Search

The elucidation of the molecular details underlying the immune properties of glycoconjugate vaccines has largely focused on the carbohydrate moiety, while very little is known on the effect of the corresponding conjugation sites. Herein we constructed a set of β-(1 → 3) glucan oligosaccharide conjugates with a well-defined glycan structure, connected to patterns of predetermined tyrosine or lysine residues onto the CRM197 carrier protein. To evaluate the effect of multivalent architecture in the glycan presentation, a novel linker enabling tyrosine-directed ligation of couples of oligosaccharides was prepared. The potential of these constructs as anti-Candida vaccines was evaluated in vivo, using as controls glycoconjugates prepared by a conventional random coupling strategy, and the structure–immune properties relationship was established. We found that: (i) the tyrosine-directed ligation resulted in higher anti-glycan IgG levels in comparison to the conjugation at predetermined lysine residues; (ii) the presentation of the carbohydrate antigen with a biantennary cluster of glycans onto specific tyrosine residues did not further increase the anti-glycan antibody level; (iii) the sera deriving from immunization with defined conjugates at tyrosine and, particularly at lysine residues, were proven stronger inhibitors of fungal adhesion to human epithelial cells in comparison to those from conjugates prepared by classic random chemistry; (iv) the presence of antibodies directed to the linkers did not affect the anti-glycan immune response. These findings suggest that a careful choice of the defined sites of conjugation and the loading density of antigens are important factors to raise high-quality anti-carbohydrate antibodies.

Some new phosphonoester-linked oligomers, stabilized analogues of the corresponding phosphate-bridged oligomers of Neisseria meningitidis A (MenA) capsular polysaccharide (CPS), were conjugated to human serum albumin (HSA), as a protein carrier model, and studied for immunological activities. We determined (i) in vitro, their biocompatibility (CAM test) and activity in inducing both T cell proliferation (CFSE method) and IL-2 release (ELISA), and (ii) in vivo, their ability to stimulate specific IgG antibody production (ELISA). All HSA-conjugated compounds induce T cell proliferation (40% of proliferation at 10(2) μM), whereas only the phosphonodisaccharide was effective (28% of proliferation at 10(2) μM) among the unconjugated forms. IL-2 release confirmed these results. In addition, the HSA-conjugated showed in vivo the capacity of eliciting the production of specific IgG antibodies. In conclusion, we obtained novel biocompatible, water-stable, and immunoactive MenA CPS analogues. A short disaccharide fragment showed the unusual behavior of triggering T cell proliferation in vitro.

Laminarin-CRM197 glycoconjugates were previously demonstrated to be immunogenic and confer protection against Candida albicans in mice. Laminarin consists of β-(1,3) glucan repeating units, with sporadic β-(1,6) branches. A set of short glucans was used to study the effect of the β-(1,6) branch on the antigenicity of linear β-(1,3) glucans. A linear β-(1,3) glucan hexasaccharide was selected as the best fragment able to inhibit the binding between laminarin and antilaminarin antibodies. The hexamer was then conjugated to CRM197 and induced, in mice, significant titers of specific antilaminarin antibodies comparable with those raised by the Lam-CRM197 conjugate.

Multicomponent constructs, obtained by coupling different glycans to the carrier protein, have been proposed as a way to co-deliver multiple surface carbohydrates targeting different strains of one pathogen and reduce the number of biomolecules in the formulation of multivalent vaccines. To assess the feasibility of this approach for anti-microbial vaccines and investigate the potential immunodominance of one carbohydrate antigen over the others in these constructs, we designed a bivalent unimolecular vaccine against serogroup A (MenA) and C (MenC) meningococci, with the two different oligomers conjugated to same molecule of carrier protein (CRM197). The immune response elicited in mice by the bivalent MenAC construct was compared with the ones induced by the monovalent MenA and MenC vaccines and their combinations. After the second dose, the bivalent construct induced good levels of anti-MenA and anti-MenC antibodies with respect to the controls. However, the murine sera from the MenAC construct exhibited good anti-MenC bactericidal activity, and very low anti-MenA functionality when compared to the monovalent controls. This result was explained with the diverse relative avidities against MenA and MenC polysaccharides, which were measured in the generated sera. The immunodominant effect of the MenC antigen was fully overcome following the third immunization, when sera endowed with higher avidity and excellent bactericidal activity against both MenA and MenC expressing strains were elicited. Construction of multicomponent glycoconjugate vaccines against microbial pathogens is a feasible approach, but particular attention should be devoted to study and overcome possible occurrence of immune interference among the carbohydrates.

An anti-candidiasis glycoconjugate vaccine was prepared via a tyrosine-selective alkynylation and a click chemistry mediated glycoconjugation sequence. It features a well-defined glycan, protein carrier, and connectivity. The construct, although with significantly lower carbohydrate loading and a shorter β-(1,3) glucan chain than the well-established anti-candidiasis vaccine derived from the random conjugation of laminarin at lysines, elicited a comparable level of specific IgG antibodies.