inconsistent with a role for gangliosides as co-receptors, we re-examined this important issue. Using TLR5-negative RAW 264.7 cells and a TLR5-enhanced yellow fluorescent protein chimera, we established an assay for specific binding of flagellin to cells. Inhibition of clatherin-mediated internalization of flagellin.TLR5-enhanced yellow fluorescent protein complexes did not impair flagellin activation of IRAK-1. Thus flagellin signal occurs at the cell surface and not intracellularly. Exogenous addition of mixed gangliosides (GM1, GD1a, and GT1b) as well as GD1a itself inhibited flagellin-induced interleukin-1 receptor-associated kinase activation as well as tumor necrosis factor alpha production in HeNC2, THP-1, and RAW 264.7 cells. Gangliosides inhibited flagellin signaling in the absence of an effect on flagellin binding to TLR5. Depletion of gangliosides in RAW 264.7 cells did not alter the concentration dependence or magnitude of flagellin signaling as measured by interleukin-1 receptor-associated kinase activation or tumor necrosis factor alpha production. Our findings are consistent with the conclusions that gangliosides are not essential co-receptors for flagellin and that the inhibitory effect of gangliosides is mediated by at least one mechanism that is distinct from any effect on the binding of flagellin to TLR5.

Inhibition of macrophage nitric oxide production by gangliosides derived from a spontaneous T cell lymphoma: the involved mechanisms

Gangliosides (DLG) derived from a spontaneous T cell lymphoma (Dalton's lymphoma) have been shown to impair the ability of lipopolysaccharide-activated macrophages to produce nitric oxide (NO). However, the mechanism and nature of this effect is not known. In this investigation, we sought to (1) determine whether the inhibitory action of DLG on macrophages is through the modulation of inducible nitric oxide synthase (iNOS) expression and (2) identify the possible mechanisms and signal transduction events underlying the inhibitory action of DLG. Immunoblot analysis of DLG-treated macrophages showed a decrease in iNOS expression. DLG also inhibited the production of monokines interleukin-1 and tumor necrosis factor by macrophages. However, the DLG-induced inhibition was reversible in nature. Studies showed that DLG-induced inhibition of macrophage activation could be blocked by sodium orthovanadate, indicating a role of phosphatase activity in ganglioside-induced inhibition.

Gangliosides inhibit the release of interleukin-1beta in amyloid beta-protein-treated human monocytic cells

Amyloid-beta protein (A beta) is known to induce microglial activation with concomitant release of cytokines. Gangliosides have documented neuritogenic and neurotrophic properties. We determined the effects of A beta on the release of interleukin-1beta (IL-1beta) from the human monocytic cell line, THP-1 cells. A beta 1-42 significantly induced the release of IL-1beta from the cells. A beta 1-40, A beta 40-1, A beta 1-38, and A beta precursor protein (beta-APP) analogs also released a small amount of IL-1beta. A beta 1-42-activated cells demonstrated approx an 18-fold higher IL-1beta release than that for control cells or A beta 1-40 (soluble; S) treated cells. The release of IL-1beta from A beta 1-42-activated cells was significantly inhibited (33-48% of activated cells; p < 0.05 for the control value) by addition of gangliosides, suggesting that gangliosides inhibit the continuous cycle of the IL-1beta production in THP-1 cells.

GM1 ganglioside potentiates trimethyltin-induced expression of interleukin-1 beta and the nerve growth factor in reactive astrocytes in the rat hippocampus: an immunocytochemical study

This study demonstrates potentiation by GM1 ganglioside treatment of trimethyltin (TMT) induced reactivity of astrocytes, and the expression of astroglial interleukin-1 beta (IL-1 beta) and nerve growth factor (NGF) immunoreactivities in the rat hippocampus. GM1 treatment also results in an increase of the number of IL-1 beta and NGF immunoreactive astrocytes. Both the intensity of gliosis and stimulation of IL-1 beta and NGF expression in astrocytes mostly occurs in the regions of heaviest neurodegeneration in the hippocampus (CA4/CA3c and CA1). It is tempting to assume that enhancement of astroglial NGF expression by GM1 ganglioside may play a role in the protective action of GM1 against neurotoxic insult.

