Cytotoxicity to tumour cells induced in human monocytes cultured in vitro in the presence of different sera.

Rodent macrophages can be stimulated in vivo and in vitro to become cytotoxic to neoplastic cells. It is shown in the present paper that cytotoxicity to a human tumour cell line is induced in human monocytes cultured in vitro in the presence of human serum. The cytotoxic ability is defined as including cytostatic ability, measured as inhibition of 3H-thymidine (3H-TdR) incorporation in tumour cells, and cytocidal ability, measured as release of radioactivity from 3H-TdR-labelled tumour cells. Monocytes cultured in medium containing 25 per cent human serum (HS-M) developed both a cytostatic and a cytocidal ability. When tumour cells were separated from these monocytes by a membrane, allowing factor-mediated interactions, a cytostatic effect was found, thus indicating that secretion of soluble factor(s) may be an important mechanism. The development of cytotoxic ability in the monocytes was accompanied by development of high capacity for phagocytosis of 125I-labelled Candida albicans, increased protein synthesis in the monocytes and microscopically observed alteration into large, well-spread monocytes with accumulation of phase-dense granules in the perinuclear region. Culture of monocytes in the presence of bovine sera induced less cytotoxic and phagocytic ability, as well as a smaller increase in protein synthesis and less morphological alterations, as compared to culture in HS-M.