Establishing Rubella virus vaccine platforms for protection against priority emerging viruses employing class II fusion protein mediated entry

The updated WHO priority list of pathogens is due for release in early 2024 and will no doubt include a range of arthropod-borne (arbo-) viruses. Arboviruses encompass a diverse range of viral threats, including flaviviruses (e.g., dengue, yellow fever, West Nile, Japanese encephalitis, and Zika), alphaviruses (e.g., chikungunya virus), and bunyaviruses (e.g., Rift Valley fever virus, Crimean-Congo haemorrhagic fever virus). These viruses are widely distributed and, with an expanding vector range, international trade, and travel, have the potential for rapid global dissemination. Importantly many of these viruses utilise class II fusion protein-mediated entry.

Measles (MV) and rubella virus (RubV) vaccines have proven safety, efficacy, and manufacturability, yet their potential as vaccine platforms is generally overlooked, due to concerns about poor immunogenicity resulting from pre-existing host immunity following vaccination or natural infection. However, recent studies have suggested that this concern is misplaced. For example, a measles-vector vaccine for chikungunya virus (now in phase III) has been shown to be highly immunogenic, with negligible impact from pre-existing immunity to measles [https://doi.org/10.1016/S0140-6736(18)32488-7].

We developed vectors to allow efficient production and release of RubV virus-like particles (VLPs), whereby the ectodomains of RubV upstream and downstream glycoproteins (E2E1) are replaced with the respective glycoprotein ectodomains from an unrelated virus that utilises class II fusion protein-entry (in this case HCV). Following optimisation of the fusion location and approach, we demonstrated efficient assembly and release of VLPs from mammalian cells and showed that HCV glycoprotein conformation and antigenicity were maintained.

Maintenance of GP conformation and antigenic landscape in class II fusion glycoprotein vaccine candidates is a major challenge. These glycoproteins mature and are retained on internal membranes, limiting their immunogenicity to B cells. To overcome this, soluble forms of one of the GP partners can be generated, but loss of the partner GP compromises immunogenicity. One advantage of RubV over other platforms is the ability of the RubV chimera to package native conformationally-correct vaccine-target GP ectodomains (due to shared GP maturation/assembly processes on internal membranes). Also, replacement of the entire RubV GP ectodomains with vaccine-target GP ectodomains overcomes potential issues with existing neutralising antibody-mediated anti-RubV immunity.

We, therefore, want to explore if this chimeric approach can be utilised more widely for viruses utilising a class II fusion-mediated entry mechanism (e.g., flaviviruses and bunyaviruses). Given the proven safety profile of RubV vaccines, especially in children, we wish to extend our pilot studies to explore the feasibility of generating fully infectious RubV chimeras. To explore the platform potential, we will focus on two viruses: Zika and Nairobi Sheep Virus. These targets have been chosen as ZIKV has a classical, whilst NSV a non-classical (complex), class II fusion protein that mediates entry. In addition, both are low bio-risk agents enabling us to explore chimeric virus generation safely and rapidly.

A rubella virus vaccine platform would represent a safe, effective means to generate vaccine candidates for a large range of emerging virus threats and has the potential to elicit superior protective immune responses due to the assembly and release of VLP or virus chimeras harbouring class II fusion glycoprotein partners in a more native conformation than is typically possible with many other vaccine approaches.