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This report compares the sensitivity of 17 tumor cell lines to cytolysis mediated by natural killer (NK) cells or by activated, bone marrow-derived macrophages (AM) from 15 inbred mouse strains. Some tumor cell lines, notably P815, were highly sensitive to AM-mediated lysis but almost completely insensitive to NK cells, whereas other cell lines were lysed by NK cells but not AM. In a genotype survey, some low-responder strains in the NK system, such as A/Sn, were high responders in the AM system, and conversely, one intermediate to high-responder strain (C3H/HeJ) in the NK system was a low responder in AM-mediated cytolysis. In addition, macrophage cytotoxicity factor was necessary to activate macrophages, but this lymphokine did not augment NK activity. Furthermore, the NK population did not contain pre-activated macrophages since pre-activated cells were removed on glass bead columns or by iron carbonyl and a magnet; treatments which have been previously shown not to affect NK cells. These results suggest that NK cells are distinct from AM in physical characteristics, target selectivity, genotype distribution and the mechanism of cytolysis.

Mouse bone marrow cells are cultivated in a liquid culture system in the presence of fibroblast conditioned medium. Under these conditions, proliferation of macrophage and granulocyte precursor cells is induced. Cells of a 5-day-old culture are shown to act as cytotoxic effector cells against tumor targets such as P815, E14, YAC and L5178Y. The effector cell is of macrophage origin since it is susceptible to treatment with the alloantiserum Mph-1.2 plus complement. The kinetics of the reaction resembles the kinetics for killer (K) lymphocyte lysis. In contrast to bone marrow cells, peritoneal macrophages do not show cytotoxic activity against antibody-coated tumor targets although they are susceptible to activation to cytotoxicity by lymphokines. The possible relationship of bone marrow effector cells and K lymphocytes is discussed.

Cooperative effects are described between 2 different mechanisms of macrophage-mediated cytotoxicity, the lymphokine-induced and the antibody-dependent cytotoxicity:Macrophages are activated to nonspecific cytotoxicity by a 24 h incubation with lymphokines. Young macrophages cultured from the bone-marrow or obtained from induced peritoneal exudates, but not the resident peritoneal macrophages, have the capacity to kill antibody-coated tumor cells. To obtain a cooperative effect between the 2 mechanisms both the lymphokine and the antibody have been titrated to a dilution, at which the macrophages did not exert any cytotoxicity. At such subthreshold dosages, however, a very good synergistic effect between the 2 mechanisms was observed. This synergism was obtained with bone-marrow macrophages and with thioglycollate-induced peritoneal macrophages. Macrophages of C3H/HeJ mice, which have been reported to be a low responder strain for lymphokine activation, showed synergistic effects with similar lymphokine concentrations as macrophages from the high responder strain C57BL/10. Thus, in this synergistic situation the low responsiveness of C3H/HeJ mice is completely compensated. Since during an immune reaction in most cases lymphokine as well as antibody are produced, this synergistic reaction will be of particular relevance for in vivo-situations.

Macrophages cultured from mouse bone marrow served as excellent indicator cells for mouse as well as for human migration inhibitory factor (MIF) preparations. The tests were performed in the capillary system and showed high reproducibility. Using mouse or human MIF-containing supernatants, we found an inhibition of 53% compared with the controls. Similar results were obtained if concanavalin A-stimulated human peripheral blood lymphocytes were mixed with mouse test macrophages in a ratio of 1:5 or 1:10.