Expression of recombinant S protein constructs The receptor-binding domain of the SARS-CoV-2 spike protein (amino acids 319-541) was produced in three cell lines using an expression vector obtained through BEI Resources, NIAID, NIH, which is vector pCAGGS containing the SARS-CoV-2, Wuhan-Hu-1 spike glycoprotein gene RBD with C-terminal Hexa-Histidine tag (NR-52309). The HEK293 derivate HEK293F cell line (Cat nr R79007, Thermo Fisher Scientific) were cultured in Freestyle 293 medium. Cells were transfected at 2 × 106 cells/mL using FectoPro transfection reagent (Polyplus transfection). Transfected HEK293F cells were kept at 37 °C. Protein-containing culture supernatants (800 mL – 1 L) were harvested when cell viability was below 80 %, which was after 90 hours (HEK293F), filtered using Polydisc AS 0.45 μm (Whatman, Maidstone, UK) and loaded onto a 5 mL HisExcel column (Cytiva, Marlborough, MA). After sample loading, the column was washed with 20 mM sodium phosphate, 0.5 M NaCl and 30 mM imidazole before elution of the protein using the same buffer with 300 mM imidazole (HEK293F-produced RBD). Pooled fractions were concentrated using 10 kDa Vivaspin concentrators (MWCO 10 kDa, Sartorius, Göttingen, Germany), passed over a HiPrep 26/10 desalting column (Cytiva) in phosphate-buffered saline and finally concentrated again.