Rina Saksena

Synthetic Fragments of Vibrio cholerae O1 Inaba O-Specific Polysaccharide Bound to a Protein Carrier Are Immunogenic in Mice but Do Not Induce Protective Antibodies

ABSTRACT Development of Vibrio cholerae lipopolysaccharide (LPS) as a cholera vaccine immunogen is justified by the correlation of vibriocidal anti-LPS response with immunity. Two V. cholerae O1 LPS serotypes, Inaba and Ogawa, are associated with endemic and pandemic cholera. Both serotypes induce protective antibody following infection or vaccination. Structurally, the LPSs that define the serotypes are identical except for the terminal perosamine moiety, which has a methoxyl group at position 2 in Ogawa but a hydroxyl group in Inaba. The terminal sugar of the Ogawa LPS is a protective B-cell epitope. We chemically synthesized the terminal hexasaccharides of V. cholerae serotype Ogawa, which comprises in part the O-specific polysaccharide component of the native LPS, and coupled the oligosaccharide at different molar ratios to bovine serum albumin (BSA). Our initial studies with Ogawa immunogens showed that the conjugates induced protective antibody. We hypothesized that antibodies specific for the terminal sugar of Inaba LPS would also be protective. Neoglycoconjugates were prepared from synthetic Inaba oligosaccharides (disaccharide, tetrasaccharide, and hexasaccharide) and BSA at different levels of substitution. BALB/c mice responded to the Inaba carbohydrate (CHO)-BSA conjugates with levels of serum antibodies of comparable magnitude to those of mice immunized with Ogawa CHO-BSA conjugates, but the Inaba-specific antibodies (immunoglobulin M [IgM] and IgG1) were neither vibriocidal nor protective in the infant mouse cholera model. We hypothesize that the anti-Inaba antibodies induced by the Inaba CHO-BSA conjugates have enough affinity to be screened via enzyme-linked immunosorbent assay but not enough to be protective in vivo.

Development of Antibodies against Anthrose Tetrasaccharide for Specific Detection of Bacillus anthracis Spores

ABSTRACT Methods for the immunological detection of Bacillus anthracis in various environmental samples and the discrimination of B. anthracis from other members of the B. cereus group are not yet well established. To generate specific discriminating antibodies, we immunized rabbits, mice, and chickens with inactivated B. anthracis spores and, additionally, immunized rabbits and mice with the tetrasaccharide β-Ant-(1→3)-α-l-Rhap-(1→3)-α-l-Rhap-(1→2)-l-Rhap. It is a constituent of the exosporium glycoprotein BclA and contains the newly discovered sugar anthrose 2-O-methyl-4-(3-hydroxy-3-methylbutamido)-4,6-dideoxy-β-d-glucose. The BclA protein is a major component of the exosporium of B. anthracis spores and is decorated by the tetrasaccharide indicated above. The anthrose-containing tetrasaccharide chain seems to be highly specific for B. anthracis, which makes it a key biomarker for the detection of these spores. The different immunizations led to anthrose-reactive polyclonal and monoclonal antibodies which were analyzed by various methods to characterize their ability to discriminate between B. anthracis and other Bacillus spp. Multiple applications, such as enzyme-linked immunosorbent assay, indirect immunofluorescence assay, and electron microscopy, revealed the specificities of the polyclonal and monoclonal antibodies generated for B. anthracis spore detection. All polyclonal antibodies were able to correctly identify the B. anthracis strains tested and showed only minimal cross-reactivities with other Bacillus strains. Moreover, the antibodies generated proved functional in a new capture assay for B. anthracis spores and could therefore be useful for the detection of spores in complex samples.

Neoglycoconjugates from synthetic tetra- and hexasaccharides that mimic the terminus of the O-PS of Vibrio cholerae O:1, serotype Inaba

A glycosyl acceptor and a glycosyl donor having the N-3-deoxy-L-glycero-tetronic acid side chain already attached have been prepared and used for the synthesis of the di-through to the hexasaccharide that mimic the upsteam terminus of the O-specific polysaccharide of Vibrio cholerae O:1, serotype Inaba. The target tetra- and the hexasaccharide, which were obtained in the form of 5-methoxycarbonylpentyl glycosides, were linked to BSA using squaric acid diester chemistry. The conjugation reactions were monitored by surface enhanced laser desorption ionization-time of flight mass spectrometry (SELDI-TOF MS). This allowed the progression of the conjugation of the synthetic oligosaccharides in a controlled way and termination of the reaction when the desired molar hapten/BSA ratio had been reached, yielding neoglycoconjugates with predetermined carbohydrate/carrier ratios. The ability to monitor the conjugation by the SELDI-TOF MS technique made it possible to prepare, from one hapten in a one-pot reaction, several neoglycoconjugates having different, predetermined carbohydrate/carrier ratios.

