CD4⁺CD25⁺ T_Regs of HCV-naive, HCV-recovered, and persistently HCV-infected chimpanzees all exert suppressive function but differ in their response to exogenous IL-2. (A) Purified CD4⁺CD25⁺ T cells (2.5 × 10⁴/well) of HCV-naive, HCV-recovered, and persistently HCV-infected chimpanzees did not proliferate in response to plate-bound anti-CD3, but suppressed the proliferation of CD4⁺CD25^– T cells. The data are representative of 15 experiments, performed with T cells from 2 HCV-naive, 4 HCV-recovered, and 3 persistently HCV-infected chimpanzees. CD4⁺CD25⁺ and CD4⁺CD25^– T-cell populations are identical to those in Figure 2. (B) IL-2 responsiveness differed between CD4⁺CD25⁺ T cells of HCV-naive chimpanzees and those of chimpanzees that had experienced HCV infection (here shown for Ch1605, Ch6412). The results are representative for 2 HCV-naive and 4 HCV-recovered chimpanzees. Error bars indicate SD. n.s. indicates not significant.