Isolation and characterization of HL-60 cell variants with different potentials for spontaneous differentiation

Although HL-60 cells, an in vitro established cell line derived from a patient with acute promyelocytic leukemia, are blocked at the promyelocytic stage of myeloid differentiation, certain chemicals can induce the cells to undergo terminal differentiation into either granulocytes or macrophages. Moreover, a small fraction of the cell population undergoes differentiation spontaneously without the addition of any inducing agent. In this paper it is demonstrated that this cell line is heterogeneous with respect to the ability of the cells to differentiate spontaneously: some clones (SD+) have a higher tendency to do so than others (SD-). In semi-solid medium, SD+ cells developed diffused colonies containing mature monocytes and macrophages, whereas SD- cells developed compact colonies of promyelocytes. Based on these morphological differences the various clones were isolated and analysed. Although only a small fraction of the population actually became differentiated at any particular time, practically all the cells in the SD+ clones had the potential to differentiate spontaneously. The clones also differ in their response to differentiation inducers; whereas some agents induced complete differentiation in both types of clones, others (e.g. actinomycin C and cytosine arabinoside) induced only SD+ clones, suggesting that differentiation induced by the latter agents is related to the ability of the cells to differentiate spontaneously. Thus the potential of leukemic cells to undergo spontaneous differentiation may be an important factor when considering differentiation-inducing therapy for leukemic patients.