Reconstituted high-density lipoprotein neutralizes gram-negative bacterial lipopolysaccharides in human whole blood

Inhibition of Lipopolysaccharide-Induced Inflammatory Responses by an Apolipoprotein AI Mimetic Peptide

Previous studies suggest that high-density lipoprotein and apoAI inhibit lipopolysaccharide (LPS)-induced inflammatory responses. The goal of the current study was to test the hypothesis that the apoAI mimetic peptide L-4F exerts antiinflammatory effects similar to apoAI. Pretreatment of human umbilical vein endothelial cells (HUVECs) with LPS induced the adhesion of THP-1 monocytes. Incubation of cells with LPS and L-4F (1 to 50 μg/mL) reduced THP-1 adhesion in a concentration-dependent manner. This response was associated with a significant reduction in the synthesis of cytokines, chemokines, and adhesion molecules. L-4F reduced vascular cell adhesion molecule-1 expression induced by LPS or lipid A, whereas a control peptide (Sc-4F) showed no effect. In contrast to LPS treatment, L-4F did not inhibit IL-1β- or tumor necrosis factor-α–induced vascular cell adhesion molecule-1 expression. The inhibitory effect of L-4F on LPS induction of inflammatory markers was associated with reduced binding of LPS to its plasma carrier molecule, lipopolysaccharide binding protein, and decreased binding of LPS to HUVEC monolayers. LPS and L-4F in HUVEC culture medium were fractionated by fast protein liquid chromatography and were localized to the same fractions, suggesting a physical interaction between these molecules. Proinflammatory responses to LPS are associated with the binding of lipid A to cell surface receptors. The current studies demonstrate that L-4F reduces the expression of inflammatory markers induced by LPS and lipid A and suggest that apoAI peptide mimetics may be useful in the treatment of inflammation associated with endotoxemia.

Effect of Short-Term Treatment With Pravastatin on Cytokines and Cytokine Receptors in Patients With Chronic Heart Failure Due to Ischemic and Nonischemic Disease

The antiinflammatory effect of lipoproteins through neutralization of circulating endotoxin has questioned the safety of lipid-lowering drugs in chronic heart failure (CHF). We measured serum levels of interleukin-6, tumor necrosis factor (TNF)-alpha, and soluble TNF-alpha receptors 1 and 2 before and after 1-month treatment with pravastatin 40 mg in 58 patients with CHF. Short-term treatment with pravastatin attenuated the immune response in patients with CHF due to ischemic or nonischemic etiology.

Dyslipidemia and inflammation: an evolutionary conserved mechanism

Inflammation leads to changes in lipid metabolism aimed at decreasing the toxicity of a variety of harmful agents and tissue repair by redistributing nutrients to cells involved in host defence. Acute phase response, mediated by cytokines, preserves the host from acute injury. When this inflammation becomes chronic, it might lead to chronic disorders as atherosclerosis and the metabolic syndrome. The activation of the inflammatory cascade will induce a decrease in HDL-cholesterol (HDL-C), with impairment in reverse cholesterol transport, and parallel changes in apolipoproteins, enzymes, anti-oxidant capacity and ATP binding cassette A1-dependent efflux. This decrease in HDL-C and phospholipids could stimulate compensatory changes, as synthesis and accumulation of phospholipid-rich VLDL which binds bacterial products and other toxic substances, resulting in hypertriglyceridemia. The final consequence is an increased accumulation of cholesterol in cells. When the compensatory response (inflammation) is not able to repair injury, it turns into a harmful reaction, and the lipid changes will become chronic, either by repeated or overwhelming stimulus, enhancing the formation of atherosclerotic lesions. Thus, the classical lipid changes associated with the metabolic syndrome (increased triglycerides and decreased HDL-C) may be envisioned as a highly conserved evolutionary response aimed at tissue repair. Under this assumption, the problem is not the response but the persistence of the stimulus.

Endotoxin promotes preferential periportal upregulation of VLDL secretion in the rat liver

Zonation affects liver parenchymal cell function and metabolism as well as nonparenchymal cell activation, but whether VLDL production is zonated has yet to be elucidated. Infection induces enhanced VLDL secretion by the liver. Ex vivo studies were undertaken to examine the liver heterogeneity for VLDL formation and secretion and their in vivo response to endotoxin. Highly pure periportal (PP) and perivenous (PV) hepatocytes were isolated from fasted lipopolysaccharide-treated, fasted, and fed rats. They were used to assess their capacity to release VLDL-apolipoprotein B (apoB) and lipid classes in relation to de novo lipid synthesis and the expression of genes crucial to VLDL production. Despite the common superior ability of PP hepatocytes for lipid release and zonal differences in lipid synthesis, zonated secretion of VLDL particles was observed in septic but not in normal fed or fasted livers. The endotoxin-induced apoB secretion was more accentuated in PP hepatocytes; this was accompanied by a preferential PP increase in apoB and microsomal triglyceride transfer protein mRNA levels, whereas lipogenesis indicators were, if anything, similarly modified in hepatocytes of either acinar origin. We conclude that PP and PV hepatocytes exhibited similar capabilities for VLDL formation/secretion in normal conditions; however, the endotoxic pressure did zonate periportally.

Potential Role of Endotoxin as a Proinflammatory Mediator of Atherosclerosis

Atherosclerosis is increasingly recognized as a chronic inflammatory disease. Although a variety of inflammatory markers (ie, C-reactive protein) have been associated with atherosclerosis and its consequences, it is important to identify principal mediators of the inflammatory responses. One potentially important source of vascular inflammation in atherosclerosis is bacterial endotoxin. Mutations in Toll-like receptor 4 (TLR-4), an integral component of the endotoxin signaling complex, are fairly common in the Caucasian population and have recently been associated with reduced incidence of atherosclerosis and other cardiovascular diseases in some studies. Moreover, epidemiological studies suggest that endotoxemia at levels as low as 50 pg/mL constitutes a strong risk factor for the development of atherosclerosis. Endotoxin concentrations in this range may be produced by a variety of common subclinical Gram-negative infections. In this article, we outline the main elements of the endotoxin signaling receptor complex that initiates proinflammatory signaling (lipopolysaccharide binding protein [LBP], CD14, TLR-4, and MD-2) and discuss how changes in expression of these molecules may affect proatherogenic responses in the vessel wall. We also describe some of the proinflammatory effects of endotoxin that may be relevant to atherosclerosis, and discuss how serum lipoproteins, especially high-density lipoprotein, may modulate endotoxin-induced inflammatory responses. Further, we discuss recent findings suggesting that the lipid-lowering statins may have an additional protective role in blocking at least some of these proinflammatory signaling pathways. Finally, we discuss species diversity with regard to endotoxin signaling that should be considered when extrapolating experimental data from animal models to humans.

Low density lipoprotein inactivates Vibrio vulnificus cytolysin through the oligomerization of toxin monomer

Blood lipoprotein has been shown to be an important defense factor against the bacterial infection. Lipoprotein is scavenger of bacterial endotoxin (lipopolysaccharide, LPS). However, there is little evidence showing its protective action against the bacterial exotoxin. We have previously demonstrated that cholesterol inactivates Vibrio vulnificus cytolysin (VVC) through oligomerization of the toxin monomer. The aim of the present study was to investigate the relationship between VVC and low-density lipoprotein cholesterol (LDL-C). LDL induced the oligomerization of VVC in a dose-dependent manner. The oligomerization of VVC monomer with LDL was demonstrated by immunoblotting, which exhibited 200-kDa bands corresponding to a tetramer of the native VVC of relative molecular weight of 51,000. Moreover, LDL inactivated hemolytic activity of VVC in a dose-dependent manner, and this response was not changed by the modifications of LDL (heat denaturation of proteins and peroxidation of phospholipids), suggesting that LDL-C is associated with the defense action against VVC. These results suggest that LDL-C inactivates VVC through the induction of toxin oligomerization.

