Antibiotics enhance binding by human lipid A-reactive monoclonal antibody HA-1A to smooth gram-negative bacteria

Minireview: Therapeutic intervention in sepsis with antibody to endotoxin: is there a future?

Within the last 24 months a number of studies that tested the efficacy of immunologic reagents in the treatment of sepsis were concluded. Among these reports were 4 studies1-4completed on 2 anti-endotoxin monoclonal antibodies (MAb, HA-1A and E-5). These clinical trials did not generate data sufficient to support product licensure. Given the attention and expectations surrounding the anti-endotoxin MAbs, the disappointing results raised the question whether the use of anti-endotoxin antibodies in the treatment of sepsis was still a viable concept.5The question was rendered even more relevant given the decade-old controversy surrounding the efficacy of polyclonal antibodies to endotoxin, particularly antibody to the J5 (Rc chemotype) mutant of Escherichia coli 0111:B4, a conceptual progenitor of the HA-1A and E-5 MAbs.6In this review we shall examine whether anti-endotoxin antibodies may yet offer any therapeutic potential in the treatment of sepsis. It will be our contention that antibodies to core glycolipid will be useful adjuncts to therapy, particularly if used as part of combination immunotherapy.

Review: Bacterial endotoxin and the human monoclonal antibody HA-IA: specificity, potential mechanisms of action, and limits to its effectiveness

Bacterial endotoxins are lipopolysaccharides present in the outer membrane of all Gram-negative bacteria (GNB). Endotoxins consist of a lipid moiety, lipid A, that is covalently linked to highly variable, serotype O-specific polysaccharide lateral chains. In contrast, the endotoxin core, which includes lipid A, is better conserved and can be recognized by antibodies showing cross-reactivity among various GNB. Such polyclonal and monoclonal antibodies have been developed in an attempt to neutralize the biological and dele. terious effects of endotoxin, thus preventing lipid A from binding to macrophages. In fact, almost all the biological activities of endotoxin are elicited by lipid A, and there is substantial evidence to the effect that the monocyte-macrophage is the principal mediator of endotoxicity. Antiserum against LPS isolated from rough mutants of GNB (expressing virtually only the central core-lipid A), has been shown to counteract the lethal effects of endotoxin in animals and humans. However, such serum or plasma contains antibodies of different specificities and isotypes which represent different effector functions, insofar as LPS is a very complex and highly heterogenous macromolecule. Because of the difficulties encountered in investigating the nature and specificity of the protection afforded by these antisera, and their limited capacity of production for therapeutic use, specific anti-lipid A monoclonal antibodies have been produced in their stead. A variety of mouse and human monoclonal antibodies against LPS have been generated and selected for their ability to cross-react with many GNB species. The most recent clinical trials involving the treatment of septic patients with human HA-IA (Centoxin) or with murine (E5) anti-lipid A monoclonal antibody showed no difference in survival rates, as compared to treatment with a placebo. However, statistical significance was demonstrated in subsets of patients suffering from documented Gram-negative septicemia or Gram-negative sepsis without refractory shock. The usefulness of anti-lipid A antibodies will undoubtedly remain controversial, since they appear to benefit only a minority of all patients treated, and also because no consensus exists regarding their specificity and modes of action. The aim of this review is to describe results which demonstrate the requirements for, difficulties in and limits to, elucidating the ability of certain antibodies to recognize structural elements present in the lipid A domain of LPS. A clear demonstration of antibody cross-reactivity was obtained only when rough LPS bacteria were used, and was markedly enhanced when smooth bacteria had been pretreated with cell wall active antibiotics. Further, new data have recently demonstrated the specific involvement of HA-IA in the immunocytoadherence assay in the presence of human complement and human red blood cells. Such phenomena may form part of the potential role for natural or monoclonal human IgM anti-lipid A antibodies, which will be to remove IgM-lipid A immune complexes through transhepatic clearance via C3b binding to the CR1 present on circulating human erythrocytes. Insofar as immunocytoadherence is a multiparameter phenomenon, various limiting factors probably interfere with its mechanism of clearance. These factors may be absent in various subsets of septic patients under treatment, thus explaining therapeutic failures with HA-IA in humans or preclinical animal studies. Several clinical settings involving defects to CR1 expression, C3b production, LPS recognition and hepatic clearance dysfunction are described. Long term, however, it will impossible to specify the patient subsets suitable for monoclonal therapy without first defining their characteristics. HA-IA may be able to inhibit one of the earliest stages in activation of the cytokine cascade by sequestrating and eliminating biologically active lipid A. The major problem today in terms of using anti-lipid A antibodies is an efficiently early detection of specific pathway defects which detract from or nullify the HA-IA therapeutic effect.

