In vivo activity of ALM and selection of CD52⁻ GPI-defective cells. 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 ALM 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 did not stain with PE-conjugated GaH, indicating that ALM was not present on the surface (light gray histograms). Preincubation of the cells with ALM before GaH staining did not increase fluorescence, demonstrating the absence of CD52 on the surface (dark gray histograms). Cells recovered from control animals stained with GaH after preincubation with ALM, illustrating normal expression of CD52. Representative histograms are shown. (C) Cells recovered from the bone marrow of treated animals showed concurrent absence of CD52 and CD55 in both cases, illustrating a GPI-defective phenotype.