THERMAL RESPONSE AND HYPERTHERMIC RADIOSENSITIZATION OF SCID MOUSE BONE-MARROW CFU-C

Purpose: Scid mice are severely immunodeficient as a result of a defec tive recombinase system. Mice with the scid mutation have been shown t o have an increased sensitivity to ionizing radiation, presumably as a result of an inability to repair DNA damage. Little is known of the i mpact of this mutation on the thermal response and on hyperthermic rad iosensitization. This investigation established the thermal response ( 42-44 degrees C), patterns of thermotolerance development, and the imp act of hyperthermia (60 min at 40 degrees C or 42 degrees C) on the ra diation response of bone marrow colony forming unit-culture cells (CFU -C) in scid mice. Methods and Materials: Anesthetized scid mice (pento barbital, 90 mg/kg) were killed by cervical dislocation and the nuclea ted marrow obtained from both tibia and femora by passing 2 mi of cold Mccoy's 5A medium supplemented with 15% fetal bovine serum through ea ch bone. Single cell suspensions of nucleated marrow were heated in 12 X 75 mm sterile tissue culture tubes at a concentration of approximat ely 5 X 10(6) cells/ml. Radiation, when used, was delivered immediatel y prior to hyperthermia by a Cs-137 irradiator (dose rate of 1.20 Gy/m in). Colony forming unit-culture were cultured in semisolid agar in th e presence of colony stimulating factor (conditioned medium from L929 cells) for 7 days. Results: The slope of the radiation dose-response c urve for CFU-C in scid mice was biphasic, the Dos (+/- SE) were 0.29 /- 0.03 Gy and 1.09 +/- 0.20 Gy, respectively. The Dos of the radiatio n dose-response curve for wild type marrow from CB-17 and Balb/c mice were 1.28 +/- 0.05 Gy and 1.47 +/- 0.15 Gy, respectively. The Dos of t he hyperthermia dose-response curves for scid mice were 75 +/- 5, 10 /- 1.4, and 4 +/- 0.2 min, respectively, for temperatures of 42 degree s, 43 degrees, and 44 degrees C. Thermotolerance development at 37 deg rees C increased to a maximum at approximately 240 min after acute hyp erthermia (15 min at 44 degrees C) and thereafter, decreased to contro l levels within 15 h. Thermotolerance did not develop in scid CFU-C du ring chronic hyperthermia at temperatures < 42.5 degrees C. Hypertherm ia (60 min at 40 degrees or 42 degrees C) immediately after ionizing r adiation did not significantly alter the terminal slope of the radiati on dose-response curve of scid CFU-C (Do = 1.28 +/- 0.08 Gy). By contr ast, hyperthermia following radiation of wild type CFU-C resulted in a decrease in the Do from 1.47 +/- 0.05 Gy (Balb/c, rad only) to 1.31 /- 0.08 or 1.06 +/- 0.18 Gy for 60 min at 40 degrees or 42 degrees C, respectively. Conclusion: These results show that the thermal response and the pattern of thermotolerance development of scid CFU-C were sim ilar to that of wild type Balb/c CFU-C, but that hyperthermia given im mediately after ionizing radiation did not alter the radiation respons e of scid CFU-C. The scid mutation does not increase hyperthermic sens itivity or change the pattern of thermotolerance development of scid m ouse CFU-C, implying that the scid mutation is not involved with therm al response, but does render the already radiation-sensitive scid cell s incapable of thermal radiosensitization.