Switching on the macrophage-mediated suppressor mechanism by tumor cells to evade host immune surveillance.

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Abstract
A unique mechanism for tumor cell-induced immunosuppression is demonstrated [using mouse lymphocytes]. In the presence of a nonsuppressive dose of tumor cells, generation of cytotoxic T [thymus-derived] cells in the mixed lymphocyte culture (MLC) is completely suppressed by adding exogenous (peritoneal) macrophages (PM.vphi.) after the initiation of the MLC. Tumor cells can switch on a suppressor mechanism through host macrophages. Suppression can be induced only if resident (splenic) macrophages (SM.vphi.) are exposed to tumor cells prior to addition of PM.vphi.. If SM.vphi. and PM.vphi. are simultaneously present with the tumor cells, induction of suppression is completely precluded. Switching on of the suppressor mechanism by tumor cells has a critical requirement for the collaboration of 2 populations of macrophages, SM.vphi. and PM.vphi., and their presence in a specific sequence (SM.vphi. preceding PM.vphi.). This may represent a mechanism by which tumor cells evade host immune surveillance.