Glycoconjugate content quantification to assess vaccine potency: A simplified approach.

Several glycoconjugate vaccines have been licensed or are currently in clinical development to prevent bacterial infections. Here we report the development of a single analytical assay to quantify the conjugated saccharide content, as alternative to two separated total and free (unconjugated) saccharide assays used so far, for a quadrivalent conjugate vaccine containing meningococcal serogroup A polysaccharide (α-1,6-linked N-acetylmannosamine phosphate repeating unit partly O-acetylated at position C3 or C4) coupled with CRM197 protein. The results confirm a high linear correlation among the two approaches (conjugated saccharide content vs. difference of total saccharide and free saccharide). Conjugated saccharide content estimation is therefore demonstrated to be a suitable method to monitor the product quality of vaccines containing meningococcal serogroup A conjugate antigen, in the final filled presentation as demonstrated here and potentially on the bulk conjugate before formulation.

Group B Streptococcus chimeric capsular polysaccharides as novel multivalent vaccine candidates.

The capsular polysaccharide of the human pathogen Group B Streptococcus is a key virulence factor and vaccine candidate that induces protective antibodies when conjugated to carrier proteins. It consists of long polymeric chains of oligosaccharide repeating units, and each of the ten capsular serotypes described so far presents a unique chemical structure with distinct antigenic properties; therefore, broad protection against this pathogen could be achieved by a combination of ten glycoconjugates. Capsular polysaccharide biosynthesis and assembly follow a polymerase-dependent pathway that is widespread in encapsulated bacteria and is encoded by a polycistronic operon. Here we exploited the sequence similarity between the capsule operons of types V and IX to generate hybrid polysaccharides incorporating epitopes of both serotypes in a single molecule, by co-expressing their specific CpsM, O, I glycosyltransferases in a single isolate. Physicochemical and immunochemical methods confirmed that an engineered strain produced a high molecular weight chimeric polysaccharide, combining antigenic specificities of both type V and IX. By optimizing the copy number of key glycosyltransferase genes, we were able to modulate the ratio between type-specific epitopes. Finally, vaccination with chimeric glycoconjugates significantly decreased the incidence of disease in pups born from immunized mice challenged with either serotype. This study provides proof of concept for a new generation of glycoconjugate vaccines that combine the antigenic specificity of different polysaccharide variants in a single molecule, eliciting a protective immune response against multiple serotype variants.

Structure of a protective epitope of group B Streptococcus type III capsular polysaccharide.

Despite substantial progress in the prevention of group B Streptococcus (GBS) disease with the introduction of intrapartum antibiotic prophylaxis, this pathogen remains a leading cause of neonatal infection. Capsular polysaccharide conjugate vaccines have been tested in phase I/II clinical studies, showing promise for further development. Mapping of epitopes recognized by protective antibodies is crucial for understanding the mechanism of action of vaccines and for enabling antigen design. In this study, we report the structure of the epitope recognized by a monoclonal antibody with opsonophagocytic activity and representative of the protective response against type III GBS polysaccharide. The structure and the atomic-level interactions were determined by saturation transfer difference (STD)-NMR and X-ray crystallography using oligosaccharides obtained by synthetic and depolymerization procedures. The GBS PSIII epitope is made by six sugars. Four of them derive from two adjacent repeating units of the PSIII backbone and two of them from the branched galactose-sialic acid disaccharide contained in this sequence. The sialic acid residue establishes direct binding interactions with the functional antibody. The crystal structure provides insight into the molecular basis of antibody-carbohydrate interactions and confirms that the conformational epitope is not required for antigen recognition. Understanding the structural basis of immune recognition of capsular polysaccharide epitopes can aid in the design of novel glycoconjugate vaccines.

Role of O-Acetylation in the Immunogenicity of Bacterial Polysaccharide Vaccines.

