Purification of acyloxyacyl hydrolase, a leukocyte enzyme that removes secondary acyl chains from bacterial lipopolysaccharides

Acyloxyacyl hydrolase deficiency induces chronic inflammation and bone loss in male mice

Hormonal homeostasis is essential in bone remodeling. Recent studies have shown that the treatment of intestinal inflammation can result in the regulation of bone resorption in distant bones. Increased intestinal permeability may lead to systemic inflammation and bone loss, also known as gut-bone axis. However, the underlying mechanism remains to be elucidated. Lipopolysaccharide (LPS) is a component of gram-negative bacteria that can increase osteoclastic differentiation in vitro. Acyloxyacyl hydrolase (AOAH) is a specific degrading enzyme of LPS, but little is known about the role of AOAH in bone metabolism. In this study, adult Aoah^-/- mice showed a chronic inflammatory state and osteopenic phenotype analyzed by micro-CT and HE staining. Tartrate-resistant acid phosphatase (TRAP) staining of femurs showed an increase in TRAP-positive cells from Aoah^-/- mice. AOAH depletion enhanced the osteoclast differentiation and bone resorption capacity of bone marrow-derived macrophages (BMMs). The enhanced osteoclast differentiation and bone resorption capacity of Aoah^-/- BMMs were reversed by rAOAH. In conclusion, the chronic inflammatory state of adult Aoah^-/- mice promotes bone resorption. AOAH participates in bone metabolism, which is mainly mediated by inhibiting osteoclast differentiation. LPS may be a key mediator of the gut-bone axis, and targeting AOAH may represent a feasible strategy for the treatment of chronic inflammatory bone resorption. KEY MESSAGES : AOAH knockout mice exhibited chronic inflammation mediated by LPS, and LPS may also serve as an important mediator in the regulation of bone metabolism in the gut-bone axis. AOAH regulated bone resorption by blocking the osteoclast differentiation via classical ERK and JNK pathways. rAOAH could rescue the enhanced osteoclast differentiation caused by AOAH deficiency.

Phospholipase activity of acyloxyacyl hydrolase induces IL-22-producing CD1a-autoreactive T cells in individuals with psoriasis

Psoriasis is a chronic inflammatory skin disease characterized by Th17 responses. Recent evidence has identified Langerhans cells to have a key role in disease pathogenesis, with constitutive high expression of CD1a and capacity to present lipid antigens to T cells. Phospholipase A2 enzymes generate neolipid antigens for recognition by CD1a-reactive T cells; however, the broader enzymatic pathways of CD1a lipid ligand generation have not been thoroughly investigated. In this study, we used immunofluorescence of skin and ELISpot analyses of CD1a-reactive T cells to investigate the role of the lipase acyloxyacyl hydrolase (AOAH) in CD1a ligand generation with relevance to the pathogenesis of psoriasis. We found that the PLA2 activity of rAOAH leads to the activation of circulating CD1a auto-reactive T cells, leading to the production of IFN-γ and IL-22. Circulating AOAH-responsive CD1a-reactive T cells from patients with psoriasis showed elevated IL-22 production. We observed that AOAH is highly expressed in psoriatic lesions compared to healthy skin. Overall, these data present a role for AOAH in generating antigens that activate circulating lipid-specific CD1a-restricted T cells and, thus, contribute to psoriatic inflammation. These findings suggest that inhibition of PLA2 activity of AOAH may have therapeutic potential for individuals with psoriasis.

A highly conserved host lipase deacylates oxidized phospholipids and ameliorates acute lung injury in mice

Oxidized phospholipids have diverse biological activities, many of which can be pathological, yet how they are inactivated in vivo is not fully understood. Here, we present evidence that a highly conserved host lipase, acyloxyacyl hydrolase (AOAH), can play a significant role in reducing the pro-inflammatory activities of two prominent products of phospholipid oxidation, 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine and 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine. AOAH removed the sn-2 and sn-1 acyl chains from both lipids and reduced their ability to induce macrophage inflammasome activation and cell death in vitro and acute lung injury in mice. In addition to transforming Gram-negative bacterial lipopolysaccharide from stimulus to inhibitor, its most studied activity, AOAH can inactivate these important danger-associated molecular pattern molecules and reduce tissue inflammation and injury.

Biochemical transformation of bacterial lipopolysaccharides by acyloxyacyl hydrolase reduces host injury and promotes recovery

Animals can sense the presence of microbes in their tissues and mobilize their own defenses by recognizing and responding to conserved microbial structures (often called microbe-associated molecular patterns (MAMPs)). Successful host defenses may kill the invaders, yet the host animal may fail to restore homeostasis if the stimulatory microbial structures are not silenced. Although mice have many mechanisms for limiting their responses to lipopolysaccharide (LPS), a major Gram-negative bacterial MAMP, a highly conserved host lipase is required to extinguish LPS sensing in tissues and restore homeostasis. We review recent progress in understanding how this enzyme, acyloxyacyl hydrolase (AOAH), transforms LPS from stimulus to inhibitor, reduces tissue injury and death from infection, prevents prolonged post-infection immunosuppression, and keeps stimulatory LPS from entering the bloodstream. We also discuss how AOAH may increase sensitivity to pulmonary allergens. Better appreciation of how host enzymes modify LPS and other MAMPs may help prevent tissue injury and hasten recovery from infection.

