Fig. 1.

Immunogens used in this study. A Protein constructs used to produce immunogens. HexaPro spike is a prefusion-stabilized trimeric spike construct containing six proline mutations and a loss-of-function mutation in the furin-cleavage site²⁶. RBD: CoV-2 S protein receptor binding domain. HRV 3 C site: HRV 3 C protease cleavage site. mSA2: mono-streptavidin sequence. B Conceptual design (cartoon) and representative electron micrographs of negative-stained prepared immunogens (bottom row). The red scale bar indicates 50 nm. Full micrographs of all immunogens, including that for poorly folded ferritin-RBD, can be found in Supplementary Figs. 1 and 2. Liposome: spike nanoparticle was prepared by mixture of 8xHis-tagged spike protein (A, spike-His) with DSPC (1,2-distearoyl-sn-glycero-3-phosphocholine) liposomes intercalated with DGS-NTA(Ni²⁺)(1,2-dioleoyl-sn-glycero-3-[(N-(5-amino-1-carboxypentyl)iminodiacetic acid)succinyl] nickel salt). C Summary of results of animal inoculation experiments for selected immunogens. Check marks (✓) and ×’s denote that an experiment was carried out successfully or unsuccessfully, respectively. Plus sign (+) and minus sign (-) indicate the inclusion or exclusion of adjuvant (AddaVax™), respectively