Analysis of the individual and combined reactivities of monoclonal antibodies H25, H366, and MY9 with normal progenitor cells and blast cells from patients with acute myeloblastic leukemia.

Recently we reported that two monoclonal antibodies (MoAbs), H25 and H366, which react with human natural killer cells and monocytes, also react with normal in vitro colony-forming cells including granulocyte-monocyte colony-forming units (CFU-GM), erythroid burst-forming units (BFU-E), and erythroid colony-forming units and with leukemic blasts in preliminary testing of cells from patients with myeloid leukemias and T cell acute lymphocytic leukemia. In the present studies we examined the reactivities of MoAbs H25, H366, and MY9 (singly or combined) with the total leukemic cell population and the leukemic clonogenic cells (L-CFC) from 28 patients with acute myeloblastic leukemia. Using cytofluorography, we found the extent of expression of antigen H25 comparable to MY9 in the majority of patients, and both were more highly expressed than antigen H366. Incubation with H25 and H366 MoAbs simultaneously did not increase the number of positive cells over that seen when stained with H25 alone; however, the amount of antibody fluorescence intensity (FI) was increased. Leukemic cells simultaneously stained with MoAbs H25, H366, and MY9 displayed the highest number of positive cells and FI. Using magnetic beads coated with sheep anti-mouse IgG for depleting antibody-binding cells, greater than or equal to 90% of L-CFC were depleted by a combination of H25 and H366 MoAbs in 76% of AML cases tested as compared to 41% of the cases with MoAb MY9. Using a MoAb cocktail of H25, H366, and MY9, greater than or equal to 90% of L-CFC were depleted in 94% of cases tested, and greater than or equal to 99% of L-CFC were removed in 76% of the cases. Using the same depletion methods for normal bone marrow cells, a combination of anti-H25 and anti-H366 removed 90%, 98%, and 84% of CFU-GM, BFU-E, and multipotent colony-forming units (CFU-GEM), respectively, whereas the cocktail of H25, H366, and MY9 MoAbs removed 98%, 99.5%, and 97% of CFU-GM, BFU-E, and CFU-GEM, respectively. Incubation of H25 and H366-depleted bone marrow cells for 2 weeks in the presence of irradiated adherent cell layers from long-term marrow cultures generated CFU-GM and some BFU-E, as did H25, H366, and MY9-depleted marrow cells, although to a much lesser extent. Based on the overall data, combinations of H25, H366, and MY9 MoAbs and immunomagnetic beads conceivably might have therapeutic potential for ex vivo elimination of leukemic cells from AML remission marrows prior to autologous transplantation.