Long-term leukemia-initiating capacity of a CD34(-) subpopulation of acute myeloid leukemia

Acute myeloid leukemia (AML) proliferation in vivo is maintained by a small fraction of progenitor cells. These cells have been assumed to express an immature phenotype and to produce most colony-forming units (CFU-AML). For one case of AML (French-American-British [FAB] M1, normal cytogenetics), we examined the capacity of the CD34(+) (25% of unseparated AML cells) and CD34(-) fractions to initiate leukemia in severe combined immunodeficient (SCID) mice. In addition, the production of CFU-AML and nucleated cells (NC) of these subsets was investigated in long-term bone marrow culture (LTBMC). The frequencies of cobblestone area-forming cells (CAFC) were also estimated; early appearing cobblestone areas (CAs) are indicative of relatively mature progenitors and late CAs represent the progeny of primitive progenitors. In mice transplanted with CD34(-) (98% pure) or CD34(+) (98% pure) grafts, similar AML cell growth was seen throughout an observation period of 106 days. The capacity to establish long-term growth from the CD34(-) cells was confirmed by renewed outgrowth after retransplantation. In vitro, the CD34(-) fraction contained both immature and mature CAFCs and produced high numbers of CFU-AML and NC in LTBMC. The CD34(+) fraction produced only small numbers of CFU-AML, NC, and mature CAFCs. Therefore, the expression of CD34 and the content of CFU-AML were not associated with long-term growth of AML. However, similar frequencies of primitive CAFCs were observed in both fractions. Thus, both CD34(-) and CD34(+) subsets of this AML sample contained immature progenitors with the capacity to initiate long-term AML growth as characterized in vivo (in SCID mice) as well as in vitro (in CAFC assay), indicating asynchrony between functional and immunophenotypical maturation of AML progenitor cell compartments. (C) 1996 by The American Society of Hematology.