Crystal structure of the mammalian lipopolysaccharide detoxifier

LPS is a potent bacterial endotoxin that triggers the innate immune system. Proper recognition of LPS by pattern-recognition receptors requires a full complement of typically six acyl chains in the lipid portion. Acyloxyacyl hydrolase (AOAH) is a host enzyme that removes secondary (acyloxyacyl-linked) fatty acids from LPS, rendering it immunologically inert. This activity is critical for recovery from immune tolerance that follows Gram-negative infection. To understand the molecular mechanism of AOAH function, we determined its crystal structure and its complex with LPS. The substrate's lipid moiety is accommodated in a large hydrophobic pocket formed by the saposin and catalytic domains with a secondary acyl chain inserted into a narrow lateral hydrophobic tunnel at the active site. The enzyme establishes dispensable contacts with the phosphate groups of LPS but does not interact with its oligosaccharide portion. Proteolytic processing allows movement of an amphipathic helix possibly involved in substrate access at membranes.

Acyloxyacyl hydrolase modulates pelvic pain severity

Chronic pelvic pain causes significant patient morbidity and is a challenge to clinicians. Using a murine neurogenic cystitis model that recapitulates key aspects of interstitial cystitis/bladder pain syndrome (IC), we recently showed that pseudorabies virus (PRV) induces severe pelvic allodynia in BALB/c mice relative to C57BL/6 mice. Here, we report that a quantitative trait locus (QTL) analysis of PRV-induced allodynia in F2_CxB progeny identified a polymorphism on chromosome 13, rs6314295 , significantly associated with allodynia (logarithm of odds = 3.11). The nearby gene encoding acyloxyacyl hydrolase ( Aoah) was induced in the sacral spinal cord of PRV-infected mice. AOAH-deficient mice exhibited increased vesicomotor reflex in response to bladder distension, consistent with spontaneous bladder hypersensitivity, and increased pelvic allodynia in neurogenic cystitis and postbacterial chronic pain models. AOAH deficiency resulted in greater bladder pathology and tumor necrosis factor production in PRV neurogenic cystitis, markers of increased bladder mast cell activation. AOAH immunoreactivity was detectable along the bladder-brain axis, including in brain sites previously correlated with human chronic pelvic pain. Finally, AOAH-deficient mice had significantly higher levels of bladder vascular endothelial growth factor, an emerging marker of chronic pelvic pain in humans. These findings indicate that AOAH modulates pelvic pain severity, suggesting that allelic variation in Aoah influences pelvic pain in IC.

Effect of inflammatory and antiinflammatory stimuli on acyloxyacyl hydrolase gene expression and enzymatic activity in murine macrophages

Acyloxyacyl hydrolase (AOAH) is an enzyme found in macrophages and neutrophils that specifically cleaves the acyloxyacyl moieties of lipopolysaccharide (LPS), thus rendering it non-toxic for human cells. In the present study, we demonstrate that LPS augments AOAH mRNA expression (10-20-fold) in murine macrophages. Following LPS treatment (100 ng/m]), AOAH mRNA was induced by 2 h, peaked at 6 h, and was sustained over 72 h. Optimal induction of AOAH mRNA was observed with as little as 0.1 ng/ml LPS. LPS also induced a concomitant increase in AOAH enzymatic activity in cytosolic extracts from murine macrophages and the ability of macrophages to deacylate LPS was not diminished in endotoxin-tolerized macrophages. LPS-stimulated AOAH mRNA expression was cycloheximide sensitive, indicating that de novo protein synthesis is required for AOAH mRNA production. Moreover, AOAH mRNA expression was also induced by IFN-γ. LPS-stimulated mRNA expression was not suppressed by either dexamethasone or IL-10. Finally, intraperitoneal challenge of mice with 25 μg of LPS resulted in increases in AOAH mRNA in both the lung (∼3-fold) and in the liver (∼6-fold). A possible role for LPS-inducible AOAH in the elimination of LPS is discussed.

A high-throughput, multiplexed assay for superfamily-wide profiling of enzyme activity

The selectivity of an enzyme inhibitor is a key determinant of its usefulness as a tool compound or its safety as a drug. Yet selectivity is never assessed comprehensively in the early stages of the drug discovery process, and only rarely in the later stages, because technical limitations prohibit doing otherwise. Here, we report EnPlex, an efficient, high-throughput method for simultaneously assessing inhibitor potency and specificity, and pilot its application to 96 serine hydrolases. EnPlex analysis of widely used serine hydrolase inhibitors revealed numerous previously unrecognized off-target interactions, some of which may help to explain previously confounding adverse effects. In addition, EnPlex screening of a hydrolase-directed library of boronic acid- and nitrile-containing compounds provided structure-activity relationships in both potency and selectivity dimensions from which lead candidates could be more effectively prioritized. Follow-up of a series of dipeptidyl peptidase 4 inhibitors showed that EnPlex indeed predicted efficacy and safety in animal models. These results demonstrate the feasibility and value of high-throughput, superfamily-wide selectivity profiling and suggest that such profiling can be incorporated into the earliest stages of drug discovery.

