Importance of antibodies to lipopolysaccharide in natural and vaccine-induced serum bactericidal activity against Neisseria meningitidis group B

Neisseria gonorrhoeae lipooligosaccharide glycan epitopes recognized by bactericidal IgG antibodies elicited by the meningococcal group B-directed vaccine, MenB-4C

Introduction

Outer membrane vesicles (OMVs) of Neisseria meningitidis in the group B-directed vaccine MenB-4C (BexseroR) protect against infections with Neisseria gonorrhoeae. The immunological basis for protection remains unclear. N. meningitidis OMV vaccines generate human antibodies to N. meningitidis and N. gonorrhoeae lipooligosaccharide (LOS/endotoxin), but the structural specificity of these LOS antibodies is not defined.

Methods

Ten paired human sera obtained pre- and post-MenB-4C immunization were used in Western blots to probe N. meningitidis and N. gonorrhoeae LOS. Post-MenB-4C sera (7v5, 19v5, and 17v5), representing individual human variability in LOS recognition, were then used to interrogate structurally defined LOSs of N. meningitidis and N. gonorrhoeae strains and mutants and studied in bactericidal assays.

Results and discussion

Post-MenB-4C sera recognized both N. meningitidis and N. gonorrhoeae LOS species, ~10% of total IgG to gonococcal OMV antigens. N. meningitidis and N. gonorrhoeae LOSs were broadly recognized by post-IgG antibodies, but with individual variability for LOS structures. Deep truncation of LOS, specifically a rfaK mutant without α-, β-, or γ-chain glycosylation, eliminated LOS recognition by all post-vaccine sera. Serum 7v5 IgG antibodies recognized the unsialyated L1 α-chain, and a 3-PEA-HepII or 6-PEA-HepII was part of the conformational epitope. Replacing the 3-PEA on HepII with a 3-Glc blocked 7v5 IgG antibody recognition of N. meningitidis and N. gonorrhoeae LOSs. Serum 19v5 recognized lactoneotetrose (LNT) or L1 LOS-expressing N. meningitidis or N. gonorrhoeae with a minimal α-chain structure of Gal-Glc-HepI (L8), a 3-PEA-HepII or 6-PEA-HepII was again part of the conformational epitope and a 3-Glc-HepII blocked 19v5 antibody binding. Serum 17v5 LOS antibodies recognized LNT or L1 α-chains with a minimal HepI structure of three sugars and no requirement for HepII modifications. These LOS antibodies contributed to the serum bactericidal activity against N. gonorrhoeae. The MenB-4C vaccination elicits bactericidal IgG antibodies to N. gonorrhoeae conformational epitopes involving HepI and HepII glycosylated LOS structures shared between N. meningitidis and N. gonorrhoeae. LOS structures should be considered in next-generation gonococcal vaccine design.

Recent advances in lipopolysaccharide-based glycoconjugate vaccines

INTRODUCTION

The public health burden caused by pathogenic Gram-negative bacteria is increasingly prominent due to antimicrobial resistance. The surface carbohydrates are potential antigens for vaccines against Gram-negative bacteria. The enhanced immunogenicity of the O-specific polysaccharide (O-SP) moiety of LPS when coupled to a carrier protein may protect against bacterial pathogens. However, because of the toxic lipid A moiety and relatively high costs of O-SP isolation, LPS has not been a popular vaccine antigen until recently.

AREAS COVERED

In this review, we discuss the rationales for developing LPS-based glycoconjugate vaccines, principles of glycoconjugate-induced immunity, and highlight the recent developments and challenges faced by LPS-based glycoconjugate vaccines.

EXPERT OPINION

Advances in LPS harvesting, LPS chemical synthesis, and newer carrier proteins in the past decade have propelled LPS-based glycoconjugate vaccines toward further development, through to clinical evaluation. The development of LPS-based glycoconjugates offers a new horizon for vaccine prevention of Gram-negative bacterial infection.

