Medullasin enhances human natural killer cell activity.

Medullasin, a new serine protease found in bone marrow cells, increased markedly human natural killer cell activity. Whereas the natural killer cell activity measured immediately after the treatment with medullasin remained almost on the same level as the control, an incubation at 37 degrees C for several hours increased markedly the natural killer cell activity of the lymphocytes treated with medullasin. Enhancement of the natural cytotoxicity was caused by the treatment with physiologic concentrations of the protease (5-20 micrograms/ml). Inhibitors of medullasin such as phenylmethylsulfonyl fluoride and elastatinal prevented the activation of natural cytotoxicity. Depletion of lymphocytes bearing Fc receptors for IgG abolished the enhancement of natural killer cell activity by medullasin. Interferon activity was not detected in the supernatant of lymphocyte cultures stimulated with medullasin. The medullasin enhanced further the natural killer cell activity of lymphocytes stimulated with interferon. Medullasin activity was detected neither in unstimulated nor stimulated (by concanavalin A or phytohemagglutinin) human lymphocytes. The protease was released easily from human mature granulocytes into culture medium. It is considered from these results that the level of human natural killer cell activity is regulated by medullasin released by mature granulocytes.