Interaction of lipopolysaccharide endotoxin produced from Escherichia coli with D-tubocurarine at the nicotinic2 receptor and adenosine 3':5' cyclic monophosphate during physiological contraction in skeletal muscle

In this report the murine model of endotoxicosis was used to evaluate hyposensitivity to the neuromuscular relaxant D-tubocurarine (dTC). This hyposensitivity was expressed in terms of a decreased potency to dTC. A rightward shift of the dose-response curve due to endotoxin was observed. Mice were subjected to cumulative intraperitoneal doses of Escherichia coli endotoxin over a 2-wk period. The interaction between endotoxin and dTC was examined during an acute (1 wk) and chronic (2 wk) period of endotoxicosis. Muscle twitch analyses were performed and samples of gastrocnemius muscle were assayed for adenosine 3':5' cyclic monophosphate (cAMP) by [125I]radioimmunoassay. A parallel shift in the dose-response curve occurred in the endotoxin group subjected to doses corresponding to one-third the dose evoking 50% lethality for 2 wk. Both skeletal muscle tension and cAMP levels decreased as cumulative endotoxin doses increased. A relationship between decreasing cAMP levels and increasing dTC and effective dose required to achieve 50% muscle paralysis values was thought to be evoked by the agonistic activity of E. coli endotoxin leading to desensitizing of adenylate cyclase. The perturbations of the classical second messenger cAMP system by endotoxin may be responsible for skeletal muscle dysfunction observed in immunocompromised patients.

Cellular functions during activation and damage by pathogens: immunogold studies of the interaction of bacterial endotoxins with target cells

Bacterial endotoxins (lipopolysaccharides or LPS) are active components of Gram-negative bacteria that act on numerous cellular functions through the processes of cell activation and damage. The molecular mechanisms involved in the "endotoxic phenomenon" are not defined yet, although extensive studies have been carried out. Immunogold and electron microscopy (EM) have contributed to identify the primary target cells of endotoxins and the subcellular systems that receive the direct action of these bacterial agents. Here, we review our studies on immunogold detection of endotoxins in cellular and subcellular systems. The analysis of the interaction between endotoxins and cells was focussed on the following aspects: (1) morphological characteristics of the LPS aqueous suspensions used in experimental work; (2) binding of endotoxins to the plasma membrane of type II pneumocytes and alveolar macrophages (two of their cellular targets), and influence of the state of aggregation of the LPS; (3) movement and distribution of endotoxins inside the cell, from the plasma membrane to the nucleoplasm; and (4) interaction of LPS with microtubules and its effects on the integrity of the microtubular network. These approaches provide information at the molecular level as well as data for the establishment of physiological models of endotoxicity.

The acute inflammatory response to lipopolysaccharide in CNS parenchyma differs from that in other body tissues

Acute inflammation is important for defence against infection, wound repair and the mediation of auto-immune tissue destruction. Myelomonocytic recruitment in acute inflammation is a stereotyped and non-specific response to tissue insult which begins within 2 h. In this study, lipopolysaccharide was injected into the murine CNS and other body sites of mice to compare the inflammatory responses. Doses of lipopolysaccharide which induced typical myelomonocytic recruitment in skin and the choroid plexus had no effect in CNS parenchyma, apart from the morphological activation of local resident microglia. The CNS parenchymal response proceeded independently of that in the choroid plexus-cerebral ventricles and had three distinct and unique phases. Initially there was minimal neutrophil exudation and a two-day delay before any increase in macrophage-microglial cell number. Next, there was a rapid increase in macrophage-microglial cell numbers during the third day, mainly due to recruitment of blood monocytes. During this phase, leukocyte recruitment was restricted to monocytes which rapidly adopted the arborized microglial phenotype. Monocytes migrated through an intact blood-brain barrier independent of changes in solute permeability. Finally, there was a florid myelomonocytic reaction predominantly in the white matter, one week after intracerebral injection of 2 micrograms lipopolysaccharide. At this time, the leukocyte reaction disrupted the blood-brain barrier, mononuclear phagocytes expressed macrophage morphology and abundant major histocompatibility complex Class II antigen, and T lymphocytes were present. Myelomonocytic entry into the CNS was partially inhibited by prior blockade of the type 3 complement receptor, known to mediate leukocyte adhesion to endothelium elsewhere. The processes which lead to rapid myelomonocytic recruitment in other tissues are absent in CNS parenchyma. Understanding the molecular mechanisms responsible could have considerable significance both for CNS pathophysiology as well as possible anti-inflammatory therapeutic application elsewhere in the body.

