Pediatric acute myelogenous leukemia cells express IL-6 receptors and are sensitive to a recombinant IL6-Pseudomonas exotoxin

We have studied IL-6 receptor (IL-6R) expression on AML cells from 15 pediatric patients by immunocytochemistry/flow cytometry, reverse-transcription polymerase chain reaction, and Scatchard analysis, High-affinity IL-SR were detected on leukemic cells from 12 (80%) patients. Binding sites per cell ranged from 140 to 3580 (median 920; mean 1240), with dissociation constants of 0.26 to 0.71 nM, We therefore assessed the in vitro sensitivity of IL-6R(+) AML cells to treatment with a recombinant IL6-Pseudomonas exotoxin fusion protein (IL6-PE4E), using the XTT cytotoxicity assay. Leukemic cells from eight patients had ID50 values (concentration of IL6-PE4E producing a 50% decrease in cell viability) of < 1000 ng/ml (median, 87 ng/ml; mean, 282 ng/ml), Sensitivity to IL6-PE4E correlated significantly with receptor number. Normal bone marrow mononuclear cells had undetectable IL6-R expression (<20 receptors/cell) and were relatively resistant 10 IL6-PE4E. To test the efficacy of IL6-PE4E for ex vivo purging in an autologous stem cell transplantation setting, we incubated primary IL-6R(+) AML cells with 10(3) ng/ml 1L6-PE4E for 24 h, followed by Inoculation into SCID mice. Mice receiving treated cells showed no leukemic engraftment, while all mice receiving untreated or control-treated cells developed leukemia with a median presymptomatic interval of 55 days. In recipients of IL6-PE4E treated cells, no evidence of occult leukemia was detected by PCR analysis of blood and bone marrow cells at 185 days post-inoculation. These data suggest that IL-6R are expressed on leukemic cells from a substantial percentage of pediatric AML patients, Furthermore, leukemic cells expressing high numbers of IL6-R may he sensitive to IL6-PE4E in an ex vivo purging protocol.