The incidence of infectious diseases caused by several bacterial pathogens such as Haemophilus influenzae type b, Streptococcus pneumoniae, and Neisseria meningitidis, has been dramatically reduced over the last 25 years through the use of glycoconjugate vaccines. The structures of the bacterial capsular polysaccharide (CPS) antigens, extracted and purified from microbial cultures and obtained with very high purity, show that many of them are decorated by O-acetyl groups. While these groups are often considered important for the structural identity of the polysaccharides, they play a major role in the functional immune response to some vaccines such as meningococcal serogroup A and Salmonella typhi Vi, but do not seem to be important for many others, such as meningococcal serogroups C, W, Y, and type III Group B Streptococcus. This review discusses the O-acetylation status of CPSs and its role in the immunological responses of these antigens.

Structure-Immunogenicity Relationship of α- and ß-Tetrasaccharide Glycoforms from Bacillus anthracis Exosporium and Fragments Thereof.

The tetrasaccharide (2-O-methyl-4-(3-hydroxy-3-methylbutamido)-4,6-dideoxy-α-d-glucopyranosyl-(1→3)-α-l-rhamnopyranosyl-(1→3)-α-l-rhamnopyranosyl-(1→2)-l-rhamnopyranose) from the major exosporium protein (BclA) of Bacillus anthracis has been proposed as a target for development of diagnostics and immune therapy or prophylaxis. While the immunodominant character of the anthrose residue has been previously elucidated, the role of the stereochemical configuration of the downstream rhamnose is unknown. Because the linkage of this residue to the GlcNAc bridging the glycan and the protein is lost during isolation of the tetrasaccharide, its α- and ß-glycoforms have been synthesized. Herein, we prepared neoglycoconjugates from a series of fragments of the tetrasaccharide, including the complete α- and ß-tetrasaccharide glycoforms, a 2-demethoxylated version of the α-tetrasaccharide, and the α- and ß-trirhamnosides and CRM197. By immunization of mice, we showed that the anti α- and ß-tetrasaccharide serum equally recognized both glycoforms. In contrast the sera produced following immunization with the α- and ß-trirhamnoside fragments exhibited higher recognition for their own antigens than for their anomeric counterparts. The anti α- and ß-tetrasaccharide sera recognized Sterne spores in a comparable fashion. ΔBclA spores not expressing the major exosporium protein were also recognized by the same sera, while mutants that produced the carbohydrate antigen with deletion of either rhamnose or anthrose were not. The tetrasaccharide could, therefore, be expressed in proteins other than BlcA. This work proves that α- and ß-tetrasaccharide are equally potent immunogens.

Copper(I) Forms a Redox-Stable 1:2 Complex with α-Synuclein N-Terminal Peptide in a Membrane-Like Environment.

α-Synuclein (αS) is the main protein component of Lewy bodies, characterizing the pathogenesis of Parkinson's disease. αS is unstructured in solution but adopts a helical structure in its extended N-terminal segment upon association with membranes. In vitro the protein binds avidly Cu(II), but in vivo the protein is N-acetylated, and Cu(II) binding is lost. We have now clarified the binding characteristics of the Cu(I) complex with the truncated αS peptide 1-15, both in N-acetylated and free amine forms, in a membrane mimetic environment and found that complexation occurs with a 1:2 Cu(I)-αS stoichiometry, where Cu(I) is bound to Met1 and Met5 residues of two helical peptide chains. The resulting tetrahedral Cu(I) center is redox-stable, does not form reactive oxygen species, and is unreactive against dopamine in the presence of O2. This suggests that, unlike cytosolic Cu(I)-αS, which retains the capacity to activate O2 and promote oxidative reactions, membrane-bound Cu(I)-αS may serve as a sink for unreactive copper.

Copper(I/II), α/ß-Synuclein and Amyloid-ß: Menage à Trois?