A fluorogenic substrate for detection of organophosphatase activity

A new fluorogenic substrate for the specific detection of organophosphatase (OPase) activity has been designed and evaluated. Our results indicate that 7-diethylphospho-6,8-difluor-4-methylumbelliferyl (DEPFMU) is hydrolyzed specifically by the OPases, mammalian serum paraoxonase and bacterial organophosphorus hydrolase (OPH). The apparent K(m) of DEPFMU is 29 microM for OPH and 91 and 200 microM for the PON1 L(55)R(192) and PON1 L(55)Q(192) isoforms of human paraoxonase, respectively. DEPFMU-based assay systems are 10-100 times more sensitive for OPH and mammalian paraoxonase detection than existing methods. Importantly, DEPFMU is poorly hydrolyzed by both serum and cellular phosphatases and, therefore, may be used as part of a robust and sensitive assay for detecting not only purified, but also highly impure, preparations of OPase such as blood samples. The superior sensitivity of DEPFMU makes it potentially useful in the search for new enzymes that may hydrolyze nerve poisons such as sarin, soman, and VX, monitoring the decontamination of organophosphates (OPs) by OPH and determining serum paraoxonase activity which appears to be important for protection against atherosclerosis, sepsis, and OP toxicity.

High-density lipoproteins in sepsis and septic shock: metabolism, actions, and therapeutic applications

Sepsis and septic shock are important causes of morbidity and lethality in noncoronary intensive care units. Circulating levels of high-density lipoproteins (HDLs) are reduced in sepsis/septic shock, and the magnitude of this reduction is positively correlated with the severity of the illness. The mechanisms underlying this phenomenon are incompletely understood, although increased levels of several acute-phase proteins, including serum amyloid A (SAA) and secretory phospholipase A2 (sPLA2), may contribute to the decrease in plasma HDLs. It has been suggested that HDLs possess anti-inflammatory properties and, hence, may play a crucial role in innate immunity by regulating the inflammatory response as well as being capable of reducing the severity of organ injury in animals and patients with septic shock. These protective effects of HDLs are mediated mainly via (a) lipopolysaccharide (LPS) binding and neutralization, (b) the HDL-associated enzymes, plasma paraoxonase (PON1) and platelet-activating factor acetylhydrolase (PAF-AH), which protect low-density lipoproteins against peroxidative damage, (c) inhibition of the expression of endothelial cell adhesion molecules and release of proinflammatory cytokines, which prevents inflammatory cell infiltration and subsequent multiple organ dysfunction, and (d) stimulation of the expression of endothelial nitric oxide synthase (eNOS). Thus, HDL exerts potent anti-inflammatory effects, some of which are independent of endotoxin binding and might be useful in the treatment of patients with not only sepsis/septic shock but also other conditions associated with an uncontrolled inflammatory response, such as ischemia-reperfusion injury and hemorrhagic shock.

Reconstituted high-density lipoprotein attenuates organ injury and adhesion molecule expression in a rodent model of endotoxic shock

The salutary effects of high-density lipoproteins (HDLs) in animal and human models of endotoxic shock have in the past been attributed to the ability of this lipoprotein to bind to lipopolysaccharide. However, the precise mechanisms for the protective effect of HDL are unclear. The first objective of this study was to determine the effects of HDLs on the organ injury and dysfunction associated with acute severe endotoxemia. Second, to gain insight into the mechanism of action of HDL, we also investigated the effect of HDLs on 1) the expression of P-selectin and intercellular adhesion molecule-1 in the kidneys of rats treated with endotoxin and 2) the rise in the plasma levels of tumor necrosis factor-alpha (TNF-alpha). Rats were given Escherichia coli lipopolysaccharide (6 mg/kg i.v.), pretreated with either vehicle (n = 9) or reconstituted HDL (rHDL; apolipoprotein A-I/phosphatidylcholine proteoliposomes, n = 10), and were monitored for 6 h. Here we report that rHDL attenuates the renal injury and dysfunction caused by endotoxin in the rat. In addition, rHDL reduced the degree of histological tissue injury in the lung, liver and intestine and attenuated the expression of P-selectin and intercellular adhesion molecule-1 in the renal glomerulus. Interestingly, pretreatment of rats with rHDL did not prevent the hypotension nor the rise in plasma levels of TNF-alpha (at 90 min) caused by endotoxin. Thus, rHDL reduces the organ injury/dysfunction, but does not affect the circulatory failure, nor the rise in plasma levels of TNF-alpha caused by endotoxin in the rat. We propose that the mechanisms of these beneficial effects of HDL may be related to direct inhibition of adhesion molecule expression.

Effects of infection and inflammation on lipid and lipoprotein metabolism: mechanisms and consequences to the host

Infection and inflammation induce the acute-phase response (APR), leading to multiple alterations in lipid and lipoprotein metabolism. Plasma triglyceride levels increase from increased VLDL secretion as a result of adipose tissue lipolysis, increased de novo hepatic fatty acid synthesis, and suppression of fatty acid oxidation. With more severe infection, VLDL clearance decreases secondary to decreased lipoprotein lipase and apolipoprotein E in VLDL. In rodents, hypercholesterolemia occurs attributable to increased hepatic cholesterol synthesis and decreased LDL clearance, conversion of cholesterol to bile acids, and secretion of cholesterol into the bile. Marked alterations in proteins important in HDL metabolism lead to decreased reverse cholesterol transport and increased cholesterol delivery to immune cells. Oxidation of LDL and VLDL increases, whereas HDL becomes a proinflammatory molecule. Lipoproteins become enriched in ceramide, glucosylceramide, and sphingomyelin, enhancing uptake by macrophages. Thus, many of the changes in lipoproteins are proatherogenic. The molecular mechanisms underlying the decrease in many of the proteins during the APR involve coordinated decreases in several nuclear hormone receptors, including peroxisome proliferator-activated receptor, liver X receptor, farnesoid X receptor, and retinoid X receptor. APR-induced alterations initially protect the host from the harmful effects of bacteria, viruses, and parasites. However, if prolonged, these changes in the structure and function of lipoproteins will contribute to atherogenesis.

Impact of sepsis-induced changes in plasma on LPS interactions with monocytes and plasma lipoproteins: roles of soluble CD14, LBP, and acute phase lipoproteins

Sepsis-induced changes in human plasma decrease LPS association with monocytes by regulating dynamic interactions among LPS, monocytes, and plasma lipoproteins. In the physiological environment of undiluted human serum, we have found that: (i) LPS binds transiently to monocytes and is released into plasma lipoproteins; (ii) the release of LPS from monocytes is dependent upon lipoprotein acceptors and is enhanced by soluble CD14 (sCD14); and (iii) both lipoproteins and sCD14 can attenuate cytokine responses in monocytes that have already bound LPS. Whereas LPS binding protein (LBP) also inhibited LPS responses after LPS had bound to monocytes, this did not require extensive release of cell-bound LPS as was observed with sCD14. In the serum of septic patients, both free LPS and monocyte-bound LPS were usually transferred to lipoproteins at an accelerated rate. In spite of a sharp decline in HDL levels, HDL remained the dominant LPS acceptor in many severely septic patients, whereas in some cases LPS binding shifted largely to a non-HDL lipoprotein fraction that co-eluted according to size with very low-density lipoprotein (VLDL). Preliminary data suggest that these lipoproteins have a very low density, and they contain apolipoprotein E and higher than normal proportions of the total lipoprotein cholesterol, phospholipid, apolipoprotein B, and serum amyloid A. The data suggest that the VLDL fraction contains acute phase lipoproteins of significantly altered composition that can replace HDL as the dominant LPS acceptor during sepsis when HDL levels are low.