Biochemical and Structural Investigation of GnnA in the Lipopolysaccharide Biosynthesis Pathway of Acidithiobacillus ferrooxidans

Lipopolysaccharide (LPS) is a crucial component in the outer membrane of Gram-negative bacteria that contributes to both pathogenicity as well as immunity against pathogenic bacteria. Typical LPS contains GlcN disaccharide as the core of lipid A. However, some bacteria such as Acidithiobacillus ferrooxidans and Leptospira interrogans contain GlcN3N in lipid A instead. This modification has been shown to dampen the host immune response and increase resistance to antimicrobial peptides. Therefore, investigation of the enzymes responsible for the biosynthesis of GlcN3N has promising applications in the development of vaccines, antibiotics, or usage of the enzymes in chemoenzymatic synthesis of modified LPS. Here, we describe biochemical and structural investigation of GnnA from A. ferrooxidans (AfGnnA) that is responsible for oxidation of UDP-GlcNAc, which subsequently undergoes transamination to produce UDP-GlcNAc3N as a precursor for LPS biosynthesis. AfGnnA is specific for NAD⁺ and UDP-GlcNAc. The crystal structures of AfGnnA in combination with molecular dynamics simulation and mutational analysis suggest the substrate recognition mode and the catalytic mechanism. K91 or H164 is a potential catalytic base in the oxidation reaction. The results will not only provide insights into the biosynthesis of unusual LPS but will also lay the foundation for development of more immunogenic vaccines, novel antibiotics, or utilization of GnnA in the synthesis of UDP-sugars or modified LPS.

Mechanistic research holds promise for bacterial vaccines and phage therapies for Pseudomonas aeruginosa

Vaccines for Pseudomonas aeruginosa have been of longstanding interest to immunologists, bacteriologists, and clinicians, due to the widespread prevalence of hospital-acquired infection. As P. aeruginosa becomes increasingly antibiotic resistant, there is a dire need for novel treatments and preventive vaccines. Despite intense efforts, there currently remains no vaccine on the market to combat this dangerous pathogen. This article summarizes current and past vaccines under development that target various constituents of P. aeruginosa. Targeting lipopolysaccharides and O-antigens have shown some promise in preventing infection. Recombinant flagella and pili that target TLR5 have been utilized to combat P. aeruginosa by blocking its motility and adhesion. The type 3 secretion system components, such as needle-like structure PcrV or exotoxin PopB, are also potential vaccine targets. Outer membrane proteins including OprF and OprI are newer representatives of vaccine candidates. Live attenuated vaccines are a focal point in this review, and are also considered for novel vaccines. In addition, phage therapy is revived as an effective option for treating refractory infections after failure with antibiotic treatment. Many of the aforementioned vaccines act on a single target, thus lacking a broad range of protection. Recent studies have shown that mixtures of vaccines and combination approaches may significantly augment immunogenicity, thereby increasing their preventive and therapeutic potential.

Effect of fresh frozen plasma on the in vitro activation of U937 monocytes: a potential role for the age of blood donors and their underlying cytokine profile

BACKGROUND

Fresh frozen plasma (FFP) administration may increase the risk of nosocomial infections in parallel with the development of immune modulation. This could be driven by soluble mediators, possibly influencing the in vitro activation of human U937 monocyte cells, in a manner dependent on the age of the donors.

METHODS

FFP donors were stratified into groups of 19-30 years, 31-40 years or 41-50 years, and U937 cells were cultured with FFP (alone or plus lipopolysaccharide-LPS) for 24 h. Both in FFP and supernatants, TNF, IL-1β, IL-6, and IL-10 levels were measured by ELISA. Additionally, CD11B, TLR2, and CASP3 gene expression were measured by qtPCR in U937 cells. Total phagocytic activity was also assayed.

RESULTS

Elevated IL-10, but low TNF and IL-1β levels were measured in FFP from individuals aged 19-40 years, whereas in individuals aged 41-50 years FFP were characterized by equalized TNF and IL-10 levels. Elevated IL-6 levels were found in all FFP samples, especially in those from the oldest individuals. FFP stimulation was associated with striking modifications in cytokine production in an age-dependent way. Exposure to FFP attenuates the response to LPS. TLR2 and CD11B expression were enhanced regardless of the age of plasma donors, although CASP3 expression was increased only when FFP from individuals aged 19-40 years were tested. Phagocytosis decreased after exposure to FFP regardless of donor age.