Copper binding to α-synuclein (aS) and to amyloid-ß (Ab) has been connected to Parkinson's and Alzheimer's disease (AD), respectively, because Cu ions can modulate the peptide aggregation, and these Cu ⋅ peptide complexes can catalyse the production of reactive oxygen species (ROS). In a significant proportion of AD brains, aggregation of aS and Ab has been detected, and it was proposed that Ab and aS interact with each other. Thus, we investigated the potential interactions of Ab and aS through their binding of copper(I) and copper(II). Additionally, ß-synuclein (bS) was investigated, due to its additional methionine residue, a potential Cu(I) ligand. We found that: 1) the peptides containing the Cu-binding domains Ab1-16, aS1-15 and bS1-15 have similar affinities towards Cu(II) and towards Cu(I), with Ab1-16 being slightly stronger, 2) in the case of Cu(I), the additional Met residue in bS1-15 increased the affinity slightly, 3) the exchange of Cu(I/II) between the two peptides is rapid (≤ ms), 4) a/bS1-15 and Ab1-16 form a heterodimeric complex with Cu(II), 5) Cu(I) probably promotes a transient ternary complex, 6) the different Cu(I/II) coordination of Ab1-16, aS1-15 and bS1-15 impacts the capacity to produce ROS and to oxidise catechol, and 7) when Ab1-16, aS1-15 and Cu are present, the ROS production more closely resembles that by Ab1-16. The work gives insights into the coordination chemistry of these related peptides, and the relevance of coordination differences, the ternary complex and ROS production are discussed.

Differences in the binding of copper(I) to α- and ß-synuclein.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the presence of abnormal α-synuclein (αS) deposits in the brain. Alterations in homeostasis and metal-induced oxidative stress may play a crucial role in the progression of αS amyloid assembly and pathogenesis of PD. Contrary to αS, ß-synuclein (ßS) is not involved in the PD etiology. However, it has been suggested that the ßS/αS ratio is altered in PD, indicating that a correct balance of these two proteins is implicated in the inhibition of αS aggregation. αS and ßS share similar abilities to coordinate Cu(II). In this study, we investigated and compared the interaction of Cu(I) with the N-terminal portion of ßS and αS by means of NMR, circular dichroism, and X-ray absorption spectroscopies. Our data show the importance of M10K mutation, which induces different Cu(I) chemical environments. Coordination modes 3S1O and 2S2O were identified for ßS and αS, respectively. These new insights into the bioinorganic chemistry of copper and synuclein proteins are a basis to understand the molecular mechanism by which ßS might inhibit αS aggregation.

Remote His50 Acts as a Coordination Switch in the High-Affinity N-Terminal Centered Copper(II) Site of α-Synuclein.

Parkinson's disease (PD) etiology is closely linked to the aggregation of α-synuclein (αS). Copper(II) ions can bind to αS and may impact its aggregation propensity. As a consequence, deciphering the exact mode of Cu(II) binding to αS is important in the PD context. Several previous reports have shown some discrepancies in the description of the main Cu(II) site in αS, which are resolved here by a new scenario. Three Cu(II) species can be encountered, depending on the pH and the Cu:αS ratio. At low pH, Cu(II) is bound to the N-terminal part of the protein by the N-terminal amine, the adjacent deprotonated amide group of the Asp2 residue, and the carboxylate group from the side chain of the same Asp2. At pH 7.4, the imidazole group of remote His50 occupies the fourth labile equatorial position of the previous site. At high Cu(II):αS ratio (>1), His50 leaves the coordination sphere of the first Cu site centered at the N-terminus, because a second weak affinity site centered on His50 is now filled with Cu(II). In this new scheme, the remote His plays the role of a molecular switch and it can be anticipated that the binding of the remote His to the Cu(II) ion can induce different folding of the αS protein, having various aggregation propensity.

Analytical Insights into Protein-Alum Interactions and Their Impact on Conformational Epitope.

