Potential targets for next generation antimicrobial glycoconjugate vaccines

Abstract

Cell surface carbohydrates have been proven optimal targets for vaccine development. Conjugation of polysaccharides to a carrier protein triggers a T-cell-dependent immune response to the glycan moiety. Licensed glycoconjugate vaccines are produced by chemical conjugation of capsular polysaccharides to prevent meningitis caused by meningococcus, pneumococcus and Haemophilus influenzae type b. However, other classes of carbohydrates (O-antigens, exopolysaccharides, wall/teichoic acids) represent attractive targets for developing vaccines. Recent analysis from WHO/CHO underpins alarming concern toward antibiotic-resistant bacteria, such as the so called ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) and additional pathogens such as Clostridium difficile and Group A Streptococcus. Fungal infections are also becoming increasingly invasive for immunocompromised patients or hospitalized individuals. Other emergencies could derive from bacteria which spread during environmental calamities (Vibrio cholerae) or with potential as bioterrorism weapons (Burkholderia pseudomallei and mallei, Francisella tularensis). Vaccination could aid reducing the use of broad-spectrum antibiotics and provide protection by herd immunity also to individuals who are not vaccinated.This review analyzes structural and functional differences of the polysaccharides exposed on the surface of emerging pathogenic bacteria, combined with medical need and technological feasibility of corresponding glycoconjugate vaccines.

Figure 1.

Structures of the cell walls of Gram-negative and Gram-positive bacteria. Both classes of bacteria can produce a capsule (CPS). Gram-negative bacteria express lipopolysaccharide (LPS) or lipooligosaccharide (LOS). Unlike Gram-negative bacteria which possess an outer membrane with an outmost layer rich of phospholipids and LPS molecules, Gram-positive bacteria lack of the outer membrane and possess lipoteichoic acids (LTA) and the more exposed wall teichoic acids (WTA).

Figure 2.

Mechanisms for polysaccharide biosynthesis in bacteria. (A) In the Wzx/Wzy-dependent pathway, the polysaccharide is built on a undecaprenol diphosphate (UndPP) acceptor, on which cytosolic sugar nucleotides are attached by glycosyltransferase (GT) catalyzed reactions an then exported across the membrane by a flippase Wzx protein for final polymerization by a Wzy polymerase, under the control of a polysaccharide copolymerase (PCP). (B) In the ABC transport-dependent pathway, the polysaccharide is built up at the cytoplasmic face of the inner membrane by GTs, and then exported by the ABC transporter. (C) In the synthase-dependent pathway, the polysaccharide is assembled at the cytoplasmic face of the inner membrane by a synthase that is also involved in its transportation across the membrane.

Figure 3.

General structures of bacterial surface polysaccharides. (A) Capsules are homopolymeric or heteropolymeric carbohydrate chains inserted into the membrane. (B) LPS is made of three components: lipid A, core-oligosaccharide and O-polysaccharide or O-antigen. LPS lacking of the O-antigen is termed LOS. (C) Teichoic acids are differentiated into lipoteichoic acids (LTA) and wall teichoic acids (WTA).

Figure 4.

Approaches for the production of glycoconjugate vaccines include (A) engineering of E. coli for expression of carbohydrate, carrier protein and in vivo conjugation, resulting in glycans radially oriented relative to protein; (B) polysaccharides activation of sugar residues along the chain and conjugation to the carrier protein, resulting in cross-linked structures; (C) polysaccharide fragmentation (hydrolysis or other methods discussed in the text), sizing and conjugation via end terminal residues, resulting in glycans radially oriented relative to protein; (D) construction of the oligosaccharide from appropriate building blocks with an in-built linker for conjugation, also resulting in glycans radially oriented relative to protein.