Advancing Homogeneous Antimicrobial Glycoconjugate Vaccines.

Since 2004, when the first synthetic glycoconjugate vaccine against the pneumonia and meningitis causing bacterium Haemophilus influenza type b (Hib) approved for human use in Cuba was reported, 34 million doses of the synthetic vaccine have been already distributed in several countries under the commercial name of Quimi-Hib. However, despite the success of this product, no other synthetic glycoconjugate vaccine has been licensed in the following 13 years. As well as avoiding the need to handle pathogens, synthetic glycoconjugates offer clear advantages in terms of product characterization and the possibility to understand the parameters influencing immunogenicity. Nevertheless, large scale application of synthetic sugars has been perceived as challenging because of manufacturing costs and process complexity compared to natural polysaccharides. Chemoenzymatic approaches, one-pot protocols, and automated solid-phase synthesis are rendering carbohydrate production considerably more attractive for industrialization. Here we identify three areas where chemical approaches can advance this progress: (i) chemical or enzymatic methods enabling the delivery of the minimal polysaccharide portion responsible for an effective immune response; (ii) site-selective chemical or enzymatic conjugation strategies for the exploration of the conjugation point in immune responses against carbohydrate-based vaccines, and the consistent preparation of more homogeneous products; (iii) multicomponent constructs targeting receptors responsible for immune response modulation in order to control its quality and magnitude. We discuss how synthesis of bacterial oligosaccharides is useful toward understanding the polysaccharide portion responsible for immunogenicity, and for developing robust and consistent alternatives to natural heterogeneous polysaccharides. The synthesis of sugar analogues can lead to the identification of hydrolytically more stable versions of oligosaccharide antigens. The study of bacterial polysaccharide biosynthesis aids the development of in vitro hazard-free oligosaccharide production. Novel site-selective conjugation methods contribute toward deciphering the role of conjugation sites in the immunogenicity of glycoconjugates and prove to be particularly useful when glycans are conjugated to protein serving as carrier and antigen. The orthogonal incorporation of two different carbohydrate haptens enables the reduction of vaccine components. Finally, coordinated conjugation of glycans and small molecule immunopotentiators supports simplification of vaccine formulation and localization of adjuvant. Synergistic advancement of these areas, combined with competitive manufacturing processes, will contribute to a better understanding of the features guiding the immunological activity of glycoconjugates and, ultimately, to the design of improved, safer vaccines.

6-O-Branched Oligo-ß-glucan-Based Antifungal Glycoconjugate Vaccines.

With the rapid growth in fungal infections and drug-resistant fungal strains, antifungal vaccines have become an especially attractive strategy to tackle this important health problem. ß-Glucans, a class of extracellular carbohydrate antigens abundantly and consistently expressed on fungal cell surfaces, are intriguing epitopes for antifungal vaccine development. ß-Glucans have a conserved ß-1,3-glucan backbone with sporadic ß-1,3- or ß-1,6-linked short glucans as branches at the 6-O-positions, and the branches may play a critical role in their immunologic functions. To study the immunologic properties of branched ß-glucans and develop ß-glucan-based antifungal vaccines, three branched ß-glucan oligosaccharides with 6-O-linked ß-1,6-tetraglucose, ß-1,3-diglucose, and ß-1,3-tetraglucose branches on a ß-1,3-nonaglucan backbone, which mimic the structural epitopes of natural ß-glucans, were synthesized and coupled with keyhole limpet hemocyanin (KLH) to form novel synthetic conjugate vaccines. These glycoconjugates were proved to elicit strong IgG antibody responses in mice. It was also discovered that the number, size, and structure of branches linked to the ß-glucan backbone had a significant impact on the immunologic property. Moreover, antibodies induced by the synthetic oligosaccharide-KLH conjugates were able to recognize and bind to natural ß-glucans and fungal cells. Most importantly, these conjugates elicited effective protection against systemic Candida albicans infection in mice. Thus, branched oligo-ß-glucans were identified as functional epitopes for antifungal vaccine design and the corresponding protein conjugates as promising antifungal vaccine candidates.

Synthesis and immunological studies of linear oligosaccharides of ß-glucan as antigens for antifungal vaccine development.

Antifungal vaccines have recently engendered considerable excitement for counteracting the resurgence of fungal infections. In this context, ß-glucan, which is abundantly expressed on all fungal cell surfaces, functionally necessary for fungi, and immunologically active, is an attractive target antigen. Aiming at the development of effective antifungal vaccines based on ß-glucan, a series of its oligosaccharide derivatives was designed, synthesized, and coupled with a carrier protein, keyhole limpet hemocyanin (KLH), to form new semisynthetic glycoconjugate vaccines. In this article, a convergent and effective synthetic strategy using preactivation-based iterative glycosylation was developed for the designed oligosaccharides. The strategy can be widely useful for rapid construction of large oligo-ß-glucans with shorter oligosaccharides as building blocks. The KLH conjugates of the synthesized ß-glucan hexa-, octa-, deca-, and dodecasaccharides were demonstrated to elicit high titers of antigen-specific total and IgG antibodies in mice, suggesting the induction of functional T cell-mediated immunity. Moreover, it was revealed that octa-, deca-, and dodeca-ß-glucans were much more immunogenic than the hexamer and that the octamer was the best among these. The results suggested that the optimal oligosaccharide sequence of ß-glucan required for exceptional immunogenicity was a hepta- or octamer and that longer glucans are not necessarily better antigens, a finding that may be of general importance. Most importantly, the octa-ß-glucan-KLH conjugate provoked protective immunity against Candida albicans infection in a systemic challenge model in mice, suggesting the great potential of this glycoconjugate as a clinically useful immunoprophylactic antifungal vaccine.

