Cytotoxic effect of lymphocytes from pregnant mice on cultivated tumor cells. II. Blocking and unblocking of cytotoxicity

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Abstract
Sera from BALB/c mice transplanted with either of two syngeneic, methylchol‐anthrene‐induced sarcomas (1315 and 1321) were found to block (i.e. abrogate) the cytotoxic effect of lymph‐node cells (LNC) from multiparous mice on cultivated tumor cells. The blocking activity was seen even at high serum dilutions (up to 1:320). It was found to persist in mice whose tumors had been removed 8–11 days previously. These mice had transplantation resistance against individually unique antigens of the respective sarcoma to which they had been immunized, but not against any common, putatively embryonic ones. In keeping with this finding, their sera did not block LNC‐mediated cytotoxicity to the unique transplantation antigens, when tested 8–11 days after removal of the immunizing tumor. Sera from rabbits immunized with BALB/c embryonic tissues and subsequently absorbed with normal adult BALB/c cells, were found to cancel („unblock”︁) the blocking activity of serum from multiparous mice. The specificity of this unblocking effect and its potential usefulness as a tool to study immune reactions to embryonic tumor antigens need to be established.