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.