Modulation of NK activity by thymic hormones: in vitro effects of thymostimulin.

Plastic-adherent depleted or not depleted peripheral mononuclear blood cells (PMBC) from healthy donors showed enhanced lytic activity against 51Cr-labelled K562 target cells when exposed to thymostimulin (TS), 1 microgram ml-1, for 3 h, washed and incubated in TS-free medium before testing for natural killer (NK) cytotoxicity. No modification of NK cell activity was seen when effector cells were treated with placebo (splenic extract). The NK boosting activity of TS was lost when effector cells were treated for 3 h immediately before the performance of the cytotoxic test or when this thymic extract was added directly to the mixture of effector and target cells during the lytic phase of 51Cr release assay.