The development and magnitude of thermotolerance during chronic hyperthermia in murine granulocyte—macrophage progenitors: II

Abstract

We have previously reported that murine granulocyte-macrophage progenitors (CFU-GM) are capable of developing thermotolerance during chronic hyperthermia at temperatures of 40 to 42d`C. However, a differential profile of intrinsic thermal response and, in particular, the capability of developing thermotolerance during chronic heating was identified between CFU-GM and macrophage colony-forming units (CFU-M) stimulated respectively, by lung conditioned medium (LCM) and L929 cell conditioned medium (CCM). Nucleated marrow cells treated in vitro were cultured in McCoy's 5A medium plus 15% fetal bovine serum (FBS) in semisolid agar with 10% of CCM. Two different treatment protocols were used in this study to determine the kinetics of thermotolerance in CFU-M: (1) nucleated marrow from mouse tibia and femur were chronically heated in vitro at temperatures of 40, 41 and 42d`C (up to 480 min) or (2) nucleated marrow cells were heated over a period of 90 min stepwise from 37 to 42d`C, at a heating rate of 0.056d`C/min, before exposure to 42d`C. The amount of thermotolerance developed was analysed at various times after chronic incubation at 40–42d`C by a challenge with 15 min at 44d`C. In contrast to CFU-GM, the surviving fraction of CFU-M heated with 15 min at 44d`C did not increase during chronic hyperthermia at 40d`C for up to 480 min indicating failure to develop thermotolerance. However, CFU-M were able to develop thermotolerance during prolonged incubation at 41 and 42CC, although to a much less extent than observed in CFU-GM. In other words, there was much less development of thermotolerance in murine CFU-M compared to that in CFU-GM. Furthermore, a slow temperature transit from 37 to 42CC over 90 min before exposure to 42d`C induced CFU-M to develop thermotolerance. The thermotolerance ratio (TTR, the ratio of the surviving fraction at maximum tolerance versus normotolerance) increased from a maximum of 3.5 after 180 +min at 42d`C (no warm-up) to a maximum of 4.1 after 60 min at 42d`C when the cells received a slow warmup to 42CC. This implies that in the murine bone marrow granulocyte/macrophage lineage, CFU-M does not normally develop thermotolerance during hyperthermia and that the colony forming unitgranulocyte (CFU-G) and CFU-GM play a more critical role than CFU-M in the initiation and promotion of thermotolerance during chronic hyperthermia. However, in a situation that simulates the slow heatup used clinically in wholebody hyperthermia, e.g., the 90 min slow warmup from 37 to 42d`C, stimulated CFU-M to develop greater thermotolerance more rapidly than during rapid heating.

Key Words:

• Thermotolerance
• CFU-M
• wholebody hyperthermia
• rate of heating