Immunity against MOPC 104E plasmacytoma: effects of tumor size and time post therapy on in vivo tumor immunity.

BALB/c mice with the plasmacytoma MOPC 104E were cured of palpable tumors (6-15x10(7) cells) with a single injection of cyclophosphamide (10 mg/kg). Animals cured of tumor showed a considerable increase in their ability to reject secondary challenge with graded numbers of viable tumor cells. Mice with palpable subcutaneous tumors were cured therapeutically and rechallenged 22, 44, or 120 days post therapy. The ability of such animals to reject secondary tumor cell challenge was similar in all groups, which implied that in vivo tumor immunity remained relatively constant for at least 4 months post therapy. A second group of animals was treated therapeutically (10 mg cyclophosphamide/kg) 4, 11, or 20 days post tumor cell injection. These therapeutically treated animals were then rechallenged with various numbers of viable tumor cells 30 days post therapy. Mice given cyclophosphamide 4, 11, or 20 days post tumor injection rejected 6, 60, or 400 times as many tumor cells, respectively, as did control animals. These results implied that, over the range of tumor sizes investigated, exposure to greater amounts of tumor antigen resulted in increasing amounts of residual tumor immunity following cure.