Abstract
Monocytes were separated from peripheral blood mononuclear cells of normal human donors by adherence on plastic conditioned by cell lines (microexudatecoated plastic) and harvested by exposure to ethylene diamine tetra‐acetic acid. Cytolytic activity was tested by incubating effector cells for 48 h with the murine SV40‐transformed TU5 kidney line or the human lung cancer‐derived CaLu line prelabelled with tritiated thymidine. Lymphokine‐containing supernatants were obtained from in vitro cultures of lymphoid cells with phytohemagglutinin (PHA)^1^, purified protein derivative (PPD), or with Corynebacterium parvum strains CN6134 or CN5888. The monocytes had significant levels of spontaneous cytotoxicity and exposure to lymphokine supernatants markedly enhanced their tumoricidal activity. Augmentation of monocyte‐mediated cytotoxicity required a minimal exposure to lymphokine supernatants for 4 h and was maximal after 24 h of preincubation. Treatment of the effector cells with anti‐human T‐cell serum and complement did not affect either their spontaneous or their lymphokine‐stimulated cytotoxicity, whereas silica impaired both reactivities. Supernatants of cultures with PHA, PPD and C. parvum CN6134 had significant levels of interferon (IF). Since partially purified human fibroblast or leukocyte IF was able to stimulate monocyte‐mediated cytotoxicity, the IF in these supernatants could play some role in the stimulation of the monocytes. However, C. parvum CN5888 supernatants, which had little IF, enhanced monocyte cytotoxicity as effectively as the C. parvum CN6134 supernatants, strongly suggesting that lymphocyte mediators other than IF can augment the tumoricidal activity of these effector cells. Mature macrophages obtained by in vitro cultivation of monocytes for 4–7 days retained natural cytolytic activity and showed enhanced cytotoxicity in the presence of lymphokines. However, more prolonged in vitro cultivation (> 10 days) resulted in cultures of epithelioid and multinucleated cells which had little natural cytotoxicity and were not responsive to lymphokines.