Human cytomegalovirus (CMV) is associated with significant disease burden, particularly in congenital infections, the immunocompromised and transplant recipients. Currently there is no licenced vaccine for human CMV and the development of an effective vaccine has been recognised as highest priority by the US National Institute of Medicine. Previous studies with a subunit vaccine based CMV-encoded glycoprotein B (gB) in combination with MF59 adjuvant showed up to 50% efficacy in preventing acquisition of the viral infection. To further enhance potential efficacy of this vaccine, we have developed a novel bivalent vaccine formulation that includes an engineered recombinant gB protein, polyepitope protein and TLR9 agonist (CpG1018). Our preclinical mouse models demonstrate this formulation generates robust CMV-specific neutralising antibodies and CD4+ and CD8+ T cell responses.
Utilising cryogenic electron microscopy (CryoEM), in combination with cutting-edge AI algorithms and functional immune analysis, we have resolved the structure of engineered recombinant gB protein to a resolution of 3.9 Å. CryoEM structure of this protein revealed a trimeric molecule resembling a post-fusion conformation, with exposed surface in domain I (DI), which includes fusion loops, critical for viral entry. These fusion loops are concealed by the membrane proximal region (MPR) in previously published structures of pre-fusion and post-fusion forms of gB. As our engineered gB protein lacks MPR and transmembrane domain, these fusion loops are no longer concealed and allows more efficient immune recognition. Indeed, a fine mapping of gB-specific immune response showed broad recognition of immunogenic epitopes recognized by B cells, CD4+ and CD8+ T cells from HLA transgenic mice immunised CMV vaccine and healthy seropositive individuals.
Together these observations provides insight into the molecular mechanisms underpinning the protective responses generated by bivalent CMV vaccine formulation based on a trimeric post-fusion gB protein.