In Vivo Radiosensitizing Effects of Recombinant Interleukin 6 on Radiation Resistant BCL-1 B-Lineage Leukemia Cells in a Murine Syngeneic Bone Marrow Transplant Model System

Abstract

The ability of total body irradiation (TBI) to eradicate clonogenic leukemia cells from B-lineage acute lymphoblastic leukemia patients prior to bone marrow transplantation (BMT) is greatly hampered by their inherent or acquired radiation resistance. The radiorefractory nature of these cells is believed to contribute to the high relapse rate subsequent to TBI and BMT in patients with B-lineage acute lymphoblastic leukemia (ALL). A method by which clonogenic leukemia cells could be radiosensitized in vivo could be clinically beneficial. In the present study, we used a highly radiation resistant sub-clone of the murine B-lineage leukemia cell line BCL-I in a syngeneic BMT model to investigate if any of the Bell stimulatory cytokines interleukin 2, interleukin 4, interleukin 5, or interleukin 6 could have radiosensitizing effects. All untreated BALB/c mice (N = 33) inoculated with 1 × 10^th BCL-I cells died of disseminated leukemia within 24 days with a median survival of 13.3 days. TBI (700 cGy = LD_100/30 for BALB/c mice) followed by syngeneic BMT (N = 70) extended the median survival to 23.6 days (P < 0.001 by log-rank test). A single intraperitoneal bolus injection of 100 ng, 500 ng, or 2500 ng recombinant murine interleukin 6(rmlL-6) 2-4 hours before TBI extended the median survival to 32.5 days, 31.0 days, and 30.5 days, respectively (P < 0.01 by log-rank test for a11 dose groups). The improved survival was not due to any direct anti-leukemic activity of rmlL-6 and all control BALB/c mice (N = 15) that received the same doses of rmlL-6 but did not undergo TBI and BMT died of BCL-I leukemia within 28 days with a median survival of 13.6 days. In contrast to rmlL-6, recombinant murine interleukin 5 (rmlL-5) had minimal radiosensitizing effects. A single intraperitoneal bolus injection of 100 ng, 500 ng, or 2500 ng rmlL-5 2-4 hours before TBI extended the median survival only to 27.0 days (P = 0.02 by log-rank test). 25.5 days, and 26.0 days (P < 0.01 by log-rank test), respectively. Neither recombinant interleukin 2 (N = 15) nor recombinant interleukin 4 (N= 15) had any radiosensitizing effects when administered at similar doses and schedules (range of median survival times: 22.4 days to 25.9 days). In contrast to the radiosensitizing effects of short-term rmlL-6 preconditioning, administration of this cytokine (or any of the other cytokines) 24 hours before TBI did not improve the outcome of TBI/BMT. Additional experiments demonstrated that rmlL-6 administration two hours prior to TBI enhances radiation induced apoptosis of BCL-I leukemia cells in vivo, thereby providing a cogent explanation for the favorable impact of rmlL-6 on the outcome of TBVBMT in mice challenged with BCL-I leukemia. To our knowledge. this report represents (a) the first detailed comparative analysis of the effects of recombinant interleukins 2, 4, 5. and 6 on radiation responses of B-lineage leukemia cells. and (b) the first demonstration that a single bolus dose of a recombinant cytokine when administered before TBI can amplify radiation induced apoptosis of leukemia cells and confer extended survival following BMT. Our results indicate that recombinant interleukin 6 may be useful as a radiosensitizing agent against IL-6 receptor positive lymphoid malignancies.

Key Words:

• In vivo radiosensitization
• recombinant interleukin-6
• BCL-I
• B-lineage leukemia
• Syngeneic bone marrow transplant model
• murine leukemia