Single-dose immunisation with a multimerised SARS-CoV-2 receptor binding domain (RBD) induces an enhanced and protective response in mice

The COVID‐19 pandemic, caused by the SARS‐CoV‐2 coronavirus, has triggered a worldwide health emergency. Here, we show that ferritin‐like Dps from hyperthermophilic Sulfolobus islandicus, covalently coupled with SARS‐CoV‐2 antigens via the SpyCatcher system, forms stable multivalent dodecameric vaccine nanoparticles that remain intact even after lyophilisation. Immunisation experiments in mice demonstrated that the SARS‐CoV‐2 receptor binding domain (RBD) coupled to Dps (RBD‐S‐Dps) elicited a higher antibody titre and an enhanced neutralising antibody response compared to monomeric RBD. A single immunisation with RBD‐S‐Dps completely protected hACE2‐expressing mice from serious illness and led to viral clearance from the lungs upon SARS‐CoV‐2 infection. Our data highlight that multimerised SARS‐CoV‐2 subunit vaccines are a highly efficacious modality, particularly when combined with an ultra‐stable scaffold.
Preparation and quality control of coupled antigen–Dps complexes (Ag‐S‐Dps). (A) SDS/PAGE of the three expressed and purified antigens as introduced in Fig. 1C, Coomassie stained. Glycosylation of Spike leads to a fuzzy appearance of its band. RBD‐SpyT2 and Spike‐SpyT2 were expressed in mammalian cells, and SpyT2‐NP was expressed in bacteria, as was the SpyC‐Dps scaffold. (B) Size exclusion chromatography to separate excess antigens after the SpyCatcher/Spytag2 coupling reactions; Superose 6 Increase in PBS. (C) SDS/PAGE of the coupled and purified Ag‐S‐Dps complexes. ‘RT’, no heating; ‘99’, heated to 99 °C. The SpyC‐Dps scaffold alone, as well as all the three coupled complexes show high‐molecular weight complexes, presumably dodecameric, that disappear only after heating of the samples in SDS loading buffer (Coomassie stained). (D) Negative‐stain electron microscopy analyses of the three multimeric Ag‐S‐Dps complexes, showing that all samples form defined and monodisperse spheres that display the antigens on their surface, leading to particles of different sizes for the three differently sized antigens.