Artificial disaccharide-protein conjugates as immunogens for the preparation of specific anti-Salmonella O-antisera

Classical and novel strategies to develop a Shigella glycoconjugate vaccine: from concept to efficacy in human

Shigella are gram-negative bacteria that cause severe diarrhea and dysentery, with a high level of antimicrobial resistance. Disease-induced protection against reinfection in Shigella-endemic areas provides convincing evidence on the feasibility of a vaccine and on the importance of Shigella lipopolysaccharides as targets of the host humoral protective immune response against disease. This article provides an overview of the original and current strategies toward the development of a Shigella glycan-protein conjugate vaccine that would cover the most commonly detected strains. Going beyond pioneering “lattice”-type polysaccharide-protein conjugates, progress, and challenges are addressed with focus on promising alternatives, which have reached phases I and II clinical trial. Glycoengineered bioconjugates and “sun”-type conjugates featuring well-defined synthetic carbohydrate antigens are discussed with insights on the molecular parameters governing the rational design of a cost-effective glycoconjugate vaccine efficacious in preventing diseases caused by Shigella in the most at risk populations, young children living in endemic areas.

Pathogen specific carbohydrate antigen microarrays: a chip for detection of Salmonella O-antigen specific antibodies

A Salmonella O-antigen microarray was developed by covalent coupling of oligosaccharide antigens specific for serogroups Salmonella enterica sv. Paratyphi (group A), Typhimurium (group B) and Enteritidis (group D). Antibodies were correctly detected in sera from patients with culture verified salmonellosis. High serogroup-specificity was seen with the disaccharide antigens. With the larger antigens, containing the backbone sequence Manalpha1-2Rhaalpha1-2Gal (MRG), common backbone-specific antibodies (O-antigen 12) were also detected. This is "proof of principle" that pathogen-specific carbohydrate antigen microarrays constitute a novel technology for rapid and specific serological diagnosis in either individual patients or larger sero-epidemiological and vaccine studies.

Anti-Salmonella lipopolysaccharide monoclonal antibodies: characterization of Salmonella BO-, CO-, DO-, and EO-specific clones and their diagnostic usefulness

To facilitate the identification and serotyping of Salmonella species, we established a wide variety of murine monoclonal antibodies (MAbs) that were reactive with the lipopolysaccharides (LPSs) of Salmonella serogroups B to E. An effective approach for generating LPS-reactive hybridomas was used; this required immunization of mice with LPS-coated bacteria. To screen for diagnostically useful MAbs, the MAbs were tested by enzyme-linked immunosorbent assay against a set of purified LPSs from smooth and rough Salmonella strains. At least four major groups of antibody specificities were identified: Salmonella (i) BO specific, (ii) CO specific, (iii) DO specific, and (iv) EO specific. For a more detailed epitope analysis, a panel of eight different serogroup-specific MAbs which were shown to bind the O-antigenic polysaccharide chains, yielding characteristic ladder patterns in Western blots (immunoblots) against the LPS of Salmonella serogroups B to E, were selected. The availability of various chemically defined LPS structures and Salmonella O-antigen glycoconjugates permitted the definition of O-antigenic polysaccharide epitopes recognized by each MAb that serologically corresponded to factors O3, O4, O5, O6, O7, O8, O9, and O10 on the basis of the Kauffmann-White scheme for Salmonella classification. The diagnostic accuracy of these immunochemically defined O-specific MAbs for Salmonella serotyping was demonstrated by correct identification of all 167 salmonellae (including 72 serotypes from serogroups B to E) among the 294 bacterial strains in a slide agglutination test. No false-positive reactions were detected.

Production of antibody to lipopolysaccharide (LPS) after immunization with a LPS-polymyxin B-agarose immunogen

A method was devised to produce antibodies to lipopolysaccharide (LPS) in guinea-pigs following a single immunization. The antigen was prepared by mixing polymyxin B-agarose with LPS from Escherichia coli O55:B5. Use of the agarose support allowed purification of the complex by simple washing procedures. Twenty-nine days after a single injection of the immunogen mixed with Freund complete adjuvant all animals demonstrated antibody to the LPS portion of the complex. No antibodies were detected to the polymyxin B component. Typical titres of LPS as measured by ELISA were 2(11). After, a booster immunization, titres of LPS antibody were further increased and a greater avidity was noted. In contrast to other methods which have been employed for production of antibody to LPS, use of the polymyxin B-agarose complex has the following advantages: ease of antigen preparation, ready purification of the complex, potent immunostimulation, and under the conditions employed here, LPS-specific antibody production, without accompanying antibody to polymyxin B.