Several alum-adjuvanted vaccines have been licensed in the past 40 years. Despite its extensive and continuous use, the immune mechanism of action of alum adjuvants is not yet completely understood. Many different variables during the formulation process have been assessed as critical for alum-adjuvanted vaccines, although most of them are still not yet fully understood. The absence of a clear understanding of all the possible variables regulating the mechanism of action and the behavior that alum adjuvant imposes on the protein antigen may also be related to analytical challenges. For this reason, there is an urgent need for a fast and simple tool that is possible without a preliminary sample manipulation and is able to control the amount and the degree of antigen adsorption levels and their consistency across different production processes. This work attempts to develop new analytical tools with the aim of directly quantifying and assessing both the content and/or the purity of formulated alum-adsorbed antigens, without any preliminary sample manipulation (e.g., antigen desorption) being reported. In addition, the different confirmation/behavior in terms of the response to specific monoclonal antibodies in the presence of different ratios of alum-OH adsorbent antigens have been investigated. As a proxy to develop new analytical tools, three recombinant protein adsorbed models were used as follows: Neisseria adhesin A (NadA), Neisserial Heparin Binding Antigen (NHBA), and factor H binding protein (fHbp) as antigens, as well as aluminum hydroxide (AH) as an adjuvant system. The selection of the adjuvanted system model was dictated due to the substantial quantity of the literature regarding the protein structure and immunological activities, meaning that they are well characterized, including their adhesion rate to alum. In conclusion, three different analytical tools were explored to quantify, detect, and study the behavior of antigens in the presence of the alum adjuvant.

Innovative Reversed-Phase Chromatography Platform Approach for the Fast and Accurate Characterization of Membrane Vesicles' Protein Patterns.

Outer membrane vesicles (OMVs) have been widely explored to develop vaccine candidates for bacterial pathogens due to their ability to combine adjuvant properties with immunogenic activity. OMV expresses a variety of proteins and carbohydrate antigens on their surfaces. For this reason, there is an analytical need to thoroughly characterize the species expressed at their surface: we here present a simple and accurate reversed-phase ultrahigh-performance liquid chromatography (RP-UPLC) method developed according to quality by design principles. This work provides an analytical alternative to the classical sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) characterization. The higher selectivity and sensitivity of the RP-UHPLC assay allow for the identification of additional protein species with respect to SDS-PAGE and facilitate its precise relative abundance quantification. According to validation results, the assay showed high accuracy, linearity, precision, repeatability, and a limit of quantification of 1% for less abundant proteins. This performance paves the way for improved production campaign consistency while also being analytically simple (no sample pretreatment required), making it suitable for routine quality control testing. In addition, the applicability of the assay to a wider range of vesicle classes (GMMA) was demonstrated.

Development of a New Solid-Phase Extraction Base Method for Free Saccharide Content Estimation of Meningococcal Conjugate Vaccines.

GlaxoSmithKline (GSK) is currently developing a fully liquid presentation to ease the administration of the licensed quadrivalent conjugate vaccine (Menveo) against meningococcal serogroup A, C, W, and Y (MenACWY) infections. Herein, we report a new method for determining the free saccharide (FS) content of CRM197-MenACWY conjugated antigens, with the aim of improving accuracy and reproducibility. Mathematical models have been used to support technical knowledge in reducing the need for experimental development. This results in an improved, faster, and platform-based technique for FS separation with one single pretreatment applicable to all antigens of the multivalent meningococcal vaccine.

Combined Chemical Synthesis and Tailored Enzymatic Elongation Provide Fully Synthetic and Conjugation-Ready Neisseria meningitidis Serogroup X Vaccine Antigens.

