Effect of CD4⁺CD25⁺ cells on expansion of HCV-specific, peptide-stimulated CD8 T-cell populations. Depletion of CD25⁺ T cells from PBMCs of HCV-recovered chimpanzees (A) and persistently HCV-infected chimpanzees (B) resulted in increased expansion of tetramer-positive, HCV-specific CD8⁺ T cells on stimulation with cognate peptide. Seven representative experiments of a total of 15 are shown.