Influence of tumor antigen on maintenance versus depression of tumor-specific immunity.

The development of immunity to tumor-specific transplantation antigens has been studied with a syngeneic methylcholanthrene-induced fibrosarcoma. Of several immunization procedures tested, live tumor implantation followed by resection gave the strongest immune resistance to challenge implantation of tumor cells. The immune resistance persisted with only slight decline for at least 100 days after the sensitizing tumor was cured by surgery or irradiation. If the sensitizing tumor was left in situ past a size of about 7 to 8 mm diameter, the immune resistance of the host declined rapidly but was not abrogated. The depression was related to the size of the sensitizing tumor present at the time of challenge and could be maintained by a residual small tumor mass or by injections of killed tumor cells after a large tumor had been removed. The recovery of immune resistance following the cure of a large tumor was more rapid if the mice were cured by excision than if the mice were cured by local radiotherapy. Depression of immune resistance by injection of killed tumor cells was not seen early in the immune response or 3 weeks after resection of a sensitizing tumor, nor was depression seen at any time if the mice were challenged i.v. The injection of suspensions of killed tumor cells appeared to boost the level of immune resistance, which was in slight decline when it was tested from 20 to 101 days after the removal of a sensitizing primary tumor.