REDUCTION OF MITOGEN-INDUCED RESPONSIVENESS OF HUMAN AND MURINE LEUKOCYTES IN-VITRO BY YOHIMBINE AND CORYNANTHINE

This work was undertaken to assess the effects of yohimbine and coryna nthine firstly on the in vitro proliferation of human peripheral blood lymphocytes stimulated with various concentrations of phytohemaggluti nin (PHA), concanavalin A (Con A) and pokeweed mitogen (PWM). Mononucl ear cells were cultured in flat-bottomed 96-well microplates at 37 deg rees C for 96 (PHA and Con A) or 144 hours (PWM) in the presence of on e mitogen and of one alkaloid. Concentrations of yohimbine and corynan thine tested in cultures were 15 to 60 mu g/ml and 30 to 120 mu g/ml, respectively. Cell growth was measured by the incorporation of tritiat ed thymidine into lymphocyte DNA. We observed that both alkaloids inhi bited lymphocyte DNA-synthesis in response to all mitogens. However, t he effects were less pronounced when PWM was the lymphocyte stimulator , indicating that B cells are less susceptible to the drugs. Furthermo re, yohimbine caused a greater inhibition than corynanthine for a give n concentration of the drugs. The alkaloids were added to cultures at different times after the beginning of the incubation period. The effe ct of corynanthine occurred when the alkaloid was added during the fir st 24 hours of the cultures, whereas the effect of yohimbine was evide nt even 48 hours after addition to PHA- and Con A-stimulated lymphocyt e cultures. We also tested the influence of both alkaloids (30 to 120 mu g/ml) on the in vitro proliferation of murine spleen cells treated with 1 % PHA and 50 mu g/ml Con A and interleukin 2 (IL-2) secretion. In this case cell proliferation was assessed by the MTT [4,5-dimethylt hiazol-2-yl]-2,5-diphenyltetrazolium bromide) method and IL,-2 product ion was analysed by ELISA. Similarly to what was observed with human p eripheral lymphocytes, corynanthine and yohimbine also inhibited murin e leukocyte reactivity. Furthermore, the inhibition caused by the drug s was simultaneous to a suppression of IL-2 production. The present st udy therefore demonstrates that the stereoisomers indole alkaloids yoh imbine and corynanthine modulate DNA synthesis in mitogen-stimulated h uman lymphocytes.