Role of a non-committed accessory cell in the in vivo suppression of a syngeneic tumour by immune lymphocytes.

CBA/J mice bearing a methylcholanthrene-induced sarcoma exhibited concomitant immunity to the tumour. The cellular basis for this immunity was investigated by local transfer of mixtures of lymphoid and tumour cells into the footpads of syngeneic mice. Peritoneal exudate cells obtained from tumour-bearing mice 1 day after intraperitoneal injection of saline had a marked tumour-suppressive effect, whereas normally resident peritoneal cells did not. Peritoneal exudate cells from which adherent cells had been removed ('lymphocytes') had a suppressive effect at high or low lymphocyte: tumour cell ratios in normal recipients. In irradiated (450 R) recipients the lymphocytes were suppressive at high ratios only. At low ratios the lymphocytes were suppressive only if the irradiated recipients had been partially shielded or if the lymphocytes were admixed with peritoneal exudate cells from normal mice. It is concluded that an amplifying mechanism, possibly involving macrophages, can operate in tumour cell suppression initiated by immune lymphocytes.