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.