Diversity in Mycobacterium tuberculosis mannosylated cell wall determinants impacts adaptation to the host

Mycobacterium tuberculosis (the causal agent of TB) has co-evolved with humans for centuries. It infects via the airborne route and is a prototypic highly adapted intracellular pathogen of macrophages. Extensive sequencing of the M. tuberculosis genome along with recent molecular phylogenetic studies is enabling us to gain insight into the biologic diversity that exists among bacterial strains that impact the pathogenesis of latent infection and disease. The majority of the M. tuberculosis cell envelope is comprised of carbohydrates and lipids, and there is increasing evidence that these microbial determinants that are readily exposed to the host immune system play critical roles in disease pathogenesis. Studies from our laboratory and others have raised the possibility that M. tuberculosis is adapting to the human host by cloaking its cell envelope molecules with terminal mannosylated (i.e. Man-alpha-(1-->2)-Man) oligosaccharides that resemble the glycoforms of mammalian mannoproteins. These mannosylated biomolecules engage the mannose receptor (MR) on macrophages during phagocytosis and dictate the intracellular fate of M. tuberculosis by regulating formation of the unique vesicular compartment in which the bacterium survives. The MR is highly expressed on alveolar macrophages (predominant C-type lectin on human cells) and functions as a scavenger receptor to maintain the healthiness of the lung by clearing foreign particles and at the same time regulating dangerous inflammatory responses. Thus M. tuberculosis exploits MR functions to gain entry into the macrophage and survive. Key biochemical pathways and mycobacterial determinants involved in the development and maintenance of the M. tuberculosis phagosome are being identified. The phylogenetic diversity observed in M. tuberculosis strains that impact its cell wall structure together with the genetic diversity observed in human populations, including those elements that affect macrophage function, may help to explain the extraordinary evolutionary adaptation of this pathogen to the human host. Major developments in these areas are the focus of this review.

Chapter 2: Kill the bacteria...and also their messengers?

We consider here a previously neglected aspect of recovery from infectious diseases: how animals dispose of the dead microbes in their tissues. For one of the most important disease-causing microorganisms, Gram-negative bacteria, there is now evidence that the host catabolism of a key microbial molecule is essential for full recovery. As might be expected, it is the same bacterial molecule that animals sense to detect the presence of Gram-negative bacteria in their tissues, the cell wall lipopolysaccharide (LPS). Here, we discuss current knowledge about LPS sensing with emphasis on the host enzyme that inactivates this microbial "messenger" molecule. We also consider the possibility that the rate at which stimulatory microbial molecules undergo inactivation may influence the duration and severity of diseases caused by other infectious agents.

Host defense genes in asthma and sepsis and the role of the environment

PURPOSE OF REVIEW

There is growing evidence that innate immunity genes contribute to asthma pathogenesis. At the core of the innate immune response are ubiquitous, soluble fragments of bacterial lipopolysaccharide or endotoxin, and chronic exposure to domestic endotoxin has been shown to influence asthma severity. Asthmatic and atopic individuals are more sensitive to endotoxin than nonallergic individuals, suggesting a role for genetics in the innate immunity response, and the potential for gene-environment interactions. Variants in genes associated with classic innate immunity-related disorders, such as sepsis, may be unique candidates for asthma susceptibility.

RECENT FINDINGS

Candidate genes for asthma and allergic diseases co-associated with sepsis including innate immunity receptors and related molecules (CD14, TLR4 and AOAH) and novel genes such as MYLK provide good examples of pleitropic effects of innate immunity genes, where variants conferring risk to specific traits (i.e. sepsis) under one set of genetic and environmental circumstances confer a reduced risk in a different (but possibly related) clinical outcome (i.e. allergic asthma), and support the 'common variant/multiple disease' hypothesis.

SUMMARY

Collectively, these observations suggest a greater role for the innate immunity response in allergic asthma than previously assumed, and implicate host defense genes in disease pathology.

Endotoxin-binding Proteins Modulate the Susceptibility of Bacterial Endotoxin to Deacylation by Acyloxyacyl Hydrolase

Acyloxyacyl hydrolase (AOAH) is an eukaryotic lipase that partially deacylates and detoxifies Gram-negative bacterial lipopolysaccharides and lipooligosaccharides (LPSs or LOSs, endotoxin) within intact cells and inflammatory fluids. In cell lysates or as purified enzyme, in contrast, detergent is required for AOAH to act on LPS or LOS (Erwin, A. L., and Munford, R. S. (1990) J. Biol. Chem. 265, 16444-16449 and Katz, S. S., Weinrauch, Y., Munford, R. S., Elsbach, P., and Weiss, J. (1999) J. Biol. Chem. 274, 36579-36584). We speculated that the sequential interactions of endotoxin (E) with endotoxin-binding proteins (lipopolysaccharide-binding protein (LBP), CD14, and MD-2) might produce changes in endotoxin presentation that would allow AOAH greater access to its substrate, lipid A. To test this hypothesis, we measured the activity of purified AOAH against isolated, metabolically labeled meningococcal LOS and Escherichia coli LPS that were presented either as aggregates (LOSagg or LPSagg)+/-LBP or as monomeric protein (sCD14 or MD-2)-endotoxin complexes. Up to 100-fold differences in the efficiency of endotoxin deacylation by AOAH were observed, with the following rank order of susceptibility to AOAH: E:sCD14>or=endotoxin aggregates (Eagg):LBP (molar ratio of E/LBP 100:1)>>Eagg, Eagg:LBP (E/LBP approximately 1, mol/mol), or E:MD-2. AOAH treatment of LOS-sCD14 produced partially deacylated LOS still complexed with sCD14. The underacylated LOS complexed to sCD14 transferred to MD-2 and thus formed a complex capable of preventing TLR4 activation. These findings strongly suggest that LBP- and CD14-dependent extraction and transfer of endotoxin monomers are accompanied by increased exposure of fatty acyl chains within lipid A and that the acyl chains are then sequestered when LOS binds MD-2. The susceptibility of the monomeric endotoxin-CD14 complex to AOAH may help constrain endotoxin-induced TLR4 activation when endotoxin and membrane CD14 are present in excess of MD-2/TLR-4.

