Leukemic B-cell precursors constitutively express functional receptors for human interleukin-1

Abstract

This study analyzes the expression of functional interleukin-1 (IL-1) receptors on leukemic B-cell precursors (BCPs) from B-cell precursor acute lymphoblastic leukemia (BCP ALL) patients. We first investigated the specific binding of 125I-labeled recombinant IL-1 (125I-rIL-1) (4 x 1017 cpm/mol) to fresh marrow blasts from 11 BCP ALL patients. In five of 11 cases, the binding of 125I-rIL-1 was significantly blocked by excess cold rIL-1. In these five cases, the cell-bound radioactivity ranged from 146 cpm/106 cells to 2,412 cpm/106 cells (mean ± SE = 782 ± 414 cpm/106 cells), indicating that 4 to 60 femtomols (mean ± SE = 20 ± 10 femtomols) of 125I-rIL-1 were specifically bound per 107 cells. The estimated number of 125I-rIL-1 molecules bound per cell ranged from 219 to 3,618 (mean ± SE = 1173 ± 621). In all five cases, BCP colony formation was stimulated by 10 ng/mL (570 femtomolar) rIL-1, and the background-subtracted colony numbers ranged from 130 to 298 (mean ± SE = 226 ± 31). In contrast, no stimulation was observed in six cases that showed no significant 125I-rIL-1 binding. Hence, there was a high correlation between 125I-rIL-1 binding and IL-1 responsiveness, indicating that functional IL-1 receptors were detected in ligand binding assays. Scatchard plot analysis of the specific equilibrium binding data for leukemic BCPs from two IL-1 - responsive BCP ALL cases yielded straight linear regression lines, indicating the existence of a single class of 132 to 154 high affinity IL-1 receptors/cell. The apparent affinity constants (Ka) values ranged from 5.2 x 109 mol/L-1 to 1.2 x 1010 mol/L-1. Notably, the concentrations of IL-1 required for half-maximal receptor occupancy (kd = 83 pmol/L to 190 pmol/L) were approximately three orders of magnitude higher than those needed to elicit a half-maximal proliferative response of leukemic BCPs in colony assays (0.1 to 1.0 ng/mL = 5.7 to 57 femtomolar), indicating that only a small fraction of IL-1 receptors need to be occupied to stimulate leukemic BCPs. Notably, IL-1 unresponsive leukemic BCPs from one BCP ALL patient and two BCP ALL cell lines (REH, KM-3) did not exhibit any significant IL-1 binding (< 10 IL-1 binding sites/cell), and two additional IL-1 unresponsive BCP ALL cell lines (NALM-6, HPB-NULL) expressed only 24 to 54 IL-1 binding/sites cell with a Ka of 7.8 to 9.8 x 109 mol/L-1. Fluorescence-activated cell sorter (FACS) isolated pure populations of leukemic BCPs from IL-1 - responsive BCP ALL patients also showed a proliferative response to rIL-1, providing direct evidence that the observed rIL-1 effects were not mediated by non-leukemic accessory cells. These data provide direct and unprecedented evidence that leukemic BCPs from some BCP ALL patients constitutively express functional high affinity IL-1 receptors. By comparison, FACS sorted CD10/CALLA+ normal BCPs from two fetal livers (FLs) failed to proliferate in response to rIL-1. In 21 BCP ALL patients, we compared the bioactivity of rIL-1 on leukemic BCPs to the bioactivities of biochemically purified HMW-BCGF and LMW-BCGF. rIL-1 stimulated the proliferative activity of LPC in ten of 21 cases without inducing differentiation and the mean (± SE) plating efficiency was 0.272% ± 0.053%. Of these ten IL-1 responsive cases, all ten responded to LMW-BCGF with a mean plating efficiency of 0.424% ± 0.117%, and eight responded to HMW-BCGF with a mean plating efficiency of 0.379% ± 0.232%, providing evidence that B-lineage LPC in a significant number of BCP ALL patients coexpress functional receptors for rIL-1, LMW-BCGF, and HMW-BCGF. There was a marked interpatient variation in growth factor response patterns of B-lineage LPC and the growth factor responsiveness correlated with disease status. While only two of 15 newly diagnosed cases were able to respond to rIL-1, LMW-BCGF, as well as HMW-BCGF, six of six relapsed cases responded to all three growth factors. Also provided is circumstantial evidence for the existence of multiple immunologically distinct LPC subpopulations that differ in their growth factor requirements.