In mice, giving a thymic peptide called thymostimulin (or thymosin‑alpha‑1) repeatedly for a few days makes natural killer (NK) cells more active, especially if the mice are young. The boost is strongest a couple of days after stopping the treatment. This hints the peptide could be used to ramp up immune defenses, but we don’t yet know the right dose or schedule for people.

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 micrograms/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 alpha 1). 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.