Characterization of lineage-negative blast subpopulations derived from normal and chronic myelogenous leukemia bone marrows and determination of their responsiveness to human c-kit ligand.

Lineage-negative (lin-) normal and chronic myelogenous leukemia (CML) marrow blast populations were obtained by negative selection and subsequently separated on the basis of size by velocity sedimentation. The three subpopulations of lin- blasts obtained were enriched for F8 (the more primitive small blasts), F11 (blasts intermediate in size), and F13 (the more mature large blasts). We examined the morphological and phenotypic characteristics and cell cycle status of the subpopulations and determined the responsiveness of granulocyte-monocyte progenitors (colony-forming units/granulocyte-macrophage) derived from each subpopulation to mast cell growth factor in combination with granulocyte (G-CSF) or granulocyte-macrophage (GM-CSF) colony-stimulating factors alone and in combination. Morphological assessment revealed that an increased proportion of CML lin- blasts exhibited early cytoplasmic maturation as evidenced by the appearance of azurophilic (nonspecific) granules in the cytoplasm. Although the percentages of CML and normal small blasts expressing CD34 were similar, the proportion of CML lin- blasts expressing CD34 declined in the intermediate and more mature large lin- blast subpopulations by about 50%, whereas the percentage of CD34+ normal blasts remained essentially the same, indicating an earlier loss of CD34 expression by CML lin- blasts. In addition, the percentages of CML small blasts expressing CD33 were higher than normal (26-61% versus 0-16%, respectively), indicating that a higher proportion of CML small lin- blasts had a more mature phenotype. Mast cell growth factor addition to cultures stimulated by G-CSF, GM-CSF, or G-CSF plus GM-CSF, exerted the greatest synergistic effect (increased colony number and size) in the normal small and intermediate lin- blast cultures, but mast cell growth factor had considerably less effect, or no effect, in cultures of comparable CML subpopulations, indicating that CML lin- progenitors had a somewhat lower requirement for multiple growth factors. The findings suggest that the differences observed between normal and CML marrow subpopulations are proportional differences and that a greater proportion of CML lin- blast subpopulations exhibit characteristics associated with a more advanced stage of maturation than comparable normal lin- blast subpopulations.