Stimulation of primary monocytes and Ba/F3-FcγRI cells results in enhanced IC binding. Percentage of Ba/F3-FcγRI cells binding RBCs opsonized with IgG2a (A-C) or human IgG3 (D-E). Open squares represent starved cells, filled squares represent starved Ba/F3-FcγRI cells stimulated with IL-3 for 1 hour. (B) EA rosette assay in the presence of FcγRI-specific mAb 10.1 (10.1) or isotype control (iso). **P < .01; NS: not significant, t test. (C) Effect of duration of IL-3 stimulus on IC binding. *P < .05; **P < .01, t test. (D) Binding of 1 or more RBCs to Ba/F3 cells. (E) Binding of 2 or more RBCs to Ba/F3 cells. Datapoints represent the means of at least 3 replicates. Similar results were obtained in 3 independent experiments. ***P < .001. (F) Binding of Ba/F3-FcγRI cells to BSA or IgG1-coupled beads in the presence of 10.1 mAb (10.1) or isotype control (ISO). Similar results were obtained in 3 independent experiments. **P < .01, t test. (G) Rosetting of BSA and IgG1-coupled beads with Ba/F3-FcγRI cells visualized with fluorescence microscopy. Beads are detected in the red channel, Ba/F3-FcγRI in the green channel. (H) Binding of unopsonized (unopson) and opsonized (opson) erythrocytes to primary monocytes in the presence of 10.1mAb (10.1) or isotype control (iso). Data represent means of triplicates measured from 1 donor. Similar results were seen in 3 of 4 donors. *P < .05; **P < .01. (I) FcγRI (CD64) and CD11b expression on CD14⁺ gated monocytes. Shown is the fold increase in surface expression relative to unstimulated (medium) cells. Data represent mean from triplicates.