Background: Group A Streptococcus (GAS) causes 500 000 deaths with no available vaccine. Several GAS vaccines are currently developed based on a surface protein – the M protein. Recent epidemiological data from all around the world showed that the global coverage would not be optimal, mostly in low- and middle-income settings where the M diversity is higher than in high income settings. M-like proteins, Enn and Mrp, are present on 85% of GAS strains suggesting their biological relevance. Both M-like proteins are structurally like the M protein, composed of conserved C-terminus, central heptad-repeat regions and a hyper variable region (HVR) in N-terminus. 276 unique Enn proteins can be phylogenetically classified in biologically relevant clusters.
Methods: 10 most relevant Enn were chosen, one per cluster, and rabbits were immunized with 10 ENNHVR peptides to study their immunogenicity via ELISA. Cross-recognition between Enn from different clusters was tested. Finally, functionality of anti-Enn sera was assessed using a whole blood killing assay (WBKA) with a wide range of GAS strains.
Results: We observed high specific antibody titers against injected Enn proteins. Moreover, for some Enn proteins, we also observed an inter-cluster recognition. As well, our data showed an increased killing of GAS strains expressing Enn proteins in the presence of immunized sera in a phagocytosis-dependent manner. Most importantly, we also observe intra and inter cluster cross-protection using anti-Enn sera.
Conclusion: Enn proteins possess immunogenic and protective properties and should be further considered as complementary antigens in new vaccine formulation to reach global coverage.