Detection of the lst gene in different serogroups and LOS immunotypes of Neisseria meningitidis

Analysis of Bacterial Lipooligosaccharides by MALDI-TOF MS with Traveling Wave Ion Mobility

Lipooligosaccharides (LOS) are major microbial virulence factors displayed on the outer membrane of rough-type Gram-negative bacteria. These amphipathic glycolipids are comprised of two domains, a core oligosaccharide linked to a lipid A moiety. Isolated LOS samples are generally heterogeneous mixtures of glycoforms, with structural variability in both domains. Traditionally, the oligosaccharide and lipid A components of LOS have been analyzed separately following mild acid hydrolysis, although important acid-labile moieties can be cleaved. Recently, an improved method was introduced for analysis of intact LOS by MALDI-TOF MS using a thin layer matrix composed of 2,4,6-trihydroxyacetophenone (THAP) and nitrocellulose. In addition to molecular ions, the spectra show in-source "prompt" fragments arising from regiospecific cleavage between the lipid A and oligosaccharide domains. Here, we demonstrate the use of traveling wave ion mobility spectrometry (TWIMS) for IMS-MS and IMS-MS/MS analyses of intact LOS from Neisseria spp. ionized by MALDI. Using IMS, the singly charged prompt fragments for the oligosaccharide and lipid A domains of LOS were readily separated into resolved ion plumes, permitting the extraction of specific subspectra, which led to increased confidence in assigning compositions and improved detection of less abundant ions. Moreover, IMS separation of precursor ions prior to collision-induced dissociation (CID) generated time-aligned, clean MS/MS spectra devoid of fragments from interfering species. Incorporating IMS into the profiling of intact LOS by MALDI-TOF MS exploits the unique domain structure of the molecule and offers a new means of extracting more detailed information from the analysis. Graphical Abstract ᅟ.

How the Knowledge of Interactions between Meningococcus and the Human Immune System Has Been Used to Prepare Effective Neisseria meningitidis Vaccines

In the last decades, tremendous advancement in dissecting the mechanisms of pathogenicity of Neisseria meningitidis at a molecular level has been achieved, exploiting converging approaches of different disciplines, ranging from pathology to microbiology, immunology, and omics sciences (such as genomics and proteomics). Here, we review the molecular biology of the infectious agent and, in particular, its interactions with the immune system, focusing on both the innate and the adaptive responses. Meningococci exploit different mechanisms and complex machineries in order to subvert the immune system and to avoid being killed. Capsular polysaccharide and lipooligosaccharide glycan composition, in particular, play a major role in circumventing immune response. The understanding of these mechanisms has opened new horizons in the field of vaccinology. Nowadays different licensed meningococcal vaccines are available and used: conjugate meningococcal C vaccines, tetravalent conjugate vaccines, an affordable conjugate vaccine against the N. menigitidis serogroup A, and universal vaccines based on multiple antigens each one with a different and peculiar function against meningococcal group B strains.

Mutant Native Outer Membrane Vesicles Combined with a Serogroup A Polysaccharide Conjugate Vaccine for Prevention of Meningococcal Epidemics in Africa

Background

The meningococcal serogroup A (MenA) polysaccharide conjugate vaccine used in Sub-Saharan Africa does not prevent disease caused by MenW or MenX strains, which also cause epidemics in the region. We investigated the vaccine-potential of native outer membrane vesicles with over-expressed factor H-binding protein (NOMV-fHbp), which targeted antigens in African meningococcal strains, and was combined with a MenA polysaccharide conjugate vaccine.

