Structure‐guided design of a Group B Streptococcus type III synthetic glycan‐conjugate vaccine

Identification of glycan functional epitopes is of paramount importance for rational design of glycoconjugate vaccines. We recently mapped the structural epitope of the capsular polysaccharide from type III Group B Streptococcus (GBSIII), a major cause of invasive disease in newborns, by using a dimer fragment obtained by depolymerization complexed with a protective mAb. While literature data had suggested a highly complex epitope contained in a helical structure composed of more than four repeating units, we showed that such dimer conjugated to a carrier protein with a proper glycosylation degree elicited functional antibodies comparably to the full-length conjugated polysaccharide. Here, starting from the X-ray crystallographic structure of the polysaccharide fragment-mAb complex, we synthesized a hexasaccharide comprising exclusively the relevant positions involved in binding. Combining competitive Surface Plasmon Resonance, Saturation Transfer Difference NMR and in silico modelling, we demonstrated that this synthetic glycan was recognized by the mAb similarly to the dimer. The hexasaccharide conjugated to CRM197, a mutant of diphtheria toxin, elicited a robust functional immune response non inferior to the polysaccharide conjugate, indicating to suffice as vaccine antigen. This is the first evidence of an X-ray crystallography guided design of a synthetic carbohydrate based conjugate vaccine.