Effect of parenteral administration of GM1 on cytokines and anti-ganglioside antibody patterns. Preliminary report in normal human individuals

To assess the effects of monosialoganglioside GM1 on some immunological parameters, 12 healthy men were treated with 100 mg GM1 i.m. daily for 15 days. Before and after treatment, the following were studied: (1) serum levels of antibodies against GM1, asialo-GM1 (aGM1), GM2 and GD1b; (2) serum levels of interleukin (IL)-1 beta, IL-2, soluble IL-2 receptor (sIL-2R), tumor necrosis factor-alpha (TNF-alpha), and interferon-gamma (IFN-gamma); (3) IL-1 beta and TNF-alpha production by peripheral blood monocytes (PBMO). Anti-ganglioside antibody and cytokine serum levels were not affected by exogenous GM1 administration with the exception of a transient increase in anti-GM1 antibody titer observed in one subject. In addition, no inhibition of IL-1 beta and TNF-alpha production by PBMO was observed. These preliminary data do not support a potential immunogenic or immunomodulatory function for in vivo administered GM1.

Effect of micellar and bilayer gangliosides on proliferation of interleukin-2-dependent lymphocytes

Micellar gangliosides are potent inhibitors of the proliferation of the murine interleukin-2-dependent cell lines HT-2 and CTLL-2 in vitro. The glycolipids abolished both DNA and protein synthesis, and depressed cellular expansion, without affecting viability. These effects were reversible for at least 12 hr following ganglioside treatment. Highly sialylated gangliosides were more inhibitory, while structurally related molecules, including ganglioside oligosaccharides, simple and complex neutral glycosphingolipids, sulfatides, sphingomyelin, ceramides, and sphingosine had only small suppressive effects. Gangliosides were most effective as inhibitors when added during the first 4 hr of culture with the growth factor. Inhibition of DNA synthesis by gangliosides could be partially reversed by high concentrations of exogenous interleukin-2. Gangliosides incorporated into lipid bilayers, both multilamellar liposomes and unilamellar vesicles, were also effective inhibitors of interleukin-2-induced proliferation. Competition studies showed that both ganglioside micelles and lipid vesicles containing gangliosides prevented binding of 125I-interleukin-2 to high-affinity receptors on the lymphocyte surface. We have recently shown that gangliosides, in both micelles and lipid bilayer vesicles, are able to bind interleukin-2 (J. W. K. Chu and F. J. Sharom, Biochim, Biophys. Acta 1028, 205, 1990). Taken together, these results strongly suggest that inhibition of lymphocyte proliferation by gangliosides in micelles and vesicles arises as a direct result of competition between the glycolipids and high-affinity receptors for available interleukin-2.

An evolutionary perspective of endotoxin: a signal for a well-adapted defense system

In the evolutionary view of endotoxin presented here, endotoxin is the primary signal animals use to detect gram negative (Gr-) bacteria. Since endotoxin, or lipopolysaccharide (LPS), is an integral part of the surface of all Gr- bacteria, it was excellent evolutionary 'choice' for the signal. The concept of an 'endotoxin response system' (ERS) is introduced. The ERS protects against Gr- bacteria by employing many of the body's defenses to both detect and react against LPS. The intensity of the response has evolved to maximize protection while minimizing the biological cost and self-damaging effects. The setting of the response, here termed the 'endostat', is programmed by natural selection and fine tuned by feedback mechanisms. Other potentially invasive organisms are detected by different signals, but the effector components of the defenses are similar. This evolutionary view of LPS offers a framework for the seemingly contradictory findings on endotoxin and suggests new avenues of productive research.

