Development and evaluation of a dual target glycoconjugate vaccine against Shigella sonnei

BackgroundShigellosis morbidity and mortality, combined with the increase in multidrug-resistant infections make Shigella vaccine development a global imperative. Glycoconjugate vaccines that couple immunogenic O-antigen to protein derived from Shigella may provide broader protection across Shigella species and serogroups. Such an approach also circumvents immunotolerance arising from repeated use of the same carrier. Here we use bioconjugation, exploiting an oligosaccharyltransferase (OST) enzyme to couple O-antigen and carrier protein in vivo, to generate a "double-hit" Shigella glycoconjugate vaccine. MethodGlycoconjugates were synthesised in E. coli SDB1 cells expressing S. sonnei O-antigen, the OST PglS, and one of two Shigella carrier proteins. Recombinant glycoconjugate was purified using anion exchange chromatography and then used to immunise mice. Antibody responses were measured and compared by ELISA. ResultsWhen co-produced in E. coli, PglS was able to transfer the cloned S. sonnei O-antigen onto three carrier proteins, modified to accept glycans from the PglS transferase enzymes- the standard bioconjugate carrier ExoA and two immunogenic Shigella-specific outer membrane proteins, EmrK and MdtA. Production of MdtA or ExoA glycoconjugates for immunisation studies utilised successive rounds of anion exchange chromatography, to remove unglycosylated material and obtain highly purified glycoconjugate proteins for us in vaccination. Analysis of murine sera following immunisation revealed an IgG response was raised against both carrier protein and the S. sonnei O-antigen for each glycoconjugate. ConclusionA novel, conserved Shigella protein can be utilised as an effective carrier for the generation of a "double-hit", immunogenic Shigella glycoconjugate vaccine that elicits IgG responses to both carrier protein and S. sonnei O-antigen.