The enzymatic synthesis of lipid A: molecular structure and biologic function of monosaccharide precursors

Hit 'em Where It Hurts: Gram-Negative Bacterial Lipopolysaccharide as a Vaccine Target

Infections with antimicrobial-resistant (AMR) bacteria pose an increasing threat to the ability to perform surgical procedures, organ transplantation, and treat cancer among many other medical conditions. There are few new antimicrobials in the development pipeline. Vaccines against AMR Gram-negative bacteria may reduce the use of antimicrobials and prevent bacterial transmission. This review traces the origins of lipopolysaccharide (LPS)-based vaccines against Gram-negative bacteria, the role of O polysaccharides and LPS core regions as potential vaccine targets, the development of new vaccine technologies, and their application to vaccines in current development.

Development of an anti-endotoxin vaccine for sepsis

Gram-negative bacterial lipopolysaccharide (LPS, endotoxin) is an important initiator of sepsis, a clinical syndrome that is a leading cause of death in intensive care units. Vaccines directed against core LPS structures that are widely conserved among Gram-negative bacteria (GNB) have been developed for the treatment and/or prevention of sepsis. Killed whole bacterial vaccines (E. coli O111:B4, J5 [Rc chemotype] mutant and S. minnesota, Re chemotype) protected mice against experimental sepsis. Human J5 immune antisera reduced the mortality from GNB sepsis in a large controlled clinical trial; however, subsequent clinical studies with antiendotoxin antibodies did not demonstrate protective efficacy in sepsis. Multiple clinical studies have since demonstrated a correlation between the level of circulating antibodies to LPS core and morbidity and mortality in different clinical settings. We therefore developed a subunit vaccine by combining detoxified J5 LPS (J5 dLPS) with the outer membrane protein (OMP) from group B N. meningitidis. This vaccine was highly efficacious in experimental models of sepsis and progressed to phase 1 clinical trial. While well-tolerated, this vaccine induced only 3-4-fold increases in anti-J5 dLPS antibody. Addition of the TLR9 agonist, oligodeoxynucleotide with a CpG motif, as adjuvant to the vaccine increased antibody levels in mice and the vaccine/CpG combination will progress to phase 1 human study. Additional vaccines in which the core glycolipid was either conjugated to carrier protein or incorporated into liposomes have been developed, but have not progressed to clinical trial. Should an antiendotoxin vaccine become available, a new immunization strategy directed towards distinct populations at risk will be required.

Vaccines and antibodies in the prevention and treatment of sepsis

Antibodies to various core glycolipid antigens have been shown to correlate with survival from Gram-negative sepsis. Recent preclinical data also support efficacy of the anti-core glycolipid antibodies in the treatment of sepsis. Failure of some of the previous clinical trials with anti-core glycolipid antibody was probably due to inadequate levels of antibody in those preparations. Future clinical trials must ensure that sufficient amounts of anti-core glycolipid antibodies are present in the circulation of patients with sepsis.

A glycolipid precursor of bacterial lipopolysaccharide (lipid X) lacks activity against endothelial cells in vitro and is not toxic in vivo

Lipid X (2,3-diacylglucosamine-1-phosphate) accumulates in mutants of Escherichia coli incapable of assembling the disaccharide backbone of lipid A, the principle endotoxic moiety of bacterial lipopolysaccharide (LPS). We compared the effects of lipid X on cultured bovine aortic endothelial cell (BEC) viability and prostacyclin (PGI2) release with those of lipid A and LPS. At 10(-5) M, both LPS and lipid A produced significant BEC cytotoxicity (percentage cytotoxicity 69 +/- 4 for LPS and 51 +/- 11 for lipid A) and induced a variable but consistent increase in the release of PGI2 (11- to 73-fold increase for LPS and 4- to 6-fold increase for lipid A). Lipid X, in contrast, was not toxic and did not induce PGI2 release at 10(-4) M. Pretreatment and coincubation of BEC with lipid X, at a concentration 100 times greater than LPS, failed to prevent LPS-mediated cytotoxicity. Intravenous infusion of lipid X in goats had no effect except for a modest elevation in the pulmonary artery pressure during the period of infusion. Moreover, pretreatment of goats with lipid X (70 micrograms/kg) did not block the effects of a subsequent infusion of LPS (5 micrograms/kg). These data suggest that a fatty acid-substituted disaccharide is the minimal molecular requirement for the numerous effects in vivo and activity in vitro induced by LPS. Furthermore, these effects are not prevented by pretreatment with a monosaccharide precursor of lipopolysaccharide, lipid X, at a dose 10- to 100-fold greater than that of LPS.

