Background/Aims: A human cytomegalovirus (HCMV) vaccine remains high priority status. Our ongoing studies of the humoral immune response to gB/MF59 vaccine have identified a novel antigenic domain (AD-6) in HCMV glycoprotein B (gB) which correlated with protection in a phase II study. Subsequently, we demonstrated that a rabbit pAb directed against AD-6 reduced HCMV cell-to-cell spread in fibroblasts as a potential mechanism of anti-viral activity and now we also demonstrate that the AD-6 antibody potently blocks HCMV reactivation – which is a likely source of viraemia in vivo further underpinning the importance of the AD-6 response.

Our overarching aim is to combine in silico and wet lab studies to comprehensively understand the clinical, immunological and functional importance of AD-6 within gB. First we utilised the high conservation of gB structure to identify analogues of AD-6 in other HHV gBs despite minimal sequence conservation. Intriguingly, molecular frustration analyses revealed that AD-6 within gB can exist in an energetically unfavourable state in the monomer driving trimerisation. In silico interrogation of HHV gBs predicts overlapping immunogenicity within AD-6 suggesting that conformational epitopes could be important immunologically. Consistent with conformation-specific antibodies being important, the pAb raised against HCMV AD-6 was also anti-viral against HSV-1 whereas a mAb raised against a linear (thus sequence dependent) epitope in HCMV AD-6, and active against HCMV, is unable to prevent the cell-to-cell spread of HSV-1. Importantly, we demonstrated that human AD-6 antibodies generated in response to gB/MF59 vaccination also recognise conformational epitopes by demonstrating a correlation between binding HCMV/HSV-1 AD-6 peptides for individual vaccine recipients.

We hypothesise AD-6 plays a key role in gB function which is inhibited, in part, by the binding of conformation-specific antibodies. Finally, it suggests the provocative concept that targeting cognate AD-6s could be a general strategy for HHV vaccination.