A host lipase detoxifies bacterial lipopolysaccharides in the liver and spleen

Much of the inflammatory response of the body to bloodborne Gram-negative bacteria occurs in the liver and spleen, the major organs that remove these bacteria and their lipopolysaccharide (LPS, endotoxin) from the bloodstream. We show here that LPS undergoes deacylation in the liver and spleen by acyloxyacyl hydrolase (AOAH), an endogenous lipase that selectively removes the secondary fatty acyl chains that are required for LPS recognition by its mammalian signaling receptor, MD-2-TLR4. We further show that Kupffer cells produce AOAH and are required for hepatic LPS deacylation in vivo. AOAH-deficient mice did not deacylate LPS and, whereas their inflammatory responses to low doses of LPS were similar to those of wild type mice for approximately 3 days after LPS challenge, they subsequently developed pronounced hepatosplenomegaly. Providing recombinant AOAH restored LPS deacylating ability to Aoah(-/-) mice and prevented LPS-induced hepatomegaly. AOAH-mediated deacylation is a previously unappreciated mechanism that prevents prolonged inflammatory reactions to Gram-negative bacteria and LPS in the liver and spleen.

Bile acids in physico-chemical host defence

The discovery of the physico-chemical host defence is closely connected with the endotoxin research. It is well known that the toxic effects of endotoxins under experimental conditions can be induced only when they are administered parenterally. However, in naturally occurring entero-endotoxemic diseases (e.g. septic and various shocks, etc.), the endotoxin is absorbed from the intestinal tract. The cause and mode of translocation have been unknown. The generally used experimental shock models differ from natural diseases only in the mode by which endotoxin enters the blood circulation. If the common bile duct of rats was chronically canulated (bile-deprived animals) orally administered endotoxin was absorbed from the intestinal tract into blood circulation and provoked endotoxin shock. This translocation of endotoxins and the consequent shock can be prevented by sodium deoxycholate or natural biles. The bile acids split the endotoxin macromolecule into atoxic fragments. A similar detoxifying detergent action plays a significant role in host defence against infectious agents with outer lipoprotein structure (e.g. so-called 'big' viruses). This defence mechanism of macroorganisms based on the detergent activity of bile acids (end-products of the cholesterol metabolism) is called as physico-chemical defence system. Therefore, bile deficiency and the consequent endotoxemia are important components in the pathogenesis of certain diseases (e.g. sepsis, intestinal syndrome of radiation disease, hepato-renal syndrome, parvovirus infection, herpes, psoriasis, atherosclerosis, etc.). Bile acids may be used for the prevention and/or therapy of the above mentioned clinical conditions.

Deletion of the GPIdeAc Gene Alters the Location and Fate of Glycosylphosphatidylinositol Precursors in Trypanosoma brucei

Glycosylphosphatidylinositol (GPI) membrane anchors are ubiquitous among the eukaryotes. In most organisms, the pathway of GPI biosynthesis involves inositol acylation and inositol deacylation as discrete steps at the beginning and end of the pathway, respectively. The bloodstream form of the protozoan parasite Trypanosoma brucei is unusual in that these reactions occur on multiple GPI intermediates and that it can express side chains of up to six galactose residues on its mature GPI anchors. An inositol deacylase gene, T. brucei GPIdeAc, has been identified. A null mutant was created and shown to be capable of expressing normal mature GPI anchors on its variant surface glycoprotein. Here, we show that the null mutant synthesizes galactosylated forms of the mature GPI precursor, glycolipid A, at an accelerated rate (2.8-fold compared to wild type). These free GPIs accumulate at the cell surface as metabolic end products. Using continuous and pulse-chase labeling experiments, we show that there are two pools of glycolipid A. Only one pool is competent for transfer to nascent variant surface glycoprotein and represents 38% of glycolipid A in wild-type cells. This pool rises to 75% of glycolipid A in the GPIdeAc null mutant. We present a model for the pathway of GPI biosynthesis in T. brucei that helps to explain the complex phenotype of the GPIdeAc null mutant.

Receptors, mediators, and mechanisms involved in bacterial sepsis and septic shock

Bacterial sepsis and septic shock result from the overproduction of inflammatory mediators as a consequence of the interaction of the immune system with bacteria and bacterial wall constituents in the body. Bacterial cell wall constituents such as lipopolysaccharide, peptidoglycans, and lipoteichoic acid are particularly responsible for the deleterious effects of bacteria. These constituents interact in the body with a large number of proteins and receptors, and this interaction determines the eventual inflammatory effect of the compounds. Within the circulation bacterial constituents interact with proteins such as plasma lipoproteins and lipopolysaccharide binding protein. The interaction of the bacterial constituents with receptors on the surface of mononuclear cells is mainly responsible for the induction of proinflammatory mediators by the bacterial constituents. The role of individual receptors such as the toll-like receptors and CD14 in the induction of proinflammatory cytokines and adhesion molecules is discussed in detail. In addition, the roles of a number of other receptors that bind bacterial compounds such as scavenger receptors and their modulating role in inflammation are described. Finally, the therapies for the treatment of bacterial sepsis and septic shock are discussed in relation to the action of the aforementioned receptors and proteins.