Lipid X protects mice against fatal Escherichia coli infection

Lipid X, the major monosaccharide precursor of lipid A, is nontoxic and has previously been shown to protect mice and sheep from the harmful effects of endotoxin. To test the hypothesis that lipid X could be therapeutic against infections with gram-negative organisms, neutropenic ICR mice were infected by intramuscular inoculation of Escherichia coli and subsequently treated with lipid X alone or in combination with the antibiotic ticarcillin. Lipid X slightly prolonged survival; treated mice had a significantly improved rate of survival 18 h after intramuscular inoculation as compared with controls (P less than 0.025). By 24 h, however, this difference disappeared. When lipid X was combined with ticarcillin, survival differences were both significant and prolonged. Treatment of mice with one to two doses of lipid X for a total dose of 1 mg intravenously and with 1,200 mg of ticarcillin per kg every 6 h improved survival over a 48-h treatment period from 5 to 23% (P less than 0.0025). Treatment with lipid X and ticarcillin over a broad range of antibiotic dosages in 362 mice demonstrated improved survival of two- to fourfold (P less than 0.0001 at 24 h after inoculation, P less than or equal to 0.0005 at 48 h, and P less than or equal to 0.0001 at 5 days). Lipid X enabled the dose of ticarcillin necessary to protect 50% of mice from death to be reduced by two- to fivefold. Pretreatment with lipid X was not necessary to improve survival: 16 of 17 (94%) infected and visibly ill animals that received lipid X and ticarcillin 6 h after thigh inoculation survived versus 30 of 44 (68%) control animals treated with ticarcillin alone (P less than 0.0001). Lipid X had no antimicrobial activity in vitro. Lipid X is a novel agent that enhances survival in an animal model of severe infection with gram-negative organisms.

Elimination and tissue distribution of the monosaccharide lipid A precursor, lipid X, in mice and sheep

Lipid X (2,3-diacylglucosamine 1-phosphate) is a novel monosaccharide precursor of lipid A (the active moiety of gram-negative endotoxin) and has been found to be protective against endotoxin administered to mice and sheep and against life-threatening gram-negative infections in mice. Because of the need to design optimal dosing regimens in experimental models of ovine and murine septicemia, the pharmacokinetic profile of lipid X was investigated in sheep and in two strains of mice by using 32P-labeled lipid X. In sheep, peak whole blood lipid X levels after a bolus injection of 100 micrograms of lipid X per kg were 900 ng/ml. An initial rapid distribution phase of 7.98 +/- 0.1 min was observed, followed by a prolonged elimination phase of 3.0 +/- 0.5 h; the area under the curve from time zero to infinity was 428 +/- 27 ng.h/ml. The serum half-lives of lipid X were slightly shorter than whole blood half-lives, suggesting that lipid X associates with cellular elements. Metabolites of lipid X could not be detected in serum over a 4-h period. Lipid X appears to accumulate mainly in the liver, and the tissue distribution of lipid X resembles that of lipopolysaccharide. The elimination rate of lipid X in mice was approximately four times as rapid as that seen in sheep. Lipid X pharmacokinetics in lipopolysaccharide-sensitive DBA/2J mice were virtually identical with those seen in endotoxin-resistant C3H/HeJ mice. The pharmacokinetics described here should greatly aid in the design and interpretation of animal studies investigating the therapeutic applications of lipid X in gram-negative septicemia.