Chemoenzymatic synthesis of immunogenic meningococcal group C polysialic acid-tetanus Hc fragment glycoconjugates

Vaccination with meningococcal glycoconjugate vaccines has decreased the incidence of invasive meningitis worldwide. These vaccines contain purified capsular polysaccharides attached to a carrier protein. Because of derivatization chemistries used in the process, conjugation of polysaccharide to protein often results in heterogeneous mixtures. Well-defined vaccines are needed to determine the relationship between vaccine structure and generated immune response. Here, we describe efforts to produce well-defined vaccine candidates by chemoenzymatic synthesis. Chemically synthesized lactosides were substrates for recombinant sialyltransferase enzymes from Camplyobacter jejuni and Neisseria meningitidis serogroup C. These resulting oligosialic acids have the same α(2-9) sialic acid repeat structure as Neisseria polysaccharide capsule with the addition of a conjugatable azide aglycon. The degree of polymerization (DP) of carbohydrate products was controlled by inclusion of the inhibitor CMP-9-deoxy-NeuNAc. Polymers with estimated DP < 47 (median DP 25) and DP < 100 (median DP 51) were produced. The receptor binding domain of the tetanus toxin protein (TetHc) was coupled as a carrier to the enzymatically synthesized oligosialic acids. Recombinant TetHc was derivatized with an alkyne squarate. Protein modification sites were determined by trypsin proteolysis followed by LC/MS-MSE analysis of peptides. Oligosialic acid azides were conjugated to modified TetHc via click chemistry. These chemoenzymatically prepared glycoconjugates were reactive in immunoassays with specific antibodies against either group C polysaccharide or TetHc. Sera of mice immunized with oligosialic acid-TetHc glycoconjugates contained much greater levels of polysaccharide-reactive IgG than the sera of control mice receiving unconjugated oligosialic acids. There was no apparent difference between glycoconjugates containing oligosaccharides of DP < 47 and DP < 100. These results suggest that chemoenzymatic synthesis may provide a viable method for making defined meningococcal vaccine candidates.

Multimeric bivalent immunogens from recombinant tetanus toxin HC fragment, synthetic hexasaccharides, and a glycopeptide adjuvant

Using recombinant tetanus toxin HC fragment (rTT-HC) as carrier, we prepared multimeric bivalent immunogens featuring the synthetic hexasaccharide fragment of O-PS of Vibrio cholerae O:1, serotype Ogawa, in combination with either the synthetic hexasaccharide fragment of O-PS of Vibrio cholerae O:1, serotype Inaba, or a synthetic disaccharide tetrapeptide peptidoglycan fragment as adjuvant. The conjugation reaction was effected by squaric acid chemistry and monitored in virtually real time by SELDI-TOF MS. In this way, we could prepare well-defined immunogens with predictable carbohydrate–carrier ratio, whose molecular mass and the amount of each saccharide attached could be independently determined. The ability to prepare such neoglycoconjugates opens unprecedented possibilities for preparation of conjugate vaccines for bacterial diseases from synthetic carbohydrates.

One-pot preparation of a series of glycoconjugates with predetermined antigen-carrier ratio from oligosaccharides that mimic the O-PS of Vibrio cholerae O:1, serotype Ogawa

Di-through the pentasaccharide that mimic the upstream terminus of the O-specific polysaccharide of Vibrio cholerae O:1, serotype Ogawa were synthesized in the form of 5-methoxycarbonylpentyl glycosides and linked to BSA using squaric acid diester chemistry. The conjugation reactions were monitored by surface-enhanced laser-desorption/ionization-time-of-flight mass spectrometry (SELDI-TOF MS), which allowed conducting the conjugation of the synthetic oligosaccharides in a controlled way and termination of the reaction when the desired molar hapten-BSA ratio had been reached. This made it possible to prepare, from one hapten in a one-pot reaction, a series of neoglycoconjugates having different, predetermined carbohydrate-carrier ratios. The accuracy of molecular mass determination in SELDI-TOF MS analysis could be increased by using the carrier protein as the internal standard.

Transcutaneous immunization with a synthetic hexasaccharide-protein conjugate induces anti-Vibrio cholerae lipopolysaccharide responses in mice.

Antibodies specific for Vibrio cholerae lipopolysaccaride (LPS) are common in humans recovering from cholera, and constitute a primary component of the vibriocidal response, a serum complement-mediated bacteriocidal response correlated with protection against cholera. In order to determine whether transcutaneous immunization (TCI) with a V. cholerae neoglycoconjugate (CHO-BSA) comprised of a synthetic terminal hexasaccharide of the O-specific polysaccharide of V. cholerae O1 (Ogawa) conjugated with bovine serum albumin (BSA) could induce anti-V. cholerae LPS and vibriocidal responses, we applied CHO-BSA transcutaneously in the presence or absence of the immune adjuvant cholera toxin (CT) to mice. Transcutaneously applied neoglycoconjugate elicited prominent V. cholerae specific LPS IgG responses in the presence of CT, but not IgM or IgA responses. CT applied on the skin induced strong IgG and IgA serum responses. TCI with neoglycoconjugate did not elicit detectable vibriocidal responses, protection in a mouse challenge assay, or stool anti-V. cholerae IgA responses, irrespective of the presence or absence of CT. Our results suggest that transcutaneously applied synthetic V. cholerae neoglycoconjugate is safe and immunogenic, but predominantly induces systemic LPS responses of the IgG isotype.