Synthesis of antifungal vaccines by conjugation of ß-1,2 trimannosides with T-cell peptides and covalent anchoring of neoglycopeptide to tetanus toxoid.

Selective strategies for the construction of novel three component glycoconjugate vaccines presenting Candida albicans cell wall glycan (ß-1,2 mannoside) and polypeptide fragments on a tetanus toxoid carrier are described. The first of two conjugation strategies employed peptides bearing an N-terminal thiopropionyl residue for conjugation to a trisaccharide equipped with an acrylate linker and a C-terminal S-acetyl thioglycolyl moiety for subsequent linking of neoglycopeptide to bromoacetylated tetanus toxoid. Michael addition of acrylate trisaccharides to peptide thiol under mildly basic conditions gave a mixture of N- and C- terminal glyco-peptide thioethers. An adaptation of this strategy coordinated S-acyl protection with anticipated thioester exchange equilibria. This furnished a single chemically defined fully synthetic neoglycopeptide conjugate that could be anchored to a tetanus toxoid carrier and avoids the introduction of exogenous antigenic groups. The second strategy retained the N-terminal thiopropionyl residue but replaced the C-terminal S-acetate functionality with an azido group that allowed efficient, selective formation of neoglycopeptide thioethers and subsequent conjugation of these with propargylated tetanus toxoid, but introduced potentially antigenic triazole linkages.

Attempts at a peptide vaccine against paracoccidioidomycosis, adjuvant to chemotherapy.

Chemotherapy is the basis of treatment of paracoccidioidomycosis in its various forms. Depending on the Paracoccidioides brasiliensis virulence, the status of host immunity, the degree of tissue involvement and fungal dissemination, treatment can be extended for long periods with an alarming frequency of relapses. Association of chemotherapy with a vaccine to boost the cellular immune response seemed a relevant project not only to reduce the time of treatment but also to prevent relapses and improve the prognosis of anergic cases. The candidate immunogen is the gp43 major diagnostic antigen of P. brasiliensis and more specifically its derived peptide P10, carrying the CD4+ T-cell epitope. Both gp43 and P10 protected Balb/c mice against intratracheal infections with virulent P. brasiliensis strain. P10 as single peptide or in a multiple-antigen-peptide (MAP) tetravalent construction was protective without adjuvant either by preimmunization and intratracheal challenge or as a therapeutic agent in mice with installed infection. P10 showed additive protective effects in drug-treated mice stimulating a Th-1 type immune response with high IFN-gamma and IL-12. P10 and few other peptides in the gp43 were selected by Tepitope algorithm and actually shown to promiscuously bind several prominent HLA-DR molecules suggesting that a peptide vaccine could be devised for a genetically heterogenous population. P10 was protective in animals turned anergic, was effective in a DNA minigene vaccine, and increased the protection by monoclonal antibodies in Balb/c mice. DNA vaccines and peptide vaccines are promising therapeutic tools to be explored in the control of systemic mycoses.

Synthesis and immunochemical characterization of S-linked glycoconjugate vaccines against Candida albicans.

Replacement of the glycosidic oxygen atom by a sulphur atom is a promising technique for creating glycoconjugates with increased resistance to hydrolysis by endogenous glycosidases. The synthesis and antigenic properties of two distinct (1-->2)-beta-mannan trisaccharides with inter residue-S-linked mannopyranose residues are described. Syntheses were based on an oxidation-reduction strategy to construct the O-linked beta-mannopyranoside bonds and a SN2 inversion to provide 1-thio-beta-mannopyranoside residues. Subsequently the allyl trisaccharide glycosides were subjected to photo addition with cysteine amine and coupled to tetanus toxoid and bovine serum albumin with good efficiency via an adipic acid tether. Rabbit immunization studies revealed that the antibodies elicited by the two glycoconjugates were able to recognize the corresponding O-linked trisaccharide epitope conjugated to BSA and the native cell wall antigen of Candida albicans.

Synthesis and immunochemical studies on a Candida albicans cluster glycoconjugate vaccine.