Influence of drug class and dose size on antibiotic-induced endotoxin/IL-6 release and impact on efficacy of anti-endotoxin antibody

There are limited comparative data on antibiotic-mediated endotoxin release and cytokine liberation in septic hosts. To determine the effect of antimicrobial drug class and dose size on the relative concentration kinetics of free endotoxin and interleukin-6 (IL-6), CF-1 mice were made septic following a thermal injury and low-dose (103 CFU) subeschar challenge with Klebsiella pneumoniae serotype K2. Single intraperitoneal (i.p.) doses (in mg/kg) of ceftazidime (TAZ, 25, 200), aztreonam (AZT, 200), piperacillin (PIP, 200), meropenem (MER, 200), imipenem (IMI, 25, 100, 200), ciprofloxacin (CIP, 25) and gentamicin (GEN, 25) were administered at 72 h post burn and infection, when mice were septic with organ dysfunction. AZT, TAZ, MER, PIP (each at 200 mg/kg) and IMI (100 mg/kg) resulted in fold-increases in median endotoxin levels of 15.3, 14.9, 13.1, 8.2 and 12.4, respectively. All were significantly greater than predose baseline values (P < 0.01), however differences among agents did not reach statistical significance. The increases in free endotoxin levels for all of the beta lactams (8.2-15.3-fold) and CIP (7.7-fold) were significantly greater than for GEN (3.9-fold, P < 0.01). The fold-rise in median IL-6 concentrations from baseline for the beta lactams ranged from 3.0-7.7. All of the beta lactams resulted in statistically greater IL-6 release as compared with CIP (1.9-fold, P < 0.01) and GEN (1.4-fold, P < 0.01). The median endotoxin concentrations were significantly higher for the 100 mg/kg (668 EU/ml) and 200 mg/kg (862 EU/ml) doses of IMI compared to the 25 mg/kg dose (378 EU/mL, P < 0.05). There was also a significant increase in endotoxin levels with a dose increase of TAZ from 25 to 200 mg/kg (597 vs 1030 EU/ml, P= 0.017). The addition of antiendotoxin monoclonal antibody (E5, 2 mg/kg i.p. four times daily [qid]) to AZT (75 mg/kg qid) or TAZ (10 mg/kg qid) for 2 days significantly reduced the mortality by ∼20-40% for a 2-3 day period (P < 0.05) compared to AZT or TAZ alone. In contrast, the administration of E5 with either CIP (10 mg/kg) or GEN (10 mg/kg) by the same qid dosing schedule did not improve survival (P> 0.05). In conclusion, data from the present study, generated in a physiologically relevant model of sepsis, found relatively greater release of unbound endotoxin and IL-6 with the beta lactams, intermediate for CIP and lowest for GEN. The size of the dose of two beta lactams was also found to be a potentially important variable, suggesting that dose optimization might be possible to minimize antibiotic-associated endotoxin release. Furthermore, under our experimental conditions, the efficacy of E5 appears to be dependent upon the antibiotic with which it is administered. In order to understand better the potential significance of antibiotic-induced endotoxin release, the pharmacologic variables of the antimicrobial should be systematically evaluated in preclinical and clinical trials.

Multiparameter flow cytometric analysis of antibiotic effects on membrane potential, membrane permeability, and bacterial counts of Staphylococcus aureus and Micrococcus luteus

