Influence of limb restraint on the thermal response of bone marrow CFU-GM heated in situ

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

We have previously reported that murine granulocyte-macrophage progenitors (CFU-GM) are capable of developing thermotolerance during chronic hyperthermia at temperatures of 40 to 42 degrees 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 42 degrees C (up to 480 min) or (2) nucleated marrow cells were heated over a period of 90 min stepwise from 37 to 42 degrees C, at a heating rate of 0.056 degrees C/min, before exposure to 42 degrees C. The amount of thermotolerance developed was analysed at various times after chronic incubation at 40-42 degrees C by a challenge with 15 min at 44 degrees C. In contrast to CFU-GM, the surviving fraction of CFU-M heated with 15 min at 44 degrees C did not increase during chronic hyperthermia at 40 degrees 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 42 degrees C, 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 42 degrees C over 90 min before exposure to 42 degrees 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 42 degrees C (no warm-up) to a maximum of 4.1 after 60 min at 42 degrees C when the cells received a slow warm-up to 42 degrees C. 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 unit-granulocyte (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 heat-up used clinically in wholebody hyperthermia, e.g., the 90 min slow warm-up from 37 to 42 degrees C, stimulated CFU-M to develop greater thermotolerance more rapidly than during rapid heating.

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

Murine bone marrow granulocyte-macrophage progenitors (CFU-GM) are capable of developing thermotolerance during exposure to temperatures < 42.5 degrees C. Bone marrow from the tibia and femora was heated to 40-42 degrees C (i.e. chronic hyperthermia), and challenged immediately with 15 min at 44 degrees 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 lung-conditioned 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 40-41.5 degrees C were similar. Survival increased rapidly to a maxima by approximately 120 min of hyperthermia (temperatures of 40-41.5 degrees C) and thereafter decreased with a slope similar to the controls. Normalization for cell killing by chronic hyperthermia that occurred during "step-up' heating permitted analysis of thermotolerance in the surviving cells. The surviving fraction after 15 min at 44 degrees C, during incubation at 40, 41 and 41.5 degrees C increased from 0.13 to maxima of 0.56 +/- 0.04, 0.71 +/- 0.03 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 44 degrees C during incubation at 42 degrees C increased more slowly than during incubations at 40-41.5 degrees C. The survival of thermotolerant cells after exposure to 15 min at 44 degrees C during 42 degrees C chronic hyperthermia was maximal at 0.87 +/- 0.08 by 120 min and then decreased after approximately 150 min of exposure to 42 degrees C. The thermotolerance ratios (TTR's) were 4.0, 5.4, 6.7 and 6.9 for temperatures of 40, 41, 41.5 and 42 degrees C respectively. The results suggest that chronic hyperthermia temperatures (i.e. 40-42 degrees 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.

Intrinsic thermal response, thermotolerance development and stepdown heating in murine bone marrow progenitor cells

Thermal response, thermotolerance development and stepdown heating (SDH) in the murine bone marrow granulocyte-macrophage (CFU-GM) progenitors were determined in vitro. Marrow was removed from femora and tibia, heated in McCoy's 5A medium plus 15% FBS and cultured in soft agar in the presence of three different sources of colony stimulating factor. D0's (+/- SE) for survival curves of CFU-GM heated in vitro were 147 +/- 13, 71 +/- 9, 37 +/- 2, 19 +/- 0.7, 11 +/- 1, and 4.3 +/- 0.3 min, for temperatures of 41.8, 42, 42.3, 42.5, 43 and 44 degrees C, respectively. Arrhenius analysis showed inactivation enthalpies of 812 +/- 9 KJoules/mole (193 +/- 2 Kcal/mole) above, and 2142 +/- 157 KJoules/mole (509 +/- 37 Kcal/mole) below, an inflection at 42.5 degrees C. Thermotolerance development was evident during prolonged hyperthermia exposure at temperatures below 42.5 degrees C (chronic hyperthermia) as a change in the slope of the survival curves after approximately 110 min of heating. Thermotolerance development at 37 degrees C after exposure to temperatures of 43 degrees C or greater (acute hyperthermia) was assessed by fractionated heat treatments consisting of an initial heat treatment (15 min at 44 degrees C) followed by incubation at 37 degrees C and challenge with 15 min or 25 min at 44 degrees C. Maximum thermotolerance occurred after 210 and 330 min at 37 degrees C, respectively. The half-time for maximum thermotolerance development was 36 min. Depending on the amount of heat damage and the maximum amount of thermotolerance development, the decay of thermotolerance was complete after approximately 48-72 h at 37 degrees C. An exposure of 10 min at 44 degrees C before incubation at 40 or 41 degrees C (stepdown heating) reduced the slope of the 40 or 41 degrees C survival curves by inhibiting thermotolerance development that would have otherwise occurred. D0's were 100 +/- 19 and 45 +/- 5 min for 40 and 41 degrees C incubation preceded by 10 min at 44 degrees C, respectively. These studies indicate that whole-body or regional hyperthermia protocols designed either to treat solid tumours or to purge leukemic stem cells from marrow ex vivo should avoid inadvertent temperature elevations to large volumes of marrow. Although, marrow progenitors are capable of thermotolerance development during exposure to temperatures up to 42.3 degrees C, results suggest that conditions of stepdown heating may prevent thermotolerance development.