THP-1 phagocytosis of beads coated with PLL-conjugated polysaccharides was performed as previously described (53 (link)). Briefly, PLL-conjugated polysaccharides from 3, 6A, and 19A were biotinylated with 50-fold excess biotin with EZ-Link NHS long-chain biotin (Thermo Fisher Scientific) following the manufacturer’s instructions and then adsorbed onto 1-μm fluorescent neutravidin beads (Invitrogen) at a 1:1 (micrograms:microliters) ratio of biotinylated polysaccharide to beads. Ten microliters of a 1:100 suspension of antigen-coupled beads were added to each well of a 96-well plate along with equal volume of serum diluted 1:50, and plates were incubated for 2 hours at 37°C and then washed with PBS. A total of 25,000 THP-1 cells (human acute monocytic leukemia cell line, American Type Culture Collection, RRID:CVCL_0006) were added and incubated at 37°C for 18 to 20 hours. Cells were fixed with 4% paraformaldehyde solution (Santa Cruz Biotechnology) before data acquisition. Phagocytosis was measured by iQue Screener PLUS (Intellicyt). Phagocytic scores were calculated as (percent bead-positive cells) × (geometric mean fluorescence intensity)/10,000. Specificity of the assay was confirmed using control monoclonal antibodies targeted to each serotype. Each sample was assayed in two independent technical replicates and averaged.