Decline of Natural Killer Cell Activity in Sublethally Irradiated Mice2

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Abstract
Spleen cells of (C57BL/6 × C3H/He)F1 mice were assayed for natural killer (NK) cell activity against YAC-1 and FBL-3 lymphoma targets at several intervals after total-body exposure to a high sublethal dose of137Cs or60Co 7-rays. NK cell activity did not decline for the first 12 days but decreased sharply thereafter and remained low until day 24. The recovery of splenic NK cell activity was delayed. Beginning on day 28, the activity was slowly increased, reaching near-normal levels (80% of controls) 41–59 days after irradiation. Suppressor cells detectable during the period of lowest NK cell activity, i.e., on days 17 and 19, may have been responsible for the delayed and slow recovery. These studies indicated that a) mature effectors of natural cytotoxicity are relatively radioresistant renewable cells with a lifespan of about 2 weeks whose progenitors are radiosensitive cells and b) the kinetics of decline and especially of recovery of NK cell activity may be influenced by suppressor cells. Should NK cell activity confer resistance to autochthonous lymphomas in vivo, it may be a significant consideration for strategies of tumor therapy by cytotoxic agents that reconstitution of the NK cell pool is a slow process and that suppressor cell function must be overcome for full recovery.