THE DEVELOPMENT AND MAGNITUDE OF THERMOTOLERANCE DURING CHRONIC HYPERTHERMIA IN MURINE GRANULOCYTE-MACROPHAGE PROGENITORS .2.

We have previously reported that murine granulocyte-macrophage progeni tors (CFU-GM) are capable of developing thermotolerance during chronic hyperthermia at temperatures of 40 to 42 degrees C. However, a differ ential profile of intrinsic thermal response and, in particular, the c apability of developing thermotolerance during chronic heating was ide ntified between CFU-GM and macrophage colony-forming units (CFU-M) sti mulated respectively, by lung conditioned medium (LCM) and L929 cell c onditioned medium (CCM). Nucleated marrow cells treated in vitro were cultured in McCoy's 5A medium plus 15% fetal bovine serum (FBS) in sem isolid agar with 10% of CCM. Two different treatment protocols were us ed in this study to determine the kinetics of thermotolerance in CFU-M : (1) nucleated marrow from mouse tibia and femur were chronically hea ted 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 st epwise from 37 to 42 degrees C, at a heating rate of 0.056 degrees C/m in, before exposure to 42 degrees C. The amount of thermotolerance dev eloped 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 degre es 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 thermoto lerance in murine CFU-M compared to that in CFU-GM. Furthermore, a slo w temperature transit from 37 to 42 degrees C over 90 min before expos ure to 42 degrees C induced CFU-M to develop thermotolerance. The ther motolerance ratio (TTR, the ratio of the surviving fraction at maximum tolerance versus normotolerance) increased from a maximum of 3.5 afte r 180 min at 42 degrees C (no warm-up) to a maximum of 4.1 after 60 mi n 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 hyper thermia and that the colony forming unit-granulocyte (CFU-G) and CFU-G M play a more critical role than CFU-M in the initiation and promotion of thermotolerance during chronic hyperthermia. However, in a situati on that simulates the slow heat-up used clinically in wholebody hypert hermia, e.g., the 90 min slow warm-up from 37 to 42 degrees C, stimula ted CFU-M to develop greater thermotolerance more rapidly than during rapid heating.