CONCLUSION

Our results suggest that soluble mediators in FFP may modulate the functioning of monocytes. Interestingly, this effect appears to be partially influenced by the age of donors.

The role of anti-NHba antibody in bactericidal activity elicited by the meningococcal serogroup B vaccine, MenB-4C

BACKGROUND

MenB-4C (Bexsero®) is a multicomponent serogroup B meningococcal vaccine. For vaccine licensure, efficacy was inferred from serum bactericidal antibody (SBA) against three antigen-specific indicator strains. The bactericidal role of antibody to the fourth vaccine antigen, Neisserial Heparin binding antigen (NHba), is incompletely understood.

METHODS

We identified nine adults immunized with two or three doses of MenB-4C who had sufficient volumes of sera and >3-fold increases in SBA titer against a strain with high NHba expression, which was mismatched with the other three MenB-4C antigens that elicit SBA. Using 1month-post-immunization sera we measured the effect of depletion of anti-NHba and/or anti-Factor H binding protein (FHbp) antibodies on SBA.

RESULTS

Against three strains matched with the vaccine only for NHba, depletion of anti-NHba decreased SBA titers by an average of 43-79% compared to mock-adsorbed sera (P<0.05). Despite expression of sub-family A FHbp (mismatched with the sub-family B vaccine antigen), depletion of anti-FHbp antibodies also decreased SBA by 45-64% (P<0.05). Depletion of both antibodies decreased SBA by 84-100%. Against a strain with sub-family B FHbp and expression of NHba with 100% identity to the vaccine antigen, depletion of anti-NHba decreased SBA by an average of 26%, compared to mock-adsorbed sera (P<0.0001), and depletion of anti-FHbp antibody decreased SBA by 92% (P<0.0001).

CONCLUSIONS

Anti-NHba antibody can contribute to SBA elicited by MenB-4C, particularly in concert with anti-FHbp antibody. However, some high NHba-expressing strains are resistant, even with an exact match between the amino acid sequence of the vaccine and strain antigens.

Recent Progress in the Prevention of Serogroup B Meningococcal Disease

The widespread use of meningococcal polysaccharide conjugate vaccines has highlighted the challenge of providing protection against serogroup B disease. Over a period of 4 decades, vaccine development has focused on subcapsular protein antigens, first with outer membrane vesicle (OMV) vaccines against epidemic outbreaks, and more recently on new multicomponent vaccines designed to offer better cross-protection against the antigenically diverse strains responsible for endemic disease. Because of the low incidence of meningococcal disease, the protective efficacy of these vaccines has not been determined in clinical studies, and their licensure has been based on serological data; however, the serological assays used to predict protective coverage have limitations. As a result, evidence of the effectiveness of these vaccines against different strains and the contribution of specific antigens to protection can only be provided by epidemiological analyses following their implementation in sufficiently large populations. The recent inclusion of the four-component meningococcal serogroup B (4CMenB) vaccine, Bexsero, in the infant immunization program in the UK has provided preliminary evidence that the vaccine is effective. Ongoing surveillance will provide valuable data on its longer-term impact and antigenic coverage. Further development of protein-based vaccines against meningococcal disease is anticipated to improve antigenic coverage and adjust to changes in circulating strains. At the same time, alternative immunization strategies may be explored to improve overall vaccine effectiveness by, for example, protecting the youngest infants or providing herd protection.

Opc expression, LPS immunotype switch and pilin conversion contribute to serum resistance of unencapsulated meningococci

