Phenotypic and functional characterization of CD4⁺CD8⁺ cells in a chimpanzee during the course of HCV infection. (A) Absolute number of CD4⁺ and CD8⁺ cells and CD4⁺CD8⁺ cells per centimeter liver biopsy during the course of HCV infection. The 3 arrows indicate the time point of intravenous HCV inoculation. CID₅₀ indicates 50% chimpanzee infectious dose. (B) Activation status of CD4⁺ and CD8⁺ cells and CD4⁺CD8⁺ cells as assessed by expression of HLA-DR. + and - indicate presence and absence of serum HCV RNA as determined by RT-PCR. (C) Phenotype of CD4⁺CD8⁺ liver-derived cell lines. In the left panel, cells were gated on CD3⁺ T cells. In the middle panel, CD3⁺CD4⁺ T cells were gated and examined for CD8 α and β chain expression. The anti-CD8α antibody recognizes the α chain either in the αα homodimer or in the αβ heterodimer. The anti-CD8β antibody recognizes only the β chain. In the right panel, cells were gated on CD4⁺CD8⁺ cells and examined for TCR αβ or γδ expression. (D) Proliferative responses of the respective CD4⁺CD8⁺ T-cell lines to HCV proteins. Dotted lines represent the cutoff of positivity.