Genes encoding VHH-human apoferritin fusion, Fc fusions, Fabs, IgG, and RBD mutants were synthesized and cloned into the pcDNA3.4 expression vector. All constructs were expressed transiently in HEK 293F at 0.8 × 10^6 cells/mL with 50 μg of DNA per 200 mL of cells using FectoPRO in a 1:1 ratio unless specified otherwise. After 6–7 days of incubation, cell suspensions were harvested by centrifugation and supernatants were filtered. Fabs and IgGs were transiently expressed by co-transfecting 90 μg of the LC and the HC in a 1:2 ratio and purified using KappaSelect affinity column and HiTrap Protein A HP column. Fc fusions of ACE2 and VHH-72 were purified the same way as IgGs. Wild type and mutant RBDs, the prefusion S ectodomain and Fc receptors from mouse and human were purified using a HisTrap Ni-NTA column. Transient transfection of the Multabodies in HEK-293F cells were obtained by mixing 66 μg of the plasmids in different ratio. The DNA mixture was filtered and incubated at room temperature (RT) with 66 μl of FectoPRO before adding to the cell culture. Split Multabodies were purified by affinity chromatography using a HiTrap Protein A HP column.

SARS-CoV-2, the virus responsible for COVID-19, has caused a global pandemic. Antibodies can be powerful biotherapeutics to fight viral infections. Here, we use the human apoferritin protomer as a modular subunit to drive oligomerization of antibody fragments and transform antibodies targeting SARS-CoV-2 into exceptionally potent neutralizers. Using this platform, half-maximal inhibitory concentration (IC₅₀) values as low as 9 × 10^-14 M are achieved as a result of up to 10,000-fold potency enhancements compared to corresponding IgGs. Combination of three different antibody specificities and the fragment crystallizable (Fc) domain on a single multivalent molecule conferred the ability to overcome viral sequence variability together with outstanding potency and IgG-like bioavailability. The MULTi-specific, multi-Affinity antiBODY (Multabody or MB) platform thus uniquely leverages binding avidity together with multi-specificity to deliver ultrapotent and broad neutralizers against SARS-CoV-2. The modularity of the platform also makes it relevant for rapid evaluation against other infectious diseases of global health importance. Neutralizing antibodies are a promising therapeutic for SARS-CoV-2.