Acute inflammation and infection maintain circulating phospholipid levels and enhance lipopolysaccharide binding to plasma lipoproteins

Circulating lipoproteins are thought to play an important role in the detoxification of lipopolysaccharide (LPS) by binding the bioactive lipid A portion of LPS to the lipoprotein surface. It has been assumed that hypocholesterolemia contributes to inflammation during critical illness by impairing LPS neutralization. We tested whether critical illness impaired LPS binding to lipoproteins and found, to the contrary, that LPS binding was enhanced and that LPS binding to the lipoprotein classes correlated with their phospholipid content. Whereas low serum cholesterol was almost entirely due to the loss of esterified cholesterol (a lipoprotein core component), phospholipids (the major lipoprotein surface lipid) were maintained at near normal levels and were increased in a hypertriglyceridemic subset of septic patients. The levels of phospholipids found in the LDL and VLDL fractions varied inversely with those in the HDL fraction, and LPS bound predominantly to lipoproteins in the LDL and VLDL fractions when HDL levels were low. Lipoproteins isolated from the serum of septic patients neutralized the bioactivity of the LPS that had bound to them. Our results show that the host response to acute inflammation and infection tends to maintain lipoprotein phospholipid levels and that, despite hypocholesterolemia and reduced HDL levels, circulating lipoproteins maintain their ability to bind and neutralize an important bacterial agonist, LPS.

C1 inhibitor prevents endotoxin shock via a direct interaction with lipopolysaccharide

C1 inhibitor (C1INH) is beneficial in animal models of endotoxemia and sepsis. However, the mechanism(s) of C1INH protection remain(s) ill-defined. In this study, we demonstrated that both active C1INH and reactive center-cleaved, inactive C1INH protected mice from lethal Gram-negative endotoxemia. Both forms of C1INH blocked the LPS-binding protein-dependent binding of Salmonella typhimurium LPS to the murine macrophage cell line, RAW 264.7, and suppressed LPS-induced TNF-alpha mRNA expression. Inhibition of LPS binding to RAW 264.7 cells was reversed with anti-C1INH Ab and was more efficient when C1INH was incubated first with LPS rather than with the cells. C1INH also suppressed LPS-induced up-regulation of TNF-alpha mRNA in whole human blood. The interaction of C1INH with LPS was directly demonstrated both by ELISA and by nondenaturing PAGE, but deletion of the amino-terminal 97-aa residues abrogated this binding. Therefore, C1INH, in addition to its function as a serine protease inhibitor, has a novel anti-inflammatory function mediated via its heavily glycosylated amino-terminal non-serpin domain.

A single intravenous dose of endotoxin rapidly alters serum lipoproteins and lipid transfer proteins in normal volunteers

Endotoxemia is associated with rapid and marked declines in serum levels of LDL and HDL by unknown mechanisms. Six normal volunteers received a single, small intravenous (iv) dose of endotoxin (Escherichia coli 0113, 2 ng/kg) or saline in a random order, cross-over design. After endotoxin treatment, volunteers had mild, transient flu-like symptoms and markedly increased serum levels of tumor necrosis factor and its soluble receptors, interleukin-6, cortisol, serum amyloid A, and C-reactive protein. Triglyceride (TG), VLDL-TG, and nonesterified fatty acid increased (peak at 3-4 h), then TG declined (nadir at 9 h), and then cholesterol, LDL cholesterol, apolipoprotein B (apoB), and phospholipid declined (nadirs at 12-24 h). HDL cholesterol and apoA-I levels were not affected, but half of the decrease in phospholipid was HDL phospholipid. Lipopolysaccharide binding protein (LBP) rose 3-fold (peak at 12 h), with smaller and later decreases in the activities of phospholipid transfer protein and cholesteryl ester transfer protein. In conclusion, a decline in LDL was rapidly induced in normal volunteers with a single iv dose of endotoxin. The selective loss of phospholipid from HDL may have been mediated by LBP and, after more intense or prolonged inflammation, could result in increased HDL clearance and reduced HDL levels.

Safety and pharmacokinetics of an endotoxin-binding phospholipid emulsion

BACKGROUND

Lipids and lipoproteins have been shown to bind and neutralize endotoxin and to improve outcomes in animal models of sepsis.

OBJECTIVE

To provide safety and pharmacokinetic data for a protein-free, phospholipid-rich emulsion developed as an agent to neutralize endotoxin, and to study the changes in lipids and lipoproteins following emulsion administration.

METHODS

Thirty healthy male volunteers (aged 18-45 y) were given an emulsion containing 92.5% soy phospholipid, 7.5% soy triglyceride, and 18 mM sodium cholate using a double-blind, placebo-controlled crossover protocol. Emulsion at 3 escalating doses (75, 150, 300 mg/kg) based on phospholipid content was administered by intravenous infusion over 2 hours in the low- and mid-dose groups and 6 hours in the high-dose group.

RESULTS

All subjects completed the protocol without significant toxicities. A slight dose-dependent increase in indirect bilirubin at the 24-hour time point was observed in the emulsion treatment period, with a maximum difference between placebo and emulsion of 0.9 mg/dL. Mean +/- SD peak phospholipid levels were 316 +/- 30, 533 +/- 53, and 709 +/- 86 mg/dL, and phospholipid half-lives were 5.4 +/- 0.6, 5.4 +/- 0.5, and 8.0 +/- 0.8 hours for the low, mid, and high doses, respectively. Increases in total cholesterol, low-density lipoprotein cholesterol and apolipoprotein A-I and B levels were observed. High-density lipoprotein cholesterol decreased immediately following emulsion infusion, but rebounded to above placebo levels by 24 hours.

CONCLUSIONS

A unique phospholipid-rich emulsion was shown to have a favorable safety profile and to expand the blood lipid and lipoprotein pool without the use of human-derived blood products. Lipid levels expected to protect against the physiologic effects of bacterial endotoxin were achieved.

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.

Binding and internalization of lipopolysaccharide by Cla-1, a human orthologue of rodent scavenger receptor B1

Scavenger receptor, class B, type I (SR-BI) mediates selective uptake of high density lipoprotein (HDL) cholesteryl ester. SR-BI recognizes HDL, low density lipoprotein (LDL), exchangeable apolipoproteins, and protein-free lipid vesicles containing negatively charged phospholipids. Lipopolysaccharides (LPS) are highly glycosylated anionic phospholipids contributing to septic shock. Despite significant structural similarities between anionic phospholipids and LPS, the role of SR-BI in LPS uptake is unknown. Cla-1, the human SR-BI orthologue, was determined to be a LPS-binding protein and endocytic receptor mediating the binding and internalization of lipoprotein-free, monomerized LPS. LPS strongly competed with HDL, lipidfree apoA-I and apoA-II for HDL binding to the mouse RAW cells. Stably transfected HeLa cells expressing Cla-1-bound LPS with a Kd of about 16 microg/ml, and had a 3-4-fold increase in binding capacity and LPS uptake. Bodipy-labeled LPS uptake was found to initially accumulate in the plasma membrane and subsequently in a perinuclear region identified predominantly as the Golgi complex. Bodipy-LPS and Alexa-apoA-I had staining that colocalized on the cell surface and intracellularly indicating similar transport mechanisms. When associated with HDL, LPS uptake was increased in Cla-1 overexpressing HeLa cells by 5-10-fold. Cla-1-associated 3H-LPS uptake exceeded 125I-apolipoprotein uptake by 5-fold indicating a selective LPS uptake. Upon interacting with Cla-1 overexpressing HeLa cells, the complex (Bodipy-LPS/Alexa 488 apolipoprotein-labeled HDL) bound and was internalized as a holoparticle. Intracellularly, LPS and apolipoproteins were sorted to different intracellular compartments. With LPS-associated HDL, intracellular LPS co-localized predominantly with transferrin, indicating delivery to an endocytic recycling compartment. Our study reveals a close similarity between Cla-1-mediated selective LPS uptake and the recently described selective lipid sorting by rodent SR-BI. In summary, Cla-1 was found to bind and internalize monomerized and HDL-associated LPS, indicating that Cla-1 may play important role in septic shock by affecting LPS cellular uptake and clearance.

