Hyperthermia: the immune response and tumor metastasis

Evidence is circumstantial that in animals and, to some extent, in man, an antitumor immune response may be generated after curative hyperthermia that leads to disappearance of metastases (abscopal response) and the acquisition of host immunity. In rodents, tumor regression after heating does not occur in immunosuppressed hosts, and the tumor cure rate is significantly reduced by inhibition of macrophage activity with silica; cured immune rats succumb to tumor inoculation when immunosuppressed. Quantitative data for cellular and humoral immunocompetence in these situations exist for only a few tumor types, and the bulk of evidence indicates that host response following tumor heating is nonspecific in type, with a major macrophage component. Little is distinctive about the regression of heated tumors, i.e., an abscopal response can follow treatment of tumors by excision or hypothermia. In man, an immune response is seldom evoked by the heating of the common solid tumors. Whole-body heating in animals can cause immunosuppression, probably from a direct damaging effect on lymphoid tissue, and enhanced metastatic spread may follow in the tumor-bearing host, but this has not been proved in man. The differences in response of tumors to heat in animals and man may be due to variations in tumor immunogenicity and host tolerance to heat.