In vivo activity of RTX and persistence of cells with a reversible CD20^low phenotype. Animals were inoculated with primary leukemic cells from patient 24 (top) or patient 15 (bottom). Starting from the time points indicated by the arrows, animals received 250 μg rituximab or saline daily for 5 days per week for a total of 3 weeks. For each case, 2 independent experiments were performed with 3 treated animals and 3 control animals per group. Data are mean ±SD of all measurements. (A) Leukemic cell counts in peripheral blood (PB) during treatment, and leukemic cell content of PB, spleen (SP), and bone marrow (BM) at the experimental endpoint. *P < .01 by 2-tailed Student t test. (B) Flow cytometric analysis of cells recovered from BM. Cells recovered from treated animals slightly stained with PE-conjugated GaH, indicating that low levels of RTX were present on the surface (light gray histograms). Preincubation 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. (C) Expression of CD20 was restored after passage through secondary recipients.