Immune responses to weakly immunogenic virally induced tumors II. Suppressive effects of the in vivo carried tumor YAC

Unprimed spleen cells from A and C57BL/6 mice could not produce cytotoxic responses to their syngeneic tumors: a Moloney virus‐induced in vitro subline YAC‐1 and a Rauscher virus‐induced in vitro subline RBL5, respectively. Spleen cells from A and C57BL/6 mice immunized with YAC‐1 or RBL5 (which cross‐react serologically) generated significant syngeneic cytotoxicities after cultivation in vitro. The in vivo carried tumor of A mice, unlike the in vitro sublines, could not stimulate a priming effect. In contrast, YAC stimulated the formation of suppressor cells in both A and C57BL/6 mice. The suppressor cells abrogated the priming effect of the syngeneic tumors, but not the priming effect of the allogeneic tumors. Furthermore, YAC did not suppress normal allogeneic anti‐tumor responses. The theoretical and the practical implications of these studies are discussed.