Immune response to weakly immunogenic virally induced tumors. IX. Mice injected with the in vitro variant of YAC tumor (YAC-1) resist lethal doses of the tumorigenic YAC cells.

The ability of the in vitro propagated syngeneic tumor YAC-1 and the allogeneic tumor RBL5 to stimulate cytotoxic cells in A mice, was in correlation with the ability of these tumors to impose in vivo resistance to the viable YAC cells. Similarly, the ability of allogeneic YAC-1 tumor and syngeneic RBL5 tumor to stimulate cytotoxic cells in C57BL/6 mice was in correlation with the ability of these tumors to impose in vivo resistance to the viable RBL5 cells. In contrast, the original in vivo carried YAC tumor cells that induce the appearance of suppressor cells, as measured by in vitro assay, stimulated the in vivo enhancement of tumor growth in A mice. Adult thymectomy delayed the death of A mice injected with viable YAC cells, suggesting that at least some of the cells participating in the tumor enhancement are thymus derived cells.