The use of P815: a warning.

It is generally known that many malignancies are accompanied by forms of impairment of the immune system of the host. Also P815 mastocytoma cells can exert suppressive effects on the immune response reaction in vivo and in vitro. However, we have found a product released by P815 cells, which is able to activate macrophages non-specifically; macrophages normally not cytotoxic against tumor cells become cytotoxic against various tumor cell lines upon incubation with the P815 product. Nevertheless the same product which induces macrophage activation in vitro, seems to cause enhancement of tumor growth in vivo. We feel, if the activity of a tumor product depends so strongly on the experimental conditions used, one should be very careful in interpreting the results of experiments in which P815 cells are used as target cells.