Heterogeneity of lipopolysaccharides of Neisseria meningitidis revealed by thin-layer chromatography combined with monoclonal antibodies

Cellular activating properties and morphology of membrane-bound and purified meningococcal lipopolysaccharide

Neisseria meningitidis, the cause of epidemic meningitis and acute lethal sepsis, synthesizes surplus lipopolysaccharides (LPSs) during growth, which are released as outer membrane vesicles (OMV) or `blebs'. Meningococcal disease severity is related to plasma LPS levels. We have compared the biological activities of native outer membrane vesicles (nOMV) to those of purified Nm-LPS (Nm-LPS) and LPS-depleted OMV (dOMV) prepared from N. meningitidis. The LPS content of nOMV was determined spectrophotometrically by quantifying KDO and by silver-stained SDS-PAGE gels. The morphology of the preparations was studied by transmission electron microscopy. The Limulus amoebocyte lysate (LAL) assay was used to quantify LPS in the plasma solutions. The preparations were diluted in endotoxin-free heparin plasma to equal amounts of LPS (w/w) in the range 50—5000 pg/ml. The biological reactivity was tested by: (i) a monocyte target-assay (monocyte purity ≥ 96%); and (ii) a whole blood model, measuring the secretion of TNF-α and IL-6 induction of procoagulant activity in monocytes (PCA). In both models, nOMV induced dose-dependent cell responses (TNF-α, IL-6, PCA) similar to purified Nm-LPS, whereas dOMV induced minimal responses. However, LAL activity was significantly higher for nOMV than for purified Nm-LPS and dOMV. The cellular responses of purified Nm-LPS and nOMV were reduced (>95%) by a specific anti-CD 14-antibody.

Invited review: Neisseria meningitidis lipopolysaccharides in human pathology

Neisseria meningitidis causes meningitis, fulminant septicemia or mild meningococcemia attacking mainly children and young adults. Lipopolysaccharides (LPS) consist of a symmetrical hexa-acyl lipid A and a short oligosaccharide chain and are classified in 11 immunotypes. Lipid A is the primary toxic component of N. meningitidis . LPS levels in plasma and cerebrospinal fluid as determined by Limulus amebocyte lysate (LAL) assay are quantitatively closely associated with inflammatory mediators, clinical symptoms, and outcome. Patients with persistent septic shock, multiple organ failure, and severe coagulopathy reveal extraordinarily high levels of LPS in plasma. The cytokine production is compartmentalized to either the circulation or to the subarachnoid space. Mortality related to shock increases from 0% to > 80% with a 10-fold increase of plasma LPS from 10 to 100 endotoxin units/ml. Hemorrhagic skin lesions and thrombosis are caused by up-regulation of tissue factor which induces coagulation, and by inhibition of fibrinolysis by plasminogen activator inhibitor 1 (PAI-1). Effective antibiotic treatment results in a rapid decline of plasma LPS (half-life 1—3 h) and cytokines, and reduced generation of thrombin, and PAI-1. Early antibiotic treatment is mandatory. Three intervention trials to block lipid A have not significantly reduced the mortality of meningococcal septicemia.

Selection and Characterization of Cyclic Peptides that Bind to a Monoclonal Antibody Against Meningococcal L3,7,9 lipopolysaccharides

There is still no general vaccine for prevention of disease caused by group-B meningococcal strains. Meningococcal lipopolysaccharides (LPSs) have received attention as potential vaccine candidates, but concerns regarding their safety have been raised. Peptide mimics of LPS epitopes may represent safe alternatives to immunization with LPS. The monoclonal antibody (MoAb) 9-2-L3,7,9 specific for Neisseria meningitidis LPS immunotype L3,7,9 is bactericidal and does not cross-react with human tissue. To explore the possibility of isolating peptide mimics of the epitope recognized by MoAb 9-2-L3,7,9, we have constructed two phage display libraries of six and nine random amino acids flanked by cysteines. Furthermore, we developed a system for the easy exchange of peptide-encoding sequences from the phage-display system to a hepatitis B core (HBc) expression system. Cyclic peptides that specifically bound MoAb 9-2-L3,7,9 at a site overlapping with the LPS-binding site were selected from both libraries. Three out of four tested peptides which reacted with MoAb 9-2-L3,7,9 were successfully presented as fusions to the immunodominant loop of HBc particles expressed in Escherichia coli. However, both peptide conjugates to keyhole limpet haemocyanin and HBc particle fusions failed to give an anti-LPS response in mice.