The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues

Because cholesterol is a precursor for the synthesis of steroid hormones, steroidogenic tissues have evolved multiple pathways to ensure adequate supplies of cholesterol. These include synthesis, storage as cholesteryl esters, and import from lipoproteins. In addition to endocytosis via members of the low-density lipoprotein receptor superfamily, steroidogenic cells acquire cholesterol from lipoproteins by selective lipid uptake. This pathway, which does not involve lysosomal degradation of the lipoprotein, is mediated by the scavenger receptor class B type I (SR-BI). SR-BI is highly expressed in steroidogenic cells, where its expression is regulated by various trophic hormones, as well as in the liver. Studies of genetically manipulated strains of mice have established that SR-BI plays a key role in regulating lipoprotein metabolism and cholesterol transport to steroidogenic tissues and to the liver for biliary secretion. In addition, analysis of SR-BI-deficient mice has shown that SR-BI expression is important for alpha-tocopherol and nitric oxide metabolism, as well as normal red blood cell maturation and female fertility. These mouse models have also revealed that SR-BI can protect against atherosclerosis. If SR-BI plays similar physiological and pathophysiological roles in humans, it may be an attractive target for therapeutic intervention in cardiovascular and reproductive diseases.

Lipid composition and lipopolysaccharide binding capacity of lipoproteins in plasma and lymph of patients with systemic inflammatory response syndrome and multiple organ failure

BACKGROUND

Lipopolysaccharide (LPS), the major glycolipid component of Gram-negative bacterial outer membranes, is a potent endotoxin responsible for many of the directly or indirectly induced symptoms of infection. Lipoproteins (in particular, high-density lipoproteins) sequester LPS, thereby acting as a humoral detoxification mechanism.

PATIENTS

Differences in the lipoprotein composition in human plasma and lymph of a control patient group (n = 5) without systemic inflammatory response syndrome (non-SIRS/MOF) and patients with SIRS and multiple organ failure (MOF, n = 9) were studied. The LPS binding capacity of the lipoproteins in SIRS/MOF and non-SIRS/MOF patients was investigated by rechallenge of the plasma and lymph with fluorescently labeled LPS ex vivo. The lipoprotein composition was analyzed using immunochemical techniques and high-performance gel permeation chromatography.

RESULTS

In the non-SIRS/MOF patient group, plasma and lymph levels of apolipoprotein A-I (600 and 450 mg/L, respectively), apolipoprotein B (440 and 280 mg/L, respectively), total cholesterol (2.88 and 1.05 mM, respectively), and total triglycerides (0.67 and 0.97 mM, respectively) were observed. In the SIRS/MOF group, a decrease of apolipoprotein A-I (-55% in plasma and lymph), a decrease of apolipoprotein B (-43% in plasma and -38% in lymph), and a decrease of total cholesterol levels (-54% in plasma and -37% in lymph) were demonstrated. However, the triglyceride levels in the SIRS/MOF group showed a 30% increase in plasma and a 47% decrease in lymph compared with the non-SIRS/MOF patients. In SIRS/MOF patients, a 2.8-fold increase in plasma and a 1.8-fold increase in lymph of the LPS low-density lipoprotein/high-density lipoprotein ratio was observed, indicating that the relative LPS binding capacity of the lipoproteins in the SIRS/MOF patient group showed a trend to be shifted mainly toward low-density lipoproteins. Furthermore, in plasma and lymph of four SIRS/MOF patients, a novel cholesterol-containing high-density lipoprotein-like particle was found that barely had LPS binding capacity (<5%).

CONCLUSIONS

In the SIRS/MOF patients, the changes in lipoprotein composition in lymph are a reflection of those in plasma, except for the triglyceride levels. In comparison with the non-SIRS/MOF patients, the SIRS/MOF patients show a shifted LPS binding capacity of high-density lipoproteins toward low-density lipoproteins in plasma and in lymph. Moreover, in plasma and lymph, novel cholesterol-containing particles, resembling high-density lipoprotein, were identified in the SIRS/MOF patient group.

Lipoprotein metabolism in patients with severe sepsis

OBJECTIVE

Lipoproteins have been implicated to play a role in innate immunity. Changes in lipoprotein levels have been reported in a variety of inflammatory disorders. Not much is known about lipoprotein metabolism in patients with severe sepsis. We conducted an ancillary study in a multiple-center phase III sepsis trial to investigate the dynamics of plasma lipoproteins in patients with severe sepsis.

DESIGN

Prospective analysis in patients meeting criteria for severe sepsis as part of a multiple-center sepsis study (KyberSept) with antithrombin III (Kybernin P).

SETTING

University hospital intensive care unit.

PATIENTS

Seventeen patients were included in the study.

INTERVENTIONS

Randomized patients received a loading dose of 6000 IU of antithrombin III (Kybernin P) or placebo followed by a 96-hr continuous infusion of 250 IU/hr antithrombin III (Kybernin P) or placebo. In each patient, serial blood samples for total cholesterol, lipoprotein cholesterol, triglycerides, apolipoprotein A-1, apolipoprotein B, and C-reactive protein determination as well as clinical data were collected over 28 days.

MEASUREMENTS AND MAIN RESULTS

Plasma cholesterol levels rapidly decreased from 2.67 +/- 2.02 mmol/L on day 0 to a nadir of 1.41 +/- 0.70 mmol/L on day 3, followed by a slow increase to 4.18 +/- 1.94 mmol/L on day 28. High-density lipoprotein (HDL) cholesterol concentrations decreased rapidly from 0.84 +/- 0.92 mmol/L to a nadir of 0.42 +/- 0.35 mmol/L on day 3, to show a slow increase during the following 4 wks to 0.84 +/- 0.42 mmol/L. The low-density lipoprotein (LDL) cholesterol concentrations were already low (0.94 +/- 0.81 mmol/L) at study entry, to show a progressive increase to subnormal values (2.01 +/- 0.94 mmol/L) at 4 wks. Nadir and recovery lipoprotein concentrations were significantly different (paired Student's t-test, p <.05). A significant correlation was found between HDL cholesterol and apolipoprotein A-1 (r =.714, p <.05) and between LDL cholesterol and apolipoprotein B (r =.733, p <.05). There was no statistical difference in lipoprotein concentrations either between survivors and nonsurvivors or between patients receiving antithrombin III or placebo. Serum amyloid A was a major apoprotein (45%) in HDL at the start of the sepsis and was slowly replaced by apolipoprotein A-1 during recovery. A positive correlation was found between plasma C-reactive protein concentrations and serum amyloid A concentrations in HDL (r =.684, p <.05). No other relevant correlations were found between inflammatory and lipoprotein parameters.

CONCLUSIONS

In patients with severe sepsis, lipoprotein concentrations rapidly change and can be reduced to 50% of recovery concentrations. The pattern of early rapid decline is found primarily in the HDL and a slow recovery in both HDL and LDL fractions. The correlation between apolipoprotein and lipoprotein cholesterol concentrations suggests a decline in lipoprotein particles. During severe sepsis, HDL is shifted to acute phase HDL, which is enriched in serum amyloid A and depleted of cholesterol and apolipoprotein A-1. Lipoprotein concentrations are unable to discriminate between survivors and nonsurvivors.

