Mosaic receptor-binding domain nanoparticles induce protective immunity against SARS-CoV-2 challenges

Recurrent spillovers of - and {beta}-coronaviruses (CoV) such as acute respiratory syndrome (SARS)-CoV, Middle East respiratory syndrome (MERS)-CoV, SARS-CoV-2, and possibly human CoV (NL63, 229E, OC43, and HKU1) have caused serious morbidity and mortality worldwide. Six receptor binding domains (RBDs) derived from - and {beta}-CoV that are considered to have originated from animals and cross-infected humans were linked to proliferating cell nuclear antigen (PCNA) heterotrimeric subunits, PCNA1, PCNA2, and PCNA3. These were used to form a scaffold-based mosaic multivalent antigen, 6RBD-np. Electron microscopic and atomic force microscopic images show a ring-shaped disk with six protruding RBDs, like jewels in a crown, with a size of 40 nm. Prime-boost immunizations with 6RBD-np in BALB/c mice elicited strong, dose-dependent antibody responses. In human angiotensin converting enzyme 2-transgenic mice, the same immunization induced full-protection against SARS-CoV-2 wild type and Delta challenges, resulting in a 100% survival rate. The mosaic 6RBD-np provides a potential platform for developing a pan-CoV vaccine against newly emerging SARS-CoV-2 variants and future CoV spillovers. SignificanceDespite the arsenal of COVID-19 vaccines, hospitalization and mortality associated with SARS-CoV-2 (acute respiratory syndrome coronavirus 2) variants remain high. There is an urgent need to develop next-generation COVID vaccines that provide broad protection against diseases by current and newly emerging SARS-CoV-2 variants. In this study, six receptor binding domains (RBDs) derived from - and {beta}-CoV were linked to proliferating cell nuclear antigen (PCNA) heterotrimeric scaffolds. They assemble to create a stable mosaic multivalent nanoparticle, 6RBD-np, displaying a ring-shaped disk with six protruding antigens. The prime-boost immunization in BALB/c and human angiotensin converting enzyme 2-transgenic mice with the 6RBD-np elicited strong, dose-dependent antibody responses and induced full-protection against both the SARS-CoV-2 wild type (WT) and Delta challenges. This study provides proof-of-concept that the mosaic 6RBD-np induces 100% protection against SARS-CoV-2 WT and Delta. It provides the potential of co-displaying heterologous antigens for novel vaccine designs, which can be deployed countering future pandemics.