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Eluates of SEYF-a cells (from polyoma virus-induced sarcoma grown in A.BY mice) containing cytotoxic anti-SEYF-a antibodies mediated complement-dependent cytotoxicity of normal lymph node cells. Some lymphocytotoxicity was also mediated by sera of tumor-bearing mice, but the pattern of cytotoxicity was different from that exhibited by tumor eluates. The titer of lymphocytotoxic antibodies in the serum was lower than the titer of anti-SEYF-a antibodies, whereas eluates exhibited similar titers toward both target cells. These results indicated that tumor cells selectively absorbed the lymphocytotoxic antibodies from the serum. Tumor eluates had cytotoxic activity also against thymocytes, splenocytes, and even bone marrow cells. The strain distribution of sensitive thymocytes to the antibodies present in eluates indicated that Thy 1 was not involved. Unfractionated eluates as well as IgG fractions isolated from eluates were active.

Summary Three types of natural immune responses against malignant cells were studied in vitro: Cytotoxicity mediated by splenic NK cells; cytostasis mediated by splenocytes and binding of naturally occurring antibodies to various tumour targets. These responses were studied in untreated 3 and 12 month old mice and in mice of both age groups inoculated with B16 melanoma cells. The results showed that in normal mice NK activity decreases with age, cytostatic activity remains unchanged and the titre of natural antibodies increases.Twelve-month old mice were shown to be appreciably more resistant than 3 month old mice to the development of tumours from subthreshold numbers of B16 tumour cells.In mice injected with threshold amounts of the B16 tumour, there was no change in any of the responses in the tumour-free period, but there was a decrease in NK activity and an increase in cytostatic activity when a large tumour mass developed. An increase in the titre of natural antibodies in young mice injected with the tumour was also seen. The correlation between these changes and tumour appearance and development is discussed.