CB was shown to inhibit the PHA-induced activation of rabbit lymph node lymphocytes as assessed by the incorporation of 3H-uridine into RNA or 3H-thymidine into DNA. This suppression was dose dependent with an optimum of 10 µg CB/ml (20. 8 µM). Mitogen-activated lymphocytes escaped the inhibitory effect of the drug when CB was added later than 1 hr after the addition of PHA. CB also suppressed the activation of membrane phospholipid metabolism which occurs among the earliest detectable changes in activated lymphocytes. Thus the incorporation of 14C-choline, 14C-acetate, or 14C-oleate into lecithin in the presence of PHA and CB was the same as the level of their incorporation in unstimulated lymphocytes. The increased incorporaion of 14C-oleate into lecithin of the plasma membrane of activated lymphocytes was similarly prevented in the presence of CB. In contrast, CB exhibited no or only a marginal effect on the phospholipid turnover of unstimulated lymphocytes. Our results point to plasma membrane phospholipid metabolism as the possible site of CB interference with the lymphocyte activation.

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