Methodology/Principal Findings

The NOMV-fHbp vaccine was prepared from a mutant African MenW strain with PorA P1.5,2, attenuated endotoxin (ΔLpxL1), deleted capsular genes, and over-expressed fHbp in variant group 1. The NOMV-fHbp was adsorbed with Al(OH)₃ and used to reconstitute a lyophilized MenA conjugate vaccine, which normally is reconstituted with liquid MenC, Y and W conjugates in a meningococcal quadrivalent conjugate vaccine (MCV4-CRM, Novartis). Mice immunized with the NOMV-fHbp vaccine alone developed serum bactericidal (human complement) activity against 13 of 15 African MenA strains tested; 10 of 10 African MenX strains, 7 of 7 African MenW strains, and 6 of 6 genetically diverse MenB strains with fHbp variant group 1 (including 1 strain from The Gambia). The combination NOMV-fHbp/MenA conjugate vaccine elicited high serum bactericidal titers against the two MenA strains tested that were resistant to bactericidal antibodies elicited by the NOMV-fHbp alone; the combination elicited higher titers against the MenA and MenW strains than those elicited by a control MCV4-CRM vaccine (P<0.05); and high titers against MenX and MenB strains. For most strains, the titers elicited by a control NOMV-fHbp knock out vaccine were <1∶10 except when the strain PorA matched the vaccine (titers >1∶000).

Conclusion/Significance

The NOMV-fHbp/MenA conjugate vaccine provided similar or higher coverage against MenA and MenW strains than a quadrivalent meningococcal conjugate vaccine, and extended protection against MenX strains responsible for epidemics in Africa, and MenB strains with fHbp in variant group 1.

fH-dependent complement evasion by disease-causing meningococcal strains with absent fHbp genes or frameshift mutations

Meningococci bind human fH to down-regulate complement, which enhances survival of the bacteria in serum. A major fH ligand is the vaccine candidate, factor H-binding protein (fHbp). Although fHbp has been considered an essential meningococcal virulence factor, rarely, invasive isolates with absent fHbp genes or frameshift mutations have been identified. In previous studies fH binding to these isolates was not detected. The aim of the present study was to investigate fH binding and complement evasion by invasive meningococcal serogroup B clinical isolates with absent fHbp genes or frameshift mutations. Four of the seven isolates tested bound human fH by flow cytometry and survived in IgG-depleted human serum. In all four, fH binding was decreased after inactivating the gene encoding NspA. Binding of fH to fHbp and NspA is specific for human fH. To investigate fH-dependent evasion of host defenses, human fH transgenic infant rats, or control littermates negative for human fH, were challenged IP with 10(3)-10(4)CFU of two of the isolates with no detectable fH binding by flow cytometry. At 6h, bacteremia caused by both strains was higher in human fH transgenic rats than in control rats (P<0.002). In conclusion, six of the seven isolates had evidence of fH binding and/or human fH-dependent complement evasion in transgenic rats. In four, NspA was as an alternative fH ligand. fHbp vaccination may select for mutants that do not require fHbp for complement evasion. Inclusion of additional target antigens in vaccines containing fHbp may delay emergence of these mutants.

The meningococcal vaccine candidate neisserial surface protein A (NspA) binds to factor H and enhances meningococcal resistance to complement

Complement forms an important arm of innate immunity against invasive meningococcal infections. Binding of the alternative complement pathway inhibitor factor H (fH) to fH-binding protein (fHbp) is one mechanism meningococci employ to limit complement activation on the bacterial surface. fHbp is a leading vaccine candidate against group B Neisseria meningitidis. Novel mechanisms that meningococci employ to bind fH could undermine the efficacy of fHbp-based vaccines. We observed that fHbp deletion mutants of some meningococcal strains showed residual fH binding suggesting the presence of a second receptor for fH. Ligand overlay immunoblotting using membrane fractions from one such strain showed that fH bound to a ∼17 kD protein, identified by MALDI-TOF analysis as Neisserial surface protein A (NspA), a meningococcal vaccine candidate whose function has not been defined. Deleting nspA, in the background of fHbp deletion mutants, abrogated fH binding and mAbs against NspA blocked fH binding, confirming NspA as a fH binding molecule on intact bacteria. NspA expression levels vary among strains and expression correlated with the level of fH binding; over-expressing NspA enhanced fH binding to bacteria. Progressive truncation of the heptose (Hep) I chain of lipooligosaccharide (LOS), or sialylation of lacto-N-neotetraose LOS both increased fH binding to NspA-expressing meningococci, while expression of capsule reduced fH binding to the strains tested. Similar to fHbp, binding of NspA to fH was human-specific and occurred through fH domains 6–7. Consistent with its ability to bind fH, deleting NspA increased C3 deposition and resulted in increased complement-dependent killing. Collectively, these data identify a key complement evasion mechanism with important implications for ongoing efforts to develop meningococcal vaccines that employ fHbp as one of its components.