Stimulus-dependent deacylation of bacterial lipopolysaccharide by dendritic cells

We describe here a previously unrecognized property of dendritic cells (DCs), the ability to deacylate the lipid A moiety of gram-negative bacterial LPSs. Both immature DCs of the XS52 cell line and bone marrow-derived DCs produce acyloxyacyl hydrolase, an enzyme that detoxifies LPS by selectively removing the secondary acyl chains from lipid A. Acyloxyacyl hydrolase expression decreased when DCs were incubated with IL-4, IL-1 beta, TNF alpha, and an agonistic CD40 antibody (maturation cocktail), and increased after treatment with LPS, CpG oligodeoxynucleotides, or a gram-positive bacterium (Micococcus luteus). Maturation cocktail treatment also diminished, whereas LPS treatment enhanced or maintained the cells' ability to kill Escherichia coli, deacylate LPS, and degrade bacterial protein. Enzymatic deacylation of LPS is an intrinsic, regulated mechanism by which DCs may modulate host responses to this potent bacterial agonist.

Salmonella-type heptaacylated lipid A is inactive and acts as an antagonist of lipopolysaccharide action on human line cells

The stimulation of both THP-1 and U937 human-derived cells by Salmonella lipid A preparations from various strains, as assessed by TNF-alpha induction and NF-kappaB activation, was found to be very low (almost inactive) compared with Escherichia coli lipid A, but all of the lipid As exerted strong activity on mouse cells and on Limulus gelation activity. Experiments using chemically synthesized E. coli-type hexaacylated lipid A (506) and Salmonella-type heptaacylated lipid A (516) yielded clearer results. Both lipid A preparations strongly induced TNF-alpha release and activated NF-kappaB in mouse peritoneal macrophages and mouse macrophage-like cell line J774-1 and induced Limulus gelation activity, although the activity of the latter was slightly weaker than that of the former. However, 516 was completely inactive on both THP-1 and U937 cells in terms of both induction of TNF-alpha and NF-kappaB activation, whereas 506 displayed strong activity on both cells, the same as natural E. coli LPS. In contrast to the action of the lipid A preparations, all the Salmonella LPSs also exhibited full activity on human cells. However, the polysaccharide portion of the LPS neither exhibited TNF-alpha induction activity on the cells when administered alone or together with lipid A nor inhibited the activity of the LPS. These results suggest that the mechanism of activation by LPS or the recognition of lipid A structure by human and mouse cells may differ. In addition, both 516 and lipid A from Salmonella were found to antagonize the 506 and E. coli LPS action that induced TNF-alpha release and NF-kappaB activation in THP-1 cells.

Deacylation of lipopolysaccharide in whole Escherichia coli during destruction by cellular and extracellular components of a rabbit peritoneal inflammatory exudate

Deacylation of purified lipopolysaccharides (LPS) markedly reduces its toxicity toward mammals. However, the biological significance of LPS deacylation during infection of the mammalian host is uncertain, particularly because the ability of acyloxyacyl hydrolase, the leukocyte enzyme that deacylates purified LPS, to attack LPS residing in the bacterial cell envelope has not been established. We recently showed that the cellular and extracellular components of a rabbit sterile inflammatory exudate are capable of extensive and selective removal of secondary acyl chains from purified LPS. We now report that LPS as a constituent of the bacterial envelope is also subject to deacylation in the same inflammatory setting. Using Escherichia coli LCD25, a strain that exclusively incorporates radiolabeled acetate into fatty acids, we quantitated LPS deacylation as the loss of radiolabeled secondary (laurate and myristate) and primary fatty acids (3-hydroxymyristate) from the LPS backbone. Isolated mononuclear cells and neutrophils removed 50% and 20-30%, respectively, of the secondary acyl chains of the LPS of ingested whole bacteria. When bacteria were killed extracellularly during incubation with ascitic fluid, no LPS deacylation occurred. In this setting, the addition of neutrophils had no effect, but addition of mononuclear cells resulted in removal of >40% of the secondary acyl chains by 20 h. Deacylation of LPS was always restricted to the secondary acyl chains. Thus, in an inflammatory exudate, primarily in mononuclear phagocytes, the LPS in whole bacteria undergoes substantial and selective acyloxyacyl hydrolase-like deacylation, both after phagocytosis of intact bacteria and after uptake of LPS shed from extracellularly killed bacteria. This study demonstrates for the first time that the destruction of Gram-negative bacteria by a mammalian host is not restricted to degradation of phospholipids, protein, and RNA, but also includes extensive deacylation of the envelope LPS.

