The state in which macrophages (Mphi) from regressing Moloney sarcomas could kill tumor target cells was a highly labile one which decayed rapidly in vitro. Thereafter, regressor Mphi were noncytolytic. Mphi from several different progressing sarcomas failed to kill, even when challenged with target cells immediately after explantation. Similarly, thioglycollate-induced peritoneal Mphi (TG-Mphi) did not kill. Noncytolygic Mphi derived either from progressing sarcomas or from long-term (up to 96 h) cultures of regressor Mphi were exquisitely sensitive to stimulation by bacterial lipopolysaccharide (LPS); picogram/milliliter amounts induced killing. Similar concentrations of LPS had no demonstrable effect on TG-Mphi. Thus, tumor Mphi generally appeared to have been primed in vivo, with those in regressing sarcomas having additionally acquired cytolytic activity. Inability of progressor Mphi to kill apparently stemmed from lack of, or failure to respond to, the signal needed in vivo to trigger cytolytic activity, rather than the total absence of activation.

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