The development and magnitude of thermotolerance during chronic hyperthermia in murine bone marrow granulocyte-macrophage progenitors: I

Abstract

Murine bone marrow granulocyte-macrophage progenitors (CFU-GM) are capable of developing thermotolerance during exposure to temperatures <42–5d`C. Bone marrow from the tibia and femora was heated to 40–42d`C (i.e. chronic hyperthermia), and challenged immediately with 15 min at 44d`C at regular intervals during treatment (step-up heating). CFU-GM were heated and cultured in McCoy's 5A medium + 15% FBS (fetal bovine serum) and lungconditioned medium (source of colony stimulating factor) in semisolid agar. The kinetics of thermotolerance development and decay, and the magnitude of the thermotolerance during chronic heating with temperatures of AQ-AX -5d`C were similar. Survival increased rapidly to a maxima by approximately 120 min of hyperthermia (temperatures of 40–41 .5d`C) and thereafter decreased with a slope similar to the controls. Normalization for cell killing by chronic hyperthermia that occurred during ‘stepup’ heating permitted analysis of thermotolerance in the surviving cells. The surviving fraction after 15 min at 44CC, during incubation at 40,41 and 41 .5d`C increased from 0.13 to maxima of 0.56 ± 0.04,0.71 ± 003 and 0.82 ± 0.03 respectively, by 150 min and did not decrease for up to 480 min during chronic hyperthermia. The surviving fraction after 15 min at 44d`C during incubation at 42d`C increased more slowly than during incubations at 40–41 .5d`C. The survival of thermotolerant cells after exposure to 15 min at 44d`C during 42d`C chronic hyperthermia was maximal at 0.87 ± 0.08 by 120 min and then decreased after approximately 150 min of exposure to 42d`C. The thermotolerance ratios (TTR's) were 4.0, 5.4, 6.7 and 6.9 for temperatures of 40, 41, 41.5 and 42d`C respectively. The results suggest that chronic hyperthermia temperatures (i.e. 40–2d`C) induce rapid thermotolerance development in CFU-GM during the thermal exposure and protect this normal marrow progenitor during whole body hyperthermia or ex vivo purging of leukaemic cells.

Key Words:

• Thermotolerance
• CFU-GM
• whole body hyperthermia