Pokeweed-mitogen induced lymphocyte proliferation: the effect of stimulation on mononuclear phagocytic cells.

Human peripheral blood mononuclear cells were stimulated with pokeweed mitogen (PWM) to study the role of mononuclear phagocytic cells (MNP) in lymphocyte proliferation. MNP were identified by cytoplasmic alpha-naphthyl acetate esterase, by the capacity of phagocytosis and by lysozyme synthesis. It appeared that after 3 days of stimulation with PWM all MNP disappeared from the cultures and remained absent during prolonged culture in mitogen-free medium. In non-stimulated cultures MNP remained. The disappearance of MNP from cell cultures was caused by a lymphocyte-derived factor, which was transferable by cell-free supernatants of stimulated mononuclear cells. From experiments in which cultures were treated with different concentations of PWM and from pre-culture experiments, it could be shown that in vitro lymphocyte proliferation required both non-stimulated lymphocytes and freshly prepared MNP. In addition, the decreasing concentration of PWM as stimulating agent, resulted in a decreasing proliferation of lymphocytes, which was inversely proportional to the presence of MNP.