Perforin-expressing CD8⁺ T cells are sufficient to suppress host T-cell activation in vivo and viral antigen presentation by endogenous DCs. Naive, polyclonal CD8⁺ T cells from WT or prf^−/− donors (CD45.1 congenic) were transferred into prf^−/− mice to achieve high levels of donor T-cell chimerism (20% to 30%; see “Methods”). Two weeks later, recipients were infected with LCMV. (A) Seven days after infection, in vivo IFN-γ production by T cells was assessed (as in Figure 1). Live gated, endogenous (host) CD8⁺ T cells are shown. (B) Antigen presentation by splenic DCs from recipient mice was assessed by sorting DCs 7 days after infection and culturing with LCMV-specific CD8⁺ T cells. *P < .01.