DIPHTHERIA-TOXIN FUSED TO GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR ELIMINATES ACUTE MYELOID-LEUKEMIA CELLS WITH THE POTENTIAL TO INITIATE LEUKEMIA IN IMMUNODEFICIENT MICE, BUT SPARES NORMAL HEMATOPOIETIC STEM-CELLS

We studied the cell kill induced by granulocyte-macrophage colony-stim ulating factor (GM-CSF) fused to Diphtheria Toxin (DT-GM-CSF) in acute myeloid leukemia (AML) samples and in populations of normal primitive hemopoietic progenitor cells. AML samples from three patients were in cubated in vitro with 100 ng/mL DT-GM-CSF for 48 hours, and AML cell k ill was determined in a proliferation assay, a clonogenic assay colony -forming unit-AML (CFU-AML) and a quantitative long-term bone marrow ( BM) culture ie, the leukemic-cobblestone area forming cell assay (L-CA FC). To measure an effect on cells with int vivo leukemia initiating p otential DT-GM-CSF exposed AML cells were transplanted into immunodefi cient mice. In two out of three samples it was shown that all AML subs ets, including those with long-term abilities in vivo (severe combined immunodeficient mice) and in vitro (L-CAFC assay) were reduced in num ber by DT-GM-CSF. Cell kill induced by DT-GM-CSF could be prevented by coincubation with an excess of GM-CSF, demonstrating that sensitivity to DT-GMCSF is specifically mediated by the GM-CSF receptor. Therefor e, binding and internalization of GM-CSF probably occur in immature AM L precursors of these two cases of AML, The third AML sample was not r esponsive to either GM-CSF or DT-GM-CSF. The number of committed proge nitors of normal bone marrow (burst-forming unit-erythroid, colony-for ming unit granulocyte-macrophage, and cobble stone area forming cell [ CAFC] week 2) and also the number of cells with long-term repopulating ability, assayed as week 6 CAFC, were unchanged after exposure to DT- GM-CSF (100 ng/mL, 48 hours). These studies show that DT-GM-CSF may be used to eliminate myeloid leukemic cells with long-term potential in vitro and in immunodeficient mice, whereas normal hemopoietic stem cel ls are spared. (C) 1997 by The American Society of Hematology.