An effective,direct immunomagnetic procedure for purging acute lymphoblastic leukemia cells from human bone marrow

The AB4 monoclonal antibody, which recognizes an HLA-DR epitope, was found to bind to a high percentage of malignant blast cells in samples obtained from 27 patients with ALL. These included 11 of 11 cases with c-ALL, 3 of 7 with pre-pre-B, and 8 of 9 cases with pre-B ALL. AB4 was used together with anti CD10 and anti CD19 antibodies and super-paramagnetic particles for developing a direct immunomagnetic procedure for purging human bone marrow of leukemic cells. In model experiments with KM3 cells admixed to mononuclear bone marrow cells, the individual antibodies each removed 2.8–3.1 logs and 3.6–4.1 logs of tumor cells with one and two purging cycles, respectively. In comparison, the efficacy of a mixture of the three antibodies was 4.4 logs with one treatment cycle, and > 5 logs with repeated treatments. Whereas the use of a commercially available anti-HLA-DR antibody resulted in a 90% reduction in the survival of CFU-GMs and normal blast colonies, AB4 had only a moderate effect on the progenitor cells (46% and 30% reduction). In conjunction with autologous transplantation, bone marrow from a patient was purged with the antibody mixture and 50% of the CFU-GMs and 47% of the CD34⁺ cells remained after treatment. The patient showed a normal engraftment, reaching a level of 0.5 × 1071 neutrophils by day 20 and 20 × 107 9/1 platelets by day 30. It is concluded that the antibody cocktail may safely and effectively be used for rapid autograft purging in patients with c-ALL, and also in phenotypically selected cases with other subtypes of ALL. This work was supported by the Norwegian Cancer Society