Although flow cytometry has been used to study antibiotic effects on bacterial membrane potential (MP) and membrane permeability, flow cytometric results are not always well correlated to changes in bacterial counts. Using new, precise techniques, we simultaneously measured MP, membrane permeability, and particle counts of antibiotic-treated and untreated Staphylococcus aureus and Micrococcus luteus cells. MP was calculated from the ratio of red and green fluorescence of diethyloxacarbocyanine [DiOC(2)(3)]. A normalized permeability parameter was calculated from the ratio of far red fluorescence of the nucleic acid dye TO-PRO-3 and green DiOC(2)(3) fluorescence. Bacterial counts were calculated by the addition of polystyrene beads to the sample at a known concentration. Amoxicillin increased permeability within 45 min. At concentrations of <1 microg/ml, some organisms showed increased permeability but normal MP; this population disappeared after 4 h, while bacterial counts increased. At amoxicillin concentrations above 1 microg/ml, MP decreased irreversibly and the particle counts did not increase. Tetracycline and erythromycin caused smaller, dose- and time-dependent decreases in MP. Tetracycline concentrations of <1 microg/ml did not change permeability, while a tetracycline concentration of 4 microg/ml permeabilized 50% of the bacteria; 4 microg of erythromycin per ml permeabilized 20% of the bacteria. Streptomycin decreased MP substantially, with no effect on permeability; chloramphenicol did not change either permeability or MP. Erythromycin pretreatment of bacteria prevented streptomycin and amoxicillin effects. Flow cytometry provides a sensitive means of monitoring the dynamic cellular events that occur in bacteria exposed to antibacterial agents; however, it is probably simplistic to expect that changes in a single cellular parameter will suffice to determine the sensitivities of all species to all drugs.

Anti-lipid A monoclonal antibody centoxin (HA-1A) binds to a wide variety of hydrophobic ligands

This note describes the binding specificities of four lipid A monoclonal antibodies (MAbs) including Centoxin (HA-1A); these MAbs display similar binding properties. MAbs reacted with lipid A and heat-killed smooth bacteria, whereas no reactivity was observed with smooth lipopolysaccharide (LPS). Immunoblotting of bacterial extracts separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the MAbs bound to many polypeptide bands including the molecular weight markers. Denaturation of bovine serum albumin (BSA) by boiling or dithiothreitol treatment unmasked antibody epitopes. In addition, binding both to a hydrophobic aliphatic C12 chain covalently coupled to BSA and to single-stranded DNA was observed. The polyreactivity of these clones is most likely mediated by a preferential reactivity with hydrophobic molecular patches.

Endotoxemia: methods of detection and clinical correlates

As an assay for endotoxin, the Limulus amebocyte lysate assay has several desirable properties: sensitivity, specificity, and potential for adaptation to a quantitative format. Several modifications have been developed to enhance its potential for clinical application. The modifications that allow quantitative measurement of endotoxin and also improve its application to blood samples are described in this review. In fluids other than blood, the detection of endotoxin with the Limulus amebocyte lysate assay can be used as an aid to identify the presence of gram-negative bacteria, and the assay has established utility. With blood, however, there are a range of factors that interfere with the detection of endotoxemia and there are disparate views with respect to the diagnostic and prognostic significance of the test results. In general, the clinical significance of the finding of endotoxemia broadly parallels the frequency and importance of gram-negative sepsis in the patient groups studied and a decline in endotoxin levels accompanies clinical improvement. However, with therapies designed to reduce levels of endotoxin, or to antagonize its effects, it is unclear whether clinical improvement occurs as a consequence of changes in the levels of endotoxemia.

Effect of HA-1A monoclonal IgM antibody on endotoxin-induced proliferation of cultured rat middle ear epithelium

The effect of human monoclonal antibody HA-1A (Centoxin) on the effect of endotoxin on cultured rat middle ear epithelium was investigated. The addition of endotoxin to the standard culture medium revealed a concentration-related proliferative effect on cultured rat middle ear epithelium, leading to cobblestone cells, cell tracks, and stratification of epithelium, whereas rat middle ear epithelium cultured in standard medium grew as a monolayer composed of flat polygonal cells. Addition of HA-1A to standard medium supplemented with endotoxin gave rise to a statistically significant suppression of the proliferative effects of endotoxin on these cells. The morphology of rat middle ear epithelium cultured in the presence of HA-1A and endotoxin showed that these cells still had a tendency to form cobblestone-like cells and cell tracks, but to a substantially lower degree. The present results support the hypothesis that HA-1A suppresses the proliferative and morphological effects of endotoxin on rat middle ear epithelium and may play an important role in the pathogenesis of otitis media.

