Histologic aspects of concomitant resistance induced by nonimmunogenic murine tumors.

Concomitant resistance to a second tumor implant was induced in both conventional and nude BALB/c mice by two nonimmunogenic syngeneic tumors of spontaneous origin, an epidermoid carcinoma and a lymphoid leukemia. In the secondary tumor, which was significantly inhibited by concomitant resistance, histologic examination revealed the presence of well-preserved tumor cells without any sign of necrosis and without any host cell infiltration, contrasting with classical immunologic rejection. Tumor cell proliferation as evaluated by the number of mitoses per high-power field was significantly inhibited in the secondary tumor as compared with the corresponding controls. No effect of concomitant resistance could be detected on primary tumor growth.