Demonstration of a human lymphokine which renders macrophages cytotoxic.

Incubation of human peritoneal macrophages with cell free supernatants of mixed human lymphocyte cultures (MLC) renders them cytotoxic to labelled target tumour cells. The factor responsible for this effect is found as early as the second day of MLC. The production of this factor in man does not seem to require previous sensitization. In contrast with observations in mice, we have not yet demonstrated in man any clear-cut difference between the macrophage cytotoxicity index observed with supernatants from normal MLC and from MLC between the donor and the recipient of a skin allograft. Human blood monocytes can also be rendered cytotoxic by MLC acellular supernatants, but their cytotoxicity is neither as consistent nor as strong as with macrophages. This human factor does not render mouse macrophages cytotoxic.