Antibiotic-induced release of endotoxin. A therapeutic paradox

There is clear experimental evidence that antibiotics increase the bioavailability of endotoxin from Gram-negative bacteria. In this review, data for 2 variables, level of endotoxin and level of bacteria, at the time point closest to 2 hours post-antibiotic exposure were abstracted as a change from baseline readings from each available study, to enable presentation in a graphical overview. This overview indicates that the phenomenon is not limited to beta-lactam agents nor is it apparent only for the more rapidly bactericidal agents. However, evidence that this phenomenon is of clinical importance is scant. With the Jarisch-Herxheimer reaction (JHR), there is clear evidence for an acute deterioration with the initiation of antibiotic therapy and yet uncertainty as to the nature of the bacterial mediator(s) of this reaction. There is no evidence to support the commonly stated concern that therapy with antibiotics with a more rapid bactericidal action may result in the sudden lysis of bacteria with the release of cell wall components and cause a deterioration that might be avoidable through the use of antibiotics with a slower time course of action.

Differential antibiotic-induced release of endotoxin from gram-negative bacteria

Treatment of log phase cultures of Escherichia coli with cell wall active antibiotics results in increased exposure of immunologically reactive lipid A epitopes of lipopolysaccharide (LPS) and release of soluble LPS into culture supernatants. Comparison of the efficacy of two cell wall active antibiotics, ceftazidime, a penicillin-binding protein 3 selective antibiotic, and imipenem, a penicillin-binding protein 2 selective antibiotic, for their relative efficacy in mediating LPS release indicated quantitative but not qualitative differences, with the former antibiotic manifesting a significantly broader range of concentrations at which LPS release could be demonstrated. Comparison of the relative efficacy of these two antibiotics in a mouse bacteraemia model in which animals were made hypersensitive to the lethal effects of endotoxin by treatment with D-galactosamine indicated that the latter antibiotic may provide a greater level of protection. These studies suggest that the release of endotoxin mediated by antibiotic treatment may contribute to the pathogenesis of disease in infectious due to gram-negative organisms.

Biological response modifiers and infectious diseases: actual and potential therapeutic agents

Biological response modifiers (BRMs) are agents which can modify the immune response to cancer or invasion of the organism by infectious agents. An explosive appearance of new BRMs has resulted from the development of recombinant gene technology and the availability of monoclonal antibodies. Colony-stimulating factors first became available for the prevention of neutropenia but may also have a role in the treatment of infections. Interleukin-1 is being tested as a modular of hematopoiesis and may be useful as a helper factor for T- and B-cell function. Immunoglobulins are being used against viral and bacterial infections while interferons can prevent viral upper respiratory infections and suppress or irradicate some viral hepatitides. Other BRMs which show promise include chemical agents and traditional herbal medicines.

Reactivity of the human antiendotoxin immunoglobulin M monoclonal antibody HA-1A with lipopolysaccharides from rough and smooth gram-negative organisms

Clinical data suggest that the human immunoglobulin M antiendotoxin antibody HA-1A reduced mortality in patients diagnosed with gram-negative bacteremia and bacteremia with shock. Previous studies have demonstrated that HA-1A binds to the lipid A domain of lipopolysaccharide (LPS). The present study evaluated the ability of HA-1A to interact with LPs isolated from various strains of gram-negative bacteria by using liquid-phase rate nephelometry and solid-phase immunoblotting assays. HA-1A formed immune complexes in solution with LPSs isolated from both rough and smooth gram-negative organisms. Western blot (immunoblot) analysis of these LPS preparations revealed that HA-1A bound to LPS isolated from rough gram-negative organisms and to a rough LPS-like component present in smooth LPS. HA-1A also bound to LPS-protein complexes found in certain commercial rough LPS preparations. Preincubation of HA-1A with lipid A completely blocked subsequent binding of HA-1A to LPS in both liquid- and solid-phase assay formats, suggesting that the interaction of HA-1A with LPS is through the lipid A domain. Evidence that the binding of HA-1A to LPS was mediated through the antigen-combining (Fv) region of the antibody was provided by the finding that a murine anti-idiotypic antibody to HA-1A inhibited binding. These findings suggested that the broad antiendotoxin reactivity exhibited by HA-1A appeared to be due to the ability of HA-1A to bind to the conserved lipid A moiety of LPSs derived from both smooth- and rough-phenotype gram-negative bacterial strains.