Yellow green neutravidin fluorescent beads

This assay was described previously.²⁶ (link) Briefly, biotinylated SARS-CoV-2 Delta spike trimer (ECD; SinoBiological, Beijing, China, cat. no. 40589-V08H10) was incubated with yellow-green neutravidin-fluorescent beads (Molecular Probes, Eugene, OR, USA, cat. no. F8776) for 2 h at 37 °C. 10 μl of a 100-fold dilution of beads–protein was incubated 2 h at 37 °C with 100 μl of diluted plasma (900-fold) before addition of 25,000 cells per well THP-1 cells (RRID:CVCL_0006; Millipore Sigma, Burlington, MA, USA, cat. no. 88081201). After 19 h incubation at 37 °C, the cells were fixed with 2% formaldehyde solution (Tousimis, Rockville MD USA, cat. no. 1008B) and fluorescence was evaluated on a LSRII (BD Bioscience). The phagocytic score was calculated by multiplying the percentage of bead-positive cells by the geometric mean fluorescence intensity (MFI) of the bead-positive cells and dividing by 10⁴.