Antisera specificities to beta-D-galactopyranoside cluster ligands

Mono-, di-, and tri-beta-D-galactopyranosides of 2-(5-hydrazinocarbonylpentanamido)-2-(hydroxymethyl)-1,3-propan edi ol [(Gal)n-TA] have been conjugated to bovine serum albumin (BSA), and used to study the binding specificities to the Gal receptors of liver parenchymal cells. In this study, rabbit antisera produced to the (Gal)n-TA-BSA were characterized by using an enzyme-linked, immunosorbent assay under conditions that allow only the antibodies directed to the carbohydrate part of the antigen to react with the solid-phase (Gal)n-TA-BSA antigens. Inhibition assays using (Gal)n-TA-BSA conjugates showed a relative specificity of the antisera for the number of Gal residues on the TA bridging group to the BSA carrier-protein, indicating that antibodies having specificities to oligosaccharide branch points can be produced. Inhibition assays with (Gal)n-TA haptens, Gal, and methyl beta-D-Gal indicated that the antibody combining-sites interact mainly with the Gal units; no inhibition was observed with the TA bridging group used as a hapten inhibitor. The spatial distances of the Gal units were apparently important for interaction with the anti-(Gal)n-TA-BSA antibody-combining-sites, as (Lac)3-TA-BSA and (Lac)3-TA exhibited relatively little inhibitory activity.

Antibody response in rabbits to gonococcal lipopolysaccharide-bovine serum albumin conjugates

The elicitation of antibodies to gonococcal lipopolysaccharide (LPS) was studied. Rabbits were immunized with whole cells of gonococci, purified LPS, or LPS linked to bovine serum albumin with glutaraldehyde. Purified LPS was not immunogenic. Anti-LPS antibodies were produced by rabbits receiving the LPS-BSA conjugate. These animals showed an earlier IgG anti-LPS antibody response than animals receiving whole bacterial cells. Antiserum to the LPS-BSA conjugate gave rise to a single precipitation line against ultrasonically disrupted gonococci, and agglutinated heat-treated cells of the bacteria.

Antigenic determinants of Salmonella serogroups A and D1. Synthesis of trisaccharide glycosides for use as artificial antigens

The disaccharide glycoside 8-methoxycarbonyloctyl 4,6-O-cyclohexylidine-2-O-(tetra-O-benzyl-alpha-D-galactopyranosyl)-alpha-D-manno pyranoside (7) was used as a common intermediate to the trisaccharide determinants of both Salmonella serogroups A and D1. Acetalation of 8-methoxycarbonyloctyl alpha-D-mannopyranoside provided the 4,6-acetal derivative, which was selectively benzoylated to give the partially protected mannoside 4. Reaction of 4 with tetra-O-benzyl-alpha-D-galacto-pyranosyl chloride afforded the fully protected disaccharide, which, after transesterification, gave the selectively blocked, disaccharide glycoside (7). Addition of tyvelose by way of its 2,4-di-O-benzoyl-3,6-dideoxy-alpha-D-arabino-hexopyranosyl chloride derivative gave the blocked trisacchride determinant of Salmonella serogroup D1. 2,4,-Di-O-benzyl-3,6-dideoxy-alpha-D-ribo-hexopyranosyl chloride reacted with 7 to provide, after removal of blocking groups, the paratose-containing determinant of serogroup A.

Artificial Salmonella vaccines: Salmonella typhimurium O-antigen-specific oligosaccharide-protein conjugates elicit opsonizing antibodies that enhance phagocytosis

Outbred NMRI mice and rabbits were vaccinated with different artificial Salmonella typhimurium immunogens and the specificity and activity of elicited antibodies were studied in in vivo and in vitro phagocytosis assays. The Salmonella immunogens used were: (i) the synthetic disaccharide, abequose (formula see text) D-mannose, representative of Salmonella O antigen 4, covalently linked to bovine serum albumin (BSA); (ii) the octa- and dodecasaccharides, (formula see text) covalently linked to BSA; and (iii) whole heat-killed Salmonella. Rabbit antibodies passively administered to mice significantly enhanced the clearance of intravenously injected S. typhimurium challenge bacteria from the bloodstream. The clearance rate and the titer of anti-O-antigen-specific antibodies correlated. The clearance rate of an S. thompson (O6,7) strain, which has a different O antigen, was the same irrespective of the rabbit serum given. NMRI mice actively immunized with the various oligosaccharide-BSA conjugates had a significantly increased clearance rate of S. typhimurium only. In the in vitro assay, mouse antioligosaccharide-BSA sera promoted phagocytosis of S. typhimurium, but not S. thompson, when incubated with complement and mouse peritoneal exudate cells activated with Freund complete adjuvant.

