Abstract
Despite the considerable progress toward the eradication of meningococcal disease with the introduction of glycoconjugate vaccines, previously unremarkable serogroup X has emerged in recent years, recording several outbreaks throughout the African continent. Different serogroup X polysaccharide-based vaccines have been tested in preclinical trials, establishing the principles for further improvement. To elucidate the antigenic determinants of the MenX capsular polysaccharide, we generated a monoclonal antibody, and its bactericidal nature was confirmed using the rabbit serum bactericidal assay. The antibody was tested by the inhibition enzyme-linked immunosorbent assay and surface plasmon resonance against a set of oligosaccharide fragments of different lengths. The epitope was shown to be contained within five to six α-(1–4) phosphodiester mannosamine repeating units. The molecular interactions between the protective monoclonal antibody and the MenX capsular polysaccharide fragment were further detailed at the atomic level by saturation transfer difference nuclear magnetic resonance (NMR) spectroscopy. The NMR results were used for validation of the in silico docking analysis between the X-ray crystal structure of the antibody (Fab fragment) and the modeled hexamer oligosaccharide. The antibody recognizes the MenX fragment by binding all six repeating units of the oligosaccharide via hydrogen bonding, salt bridges, and hydrophobic interactions. In vivo studies demonstrated that conjugates containing five to six repeating units can produce high functional antibody levels. These results provide an insight into the molecular basis of MenX vaccine-induced protection and highlight the requirements for the epitope-based vaccine design.
Highlights
Neisseria meningitidis (Men) is a Gram-negative encapsulated diplococcus, capable of producing meningitis and sepsis in humans (Morelli et al, 2014; Fiebig et al, 2016; Ji et al, 2017)
Cross-Reacting Material 197 (CRM197) was selected as it is present in the Menveo vaccine and has been shown to provide strong immunogenicity to MenX capsular polysaccharide (CPS) (Micoli et al, 2013; Oldrini et al, 2018)
The specificity of MenX.01 monoclonal antibody (mAb) was further confirmed by immunostaining of the MenX-CRM197 conjugate and CRM197 conjugated to group B Streptococcus GBSII as the control (Figure 1C), where the glycoconjugate MenX-CRM197, showed its typical band on SDS PAGE
Summary
Neisseria meningitidis (Men) is a Gram-negative encapsulated diplococcus, capable of producing meningitis and sepsis in humans (Morelli et al, 2014; Fiebig et al, 2016; Ji et al, 2017). Men CPS has been covalently linked to immunogenic protein carriers, such as the chemically detofixied diphtheria or Tetanus Toxins (DT and TT, respectively) and the nontoxic mutant of the diphtheria toxin, Cross-Reacting Material 197 (CRM197), to form glycoconjugates (Morelli et al, 2014). Men glycoconjugate-based vaccines, such as Menactra, Menveo, Nimenrix, and more recently MenQuadfi (targeting MenA, C, Y, W) (Berti et al, 2021), have overcome most of the limitations of using plain Men CPS, that is, the lack of memory response, IgM-to-IgG maturation, and ineffectiveness in children below 2 years of age (Muindi et al, 2014; Fallarini et al, 2015; Harale et al, 2015; Pecetta et al, 2016; Hlozek et al, 2018). A MenA-TT conjugate, MenAfriVac, has been introduced in the so-called meningitis belt, leading to almost eradication of the disease (Trotter et al, 2017)
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