Altered responses to modulators of guanine nucleotide binding protein activity in endotoxin tolerance

The effects of cholera toxin or pertussis toxin and nonhydrolyzable GTP analogs on Salmonella enteritidis endotoxin stimulation of iTxB2 and i6-keto-PGF1 alpha synthesis in control and endotoxin tolerant rat peritoneal macrophages were determined. Pretreatment with pertussis toxin alone had no effect on basal macrophage iTxB2 or i6-keto-PGF1 alpha production, but pertussis toxin (0.1, 1.0 and 10 ng/ml) significantly inhibited endotoxin-stimulated iTxB2 and i6-keto-PGF1 alpha synthesis. Pretreatment with cholera toxin, which did not affect basal iTxB2 or i6-keto-PGF1 alpha synthesis, significantly enhanced endotoxin-induced synthesis of iTxB2 and i6-keto-PGF1 alpha. The effects of pertussis and cholera toxin with or without endotoxin were significantly (P less than 0.05) less in macrophages from endotoxin tolerant rats compared to control macrophages. GTP[gamma-S] (100 microM) significantly increased iTxB2 synthesis and significantly augmented endotoxin-stimulated iTxB2 synthesis in control macrophages (P less than 0.05). However, in macrophages from endotoxin tolerant rats the effect of GTP[gamma-S] on iTxB2 synthesis was significantly less (P less than 0.05) compared to control macrophages. Collectively, these data suggest that: (1) guanine nucleotide binding regulatory proteins mediate endotoxin-stimulated arachidonic acid metabolism in rat peritoneal macrophages; and (2) endotoxin tolerance induces alterations in guanine nucleotide binding protein activity.

Cytokine response by monocytes and macrophages to free and lipoprotein-bound lipopolysaccharide

Recent evidence suggests that bacterial lipopolysaccharide binds to serum lipoproteins in vitro and in vivo and that lipopolysaccharide in the form that is bound to lipoprotein is less biologically active in several experimental models. In order to study the mechanism of this apparent detoxification, we compared the ability of free and lipoprotein-bound lipopolysaccharide from Escherichia coli O18 to stimulate interleukin-1, interleukin-6, and tumor necrosis factor from elicited murine peritoneal macrophages and circulating human monocytes. Lipopolysaccharide bound to lipoprotein was 20- to 1,000-fold less active than the unbound form in inducing the release of each cytokine. We also studied the binding of each form of lipopolysaccharide to the macrophage surface. Lipopolysaccharide complexed to lipoprotein was unable to compete for the binding of radiolabeled heterologous lipopolysaccharide to murine macrophages, and radiolabeled lipopolysaccharide-lipoprotein complexes bound poorly compared with molar equivalents of free lipopolysaccharide. Our experiments suggest that in the process of binding to lipoproteins, lipopolysaccharide may be rendered less toxic through a mechanism of decreased ability to induce monocytes and macrophages to release cytokines, perhaps because of an altered interaction at the cell surface.

Lipopolysaccharide-induced production of cytokines by bone marrow-derived macrophages: dissociation between intracellular interleukin 1 production and interleukin 1 release

We investigated the capacity of mouse bone marrow-derived macrophages (BMDM) to produce interleukin 1 (IL 1), interleukin-6 (IL 6), and tumor necrosis factor (TNF) upon lipopolysaccharide (LPS) stimulation. BMDM were allowed to differentiate either in the presence of conditioned medium (from WEHI-3 or L cells), or in the presence of recombinant cytokines (IL 3, macrophage-colony stimulating factor [M-CSF], or granulocyte/macrophage-colony stimulating factor [GM-CSF]). Cells were maintained in culture up to 3 weeks and tested at different times. Significant spontaneous cytokine production was never observed. BMDM rapidly acquired the capacity to elaborate cytokine upon LPS activation. LPS-triggered BMDM were able to produce IL 1, IL 6, and TNF, throughout the culture period, although 2- to 3-week-old cells lost their ability to release IL 1 while accumulation of intracellular IL 1 remained unchanged. The dissociation between synthesis and release of IL 1 was not correlated with a significant modification of the specific binding of LPS onto the cell surface.

Augmentation of mouse natural killer (NK) activity by GM-1/P, a processed form of monosialoganglioside GM-1

