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

Shigellosis 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 Shigella-specific dual-target glycoconjugate.

Glycoconjugates were synthesised in E. coli SDB1 cells producing S. sonnei O-antigen, the OST PglS, and one of two Shigella carrier proteins or the standard bioconjugate carrier, ExoA. Recombinant glycoconjugate was purified using anion exchange chromatography and then used to immunise mice. Antibody responses were measured and compared by ELISA.

When 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 enzyme-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 glycosylated proteins. 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.

A novel, conserved Shigella protein can be utilised as an effective carrier for the S. sonnei O-antigen to generate a Shigella-specific glycoconjugate that elicits IgG responses to both carrier protein and S. sonnei O-antigen.