Enhanced Suppressor Macrophage Activity Associated with Termination of the L5178Y Cell Tumor-dormant State in DBA/2 Mice1

Both cytolytic T-lymphocytes and cytolytic macrophages have been implicated in the long-term maintenance of L5178Y cells in a tumor-dormant state in DBA/2 mice. Eventually, however, the tumor-dormant state terminates, and all mice develop ascitic tumors. In an evaluation of the mechanisms involved in termina tion of the tumor-dormant state, we detected in the peritoneal cavity of many tumor-dormant mice macrophages with increased capacity to suppress the in vitro generation of a secondary antiLSI 78Y cell cytolytic T-lymphocyte response. The incidence of macrophage-mediated immunosuppressive activity in individual tumor-dormant mice was related directly to the number of tumor cells in the peritoneal cavity of those mice. Furthermore, in tumordormant mice harboring fewer than 5 x 104 L5178Y cells, the detection of macrophage-mediated immunosuppressive activity was a prognostic indicator of termination of the tumor-dormant state and development of an ascitic tumor. These data suggest that peritoneal macrophage-mediated immunosuppressive activ ity, through inhibition of cytolytic T-lymphocyte generation in vivo, contributes to the termination of the tumor-dormant state and development of ascitic tumors.