Leukemic B-cell precursors express functional receptors for human interleukin-3

The purpose of this study was to analyze the expression of functional interleukin-3 (IL-3) receptors on leukemic B-cell precursors (BCPs) from 12 BCP acute lymphoblastic leukemia (ALL) patients and five BCP ALL cell lines. The specific binding of biosynthetically labeled 35S- recombinant (r) IL-3 to freshly obtained leukemic marrow blasts was initially investigated. In five of 12 BCP ALL cases, the binding of 35S- rIL-3 was markedly blocked by excess cold rIL-3, and the percentage of inhibitable binding ranged from 53% to 78% (mean +/- SE = 65% +/- 4%). In these cases, the cell-bound radioactivity ranged from 146 cpm/10(7) cells to 1,433 cpm/10(7) cells (mean +/- SE = 627 +/- 250 cpm/10(7) cells), indicating that 1 to 14 femtomole (mean +/- SE = 6 +/- 2 fms) of [35S]rIL-3/10(7) cells were specifically bound (= 60 to 840 molecules per cell). rIL-3 stimulated the proliferative activity of leukemic BCPs in a dose-dependent fashion without inducing differentiation, and the half-maximal stimulatory activity was observed at a concentration of 17 to 34 pmol/L. Fluorescence-activated cell sorter (FACS)-isolated virtually pure populations of CD10+CD19+ leukemic BCPs from two BCP ALL patients, as well as from two of five BCP ALL cell lines, showed a marked proliferative response to highly purified rIL-3, providing formal evidence that the observed IL-3 responses were not mediated by accessory cells. There was a high correlation between [35S]rIL-3 binding and proliferative response in colony assays, indicating that functional IL-3 receptors were detected in ligand binding assays. Scatchard plot analysis of the specific equilibrium binding data for IL-3-responsive leukemic BCPs from one BCP ALL patient and two BCP ALL cell lines yielded a straight linear regression line, indicating the existence of a single class of 60 to 210 high-affinity IL-3 binding sites/cell. The calculated apparent affinity constant (Ka) values ranged from 3.6 x 10(9) to 5.9 x 10(9) mol/L-1. Hence, the concentration of IL-3 required to produce 50% maximal receptor occupancy (Kd) was in the range of 168 to 280 pmol/L. These concentrations are approximately tenfold higher than those required to induce 50% maximal proliferative response from leukemic BCPs in colony assays, indicating that low receptor occupancy is sufficient for growth stimulation of leukemic BCPs by rIL-3. In comparison, less than 10 to 20 IL-3 molecules/cell were bound to IL-3 unresponsive leukemic BCPs even when the concentration of free-[35S]rIL- 3 was as high as 2 nmol/L.(ABSTRACT TRUNCATED AT 400 WORDS)