SYNERGISTIC ACTION OF INTERLEUKIN-2 AND STEEL FACTOR (SLF) ON A HUMAN T-LYMPHOBLASTOID CELL-LINE

Ten cell lines recently established from paediatric patients with acute lymphoblastic leukaemia (ALL) were examined for expression of P145(c-kit), the growth factor receptor encoded by the c-kit proto-oncogene, by immunofluorescence and flow cytometry using monoclonal antibody YB5.B8. Three of five T-ALL cell lines, but none of five B llneage ALL cell lines displayed significant binding of the antibody. The cell line with the highest level of binding was PER-423 (Kees et al, Leukemia Res 1993; 17: 51-59 which has the phenotype CD7(+), CD56(bright), CD2(-), CD4(-), CD5(-), CD8(-), CD16(-), has rearranged T cell receptor beta-chain genes, expresses cytoplasmic CD3 and is strictly dependent on interleukin 2 (IL-2) for proliferation. Recombinant human Steel factor (SLF), the ligand for P145(c-kit), was shown to act in synergy with IL-2 to promote proliferation of PER-423 cells. In five experiments, SLF increased the maximal amount of proliferation by 105 +/- 15%, and decreased the level of IL-2 required for a half-maximal response by 43 +/- 7%. The cells constitutively express the intermediate affinity IL-2 receptor (beta/gamma), but can be induced in the presence of phorbol ester to express the alpha chain (CD25, Tac) which confers high affinity binding of IL-2. In contrast, the alpha chain was not induced by SLF. The enhancement of proliferation of PER-423 cells by SLF could be prevented by inclusion in the assay of a blocking monoclonal antibody to P145(c-kit). These experiments demonstrate that SLF/P145(c-kit) can provide a significant growth stimulus for ALL cells, and PER-423 cells may be a useful system for investigating the mechanism of synergy between SLF and IL-2.