Neisseria meningitidis employs polysaccharides and outer membrane proteins to cope with human serum complement attack. To screen for factors influencing serum resistance, an assay was developed based on a colorimetric serum bactericidal assay. The screening used a genetically modified sequence type (ST)-41/44 clonal complex (cc) strain lacking LPS sialylation, polysaccharide capsule, the factor H binding protein (fHbp) and MutS, a protein of the DNA repair mechanism. After killing of >99.9% of the bacterial cells by serum treatment, the colorimetric assay was used to screen 1000 colonies, of which 35 showed enhanced serum resistance. Three mutant classes were identified. In the first class of mutants, enhanced expression of Opc was identified. Opc expression was associated with vitronectin binding and reduced membrane attack complex deposition confirming recent observations. Lipopolysaccharide (LPS) immunotype switch from immunotype L3 to L8/L1 by lgtA and lgtC phase variation represented the second class. Isogenic mutant analysis demonstrated that in ST-41/44 cc strains the L8/L1 immunotype was more serum resistant than the L3 immunotype. Consecutive analysis revealed that the immunotypes L8 and L1 were frequently observed in ST-41/44 cc isolates from both carriage and disease. Immunotype switch to L8/L1 is therefore suggested to contribute to the adaptive capacity of this meningococcal lineage. The third mutant class displayed a pilE allelic exchange associated with enhanced autoaggregation. The mutation of the C terminal hypervariable region D of PilE included a residue previously associated with increased pilus bundle formation. We suggest that autoaggregation reduced the surface area accessible to serum complement and protected from killing. The study highlights the ability of meningococci to adapt to environmental stress by phase variation and intrachromosomal recombination affecting subcapsular antigens.

Improvement of immunogenicity of meningococcal lipooligosaccharide by coformulation with lipidated transferrin-binding protein B in liposomes: implications for vaccine development

Among various meningococcal antigens, lipooligosaccharide (LOS) and recombinant lipidated transferrin-binding protein B (rlip-TbpB) are considered to be putative vaccine candidates against group B Neisseria meningitidis. In the present work, we report the development of a new liposome-based vaccine formulation containing both rlip-TbpB and L8 LOS. The endotoxic activity of the liposomal LOS was evaluated in vitro using the Limulus Amebocyte Lysate assay and compared to the endotoxic activity of free LOS. Above a 250:1 lipid/LOS molar ratio, liposomes were shown to effectively detoxify the LOS as the endotoxic activity of the LOS was reduced by more than 99%. Immunogenicity studies in rabbits showed that the presence of rlip-TbpB dramatically increased the immunogenicity of the LOS. While the formulation raised a strong anti-TbpB response, it elicited a higher anti-LOS IgG level than the liposomal LOS alone. Sera from rabbits immunized with rlip-TbpB/liposomal LOS displayed increased ability to recognize LOS on live bacteria expressing the L8 immunotype and increased anti-LOS-specific bactericidal activity compared to sera from rabbits immunized with liposomal LOS alone. Measurement of interleukin-8 (IL-8) produced by HEK293 cells transfected with Toll-like receptor (TLR) after stimulation with rlip-TbpB showed that the protein is a TLR2 agonist, which is in accordance with the structure of its lipid. Furthermore, an in vivo study demonstrated that the lipid moiety is not only required for its adjuvant effect but also has to be linked to the protein. Overall, the rlip-TbpB/LOS liposomal formulation was demonstrated to induce an effective anti-LOS response due to the adjuvant effect of rlip-TbpB on LOS.

Phase I study of a Neisseria meningitidis liposomal vaccine containing purified outer membrane proteins and detoxified lipooligosaccharide

Purified outer membrane proteins and purified deacylated lipooligosaccharide (dLOS) were formulated for use as a vaccine in three formulations for clinical use. The three vaccine formulations included (1) purified outer membrane proteins (OMPs) and L8-5 dLOS adsorbed to aluminum hydroxide; (2) purified OMPs and L8-5 dLOS incorporated into liposomes; and (3) purified OMPs and L7 dLOS incorporated into proteoliposomes. The vaccines were compared for immunogenicity and safety in a phase 1clinical study. Ten adult volunteers were vaccinated with each of the three vaccine formulations. Two 50 μg doses were given six weeks apart, and serum samples were obtained at 0, 2, 6, 8 and 14 weeks. Volunteers were evaluated for reactogenicity 30 min after vaccination and at days 1, 2, and 14 after each vaccination, and laboratory safety tests were done at 0, 2 and 6 weeks. Overall, the vaccines were well tolerated. Bactericidal assays against a homologous strain showed a four-fold or greater increase in titer in 6 of 7 volunteers in group one, 9 of 10 volunteers in group two, and 5 of 10 volunteers in group three. A quantitative enzyme linked immunosorbant assay showed increases in antibody against both OMPs and LOS antigens. The liposome formulation appeared to be particularly effective in presenting the dLOS as an antigen.