Figure 3.

CD21 expression on stable DG75 cells driven by CMV promoter is down-regulated over time. DG75 cells were stably transfected with the pcDNA3.1(+)-CD21 plasmid, and 30 d following transfection, CD21+ cells were selected by binding to anti-human CD21 IgG antibody followed by purification with anti-mouse IgG Dynabeads. The purified CD21+ DG75 cells were allowed to grow for 3–5 d and then negative selection was performed to remove the bead-bound cells. The proportion of CD21+ DG75 cells was examined by FACS staining at 7, 14, 34, and 42 d after selection, and is marked on each chart. The black line represents cells stained with anti-CD21 antibody coupled with the secondary fluorescent antibody and the light grey shading shows unstained cells. The experiment was performed in duplicate. The secondary only antibody control for DG75 cells was ignored as it was shown to be no background on DG75 cells.