RTX mediates down-modulation of surface CD20. Animals were inoculated with primary cells from patient 24 (top) or patient 15 (bottom), and after 21 or 26 days, respectively, animals received a single dose of 500 μg RTX or saline only (n = 4 for each case). (A) Leukemic cell content in peripheral blood (PB), spleen (SP), and bone marrow (BM) 24 hours after single-dose RTX. Data are mean ±SD of all measurements. (B) Flow cytometric analysis of cells recovered from BM. Cells recovered from treated animals moderately stained with PE-conjugated GaH, indicating that low levels of RTX were present on the surface (light gray histograms). Additional incubation of the cells with RTX before GaH staining did not increase fluorescence, demonstrating absence of free CD20 epitopes on the surface and hence loss of surface C20 (dark gray histograms). Cells recovered from control animals stained with GaH only after preincubation with RTX, illustrating normal expression of CD20. Representative histograms are shown.