High-level endothelial E-selectin (CD62E) cell adhesion molecule expression by a lipopolysaccharide-deficient strain of Neisseria meningitidis despite poor activation of NF-kappaB transcription factor

Binding of host inflammatory cells to the endothelium is a critical contributor to the vascular damage characteristic of severe meningococcal disease and is regulated by endothelial cell adhesion molecules such as ICAM-1, VCAM-1 and CD62E. Intact meningococci induce far higher levels of CD62E than lipopolysaccharide (LPS) alone, whereas LPS is at least as potent as meningococci at inducing both VCAM-1 and ICAM-1 expression. This suggests that meningococci possess additional factors other than LPS present in whole bacteria that result in differential adhesion molecule expression. To investigate this possibility, we studied the capacity of an LPS-deficient isogenic strain of serogroup B Neisseria meningitidis H44/76 (lpxA-) to induce endothelial cell adhesion molecule expression and translocation of the transcription factor NF-kappaB, and compared it to both parent and unencapsulated strains of both B1940 and H44/76 and purified LPS. Although the LPS-deficient isogenic mutant of strain H44/76 was found to be a poor inducer of NF-kappaB, it induced higher levels of CD62E expression than LPS alone. These data provide evidence that intact meningococci induce a range of signals in the endothelium that are distinct from those seen with purified LPS alone and that they occur in a LPS-dependent and LPS-independent manner. These signals may explain the potent effects of N. meningitidis on CD62E expression on vascular endothelium and provide a basis for the complex endothelial dysregulation seen in meningococcal sepsis.

Cross-reactive polyclonal antibodies to the inner core of lipopolysaccharide from Neisseria meningitidis

Sera from mice immunized with native or detergent-extracted outer membrane vesicles derived from lipopolysaccharide (LPS) mutant 44/76(Mu-4) of Neisseria meningitidis were analyzed for antibodies to LPS. The carbohydrate portion of 44/76(Mu-4) LPS consists of the complete inner core, Glc beta 1-->4[GlcNAc alpha 1-->2Hep alpha 1-->3]Hep alpha 1-->5KDO[4-->2 alpha KDO]. Immunoblot analysis revealed that some sera contained antibodies to wild-type LPS which has a fully extended carbohydrate chain of immunotype L3,7, as well as to the homologous LPS. Sera reacted only weakly to LPS from 44/76(Mu-3), which lacks the terminal glucose of the inner core. No binding to more truncated LPS was observed. Consequently, the cross-reactive epitopes are expressed mainly by the complete inner core. Dephosphorylation of wild-type LPS abolished antibody binding to LPS in all but one serum. Thus, at least two specificities of cross-reactive antibodies exist: one is dependent on phosphoethanolamine groups in LPS, and one is not. Detection of these cross-reactive antibodies strongly supports the notion that epitopes expressed by meningococcal LPS inner core are also accessible to antibodies when the carbohydrate chain is fully extended. Also, these inner core epitopes are sufficiently immunogenic to induce antibody levels detectable in polyclonal antibody responses. Meningococci can escape being killed by antibodies to LPS that bind only to a specific LPS variant, by altering the carbohydrate chain length. Cross-reactive antibodies may prevent such escape. Therefore, inner core LPS structures may be important antigens in future vaccines against meningococcal disease.