Changes in endotoxin-binding proteins during major elective surgery: important role for soluble CD14 in regulation of biological activity of systemic endotoxin

Assessment of circulating endotoxin during the perioperative period, which is only demonstrated by the Limulus amebocyte lysate (LAL) test, may be modulated by several endotoxin-binding proteins. Endotoxin-neutralizing capacity (ENC) and the plasma levels of soluble CD14 (sCD14), lipopolysaccharide-binding protein, and bactericidal/permeability-increasing protein (BPI) were determined in 40 patients 6 h prior to skin incision for major abdominal surgery. The bioactivity of plasma endotoxin was tested by the polymyxin B-inhibited stimulatory activity of the plasma samples on healthy monocytes as measured by the release of tumor necrosis factor alpha. Plasma endotoxin levels in almost all patients increased from 0.05 +/- 0.01 to 0.23 +/- 0.03 experimental units (EU) per ml (P < 0.001); more specifically, 17 of 40 samples showed endotoxin levels of greater than 0.2 EU per ml and corresponding reductions in ENC. Soluble CD14 plasma levels were decreased from 5. 6 +/- 0.3 to 4.6 +/- 0.3 microg per ml (P < 0.05). ENC was strongly correlated with the sCD14 plasma concentration throughout the period of observation. The addition of sCD14-neutralizing monoclonal anti-sCD14 antibodies reduced ENC both pre- and postoperatively. No correlation could be established between ENC and the plasma levels of BPI, high-density lipoproteins, or low-density lipoproteins determined by measuring the concentrations of apoprotein A and apoprotein B. Biologically active endotoxin was found in only 6 of 17 samples with endotoxin levels greater than 0.2 EU per ml in the LAL test. These samples could be characterized by their perioperative loss of at least 35% of their sCD14. No change in sCD14 was detected in the remaining 11 samples. The perioperative loss of ENC is partly caused by the loss of sCD14 resulting from its consumption by endotoxin reaching the bloodstream. This study demonstrated the role of sCD14 on the bioactivity of circulating endotoxin in a human model of endotoxemia after major abdominal surgery.

Treponema pallidum and Borrelia burgdorferi Lipoproteins and Synthetic Lipopeptides Activate Monocytic Cells via a CD14-Dependent Pathway Distinct from That Used by Lipopolysaccharide

Lipoproteins of Treponema pallidum and Borrelia burgdorferi possess potent proinflammatory properties and, thus, have been implicated as major proinflammatory agonists in syphilis and Lyme disease. Here we used purified B. burgdorferi outer surface protein A (OspA) and synthetic lipopeptides corresponding to the N-termini of OspA and the 47-kDa major lipoprotein immunogen of T. pallidum to clarify the contribution of CD14 to monocytic cell activation by spirochetal lipoproteins and lipopeptides. As with LPS, mouse anti-human CD14 Abs blocked the activation of 1,25-dihydroxyvitamin D3-matured human myelomonocytic THP-1 cells by OspA and the two lipopeptides. The existence of a CD14-dependent pathway was corroborated by using undifferentiated THP-1 cells transfected with CD14 and peritoneal macrophages from CD14-deficient BALB/c mice. Unlike LPS, cell activation by lipoproteins and lipopeptides was serum independent and was not augmented by exogenous LPS-binding protein. Two observations constituted evidence that LPS and lipoprotein/lipopeptide signaling proceed via distinct transducing elements downstream of CD14: 1) CHO cells transfected with CD14 were exquisitely sensitive to LPS but were lipoprotein/lipopeptide nonresponsive; and 2) substoichiometric amounts of deacylated LPS that block LPS signaling at a site distal to CD14 failed to antagonize activation by lipoproteins and lipopeptides. The combined results demonstrate that spirochetal lipoproteins and lipopeptides use a CD14-dependent pathway that differs in at least two fundamental respects from the well-characterized LPS recognition pathway.

Endotoxin binding and elimination by monocytes: secretion of soluble CD14 represents an inducible mechanism counteracting reduced expression of membrane CD14 in patients with sepsis and in a patient with paroxysmal nocturnal hemoglobinuria

Little is known about the role of peripheral blood mononuclear cells (PBMCs) in lipopolysaccharide (LPS) elimination. We studied the endotoxin elimination capacities (EEC) of PBMCs of 15 healthy volunteers, 13 patients with sepsis, and 1 patient suffering from paroxysmal nocturnal hemoglobinuria (PNH). Although expression of CD14, the best-characterized receptor for LPS to date, was reduced from 93.6% +/- 0.8% in healthy subjects to 50.5% +/- 6.5% in patients with sepsis and was 0.3% in a patient with septic PNH, EEC were found to be unchanged. There was no difference in the amount of tumor necrosis factor alpha (TNF-alpha) released by PBMCs of healthy donors and patients with sepsis. Anti-CD14 antibodies (MEM-18) completely suppressed EEC, binding of fluorescein isothiocyanate-labeled LPS to monocytes as determined by FACScan analysis, and TNF-alpha release in all three groups studied. The concentrations of soluble CD14 (sCD14) secreted by endotoxin-stimulated PBMCs from healthy donors and patients with sepsis amounted to 4.5 +/- 0.4 and 20.1 +/- 1.8 ng/ml, respectively. Based on our results, we suggest that PBMCs eliminate LPS by at least two different mechanisms; in healthy subjects, the membrane CD14 (mCD14) receptor is the most important factor for LPS elimination, while in patients with sepsis (including the septic state of PNH), increased sCD14 participates in LPS elimination. Secretion of sCD14 is strongly enhanced under conditions of low expression of mCD14 in order to counteract the reduction of mCD14 and maintain the function of monocytes. This sCD14 may substitute the role of mCD14 in LPS elimination during sepsis. The elimination of LPS by PBMCs correlates with the binding reaction and the secretion of TNF-alpha.

