Phagocytic Activity of SARS-CoV-2 Antibodies

Characterization of the phagocytic activity of serum antibodies was performed^{80 (link)}. Briefly, 1 μM yellow-green fluorescent beads (Thermo Fisher, F8813) were covalently conjugated to spike or RBD antigens. Beads were then incubated with serum samples for 4 h with THP-1 cells (ATCC TIB-202) at 37 °C in 5% CO₂. Cells were fixed and analyzed by flow cytometry (Supplemental Fig. 6) using a MACSQuant Analyzer (Miltenyi Biotec). Scores were calculated as the percentage of cells that phagocytosed one or more fluorescent beads multiplied by the MFI of this population. S309 and VRC01 antibodies were included as positive and negative controls, respectively. Additional control wells with no added antibody were used to determine the level of antibody-independent phagocytosis. Serum samples were assayed at three different dilutions, which were determined by an initial pilot experiment to determine the optimal dilution series for measuring signal compared to the background. Concentrated pooled polyclonal serum IgG (Sigma Aldrich I4506) was used as a positive control for endemic CoV (Supplemental Fig. 7), and samples were run in three biological replicates.