Quantification of lipopolysaccharides in outer membrane vesicle vaccines against meningococcal disease. High-performance liquid chromatographic determination of the constituent 3-hydroxy-lauric acid

A high-performance liquid chromatographic (HPLC) assay for quantification of lipopolysaccharides (LPSs, endotoxins) in outer membrane vesicle vaccines against meningococcal disease has been developed. The LPS constituent, 3-hydroxy-lauric acid, served as marker substance for the quantification. LPS from the vaccine was precipitated by ethanol and the fatty acid constituents, including 3-hydroxy-lauric acid, were released by acidic hydrolysis, collected and purified by solid phase extraction on C18 disc-cartridges and converted into phenacyl esters for UV detection at 240 nm. Quantification of the derivatized 3-hydroxy-lauric acid was achieved by HPLC using a Brownlee RP-18 reversed phase column with acetonitrile/water (68:32, v/v) as mobile phase. The method was found to be linear over the range 3-49 microg LPS/ml with a sensitivity of 1.6 (microg/ml)(-1). The repeatability (within-day precision) of the method at three levels (3-49 microg LPS/ml) was 6-14% relative standard deviation and the intermediate (between-day) precision was 7% relative standard deviation (at level 15 microg LPS/ml). The method has been successfully used in the quality control of a meningococcal B outer membrane vesicle vaccine, containing 4-8% LPS relative to protein (w/w), in our laboratory for three years.

Dendritic cell activation and cytokine production induced by group B Neisseria meningitidis: interleukin-12 production depends on lipopolysaccharide expression in intact bacteria

Interactions between dendritic cells (DCs) and microbial pathogens are fundamental to the generation of innate and adaptive immune responses. Upon stimulation with bacteria or bacterial components such as lipopolysaccharide (LPS), immature DCs undergo a maturation process that involves expression of costimulatory molecules, HLA molecules, and cytokines and chemokines, thus providing critical signals for lymphocyte development and differentiation. In this study, we investigated the response of in vitro-generated human DCs to a serogroup B strain of Neisseria meningitidis compared to an isogenic mutant lpxA strain totally deficient in LPS and purified LPS from the same strain. We show that the parent strain, lpxA mutant, and meningococcal LPS all induce DC maturation as measured by increased surface expression of costimulatory molecules and HLA class I and II molecules. Both the parent and lpxA strains induced production of tumor necrosis factor alpha (TNF-alpha), interleukin-1alpha (IL-1alpha), and IL-6 in DCs, although the parent was the more potent stimulus. In contrast, high-level IL-12 production was only seen with the parent strain. Compared to intact bacteria, purified LPS was a very poor inducer of IL-1alpha, IL-6, and TNF-alpha production and induced no detectable IL-12. Addition of exogenous LPS to the lpxA strain only partially restored cytokine production and did not restore IL-12 production. These data show that non-LPS components of N. meningitidis induce DC maturation, but that LPS in the context of the intact bacterium is required for high-level cytokine production, especially that of IL-12. These findings may be useful in assessing components of N. meningitidis as potential vaccine candidates.

Gram-negative bacteria induce proinflammatory cytokine production by monocytes in the absence of lipopolysaccharide (LPS)

Tumour necrosis factor-alpha (TNF-alpha), IL-1alpha and IL-6 production by human monocytes in response to a clinical strain of the Gram-negative encapsulated bacteria Neisseria meningitidis and an isogenic lpxA- strain deficient in LPS was investigated. Wild-type N. meningitidis at concentrations between 105 and 108 organisms/ml and purified LPS induced proinflammatory cytokine production. High levels of these cytokines were also produced in response to the lpxA- strain at 107 and 108 organisms/ml. The specific LPS antagonist bactericidal/permeability-increasing protein (rBPI21) inhibited cytokine production induced by LPS and wild-type bacteria at 105 organisms/ml but not at higher concentrations, and not by LPS-deficient bacteria at any concentration. These data show that proinflammatory cytokine production by monocytes in response to N. meningitidis does not require the presence of LPS. Therapeutic strategies designed to block LPS alone may not therefore be sufficient for interrupting the inflammatory response in severe meningococcal disease.