Binding to leukocytes and induction of TNF alpha production involve different sites within the lipid-A region of LPS-like molecules

The induction of TNF alpha synthesis in whole blood culture assay and isolated peripheral blood mononuclear cells was investigated, using LPS from Klebsiella pneumoniae and two water-soluble 34 kDa derivatives designed as acylpolygalactoside (APG) and EFA-APG, an APG molecule bearing two additional ester-linked fatty acids. Both APG and EFA-APG bind to monocytes by specific ligand receptor interaction but only EFA-APG could induce TNF alpha synthesis. It is concluded that ester-linked fatty acids are not involved in LPS binding to the cell surface, but play a critical role in the triggering of cellular responses.

Diagnostic and prognostic significance of plasma endotoxin determination in febrile patients with haematological malignancies

We evaluated the clinical utility of a new endotoxin-specific chromogenic limulus test in febrile patients with haematological malignancies. The specificity is assured by the removal of factor G, which is sensitive to (1-->3)-beta-D-glucan, from horseshoe crab amoebocyte lysate. The sensitivity and specificity of the test to systemic gram-negative bacterial infections were 69.7 and 96.3%, respectively. Meanwhile, gram-negative bacteria grew in only 39.7% of endotoxaemic samples. Thus, it seems appropriate to consider gram-negative bacteraemia and endotoxaemia as different entities. Endotoxaemia was significantly associated with septic shock and infectious death, especially in patients with neutropenia. The new test, the results of which are available within 3 h, should help physicians to recognise this ominous sign early and to initiate a prompt countermeasure to endotoxaemia.

Endotoxins and their significance for murine trypanosomiasis

The study of endotoxins is complicated by their heterogenous nature, their multiple effects and the complex methodologies required for their identification. In this brief review, Vic Pentreath summarizes how the substances have been implicated in the pathogenesis of several diseases caused by parasitic protozoa, and how the parasites (eg. Plasmodium falciparum and Trypanosoma cruzi) may themselves contain endotoxin-like materials. Recent studies have shown that, during T. b. brucei infection in mice, serum endotoxin levels become markedly elevated and that endotoxin-like substances are also present in the purified parasite extracts.

Lipo-oligosaccharides (LOS) of mucosal pathogens: molecular mimicry and host-modification of LOS

Immunochemical studies of the lipo-oligosaccharides (LOS) of the Gram-negative bacteria Neisseria gonorrhoeae and Neisseria meningitidis have revealed some interesting structural characteristics of these LOS that might relate to their roles during pathogenesis. The carbohydrate moieties of the LOS of pathogenic Neisseria mimic carbohydrates present in glycosphingolipids of human cells. Firstly, an LOS component present among a number of Neisseria species is antigenically and/or chemically identical to lactoneoseries glycosphingolipids present in human cells. The lactoneoseries LOS becomes sialylated on Neisseria gonorrhoeae when they are grown in the presence of cytidine 5'-monophospho-N-acetyl-neuraminic acid (CMP-NANA), the nucleotide sugar for sialic acid. Examination of gonococci present in exudates from males with natural infection indicates that sialylation also occurs in vivo. The mechanism for this process apparently involves a bacterial sialyltransferase scavenging available host CMP-NANA ("host-modification" of LOS) and transferring the sialic acid to the lactoneoserieslike LOS. Strains of N. meningitidis and Haemophilus influenzae also express similarly sialylated LOS suggesting that this is a common mechanism of pathogenesis among these bacteria. Additional examples of LOS that mimic other glycosphingolipid series have been identified also and the fact that multiple series can be expressed in a single population of gonococci suggests that a diverse set of LOS can be presented to the host during infection. It is possible that this diverse set of LOS serve different functions for the bacteria in various hosts and/or environments during infection.

The bactericidal/permeability-increasing protein (BPI), a potent element in host-defense against gram-negative bacteria and lipopolysaccharide

The bactericidal/permeability-increasing protein (BPI), is a ca. 55 kDa cytotoxic cationic protein of polymorphonuclear leukocytes (PMN) that is present principally in the azurophilic granules. BPI is toxic only toward Gram-negative bacteria. This target specificity is attributable to the strong attraction of BPI for the lipopolysaccharides (LPS) in the bacterial envelope. BPI also binds with high affinity (apparent Kd 2-5 nM) to a broad range of LPS species and potently inhibits the biologic activities of LPS in vitro. A proteolytically prepared or recombinant ca 25 kDa N-terminal fragment of BPI carries all the antibacterial activities of holo-BPI and is more potent than the holo-protein against more resistant bacteria with S-form LPS in their envelope. The fragment is as active as holo-BPI as an LPS-neutralizing agent in vitro and more potently inhibits cytokine induction by S-form Escherichia coli in whole blood ex vivo. Recombinant forms of both proteins protect animals against the lethal effects of administered LPS.

Hyperlipidemic response to endotoxin--a part of the host-defence mechanism

Endotoxin administration into experimental animals leads to an acute hyperlipidemic response. In addition, lipoproteins can inhibit various biological activities of endotoxin both in vitro and in vivo. The endotoxin-binding and endotoxin-inactivating abilities of lipoproteins, as well as the plasma levels of lipoproteins, are thus increased following endotoxin administration. The endotoxin-induced hyperlipoproteinemia may not only represent the consequence of endotoxemia, but may also be a physiological defence mechanism whereby the body attempts to combat the toxic effects of circulating endotoxin. This minireview discusses the interaction between lipoproteins and endotoxin, and the role of this acute hyperlipidemic response to endotoxin in the context of the host-defence mechanism.

