In experimental animals, it has been established that macrophages activated by various stimulants act as effector cells in the inhibition of tumor growth. The present study aims at determination of in vitro effect of activated human macrophages on the growth of human tumors. In the presence of human macrophages obtained from the peritoneal cavity and activated by bacterial lipopolysaccharide (LPS) in vitro, 3H-thymidine (TdR) uptake by human tumor cells was significantly inhibited but that by normal human diploid cells was not. Neither non-activated macrophages nor culture supernatant of the LPS-activated macrophages inhibited 3H-TdR uptake by the tumor cells. Under a phase microscopy, it was observed that activated macrophages attached to tumor cells and degeneration occurred afterward in the tumor cells attached by macrophages. Electronmicroscopically plasma membranes of activated macrophages fitted in closely with the plasma membranes of tumor cells, and microvilli of both cell types protruded to interdigitate each other. From all these results, it was concluded that the inhibition of tumor growth by the activated macrophages stems from the close attachment of the macrophages to the target tumor cells, but not from any mediators produced by the activated macrophages.
