Analysis of T suppressor cell-mediated tumor escape mechanisms is facilitated by the selective in vitro activation of tumor-specific Ts cells.

We have shown previously that tumorspecific T suppressor (Ts) cells were induced in vivo in BALB/c mice by the syngeneic plasmacytoma (PC) ADJ-PC-5 at very early stages of tumorigenesis [1, 2]. These Ts cells, which suppress a strong primary cytotoxic T cell response, have been characterized in detail [1-3]. There is evidence that Ts cell-inducing antigens (Ts-Ag) on ADJ-PC-5 plasmacytoma cells are expressed to some extent on normal BALB/c spleen cells and are therefore "self' antigens rather than tumor-specific neoantigens [4]. These data were subsequently confirmed by independent comparable studies using the EL4 thymoma ofC57BI/6 mice [5]. Thus, the induction of Ts cells by tumor-associated self antigens seems to be a more general rule and might be an important tumor escape mechanism. To characterize Ts-Ag in more detail we have developed an in vitro system for the selective induction of tumor-specific Ts cells. Ts cell function would be masked in the in vitro Ts assay in the presence of activated cytotoxic T cells, which, like specific cytotoxic T cell clones, are not susceptible to suppression [2]. Activation of cytotoxic T cells is prevented by pretreatment of the ADJ-PC-5 stimulator cells with glutardialdehyd (GA) (Fig. 1). In contrast, specific Ts cells were activated by this approach which suppress the