Effect of FcγRIIb1 on the rate of internalization of other mAb-ligated receptors. (A) Ramos control, FcγRIIb1-low, FcγRIIb1-medium, and FcγRIIb1-high transfectants were cultured with 5 µg/mL A488-labeled mAbs for 6 hours and the proportion of total mAb remaining on the cell surface assessed as in Figure 1. *P < .05, **P < .01. (B) Ramos FcγRIIb1 low transfectants or (C-D) primary CLL cells were incubated with 50 µg/mL AT10 for 30 minutes to block the Fc binding site of FcγRIIb, or left untreated, then cultured as in panel A and the proportion of surface mAb was assessed. AT10-treated cells were compared with untreated cells using the Wilcoxon signed-ranks test. *P < .05, **P < .01, ***P < .001; n = 6-12 (B) and 10 (C-D). Horizontal bars represent the median.