Molecular evidence for a common leukaemic progenitor in acute mixed lymphoid and myeloid leukaemia

Age-related differences in the bone marrow stem cell niche generate specialized microenvironments for the distinct regulation of normal hematopoietic and leukemia stem cells

The bone marrow (BM) microenvironment serves as a stem cell niche regulating the in vivo cell fate of normal hematopoietic stem cells (HSC) as well as leukemia stem cells (LSCs). Accumulating studies have indicated that the regeneration of normal HSCs and the process of leukemogenesis change with advancing age. However, the role of microenvironmental factors in these age-related effects are unclear. Here, we compared the stem cell niche in neonatal and adult BM to investigate potential differences in their microenvironmental regulation of both normal and leukemic stem cells. We found that the mesenchymal niche in neonatal BM, compared to adult BM, was characterized by a higher frequency of primitive subsets of mesenchymal stroma expressing both platelet-derived growth factor receptor and Sca-1, and higher expression levels of the niche cross-talk molecules, Jagged-1 and CXCL-12. Accordingly, normal HSCs transplanted into neonatal mice exhibited higher levels of regeneration in BM, with no difference in homing efficiency or splenic engraftment compared to adult BM. In contrast, in vivo self-renewal of LSCs was higher in adult BM than in neonatal BM, with increased frequencies of leukemia-initiating cells as well as higher lympho-myeloid differentiation potential towards biphenotypic leukemic cells. These differences in LSC self-renewal capacity between neonates and adults was abrogated by switching of recipients, confirming their microenvironmental origin. Our study provides insight into the differences in leukemic diseases observed in childhood and adults, and is important for interpretation of many transplantation studies involving neonatal animal models.

Characterization of genetic instability in radiation- and benzene-induced murine acute leukemia

This study, using the CBA/Ca mouse as a model, compares genetic lesions associated with radiation- and benzene-induced acute leukemias. Specific types of leukemia included in the analyses are radiation-induced acute myeloid leukemia (ML), and benzene-induced lymphoblastic leukemias, lymphomas, or mix-lineage leukemias. These leukemias have histopathological characteristics similar to those seen in human acute leukemias. G-band cytogenetic analysis showed that specific deletions involving regions D-E of one copy of mouse chromosome 2 [del(2)(D-E)] were frequently associated in both radiation- and benzene-induced acute leukemias. In addition, translocations of chr2(D-E) were also observed in some cases. These results suggest an important role of chr2 (D-E) deletions and translocations in the development of radiation- and benzene-induced murine acute leukemias. Fluorescence in situ hybridization with DNA probes specific for 2(D-E), constructed in our laboratory by means of chromosomal microdissection and PCR amplification, also demonstrate 2(D-E) deletions and/or translocations in these leukemic cells. Aneuploidy of chromosomes 3, 15, 16, and Y were also frequently detected in benzene-induced leukemic cells with or without lesions on chr2. These cytogenetic findings support the previous observations that metabolites of benzene lead to spindle-fiber disruption or abnormal cytokinesis in exposed animals. In summary, genetic instabilities observed in leukemic cells isolated from mice that had developed leukemia after exposure to radiation or benzene are syntenic with those frequently detected in patients with myelodysplastic syndrome, acute ML, and acute lymphoblastic leukemia. Thus, the CBA/Ca mouse has several characteristics that make it an excellent model for the study of radiation or benzene leukemogenesis in humans.

Emergence of T Cell Progenitors Without B Cell or Myeloid Differentiation Potential at the Earliest Stage of Hematopoiesis in the Murine Fetal Liver

It has been unclear whether the progenitors colonizing the thymus are multipotent or T cell lineage restricted. We investigated the developmental potential of hematopoietic progenitors in various populations of liver and blood cells from day 12 fetuses using the recently established in vitro experimental system effective in determining the capability of individual progenitors to generate T, B, and myeloid cells. Multipotent progenitors (p-Multi) were exclusively found in the Sca-1 high-positive (Sca-1high) subpopulation of lineage marker (Lin)−c-kit+CD45+ fetal liver cells. Restriction of developmental capacity begins at the Sca-1high stage, and a large majority of progenitors in the Sca-1low or Sca-1− population are restricted to generate T, B, or myeloid cells. Such a lineage commitment or restriction taking place in the fetal liver is independent of the thymus, because no difference in the proportion of different types of progenitors were seen between nu/nu and nu/+ fetuses. T cell lineage-restricted progenitors (p-T) were abundant in the blood of day 12 fetuses, whereas p-Multi were undetectable. It was further shown that the p-Multi generated a large number of B and myeloid cells in the thymic lobe. These results strongly suggest that it is p-T but not p-Multi that migrate into the thymus.

