Expression and activity of FcγRIIb1 and b2 isoforms in regulating the rate of internalization of anti-CD20 mAbs. (A) mRNA from CLL and MCL samples alongside Ramos-FcγRIIb1 or -FcγRIIb2 transfectants as controls was converted to cDNA and analyzed by PCR for the expression of FcγRIIb1 and FcγRIIb2. (B) Ramos cells were transfected with empty vector, FcγRIIb2, or FcγRIIb1 and stable transfectants selected expressing different levels of FcγRIIb. Control cells (filled histogram), FcγRIIb2 low (solid black line), FcγRIIb1 low (solid gray line), FcγRIIb1 medium (dotted line), and FcγRIIb1 high (dashed line) were labeled with AT10-PE and assessed by flow cytometry. (C) Ramos transfectants were cultured with 5 µg/mL A488-labeled anti-CD20 mAb for 1 hour. The proportion of total mAb remaining on the cell surface was assessed by flow cytometry after treatment of cells with anti-A488 to quench cell-surface fluorescence. Transfectants were compared with control cells using the Mann-Whitney U test. **P < .01, ***P < .001, n = 6-8. Horizontal bars represent the median. mRNA, messenger RNA.