Augmentation of human natural killer cell activity by cyclophosphamide in vitro.

Cyclophosphamide (CY) has been shown to modulate a number of immune responses. In the present study, we have examined the effect of CY on human natural killer (NK) cells in vitro. Lymphocytes from six normal volunteers were cultured alone or with CY in the medium containing human AB plasma, and their cytotoxicity was assessed against cells of K562. The results demonstrated that lymphocytes when cultured with CY had 154 to 333% increase in NK cell activity as compared to lymphocytes that were cultured in the absence of CY. Similarly, CY markedly boosted the cytotoxic activity of lymphocytes from two healthy donors against cells from cultured B-leukemia 3163. Maximal augmentation occurred in cultured lymphocytes that expressed low levels of effector NK activity. Lymphocytes from 13 of 15 children with acute leukemia in remission when cultured alone had no reactivity to autologous leukemia cells. In the presence of CY, lymphocytes from eight of 15 of these patients developed cytotoxicity against autologous leukemia cells. CY was effective in augmenting NK cell activity at concentrations ranging from 7.5 to 60 micrograms/ml. Maximal potentiation of NK cell activity was induced after 8 hr of treatment with CY, although the increase in NK activity was started within 2 hr of incubation with CY. CY-activated NK cells were found in glass wool- and nylon wool-nonadherent cells. Treatment of lymphocytes with CY did not increase the extracellular levels of interferon. The results of this study demonstrate that CY can activate NK cells in vitro. The possible mechanisms of CY-induced augmentation of NK cell activity are discussed.