Reproducibility and relation to specific and non-specific anti-tumor resistance of the tumor “sneaking through” phenomenon

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Abstract
The reproducibility and immunological specificity of the tumor “sneaking through” phenomenon and enhancement of tumor growth were studied in syngeneic and random-bred Syrian hamsters by means of a quantitative modification of the transplantation test. After primary challenge the phenomenon was neither observed in normal animals nor in animals effectively immunized against tumor. However, it was regularly observed in some “immune” animals after secondary challenge. In primary challenge of animals “sneaking through” phenomenon was most often observed in animals pretreated with large doses of heat-inactivated tumor cells. This characteristic could not be transferred with serum of pretreated animals. In contrast to specific tumor immunity, the “sneaking through” phenomenon appeared to be immunologically non-specific. This was observed in cross-transplantation tests with tumor cells bearing different TSTAs. Thus, TSTA is not an inducer and apparently not a target for a response leading to enhancement of tumor growth in pretreated hamsters. Experiments demonstrating enhanced tumor growth in pretreated animals at the same time demonstrate two other possibly more essential findings: (1) normal animals are naturally resistant to transplantation of 1 to about 1 × 103 (or more) tumor cells; and (2) this resistance can be totally abrogated by the pretreatment of normal animals with tumor cell preparations. The preliminary data demonstrate that abrogation of natural anti-tumor resistance in adult hamsters subsequently inoculated with SV40 leads to rapid development of primary tumors in such animals. The development of specific anti-tumor immune response in animals treated with inactivated tumor cell preparations was also studied. Significant non-specific inhibition of primary and secondary anti-tumor immune response in Syrian hamsters treated with inactivated syngeneic tumor cells was observed. The data obtained are considered to demonstrate two anti-tumor defense systems in the animal, i. e., nonspecific natural resistance and specific anti-tumor immunity. The first seems to be responsible for elimination of low numbers of tumor cells in the normal organism and also to be essential for effective induction and functioning of the specific anti-tumor immunity.

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