Progressive growth of the P815 mastocytoma in semisyngeneic mice evokes the generation of a T cell-mediated mechanism of immunosuppression that inhibits the capacity of passively transferred, tumor-sensitized T cells from regressing this tumor in recipient mice. This conclusion is based on two findings: (a) that it is possible to demonstrate adoptive T cell-mediated regression of established tumors, but only if the tumors are growing in T cell-deficient recipients, and (b) that adoptive T cell-mediated regression of tumors in these recipients can be inhibited by the infusion of splenic T cells from T cell-intact, tumor-bearing donors. The results of additional experiments designed to measure the effect of decreasing the number of suppressor cells and the time that they are infused, relative immune cells, indicate that the function of suppressor cells in this model is to inhibit the replication of passively transferred immune T cells. The results obtained with the P815 mastocytoma are similar to those obtained previously with a chemically induced fibrosarcoma. They show, in addition, that passively transferred immune cells are capable of destroying already seeded metastases in T cell-deficient recipients.

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