Transcutaneous Immunization with a Vibrio cholerae O1 Ogawa Synthetic Hexasaccharide Conjugate following Oral Whole-Cell Cholera Vaccination Boosts Vibriocidal Responses and Induces Protective Immunity in Mice

ABSTRACT A shortcoming of currently available oral cholera vaccines is their induction of relatively short-term protection against cholera compared to that afforded by wild-type disease. We were interested in whether transcutaneous or subcutaneous boosting using a neoglycoconjugate vaccine made from a synthetic terminal hexasaccharide of the O-specific polysaccharide of Vibrio cholerae O1 (Ogawa) coupled to bovine serum albumin as a carrier (CHO-BSA) could boost lipopolysaccharide (LPS)-specific and vibriocidal antibody responses and result in protective immunity following oral priming immunization with whole-cell cholera vaccine. We found that boosting with CHO-BSA with immunoadjuvantative cholera toxin (CT) or Escherichia coli heat-labile toxin (LT) following oral priming with attenuated V. cholerae O1 vaccine strain O395-NT resulted in significant increases in serum anti-V. cholerae LPS IgG, IgM, and IgA (P < 0.01) responses as well as in anti-Ogawa (P < 0.01) and anti-Inaba (P < 0.05) vibriocidal titers in mice. The LPS-specific IgA responses in stool were induced by transcutaneous (P < 0.01) but not subcutaneous immunization. Immune responses following use of CT or LT as an adjuvant were comparable. In a neonatal mouse challenge assay, immune serum from boosted mice was associated with 79% protective efficacy against death. Our results suggest that transcutaneous and subcutaneous boosting with a neoglycoconjugate following oral cholera vaccination may be an effective strategy to prolong protective immune responses against V. cholerae.

Glycation sites in neoglycoglycoconjugates from the terminal monosaccharide antigen of the O-PS of Vibrio cholerae O1, serotype Ogawa, and BSA revealed by matrix-assisted laser desorption-ionization tandem mass spectrometry.

We present the MALDI-TOF/TOF-MS analyses of various hapten-bovine serum albumin (BSA) neoglycoconjugates obtained by squaric acid chemistry coupling of the spacer-equipped, terminal monosaccharide of the O-specific polysaccharide of Vibrio cholerae O1, serotype Ogawa, to BSA. These analyses allowed not only to calculate the molecular masses of the hapten-BSA neoglycoconjugates with different hapten-BSA ratios (4.3, 6.6 and 13.2) but, more importantly, also to localize the covalent linkages (conjugation sites) between the hapten and the carrier protein. Determination of the site of glycation was based on comparison of the MALDI-TOF/TOF-MS analysis of the peptides resulting from the digestion of BSA with similar data resulting from the digestion of BSA glycoconjugates, followed by sequencing by MALDI-TOF/TOF-MS/MS of the glycated peptides. The product-ion scans of the protonated molecules were carried out with a MALDI-TOF/TOF-MS/MS tandem mass spectrometer equipped with a high-collision energy cell. The high-energy collision-induced dissociation (CID) spectra afforded product ions formed by fragmentation of the carbohydrate hapten and amino acid sequences conjugated with fragments of the carbohydrate hapten. We were able to identify three conjugation sites on lysine residues (Lys235, Lys437 and Lys455). It was shown that these lysine residues are very reactive and bind lysine specific reagents. We presume that these Lys residues belong to those that are considered to be sterically more accessible on the surface of the tridimensional structure. The identification of the y-series product ions was very useful for the sequencing of various peptides. The series of a- and b-product ions confirmed the sequence of the conjugated peptides.

Conjugating Low Molecular Mass Carbohydrates to Proteins 1. Monitoring the Progress of Conjugation

Publisher Summary There are many applications of neoglycoconjugates in the life sciences, of which probably the most promising is their use as immunogenic materials in developing vaccines for infectious diseases and cancer. Polysaccharides, such as the O-specific polysaccharide part of lipopolysaccharides and capsular polysaccharides, are important natural antigens but poor immunogens. They are classified as T-independent (TI) antigens, and the level and the spectrum of antibodies produced after immunization with carbohydrate antigens is insufficient to render protection. Chemical linking of carbohydrates to proteins can transform them into T cell-dependent antigens. As a result, multiple injections of neoglycoconjugates can sharply boost antibody titers way beyond those observed as a result of priming with TI antigens. This is, essentially, the rationale behind the concept of synthetic vaccines, which was pre expected to be free from some of the drawbacks of cellular vaccines. This chapter describes monitoring the progress of conjugation of synthetic, linker-equipped oligosaccharides to proteins. Practical hints, which may be useful in modifying the protocol described in the chapter to better suit a particular situation, are also included.