The immunoprotective beta-mannan of Candida albicans occurs as part of the cell wall phosphomannan N-linked glycoprotein. This macromolecule is composed of an extended alpha1,6 linked mannopyranan backbone containing alpha1,2 mannopyranan branches, to which beta1,2 mannopyranan epitopes are attached. The synthesis of beta1,2-mannan disaccharides clustered on a glucose core has been achieved as a way to imitate the multipoint display of beta-mannans in the native glycoprotein. The clustered epitopes were conjugated to tetanus toxoid and bovine serum albumin. Rabbits immunized with tetanus toxoid cluster glycoconjugate gave good antibody titres for the disaccharide cluster or simple trisaccharide epitope (coupled to BSA). The anti-sera also showed strong cross-reactivity with a Candida albicansbeta-mannan cell wall extract. These immunochemical results are compared with data obtained with non-cluster disaccharide and trisaccharide glycoconjugate antigens. The same conjugates gave substantially lower antibody levels when used to immunize mice.

IL-12 is required for induction but not maintenance of protective, memory responses to Blastomyces dermatitidis: implications for vaccine development in immune-deficient hosts.

Cellular immunity mediated by T lymphocytes, in particular CD4(+) and CD8(+) type 1 (T1) cells, is the main defense against pathogenic fungi. IL-12 initiates T1 cell development and cell-mediated immunity, but it is unclear whether IL-12 contributes to the maintenance of an antifungal T1 response. In this study, we addressed the role of IL-12 for vaccine-induced memory T cell development against experimental pulmonary blastomycosis. CD4(+) T cells absolutely required IL-12 to control a live genetically engineered attenuated strain of Blastomyces dermatitidis given s.c. as a vaccine, whereas CD8(+) T cells were significantly less dependent on IL-12. Despite differential dependency of T cell subsets on IL-12 during vaccination, neither subset acquired memory immunity in the absence of IL-12. In contrast, adoptive transfer of immune CD4 T cells from wild-type mice into IL-12(-/-) mice showed that CD4(+) T1 memory cells sustained a T1 cytokine profile and remained protective over a period of 6 mo posttransfer. Similarly, memory CD8 cells elicited in IL-12(-/-) mice with killed yeast and transient rIL-12 treatment (during vaccination) remained durable and protective after animals were rested for 3 mo. In conclusion, these studies demonstrate that once CD4 and CD8 cells have acquired a protective T1 phenotype they no longer require the presence of IL-12 to maintain antifungal protective memory.

Synthesis of glycoconjugate vaccines for Candida albicans using novel linker methodology.

[reaction: see text] The cell wall phosphomannan of Candida species is a complex N-linked glycoprotein with a glycan chain that contains predominantly alpha-linked mannose residues. However, it is the minor beta-mannan component of the phosphomannan of clinically important Candida strains that provides immunological protection in animal models of fungal disease and hence holds promise as a component of conjugate vaccines. This important antigen occurs in different forms linked to the alpha-mannan backbone via a phosphodiester bond (acid-labile beta-mannan) or directly via a glycosidic bond. To reproducibly synthesize and evaluate conjugate vaccines, a robust method for the synthesis of the different oligosaccharide epitopes is required. Here, we report the gram-scale syntheses of both types of epitopes by an approach that utilizes glucosyl trichloroacetimidate donor 2 to first create a beta-glucopyranoside linkage and then epimerizes the C-2 center via an oxidation-reduction sequence that provides an efficient multigram scale route to the beta-mannopyranosides 5, 8, and 15. Reaction of glycosides 16-18 with homobifunctional adipic acid p-nitrophenyl diesters in dry DMF gave the corresponding half esters in good yields, and of sufficient stability to permit chromatographic purification. Subsequent conjugation with BSA and tetanus toxiod (TT) under mild conjugation conditions afforded the corresponding tri- and tetrasaccharide neoglycoproteins with good efficiency. The conjugation method is also applicable to the coupling of small amounts (mg) of larger oligosaccharides with different proteins.

Synthesis and immunological studies of glycoconjugates of Cryptococcus neoformans capsular glucuronoxylomannan oligosaccharide structures.

Antibodies to the glucuronoxylomannan (GXM) component of the polysaccharide capsule of Cryptococcus neoformans are protective and GXM-protein conjugate vaccines can elicit protective immune responses. We report the synthesis of a heptasaccharide oligosaccharide representing the putative dominant motif of serotype A GXM and demonstrate that it is recognized by some monoclonal antibodies (mAbs) generated to GXM. Conjugation of the heptasaccharide to human serum albumin (HSA) resulted in an immunogenic compound that elicited high-titer IgG responses in mice when given with complete Freund's adjuvant. The antibody response elicited by the oligosaccharide conjugate vaccine had characteristics of a T-cell-dependent response. The availability of an immunogenic oligosaccharide representing a structural motif of GXM will prove useful in studies of antibody epitope specificity and represents a potential synthetic oligosaccharide vaccine against this fungal pathogen.

Synthesis of a beta1,2-mannopyranosyl tetrasaccharide found in the phosphomannan antigen of Candida albicans.

The synthesis of a portion of the challenging beta1,2-mannosyl polymer found in the cell walls of Candida albicans was undertaken to develop a conjugate vaccine against C. albicans and to facilitate NMR conformational studies of this unique polysaccharide. The novel approach to the synthesis of tetrasaccharide 1 employed the modified ulosyl bromide 11 as the glycosyl donor which provided high diastereoselectivity. A participating solvent as well as p-chlorobenzyl protection facilitated the new approach.