Plasma C-reactive protein in subjects with hypo/hyperalphalipoproteinemias

Hypoalphalipoproteinemia (Hypo-A), a lipid disorder characterized by low high-density lipoprotein (HDL)-cholesterol (HDL-C) levels, is frequently associated with an increased risk of suffering future coronary heart disease (CHD). Conversely, hyperalphalipoproteinemia (Hyper-A) is a characterized by high HDL-C concentrations and is possibly associated with longevity and protection against CHD. Whether plasma C-reactive protein (CRP) level, an emerging marker of CHD risk, may be influenced by either extremely low or high HDL-C concentrations is yet to be determined. Plasma levels of lipids and CRP have been measured in 52 middle-aged men and women, clinically free of CHD, including 20 subjects with Hypo-A, 12 with Hyper-A, and 20 healthy normolipemic age-matched controls. CRP levels were the highest in Hypo-A [0.22 mg/dL (interquartile range, 0.15 to 0.44)], the lowest in Hyper-A [0.03 mg/dL (0.02 to 0.07)], and intermediate in the control group [0.10 mg/dL (0.05 to 0.20)]. Differences in plasma CRP concentrations were significant between Hypo-A and the other 2 groups, as well as between Hyper-A and controls. Plasma CRP levels showed a particularly strong correlation with plasma HDL-C concentrations (r = -.66, P <.001). In multivariate models, HDL-C represented the only significant predictor of circulating levels of CRP. In conclusion, in subjects with Hypo-A or Hyper-A, HDL-C levels may account for plasma CRP variations independent of other potential cardiovascular risk factors.

Protein-free phospholipid emulsion treatment improved cardiopulmonary function and survival in porcine sepsis

Lipoprotein phospholipid (PL) plays a major role in neutralization of endotoxin. This study tested the hypothesis that prophylactic administration of a PL-enriched emulsion (PRE), which augments PL content of serum lipoproteins and neutralizes endotoxin in vitro, would preserve cardiovascular function and improve survival in porcine septic peritonitis. A control group was compared with low-, mid-, and high-dose treatment groups that received PRE by primed continuous infusion for 48 h. A fibrin clot containing live Escherichia coli 0111.B4 was implanted intraperitoneally 30 min after the priming dose. Survival increased in a dose-dependent manner and was correlated with serum PL. Infused PL was associated with high-density lipoprotein in the low-dose group and all serum lipoproteins at higher doses. Treatment significantly lowered serum endotoxin and tumor necrosis factor (TNF)-alpha, preserved cardiac output and ejection fraction, and attenuated increases in systemic and pulmonary vascular resistances. This study demonstrated that augmentation of lipoprotein PL via administration of PRE improved survival and offered a novel therapeutic approach to sepsis.

Novel therapies for sepsis: antiendotoxin therapies

Severe sepsis and septic shock is a common problem encountered in the critical care unit with an estimated incidence in the US of 750,000 cases/year and a mortality rate of 30-50%. Sepsis involves a complex interaction between bacterial factors and the host immune system producing a systemic inflammatory state that may progress to multiple organ failure and death. Endotoxin (a lipopolysaccharide) released from Gram-negative bacteria has been implicated as a potent, prototypical stimulus of the immune response to bacterial infection. Current antiendotoxin strategies utilise various approaches ranging from the prevention of binding to endotoxin receptors with antibodies (monoclonal or polyclonal) against endotoxin or endotoxin receptor/carrier molecules (antiCD14 or antilipopolysaccharide-binding protein antibodies), enhancing clearance or neutralisation (haemoperfusion, lipoproteins, lipopolysaccharide-neutralising proteins) or impairing cellular signalling (lipid A analogues, tyrosine kinase inhibitors). In the future, innovative therapies involving Toll-like receptors and their downstream signalling elements will be developed. This review discusses current knowledge regarding endotoxin signalling, antiendotoxin therapies currently under development, and future areas for research.

Effect of administration of a phospholipid emulsion on the initial response of horses administered endotoxin

OBJECTIVE

To evaluate the effect of a phospholipid emulsion (PLE) on the initial response of horses to administration of endotoxin.

ANIMALS

12 healthy adult horses.

PROCEDURES

Horses were assigned to 2 treatment groups (6 horses/group). The control group was administered 1 L of saline (0.9% NaCl) solution, and the treated group was administered PLE (200 mg/kg, IV); treatments were administered during a period of 120 minutes. An infusion of endotoxin was initiated in both groups starting 1 hour after initiation of the saline or PLE solutions. Physical examination and hemodynamic variables were recorded, and blood samples were analyzed for concentrations of tumor necrosis factor (TNF)-alpha, interleukin-6, thromboxane B2 (TxB2), 6 keto-prostaglandin F (PGF)1alpha, total leukocyte count, and PLE concentrations. An ANOVA was used to detect significant differences.

RESULTS

Administration of PLE resulted in significantly lower rectal temperature, heart rate, cardiac output, right atrial pressure, and pulmonary artery pressure and higher total leukocyte counts in treated horses, compared with values for control horses. The TNF-alpha concentration was significantly less in treated horses than in control horses. The TxB2 and 6 keto-PGFF1alpha concentrations were significantly different between treated and control horses at 30 minutes (TxB2) and at 30 and 60 minutes (6 keto-PGF1alpha).

CONCLUSIONS AND CLINICAL RELEVANCE

Prior infusion of PLE in horses administered a low dose of endotoxin decreased rectal temperature, heart rate, pulmonary artery pressure, and TNF-alpha concentrations. Results of this study support further evaluation of PLE for use in the treatment of horses with endotoxemia.

Effect of high-density lipoprotein on lipopolysaccharide-induced alveolar bone resorption in rats

OBJECTIVE

To determine whether treatments with high-density lipoprotein (HDL) may alter the lipopolysaccharide (LPS)-induced alveolar bone resorption in rats.

MATERIALS AND METHODS

Rats were injected with 500 microg of LPS from Escherichia coli at the alveolar mucosa of lower right first molar once every 2 days for 8 days. The negative and positive control were injected with phosphate buffered saline (PBS) and LPS alone, respectively. In HDL-treated animals various concentration of HDL were injected immediately before, after the third or the final LPS injection. The bone sections were stained with tartrate-resistant acid phosphatase (TRAP) and the numbers of both osteoclasts and preosteoclasts and the levels of alveolar bone resorption were assessed.

RESULTS

The numbers of both osteoclasts and preosteoclasts and the levels of alveolar bone resorption in animals treated with HDL before or during LPS injections were lower than those in the positive control, but higher than those in the negative control, regardless of HDL doses. Similar results were also observed in animals treated with 250 and 500 microg of HDL after the final LPS injection. Only treatments with 1000 microg of HDL after LPS injections completely reduced the number of both osteoclasts and preosteoclasts, but only partially decreased the alveolar bone resorption.

CONCLUSION

HDL treatments partially reduced the LPS-induced alveolar bone resorption in vivo in rats, suggesting that HDL may neutralize the ability of LPS to induce alveolar bone resorption.

