Expression of FcγRII in B cells from B-CLL patients and healthy donors. (A) CD19⁺ lymphocytes from peripheral blood were stained with anti-FcγRII (clone AT.10, purified mouse IgG1; generous gift from Dr M. Daeron, Institut Curie, Paris, France) and anti-mouse IgG fluorescein isothiocyanate (FITC), F(ab′)₂ fragments (Coulter-Immunotech, Marseille, France). Results are expressed as the mean fluorescence intensity (MFI) of FcγRII expression in CD19⁺ cells for each sample analyzed. MFI of control isotype ranged between 8 and 10 in all cases. (B) Representative histograms of FcγRII expression (white histograms) assessed by direct immunofluorescence analysis with anti-FcγRII (clone IV.3-FITC [whole molecule], Medarex, Annandale, NJ, or clone 2E1-PE, Coulter-Immunotech). Gray histograms indicate control isotype.