Natural cytotoxicity of mouse monocytes and macrophages

Adherent-phagocytic cells were obtained from the peripheral blood and the unstimulated peritoneal cavity of normal C3H/HeN mice and tested for their cytotoxicity in an in vitro ³H-TdR release assay and in a growth inhibition assay. These cells had appreciable cytolytic and cytostatic activity, which varied with the attacker: target (A:T) cell ratio, against TU5, and SV40 virus-transformed BALB/c tumor cell line. Cytolytic activity was first detected after 48 hr of incubation and was considerably higher at 72 hr. In contrast, this line was resistant to cytotoxicity by nonadherent lymphoid cells at the A:T ratios employed, and adherent cells from the peritoneal cavity were not cytotoxic against YAC-1 target cells in a 4-hr ⁵¹Cr release assay. The natural cytotoxicity of adherent cells from the peritoneal cavity was largely inhibited by treatment with silica. These results indicated that the natural reactivity observed here was mediated by macrophages and not by natural killer cells. The cytotoxic reactivity of the macrophages from normal mice could not be attributed to activation by a variety of possible in vivo or in vitro stimuli. Cells from germ-free nude mice had cytolytic activity, as did cells cultured in medium containing endotoxin-free fetal bovine serum or normal mouse serum. Activation by interferon produced in vitro was also unlikely since macrophages cultured in the presence of anti-interferon were also cytotoxic. As previously described for activated macrophages, peritoneal macrophages from normal C3H/HeJ mice were significantly less cytotoxic than macrophages from syngeneic C3H/HeN mice. The monocytes and macrophages seemed to have somewhat different properties since the addition of lymphokine-containing medium to the adherent peripheral monocytes increased their cytotoxicity, whereas no enhanced activity was observed by adding this supernatant to resident peritoneal macrophages. The normal macrophages and monocytes reacted against both syngeneic (1023) and allogeneic (TU5) tumor cells as well as mouse fibroblast cells transformed by SV40 virus (SV40-3T3). Similar activity was also observed against a human cell line derived from a patient with breast cancer (G11) and against human fibroblast cells transformed by SV40 (SV40-2931 and SV40-2303), but not against nontransformed fibroblasts from the same donors. Thus, normal murine peripheral moncytes and peritoneal macrophages possess natural cytotoxic activity that is selective for tumor and virus-transformed cells.