Selection of mouse macrophage-like sublines that differ in leukemogenic potential and characterization

The murine macrophage-like cell line (Mm-1), which is nonleukemogenic to syngeneic SL mice, was originally derived from spontaneously differentiated cells of a clonal line of mouse myeloid leukemia cells (M1). In the present experiment, variant cell lines with a high (Mm-A), moderate (Mm-P), and little or no (Mm-S1 and Mm-S2) leukemogenic potential were obtained from the Mm-1 cells. The mean survival times of syngeneic SL mice inoculated i.p. with 5 X 10(6) Mm-A and Mm-P cells were 17 and 33 days, respectively, whereas almost all the mice inoculated with Mm-S1 or Mm-S2 cells survived for more than 90 days. These variant cell lines did not lose their macrophage-like characteristics in vitro. These variant cell lines phagocytized latex beads and sensitized sheep erythrocytes, produced lysozyme, and adhered to culture dishes. The four variant cell lines showed no significant difference in proliferation rates in vitro in liquid medium containing 10% calf serum, but Mm-A cells could grow both in soft agar medium in the absence of ascitic fluid containing colony-stimulating factor (CSF) and in liquid medium containing 1% serum, whereas Mm-P cells could grow in the liquid medium but not in soft agar medium without ascitic fluid, and Mm-S1 and Mm-S2 cells could not grow in either medium. The ratio of the nuclear area to the cell area (NCR) of Mm-A cells was a high (51%) but those of Mm-S1 and Mm-S2 cells were low (40-41%), and that of Mm-P cells was intermediate (44%). The leukemogenicity of Mm-1 cell lines was roughly correlated with their NCR. The possibility that interactions between Mm-1 variant cells and host immune cells might be involved in the mechanisms of their different leukemogenicities was not supported by results on the in vitro susceptibilities of Mm-1 variant cells to the cytostatic actions by normal macrophages and spleen cells and on leukemogenicities of the Mm-1 variant cells in athymic nude mice. A possible method of control of the leukemogenicity of Mm-1 variant cells is discussed.