Studies on the polymerization mode of Neisseria meningitidis serogroup X capsular polymerase CsxA recently identified a truncated construct that can be immobilized and used for length controlled on-column production of oligosaccharides. Here, we combined the use of a synthetic acceptor bearing an appendix for carrier protein conjugation and the on-column process to a novel chemo-enzymatic strategy. After protein coupling of the size optimized oligosaccharide produced by the one-pot elongation procedure, we obtained a more homogeneous glycoconjugate compared to the one previously described starting from the natural polysaccharide. Mice immunized with the conjugated fully synthetic oligomer elicited functional antibodies comparable to controls immunized with the current benchmark MenX glycoconjugates prepared from the natural capsule polymer or from fragments of it enzymatically elongated. This pathogen-free technology allows the fast total in vitro construction of predefined bacterial polysaccharide fragments. Compared to conventional synthetic protocols, the procedure is more expeditious and drastically reduces the number of purification steps to achieve the oligomers. Furthermore, the presence of a linker for conjugation in the synthetic acceptor minimizes manipulations on the enzymatically produced glycan prior to protein conjugation. This approach enriches the methods for fast construction of complex bacterial carbohydrates.

Metal ion mediated transition from random coil to ß-sheet and aggregation of Bri2-23, a natural inhibitor of Aß aggregation.

Furin-dependent maturation of the BRI2 protein generates the Bri2-23 fragment that is able to arrest the aggregation of amyloidß, the peptide implicated in Alzheimer's disease (AD). Bri2-23 contains cysteines at positions 5 and 22, which are likely to bind to metal ions such as Cu(i). Metal ions may play a role in the etiology of neurodegenerative disorders such as AD, and in this work we explore the metal ion induced folding and aggregation of Bri2-23 using Hg(ii) and Ag(i) as spectroscopic probes with structural and ligand preferences similar to those of Cu(i), while not displaying redox activity under the experimental conditions. In general, interaction of Bri2-23 with soft metal ions changes the structural properties and solution behavior of the peptide that tune to increasing metal to peptide stoichiometry. Potentiometric, (199m)Hg PAC and ESI-MS data indicate that addition of up to 0.5 equivalents of Hg(ii) to Bri2-23 yields a two-coordinated HgS2 structure at the metal site. While the free peptide is inherently unstructured, the presence of Ag(i) and Hg(ii) gives rise to ß-sheet formation. NMR spectroscopy supports the formation of ß-sheet structure in the presence of 0.5 equivalents of Hg(ii), and displays an interesting and marked change in the TOCSY spectra when increasing the Hg(ii) to peptide stoichiometry from 0.5 to 0.7 equivalents, indicating the equilibrium between two structural analogues of the complex. Addition of more than 0.7 equivalents of Hg(ii) gives rise to line broadening, presumably reflecting aggregation. This is further supported by ThT fluorescence studies showing that the Bri2-23 peptide does not aggregate over 24 hours, while addition of over 0.7 equivalents of Ag(i) or Hg(ii) leads to increase of fluorescence, indicating that these metal ions induce aggregation. Thus, a model integrating all data into a coherent picture is that the metal ion binding to the two thiolates gives rise to folding of the peptide into a structure that is prone to aggregation, forming aggregates with a considerable amount of ß-sheets. Molecular dynamics simulations initiated with structures that agree with NMR data additionally support this model.

Conformation propensities of des-acyl-ghrelin as probed by CD and NMR.

Des-acyl-ghrelin is a 28 amino acid peptide secreted by both human and rat stomach. Together with ghrelin and obestatin, it is obtained by post-translational modification of a 117 aminoacid prepropeptide mainly expressed in distinct endocrine cell type in the stomach. Although its receptor has not been unambiguously identified so far, des-acyl-ghrelin is considered one of the strongest antagonists of ghrelin in activating the growth hormone secretagogue receptor (GHS-R). Here the secondary structure of des-acyl-ghrelin in different experimental conditions has been investigated and compared with that of obestatin, a bioactive peptide having similar biological functions. CD and NMR techniques have been combined for gaining the desired conformational features. The obtained structures support a steady alpha-helix structure spanning residues from 7 to 14, very similar to that observed for obestatin at the same experimental conditions, leading to suggest that a similar secondary structure can be associated with the similar biological role.