We describe the immunomodulatory activity of GM-1/P a processed form of GM-1 (monosialoganglioside) extracted from ox brain, purified and physically modified. We examined the effect of in vivo and in vitro treatment of GM-1/P on natural (NK) activity and its ability to induce the production of interleukin-2 (IL-2) in the mouse. In vivo treatment with GM-1/P (1 mg/Kg, i.v., day-1) resulted in a marked increase and in a change of distribution of NK activity, which was associated with lower density Percoll fractions. Marked increase was already observed at 18 hrs and then declined by day 4. In vitro treatment with GM-1/P (2 micrograms/ml) enhanced NK activity of B6 spleen cells, already after 6 hours of incubation, remaining at plateau levels within 18 hours. A role of IL-2 in this enhancement was suggested by the ability of an anti-IL-2 rabbit antiserum to abolish in vitro increased cytotoxicity. The presence of IL-2 in the supernatants of splenocytes from GM-1/P (1mg/Kg, i.v., ,day-1) treated mice stimulated with Con A or Con A plus TPA for 48 hrs was evaluated by proliferation of an IL-2 dependent CTLL cell line. GM-1/P by itself was unable to stimulate IL-2 production; however it markedly increased IL-2 production induced by Con A or Con A plus TPA.

Lipopolysaccharide interactions with lysozyme differentially affect lipopolysaccharide immunostimulatory activity

The effect of complex formation between lysozyme and lipopolysaccharide (LPS) on the immunostimulatory activities of LPS have been investigated in vitro. Three prototype immunostimulatory activities were examined: B-lymphocyte proliferation, B-lymphocyte differentiation and macrophage production of lymphocyte-activating factor activity. Different effects of lysozyme were noted, depending upon the structure of the LPS, even though previous studies have established that all LPS preparations readily bind lysozyme. Both Re-LPS- and lipid-A-dependent immunostimulatory activities were readily inhibited by lysozyme in a dose-dependent fashion. In contrast, S-LPS and Ra-LPS were unaffected in their immunostimulatory activities by lysozyme. These differences were not the result of quantitative differences in LPS binding of lysozyme, or effects of lysozyme on overall binding of LPS to target cells. These data suggest that the factors which dictate the initial interactions between LPS and lymphoreticular cells may not be identical for all LPS preparations and/or purified lipid A.

Dissociation of cell-associated interleukin-1 (IL-1) and IL-1 release induced by lipopolysaccharide and lipid A

The capacities of lipopolysaccharide (LPS) and lipid A to trigger mouse BALB/c peritoneal macrophages and to induce the production of cell-associated interleukin-1 (IL-1) and membrane-associated IL-1 and IL-1 release have been compared. Bordetella pertussis lipid A was 1,000 to 10,000 times less efficient than the native LPS to induce IL-1 release by freshly isolated elicited macrophages. When resident macrophages were studied, lipid A, at high concentrations (greater than 2 micrograms/ml), induced significant levels of cell-associated IL-1 but little or no IL-1 release. With synthetic lipid A built up with the Escherichia coli lipid A structure (compound 506), IL-1 activity was present in the supernatants of elicited peritoneal macrophages and to a lesser extent in those of resident macrophages. However, the release of IL-1 induced by synthetic lipid A 506 remained much lower than those induced by rough LPS. Membrane-associated IL-1 could be induced on BALB/c macrophages with LPS and natural or synthetic lipid A, the LPS being the most active. In C3H/HeJ mice, neither natural nor synthetic lipid A could induce detectable cell-associated IL-1, whereas LPS could induce cell-associated and membrane IL-1 activity but no IL-1 release. Our results indicate that fragments of endotoxins may induce the production of IL-1 but the entire structure of the LPS molecule is the most effective to induce intracellular IL-1 production, expression of membrane IL-1, and release of IL-1.

Gangliosides suppression of murine lymphoproliferation and interleukin 1 production

Studies were carried out to determine whether inhibition of gangliosides on lymphoproliferation was related to interleukin (IL)-1. The results showed that gangliosides, GM1 and GT1b were able to inhibit the proliferation of spleen lymphocytes from C57BL/6J mice in dose-dependent fashion, whereas asialo-GM1 was not inhibitory. However, gangliosides, GM1 and asialo-GM1 did not suppress the production of IL-1 in Salmonella typhosa lipopolysaccharide (LPS)-induced peritoneal adherent cells. Various types of LPS including S. enteritidis, S. minnesota and Escherichia coli 055:B5 were used to stimulate the production of IL-1 in adherent cell cultures. The IL-1 production was not affected by gangliosides, GD1a and GD1b. Although GT1b suppressed IL-1 production of human monocytes to 82% of control level it did not, however, affect the IL-1 production of murine adherent cells. Thus, the inhibitory mechanism of gangliosides on murine immune cells remains unclear, and warrants further study.