Inhibition of endotoxin-induced priming of human neutrophils by lipid X and 3-Aza-lipid X

Lipid X, a precursor of lipid A (the toxic moiety of endotoxin), has been shown to protect animals from the lethal effects of endotoxin challenge. We investigated the mechanism of action of lipid X and 3-aza-lipid X, a diamino-analogue, in vitro, using the ability of lipopolysaccharide (LPS) to prime neutrophils for an enhanced release of toxic oxygen radicals. Lipid X and 3-aza-lipid X inhibited LPS-induced neutrophil priming in a concentration-dependent manner. At high concentrations, 3-aza-lipid X was a partial agonist of priming. Lipid X was found to inhibit LPS-induced priming by directly interacting with the neutrophil in contrast to polymyxin B, which neutralized LPS by binding to it. Increasing concentrations of lipid X shifted the LPS dose response curve of neutrophils rightward but did not prevent maximum priming at higher LPS concentrations, a finding consistent with competitive inhibition. These results suggest that lipid X, a compound structurally related to lipid A, may block neutrophil priming by competing with LPS for cellular binding sites. Lipid X appears to have a novel mechanism of inhibiting LPS effect and may have efficacy in the treatment of gram-negative sepsis.

Ultrafiltration to reject human interleukin-1-inducing substances derived from bacterial cultures

Interleukin-1 (IL-1), a polypeptide cytokine, is an important mediator of host responses to infection and injury. Picogram per milliliter concentrations of bacterial products (endo- or exotoxins) stimulate human monocytes to produce IL-1 in vitro. The design of this study was based on the clinical model of bacterial contamination of fluid intended to be directly injected into humans. Physiologic saline contaminated with bacterial toxins was passed through a hollow fiber ultrafilter, and the ultrafiltrates were tested for their ability to induce human IL-1 production. The ultrafiltrates were added directly to freshly obtained human blood mononuclear cells, and after 24 h of incubation the supernatant media were assayed for the presence of IL-1. The results indicate that the IL-1-inducing material(s) present in bacterial cultures of gram-negative organisms is rejected by a factor of 100 to 100,000 by molecular size exclusion and by absorption; rejection is sustained for at least 32 liters of fluid; the rejection of Limulus-reactive material by the ultrafilter is greater for purified endotoxin than for native endotoxins derived from live bacterial cultures; and nonendotoxin IL-1-inducing toxins (molecular weight, 24,000) from Staphylococcus aureus are not rejected or absorbed. These results demonstrate that there is a considerable margin of safety with the ultrafiltration method and that it can be applied to clinical situations.

Current perspectives on septic shock

Although septic shock may be initiated by invading microbes, it is the metabolic and immunologic host responses that determine the true pathophysiology of this common critical care illness. Currently, septic shock therapeutics emphasize empiric and symptomatic treatment. Biochemical elucidation of the septic process will ultimately result in specific interventions for this ominous intensive care syndrome.

Protection of mice against lethal endotoxemia by a lipid A precursor

Lipid X, the major biosynthetic precursor of lipid A, has recently been described. Although lipid X is a mitogen and coagulates the Limulus amebocyte lysate, we found that it is not lethal for mice, even when given in large doses (2 X 10(6) micrograms/kg). Furthermore, lipid X was found to give partial protection against a 100% lethal dose of endotoxin, even if the lipid X was given as late as 6 h after endotoxin challenge.

Activation and inhibition of Limulus amebocyte lysate coagulation by chemically defined substructures of lipid A

Recent work with lipid mutants of Escherichia coli and Salmonella typhimurium has helped to elucidate the correct structure of lipid A and has suggested a biosynthetic pathway. Precursor molecules include diacylglucosamine 1-phosphates and tetraacyl disaccharide bis-phosphates. The activities of several of these compounds and of their derivatives were measured by Limulus amebocyte lysate (LAL) assay. We report that (i) both mono- and disaccharide precursors of lipid A activate LAL, (ii) two acyl chains on the monosaccharide subunit of lipid A are necessary for activation of LAL, and (iii) the monosaccharide, 2-monoacylglucosamine 1-phosphate can competitively inhibit LAL activation by diacyl monosaccharide lipid A precursors. However, 2-monoacylglucosamine 1-phosphate did not inhibit endotoxin activation of LAL. One unanticipated finding was that the activities of the monosaccharides were reduced upon storage even though their covalent structures were unchanged. Perhaps this is due to alterations in physical state. Thus, these lipid A precursors and derivatives offer some insight into the structural features required for activation of the LAL assay and may in the future provide derivatives which are competitive inhibitors of endotoxin.