Synthesis, characterization, and immunogenicity in mice of Shigella sonnei O-specific oligosaccharide-core-protein conjugates

Age-related efficacy of Shigella O-specific polysaccharide conjugates in 1-4-year-old Israeli children

BACKGROUND

Despite its high worldwide morbidity and mortality, there is yet no licensed vaccine for shigellosis. We reported the safety and immunogenicity of Shigella O-specific polysaccharide-protein conjugates in adults and young children and efficacy of Shigella sonnei conjugate in young adults.

METHODS

A double-blinded, randomized and vaccine-controlled Phase 3 evaluation of S. sonnei and Shigella flexneri 2a O-SP-rEPA conjugates, 25 microg, injected IM twice, 6 weeks apart, into healthy 1-4 years old, is reported. The children were followed for 2 years by telephone every other week and stool cultures were obtained for each episode of acute diarrhea (> or =3 loose stools/day or a bloody/mucous stool). Sera were taken randomly from 10% of the participants for IgG anti-LPS and anti-carrier levels.

RESULTS

Of the 2799 enrollees, 1433 received S. sonnei and 1366 S. flexneri 2a conjugates; 2699 (96.4%) completed the 2-year follow-up. Local reactions occurred in approximately 5% and approximately 4% had temperatures > or =38.0 degrees C lasting 1-2 days. There were no serious adverse events attributable to the vaccines. Of the 3295 stool cultures obtained, 125 yielded S. sonnei and 21 S. flexneri 2a. Immunogenicity and efficacy were age-related. The overall efficacy of the S. sonnei conjugate was 27.5%; 71.1% (P=0.043) in the 3-4 years old. The numbers for S. flexneri 2a were too few for meaningful analysis. Cross-protection by S. flexneri 2a for non-vaccine S. flexneri types was found, but the numbers were too few for statistical significance. There was an age-related rise of vaccine-specific IgG anti-LPS in both groups, peaking at about 10 weeks and declining thereafter, but remaining > or =4-fold higher than in the controls 2 years after the second dose.

CONCLUSIONS

Shigella conjugates are safe and immunogenic in 1-4 years old. The S. sonnei conjugate elicited 71.1% efficacy in the 3-4 years old and can be predicted to be efficacious in individuals older than 3 years of age. These results urge studies with our improved conjugates.

The diversity of the core oligosaccharide in lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) are the major component of the outer membrane of Gram-negative bacteria. They have a structural role since they contribute to the cellular rigidity by increasing the strength of cell wall and mediating contacts with the external environment that can induce structural changes to allow life in different conditions. Furthermore, the low permeability of the outer membrane acts as a barrier to protect bacteria from host-derived antimicrobial compounds. Lipopolysaccharides are amphiphilic macromolecules generally comprising three defined regions distinguished by their genetics, structures and function: the lipid A, the core oligosaccharide and a polysaccharide portion, the O-chain. In some Gram-negative bacteria LPS can terminate with the core portion to form rough type LPS (R-LPS, LOS). The core oligosaccharide is an often branched and phosphorylated heterooligosaccharide with less than fifteen sugars, more conserved in the inner region, proximal to the lipid A, and often carrying non-stoichiometric substitutions leading to variation and micro-heterogeneity. The core oligosaccharide contributes to the bacterial viability and stability of the outer membrane, can assure the serological specificity and possesses antigenic properties.

New and candidate vaccines for gastrointestinal infections

PURPOSE OF REVIEW

To summarize recent advances that contribute to the development of new vaccines against gastrointestinal infections.

RECENT FINDINGS

The main themes of this review include epidemiology assessing disease burden of enteric pathogens, increasing antibiotic resistance, positive effect of existing enteric vaccines in developed and developing countries, and recommendations of advisory bodies; antigen discovery and preclinical testing, including live vector systems expressing heterologous antigens, conjugation of antigen to carrier proteins, and plant-based expression systems; as well as clinical studies highlighting recently published enteric vaccine candidates, studies assessing immune correlates of protection, as well as a trend for technology transfer to developing countries where enteric vaccines can be produced less expensively.

SUMMARY

It is an exciting time for the development of novel vaccines against gastrointestinal infections. Better understanding of disease burden, interest from funding sources, identification of novel antigens, better understanding of protective immune responses, and steady progress in the conduct of clinical trials make the development of new enteric vaccines a goal that is attainable in the near future.