Modulation of lung tumor colony formation by a subcutaneously growing tumor.

When monodisperse tumor cell suspensions of the mouse sarcoma L1 were injected iv into inbred BALB/c mice bearing subcutaneous grafts of the same tumor, the number of tumor nodules developing in the lungs was highest in animals bearing the oldest primary tumor (28 days) and lowest in animals bearing the oldest primary tumor (28 days) and lowest in animals bearing 16-day-old primary tumors. The number of metastases developing in mice bearing 9-day-old subcutaneous tumors was the same as that in control animals without a subcutaneous tumor. This state of low and high susceptibility to lung metastases can be transferred from tumor-bearing mice to whole-body irradiated mice by means of spleen cells, and this transfer of susceptibility is stable for at least 3 weeks. The results indicate a) that a growing subcutaneous tumor can modulate the number of artificial metastases occurring in the lungs as a result of the iv injection of tumor cells, and b) that this modulation occurs by immune mechanisms. In the model presented here, the host is first protected and then made more susceptible to artificial lung metastases by the presence of a progressively growing subcutaneous tumor.