Conjugation of different immunogenic enterococcal vaccine target antigens leads to extended strain coverage

Enterococci have emerged as important nosocomial pathogens due to their resistance against the most commonly used antibiotics. Alternative treatments or prevention options are aimed at polysaccharides and surface-related proteins that play important roles in pathogenesis. Previously, we have shown that two Enterococcus faecium proteins, the secreted antigen A and the peptidyl-prolyl cis-trans isomerase, as well as the Enterococcus faecalis polysaccharide diheteroglycan are able to induce opsonic and cross-protective antibodies. Here, we evaluate the use of glycoconjugates consisting of these proteins and an enterococcal polysaccharide to develop a vaccine with broader strain coverage. Diheteroglycan was conjugated to these two enterococcal proteins. Rabbit sera raised against these glycoconjugates showed IgG titers towards the corresponding conjugate, as well as to the respective protein and carbohydrate antigens. Effective opsonophagocytic killing for the two sera was observed against different E. faecalis and E. faecium strains. ELISA against whole bacterial cells showed immune recognition of the sera towards 22 enterococcal strains. Moreover, sera conferred protection in a mouse infection model against E. faecalis and E. faecium strains. Our results suggest that these glycoconjugates are promising candidates for vaccine formulations with a broader coverage against these nosocomial pathogens and that the evaluated proteins are potential carrier proteins.