Comparative analysis of a flexible novel vaccine platform based on cell permeable chimeric HBV capsids for mucosal vaccination

The recent SARS-CoV-2 pandemic has demonstrated the importance of vaccines in controlling outbreaks and in preparing for emerging viruses. Platform technologies are highly relevant as they reduce the time taken for vaccine development, market authorisation and production. Here, we present a novel platform based on modified, membrane-permeable hepatitis B virus (HBV) capsids that can flexibly carry various antigens via an adapter. The SARS-CoV2 spike-derived receptor binding domain serves as the antigen. This study aims to identify the most robust system for producing stable carrier/cargo complexes by comparing various platforms. The thereby identified system was used for detailed characterization of the immune response including its capacity for enabling needle free immunization by oral/nasal application. A comparison of various adapter systems (StrepTag, AviTag, SpyTag and DogTag) for coupling the cargo antigen to the cell-permeable capsid as the carrier revealed that the DogCatcher/DogTag system outperforms the others with regard to the yield, assembly and stability of the particles, as well as antigen loading onto the carrier surface. Immunization of mice showed that antigens coupled to the carrier induce a much stronger immune response than free, uncoupled antigens. Loading the capsid interior with CpG as an adjuvant further increased the immune response. Notably, the antigen carrier's cell permeability enables oral/nasal immunization, resulting in significant titres of neutralizing IgG and IgA antibodies. Taken together, this novel platform can be used as a flexible base for quickly adapting vaccines against emerging viruses. Most interestingly, the platform's cell permeability allows for needle-free immunization via the oral/nasal route.