Hepatic endosomal trafficking of lipoprotein-bound endotoxin in rats

Triglyceride-rich lipoproteins (chylomicrons (CM), VLDL) can bind and protect against endotoxin (LPS)-induced shock and mortality in rodents. The protective effect of lipoproteins is in part due to the increased plasma clearance and biliary excretion of LPS. Specifically, CM-LPS complexes are principally removed from the circulation by the liver with a rapid plasma half-life approximating that for CM alone. Thus, we hypothesized that hepatocytes clear CM-bound LPS via known lipoprotein receptors and traffic the toxic macromolecule through the same endosomal pathway employed for the catabolism of triglyceride-rich lipoproteins. To examine the endosomal uptake and biliary excretion of LPS, we isolated early and late hepatic endosomal fractions and hepatic bile from rats following the injection of radiolabeled CM-bound LPS. The uptake of (125)I-LPS was compared in animals that overexpressed either the LDL receptor or the LDL receptor-related protein (LRP) versus untreated control with normal lipoprotein levels. Herein we present data indicating that both the LDL receptor and the LRP participate in the rapid internalization of CM-bound LPS by hepatocytes. Upregulation of the LDL receptor increased the accumulation of (125)I-LPS in late endosomes (P < 0.03). In contrast, increased levels of the LRP were associated with negligible movement of LPS into late endosomes but a trend toward the increased biliary excretion of the radiolabeled macromolecule. Taken together these data further elucidate the role of the liver in the host innate immune response to infection and potentially implicate distinct roles for the LDL receptor and LRP in the catabolism of CM-bound LPS.

Association between periodontal pockets and elevated cholesterol and low density lipoprotein cholesterol levels

BACKGROUND

Periodontitis is associated with increased prevalence of cardiovascular morbidity and mortality; however, the nature of this association is unclear. There is a rationale that indicates that the presence of periodontal pockets which can harbor pathogenic microorganisms and evoke a host response could elicit a systemic effect. The hypothesis of this study is that periodontal pockets may be associated with elevated blood lipid levels, a known risk factor for atherosclerotic disease.

METHODS

The periodontal health of 10,590 Israeli military service men and women was assessed using the Community Periodontal Index of Treatment Needs (CPITN). The relationship of blood lipids and periodontal disease and CPITN index was tested, controlling for factors that are related to elevated cholesterol levels, including high body mass index (BMI), age, diastolic blood pressure, and smoking.

RESULTS

The presence of periodontal pockets was positively associated with higher cholesterol and low density lipoprotein (LDL) cholesterol blood levels in men. No significant association was found in women.

CONCLUSIONS

In this large cohort study, the presence of periodontal pockets as measured by CPITN was positively associated with total cholesterol and LDL-cholesterol. The findings of the study support the reports linking increased prevalence of cardiovascular mortality among patients with periodontal disease.

Endogenous lipoproteins impact the response to endotoxin in humans

OBJECTIVE

To determine whether endogenous lipoproteins can abrogate the host response to lipopolysaccharide (LPS) in vivo.

DESIGN

Randomized, placebo-controlled study.

SETTING

Urban public hospital with academic affiliation.

SUBJECTS

Eighteen healthy, normolipidemic, normal weight volunteers, 21-35 yrs of age.

INTERVENTIONS

Fasting and postprandial (hypertriglyceridemic) subjects were injected with endotoxin (LPS, Lot EC-5, 4 ng/kg = 20 endotoxin units/kg) as either a bolus or following preincubation of the LPS with autologous whole blood vs. saline. In addition, LPS-induced cytokine production was determined ex vivo to examine the capacity of fasting vs. hypertriglyceridemic whole blood to attenuate the effect of large, potentially lethal concentrations of LPS in humans.

MEASUREMENTS

Vital signs were recorded and serial blood samples analyzed for changes in white blood cell count, cytokine, and stress hormone levels over 24 hrs. The distribution of lipoproteins in fasting and postprandial blood after preincubation was determined using 125I-LPS.

MAIN RESULTS

Endogenous lipoproteins abrogated the host response to LPS in vivo, but only after preincubation with the LPS. Peak oral temperature (p < .05) and white blood cell count (p < .05), and plasma tumor necrosis factor (TNF)-alpha (p < .01) and adrenocorticotropic hormone levels (p < .03) were significantly reduced in volunteers injected with LPS preincubated with whole blood vs. LPS preincubated with saline. Approximately 80% of the LPS was bound to lipoproteins after preincubation with either fasting or hypertriglyceridemic blood. Thus, protection was associated with lipoprotein binding. In addition, hypertriglyceridemic but not fasting blood inhibited the ex vivo TNF-alpha response to large, highly toxic doses of LPS (p < .05). Without the preincubation of lipoproteins with LPS, there was a trend for an exaggerated clinical and TNF-alpha response in the hypertriglyceridemic subjects.

CONCLUSION

Preincubation of LPS with whole blood promotes lipoprotein-LPS binding and is associated with an attenuated response to this toxic macromolecule. Although the clinical relevance of these data requires further elucidation, our results continue to support a lipid-based therapeutic strategy to combat gram-negative sepsis.

Plasma CD14 decreases monocyte responses to LPS by transferring cell-bound LPS to plasma lipoproteins

CD14, a myeloid cell-surface receptor and soluble plasma protein, binds LPS and other microbial molecules and initiates the innate immune response to bacterial invasion. The blood concentration of soluble CD14 (sCD14) increases during the systemic response to infection. Although high sCD14 blood levels have correlated with increased risk of dying from severe sepsis, sCD14 can diminish cell responses to LPS. We show here that in human serum, sCD14 increases the rate at which cell-bound LPS is released from the monocyte surface and binds to plasma lipoproteins. This enhanced rate of LPS efflux is associated with a significant reduction in the ability of monocytes to produce cytokines in response to LPS. Serum from septic patients reduced the LPS-monocyte interaction by as much as tenfold, and depletion of sCD14 from the serum restored LPS-monocyte binding and release kinetics to near normal levels. In serum from septic patients, monocyte-bound LPS also moved more rapidly into lipoproteins, which completely neutralized the biologic activity of the LPS that bound to them. In human plasma, sCD14 thus diminishes monocyte responses to LPS by transferring cell-bound LPS to lipoproteins. Stress-related increases in plasma sCD14 levels may help prevent inflammatory responses within the blood.

Endotoxin-neutralizing capacity of serum from cardiac surgical patients

OBJECTIVE

To determine if endotoxin core antibody (EndoCAb) from the serum of cardiac surgical patients neutralizes endotoxin in an ex vivo biologic assay.

DESIGN

Prospective blinded cohort study.

SETTING

Academic medical center.

PARTICIPANTS

Patients (n = 203) undergoing cardiac surgery.

INTERVENTIONS

Sera were obtained from patients preoperatively.

MEASUREMENTS AND MAIN RESULTS

EndoCAb levels were determined by enzyme-linked immunosorbent assay. Sera were incubated for 15 minutes at 37 degrees C with varying concentrations of endotoxin from a clinically relevant bacterium (Escherichia coli serotype O18), then tested for the presence of endotoxin activity using the validated Limulus amebocyte lysate assay. Median (interquartile range) IgM and IgG EndoCAb levels were 118 median units (range, 31 to 259 median units) and 208 median units (range, 108 to 401 medium units). Increasing levels of IgM EndoCAb were associated with increased neutralization of endotoxin (p < 0.0001). Increasing levels of IgG EndoCAb were associated with increased neutralization of endotoxin (p < 0.0001). An additive effect of IgM and IgG EndoCAb levels on endotoxin neutralization was observed without evidence of synergistic or plateau effects. EndoCAb levels did not completely predict serum neutralization capacity.

CONCLUSION

Anti-EndoCAbs of both classes (IgM and IgG) were able to neutralize lipopolysaccharide from a clinically relevant bacterium in an ex vivo model. Neither Igm nor IgG appeared to be more capable of neutralization in this model. These antibodies did not completely predict neutralization capacity; other endogenous factors in human serum must be capable of lipopolysaccharide neutralization.