Endothelial adhesion molecule expression and its inhibition by recombinant bactericidal/permeability-increasing protein are influenced by the capsulation and lipooligosaccharide structure of Neisseria meningitidis

Vascular endothelial injury is responsible for many of the clinical manifestations of severe meningococcal disease. Binding and migration of activated host inflammatory cells is a central process in vascular damage. The expression and function of adhesion molecules regulate interactions between leukocytes and endothelial cells. Little is known about how meningococci directly influence these receptors. In this study we have explored the effect of Neisseria meningitidis on endothelial adhesion molecule expression and found this organism to be a potent inducer of the adhesion molecules CD62E, ICAM-1, and VCAM-1. Exposure of endothelium to a serogroup B strain of Neisseria meningitidis, B1940, and a range of isogenic mutants revealed that lipooligosaccharide (LOS) structure and capsulation influence the expression of adhesion molecules. Following only a brief exposure (15 min) to the bacteria, there were large differences in the capacity of the different mutants to induce vascular cell adhesion molecules, with the unencapsulated and truncated LOS strains being most potent (P < 0.05). Furthermore, the pattern of cell adhesion molecule expression was different with purified endotoxin from that with intact bacteria. Meningococci were more potent stimuli of CD62E expression than was endotoxin, whereas endotoxin was at least as effective as meningococci in inducing ICAM-1 and VCAM-1. The effect of bactericidal/permeability increasing protein (rBPI(21)), an antibacterial molecule with antiendotoxin properties, was also dependent on LOS structure. The strains which possessed a truncated or nonsialylated LOS, whether capsulated or not, were more sensitive to the inhibitory effects of rBPI(21). These findings could have important implications for the use of antiendotoxin therapy in meningococcal disease.

Lipopolysaccharide heterogeneity and escape mechanisms of Neisseria meningitidis: possible consequences for vaccine development

We wanted to compare the potential protective capacity of antibodies to meningococcal lipopolysaccharides (LPS). The frequency of occurrence and degree of expression of the epitopes recognized by murine monoclonal antibodies (MAbs) to immunotypes L3,7,9 (9-2-L379) and L8 (2-1-L8) and to the LPS inner core (216-Lc and 217-Lc), were determined among 77 consecutive Norwegian meningococcal patient isolates from 1995. The immunotype L3,7,9 was strongly expressed by 95% of the isolates, whereas L8 was weakly to moderately expressed by 9%. The inner core epitopes, were widely distributed among the serogroup B organisms, but were proved weakly expressed. The bactericidal activity of the four MAbs to various selected strains, was found to correlate positively with the quantity of the LPS epitopes recognized by these four MAbs in the bacteria. When tested in the serum bactericidal assay (SBA), often a few percent of the colonies of the inocula survived high concentrations of the MAbs. The results indicate that escape from the bactericidal action could be achieved through: (i) selection of variants not expressing the LPS-epitope of the actual MAb, (ii) a relative reduction in the density of the LPS-epitope achieved by dilution with another LPS structure or (iii) other factors, not yet understood. In conclusion, antibodies to the L3,7,9 epitope seem to be of importance for protection, whereas antibodies to the epitopes of the LPS inner core or immunotype L8, are not likely to offer protection alone. However, in order to prevent escape through alteration of the LPS pattern of the microbes, various LPS structures should probably be present in the OMV vaccine.

Detection of the phosphorylcholine epitope in streptococci, Haemophilus and pathogenic Neisseriae by immunoblotting

The phosphorylcholine (PC) determinant in Streptococcus pneumoniae is known to be linked to the cell wall polysaccharides (C-Ps) and to the lipoteichoic acid (LTA) (Forssman antigen) of the plasma membrane. Western blotting with two PC specific murine monoclonal antibodies (MAbs) designated 145,F-2 (IgM) and 147,A-1 (IgA) showed a similar ladder-like pattern for all examined strains of S. pneumoniae and Streptococcus mitis. Purified antigens run in parallel indicated that this ladder pattern is due to the PC of LTA. Unlike other techniques, Western blotting thus enables the identification of only one of the streptococcal structures carrying the PC epitope. Gram-negative organisms were also examined, and six of 11 Haemophilus influenzae strains reacted with the MAbs. For this species, unlike the streptococci, only one fast moving band was detected. Analyses by thin-layer chromatography (TLC) detected the PC epitope in lipopolysaccharide (LPS) fraction from H. influenzae. Some strains of the Neisseriaceae family were also positive by Western blotting, but TLC and immunostaining did not detect the PC determinant in LPS.