Growth of transplanted rat tumors following administration of cell-free tumor antigens.

The variable effect of immunizations with cell-free tumor extract (TE) in a weakly antigenic (Morris hepatoma 3924a) and a highly antigenic (glioma 9L) inbred rat model system was described. Tumor enhancement was noted with the weakly antigenic tumor following either immunization with a low dose of TE admixed with Freund's adjuvant or immunization with high doses of TE. Enhancement was observed at four tumor cell challenge levels. Tumor enhancement with the highly antigenic tumor was found at only one level and was seen following immunization with intermediate doses of TE and a high tumor cell challenge. Tumor protection was only detected with the weakly antigenic tumor following immunizations with an intermediate dose of TE and challenge with a tumor cell dose 5-fold over threshold. This protection was tumor specific, as judged by an amputation and rechallenge experiment.