Primary growth of AMC 60 fibrosarcoma inoculated into the hind leg of ACI/N rats resulted in occasional generation of concomitant resistance to growth of a second graft of the same tumor cells in the peritoneal or pleural cavity. Using this syngeneic tumor-host system, experiments were carried out to elucidate the effect of intratumoral injections of an immunomodulator, Nocardia rubra cell wall skeleton (N-CWS), on concomitant immunity. Rats bearing a solid tumor into which N-CWS was repeatedly injected showed a significant inhibitory effect on the proliferation of the tumor cells inoculated secondarily into the peritoneal cavity, i.e., concomitant immunity, as compared to control groups of normal, N-CWS-treated and solid tumor-bearing rats. Peritoneal macrophages, when harvested after i.p. tumor inoculation into the N-CWS treated solid tumor-bearing rats, were found to be significantly potentiated for tumoricidal activity against [5-125I]iodo-2′-deoxyuridine-labeled AMC tumor cells. These potentiated macrophages were induced tumor specifically by i.p. inoculation of AMC tumor cells but not by unrelated syngeneic reticulosarcoma SL 1 tumor cells; nevertheless their tumoricidal activity was observed tumor nonspecifically for the SL 1 tumor cells. Additional experiments revealed that nonadherent peritoneal cells were only weakly tumoricidal and that the macrophage tumoricidal activity was completely abolished in the presence of carrageenan. Thus in the model presented here, it is possible to conclude that the augmentation of concomitant immunity by injection of N-CWS into a primary solid tumor is mainly due to potentiation of the tumoricidal activity of tumor-associated macrophages in the peritoneal cavity. Although the underlying mechanism by which concomitant resistance can be augmented by intratumoral injection of N-CWS remains undetermined, the existence of a tumor-specific trigger for induction of potentiated tumoricidal macrophages may indicate that N-CWS when injected repeatedly into the tumor tissue plays an important role in augmenting a pre-existing, weak, tumor-specific cell-mediated immune response leading to activation of macrophages.
Supported by a Grant in Aid for Cancer Research from Ministry of Education, Science, and Welfare.