Mechanism of Bacillus Calmette-Guérin-induced suppression of metastases in a poorly immunogenic fibrosarcoma.

Bacillus Calmette-Guérin (BCG) reduces the rate of spontaneous pulmonary metastases from a poorly immunogenic fibrosarcoma. To be effective, BCG (1 X 10(6) organisms) must be given in admixture with (1 X 10(6)) tumor cells at the time of transplantation. Reduction of metastasis at this dosage of BCG occurs without a change in the size of the primarytumor or the extent of necrosis within it. Tumors transplanted in admixture with spleen cells from BCG-exposed donors reduced the number of metastases, while spleen cells from normal or tumor-bearing donors had no effect on metastases. Fewer total tumor cells and clumps are collected from the venous effluent of tumors transplanted with BCG than from control tumors. The BCG-treated tumors have more host macrophages intimately associated with the effluent tumor cells then did controls. These data indicate that BCG can inhibit the metastatic potential of a weakly immunogenic fibrosarcoma. The mechanism of this effect appears to be a depression of entry or tumor cells into the tumor vascular channels which may be related to the interaction of tumor cells with BCG-stimulated macrophages.