Effects of flt3 ligand on acute myeloid and lymphocytic leukemic blast cells from children.

A ligand for the flt3 tyrosine kinase receptor (flt3R) has recently been cloned. Forty-three cases of childhood acute myeloid leukemia (AML) and 27 cases of childhood acute lymphocytic leukemia (ALL) were examined by flow cytometric analysis for cell-surface flt3R and proliferative response in vitro to flt3 ligand (flt3L). Flt3R was commonly expressed on the cell surface of leukemic cells from all AML subclasses and B-ALL, but we did not detect cell-surface flt3R on T-ALL. Flt3L alone induced the proliferation of the monocytic AML-M5 cells and some erythroleukemic AML-M6 cells. Some isolated instances of weak proliferative responses were also noted in other AML subclasses. Interleukin-4 (IL-4) alone inhibited the proliferation of a group of AML-M5 cells and, when combined with flt3L, suppressed the proliferative effect of flt3L. In general, B-ALL and T-ALL cells failed to respond to flt3L alone or in the presence of combinations of IL-2, IL-3, or IL-7.