Interleukin 1 increases thymidine labeling index of normal tissues of mice but not the tumor

PURPOSE: This study was conducted to investigate the action of human recombinant interleukin 1 as a radioprotector for different mouse normal cells other than bone marrow cells. METHODS AND MATERIALS: Semi-continuous injections of tritiated thymidine were administered every 6 h, over 24 h to determine thymidine labeling index. Mice were injected with recombinant human interleukin 1 24 h prior to tritiated thymidine and were compared to control animals that did not receive interleukin 1. Mice were killed 1 h after the last thymidine injection. The 24 h thymidine labeling index for normal tissues and RIF-1 tumor was determined. Labeling indices were also determined 1-14 days after a series of fractionated irradiations with or without pretreatment with a single dose of interleukin 1 administered 24 h prior to the first radiation. RESULTS: The thymidine labeling index of normal tissues was higher following the injection of recombinant human interleukin 1 24 h before radiolabeling. This was found in all normal tissues tested, including the lip and tongue mucosal basal cell layers, crypt cells of the duodenum, alveolar cells of the lung, hepatocytes, and basal skin cells. The thymidine labeling index of RIF-1 fibrosarcoma was not affected by interleukin 1 injection. A single interleukin 1 injection 24 h before the first radiation fraction also increased the thymidine labeling indices of normal tissues after localized fractionated irradiation. The thymidine labeling index of RIF-1 tumor was not increased by interleukin 1 administration except after relatively high radiation doses (20 Gy in five fractions). The ability of interleukin 1 to enhance the thymidine labeling index declined after the first day following the completion of fractionated irradiation. CONCLUSION: Recombinant human interleukin 1 increased the 24 h thymidine labeling index in normal tissues in mice, but not in RIF-1 tumor. Fractionated irradiation could maintain the effect of a single dose of interleukin 1, administered 24 h prior to the first fraction, up to 24 h after the end of radiation.