Noncytotoxic spleen cells from BALB/c mice bearing 15- to 26-mm (but not 29-mm) s.c. MOPC-315 tumors that were cultured in medium containing 2% polyethylene glycol 6000 (PEG) developed substantial levels of anti-MOPC-315 cytotoxicity as assayed by 51Cr release. The level of cytotoxicity obtained increased with progression of tumor growth. Addition of mitomycin C-treated stimulator tumor cells and PEG to the culture of tumor bearer spleen cells resulted in augmentation of antitumor cytotoxicity

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... o a level that was greater than the sum of the levels of cytotoxicity exhibited by spleen cells cultured in the presence of either mitomycin C-treated tumor cells or PEG. Maximal levels developed when the spleen cells were cultured for 5 to 6 days in 2% PEG at a responder/stimulator cell ratio of 15/1 or 30/1. Tumor bearer spleen cells that were cultured in PEG with or without added MOPC-315 stimulator cells exhibited strong anti-MOPC-315 cytotoxicity but were virtually noncytotoxic to allogeneic EL4 and syngeneic blast cells. Furthermore, these spleen cells were far superior to spleen cells cultured without PEG in mediating in in vivo antitumor activity in the local adoptive transfer assay. Thus, tumor bearer spleen cells cultured in the presence of PEG might be useful in immunotherapeutic regimens requiring histocompatible cells with augmented antitumor cytotoxicity but devoid of reactivity against normal cells.