Recombinant lipoproteins: lipoprotein-like lipid particles for drug targeting

Lipoproteins are endogenous particles that transport lipids through the blood to various cell types, where they are recognised and taken up via specific receptors. These particles are, therefore, excellent candidates for the targeted delivery of drugs to various tissues. For example, the remnant receptor and the asialoglycoprotein receptor (ASGPr), which are uniquely localised on hepatocytes, recognise chylomicrons and lactosylated high density lipopoteins (HDL), respectively. In addition, tumour cells of various origins overexpress the low density lipoprotein (LDL) receptor that recognises apolipoprotein E (apoE) on small triglyceride-rich particles and apoB-100 on LDL. Being endogenous, lipoproteins are biodegradable, do not trigger immune reactions, and are not recognised by the reticuloendothelial system (RES). However, their endogenous nature also hampers large-scale pharmaceutical application. In the past two decades, various research groups have successfully synthesised recombinant lipoproteins from commercially available natural and synthetic lipids and serum-derived or recombinant apolipoproteins, which closely mimic the metabolic behaviour of their native counterparts in animal models as well as humans. In this paper, we will summarise the studies that led to the development of these recombinant lipoproteins, and we will address the possibility of using these lipidic particles to selectively deliver a wide range of lipophilic, amphiphilic, and polyanionic compounds to hepatocytes and tumour cells. In addition, the intrinsic therapeutic activities of recombinant chylomicrons and HDL in sepsis and atherosclerosis will be discussed.

Apolipoprotein E protects against bacterial lipopolysaccharide-induced lethality. A new therapeutic approach to treat gram-negative sepsis

Septic shock is the most common cause of death in intensive care units and no effective treatment is available at present. Lipopolysaccharide (LPS) is the primary mediator of Gram-negative sepsis by inducing the production of macrophage-derived cytokines. Previously, we showed that apolipoprotein E (apoE), an established modulator of lipid metabolism, can bind LPS, thereby redirecting LPS from macrophages to hepatocytes in vivo. We now report that intravenously administered LPS strongly increases the serum levels of apoE. In addition, apoE can prevent the LPS-induced production of cytokines and subsequent death in rodents. Finally, apoE-deficient mice show a significantly higher sensitivity toward LPS than control wild-type mice. These findings indicate that apoE may have a physiological role in the protection against sepsis, and recombinant apoE may be used therapeutically to protect against LPS-induced endotoxemia.

The function of scavenger receptors expressed by macrophages and their role in the regulation of inflammation

Macrophages are able to recognise and internalise apoptotic and foreign cells from the tissue microenvironment and, where pertinent, initiate inflammation and macrophage activation. Regulation of this protective response is important so as to prevent inappropriate activation of host cells and excessive tissue damage. Scavenger receptors (SRs) are widely expressed by macrophages and are able to bind a diverse array of endogenous and foreign molecules. Studies have concentrated on the role of the receptors in atherosclerosis as they can endocytose modified lipoproteins, however, this review will detail the accumulating evidence that SRs function in innate immunity and macrophage control during inflammation.

Analysis of lipopolysaccharide (LPS)-binding characteristics of serum components using gel filtration of FITC-labeled LPS

Lipopolysaccharide (LPS)-binding components in serum play an important role in modifying LPS toxicity. We analyzed the binding characteristics of LPS in the presence of serum using gel filtration of FITC-labeled LPS (FITC-LPS) with on line detection of optical density and fluorescence. FITC-LPS separately behaves as an aggregate resulting in a low, dequenched, fluorescence. Binding of single LPS molecules, segregated from the aggregate, to serum components results in an increase in the fluorescence due to dequenching, and a comigration of fluorescence and optical density signals using gel filtration. This method, in combination with the use of specific antibodies inducing additional shifts, demonstrated that in serum high-density lipoproteins (HDL), albumin and low-density lipoproteins (LDL) were able to monomerize LPS. An ELISA on collected fractions of the gel filtration revealed binding of the recently identified LPS-binding protein, serum amyloid P component (SAP), to the high molecular weight LPS aggregate. In serum, binding of soluble CD14 (sCD14) and LPS-binding protein (LBP) to LPS could not be detected. However, this was probably due to an overshadowing effect of albumin, as an extra addition of recombinant sCD14 to serum clearly monomerized FITC-LPS. Biosensor technology revealed that, of all LPS-binding components tested, only SAP clearly bound to the LPS-coated sensor chip. These results show that gel filtration of FITC-LPS is a quick and reliable method to study the binding characteristics of LPS-binding components.

Lipopolysaccharide-binding protein and phospholipid transfer protein release lipopolysaccharides from gram-negative bacterial membranes

Although animals mobilize their innate defenses against gram-negative bacteria when they sense the lipid A moiety of bacterial lipopolysaccharide (LPS), excessive responses to this conserved bacterial molecule can be harmful. Of the known ways for decreasing the stimulatory potency of LPS in blood, the binding and neutralization of LPS by plasma lipoproteins is most prominent. The mechanisms by which host lipoproteins take up the native LPS that is found in bacterial membranes are poorly understood, however, since almost all studies of host-LPS interactions have used purified LPS aggregates. Using native Salmonella enterica serovar Typhimurium outer membrane fragments (blebs) that contained (3)H-labeled lipopolysaccharide (LPS) and (35)S-labeled protein, we found that two human plasma proteins, LPS-binding protein (LBP) and phospholipid transfer protein (PLTP), can extract [(3)H]LPS from bacterial membranes and transfer it to human high-density lipoproteins (HDL). Soluble CD14 (sCD14) did not release LPS from blebs yet could facilitate LBP-mediated LPS transfer to HDL. LBP, but not PLTP, also promoted the activation of human monocytes by bleb-derived LPS. Whereas depleting or neutralizing LBP significantly reduced LPS transfer from blebs to lipoproteins in normal human serum, neutralizing serum PLTP had no demonstrable effect. Of the known lipid transfer proteins, LBP is thus most able to transfer LPS from bacterial membranes to the lipoproteins in normal human serum.

Plasma lipoproteins promote the release of bacterial lipopolysaccharide from the monocyte cell surface

When bacterial lipopolysaccharide (LPS) enters the bloodstream, it is thought to have two general fates. If LPS binds to circulating leukocytes, it triggers innate host defense mechanisms and often elicits toxic reactions. If instead LPS binds to plasma lipoproteins, its bioactivity is largely neutralized. This study shows that lipoproteins can also take up LPS that has first bound to leukocytes. When monocytes were loaded with [(3)H]LPS and then incubated in plasma, they released over 70% of the cell-associated [(3)H]LPS into lipoproteins (predominantly high density lipoprotein), whereas in serum-free medium the [(3)H]LPS remained tightly associated with the cells. The transfer reaction could be reproduced in the presence of pure native lipoproteins or reconstituted high density lipoprotein. Plasma immunodepletion experiments and experiments using recombinant LPS transfer proteins revealed that soluble CD14 significantly enhances LPS release from the cells, high concentrations of LPS-binding protein have a modest effect, and phospholipid transfer protein is unable to facilitate LPS release. Essentially all of the LPS on the monocyte cell surface can be released. Lipoprotein-mediated LPS release was accompanied by a reduction in several cellular responses to the LPS, suggesting that the movement of LPS from leukocytes into lipoproteins may attenuate host responses to LPS in vivo.

