Spontaneous tumor regression: possible relationship to in vitro parameters of tumor immunity

Evidence is discussed from three animal systems (Shope papillomas in rabbits, Moloney sarcomas in mice, Schmidt-Ruppin-Rous sarcomas in Japanese quail) that immune reactions can be important in spontaneous tumor regression. In vitro studies performed in these systems indicate that blocking serum factors can thwart cell-mediated immune responses and that "unblocking" antibodies are often found in animals whose tumors have regressed. To what extent spontaneous regressions of human neoplasms are due to immunologic mechanisms is unknown; in 2 patients who had undergone spontaneous tumor regression, tumor cell cytotoxic lymphocytes and unblocking serum factors were detected. Better animal models and better knowledge about the mechanisms of tumor immune reactions are needed before tumor regression can be more successfully induced by immunologic manipulation (to what extent that will even be feasible in man is unknown).