Recognition of bacterial endotoxins by receptor-dependent mechanisms

Research performed during the past 5 years has provided a considerable amount of evidence to support the contention that the initial interaction of LPS (lipid A) with cells is mediated by distinct plasma membrane proteins. Some of these interactions may be solely involved in removal and eventual degradation of LPS whereas others may play a critical role in transmembrane signaling. Interactions that appear to be limited to a removal function have been assigned to the lipoprotein scavenger receptor or CD18 where R-form LPS, lipid A, or partial lipid A structures such as lipid IVa appear to be the preferred ligands; S-form LPS appears not to interact with these membrane proteins. Whether these interactions reflect events that occur in vivo remains to be definitively established. Moreover, the scavenger receptor and CD18 do not have a role in mediating LPS-induced transmembrane signaling. Photochemical crosslinking studies performed by Morrison and colleagues and by Dziarski (1991a,b) have revealed an LPS-binding membrane protein with an apparent molecular weight 70,000-80,000. This protein binds the lipid A of LPS as well as the carbohydrate backbone of peptidoglycan. Studies with monoclonal antibodies to this protein show that the presence of antibody blocks LPS binding, suggesting that engagement of this protein leads to transmembrane signaling. However, a definitive evaluation of the role of this protein in mediating LPS effects will require complete purification and/or gene cloning. Perhaps the most important advance in our understanding of how LPS acts is derived from the studies of Ulevitch, Tobias, and colleagues wherein the LBP/CD14-dependent pathway of cell stimulation has been identified. This pathway has particular importance for LPS recognition and signaling by cells such as monocytes/macrophages or polymorphonuclear leukocytes that constitutively express CD14. The importance of the LBP/CD14-dependent pathway has been definitively demonstrated by experiments using immunologic, biochemical, and molecular biologic approaches. Available data are consistent with a model for a heterodimeric LPS receptor that consists of CD14 and an as yet unidentified additional protein(s). Clearly a major goal for future research will be to elucidate fully the additional proteins involved in recognition of LPS.

Identification and characterization of lipopolysaccharide-binding proteins on human peripheral blood cell populations

Previous research in this laboratory, using photoactivatable radioiodinated lipopolysaccharide derivatized with sulfosuccinimidyl-2-(p-azidosalicylamide)-1,3'-dithiopropionate (125I-ASD-LPS), has resulted in the identification of a specific LPS receptor with a molecular mass of approximately 73 kDa on murine lymphocytes and splenic macrophages. The experiments presented in this report investigated whether a similar LPS-binding protein was also expressed on human peripheral blood populations, including monocytes, lymphocytes, neutrophils, platelets, and erythrocytes. Each cell population was incubated with 125I-ASD-LPS, UV irradiated, washed, reduced, and solubilized, and the cell lysates were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by autoradiography. On all of the cell populations, except erythrocytes, a similar 73-kDa LPS-binding protein was present. In addition, each population also expressed lower-molecular-weight secondary LPS-binding proteins, some of which were conserved among the populations. Binding of the photoactivatable LPS probe was found to be both time and temperature dependent. These data support the concept that the 73-kDa LPS-binding protein is conserved on multiple cell types from a variety of species.

Complementary DNA sequence of rabbit CAP18--a unique lipopolysaccharide binding protein

CAP18 is a novel 18 kDa cationic protein [pI approximately 10] originally purified from rabbit granulocytes using as an assay the agglutination of lipopolysaccharide (LPS) coated erythrocytes. cDNA clones encoding CAP18 were isolated from a rabbit bone marrow cDNA library using a PCR generated oligonucleotide probe derived from the N-terminal amino acid sequence. The deduced amino acid sequence reveals a putative signal sequence of 29 amino acids and a mature protein of 142 amino acid residues. The predicted size of the encoded protein is 16.6 kDa with a pI of 10. There are no N-linked glycosylation sites. The CAP18 sequence bears no homology with other known LPS-binding proteins including human bacterial permeability increasing protein (BPI)(1) and rabbit LPS binding protein (LBP)(2).

How do animal phagocytes process bacterial lipopolysaccharides?

Phagocytic cells, the front-line defense against bacterial invasion, have enzymes that deacylate and dephosphorylate the toxic lipid A moiety of gram-negative bacterial lipopolysaccharide (LPS, also called endotoxin). These enzymes may detoxify LPS without destroying its immunogenicity or antigenicity--in fact, the polysaccharide region of LPS often resists degradation in vivo and retains its antigenic epitopes for long periods of time. The net result, detoxification of LPS with preservation of its immunogenic and antigenic properties, may prevent excessive inflammatory responses while promoting the development of antibacterial immunity.

Intracellular and extracellular enzymatic deacylation of bacterial endotoxin during localized inflammation induced by Escherichia coli

Acyloxyacyl hydrolase (AOAH), an enzyme that removes the secondary acyl chains of gram-negative bacterial lipid A (endotoxin), has been identified previously in human neutrophils and mouse macrophages. We report here that bovine leukocytes also contain AOAH activity. Although bovine AOAH deacylates bacterial lipopolysaccharide in a manner similar to human AOAH, it is active in vitro over a broader pH range, from 4.0 to 7.0. By using Escherichia coli infection of the bovine mammary gland as a model of localized gram-negative bacterial disease and associated tissue inflammation, AOAH activity per leukocyte increased. In addition, AOAH activity increased in the cell-free portion of infected mammary secretions. These data indicate that AOAH activity increases in leukocytes associated with inflammation induced by gram-negative bacteria and provide additional evidence of its potential involvement in the defense against the effects of bacterial endotoxin.