In vitro synthesis of tumor‐specific factors with blocking and antibody‐dependent cellular cytotoxicity (ADC) activities

Spleen cells from BALB/c mice bearing syngeneic sarcomas and from BALB/c mice whose sarcomas had been excised were cultivated in vitro. The culture supernatants were tested for two activities: their ability (1) to suppress (“block”) specific lymph‐node cell‐mediated cytotoxic reactions directed against the respective neoplasms; and (2) to induce antibody‐dependent cellular cytotoxicity (ADC) specific for the antigens of the respective tumors. Both specific activities, blocking and induction of ADC, were detected in culture supernatants from spleens of tumor‐bearing mice, even when repeatedly harvested at intervals over a 7‐day period. Supernatants of cultured spleen cells from mice whose sarcomas had been excised 3‐4 weeks previously also had ADC but no blocking activity. Supernatants of cultures treated with inhibitors of protein synthesis lacked both blocking and ADC activities; the inhibitory effect of cycloheximide on these activities, as well as on protein synthesis, was reversible. Factors in the culture supernatants responsible for blocking and for ADC were labelled when the culture were incubated with 14 C‐leucine. The labelled material was retained by, and could be eluted from, immunoadsorbents for mouse immunoglobulins. In addition, the labelled material bound preferentially to those tumor cells for which specific blocking or ADC activities were observed. The findings indicate that factors responsible for the blocking and ADC phenomena were indeed synthesized by the spleen cells in vitro.