Enhancement of tumor outgrowth by tumor-associated blocking factors.

Transplanted lines of 3-methylcholanthrene (MCA)-induced BALB/c sarcomas were used for experiments in which tumors were mechanically suspended in Waymouth's culture medium, the suspensions centrifuged and filtered and the remaining cell-free "tumor fluids" (TF) tested for activity in vitro and in vivo. TF were found to inhibit ("block") cell-mediated cytotoxicity to tumor target cells in vitro with specificity for each tumor tested and even at high dilutions (1:1,000-1:10,000). In vivo, TF enhances the outgrowth of subcutaneously transplanted tumors in non-immune syngeneic mice. This enhancement had both specific and non-specific components, with the non-specific component being most apparent. Enhancement was best demonstrated when the TF were inoculated together with the tumor inocula. In one of two experiments in which TF were given intraperitoneally, specific enhancement was seen, whereas in the other, inhibition of tumor outgrowth was observed.