The veto activity of third-party CTLs. (A) Specific deletion of 2C effector CD8 T cells by veto CTLs following 72 hours of incubation. (i) 2C levels in the absence of veto CTLs. (ii) 2C levels in the presence of nonspecific veto CTLs of C57BL/6 background. (iii) 2C levels in the presence of veto CTLs of Balb/c background (recognized by the 2C TCR transgene). (B) Annexin staining demonstrating the induction of apoptosis by the specific veto CTLs described in Ai-iii.