Enhancement of natural killer cell activity in mice by treatment with a thymic factor

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Abstract
The administration of a thymic factor, thymostimulin (TP-1), to mice resulted in considerable augmentation of natural killer (NK) cell activity as measured in a short-term assay against 51Cr-labeled YAC-1 target cells. Conditions suitable for detection of the thymostimulin-induced boosting of NK included multiple daily exposures to TP-1 (50 μg/kg), and peak levels of reactivity were observed at 2–4 days after discontinuation of treatment. A strict age-dependency of the effect was also observed, with optimal augmentation of NK-cell activity when TP-1 was administered to mice at 4–6 weeks of age. The effect was not limited to TP-1 treatment but was also observed on administration of another thymic factor (thymosin α1). The activated cells responsible for the increased natural cell-mediated cytotoxicity appeared to be typical murine NK cells, judging by both functional and antigenic criteria.