Most Acute Myeloid Leukemia Progenitor Cells With Long-Term Proliferative Ability In Vitro and In Vivo Have the Phenotype CD34+/CD71−/HLA-DR−

Acute myeloid leukemia (AML) occurs as the result of malignant transformation in a hematopoietic progenitor cell, which proliferates to form an accumulation of AML blasts. Only a minority of these AML cells are capable of proliferation in vitro, suggesting that AML cells may be organized in a hierarchy, with only the most primitive of these cells capable of maintaining the leukemic clone. To further investigate this hypothesis, we have evaluated a strategy for purifying these primitive cells based on surface antigen expression. As an in vitro endpoint, we have determined the phenotype of AML progenitor cells which are capable of producing AML colony-forming cells (CFU) for up to 8 weeks in suspension culture (SC) and compared the phenotype with that of cells which reproduce AML in nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice. AML cells were fluorescence-activated cell sorted (FACS) for coexpression of CD34 and CD71, CD38, and/or HLA-DR and the subfractions were assayed in vitro and in vivo at various cell doses to estimate purification. While the majority of primary AML CFU lacked expression of CD34, most cells capable of producing CFU after 2 to 8 weeks in SC were CD34+/CD71−. HLA-DR expression was heterogeneous on cells producing CFU after 2 to 4 weeks. However, after 6 to 8 weeks in SC, the majority of CFU were derived from CD34+/HLA-DR− cells. Similarly, the majority of cells capable of long-term CFU production from SC were CD34+/CD38−. Most cells that were capable of engrafting NOD/SCID mice were also CD34+/CD71− and CD34+/HLA-DR−. Engraftment was not achieved with CD34+/CD71+ or HLA-DR+subfractions, however, in two patients, both the CD34+and CD34− subfractions were capable of engrafting the NOD/SCID mice. A three-color sorting strategy combining these antigens allowed approximately a 2-log purification of these NOD/SCID leukemia initiating cells, with engraftment achieved using as few as 400 cells in one experiment. Phenotyping studies suggest even higher purification could be achieved by combining lack of CD38 expression with the CD34+/CD71− or CD34+/HLA DR− phenotype. These results suggest that most AML cells capable of long-term proliferation in vitro and in vivo share the CD34+/CD71−/HLA-DR− phenotype with normal stem cells. Our data suggests that in this group of patients the leukemic transformation has occurred in a primitive progenitor, as defined by phenotype, with some degree of subsequent differentiation as defined by functional assays.

Most Acute Myeloid Leukemia Progenitor Cells With Long-Term Proliferative Ability In Vitro and In Vivo Have the Phenotype CD34+/CD71−/HLA-DR−

Acute myeloid leukemia (AML) occurs as the result of malignant transformation in a hematopoietic progenitor cell, which proliferates to form an accumulation of AML blasts. Only a minority of these AML cells are capable of proliferation in vitro, suggesting that AML cells may be organized in a hierarchy, with only the most primitive of these cells capable of maintaining the leukemic clone. To further investigate this hypothesis, we have evaluated a strategy for purifying these primitive cells based on surface antigen expression. As an in vitro endpoint, we have determined the phenotype of AML progenitor cells which are capable of producing AML colony-forming cells (CFU) for up to 8 weeks in suspension culture (SC) and compared the phenotype with that of cells which reproduce AML in nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice. AML cells were fluorescence-activated cell sorted (FACS) for coexpression of CD34 and CD71, CD38, and/or HLA-DR and the subfractions were assayed in vitro and in vivo at various cell doses to estimate purification. While the majority of primary AML CFU lacked expression of CD34, most cells capable of producing CFU after 2 to 8 weeks in SC were CD34+/CD71−. HLA-DR expression was heterogeneous on cells producing CFU after 2 to 4 weeks. However, after 6 to 8 weeks in SC, the majority of CFU were derived from CD34+/HLA-DR− cells. Similarly, the majority of cells capable of long-term CFU production from SC were CD34+/CD38−. Most cells that were capable of engrafting NOD/SCID mice were also CD34+/CD71− and CD34+/HLA-DR−. Engraftment was not achieved with CD34+/CD71+ or HLA-DR+subfractions, however, in two patients, both the CD34+and CD34− subfractions were capable of engrafting the NOD/SCID mice. A three-color sorting strategy combining these antigens allowed approximately a 2-log purification of these NOD/SCID leukemia initiating cells, with engraftment achieved using as few as 400 cells in one experiment. Phenotyping studies suggest even higher purification could be achieved by combining lack of CD38 expression with the CD34+/CD71− or CD34+/HLA DR− phenotype. These results suggest that most AML cells capable of long-term proliferation in vitro and in vivo share the CD34+/CD71−/HLA-DR− phenotype with normal stem cells. Our data suggests that in this group of patients the leukemic transformation has occurred in a primitive progenitor, as defined by phenotype, with some degree of subsequent differentiation as defined by functional assays.