Metastatic dissemination of B16 melanoma: evidence that metastases can result from nonspecific trapping of disseminated tumor cells.

Spontaneous metastasis from tumor transplants of two representative mouse B16 melanoma clones, G3.5 and G3.12, was examined experimentally to determine whether initial dissemination to the lungs, or secondary systemic spread from established lung metastases, resulted from organ-specific tropism or from nonspecific trapping of circulating tumor cells in capillary beds. In parabiosed mice, subcutaneous tumors metastasized extensively within hosts, but guests remained metastasis-free except following the rare involvement of the parabiotic junction during secondary spread. Intrasplenic tumor transplants metastasized to the liver, whereas intrarenal transplants metastasized to the lungs, reflecting patterns of venous drainage. Subcutaneous implants of neonatal lung and kidney in the flank opposite from the site of tumor initiation acquired metastases only during secondary systemic spread, and there was no evidence of organ selectivity. Metastases from various organs, and derived cell lines, when transplanted subcutaneously grew into tumors that initially metastasized exclusively to the lungs. These results indicate that both initial and secondary metastases of these B16 melanoma transplants occurred by nonspecific trapping of tumor cells in the first capillary bed encountered. In contrast, organ colonization following intravenous injection of tumor cells frequently proceeded beyond the first capillary bed.