The binding capacity of epratuzumab on different PBMCs obtained from SLE patients. (a) FACS analyses were performed on PBMCs from SLE patients using PE- labeled epratuzumab. Representative histogram of the differential binding of epratuzumab on T-cells (CD3positive, dotted line), monocytes (CD14positive, black histogram) and B-cells (CD19positive, black line). (b) PBMCs were incubated with (grey histogram) or without (black line) unlabelled F(ab')2 epratuzumab fragment for 10 minutes at 4°C. PBMCs were then stained with PE labeled-epratuzumab, and epratuzumab binding analyzed on B-cells, T-cells and monocytes (n = 3). Representative histogram of epratuzumab binding on B-cell sub-populations: CD27negative B-cells (black line), CD27positive B-cells (grey histogram) and T-cells (negative control, dotted line) are shown in (c). The results of the FACS analysis (right graph), showed higher binding capacity of epratuzumab on CD27negative B-cells compare to CD27positive B-cells (P = 0.0002). (d) To study the expression of CD22 on B-cells, PBMCs were stained with a mouse anti-CD22 mAb (Clone S-HCL-1), which recognizes a different epitope than epratuzumab (n = 5) . The FACS analysis demonstrated that CD22 is more highly expressed on CD27negative B-cells compared to CD27positive B-cells.