Diagnosis of Salmonella bacteria: antibodies against synthetic Salmonella O-antigen 8 for immunofluorescence and co-agglutination using sensitized protein A-containing staphylococci

An antiserum against the synthetic disaccharide abequose 1 leads to 3 alpha rhamnose (AR), representative of Salmonella O-antigen 8, coupled to bovine serum albumin (BSA) was used for diagnosis of Salmonella bacteria by indirect immunofluorescence (IFL) and by co-agglutination (COA) using sensitized protein A-containing staphylococci. Among the 1150 enteric bacteria tested in IFL, the antiserum correctly identified all 99 Salmonella serogroup C2 and C3 bacteria with O-antigen 8. No fluorescence was seen with 484 Salmonella bacteria belonging to other serogroups or 567 non-Salmonella enteric bacteria. The anti-AR-BSA serum was favourable as compared to a conventional Salmonella factor O8 serum as regards both titre and specificity. In the COA test, all 22 Salmonella serogroup C2 and C3 strains agglutinated strongly and within seconds, whereas no agglutination could be seen when 93 Salmonella bacteria representing other serogroups were tested.

Coupling of acid labile Salmonella specific oligosaccharides to macromolecular carriers

A coupling method for covalent attachment of acid labile oligosaccharides isolated from S. typhimurium O-polysaccharide to macromolecular carriers is described. Arylamine groups were introduced into the terminal reducing end of oligosaccharides by reacting them with 2-(4-aminophenyl)-ethylamine. After subsequent conversion to the corresponding saccharide-phenylisothiocyanato derivatives saccharides were covalently linked to free epsilon-lysylamine groups of different carrier proteins. The resulting conjugates were highly immunogenic and elicited in rabbits both anti-harptenic and anti-carrier protein specific antibodies. Some of the advantages of this coupling procedure are: (i) it can be used with oligosaccharides containing highly acid or alkali labile structures and/or glycosidic linkages, (ii) it produces conjugates with high degrees of substitution at low saccharide/protein molar input ratios, (iii) it does not grossly affect the immunogenic specificities of the carrier protein, and (iv) it is suitable for preparation of highly substituted affinity columns, e.g., coupling to a polyacrylamide matrix.

Identification of Salmonella bacteria by co-agglutination, using antibodies against synthetic disaccharide-protein antigens O2, O4 and O9, adsorbed to protein A-containing staphylococci

Protein A-containing staphylococci sensitized with antisera against synthetic Salmonella O-antigens 2, 4 and 9, representative of serogroups A, B and D, respectively, were used for identification of Salmonella bacteria by co-agglutination. Out of 416 Salmonella bacteria tested the reagents correctly identified all 24 serogroup A strains, 119 serogroup B strains and 39 serogroup D strains. Unexpected agglutination was registered with two of 144 strains belonging to serogroup C 2 with reagent containing antiserum against synthetic O antigen 4. No agglutination occurred when 24 non-Salmonella bacterial strains were tested. Approximately 10(8)bacteria were required for positive co-agglutination. As compared to standard slide agglutination with conventional anti-Salmonella O factor sera, the co-agglutination metod was favourable in that the reactions were stronger, although the concentration of antiserum used was from 20 to 200 times lower. The co-agglutination method could also be used for detection of soluble antigens in the form of lipopolysaccharides from Salmonella bacteria in concentrations of 1 microgram/ml. When the sensitivity of the co-agglutination technique was compared with indirect immunofluorescence (IFL), the IFL method was shown to be at least 1000 times more sensitive.

Synthetic disaccharide-protein antigen for production of specific O2 antiserum for immunofluorescence diagnosis of salmonella

Antisera from rabbits immunized with the synthetic disaccharide paratose 1 leads to alpha 3 mannose, representive of Salmonella O-antigen 2, covalently linked to bovine serum albumin (BSA), were used in indirect immunofluorescence studies for the identification of Salmonella serogroup A (O-antigen 1,2,12) bacteria. Among 1311 enteric bacteria tested, 497 were Salmonella. The anti-paratose 1 leads to alpha 3 mannose-BSA serum identified correctly all the 63 serogroup A strains tested. No positive reactions were recorded among 1248 strains respresenting Salmonella other than serogroup A, E. coli, Shigella, Citrobacter, Klebsiella, Enterobacter, Serratia, Proteus, Pseudomonas, Acinetobacter, Vibrio, Yersinia and Bacteroides. The study illustrates the high specificity of the antiserum elicited by immunization with the synthetic disaccharide-protein immunogen.