PEDIATRIC ACUTE MYELOGENOUS LEUKEMIA-CELLS EXPRESS IL-6 RECEPTORS ANDARE 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-tran scription polymerase chain reaction, and Scatchard analysis, High-affi nity IL-SR were detected on leukemic cells from 12 (80%) patients. Bin ding sites per cell ranged from 140 to 3580 (median 920; mean 1240), w ith dissociation constants of 0.26 to 0.71 nM, We therefore assessed t he in vitro sensitivity of IL-6R(+) AML cells to treatment with a reco mbinant IL6-Pseudomonas exotoxin fusion protein (IL6-PE4E), using the XTT cytotoxicity assay. Leukemic cells from eight patients had ID50 va lues (concentration of IL6-PE4E producing a 50% decrease in cell viabi lity) of < 1000 ng/ml (median, 87 ng/ml; mean, 282 ng/ml), Sensitivity to IL6-PE4E correlated significantly with receptor number. Normal bon e marrow mononuclear cells had undetectable IL6-R expression (<20 rece ptors/cell) and were relatively resistant 10 IL6-PE4E. To test the eff icacy of IL6-PE4E for ex vivo purging in an autologous stem cell trans plantation 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 mic e receiving untreated or control-treated cells developed leukemia with a median presymptomatic interval of 55 days. In recipients of IL6-PE4 E treated cells, no evidence of occult leukemia was detected by PCR an alysis of blood and bone marrow cells at 185 days post-inoculation. Th ese data suggest that IL-6R are expressed on leukemic cells from a sub stantial percentage of pediatric AML patients, Furthermore, leukemic c ells expressing high numbers of IL6-R may he sensitive to IL6-PE4E in an ex vivo purging protocol.