Frequency, phenotype, and function of CD4⁺CD25⁺ T cells of HCV-naive chimpanzees. (A) CD4⁺CD25⁺ T cells (bold histogram line) of a representative HCV-naive chimpanzee (Ch6497) displayed a nonactivated (CD69^–), undifferentiated (CD27⁺CD28⁺) central memory (CD45RO⁺CCR7⁺CD62L⁺) phenotype with higher levels of CD45RO and intracellular CTLA-4 expression than CD4⁺CD25^– T cells (dotted histogram line). The thin histogram line to the left in each graph indicates the negative control. (B) CD4⁺CD25⁺ but not CD4⁺CD25^– T cells express Foxp3. Numbers in the left graph indicate the percentage of events in the respective gate. (C) Purified CD4⁺CD25⁺ T cells do not proliferate in response to plate-bound anti-CD3, but suppress the proliferation of CD4⁺CD25^– T cells (left panel). Moreover, CD4⁺CD25^– T cells proliferate more vigorously in response to plate-bound anti-CD3 than total CD4⁺ T cells (right panel). Mean and SD are indicated.