Activation of macrophages for tumor cell cytotoxicity: identification of indomethacin sensitive and insensitive pathways.

Macrophages can be activated for tumor cell cytotoxicity by endotoxin- or lymphokine-containing solutions, and in both cases activation can be blocked by the addition of indomethacin. In contrast, the activation of macrophages by nonadherent or inflammatory peritoneal cells or antibody-coated tumor cells is not affected by indomethacin. These results demonstrate that there are at least 2 distinct pathways of macrophage activation, only 1 of which is affected by the addition of the drug. Activation by endotoxin that is indomethacin sensitive requires 2 steps. At present, the first is undefined and not blocked by indomethacin, but the second is inhibited by the drug and appears to require the production of E series prostaglandins. Our results also suggest that neither step alone is sufficient to activate macrophages for cytotoxicity.