Acquired resistance towards immune defense during metastatic progression represents a secondary phenomenon

The role of natural immune defense in the control of metastatic spread of tumor cells was evaluated by adapting an s.c.-grown, antigenic, but non-immunogenic rat fibrosarcoma (Bsp6S) to ascitic growth (BSp6A) in the strain of origin (BDX), where peritoneal cells display a high level of NK and macrophage activity. Parallel tests were performed to determine whether tumor cells can selectively adapt to non-specific immune defense, i.e. whether antigenicity of the non-immunogenic tumor remains unaltered, and whether this is accompanied by metastatic progression. During adaptation to ascitic growth the tumor line gradually lost susceptibility to NK cells and macrophages. This was due to the appearance of an increasing number of resistant clones (BSp6S, none; BSp6A, 50% of clones), which had lost binding structures for NK cells and macrophages. No alteration could be observed in antigenicity of the BSp6A variant as revealed by clonal analysis using LD of CTL. Neither BSp6S nor BSp6A cells metastasized. When the ascitic variant was retransplanted s.c. (BSp6AS), susceptibility to NK cells and macrophages was further decreased, in the sense that all clones of the BSp6AS variant became resistant. Furthermore, the BSp6AS variant had lost some of the tumor-associated antigens (TAAs) found on BSp6S and A variants. More important, upon s.c. transplantation BSp6AS regularly metastasized to the draining LN, contrary to BSp6S and BSp6A. When locally growing tumors were excised, rats implanted with BSp6AS frequently died with metastatic tumor burdens in LNs and lung, while other animals survived after excision of BSp6S or BSp6A. The data indicate a correlation between resistance to lysis, morphology and metastatic capacity. But the initial loss of susceptibility to NK cells and macrophages during adaptation to ascitic growth, which can be considered as an escape mechanism, was not accompanied by increased metastatic capacity. Hence, we suppose that with respect to metastatic progression of the BSp6AS variant, resistance to lysis by loss of binding structures represents only a secondary, but not a causative, element.