Detection of naïve hepatitis C virus-specific CD8+ T cells in chronically infected patients

Virus-specific CD8+ T cells play an essential role for the elimination of HCV infection; however, the exact mechanisms contributing to CD8+ T cell failure in persistent infection are not completely understood. In chronically infected patients, HCV-specific CD8+ T cells are rarely detectable ex vivo by conventional methods. Thus, it is still an open question whether the absence of these cell populations is due to a lack of priming early in infection or caused by deletion during persistent infection. To address this important question, we performed a novel tetramer enrichment protocol by using HLA-A*02-restricted NS31073- and NS31406-tetramers and a large panel of surface marker antibodies to detect and phenotypically characterize HCV-specific CD8+ T cells ex vivo in chronically infected patients (genotype 1a). By using this sensitive method, we were able to detect HCV-specific CD8+ T cells in all tested patients. Interestingly, these cells displayed two different differentiation states: in the majority of patients HCV-specific CD8+ T cells displayed an exhausted effector-memory phenotype characterized by the expression of PD-1. Surprisingly, however, 40% of HCV-specific CD8+ T cell populations displayed a naïve phenotype, suggesting that these cells have never been primed even despite ongoing viral replication. In order to analyze potential mechanisms that may explain the presence of naïve CD8+ T cells, we stimulated HCV-specific CD8+ T cells in vitro by using monocyte-derived autologous dendritic cells pulsed with respective peptides. Importantly, the majority of naïve HCV-specific CD8+ T cells could be primed and expanded in culture, suggesting the absence of intrinsic T cell defects. Next, we analyzed the corresponding viral sequence in respective patients. Consensus sequences were present in half of patients harboring naïve HCV-specific CD8+ T cells clearly suggesting that the presence of naïve HCV-specific CD8+ T cells cannot be explained by the absence of matching antigen. Taken together, our results indicate that HCV-specific CD8+ T cells are present in all chronically HCV-infected patients, that the majority of these cell populations display an exhausted effector-memory phenotype but that also a substantial fraction displays a naïve phenotype. This suggests that a lack of priming contributes to the absence of detectable HCV-specific CD8+ T cells in many chronically infected patients. Our data also suggest that the lack of priming is not due to intrinsic T cell defects or viral sequence variations. Thus, failure of HCV-specific CD8+ T cells is not limited to viral escape or T cell exhaustion but may also be explained by impaired T cell priming. Since naïve HCV-specific CD8+ T cells in chronically infected patients can sufficiently respond to antigen-specific stimulation, these data offer new insights for therapeutic immunotherapy.