Neisseria meningitidis is an important cause of bacterial meningitis and sepsis worldwide. The complement system is a family of proteins that is critical for innate immune defenses against this pathogen. In order to successfully colonize humans and cause disease, the meningococcus must escape killing by the complement system. In this study we show that meningococci can use one of its surface proteins called Neisserial surface protein A (NspA) to bind to a host complement inhibitory protein called factor H (fH). NspA is a protein vaccine candidate against group B meningococcal disease. Binding of fH limits complement activation on the bacterial surface and enhances the ability of the meningococcus to resist complement-dependent killing. Capsular polysaccharide expression decreases fH binding to NspA, while truncation of the core glycan chain of lipooligosaccharide increases fH binding to meningococcal NspA. Loss of NspA results in enhanced complement activation on the bacterial surface and increased complement-dependent killing of meningococci. Our findings have disclosed a novel function for NspA and sheds further light on how this pathogen evades killing by the complement system.

The structure of the L9 immunotype lipooligosaccharide from Neisseria meningitidis NMA Z2491

The lipooligosaccharide (LOS) from the Neisseria meningitidis prototype serogroup A strain NMA Z2491, an L9 immunotype LOS, was isolated and structurally characterized using glycosyl composition and linkage determination, mass spectrometry, and both 1- and 2-D nuclear resonance spectroscopy. The results show that the L9 LOS has an identical structure to that of an L4 LOS structure with the exception that it does not contain a sialic acid residue linked to position 3 of the lactoneotetraose terminal galactosyl residue. Further, two oligosaccharides are present in the Z2491 LOS preparation, OS1 and OS2. They differ from one another only in that OS2 contains an added glycine moiety, presumably at O-7 on the inner core Hep II residue. The structures of these oligosaccharides are as follows: where R=H or Gly.

Identification of opcA gene in Neisseria polysaccharea: interspecies diversity of Opc protein family

The gene encoding the outer membrane adhesin/invasin protein OpcA was previously described in the genomes of two pathogenic Neisseria species, N. meningitidis (Nm) and N. gonorrhoeae (Ng). In order to understand the presence or absence of opcA in nonpathogenic Neisseria species, 13 strains of N. polysaccharea (Np), four strains of N. lactamica, three strains of N. subflava and nine strains of other species were examined by DNA hybridization, polymerase chain reaction (PCR) and nucleotide sequencing. The opcA gene was found in two Np strains (85322 and 89357). The Np-opcA gene is a novel member of this gene family with 93% homology to Ng-opcA. Comparison of opcA-surrounding regions among eight Neisseria strains revealed five types of genetic organization at the opcA locus in Neisseria, which result from insertion or deletion of genetic elements at the upstream region of opcA. Comparison of the deduced peptide sequences from two Np strains, two representative Ng strains, two representative Nm strains and 13 Nm sequence variants demonstrates interspecies diversity of the OpcA protein family with conserved transmembrane regions and species-specific polymorphism at the surface-exposed loops and periplasmic turns. Reverse transcription-PCR analysis and Northern blotting showed that Np-opcA was transcribable. From an alignment of the Np-OpcA and Ng-OpcA sequences against the three-dimensional crystal structure of Nm-OpcA we conclude that there is no obvious structural reason why these proteins would not be able to form stable, folded, outer membrane proteins. The data presented here provide additional information for understanding the distribution, variation and expression of opcA in Neisseria.