Lipopolysaccharide inhibits the expression of the scavenger receptor Cla-1 in human monocytes and macrophages

Human Cla-1 is the likely homologue of the murine scavenger receptor class B type I (SR-BI). SR-BI mediates selective transfer of cholesterol to high-density lipoprotein (HDL) and the efflux of endogenously synthesized and plasma membrane sterols to HDL. HDL protects against atherosclerosis but also reduces endotoxic activity by complexation and neutralization of LPS. We found that Cla-1 is upregulated during phagocytic as well as dendritic differentiation of monocytes, indicating a function of this receptor for cholesterol homeostasis in phagocytes and antigen-presenting cells. Cla-1 expression is suppressed by the proinflammatory stimuli lipopolysaccharide, interferon-gamma, and tumor necrosis factor alpha in monocytes and macrophages. Downregulation of Cla-1 mRNA by LPS is likely due to a modification and subsequent destabilization of the mRNA. We propose that suppression of Cla-1 expression may help to stabilize the lipoprotein status in the blood compartment important for host defense.

Antiendotoxin strategies

Endotoxin is a potent stimulator of the inflammatory response and is believed to initiate the pathology in Gram-negative sepsis. Agents are being developed that bind and neutralize or block the effects of endotoxin, with the goal of improving outcome in the treatment of sepsis. Strategies discussed in this article include anti-LPS antibodies, LPS binding proteins and lipoproteins, polymyxin B conjugates, lipid A analogues, and extracorporeal techniques for endotoxin removal.

Lipopolysaccharide (LPS)-binding synthetic peptides derived from serum amyloid P component neutralize LPS

Lipopolysaccharide (LPS) is the major mediator of gram-negative septic shock. Molecules that bind LPS and neutralize its toxic effects could have important clinical applications. We showed that serum amyloid P component (SAP) neutralizes LPS. A SAP-derived peptide, consisting of amino acids 27 to 39, inhibited LPS-mediated effects in the presence of human blood. In this study, we used a pepscan of overlapping 15-mer peptides and distinguished two additional LPS-binding regions within the SAP molecule, identified in the regions spanning amino acids 61 to 75 and 186 to 200. The corresponding SAP-derived peptides, pep61-75 and pep186-200, inhibited the binding of fluorescein isothiocyanate-labeled LPS to monocytes as efficiently as a bactericidal/permeability-increasing protein (BPI)-derived 15-mer peptide comprising amino acids 85 to 99. The same SAP-derived peptides very potently inhibited LPS-induced priming of phagocytes in human blood. Also, SAP-derived pep186-200 caused a prolonged survival of actinomycin D-sensitized mice treated with LPS to induce septic shock, indicating a potential use of this peptide in the defense against serious gram-negative sepsis in humans.

A lipoteichoic acid fraction of Enterococcus hirae activates cultured human monocytic cells via a CD14-independent pathway to promote cytokine production, and the activity is inhibited by serum components

To elucidate the cellular activation mechanisms of lipoteichoic acid (LTA) compared with those of lipopolysaccharide (LPS), a quantitatively major LTA fraction, QM-1M, was prepared from hot phenol-water extracts of Enterococcus hirae (ATCC 9790) by hydrophobic octyl-Sepharose chromatography and by ion-exchange membrane (QMA-Mem Sep 1010) chromatography as a 60% 1-propanol- and 1 M NaCl-eluted fraction. Unlike the reference Escherichia coli LPS, QM-1M did not demonstrate any ability to induce cytokines in a human whole blood culture system in this study, whereas QM-1M induced a few cytokines such as interleukin (IL)-8 and tumor necrosis factor-alpha in human monocytic THP-1 cell and human peripheral mononuclear cell (PBMC) cultures in the absence of serum. Fetal calf and human sera decreased the above cytokine induction by QM-1M in THP-1 and PBMC cultures, whereas sera increased activities of the reference LPS. IL-8 induction in the absence of serum in response to QM-1M was demonstrated to proceed through a CD14-independent pathway unlike the reference LPS.

A Synthetic Lipopolysaccharide-Binding Peptide Based on Amino Acids 27–39 of Serum Amyloid P Component Inhibits Lipopolysaccharide-Induced Responses in Human Blood

LPS-binding proteins in plasma play an important role in modifying LPS toxicity. Significant properties have already been attributed to the LPS-binding protein (LBP). It accelerates LPS toxicity as well as incorporation into high-density lipoproteins, leading to neutralization of LPS in serum. A search for other LPS-binding components in serum, using LPS-coated magnetic beads, revealed a new LPS-binding protein. N-terminal microsequencing identified this protein as serum amyloid P component (SAP). Purified SAP bound to smooth and rough types of LPS via the lipid A part. SAP inhibited the binding of FITC-labeled ReLPS (LPS from Salmonella minnesota strain R595) to human monocytes and the ReLPS-induced priming of the oxidative burst of human neutrophils only in the presence of low concentrations of LBP. In search for the LPS binding site of SAP, we found that pep27–39, a 13-mer peptide consisting of amino acids 27–39 of SAP, competitively inhibited the binding of LPS to SAP. In addition, pep27–39 significantly inhibited ReLPS-induced responses in phagocytes in the presence of serum, as well as in human whole blood. Carboxamidomethylated pep27–39 showed an even more pronounced reduction of the ReLPS-induced priming of phagocytes in human blood. Performing gel filtration of FITC-labeled ReLPS incubated with soluble CD14, we showed that SAP could not prevent binding of LPS to soluble CD14, in contrast to pep27–39. The ability of pep27–39 to antagonize specifically the effects of LPS in the complex environment of human blood suggests that pep27–39 may be a novel therapeutic agent in the treatment of Gram-negative sepsis.

Plasma lipid concentrations correlate inversely with CPB-induced interleukin-6 release

PURPOSE

Cardiopulmonary bypass (CPB) is characterized by translocation of intestinal endotoxin and subsequent endogenous production of the pro-inflammatory cytokine interleukin-6 (IL-6). Plasma lipid fractions, especially high density lipoproteins, bind and neutralize endotoxin and, therefore, inhibit endotoxin-induced macrophage cytokine production, including IL-6. Increased IL-6 plasma levels have been implicated in adverse consequences associated with CPB. Previous studies demonstrated large interpatient variability in IL-6 plasma levels after CPB. The purpose of this study was to evaluate the relationship between plasma lipid concentrations and the concentrations of IL-6 following CPB in humans.

METHODS

In a prospective study, a group of 15 patients selected to exclude variables known to influence post-CPB plasma levels of IL-6 (preoperative left ventricular ejection fraction > 45%, similar durations of aortic cross clamping and total CPB time, similar temperature control during CPB, and avoidance of platelet transfusion and shed mediastinal blood re-infusion), IL-6 was measured at baseline, one and 24 hr post-CPB.

RESULTS

Interleukin-6 plasma concentrations (mean +/- SD) increased at one (142 +/- 89 pg.ml-1, P < 0.05) and 24 (129 +/- 82 pg.ml-1, P < 0.05) hr post-CPB compared with baseline (1.5 +/- 1 pg.ml-1) concentrations. An inverse correlation was found between IL-6 plasma concentrations at one hour post-CPB and plasma cholesterol concentrations (r = -0.592, P = 0.02), high density lipoprotein (r = -0.595, P = 0.02), and low density lipoprotein (r = -0.656, P = 0.01).

CONCLUSIONS

These results suggest that plasma lipids attenuate the production of IL-6 during CPB and may partly explain the variability of interpatient levels of IL-6 reported post-CPB by others.

Lipoprotein(a) inhibits lipopolysaccharide-induced tumor necrosis factor alpha production by human mononuclear cells

Lipoproteins can bind lipopolysaccharide (LPS) and decrease LPS-stimulated cytokine production. Lipoprotein(a) [Lp(a)] was as potent as low-density lipoproteins (LDL) in inhibiting LPS-stimulated tumor necrosis factor synthesis by human mononuclear cells. The kinetics of LPS inhibition by Lp(a) was similar to that of LDL. This suggests that circulating Lp(a) may be an important factor determining the amplitude of the response to LPS in humans.