In vitro expression of secondary antitumor immunity by in vitro tumor-sensitized T cells: inhibition by tumor-induced suppressor T cells.

In vitro cultivation of memory immune cells from P815- or P388-immune mice with corresponding irradiated tumor cells induced generation of cytolytic T cells (CTL). The induction of CTL generation, as well as the cytolytic activity itself, was tumor-specific. The in vitro generation of CTL from P815- or P388-immune cells was suppressed by spleen cells from mice bearing corresponding progressive tumors (tumor size 15 mm). The tumor-induced suppressor cells suppressed the in vitro generation of CTL, but did not affect their cytolytic function. The suppression was tumor-specific and was mediated by Ly1+2-L3T4+ T cells. Treatment of suppressor cell donors with cyclophosphamide or sublethal gamma-radiation completely abolished the ability of their spleen cells to inhibit the in vitro CTL generation.